ALBERT

Notes: Abstract— Hysteresis loops of a low carbon steel measured at various temperatures and plastic strain amplitudes are analysed using the statistical theory of the hysteresis loop. It is shown that the parameters of the statistical theory characterizing the saturated hysteresis loop, i.e. the probability density function and effective saturated stress, can be obtained from experiments. The probability density function is influenced mainly by strain amplitude; the effective saturated stress mainly by temperature.

Notes: Abstract— Room temperature fatigue crack propagation in a sintered tungsten alloy was studied. The fatigue crack growth rates were found to be identical for the material in the sintered and forged and as sintered conditions. The propagation rates are slower when compared with other metals due to the relatively high Young's modulus of tungsten. The value of the exponent m in Paris' power law equation was found to be 12 which is higher than for most metals. This was ascribed to the activity of a cleavage mechanism through some of the tungsten grains along with the ductile decohesion fatigue mechanism.

Notes: Abstract— The present work emphasizes the need to understand the processes involved in fatigue–creep interactions before reliable life predictions may be sensibly made. In particular, the simultaneous interaction involving a strain controlled cycle with a dwell period at maximum tensile strain is analysed and the accumulation of fatigue–creep damage computed in terms of strain range, extent of dwell and microstructure. Dominant failure modes are thus identified and the findings are used to explain the apparently different failure mechanisms previously reported for a 1Cr–Mo–V alloy steel and a Type 316 stainless steel. Due to lack of data overall mapping of regimes of dominance is not possible, but in cases where a single mechanism prevails throughout the duration of dwell accurate lifetime prediction may be achieved.

Notes: Stress intensity factors are determined for internally and externally cracked, pressurized thick cylinders with partial autofrettage (less than 100% overstrain). The solutions are based on a superposition of existing solutions which does not involve any loss of accuracy. Implications of the stress intensity factor results for the safe-life design of pressure vessels subjected to fatigue are discussed.

Notes: Fatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non-singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non-singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.

Notes: The paper presents a comprehensive review, supplemented by original data, of the engineering fatigue behaviour of copper. Variations in manufacturing route and softening treatments are shown to have little effect on the fatigue of annealed copper but the high cycle fatigue strength is increased by cold work. The high strain fatigue behaviour is defined in terms of the plastic strain range and the cyclic stress-strain characteristics are documented. Fatigue behaviour in bending and torsion is defined by data and related to that in tension by simple design rules.Notches are found to reduce the laboratory measured fatigue strength of copper by ∼ 30% and the effect of surface finish, surface distortion and surface residual stress is defined in the literature. Fatigue crack growth is defined in terms of stress intensity factor range ΔK by an upperbound law and, together with the conditions for non-growth (ΔK0), shown to relate to the equivalent conditions for steels via the ratio of the respective elastic moduli.The effect of environment on the fatigue of copper has received scant attention in the literature, such results as exist suggesting little if any reduction in strength to be brought about by gaseous or aqueous environments. The most dramatic change is the improvement of about an order of magnitude which results when tests in vacuum are compared with equivalent tests in air. Results of fatigue tests on copper in ammoniacal environments are conspicuously absent from the literature.As the test temperature is reduced below room temperature there is a predictable increase in high cycle fatigue strength, a reduction in fatigue strength occurring above room temperature. High strain fatigue test results presented in terms of plastic strain range appear insensitive to temperature although at very low strain rates and high temperatures a reduction in fatigue strength occurs. A linear life fraction cumulative damage creep-fatigue law appears sometimes to be non-conservative but much more testing is needed to evaluate fatigue damage summation laws generally for copper.Numerical data are given in support of all the aspects of the engineering fatigue behaviour reviewed in the paper.

Notes: The birth and growth of short cracks is analysed from an elastic-plastic fracture mechanics viewpoint. Low to high cumulative damage tests from the low stress to high strain regime indicate that there is no crack initiation period in the metallurgical sense and that cracks grow from the first cycle, but at a slow rate. The initiation phase terminates when one crack starts to dominate and accelerates to failure, its initial size being given by 〈displayedItem type="mathematics" xml:id="mu1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:8756758X:FFE263:FFE_263_mu1"/〉 for the medium carbon steel tested here, of grain size 56 μm.

Notes: Fractographic analyses have been used to explain the cyclic crack growth behaviour of A533-B, Ducol W30, a C-Mn steel and type 304 stainless steel in simulated light water reactor environments at ambient temperature. Fractographic observations have offered an explanation for anomalous crack growth behaviour and have also indicated where micro structural or environmental variables dominate in producing certain fracture modes and crack growth rates. An understanding of the operative corrosion fatigue mechanisms has been formulated through these fractographic analyses. Environmental crack growth in the ferritic steels has been described by a model involving both anodic dissolution and hydrogen embrittlement. Conditions where only one of these mechanisms would dominate have been identified and limits to their effect postulated. A crystallographic mode of failure observed in the austenitic type 304 stainless steel has also been explained by a selective dissolution process.

Notes: Abstract— Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine. The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm. The results in impact fatigue tests were compared to those under non-impact conditions. The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue. When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking. Such a transition will result in the acceleration of crack growth rate. The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature.

Notes: Abstract—The detection and measurement of surface microcracks in unnotched specimens are becoming more important, both from the point of view of description of crack growth by linear elastic fracture mechanics and from the classical endurance limit approach. Theoretical analyses of the current distribution in a cylindrical test specimen show promise that the a.c. potential drop method will become more sensitive for surface microcracks when higher current frequencies are applied. This effect was experimentally affirmed during fatigue tests on unnotched cylindrical specimens. With a 40 kHz current frequency semicircular surface microcracks with an area of 0.0066 mm2 (0.05% of the specimen cross-section) were detected. For accurate and reproducible crack growth measurements, a 5 kHz current frequency is preferable.

Notes: Abstract—A fully age hardened Al-4%Cu-1%Mg alloy has been subjected to plain-fatigue and fretting-fatigue and the resulting fracture surfaces compared. Crack initiation in the plain-fatigue sample occurred at cracked inter-metallic inclusions and subsequent crack growth was at 90° to the surface. Crack initiation in the fretting-fatigue sample occurred at the edge of wear scar and subsequent crack growth was at 45° to the surface. Measurement of fatigue striation spacing on the fracture surfaces showed that, for the same applied stress, the initial crack propagation rate was an order of magnitude higher in the fretting-fatigue sample. This accelerated crack propagation rate was maintained until the crack reached a depth of 0.5–1.0 mm; thereafter the growth rate was the same as that in the plain-fatigue sample.

