ALBERT

Notes: Abstract. Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin ``Isogrid'' structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis for rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.

Notes: Abstract An experimental study was performed to modify the conventional impact-echo technique to detect small diameter spherical voids in concrete. High velocity impact of steel balls was used to generate the high frequency waves in concrete necessary for detection of small voids. Elastodynamic wave propagation in concrete, type of impact source, and the size of the flaw that can be detected were studied. These laboratory studies were carried out on a 4 in. (10.2 cm) thick 4 ft. × 4 ft. (1.22 m × 1.22 m) concrete slab that contained a variety of artificial spherical flaws embedded at known locations. Analysis of the frequency spectrum of the response of the slab was used to determine the slab's thickness and presence of voids. Signal processing techniques were applied for a better interpretation of the slab's response. Autocorrelation and cross-correlation algorithms were used to determine the characteristics of the slab at different locations. The problems associated with determination of flaws and voids in concrete have been delineated. Some practical and easy to implement solutions have been proposed and ways of using the impact-echo technique more efficiently for field applications have been discussed.

Notes: Abstract The existing experimental data [Ferry and Spear 1978; Perchuk and Lavrent'eva 1983] on Mg−Fe partitioning between garnet and biotite are disparate. The underlying assumption of ideal Mg−Fe exchange between the minerals has been examined on the basis of recently available thermochemical data. Using the updated mixing parameters for the pyrope-almandine asymmetric regular solution as inputs [Ganguly and Saxena 1984; Hackler and Wood 1984], thermodynamic analysis points to non-ideal mixing in the phlogopite-annite binary in the temperature range of 550°C–950°C. The non-ideality can be approximated by a temperature-independent, one constant Margules parameter. The retrieved values for enthalpy of mixing for Mg−Fe biotites and the standard state enthalpy and entropy changes of the exchange reaction were combined with existing thermochemical data on grossular-pyrope and grossular-almandine binaries to obtain geothermometric expressions for Mg−Fe fractionation between biotite and garnet. [T in K] $$\begin{gathered} {\text{T(HW) = [20286 + 0}}{\text{.0193P - \{ 2080(X}}_{{\text{Mg}}}^{{\text{Gt}}} {\text{)}}^{\text{2}} {\text{ - 6350(X}}_{{\text{Fe}}}^{{\text{Gt}}} {\text{)}}^{\text{2}} \hfill \\ {\text{ - 13807(X}}_{{\text{Ca}}}^{{\text{Gt}}} {\text{)(1 - X}}_{{\text{Mn}}}^{{\text{Gt}}} {\text{) + 8540(X}}_{{\text{Fe}}}^{{\text{Gt}}} {\text{)(X}}_{{\text{Mg}}}^{{\text{Gt}}} {\text{)(1 - X}}_{{\text{Mn}}}^{{\text{Gt}}} {\text{)}} \hfill \\ {\text{ + 4215(X}}_{{\text{Ca}}}^{{\text{Gt}}} {\text{)(X}}_{{\text{Mg}}}^{{\text{Gt}}} {\text{ - X}}_{{\text{Fe}}}^{{\text{Gt}}} {\text{)\} + 4441}}{{{\text{(2X}}_{{\text{Mg}}}^{{\text{Bt}}} {\text{ - 1)]}}} \mathord{\left/ {\vphantom {{{\text{(2X}}_{{\text{Mg}}}^{{\text{Bt}}} {\text{ - 1)]}}} {{\text{[13}}{\text{.138}}}}} \right. \kern-\nulldelimiterspace} {{\text{[13}}{\text{.138}}}} \hfill \\ {\text{ + 8}}{\text{.3143 InK}}_{\text{D}} {\text{ + 6}}{\text{.276(X}}_{{\text{Ca}}}^{{\text{Gt}}} ){\text{(1 - X}}_{{\text{Mn}}}^{{\text{Gt}}} )] \hfill \\ {\text{T(GS) = [13538 + 0}}{\text{.0193P - \{ 837(X}}_{{\text{Mg}}}^{{\text{Gt}}} )^{\text{2}} {\text{ - 10460(X}}_{{\text{Fe}}}^{{\text{Gt}}} )^2 \hfill \\ {\text{ - 13807(X}}_{{\text{Ca}}}^{{\text{Gt}}} )(1{\text{ - X}}_{{\text{Mn}}}^{{\text{Gt}}} {\text{) + 19246(X}}_{{\text{Fe}}}^{{\text{Gt}}} ){\text{(X}}_{{\text{Mg}}}^{{\text{Gt}}} ){\text{(1 - X}}_{{\text{Mn}}}^{{\text{Gt}}} ) \hfill \\ {\text{ }}{{{\text{ + 5649(X}}_{{\text{Ca}}}^{{\text{Gt}}} ){\text{(X}}_{{\text{Mg}}}^{{\text{Gt}}} {\text{ - X}}_{{\text{Fe}}}^{{\text{Gt}}} ){\text{\} + 7972(2X}}_{{\text{Mg}}}^{{\text{Bt}}} {\text{ - 1)]}}} \mathord{\left/ {\vphantom {{{\text{ + 5649(X}}_{{\text{Ca}}}^{{\text{Gt}}} ){\text{(X}}_{{\text{Mg}}}^{{\text{Gt}}} {\text{ - X}}_{{\text{Fe}}}^{{\text{Gt}}} ){\text{\} + 7972(2X}}_{{\text{Mg}}}^{{\text{Bt}}} {\text{ - 1)]}}} {{\text{[6}}{\text{.778}}}}} \right. \kern-\nulldelimiterspace} {{\text{[6}}{\text{.778}}}} \hfill \\ {\text{ + 8}}{\text{.3143InK}}_{\text{D}} {\text{ + 6}}{\text{.276(X}}_{{\text{Ca}}}^{{\text{Gt}}} )(1{\text{ - X}}_{{\text{Mn}}}^{{\text{Gt}}} )] \hfill \\ \end{gathered} $$ The reformulated geothermometer is an improvement over existing biotite-garnet geothermometers because it reconciles the experimental data sets on Fe−Mg partitioning between the two phases and is based on updated activity-composition relationship in Fe−Mg−Ca garnet solid solutions.

