Ghatu Subhash

Evolution of microstructure and shear-band formation in α-hcp titanium

The evolution of the microstructure generated by high strain-rate plastic deformation of titanium was investigated. A testing geometry generating controlled and prescribed plastic strains under an imposed stress state close to simple shear was used; this testing procedure used hat-shaped specimens in a compression Kolsky bar which constrains the plastic deformation to a narrow region with approximately 200 Ixm width. Within this band, localization sets in, initiated at geometrical stress concentration sites, at a shear strain of approximately 1.4. The shear-band widths vary from 3 to 20 Ixm and increase with plastic strain. High strain-rate deformation induces, at lower plastic strains (7 < 1.4), planar dislocation arrays and profuse twinning in titanium. In the vicinity of the shear band, elongated cells are formed, which gradually transform into sub-grains. The break-up of these sub-grains inside the band leads to a microstructure composed of small grains ( ~ 0.2 txm) with a relatively low dislocation density. The combined effects of plastic strain and temperature on the microstructural recovery processes (dynamic recovery and recrystallization) are discussed. The experimental results are compared with predictions using a phenomenological constitutive equation and parameters obtained from compression experiments conducted over a wide range of strain rates. The experimental results indicate that the formation of shear bands occurs in two stages: (a) instability, produced by thermal softening and the enhancement of the thermal assistance in the motion of dislocations; (b) localization, which requires softening due to major microstructural changes (recovery and recrystallization) in the material. The calculated temperature rises required for instability and localization are 350 K and 776 K, respectively. Whereas instability may occur homogeneously throughout the entire specimen, localization is an initiation and propagation phenomenon, starting at geometrical (stress concentration sites) or microstructural inhomogeneities and propagating as a thin (3-20 ixm) band.

Dynamic Vickers indentation of brittle materials

Abstract Static and dynamic Vickers indentations were performed on brittle materials to investigate the rate effects in hardness, induced crack morphologies, and fracture toughness. The dynamic indentations were performed on a hardness tester, which utilizes elastic stress wave propagation phenomena in a slender rod that can deliver indentation loads of 100 μs durations. Under dynamic indentations, an increase in hardness was observed in all the brittle materials compared to their static hardness measurements. Analysis of the evolved crack morphology revealed an increase in fracture toughness for zirconia ceramics and a decrease in fracture toughness for pyrex glass under dynamic indentations. The implications of the rate dependence of hardness and fracture toughness on material removal mechanisms are discussed.

Characterization of uniaxial compressive response of bulk amorphous Zr–Ti–Cu–Ni–Be alloy

Uniaxial compressive response of bulk amorphous Zr–Ti–Cu–Ni–Be alloy, also called as Vitreloy-1, was investigated at quasistatic and high strain rates in the range of 10−3 and 103 s−1, respectively. The Vitreloy-1 specimens exhibited elastic response followed by catastrophic fracture along a narrow shear band. The ultimate strength of the specimens varied between 1800 and 2200 MPa irrespective of the strain rate and independent of the aspect ratio of the specimens. The quasistatically deformed specimens fractured into two or three large fragments. The fracture surfaces were relatively smooth and revealed well developed and uniformly distributed veinal pattern. The dynamically loaded specimens, on the other hand, fractured into several fragments with relatively rough fracture surfaces containing nonuniformly distributed and partially developed veinal patterns. Evidence of melting in the form of ‘liquid bubbles’ was also observed along the cracks on the fracture surfaces of the specimens subjected to high strain-rate loading. A comparison of the mechanical response of Vitreloy-1 with other bulk metallic glass systems is also presented.

A plasticity-based model of material removal in chemical-mechanical polishing (CMP)

It is well known that the chemical reaction between an oxide layer and a water-based slurry produces a softer hydroxylated interface layer. During chemical-mechanical polishing (CMP), it is assumed that material removal occurs by the plastic deformation of this interface layer. In this paper, the behavior of the hydroxylated layer is modeled as a perfectly plastic, material, and a mechanistic model for material removal rate (MRR) in CMP is developed. The deformation profile of the soft pad is approximated as the bending of a thin elastic beam. In addition to the dependence of MRR on pressure and relative velocity, the proposed plasticity-based model is also capable of delineating the effects of pad and slurry properties. The plasticity-based model is utilized to explore the effects of various design parameters (e.g., abrasive shape, size and concentration, and pad stiffness) on the MRR. Model predictions are compared with existing experimental observations from glass polishing, lapping, and CMP.

