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RESEARCH PAPERS

J. Eng. Mater. Technol. 1994;116(2):133-141. doi:10.1115/1.2904263.

A series of tests for creep, stress relaxation, and biaxial ratchetting of type 304 stainless steel after cyclic preloading were carried out to investigate their interaction. The interesting fact was pointed out that back stress in cyclic plasticity played an important role to describe creep, relaxation, and biaxial ratchetting following cyclic preloading. Then, the test results showed that the material behavior due to creep after cyclic preloading could be represented by the modified Bailey-Norton law with stress levels evaluated from the current center of the yield surface, i.e., back stress which was determined by the hybrid constitutive model for cyclic plasticity proposed by the authors. In addition, biaxial ratchetting of axial strain induced by cyclic shear straining after cyclic preloading was expressed by the shear stress amplitude, the number of cycle and the axial stress level from the current center.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):142-147. doi:10.1115/1.2904264.

Anisotropic creep behavior of polycrystalline metals under repeated stress changes is modeled from a phenomenological point of view. The creep model consists of basic constitutive equations (BCE) and an auxiliary hardening rule (AUX) to enhance the predictive capability of the BCE. The BCE is characterized by a kinematic hardening variable which is defined as the sum of two component variables; one represents the back stress and the other a flow resistance in the opposite direction of the stress deviator. The AUX is governed by a memory region in which only the evolution of the back stress takes place. The validity of the creep model is discussed on the basis of simulations for multiaxial nonproportional repeated creep of type 304 stainless steel at 650°C.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):148-154. doi:10.1115/1.2904265.

Biaxial plane stress experiments have been performed on cruciform specimens made of graphite grey cast iron. Different ratios of tensile and compressive loads were applied in two perpendicular directions. The primary objective of this investigation is to determine the locus of the yield surface (yield curve) under plane stress, and to establish yield functions that could model the elastoplastic behavior of grey cast iron with reasonably good accuracy. The experiments show that a sufficiently accurate description is obtained by using the ordinary von Mises yield function in the compressive-compressive region, and elsewhere, the von Mises yield function modified with a term containing the first stress invariant. It was also found that for tensile loadings nonelastic deformations develop at low stress levels. Use of the above yield function must therefore be accompanied by a very large hardening modulus for tensile loads.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):155-161. doi:10.1115/1.2904266.

Marchand and Duffy tested thin-walled steel tubes in a split Hopkinson torsion bar at a nominal strain-rate of approximately 1600/s and could not determine conclusively whether a shear band initiating at a point in the tube propagated around the circumference in one direction or in both directions. They estimated the speed of propagation to be 520 m/s in the former case and 260 m/s in the latter. Here we simulate their test numerically, and find that the shear band propagates in both directions around the circumference of the tube. When the tube is twisted at a nominal strain-rate of 5000/s, the band speed varies from 180 m/s at the site of the initiation to approximately 1000 m/s at the nearly diametrically opposite point. The band speed increases with an increase in the nominal strain-rate. The material defect is modeled by assuming that a small region near the center of the tubular surface is made of a material weaker than that of the rest of the tube.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):162-167. doi:10.1115/1.2904267.

Circular plates, under unknown clamping conditions and containing simulated defects in the form of circular localized thinning or thickening, are inspected by double-exposure holography. With an incremental uniform pressure applied between exposures, eccentric defects are readily revealed from the distinct irregular fringe patterns. In the case of central circular defects, however, the absence of distinct irregular fringe patterns does not enable easy visual detection of the defects. The simple method of analysis described in this paper, based on the fact that the displacement in a defective plate differs from that in a defect-free plate, allows easy deduction of central and eccentric defects from the fringe patterns. Furthermore, this method enables identification of the type of defect (localized thinning or thickening), the extent of thickness variation, as well as an accurate estimation of the location and size of the defect.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):168-172. doi:10.1115/1.2904268.

Crack growth rate experiments are conducted by applying an intermediate single overload cycle in constant amplitude load (CAL) tests. For a particular overload ratio, three to four tests are conducted by applying the overload cycle at different crack lengths. The loads are selected in such a way that for a given overload ratio, the size of the overload and CAL monotonic plastic zones are the same at each crack length. A functional form for the crack growth during the retardation was developed that accurately describes all the tests. For comparison, the corresponding CA-load tests are also conducted separately. Finally, a crack growth rate model is developed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):173-180. doi:10.1115/1.2904269.

An analytical method is developed to estimate notch root strains in a notched bar of elastic-plastic, isotropic material subjected to proportional and nonproportional multiaxial nominal loading. The method uses anisotropic plasticity theory to define a structural yield surface in nominal stress space that incorporates both the isotropic material properties and the anisotropic geometry factors of the notch. Notch root plastic strain increments and anisotropic work-hardening effects are then related to this yield surface using standard methods of plasticity. Comparisons of the proposed method with previously published strain estimates using the finite element method for a notched shaft under proportional nominal bending and torsion, and with strain gage measurements of a circumferentially notched solid steel shaft subjected to a series of box-shaped nonproportional loading paths in tension-torsion nominal stress space are presented. The strain calculations agree well both qualitatively and quantitatively using an appropriate nominal load-notch plastic strain relationship, and are suitable for strain-life fatigue calculations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):181-186. doi:10.1115/1.2904270.