Notes: Abstract—Ultrasonic bulk measurement technique was used to measure crack closure and a clip-on gage surface measurement technique was used to measure residual displacement at the crack tip of a propagating fatigue crack in 2219 and 7075 aluminum alloys. Experiments were carried out in vacuum, humid air, dry O2 gas, N2 gas, and deuteriated water vapor environments to evaluate effects of environments on residual strain and crack closure loads. It was observed that the closure load detected by ultrasonics decreased with introduction of humidity while the residual strain parameter dincreased with the humidity. The results are modeled as a build-up of oxide layer in addition to strengthening of asperities on the fracture surface. In the light of these observations, the effectiveness of surface and bulk measurements is evaluated. The results demonstrate that surface measurements are not always representative of the bulk behavior and use of such measurements in predictive analysis for fatigue can be misleading.

Notes: Abstract —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.

Notes: Abstract— Optical and scanning electron microscopy have revealed the existence of grain boundary cavities in a series of specimens which had been cyclically deformed in the strain range ±0.25% with hold times ranging from 0 to 1000 min. The way in which these defects increase in size and number has been measured and found to correlate with the creep strain accumulated during the hold periods.A further observation is that a critical amount of deformation is required before either fatigue or creep type damage is nucleated. These observations lead to an alternative method to the currently accepted linear damage summation rule for estimating a lower bound of the creep-fatigue endurance.

Notes: This paper puts forward a new method for analysing the behaviour of very short fatigue cracks. A probability function is introduced into the definition of the growth threshold, which rationalises the scatter in experimental data produced using an aluminium bronze alloy. This probability function can be visualised in terms of the microstructure of the material.It is shown that, in this material as in mild steels, fatigue crack initiation is not the critical stage. Initiation occurs relatively easily, but the cracks so formed may grow to only a few grain diameters in length before being arrested; thus it is the behaviour of cracks of this length which is critical in determining the fatigue strength of the material.These observations, when combined with the probability functions, allow estimation of the probability of failure of a component or structure in service with greater confidence than the methods used at present.

Notes: The high temperature low cycle fatigue properties of modified 9Cr-1Mo ferritic steel in a hot forged and a hot rolled condition have been evaluated. The hot forged material exhibited inferior fatigue properties as compared to the finer grained hot rolled material. Analysis of the data indicates that a larger grain size adversely affects the initiation stage but has little effect on the propagation stage. A steeper slope on the Coffin-Manson plot results when the number of cycles to initiation is reduced.

Notes: The overload induced fatigue crack propagation behavior of several aluminum and steel alloys was examined as a function of the baseline stress intensity factor range (δKb). In order to gain a clearer understanding of the parameters which influence the cyclic delay phenomenon, under both plane strain and plane stress conditions, tests were conducted at δKb values ranging from the near threshold regime to high δK levels approaching fast fracture. Large amounts of overload induced cyclic delay (˜100,000 cycles) were observed at both high and low δK levels (provided the plastic zone size/thickness ratio and plastic zone size/grain size ratio approached unity, respectively) with significantly less delay occurring at intermediate δK values. All alloys examined exhibited this type of delay behavior which can be described by a “U-shaped” plot. The delay phenomenon at high δKb levels under plane stress conditions was attributed to increased crack closure associated with large tensile displacements in the wake of the advancing crack. At low δKb levels increasing cyclic delay was attributed to an increased effective overload ratio as δK approached δKth.

Notes: Changes in the crack growth behavior of 7075-T651 aluminum specimens which had been exposed to temperatures between 121 and 179°C were evaluated. Specimens were fatigue tested at room temperature under flight-by-flight loading conditions. Results from these tests were then compared with data from the as-received material. Exposure to 141°C or more produced a definite increase in specimen life, apparently due to a decrease in crack growth rate. In contrast, preliminary experiments using constant amplitude loading did not show noticeable effects of these thermal treatments on fatigue lifetime. The thermal treatments were found to cause overaging of the metastable precipitate microstructure, which was thought to be responsible for the changes in fatigue response.

Notes: The initiation stage and short crack behaviour in torsional fatigue of a 0.4% C steel was investigated by a replication technique.The fatigue cracks initiated and propagated in the ferrite phase which is located at the prior austenite grain boundaries in the form of long allotriomorphs. At this stage of crack development it is proposed that crack growth rate depends on the extent and intensity of plasticity at the tip of the crack. Crack growth per cycle is correspondingly proportional to the strength of the slip band. The ferrite-pearlite boundaries are strong barriers to crack propagation, which is manifested by a deceleration of growth and possible arrest. On raising the stress level the previously non-propagating cracks may continue to grow by branching or joining with other cracks in the ferrite phase. This process is repeated until the stress fields of one or more dominant cracks attain a critical value to sustain continued growth that leads to failure.

Notes: CT type specimens containing two layers of 309L and 308L cladding stainless steels welded to A508 carbon steel and 316 stainless steel were specially devised to test the influence of R ratio and environment on the crack propagation rate behaviour of cladding materials at 300°C. Large effects are shown. The crack growth rate under vacuum can be smaller by more than one order of magnitude as compared to air. Large differences are also shown on the fracture surfaces, where it is observed than vacuum promotes the formation of large crystallographic {111} facets. The effect of environment is briefly discussed on the basis of existing gas adsorption models.

Notes: The monitoring of fatigue crack initiation mechanisms and short fatigue crack growth usually involves interrupting otherwise continuous load cycling for examination of the test-piece. This permits attainment of high resolution over a large area of surface where the fatigue cracks could initiate. However, breaks in the fatigue cycling can affect the fatigue endurance of the test-piece and in the present study of Ti-6A1-4V this has been shown to depend on the duration of the rests, the number of cycles between rests, the type of examination employed and the hydrogen content of the material. The results of the work are discussed in terms of the possible influence of the monitoring technique on the fatigue process.

Notes: Abstract— Fatigue crack tip opening displacements and strains in the material very close to the crack tip have been determined from measured displacements for cracks grown in both vacuum and humid air environments. The environment alters both the relations between crack tip opening displacement and crack tip strain, and the effect of cyclic stress intensity on these factors. Results of dynamic observation of intermittent crack growth are correlated with fractographic evidence. The relationships between crack tip parameters are used in a previously developed mathematical model. The effect of wet air on fatigue crack growth is found to be a reduction in crack tip plasticity.

Notes: Abstract— Unexpected arrest of long fatigue cracks was observed in mild steel single edge notch three-point bend specimens tested under constant amplitude loading. Arrest was associated with a low, but still positive, slope of the crack length against stress intensity factor curve, and can be explained using the R -curve concept for fatigue-crack growth. At a stress ratio of 0.1, the fatigue threshold was 6.6, 7.3 or 8.0 MN/m3/2 depending on the definition of threshold used. This has obvious implications for both the development of a standard test method for the fatigue threshold and the application of data to practical problems.

