Showing papers on "Stress–strain curve published in 1985"

Analysis of stress-strain, fracture, and ductility behavior of aluminum matrix composites containing discontinuous silicon carbide reinforcement

Abstract: Mechanical properties and stress-strain behavior were evaluated for several types of commercially fabricated aluminum matrix composites, containing up to 40 vol pct discontinuous SiC whisker, nodule, or particulate reinforcement. The elastic modulus of the composites was found to be isotropic to be independent of type of reinforcement, and to be controlled solely by the volume percentage of SiC reinforcement present. The yield/tensile strengths and ductility were controlled primarily by the matrix alloy and temper condition. Type and orientation of reinforcement had some effect on the strengths of composites, but only for those in which the whisker reinforcement was highly oriented. Ductility decreased with increasing reinforcement content; however, the fracture strains observed were higher than those reported in the literature for this type of composite. This increase in fracture strain was probably attributable to cleaner matrix powder, better mixing, and increased mechanical working during fabrication. Comparison of properties with conventional aluminum and titanium structural alloys showed that the properties of these low-cost, lightweight composites demonstrated very good potential for application to aerospace structures.

Analysis of Progressive Matrix Cracking in Composite Laminates I. Thermoelastic Properties of a Ply with Cracks

Abstract: Overall stiffnesses and compliances, thermal expansion coefficients, and stress and strain averages are evaluated for a fibrous composite lamina which contains a given density of open transverse cracks and is subjected to uniform mechanical loads and thermal changes. The evaluation procedure is based on the self-consistent method and is similar, in principle, to that used in finding elastic constants of unidirectional com posites.

A constitutive model for the stress–strain–time behaviour of ‘wet’ clays

Abstract: A constitutive model is developed to describe the stress–strain–time behaviour of ‘wet’ clays subjected to three-dimensional states of stress and strain. The model is based on Bjerrum's concept of total strain decomposition into an immediate (time-independent) part and a delayed (time-dependent) part generalized to three-dimensional situations. The classical theory of plasticity is employed to characterize the time-independent stress–strain behaviour of cohesive soils using the ellipsoidal yield surface of the modified Cam Clay model presented by Roscoe and Burland. The time-independent strain is divided into an elastic part and a plastic part. The plastic part is evaluated using the normality condition and the consistency requirement on the yield surface. The time-dependent (creep) component of the total strain is evaluated by employing the normality rule on the same yield surface as in the time-independent model and the consistency requirement which requires that the creep strain rate reduces to phenome...

Temperature and strain rate dependence of stress-strain behavior in a nickel-base superalloy

Abstract: The temperature and strain rate dependence of yield and work hardening behavior in elevated temperature stress-strain testing was investigated in the nickel-base superalloy UDIMET 115. This alloy was heat treated to produce a bimodal distribution of hyperfine and coarse γ1 precipitates. Yield behavior is shown to be controlled primarily by dislocation pair cutting of the hyperfine precipitates. Stage II work hardening appears to be governed mainly by the dislocation networks generated on the coarse precipitates as these are bypassed under the Orowan mechanism. Analysis of Stage III work hardening using ϕvs σ plots shows that a steady state exists for the stress-strain deformation of this system. Results are also reported on extremely strong serrated flow at moderately elevated temperatures and a high temperature ductility minimum.

Analysis of necking and neck propagation in polymeric materials

Abstract: N ecking and neck propagation as observed in polymers which “cold draw” is analyzed numerically for a circular cylindrical tensile specimen. The entire load-deformation history of the bar is computed using the finite element method in conjunction with hill's (1958, 1959) variational principle. Rate-independent elastic-plastic material behaviour is assumed. Results are given for the overall load-elongation response of the bar, as well as for the evolution of the specimen profile and the stress and strain distributions in the bar at various stages of the deformation process. The implications of our results on conventional methods used to analyze tension data for polymers are also discussed.

Polyaxial Yielding of Granular Rock

Abstract: Results from true triaxial testing on a granular dense marble are analyzed with emphasis on the incremental relations during strain-hardening (initial yield to peak strength) at constant intermediate and minor stresses. The strong dependency of the behavior on the value of intermediate principal stress is confirmed. Consistency of experimental flow rule was revealed for the examined yield states corresponding to initial yield, to a given value of plastic strain invariant, and to peak strength. A stress invariant and the corresponding one of plastic strain increment were almost equal, postulating a simple stress strain relation. The vector of plastic strain increment and the normal vector on the yield surface at the relative point formed an angle ranging from 10 to 23 degrees. On the basis of the preceding observations, a work balance equation, the resulting flow rule, and yield equation were extended to model three-dimensional behavior. Reasonable agreement was obtained between the predicted and experimental stress-strain relations.

Hardening non-linear behaviour in longitudinal tension of unidirectional carbon composites

Abstract: Uniaxial tensile tests of unidirectional carbon-epoxy coupons are conducted in the longitudinal direction. It is observed that the longitudinal modulus increases with axial stress or strain up to the intermediate level of tension. A fractional constitutive relation with a quadratic denominator is derived by the method of the theory of non-linear elasticity. This equation adopting the estimated higher-order compliance coefficients exhibits an excellent agreement with the experimental results. An empirical strain-based equation is also proposed as a simpler alternative. Averaging formulae for both types of relation are provided for a practical application. The present phenomenon includes the behaviour in a low-stress region discovered by some early work. The consideration of the present non-linear behaviour improves the correlation between theory and experiments in stress-strain relationships of fabric composites with carbon fibres.

