Q. Xue

Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models.

Self-organization of shear bands in titanium and Ti-6Al-4V alloy

The evolution of multiple adiabatic shear bands was investigated in commercially pure titanium and Ti–6Al–4V alloy through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (10 4 s 1 ). The shearband initiation, propagation, as well as spatial distribution were examined under different global strains. The shear bands nucleate at the internal boundary of the specimens and construct a periodical distribution at an early stage. The shear bands are the preferred sites for nucleation, growth, and coalescence of voids and are, as such, precursors to failure. The evolution of shear-band pattern during the deformation process reveals a self-organization character. The differences of mechanical response between the two alloys are responsible for significant differences in the evolution of the shear band patterns. The number of shear bands initiated in Ti (spacing of 0.18 mm) is considerably larger than in Ti–6Al–4V (spacing of 0.53 mm); on the other hand, the propagation velocity of the bands in Ti–6Al–4V (556 m/s) is approximately three times higher than in Ti (153 m/s). The experimental shear-band spacings are compared with theoretical predictions that use the perturbation analysis and momentum diffusion; the shortcomings of the latter are discussed. A new model is proposed for the initiation and propagation that incorporates some of the earlier ideas and expands them to a two-dimensional configuration. The initiation is treated as a probabilistic process with a Weibull dependence on strain; superimposed on this, a shielding factor is introduced to deal with the deactivation of embryos. A discontinuous growth mode for shear localization under periodic perturbation is proposed. The propagating shear bands compete and periodically create a new spatial distribution.  2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Evaluation of the collapsing thick-walled cylinder technique for shear-band spacing

The thick-walled cylinder (TWC) technique was successfully used to investigate the shear-band patterning in AISI 304 stainless steel. Several factors that may influence the shear-band distribution and spacing in the TWC configuration were examined. The role of machining, annealing, and shrink fitting, as well as the variation of the shear-band distribution along the longitudinal axis of the cylindrical specimen were evaluated. Experimental results indicate that the machined surface at the internal boundary of the cylindrical specimen, where shear bands initiate, provides a strain-hardened layer that significantly changes the condition for their initiation. Specimens with such a layer have a higher density of bands with a smaller spacing, in comparison with those without a work-hardened layer. The nature of contact interface in the cylindrical specimen assembly, either causing a clearance that changes the initial loading conditions or introducing a pre-strained layer with shrink-fitting technique, does not influence the spacing of shear bands, but does affect the evolution and development of multiple shear bands at the initial stage. The distribution of shear bands along the cylinder has a constant spacing but the maximum lengths of bands are sensitive to the position. The collapse process of the cylindrical specimen was simulated by using the RAVEN hydrocode. The deformation, temperature, and velocity histories during the cylinder collapse were calculated. The calculated results are in good agreement with the previous experimental data.

Constitutive response of welded HSLA 100 steel

The dynamic mechanical behavior of a welded joint of high-strength, low alloy (HSLA) 100 steel was investigated by both quasistatic (103 s � 1 ) and high strain rate (103 s � 1 ) tension loadings at ambient and low temperatures. The constitutive responses for the microstructurally different weld and base steels, along with the interface, which included the heat-affected zone (HAZ), were analyzed and compared. This response is successfully modeled by the mechanical threshold stress (MTS) constitutive equation for different regions of the welded joint, which shows qualitatively different behavior. The necking and failure occurred uniformly within the weld metal but not in the HAZ. The main mechanism for the failure of the welded joints is void growth. Microstructural characterization revealed that the nucleation of voids occurred mainly at the interface between the base and the weld metal, and initiated at inclusions. Measurements of damage distributions across HAZ were made to evaluate the contribution of porosity variation to the constitutive response. In both the quasi-static and dynamic tests, the deformation localization in the form of necking first appeared in the weld metal. Fractographic observation demonstrates that void evolution is a dominant factor in the macroscopic mechanical response. The Gurson � /Tvergaard model was included in the modeling effort to incorporate the effect of void opening on the mechanical response as well as tensile instability. The MTS constitutive model was successfully implemented to the tensile regime of loading. # 2003 Elsevier Science B.V. All rights reserved.

Microstructural evolution in adiabatic shear localization in stainless steel

Shear bands were generated under prescribed and controlled conditions in an AISI 304L stainless steel (Fe-18%Cr-8%Ni). Hat-shaped specimens were deformed in a Hopkinson bar at strain rates of ca 10(4) s(-1) and shear strains that could be varied between I and 100. Microstructural characterization was performed by electron backscattered diffraction (EBSD) with orientation imaging microscopy (OIM), and transmission electron microscopy (TEM). The shear-band thickness was ca 1-8 mum. This alloy with low-stacking fault energy deforms, at the imposed strain rates (outside of the shear band), by planar dislocations and stacking fault packets, twinning, and occasional martensitic phase transformations at twin-band intersections and regions of high plastic deformation. EBSD reveals gradual lattice rotations of the grains approaching the core of the band. A [110] fiber texture (with the [110] direction perpendicular to both shear direction and shear plane normal) develops both within the shear band and in the adjacent grains. The formation of this texture, under an imposed global simple shear, suggests that rotations take place concurrently with the shearing deformation. This can be explained by compatibility requirements between neighboring deforming regions. EBSD could not reveal the deformation features at large strains because their scale was below the resolution of this technique. TEM reveals a number of features that are interpreted in terms of the mechanisms of deformation and recovery/recrystallization postulated,. They include the observation of grains with sizes in the nanocrystalline domain. The microstructural changes are described by an evolutionary model, leading from the initial grain size of 15 mum to the final submicronic (sub) grain size. Calculations are performed on the rotations of grain boundaries by grain-boundary diffusion, which is three orders of magnitude higher than bulk diffusion at the deformation temperatures. They indicate that the microstructural reorganization can take place within the deformation times of a few milliseconds. There is evidence that the unique microstructure is formed by rotational dynamic recrystallization. An amorphous region within the shear band is also observed and it is proposed that it is formed by a solid-state amorphization process; both the heating and cooling times within the band are extremely low and propitiate the retention of non-equilibrium structures. Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Evolution in the Patterning of Adiabatic Shear Bands

The evolution of multiple adiabatic shear bands was investigated in stainless steel (different grain sizes: 30 and 140 μm), titanium, and Ti‐6Al‐4V alloy through the radial collapse of a thick‐walled cylinder under high‐strain‐rate deformation (∼104 s−1) and different global strains(up to 0.9). Ti and Ti‐6Al‐4V displayed drastically different patterns of shear bands. The shear‐band spacing is compared with one‐dimensional theoretical predictions based on perturbation (Ockendon‐Wright and Molinari) and momentum diffusion (Grady‐Kipp) concepts. The experimentally observed spacing reveals a two‐dimensional character of self‐organization, not incorporated into the existing theories. A novel analytical description is proposed, in which embryos(potential initiation sites) are activated as a function of strain (greater than a threshold) according to a Weibull‐type distribution. The model incorporates embryo disactivation by stress shielding as well as selective growth of shear bands. The imposed strain rate, emb...

Self-organization of adiabatic shear bands in Ti, Ti-6Al-4V and stainless steel

The self-organization of multiple adiabatic shear bands (SB) was investigated through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (∼104 s−1). Materials with different properties, stainless steel 304L, Ti, and Ti-6Al-4V alloy, were used to examine the shear-band initiation, propagation, as well as spatial distribution. Shear-band spacing is compared with existing theories.