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Suitability of plastic-encapsulated solid-state electronic devices for use in spacecraft discussed. Conclusion of preliminary study was plasticencapsulated parts sufficiently reliable to be considered for use in lowcost equipment used at moderate temperature and low humidity. Useful to engineers as guides to testing or use of plastic encapsulated semiconductors in severe terrestrial environments.
A uniaxial, uncoupled constitutive model to predict the response of thermal and rate dependent elastic-plastic material behavior is presented. The model is based on an incremental classicial plasticity theory extended to account for thermal, creep, and transient temperature conditions. Revisions to he combined hardening rule of the theory allow for better representation of cyclic phenomenon including the high rate of strain hardening upon cyclic reyield and cyclic saturation. An alternative approach is taken to model the rate dependent inelastic deformation which utilizes hysteresis loops and stress relaxation test data at various temperatures. The model is evaluated and compared to experiments which involve various thermal and mechanical load histories on 5086 aluminum alloy, 304 stainless steel and Hastelloy-X.
Plastic deformation of either a sample or a gasket between diamond anvils leads to large pressure concentrations, i.e., the maximum pressure can be many times the average pressure. This behavior is discussed using elementary plasticity theory for the case where the pressures are sufficiently low that the yield stress can be assumed not to vary with pressure. It is then shown that the pressure concentration factor can be even much larger when the yield stress of the sample at the highest pressure is much greater than the yield stress at the lowest pressure. This is illustrated with solid xenon where it is shown that the assumption that Nelson and Ruoff made about the pressure distribution in their xenon samples is incorrect. The pressure distribution is shown to be much steeper than assumed. Thus, the pressure they observed electrical conduction in xenon was above 1 Mbar.
Many investigations have considered structural collapse from strictly the transient point of view. While such an approach is ideally correct, certain difficulties have to be overcome in its implementation. The present investigation is concerned with the development of self-adaptive algorithms which make it possible to conduct the analysis of both static elastic and elastic-plastic postbuckling, as well as static loading to the onset of buckling followed by subsequent dynamic postbuckling. The approach employed to solve the static portion of loading is to extend the constrained Incremental Newton-Raphson (INR) algorithm by incorporating elastic-plastic constitutive characterizations. Large deformation moderate strain theory is adopted to establish the overall strategy. Attention is given to governing field equations, aspects of algorithmic development, and numerical experiments conducted to illustrate the efficiency and stability of the developed schemes.
A full asymptotic solution is presented for the fields in the neighborhood of the tip of a steadily advancing crack in an incompressible elastic-perfectly-plastic solid. There are four findings for mode I crack growth in the plane strain condition. The first is that the entire crack tip in steady crack growth is surrounded by a plastic region and that no elastic unloading is predicted by the complete dynamic asymptotic solution. The second is that, in contrast to the quasi-static solution, the dynamic solution yields strain fields with a logarithmic singularity everywhere near the crack tip. The third is that whereas the stress field varies throughout the entire crack tip neighborhood, it does not exhibit behavior that can be approximated by a constant field followed by an essentially centered-fan field and then by another constant field, especially for small crack growth speeds. The fourth finding is that there are two shock fronts emanating from the crack tip across which certain stress and strain components undergo jump discontinuities. After reviewing the mode III steady-state crack growth, it is concluded that ductile fracture criteria for nonstationary cracks must be based on solutions that include the inertia effects and that for this purpose quasi-static solutions may be inadequate.
Swaging between 750 and 1050 C has been investigated as a means to introduce work into the directionally solidified eutectic alloy gamma/gamma prime-alpha (Ni-32.3 wt percent Mo-6.3 wt percent Al) and increase the elevated temperature creep strength. The 1000 C slow plastic compressive flow stress-strain rate properties in air of as-grown, annealed, and worked nominally 10 and 25 percent materials have been determined. Swaging did not improve the slow plastic behavior. In fact large reductions tended to degrade the strength and produced a change in the deformation mechanism from uniform flow to one involving intense slip band formation. Comparison of 1000 C tensile and compressive strength-strain rate data reveals that deformation is independent of the stress state.
A simplified inelastic analysis computer program (ANSYPM) was developed for predicting the stress-strain history at the critical location of a thermomechanically cycled structure from an elastic solution. The program uses an iterative and incremental procedure to estimate the plastic strains from the material stress-strain properties and a plasticity hardening model. Creep effects are calculated on the basis of stress relaxation at constant strain, creep at constant stress or a combination of stress relaxation and creep accumulation. The simplified method was exercised on a number of problems involving uniaxial and multiaxial loading, isothermal and nonisothermal conditions, dwell times at various points in the cycles, different materials and kinematic hardening. Good agreement was found between these analytical results and nonlinear finite element solutions for these problems. The simplified analysis program used less than 1 percent of the CPU time required for a nonlinear finite element analysis.
With the aim of applying the Dugdale model to the compact specimen, the equations for stress intensity factor and crack surface displacement are obtained for a pair of pin loads and a uniform stress acting on a segment of the crack surface. The plastic zone size is calculated from the 'finiteness' condition of Dugdale, and the results agree well with collocation results from Terada (1983). The load that causes incipient yielding at the compression point of the compact specimen is calculated from a finite element strip yield analysis for an elastic-perfectly plastic material. The crack tip opening displacement for the compact specimen is calculated by adding the displacement at the tip of the physical crack length due to the pin load and due to the uniform stress. The results are within 1.5 percent of current collocation results.