Notes: Abstract— The influence of age-hardening on the middle and low crack growth rates of a 7075 Al alloy is studied in vacuum. A transition in fracture surfaces morphology and crack growth curves is observed with the T 651 and T 7351 treatments in the near-threshold regime. Measurements of crack closure show its dependance on surfaces roughness and explain the lack of dependance of ΔKth with load ratio, except for the T 7351 alloy. An equation of crack growth rate to the fourth power of ΔKeff is in good agreement only with the crack propagation curves obtained for microstructure with an homogeneous deformation mode.

Notes: Abstract— Fatigue crack growth measurements are usually made on standard specimens containing long cracks (∼10 mm) although in most practical situations, a large part of the fatigue life is spent with much shorter dimensions. The purpose of the present study is a comparison of crack growth behaviour for long cracks (∼13–16 mm) in CT specimens and smaller ones (∼0.3–0.5 mm) in four point bend specimens. Large effects are noticed indicating that, at a given stress intensity factor amplitude, the crack growth rate is significantly higher in specimens with short cracks. Mouth displacement measurements for both specimen configurations show that the crack closure phenomenon accounts for the observed effect. Crack closure is likely to be associated with fracture surface roughness as shown by partly machining the material left behind the crack tip in CT specimens.

Notes: Abstract— The fatigue crack closure response was investigated for a surface crack in BS4360 50B structural steel, subjected to (1) constant amplitude loading and (2) constant amplitude loading interrupted by a single peak overload. A variety of compliance techniques was employed to determine closure behaviour. The crack mouth gauge measured the bulk, plane strain closure load, while the near tip strain gauge indicated the surface, plane stress closure response. For constant amplitude loading it was found that the surface regions of a surface crack are closed for a greater portion of the load cycle than the maximum depth point. A single peak overload caused different closure and growth rate transients at the surface of the thumbnail crack and at the maximum depth point. For growth rates above 10-6 mm/cycle, such behaviour agrees with the response of a through crack when subjected to constant amplitude loading, and a single peak overload.

Notes: Abstract— Fatigue cracking of complex structure often involves several interacting cracks developing in a sequence of crack growth, arrest and reinitiation. A “combined” method of damage tolerance analysis is presented which employs fracture mechanics concepts to calculate crack growth and fatigue data from notched coupons with the appropriate notch radius for the crack initiation phase. The notched coupon data, plotted as peak elastic notch stress vs cycles to crack initiation, are shown to be applicable even when limited yielding occurs at the notch root. For several practical reasons it is recommended to select the initial crack size, ai, for the crack growth phase to be as large as possible, but in accord with two selection criteria. First, ai, must be within a notch-root region wherein the elastic stress distributions near a variety of notches are virtually identical. Secondly, ai must be small enough not to significantly influence the stress distributions for other cracks. The Combined Method is illustrated by means of an example involving fatigue crack growth along a widthwise row of holes in 305 mm wide test panels.

Notes: Abstract— Crack shape evolution has been studied in high strength, rolled thread, bolts. It has been found that, using the Crack Microgauge, it is possible to detect and size fatigue cracks, located at the thread root, from depths of less than 0.1 mm up to fracture. This work required a special probe system to be developed and also a theoretical solution for the a.c. field distribution in a thread. Some of the work and results are briefly described in this technical note.

Notes: Abstract— The paper shows the practical significance of theoretical studies of fatigue crack propagation in real structures, and a comparison of the results of fracture mechanics theory with the actual failure mode of structures. It follows from these conclusions that neither the influence of the magnitude and complexity of structure, nor that of the random character of loading can be overlooked. Therefore, when applying laboratory test data the character of the structure under consideration and the loading conditions should be thoroughly appreciated.

Notes: Abstract— The problem associated with short crack growth, defined as situations in which the intensity of the crack tip field is underestimated by linear elastic fracture mechanics analyses, is briefly reviewed.Two cases are identified, cracks growing in plastically strained materials, such as occurs in high strain fatigue studies and at notch roots, and small cracks growing in single grains as occurs close to the fatigue limit in plain specimens.Important mechanical and metallurgical features of short cracks are discussed with particular reference to the upper and lower bound definition of a short crack.

Notes: Abstract— Stress and strain distributions and crack opening displacement characteristics of short cracks have been studied in single edge notch bend and centre cracked panel specimens using elastic–plastic finite element analyses incorporating both a non strain hardening and a power law hardening behaviour. J contour integral solutions to describe stress strain conditions at crack tips for short cracks differ from those for long cracks. The analyses show that (i) short cracks can propagate at stress levels lower than those required for long cracks and (ii) a two-parameter description of crack tip fields is necessary for crack propagation.

Notes: Abstract—The application of dimensional analysis and similarity methods to the study of the speed of fatigue crack growth is considered. It is shown that the Paris range of the crack propagation diagram is an intermediate-asymptotic stage of the crack growth process. Over this stage the influence of the initial conditions on the process of fatigue crack growth has disappeared but the influence of the instability has not yet intruded. So-called incomplete self-similarity prevails at this stage with respect to a basic similarity parameter, equal to the ratio of the stress intensity factor amplitude to the fracture toughness. It is shown that for a certain material under fixed external loading conditions the exponent in the Paris power law is a universal function of the ratio of specimen thickness to the ultimate size of the cyclic plastic zone. Processing of available experimental data confirmed the results obtained by this approach.

Notes: Abstract— Fracture surfaces of both service and laboratory fatigue fractures frequently show dark tongue-shaped marks. In fatigue tests on 7075–T6 specimens such tongues were produced by high peak loads. Measurements indicated that a tongue is not formed during a single burst of crack extension but is the result of a number of successive pop-ins requiring an increasing load. Therefore tongue formation is a quasi-stable phenomenon. The tunnelling fracture in the centre of plate thickness is accompanied by unfailed ligaments at the plate surfaces which reduce the stress intensity at the crack tip. The effect of different material conditions and loading direction on tongue forming was studied. A new model was developed to describe the growth of a tongue. The model was in good agreement with the various test results. The analysis of the problem has some relevance to validity requirements for K1c. A formula pertaining to tongues proposed by Forsyth was slightly modified and found to be approximately correct for the present results.

Notes: Abstract— A theoretical development based on a simple physical model is proposed to help the designer predict high strain multi-axial fatigue behaviour. This approach hypothesises that the maximum shear strain γ*, on planes driving the crack through the thickness, controls the fatigue crack propagation rate and hence the life. The direct strain δ*n acting normal to the plane of γ* can exert a secondary modifying influence. Experimental results from several research laboratories have been analysed in this manner with some success.