The role of plasticity as a limiting factor in the compressive failure of high strength ceramics

Abstract The behavior of aluminum oxide under compressive loading is investigated over a wide range in strain rate and degrees of confinement. It is shown that plastic flow can be generated in Al 2 O 3 at all strain rates if confinement is sufficient to prevent premature failure via microfracture. Moreover, plastic flow is itself a source of microfracture, and the threshold for multiple slip apparently constitutes the practical ultimate strength for the ceramic. Thus, for sufficiently fine-grained alumina tested under optimum conditions, no confinement is required to generate plastic flow, at which stress the material fails via dislocation-induced general microfracture.

Investigation of the overall friction coefficient in single-pass scratch test

Abstract Single-pass scratch test on bilinear elastic–plastic materials with a conical indenter was simulated using a three-dimensional finite element model. The influence of the interfacial friction coefficient μs and the apical angle α of the indenter on the induced maximum tangential force FT, normal force FN, and the overall friction coefficient μ=FT/FN, were systematically studied. It was found that the induced tangential force is greater than the normal force when the apex is small and vice versa when the apex is large. The tangential force increases with μs, but the normal force decreases with μs. The overall friction coefficient μ was found to increase linearly with μs and tangent of the attack angle of the indenter. The relationship between the adhesion frictional component (μa), the plowing frictional component (μp), and the interfacial friction coefficient μs was analyzed. An analytical model for the overall friction coefficient μ was also developed based on the interaction between the indenter and the specimen and compared to the numerical results. The model was found to yield a good agreement with the finite element simulation results.

An elastic-plastic-cracking model for finite element analysis of indentation cracking in brittle materials

Abstract An ‘elastic–plastic-cracking’ (EPC) constitutive model was developed and incorporated into the commercial explicit finite element package abaqus to analyze the fracture characteristics of brittle materials subjected to indentation loads. The analysis indicated that the EPC model can capture the development of median cracks during the loading phase and the development of lateral cracks during the unloading phase of the Vickers indentation cycle. The influence of material properties on induced damage zone characteristics was analyzed by defining a non-dimensional brittleness parameter. The model predictions of hardness as well as load–depth ( P – h ) relationship during an indentation cycle were found to agree well with the experimental trends presented elsewhere in the literature.

Influence of lateral confinement on dynamic damage evolution during uniaxial compressive response of brittle solids

Abstract A dynamic damage growth model applicable to brittle solids subjected to biaxial compressive loading is developed. The model incorporates a dynamic fracture criterion based on wing-crack growth model with a damage evolution theory based on a distribution of pre-existing microcracks in a solid. Influences of lateral confinement pressure (dynamic or static) as well as frictional coefficient on the rate dependence of fracture strength of basalt-rock are investigated systematically. It is found that the failure strength, damage accumulation and wing-crack growth rate are strongly influenced by the nature and the magnitude of confinement pressure. It is also verified that the effect of strain rate on fracture strength of brittle solids is independent of confinement pressure in a certain range of strain rate.

Consolidation and high strain rate mechanical behavior of nanocrystalline tantalum powder

Abstract High ductility and strength exhibited in nanograined materials can potentially be exploited in explosively formed penetrator liner applications. Both coarse and nanocrystalline tantalum powder were consolidated by Plasma Pressure Compaction (P 2 C) to study the effect of grain size on dynamic compression properties. The powders were consolidated rapidly with 1 minute of isothermal holding time to retain initial microstructure. The P 2 C consolidated specimens were cut by electric discharge machining (EDM), polished for SEM characterization, and tested in dynamic compression using a Kolsky apparatus. The effect of grain size on yield stress and strain was investigated at various strain rates for a coarse grained and a nanograined specimen. Especially, the high strain rate response of nanocrystalline tantalum is discussed in this paper.

Dynamic indentation hardness and rate sensitivity in metals

An experimental technique for determining the dynamic indentation hardness of materials is described. Unlike the traditional static hardness measurements, the dynamic hardness measurements can capture the inherent rate dependent material response that is germane to high strain rate processes such as high speed machining and impact. The dynamic Vickers hardness (DHV) of several commonly used engineering materials is found to be greater than the static Vickers hardness (HV). The relationship between the hardness and yield stress under static conditions, i.e., HV = 3σ y , is also found to be valid under dynamic conditions. It is suggested that this simpler technique can be used to assess the rate sensitive nature of engineering materials at moderate strain rates in the range of around 2000/s.