Ultra high modulus polyethylene composites

Abstract: The availability of adequate quantities of ultra high modulus polyethylene (UHMPE) fibres has provided the possibility of creating a new range of polymer composites with high energy absorption capabilities. In this paper several aspects of recent research at Leeds University are described. First, there are the possible requirements of the UHMPE fibres in terms of strength, resistance to creep and adhesion to suitable polymeric resins. It has been found that plasma treatment of the fibres with oxygen as the carrier gas produces substantial improvements in adhesion. Although the interlaminar shear strength of composites made with treated fibres is increased it is found that there are only small changes in the mechanical behaviour of the composites. Secondly, comparative data are presented for unidirectional fibre composites where the fibre phase is either wholly UHMPE, Kevlar 49, Carbon EXAS or E—type glass. Results are also presented for hybrid composites which incorporate both UHMPE fibre and one of the commercial fibres. It is shown that these hybrid composites can combine in a useful way the good qualities of UHMPE fibres, especially their very high energy absorption, with other good qualities of the commercial fibres.

Strain Distribution Patterns During Plane Strain Compression

Abstract: A method is described for the determination of the distribution of strain within a specimen undergoing large finite, nonuniform deformations under plane strain compression. The method has been applied to the hot deformation of an aluminum alloy. Results show that prior strain history has no effect on the strain distribution and that specimens of a larger tool width to specimen thickness ratio provide more homogeneous strain distributions. It is proposed that variations in stress-strain curves observed for different tool width to specimen thickness ratios arise from the inhomogeneity of deformation and that these variations are greater in work-hardening materials. Implications for metallographic studies are drawn.

Mechanical properties of blends of nylon 6 with a chemically modified polyolefin

Abstract: Both tensile and impact properties were measured of a heterogeneous polymer blend system, consisting of nylon 6 and a chemically modified polyolefin, DuPont CXA3095, which is an ethylene-based multifunctional polymer. It was found, from the tensile testing, that the blends exhibited no signs of necking, and the addition of a soft resin (CXA3095) reduced the modulus and the tensile strength of nylon 6, whereas the percent elongation at break went through a minimum. When 20 wt % of CXA3095 was added to nylon 6, the impact strength was increased approximately three times. When the factors describing the interfacial adhesion were incorporated, the existing models for predicting the tensile modulus of blends were found to describe the experimental data rather well. In order to help explain the mechanical behavior observed, photomicrographs were taken of the fracture surfaces, using a scanning electron microscope.

Structural effects on grain boundary segregation, hardening and fracture

Abstract: Discussion d'etudes experimentales recentes des effets structuraux dans les metaux et alliages

Inelastic Behavior of Type 316 Stainless Steel Under Multiaxial Nonproportional Cyclic Stressings at Elevated Temperature

Abstract: The stress-range and path-shape dependencies of multiaxial nonproportional cyclic hardening were studied for annealed type 316 stainless steel at 600/sup 0/C by means of stress controlled tests. Cyclic experiments along circular stress paths with constant effective stresses in the axial-torsional stress plane were first performed. The significant cyclic hardening and its stress-range dependency observed for the circular stress cyclings were quantitatively shown in reference to the cyclic stress-strain curves resulted from uniaxial stress cyclings. Then, to discuss the effect of pathshape, the cyclic tests along square stress paths inscribed by the above circular paths, as well as the tests where uniaxial cyclings and torsional ones were alternated, were also carried out. As a result of these tests, the cyclic hardenings for square paths were found to be almost equivalent to those for their circumscribed circular paths. The other type of stress cyclings caused almost the same amount of cyclic hardenings as those for the circular cyclings of the identical stress-ranges.

Nonlinear Stress-Strain Behavior of Graphite/Epoxy Laminates

Abstract: The nonlinear stress-strain behavior of graphite/epox y was investigated. One hundred and eighty-one coupon specimens of the [ ±0] s family, with 0 equal to 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 75°, and 90°, were tested in uniaxial tension. All specimens were unnotched. Three types of tests were conducted: monotonic-to-failure; load, unload, reload-to-failure; and 100 load-unload cycles followed by a load-to-failure. In the latter two cases, the load cycle entered the nonlinear stress-strain region of the specimen. Two basic loadings were found to cause nonlinear stress-strain behavior in a unidirectional ply: longitudinal tension, which causes "stiffening," and shear, which causes "softening." These effects combined to cause overall stiffening in the [ ±8] s laminates for 0 less than 20°, and softening for 6 greater than 20°. The load, unload, reload tests showed three important results. First, there was a "permanent strain" left at zero load for those laminates that softened. Second, the reload path has the same modulus as the original load path and rejoins the basic stressstrain curve at the point to which the laminate was previously loaded. Third, the laminates show no memory of one previous loading after being reloaded to the original load point as further loading produces a stress-strain curve unaltered by one load, unload, reload cycle including no change in fracture stress. Implications for general laminates are discussed.