Notes: Abstract— Strain controlled continuous cycling fatigue tests are reported on three casts of Type 316 steel; the results obtained are shown to be consistent with published data.Strain controlled creep/fatigue tests are reported involving a hold period per cycle of between 0·2 and 168 h on one batch of Type 316 steel. An empirical extrapolation of the data and one involving a linear damage summation suggest that the existing Code Case N47 creep/fatigue design curve and rules are over-conservative. A stress relaxation/fatigue endurance correlation shows some promise for predictive purposes. Although many of the mechanical test and metallurgical features observed are consistent with a fundamental crack growth extrapolation approach, further consideration of detailed aspects are necessary before it can be used with confidence. It is concluded that there are no entirely reliable methods of extrapolation currently available for Type 316 steel and longer term tests are therefore essential to reduce the extrapolation uncertainties.

Notes: Abstract— Fatigue crack propagation tests were carried out in different environments on 7075–T6 and 2024–T3 centre-cracked sheet specimens. Observations were made on the macroscopic transition from tensile mode to shear mode. The transition is suppressed by an aggressive environment, whereas it is promoted by an inert environment. As a consequence there is no unique correlation between the state of stress and the mode of cracking. Both the state of stress and the environment have a significant effect on the mode of cracking. A simple model for the effect of environment on fatigue crack growth is presented. The implications for crack growth under corrosion fatigue conditions are discussed.

Notes: Fatigue crack growth rates have been determined on standard specimens containing long cracks (∼5–10mm) and on specimens containing two-dimensional short cracks (∼0.10–0.50mm). Large differences have been observed indicating that at a given stress intensity factor short cracks propagate much faster than long cracks. Mouth opening displacement measurements for both specimen geometries have shown that the crack closure effect is largely responsible for the observed effect. These results are used to rationalize the behaviour of short cracks initiated from natural sites which were either graphite nodules or microshrinkage pores. The three-dimensional aspect of these natural small cracks is analysed and discussed in detail.

Notes: For ultra-high strength steels and aluminium alloys, a fatigue crack could initiate from a notch tip under cyclic compressive load. The threshold value for fatigue crack initiation under compressive load can be as great as four times that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing altogether under cyclic compressive load with a maximum length of 0.2-0.5 mm. If the minimum compressive load was near zero, i.e. compression to zero load cycling, the threshold value was near that under tensile loading and the compressive fatigue crack could continue to grow; however, the crack growth rate under compression to zero load fatigue was 10–100 times less than that under the tensile fatigue loading.

Notes: An investigation has been carried out on austenitic stainless steel 0Kh16N15M3B under normal conditions and also to neutron irradiation of 6.8 × 1016nm-2s-1 (E 〉 0.1 MeV) intensity. Thin-walled torsion cylindrical specimens were tested in strain-controlled fully reversed loading mode at 923 K. Various ranges of strain, pre-loading fluences and half-cycle hold times (1, 5 and 30min) were applied. Neutron irradiation was found to result in hardening of the steel, stimulating cyclic stress relaxation and a reduction in cyclic life. When acting together, neutron irradiation and static loads cause a more significant reduction in the number of cycles to failure than if summed up as independent factors. Application of a kinetic failure criterion based on a damage parameter enables an estimation to be made of the limiting state of the steel under high-temperature cyclic loading with hold periods.

Notes: Fatigue crack propagation tests have been made on 150 mm wide panels of 1.6 mm thick 7475-T761 clad aluminium alloy sheet with and without adhesively bonded patches of pre-formed carbon fibre reinforced plastic (CFRP). The test frequency was 10 Hz, the minimum stress: maximum stress ratio, R, was 0.1 and the peak applied fatigue stress was 60 MPa.The tests were undertaken to assess the possibility of preventing the growth of fatigue cracks, or reducing their rate of growth, by the application of CFRP patches to one face only of pre-cracked aluminium alloy sheet. The variables examined included the type of patch and adhesive; the size, shape and thickness of the patch; and the effect of chamfering the edges of the patch and the removal of the cladding prior to patching.Results indicated that correctly designed and bonded CFRP patches substantially decreased the subsequent crack growth rate. The size and thickness of the patch had significant effects upon the reduction of fatigue crack growth rate whereas the shape of the patch, chamfering and the removal of the cladding prior to patching had little influence.

Notes: An experimental investigation is reported of the possible effects of adiabatic shear bands, caused by projectile impact, on the fatigue strength of a titanium alloy (Ti-6%A1-4%V). No significant reduction in fatigue strength due to the presence of adiabatic shear bands was found, nor did the fatigue cracks initiate from the bands. The results are relevant to the problem of foreign object impact damage to compressor blades in gas turbine aeroengines.

Notes: In order to study creep-fatigue interactions under multiaxial stress states, both push-pull and reversed torsion low-cycle fatigue tests were carried out using an austenitic stainless steel, SUS 304, at 923 K in air. From the tests, it is concluded that the hold-times introduced at the peak strain reduce low-cycle fatigue lives in the push-pull mode, but in the torsional mode they were not so harmful. This difference in the hold-time effect is discussed from considerations of crack formation and propagation and the stress amplitude applied to the specimen.Both maximum principal strain range, Δε1, and the von Mises' equivalent strain range, Δεeq, provide a nearly adequate comparison base for the assessment of biaxial low-cycle fatigue lives in tests without strain hold-time but are inadequate for hold-time tests. An equivalent stress range, Δσ*, which includes the effect of the stress parallel to the fatigue crack and which was previously proposed by the authors for no hold-time tests, is applied to the hold-time tests in the biaxial stress state. It is found that Δσ* is a good parameter for the correlation of biaxial low-cycle fatigue data in both no hold-time and hold-time tests.

Notes: Small fatigue crack growth behaviour in a low alloy steel was investigated under two stress step multiple loading in which the secondary stress was below the fatigue limit. Crack growth rates were presented in terms of a stress intensity factor and compared with data obtained under constant amplitude loading. In the higher ΔK region, crack growth rates increased monotonously with increasing ΔK even though the stress level was below the fatigue limit, and tended to be lower than those for constant amplitude loading. In the lower ΔK region, cracks showed a complicated behaviour, that is, an initial high growth rate was observed followed by an arrest or a drop to a minimum value and then a gradual increase. The average crack growth rates per cycle at both primary and secondary stresses in each block were approximately consistent with the da/dn-ΔK relation for constant amplitude loading. A suggestion for the prediction of crack growth life is given.

Notes: Abstract— Tests for fatigue crack initiation were carried out on two different aluminium alloys. Results and analysis of initiation under constant amplitude loading are presented; elastic and elastoplastic analyses are applied. Initiation under programmed block loading is investigated and damage accumulation is discussed. Tests were performed on two notch root radii:5 and 0.5mm. The electric potential method was used to detect fatigue crack initiation. Three point bending tests on smooth specimens were carried out to follow the evolution of damage during the crack initiation phase.