In the present paper, an improved strain-energy-density criterion presented recently for the commonly used fracture criterion, the minimum strain-energy-density criterion, is extended to the case of cyclic loading to predict mixed-mode fatigue crack growth in materials with different yield strengths in tension and compression. The analysis of the mixed-mode fatigue crack growth process is very complex. For the purpose of more precisely predicting the mixed mode fatigue crack growth process, we developed a numerical scheme in which the improved fatigue crack growth criterion is combined with the displacement discontinuity method, a boundary element method. In the fatigue crack growth analysis of an inclined crack under uniaxial cyclic loading, the stress intensity factors for each increment of the crack growth are calculated by means of the displacement discontinuity method. Fatigue growth analysis of an inclined crack under uniaxial cyclic loading in materials with different yield strengths in tension and compression is carried out.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):187-192. doi:10.1115/1.2904271.

Some mechanical components cease to function satisfactorily, failing either under excessive elastic deformation or extensive plastic yielding. In the case of constrained plastification, the researcher is faced with some difficulties in evaluating plastic and elastic-plastic strain behavior near the crack tip. In the present study local strains are measured by microstrain gages, mounted near the crack tip on CT specimens made from the high strength aluminum alloy 2024-T351 under cyclic loading at constant ΔK. The behavior and the evolution of the elastic-plastic zone are studied as a function of the stress ratio R, the thickness of the specimen and the level of ΔK. The experimental results are compared with those given by numerical and theoretical analyses based on the concepts of linear elastic fracture mechanics (LEFM).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):193-199. doi:10.1115/1.2904273.

Detailed investigations have been performed for assessing the importance of weld discontinuities in strain controlled low cycle fatigue (LCF) behavior of 308 stainless steel (SS) welds. The LCF behavior of 308 SS welds containing defects was compared with that of type 304 SS base material and 308 SS sound weld metal. Weld pads were prepared by shielded metal arc welding process. Porosity and slag inclusions were introduced deliberately into the weld metal by grossly exaggerating the conditions normally causing such defects. Total axial strain controlled LCF tests have been conducted in air at 823 K on type 304 SS base and 308 SS sound weld metal employing strain amplitudes in the range from ±0.25 to ±0.8 percent. A single strain amplitude of ±0.25 percent was used for all the tests conducted on weld samples containing defects. The results indicated that the base material undergoes cyclic hardening whereas sound and defective welds experience cyclic softening. Base metal showed higher fatigue life than sound weld metal at all strain amplitudes. The presence of porosity and slag inclusions in the weld metal led to significant reduction in life. Porosity on the specimen surface has been found to be particularly harmful and caused a reduction in life by a factor of seven relative to sound weld metal. Defect combination of porosity and slag inclusions was found to be more deleterious than the case when either the slag inclusions or porosity was present alone. Discontinuties acted as crack initiation sites and also enhanced crack propagation. The LCF properties of weld samples containing discontinuities have been correlated with the damage and fracture behavior.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):200-207. doi:10.1115/1.2904274.

The average thermally induced electroelastic fields and the effective thermal expansion and pyroelectric coefficients of two-phase composite materials are obtained by applying the Mori-Tanaka mean-field theory to the coupled response of electroelastic composites through a field superposition scheme. Results are obtained for composites reinforced by ellipsoidal piezoelectric and pyroelectric inhomogeneities and thus are applicable to a wide range of microstructural geometry including lamina, spherical particle, and continuous fiber reinforcement. The results are shown to obey the recently derived Levin-type equations relating the effective thermal expansion and pyroelectric coefficients of a two-phase composite to those of the constituents and the electroelastic moduli of the constituents and the composite. The analysis is developed in a matrix formulation convenient for numerical computation in which the electroelastic (elastic, piezoelectric, and dielectric) moduli are represented by a 9×9 matrix and the thermal expansion and pyroelectric coefficients by a 9×1 column vector. A limited parametric study is performed to illustrate the interesting behavior exhibited by some typical composite microstructures. Finally, analytical predictions are examined in light of existing experimental observations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):208-214. doi:10.1115/1.2904275.

As the first step to understand the creep behavior of a unidirectional SCS-6/Ti-15-3 metal matrix composite, creep tests were performed at 450°C using specimens reinforced with fibers oriented at 0 deg (longitudinal), 45 deg (off-axis), and 90 deg (transverse) to the specimen axis. Measuring the elongation in the gage section, we found that creep deformation and rupture can occur even in the longitudinal creep at stress levels much lower than the tensile strength although the SCS-6 fiber itself does not creep at all, and that the 45 deg off-axis creep has some ductility, which is greater at lower stress, whereas the transverse creep behavior is relatively brittle. Results of the longitudinal creep tests were discussed on the basis of the relaxation of stress in the matrix as well as the bundle strength of fibers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):215-221. doi:10.1115/1.2904276.