Notes: Summary Laboratory model test results are presented that determine the effectiveness of using layers of geogrids as reinforcement in sand to reduce the settlement of square surface foundations subjected to transient loading. The model tests were conducted with only one type of geogrid at one relative density of compaction of sand. The maximum intensity of the transient load applied always exceeded the static ultimate bearing capacity of the foundation when supported by unreinforced sand. The settlement reduction factors for various depths of reinforcement have been determined.

Notes: Summary The Fracture process zone in compact tension specimens of Indiana limestone was investigated to study its effect on the fracture mechanics parameters in such materials. Specimens were tested up to the peak load, and propagation of the crack from a preexisting notch was monitored. Experiments were designed to study the two features of the fracture process zone in rocks: ligament connections and microcracking. To observe this zone with high sensitivity and accuracy, laser interferometry methods were adopted. Holographic Interferometry was used to observe initial crack propagation. To obtain more quantitative measurements of the displacement field, in realtime, the recently developed technique of electronic speckle pattern interferometry was applied. This technique can provide continuous video recording of the interferometric fringe pattern, depict the evolution of the fracture process, and measure profiles of crack opening displacements. The macroscopic observations of full-field displacement by the laser techniques were supplemented by post mortem observation of the fracture region under a scanning electron microscope. Regions around the crack were studied after the test for possible presence of microcracks. An interactive finite element code was used to compute the stress intensity factors of the propagating crack-tip and displacements. Finite element computations were used to evaluate the effect of the process zone on crack propagation.

Notes: Abstract Acoustic emissions from reinforced-concrete beams, reinforcing bars and plain concrete cylinders were monitored. Acoustic-emission events were used in a study of source locations, frequency characteristics, and other analytical methods that have found use in the past for evaluating acoustic-emission data in other fields of engineering. Tests were done on reinforced-concrete beams under flexural loading, individual reinforcing bars under pure tension, concrete cylinders under compression, and reinforcing bars subject to pullout tests. The experimental data were first analyzed with conventional acoustic-emission methodology. A critical look at many acoustic-emission techniques currently used in other materials (metals, composites, etc.) demonstrated some of the difficulties of applying the same techniques to reinforced concrete. More importantly, it illustrated the limitations of signal processing and parameter estimation of acoustic-emission events as viable nondestructive-evaluation (NDE) techniques for reinforced-concrete structures. Subsequently, on the basis of the experimental results, some of the more promising aspects of developing acoustic emission into a structural monitoring tool are discussed.