Notes: Abstract— High strain fatigue properties of AISI 316, AISI 316N and Sandvik 253MA have been investigated at 600°C. The two latter alloys, which contain significant amounts of N, exhibit a higher resistance to fatigue than 316. This effect is accompanied by a planar dislocation slip mode in 316N and 253MA as opposed to a wavy slip mode in 316. The results provide strong evidence that N improves fatigue strength in austenitic stainless steels, by inhibiting cross-slip of screw dislocations.

Notes: Abstract— A phenomenological model of cumulative creep damage combining simulated grain boundary cavitation with internal redistribution of stress is developed and matched to constant load tensile creep data for an AISI 316 stainless steel tested at 550°C. The model is shown to predict the creep life of the material when it is subjected to single step changes in load provided the strain rates subsequent to the change are imposed in the model. It is inferred that this supports current suggestions that cavitation failure may be strain controlled.

Notes: Abstract— Sharply notched specimens of a structural low-carbon steel were fatigued under several ratios of the maximum to minimum loads. The growth behavior of a short fatigue crack near the notch tip was analyzed based on crack closure measurements. A fatigue crack first decelerates with increasing crack length, and then accelerates or becomes non-propagating depending on the applied stress. A similar deceleration is seen when the rate is correlated to the stress intensity range. The effective stress intensity range is a unique parameter in correlating the growth rate of a short crack for all the stress levels examined, and the relation is identical to that obtained for a long crack. By considering the increase in crack closure with crack length, a quantitative method is proposed for predicting the non-propagating crack length and the fatigue limit of notched specimens as a function of the applied stress and the notch geometry.

Notes: Abstract— Segmented fitting of fatigue crack growth rate data is shown to give a more accurate prediction of fatigue life with less statistical scatter than by a single fitting using the straight line of a Paris-type plot. Changes in the slope of the log (da/dN) vs log ΔK data are seen to correlate well with changes in the mode and mechanism of fracture.

Notes: Abstract— Strains very near a growing fatigue crack have been computed from measured material displacements, both for a very dry environment and for humid air. Significant differences in crack tip strains arid crack tip opening displacements occur in these environments, with crack tip deformation in water vapor evidencing less plasticity than the dry environment. These quantitative results are incorporated into a previously developed mathematical model which is based, in part, on dynamic observations of intermittent crack growth. Fractography is shown to support the concepts of intermittent crack advance and decreased crack tip plasticity due to environmental water vapor.

Notes: Abstract— A model for fatigue threshold has been proposed based on the dislocation subgrain cell structure that evolves at the crack tip in steels during the fatigue deformation process. The stabilized subgrain cells that develop in the material act as impenetrable barriers to dislocations in slip band pile-ups that emanate from the fatigue crack tip. The blocking of these dislocations tends to limit crack growth that occurs by crack tip emission of dislocations, thereby leading ultimately to the fatigue threshold condition. The grain size effect on threshold is deduced to be an indirect effect as it is proposed that the subgrain cell size is the controlling substructural parameter at the threshold stress intensity level. The subgrain cell size is shown to be proportional to the one-third power of the initial grain size.

Notes: Abstract— The fatigue strength of notched specimens of a rotor steel was examined under variable torsional loading which simulates turbine-generator oscillations resulting from the high speed reclosing of transmission-line circuit breakers. The local stress-strain response at a notch root was analysed using Neuber's rule and the resulting complex strain sequences applied to smooth specimens. Using the rain flow analysis and the linear summation rule, fatigue lives of the smooth specimens were successfully predicted from constant amplitude fatigue life data in association with the cyclic stress-strain curve obtained by the incremental step method. Experimental crack initiation lives for notched specimens subjected to variable torsional loading were in excellent agreement with the theoretical curves derived from results on smooth specimens. According to the view that fatigue damage is equated to crack length, the propagation life of a mode II crack along the notch root was predicted to be actually coincident with the life to crack initiation at the notch root denned in this study, i.e. the life at the stage of finding a continuous circumferential crack.

Notes: Abstract— A crack-tip-zone interaction model for creep-fatigue crack growth is proposed. The basic details of the model are described. Experimental evidence in support of this model is presented. Existing data on creep-fatigue crack growth interaction agree qualitatively with the model. Some quantitative evidence for the model is presented and more is being developed in current work.

Notes: Abstract— Fatigue crack propagation behaviour in a cast Mg-Al-Zn alloy was examined. The fatigue crack growth rate of the alloy under constant amplitude loading and the threshold resulting from a number of tests are given. Delayed retardation after the application of a single tension overload is explored in detail and described by using the model proposed by Matsuoka and Tanaka. From these observations, it may be seen that the Matsuoka model can be applied to the cast magnesium alloy as well as steels and aluminium alloys. Changes in the extent of retadation and the overload affected zone size with respect to the ratio of peak-to-baseline stress intensity factor range are discussed. Two phenomena, delayed arrest and acceleration in the later stages of retardation are reported.

Notes: Abstract— Fatigue crack growth in the intermediate range can be described by the Paris law of linear elastic fracture mechanics. However, if gross plastic deformation is involved, the crack growth rate is often underestimated by LEFM. In the present study the increased crack growth rate due to plastic deformation is determined by using the J–R curve. This model was found to give very good results over a wide range of high crack growth rates for three different geometries of two different steels. A way to predict instability under controlled cyclic load is discussed and is proved to work well in practice. Instability under displacement control is also discussed.

Notes: Abstract— The alloy IMI 829 is very sensitive to fretting fatigue damage at 20°C but its fretting fatigue strength increases as the temperature is raised to 600°C due to the formation of a glaze type oxide. Failure occurs by cracking of the glaze as a result of local creep. These cracks propagate into the bulk material. Local creep results from increased stresses in the fretting contact region, frictional heating, and preferential removal of alloying elements into the oxide film leading to reduced creep properties.

Notes: Abstract— The influence of steady mode III on mode I fatigue growth behavior is investigated in four materials–a plain carbon steel, a Ni–Cr–Mo–V rotor steel, and titanium alloys, TA6V and TA5E ELI. It is shown that these loading conditions give rise to two main effects: (i) a strong reduction in propagation rate and (ii) a modification in crack path, the fatigue crack adopting a characteristic “factory-roof’ aspect. In 2024 Al alloy, it is shown that the superimposition of steady mode II to cyclic mode I leads to crack bifurcation, the angle θ being a function of Ka/Ktmax. These observations are discussed in the light of a new criterion which is introduced. This criterion is based on two main assumptions: (i) Fatigue cracking is assumed to occur only under the effect of local mode I opening. (ii) It is postulated that a fatigue crack grows in a direction where the crack propagation rate is maximum. A number of limitations of this approach, associated with crack closure phenomenon, are discussed.