This paper examines the effect of varying the microstructural composition of titanium aluminide on the evolution of residual stresses in titanium aluminide matrix composites. An analytical model is developed to determine residual stresses in fiber and matrix phases of unidirectional, SiC/Ti-Al composites subjected to axisymmetric thermal loading. The model uses elements of the concentric cylinder model and the method of cells to calculate residual thermal stresses in the presence of temperature-dependent and inelastic behavior of the fiber and matrix phases. The concentric cylinder model is employed as a geometric model for the unidirectional composite, whereas the method of cells is employed in modeling the microstructure of the titanium aluminide matrix phase. The titanium aluminide matrix consists of distinct brittle and ductile α and β phases whose volume content is varied in the present scheme to understand how the resulting residual stresses can be altered. Both spatially uniform and nonuniform variations of the α and β phases are considered. The results explain the occurrence of radial microcracks in SiC/Ti-Al composites in the presence of a β-depleted region at the fiber/matrix interface, and validate the potential of engineering the matrix phase to reduce residual stresses in these composites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):222-232. doi:10.1115/1.2904277.

Conceptually, fabric composites have some structural advantages over conventional laminates. However, deformation and failure analyses become more complex with the additional anisotropy introduced by the weaving geometry. A micromechanistic deformation model, that could realistically be incorporated into structural finite element codes, is proposed where loading direction and weave parameters are allowed to vary. Comparisons are made to previous models and experimental results for woven materials, indicating that the proposed model provides improved estimates for the linear elastic stiffness. The model further provides predictions for internal stresses in the longitudinal, transverse, and interlace regions of the woven laminate which qualitatively correspond to the experimentally observed failure mechanisms. The experimental program investigates deformations behavior and failure mechanisms of 5-harness 0/90 weave Graphite/Epoxy laminates under tension, compression, and 3-point and 4-point bending loading. Under these conditions the woven laminates exhibit orientation dependent mechanical properties and strength.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):233-237. doi:10.1115/1.2904278.

An experimental investigation of the local compression flange buckling and failure of commercially produced pultruded fiber-reinforced plastic (FRP) I-shaped beams is described in this paper. Results of tests on pultruded E-glass/polyester and E-glass/vinylester composite material beams are described. The test configuration was designed to cause local buckling and ultimate failure of the compression flange of the beams and to prevent global lateral-torsional buckling. The beams were stiffened to prevent crippling and warping at the supports, and local tensile failure at the load points. All beams were monitored with strain gages and LVDT’s. Buckling loads, failure loads, buckling stresses, deflections, and failure modes are reported. Effective mechanical properties of the beams, obtained from overall flexural and shear strain data, are presented. A discussion of the different failure characteristics of the polyester and the vinylester beams is provided.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):238-243. doi:10.1115/1.2904279.

Zee-stiffened compression test panels, fabricated with dispersion-strengthened, high-temperature 8009 aluminum alloy sheet, were evaluated to determine the alloy’s feasibility for compression-critical applications. A compression panel design configuration was obtained using a strength analysis program that predicts the post-skin buckling strength of flat or curved-skinned, metallic-stiffened structure. Three short-column panels were tested to failure at room temperature: (a) a baseline riveted panel fabricated with 2024-T62 aluminum zee stringers and a 2024-T81 aluminum skin, (b) a riveted panel fabricated with 8009 aluminum zee stringers and skin, and (c) a resistance spot-welded panel fabricated with 8009 aluminum zee stringers and skin. The 8009 alloy exhibited pronounced, compressive strength anisotropy, necessitating panel orientation to take advantage of the higher compressive yield in the sheet transverse direction. Compression test results were in good agreement with the predicted compression allowables since they were within 5 percent of the test strength. The 8009 aluminum riveted panel exhibited superior skin buckling resistance and failed in the wrinkling mode, as predicted, at a load approximately 15 percent higher than that of the baseline 2024 panel. The spotwelded 8009 panel did not fail in the wrinkling mode since the spot welds failed in tension shortly after the skin locally buckled. The latter test indicates that the spot welded skin-stringer combinations should not be used above the buckling stress. Due to its excellent microstructural stability at elevated temperatures, high-temperature compression panels of 8009 alloy offer potential weight savings of 25 percent compared with conventional aluminum alloys.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):244-249. doi:10.1115/1.2904280.

A dynamic model of injection molding developed from physical considerations is used to select PID gains for pressure control during the packing phase of thermo-plastic injection molding. The relative importance of various aspects of the model and values for particular physical parameters were identified experimentally. The controller gains were chosen by pole-zero cancellation and root-locus methods, resulting in good control performance. Both open and closed-loop system responses were predicted and verified, with good overall agreement.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1994;116(2):250-254. doi:10.1115/1.2904281.

The electromagnetic field and dynamic deformation analyses for tube expansion by electromagnetic forming were performed by the finite element method. A realistic pressure distribution was calculated by taking into account both coil and workpiece. The calculated values of displacement along the tube axis and with time were in very good agreement with the measured ones.

Commentary by Dr. Valentin Fuster

ERRATA

Commentary by Dr. Valentin Fuster

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