Notes: Abstract Electronic shearography (ES), a laser interferometry technique, has the potential for large-scale structural inspection and for identifying cracks and strain anomalies. A system based on this technology could possibly be used for noninvasive inspection of structures with high insensitivity and robustness. One major problem is the existence of largerigid body motions in typical engineering structures such as bridges and high-rise buildings. These rigid-body motions are large enough to cause a complete decorrelation of the characteristic speckle pattern that is obtained by illuminating the object surface. This nullifies any possibility of interference between speckled images obtained at different stages of loading, as is necessary for interferometry applications. A systematic study was conducted to characterize the speckle pattern obtained from typical civil structures as a function of the illumination and imaging system. Experiments were carried out to quantify speckle decorrelation as a function of object motion and instrumentation characteristics. A fracture mechanics based finite element (FEM) analysis was carried out on an existing fractured bridge to determine the strains and displacements. The results of these studies were subsequently used to define the parameters of a structural inspection system based on ES for field applications.

Notes: Abstract Fracture in rocks is influenced by anisotropy and existence of faults. Fracture initiation and propagation is often under the combined presence of sliding and opening of preexisting cracks. Linear-elastic fracture-mechanics (LEFM) has been used as a model for describing the propagation of a main crack in materials such as rocks, concrete, ceramics, etc. However, the presence of the fracture process zone which includes interlocking of grains and ligament connections results in deviations from perfectly brittle behavior. These effects are more pronounced in mixed-mode fracture, which involves crack initiation under the combined presence of tension-shear or compression-shear stresses. Specimens of Indiana limestone with a preexisting inclined notch were studied to observe the fracture process under a mixed-mode state of stress. Experimental monitoring involved using the electronic speckle-pattern interferometry (ESPI) technique to monitor strains and crack-propagation paths with high sensitivity. A scanning electron microscope (SEM) was used to examine the specimen for the presence of microcracks. Experimental results were subsequently evaluated using a mixed-mode fracture theory and finite-element computations. It was possible to visually observe the pre-peak and post-peak development of the fracture process zone. Developments in the crack-tip strain concentration were observed at and beyong the peak load. While the experiments conducted involved tension-shear cracks, the possibility of extending the concepts to compression-shear cracks was also explored. The possibilities and limitations of using the fracture-mechanics approach to understanding fracture in rocks were subsequently discussed.

Notes: Abstract The shape inaccuracies of inflatable antennas and the potential shape control of the surface of those structures are investigated. Surface shape inaccuracies are due to geometric nonlinear deformation. Correcting the shape of these inflatables focused on the integration of piezopolymer actuators on the membranes. The out-of-plane displacements of a membrane structure were assessed with the shadow moiré method. The experimentally measured shape of the structure confirmed the extent of deviation from the ideal optical surface, a paraboloid of revolution. Active control of the shape of the membrane was tested using a piezoelectric material, polyvinylidene fluoride (PVDF). The deformation caused by actuation of the membrane structure was evaluated using electronic speckle pattern interferometry. An analytical solution was developed to verify the extent of shape correction that can be achieved by embedded PVDF actuators. It was confirmed that micron-level shape corrections are possible for future space-based sensors that use inflatable antennae technology.

Notes: Abstract It is necessary to study the microlevel failure mechanisms of a material in order to improve its quality and to develop a rational constitutive model to describe the material. Nonlinearity and strain-softening behavior of concrete has to be incorporated into any model which can be implemented into efficient design. Acoustic-emission (AE) techniques are useful for obtaining information pertaining to internal cracking and investigating the applicability of a particular material model. The process of localization of cracks and movement of the fracture process zone was studied using acoustic-emission techniques. The rate of acoustic-emission events and sources of acoustic-emission activity were studied for plain-mortar and model-concrete specimens loaded in direct tension. The study shows that acoustic-emission events localize to a region near the notch before peak load is attained. The region of activity progresses through the specimen during further loading and subsequent strain softening. Acoustic-emission events were used to locate the fracture-process zone (FPZ), and to check this location against the location of the effective crack tip as evaluated by a modified linear-elastic fracturemechanics model for concrete as well as by microscopical observations.