The path independent (P-I) integrals in elastic plastic fracture mechanics which have been proposed in recent years to overcome the limitations imposed on the J integral are reviewed. The P-I integrals considered herein are the J integral by Rice, the thermoelastic P-I integrals by Wilson and Yu and by Gurtin, the J* integral by Blackburn, the J sub theta integral by Ainsworth et al., the J integral by Kishimoto et al., and the delta T sub p and delta T* sub p integrals by Atluri et al. The theoretical foundation of these P-I integrals is examined with emphasis on whether or not path independence is maintained in the presence of nonproportional loading and unloading in the plastic regime, thermal gradients, and material inhomogeneities. The similarities, differences, salient features, and limitations of these P-I integrals are discussed. Comments are also made with regard to the physical meaning, the possibility of experimental measurement, and computational aspects.
A simplified inelastic analysis computer program (ANSYPM) was developed for predicting the stress-strain history at the critical location of a thermomechanically cycled structure from an elastic solution. The program uses an iterative and incremental procedure to estimate the plastic strains from the material stress-strain properties and a plasticity hardening model. Creep effects are calculated on the basis of stress relaxation at constant strain, creep at constant stress or a combination of stress relaxation and creep accumulation. The simplified method was exercised on a number of problems involving uniaxial and multiaxial loading, isothermal and nonisothermal conditions, dwell times at various points in the cycles, different materials and kinematic hardening. Good agreement was found between these analytical results and nonlinear finite element solutions for these problems. The simplified analysis program used less than 1 percent of the CPU time required for a nonlinear finite element analysis.
This paper discusses the modelling by mechanical subelements, the general plasticity, the viscoplasticity, and the creep behavior of solids under multiaxial loading conditions. The formulation of a time-independent elastic-plastic analysis is based on the viscoplasticity theory and the assumed stress finite element method. An example of an in-plane stress problem is included.
The distribution of slip dislocations in silicon dendritic web ribbons due to plastic deformation during the cooling phase of the growth was studied. The results show the existence of two distinguishable stress regions across the ribbon formed during the plastic deformation stage, namely, shear stress at the ribbon edges and tensile stress at the middle. In addition, slip dislocations caused by shear stress near the edges appear to originate at the twin plane.
The mechanics of continua, which is based on the general stress model of Cauchy, up to the present has almost exclusively been applied to liquid and solid elastic bodies. Saint-Venant has developed a theory for the plastic or remaining form changes of solids, but it does not give the required number of equations for determining motion. A complete set of equations of motion for plastic deformable bodies is derived. This is done within the framework of Cauch mechanics. And it is supported by certain experimental facts which characterize the range of applications.
Thermocycling of ceramic-coated turbomachine components produces high thermomechanical stresses that are mitigated by plasticity and creep but aggravated by oxidation, with residual stresses exacerbated by all three. These residual stresses, coupled with the thermocyclic loading, lead to high compressive stresses that cause the coating to spall. A ceramic-coated gas path seal is modeled with consideration given to creep, plasticity, and oxidation. The resulting stresses and possible failure modes are discussed.
A computerized procedure for symbolic manipulations and FORTRAN code generation of elastoplastic material matrix for finite element applications is presented. Special emphasis is placed on expression simplifications during intermediate derivations, optimal code generation, and interface with the main program. A systematic procedure is outlined to avoid redundant algebraic manipulations. Symbolic expressions of the derived material stiffness matrix are automatically converted to RATFOR code which is then translated into FORTRAN statements through a preprocessor. To minimize the interface problem with the main program, a template file is prepared so that the translated FORTRAN statements can be merged into the file to form a subroutine (or a submodule). Three constitutive models; namely, von Mises plasticity, the Drucker-Prager model, and a concrete plasticity model, are used as illustrative examples.
Fixture enables thermal forming of custom-contoured plastic covers in half hour. In assembled cover-forming fixture, spring-loaded plate presses plastic sheet toward baseplate. Finished covers stacked at left side of table.
Elastic and inelastic properties of AS4/APC-2 composites were characterized with respect to temperature variation by using a one-parameter orthotropic plasticity model and a one parameter failure criterion. Simple uniaxial off-axis tension tests were performed on coupon specimens of unidirectional AS4/APC-2 thermoplastic composite at various temperatures. To avoid the complication caused by the extension-shear coupling effect in off-axis testing, new tabs were designed and used on the test specimens. The experimental results showed that the nonlinear behavior of constitutive relations and the failure strengths can be characterized quite well using the one parameter plasticity model and the failure criterion, respectively.
Plastic deformation accompanying the wear process in fully dense and porous metals was studied in order to develop more abradable gas path seal material systems for turbine engine applications. The finite element method was used to model high-rate viscoplastic deformation in the vicinity of a moving contact. Techniques were developed for studying such deformation in both solid (fully dense) and porous metals. Results of the analysis were verified by comparison with experimental data obtained using a pendulum-type rub test apparatus and a pin-on-disk sliding wear tester. It was found that the near-surface plastic deformation in fully dense solids was different from that for porous materials because of the compressibility of the latter. Both types of materials exhibit deformation features which can be taken advantage of in the design of more abradable material systems.