Metal cutting simulation of 4340 steel using an accurate mechanical description of meterial strength and fracture

Strength and fracture constitutive relationships containing strain rate dependence and thermal softening are important for accurate simulation of metal cutting. The mechanical behavior of a hardened 4340 steel was characterized using the von Mises yield function, the Mechanical Threshold Stress model and the Johnson- Cook fracture model. This constitutive description was implemented into the explicit Lagrangian FEM continuum-mechanics code EPIC, and orthogonal plane-strain metal cutting calculations were performed. Heat conduction and friction at the toolwork-piece interface were included in the simulations. These transient calculations were advanced in time until steady state machining behavior (force) was realized. Experimental cutting force data (cutting and thrust forces) were measured for a planning operation and compared to the calculations. 13 refs., 6 figs

Materials response to large plastic deformation

Strain hardening at large plastic strains cannot be inferred from small-strain tensile tests. Most metals and alloys at room temperature do not reach steady state saturation at strain levels of 3 to 5. Typically, some disturbing influence offsets the balance between dislocation generation and annihilation. The most prominent of these appears to be texture formation. However, grain size, second-phase particles, and deformation on shear bands are also important. The effect on hardening of most of these features depends on geometry (or deformation mode) and, hence, no single intrinsic hardening curve can be expected at large strains. It should be noted that high material purity and a torsional deformation mode favor saturation. 42 references, 15 figures

Strain hardening of heavily cold-worked metals

It is demonstrated that strain hardening in torsion cannot be correlated with axisymmetric deformation by the von Mises effective stress strain criterion. In fcc materials, the flow stress levels and strain hardening rates are typically lower in torsion and saturation, only at lower stress levels. In bcc iron, a low saturtion stress is observed for torsion, whereas linear hardening is observed for axisymmetric extension. Much of the discrepancy in flow curves can be explained by texture. It is demonstrated that a crystallographic effective stress-strain criterion based on evolving average Taylor factors provides the proper magnitude correction for torsional flow curves in fcc materials. The simple crystallographic analysis does not fully explain the hardening response following deformation path changes and multidirectional loading. 96 references, 42 figures

The influence of texture on strain hardening

It is well known that the strain hardening behavior of metals is not the same in tension, compression, torsion and rolling, for example. We report on a new set of experiments, comprising wire-drawing interrupted by tensile tests, free compression, channel-die compression, and short-tube torsion in aluminum, an Al-Mg alloy, copper, silver, and 70:30 brass. The texture was measured before straining and at vonMises strain levels of roughly 1.0 and 2.0. Computer simulations of the deformation starting from a set of random grains weighted by observed initial texture, predicted deformation textures in qualitative agreement with the observed ones in most cases. Quantitatively the simulations yielded the Taylor factors as a function of strain for all paths and, with an assumed hardening law for the representative grain, the macroscopic stress/strain curves. The grain hardening rate as a function of resolved shear stress was described in tabular form such as to match one of the macroscopic curves, and then used to predict the others. The eventual fit was quite good; we will describe what judgments needed to be made to achieve this result. The conclusion is that the strain-path dependence of work hardening can be explained simply as a consequence of texture development. 13 refs., 5 figs., 1 tab

Experiments on plastic deformation at finite strains

The strain hardening behavior of metals at large plastic strains is difficult to assess experimentally. Consequently, many different techniques have been used to study such behavior and no clear experimental picture has evolved. In this paper experiments are reviewed on finite plastic deformation with emphasis on work reported since the comprehensive review of Gil Sevillano, van Houtte, and Aernoudt. The macroscopic strain hardening behavior is the primary concern, but its dependence on crystal structure, purity, alloying, microstructure, stacking fault energy, grain size, and deformation mode, is also discussed

The effect of interface damage on the microbuckling of unidirectional fiber-reinforced composites

Fiber microbuckling is the primary failure mechanism in unidirectional fiber-reinforced composites under compression. Due to processing or service conditions, damage (e.g., microcracks) exists at fiber/matrix interfaces. The effect of damage on the microbuckling of fibers is investigated in the present study. Based on the micromechanics analysis, the damage at interfaces is modeled as a linear spring against interface sliding, and the spring constant depends on the damage level. It is established that the critical strain for fiber microbuckling is relatively insensitive to the interface damage, but increases rapidly with the fiber volume fraction

Comparison of finite-element stress analysis with experimental copper sphere impacts. [NONSAP computer code]

Three copper spheres were impacted on targets of varying surface finishes at 100 m/s. Impact face friction was varied for each test and the impact was photographed with a high-speed camera. Postimpact strains and deformation were measured. A finite-element computer code, NONSAP, was used to model the impact. The best agreement between computer prediction and experiment was obtained using isoparametric elements, a graded mesh, and actual high-strain-rate copper stress-strain data. Frictional conditions at the impact face were also modeled by altering the standard NONSAP code. The most critical test of NONSAP was accurate prediction of experimental impact strains. The best agreement we could obtain had a maximum point-to-point error of 20%, although in general, the comparison was much better. Results of this research indicate that we must know more about material and impact interface friction in order to obtain reliable numerical predictions

Materials Dependence of Deformation Texture Development in Various Deformation Modes

Attempts to understand the development of deformation textures have long been hampered by the following dichotomy: in theory, one expects orientation changes to be governed entirely by the geometry of deformation and thus to be the same for all materials of the same lattice structure and the same deformation modes; yet in experiments, one observes differences between different materials of the same lattice structure. The current work represents an effort to address this problem anew. Four fcc single-phase materials (aluminium, copper, silver, and 70:30 brass) were deformed along three strain-paths (wire-drawing, compression, and torsion), with complete texture determinations before deformation and after strains of roughly 1.0 and 2.0. In parallel, 800 randomly oriented but weighted grains were subjected to simulation of these same tests by the Los Alamos polycrystal plasticity (LApp) code, taking account of the initial textures. An ancillary question which could be addressed in this way is how long initial textures can be remembered by the material; the answer depends on the circumstances and is not always small strains. 8 refs., 3 figs., 1 tab