Notes: Two parameters describing the growth of fatigue cracks are compared. They are the cyclic J integral ΔJ and the strain intensity expressed as an equivalent stress intensity ΔKeq-. By referring to cyclic stress-strain data obtained from hysteresis loops in high strength ferritic steels at room temperature and austenitic and ferritic steels at elevated temperature it is shown that: (i) for short cracks the parameters are simply related and (ii) both parameters adequately link fatigue crack growth rates observed in the separate high strain fatigue (HSF) and linear elastic fracture mechanics (LEFM) regimes. Correction factors for thumbnail cracks and the conditions under which the relations need further modification are discussed.

Notes: Abstract— The growth of ‘small’, half-penny shaped surface fatigue cracks in a precipitation hardened aluminum alloy is compared with the growth of ‘large’cracks in fracture mechanics type specimens. It is found that the small cracks grow much faster than LEFM equivalent large ones, and also experience growth rate perturbations. It is suggested that localized microplasticity in nominally elastic specimens is responsible for the rapid growth of small cracks and that grain size limitations on the microplastic regions cause transient decelerations and sometimes permanent arrest, in crack growth.

Notes: Abstract— Cyclic loading of mild steels in 4 N NaNO3 produces intergranular cracking similar to that associated with stress corrosion cracking at static loads, even when the cyclic loads are below the threshold for cracking under static conditions. However, under some circumstances cyclic loading promotes transgranular crack growth, the velocity of which may increase or decrease with increasing stress intensity factor. The transition from intergranular to transgranular cracking, or vice versa, is determined by the relative velocities of the two modes of failure, that observed having the higher velocity. The suppression of transgranular cracking as the stress intensity factor increases is only observed when the loading involves a compressive component and the effect is explained in terms of corrosion products interfering with the cracking process. Tension-tension loading in nitrates produces crack growth characteristics similar to those observed in oil.

Notes: Abstract— The influence of cyclic loading on diffuse light transmission of GRP laminates was investigated. Light transmission decreased due to the presence of matrix microcracks and interface debonding. The dependence of defects caused by cyclic loading on the variation of light transmission was established by methods of scattering medium optics. This dependence can be applied both in investigations of the kinetics of GRP laminate failure and in the estimation of cumulative fatigue damage.

Notes: Abstract— Near threshold fatigue crack growth under mixed-mode loading and elastic plane-strain conditions has been studied in 316 stainless steel in laboratory air at room temperature. Particular emphasis was placed on the influence of the mode II component. Crack growth from the starter crack, although initially coplanar, branches to be perpendicular to the maximum normal stress. However the threshold for the branched crack growth is controlled not only by mode I displacement, but also by the mode II component. Upper and lower bound curves are obtained for the threshold condition and discussed in terms of crack tip reversed plastic deformation, crack surface rubbing and oxide-induced closure. A theoretical method for predicting the lower bound curve is proposed and compared with the maximum normal stress and strain energy density criteria. The new theory shows the best agreement with experimental results, giving a safe prediction for design purposes.

Notes: Abstract— Starting from a knowledge of inhomogeneous dislocation structures observed in cyclically strained metals, a model for cyclic straining is developed. A distribution of volumes with different internal critical flow stresses is assumed characterized by a probability density function. A generalization which includes a thermally activated component of the flow stress is derived assuming that the saturated microscopic effective stress, μes, is equal in all volumes. The relations to obtain the probability density function from experimental data are derived. The theory yields the macroscopic internal stress, σe and the macroscopic effective stress, σe, along the hysteresis loop. Experimental observations on cyclically strained metals can be explained using this statistical theory.

Notes: Abstract— The effects of dry hydrogen, moist air, distilled water and hydrazine environments on the fatigue crack propagation behavior of 2021 aluminum alloy have been investigated over a wide range of growth rates spanning about six orders of magnitude. Environmental interactions in the intermediate and near-threshold crack growth regions are shown to be associated with different fracture characteristics and mechanisms. Scanning Auger and X-ray photoelectron spectroscopic analysis of fracture surface corrosion deposits revealed that oxide induced crack closure phenomena, which considerably influence the near threshold corrosion fatigue behavior of low strength steels and some aluminum alloys, are not of importance for the present 2021-T6 aluminum alloy. The mechanistic aspects of environmentally influenced fatigue behavior of the alloy are discussed in the light of hydrogen embrittlement, chemical reactions and crack closure concepts.

Notes: Recently a controversy has developed over whether crystallographic crack growth near threshold in Ni-base superalloys occurs along {111} slip planes or {100} planes at room temperature. In this work on Nimonic API crack propagation is shown to occur on both {100} and {111} planes. The most common facet plane is {111} and this is the only orientation observed at the lowest stress intensities, but at higher stress intensities occasional {100} facets are also produced. This behaviour is compared with similar results in aluminium alloys.

Notes: A crack nucleation model is constructed based on the random slip of the dislocations within the fatal persistent slip band (PSB). From such a model, we derive the notch-peak probability function. The crack is considered to nucleate when a critical notch depth is attained. This depth is constant for applied strains corresponding to the plateau of the cyclic stress- strain curve because the stress is constant. The PSBs behave in such a way that the more concentrated their localized slip the fewer the cycles required to attain the critical notch depth. Using the experimental relation between the applied strain and the magnitude of the slip offsets (reported in Part I) in conjunction with observations of the notch distributions along the gage surface, we have obtained the relation between the applied strain and the cycles for crack nucleation. A log-log plot of this relation shows a slightly different slope from that of the Coffin-Manson life data, previously reported. However, by allowing for cycles spent in Stage II propagation, good agreement is obtained.

Notes: The statistical nature of fatigue crack growth rate has been examined for G41400 steel exposed to different heat treatments and for different loading and environmental conditions. Techniques for determining the intrinsic value of the crack propagation rate are discussed. The intrinsic scatter due to the material has been evaluated and considered in terms of its effect on fatigue life prediction, both normal and log normal distributions of fatigue crack growth data being considered. It appears that the intrinsic material scatter in crack propagation rate is closely related to the scatter in total life.

Notes: A d.c. potential drop formula has been established for monitoring crack growth in circular notched members subjected to torsion. Changes in crack depth of 20 μm can be detected in cracks growing up to 2 mm in depth.

