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.