Notes: Tensile specimens of Ti-6A1-4V with four levels of interstitial oxygen content and a Ti-8Al-1Mo-1V alloy with different heat treatments to alter grain size and/or microstructural character are subjected to slow strain-controlled cyclic deformation leading to rupture in the 5-500 cycle range. Indication of crack initiation as well as rupture life are compared, relative to the plastic excursion strain. On this basis, the effects of grain size and oxygen content are not clearly discriminated. Yet, some of the materials exhibit markedly superior performance. This improvement seems to be related to a characteristic evolution in the shape of the cyclic stress-strain curve. Here, relative to a full convex hysteresis loop of early cycles, the later cycles exhibit a reduced stress level, or cyclic softening, in the first half of the excursion, followed by a resurgence of strength to initial stress levels in the latter portion. The enhanced strain hardening rate enabling this terminal strength restoration is thought to stabilize the deformation, reducing the amount of stress-relaxation-induced tensile strain. Taking such strain as an increment of damage in a cumulative cyclic creep strain criterion provides a correlation between the evolving shape of the cyclic stress-strain curve and the low cycle fatigue endurance. Results indicate the absolute increase in the terminal plastic strain hardening rate to be a constant of a material, independent of the cyclic strain excursion.

Notes: Substantial effects of hydrogen are found on the DBTT and cleavage fracture stress in both cathodically and gas phase charged samples of a BCC Ti alloy. Fatigue thresholds when plotted as a function of temperature exhibit a peak and this peak shifts towards higher temperatures when the hydrogen level is increased. An elastic calculation of static threshold stress intensities using experimentally observed fracture stress parameters show a similar trend to fatigue threshold stress intensities.

Notes: Abstract —An experimental method was developed to quantify the formation and subcritical propagation of small cracks emanating from artificial surface defects. Continuous crack depth information was obtained from dc electrical potential measurements employing an analytical model. Fatigue experiments were conducted to evaluate the crack monitoring procedures for conditions relevant to the elevated temperature defect tolerance of superalloys. Cracking progressed uniformly and predictably from small surface defects (0.1 mm deep and 1.4 mm long) in A286, 304 and 10Ni steels and in René 95 and MP–159 superalloys. Crack depths, computed based on analytical calibration of measured electrical potentials, agreed to within ±18% of corresponding values measured optically. Similar results were obtained for a penny-shaped defect, 75 μm deep. Applications of the technique were investigated, including characterizations of fatigue crack formation and growth in René 95.

Notes: Abstract —Fatigue tests under four-point bending were carried out on low alloy steel welded I-beams under constant and variable amplitude loading. Four different types of stiffeners welded to flanges and webs were analysed. Particular attention was paid to the effect of configuration at the stiffener-ends. It was shown that the stiffener-ends were the most critical points. Fatigue cracks initiated and propagated in both tension and compression flanges. The fatigue life to crack initiation was calculated on the basis of the local stress-strain analysis. Both theoretically and experimentally estimated stress concentration factors were used for calculation. The predicted fatigue lives were compared with the respective values obtained in experiments.

Notes: Abstract—Measurement of the energy expended during growth of a fatigue crack in the near threshold region and how it is affected by a water vapor environment is coupled with direct observation of the crack tip to produce a modification of the damage accumulation model for fatigue crack growth which incorporates threshold and environmental effects.

Notes: Abstract— On the basis of fatigue data derived from a large number of experiments, the load progression methods of Locati and Prot as well as a method operating on a thermometric basis have been comparatively evaluated in an experimental and analytical (statistical) study. The last-named thermo-method can provide useful estimates for a fatigue limit corresponding with ca. 0 to 5% probability of fracture with only 1 to 3 experiments.The Prot-method, set up and evaluated in a certain way, provides a useful estimate of the fatigue limit and, additionally, an approximate indication of the width of the transition range of the corresponding Wöhler-type tests after experiments on not less than about ten specimens.The load progression, contrary to the original Prot-concept, must not necessarily be applied in a continuous mode, but can occur stepwise with increments Δσ smaller than ca. 15 Nm-2.

Notes: Abstract— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.

Notes: Abstract— In this paper the strain distribution and the plastic zone size at a fatigue crack tip are discussed. From this analysis and considering the critical condition for plastic-blunting, a simple analytical formula for calculating the threshold stress intensity range for fatigue crack propagation at R= 0 is proposed. It is found that calculated values are in good agreement with existing experimental results.

Notes: Abstract— The plastic work to propagate a fatigue crack by a unit area, U, measured by the foil strain gage technique requires an extrapolation to estimate the contribution closer than 100 μm to the crack tip. This is due to the size of the strain-gages used, 200 × 210 μm. Conversely, the electron channeling technique for determining U is useful mainly close to the crack tip where subgrains form. In the present work U was measured by both techniques in the same low carbon steel at ΔK= 8 MN/m3/2. The contribution to U from closer than 100 μm of the crack tip was determined to be 1·7 × 106 J/m2 using electron channeling and 2·0 × 106 J/m2 by extrapolation. The measured contribution to U from further than 100 μm from the crack tip was 3·6 × 106 J/m2 giving 5·3 × 106 J/m2 for U. Thus, a large amount of energy is absorbed outside the region where sub-grains form. The non-hysteretic plastic work was found to be about four orders of magnitude smaller than the hysteretic plastic work, and may be neglected. A map of the plastic zone results from the strain-gage measurement. Rice's theory predicts the measured plastic zone sizeif the proper material's strength is employed in the formula.

Notes: Abstract— A method of combining long term creep data with relatively short term mechanical behaviour to provide an estimate of creep-fatigue endurance is presented. It is proposed that the creep-fatigue effect in high temperature cyclic deformation is governed by a difference in strain rate around the cycle and the associated variation in ductility with strain rate.

Notes: Abstract— Accurate knowledge of defect sizes and crack shape development from initiating defects is essential for a realistic application of fracture mechanic? to the fatigue life prediction of welded joints. Defects at the toes of high quality longitudinal non-load-carrying fillet welded joints have been examined using two methods, and compared with those observed in plain unwelded plate. The defects observed at the weld toe were, on average, four times deeper than the largest observed in the unwelded plate but had similar root radii. For this particular geometry the defects could be considered continuous at the expected initiation site. A simple estimate of the stress concentrating effect of a single composite defect, coupled with experimental evidence, led to the conclusion that the proportion of total life spent in initiation is negligible for all stress levels which cause fatigue failure. Subsequent growth and crack shape development was observed to depend on the initial defect distribution and the complex stress field at the weld toe.

Notes: Abstract— Fatigue crack growth rate tests have been performed on Nimonic AP1, a powder formed Ni-base superalloy, in air and vacuum at room temperature. These show that threshold values are higher, and near-threshold (faceted) crack growth rates are lower, in vacuum than in air, although at high growth rates, in the “structure-insensitive” regime, R-ratio and a dilute environment have little effect. Changing the R-ratio from 0.1 to 0.5 in vacuum does not alter near-threshold crack growth rates very much, despite more extensive secondary cracking being noticeable at R= 0.5.In vacuum, rewelding occurs at contact points across the crack as ΔK falls. This leads to the production of extensive fracture surface damage and bulky fretting debris, and is thought to be a significant contributory factor to the observed increase in threshold values.

Notes: Abstract— Fatigue thresholds and crack growth rates were studied in the experimental alloy, Ti—5Al—4Mo, as a function of temperature, grain size and hydrogen concentration. Deformation is confined to planar slip bands along which fracture occurred at low hydrogen concentrations. Hydrogen accelerated crack growth rates at various combinations of temperature, grain size and hydrogen with a corresponding change in fracture from “cleavage’ to interface phase fracture. In addition, hydrogen was found to promote interface phase formation. It is proposed that stress-assisted hydrogen accumulation increases the interface phase hydrogen concentration which reduces the interface phase fracture stress. This process depends on the local stress and β-phase hydrogen concentrations, temperature and the time under load. At 340 K, hydrogen had a relatively small effect on crack growth due to a change in slip behavior with increased hydrogen concentration.

Notes: Several crack tip stress intensity factor solutions have been published for semi-elliptic, surface breaking cracks in plates subjected to tension or bending forces. These solutions do not agree with each other particularly well and the basis for choosing which one is the best has not been established. In this paper, the development of fatigue crack shape is used as a diagnostic tool to test the accuracy of these theoretical stress intensity solutions in predictive fatigue crack growth calculations. Those solutions giving the best engineering estimate of crack tip stress intensity factors are identified. Single equations are also given for each loading case at the deepest point or surface intersection point of semi-elliptic cracks in order to facilitate calculations on programmable calculators. A rational basis for calculating the progress of a crack which snaps through the thickness and continues to propagate in a stable way by fatigue is suggested.

Notes: In low strain fatigue, cracks often form in persistent slip bands (PSB) in a wide variety of materials. Observations of crack nucleation involving PSB have been made in copper single crystals by careful interferometric studies. The width, length, step height and thus strain localization in the PSBs remain very stable during saturation, even up to three-quarters of the life. For a given applied strain amplitude, the specimen will form PSBs having a certain distribution of slip offsets. The fatal crack is found to nucleate in the group of micro-PSBs having the largest offsets, and the largest strain localization. The PSB volume fraction for a given strain amplitude depends only very slightly on crystal orientation, and the slip offsets not at all. Thus the kinetics of crack nucleation are not affected by orientation. The mechanism of nucleation is concluded to be one of random walk in the slip offsets of PSBs.

Notes: The cyclic strain hardening occurring during fatigue testing at 550°C was studied in terms of the dislocation structures developed. The dislocations are collected in dense tangles which form the boundaries of a cell structure. The stress required to push through, or eject, a dislocation from the cell boundary predicts the flow stress. These values showed good correlation with the cyclic shear stresses obtained by applying the Tresca criterion, which underlies its usefulness in design under cyclic loads. Carbide particles are shown to precipitate on grain and cell boundaries.

Notes: The results of high temperature low cycle fatigue (HTLCF) on IN 738, obtained from two different sources, were used to study the applicability of strain-range partitioning (SRP) to this alloy. In spite of the difficulty in separating the plastic-plastic strain component from the different strain-life relations, an attempt was made to determine the coefficients and exponents for each of these relations from both sets of data. The observation that compressive creep, balanced by tensile plastic deformation, was more damaging than the reverse, could be related to the behaviour of the material. The evidence of the effect of temperature on the plastic-plastic strain-life relation, reported by some researches, was shown to be not decisive in the reported temperature range. The reasonable agreement and the similarity in the material's behaviour, obtained from the analysis of the high temperature low cycle fatigue of the two sets of data, substantiates the applicability of the strain-range partitioning method to IN 738 alloy.

Notes: A literature search has revealed that the most extensive study of impact fatigue reported is still that of Stanton and Bairstow (1908) who were the first to work on this topic. Their results, which were on pearlitic plain carbon steels, have been reanalyzed in an attempt to deduce from them the effects of chemical composition on impact fatigue in this class of materials.The results show that when the number of impacts to failure, Nf, is greater than about 103, Nf and the energy absorbed per impact, Ei, are related by the equation: Ei, =Eo+EkNf−p, where Eo appears to be the impact endurance limit. It is suggested that Ek and p be named respectively the “impact fatigue parameter” and “impact fatigue exponent”. Ek is independent of composition except that it is lower by a factor of about four for two materials which probably failed by an intercrystalline rather than transcrystalline fracture; p has a value of 0.6; and Eo increases monotonically with silicon content but does not vary systematically with the carbon, manganese, sulphur or phorphorus contents. The significance of these results is discussed.

Notes: –The article describes a method developed at ONERA for predicting the lifetime of gas turbine blades. This method makes use of non-linear viscoplastic constitutive laws, the problem being solved from a plane cross section assumption and through a time-step linearization. The initiation of the first macroscopic crack is calculated from a stabilized cycle condition. A blade test rig has been developed for checking the method under load and temperature conditions that are as close as possible to operational ones. The results of two series of tests on convection cooled IN100 blades prove the method to be acceptable to design engineers.

Notes: Abstract —This report presents the results of a round robin study on the fatigue crack arrest threshold (δKth) of 2618 aluminium alloy and AISI 316 steel.The main purpose of this work was to develop a method for the determination of δKth, and to examine the influence of various test parameters on this threshold.Among the parameters considered, only the load ratio (R) and the environment (vacuum) appear to have a significant influence on very slow fatigue crack growth rates (FCGR). Moreover, while the results obtained with the 316 steel show great scatter, the importance of the adopted procedure is pointed out.

Notes: Abstract —Mixed mode fatigue crack growth is analysed using Sih's strain energy density approach. A centre crack panel geometry loaded under uniaxial cyclic tension is considered. The crack angle is varied from 30° to 90°. A procedure for the determination of crack propagation life is outlined. The crack trajectory due to cyclic loading is predicted. The crack growth rate, the cyclic life and the cyclic life ratio are discussed, for an aluminium alloy and a steel, as a function of initial crack angle, crack length, stress amplitude, and the strain energy density factor.

Notes: Abstract —In recent years the low fatigue growth rate regime has been the subject of extensive study revealing its complex and often seemingly inconsistent nature. The value of alternating stress intensity at threshold ΔKTh) is sensitive to a range of variables which include mean stress, stress history, monotonie and cyclic yield strength, grain size and environment. A degree of coherency may be achieved by interpreting the data in terms of a crack closure mechanism which is a function of surface roughness. Experiments, in which the surface roughness (φ) has been altered by the introduction of non-metallic inclusions, give insight into the significance of crack closure in this regime.