Anna K. Zurek

The study of adiabatic shear band instability in a pearlitic 4340 steel using a dynamic punch test

The formation of adiabatic shear band instabilities in a pearlitic 4340 steel using a dynamic punch test has been studied. The dynamic punch-impact test produced white-etching adiabatic shear bands. The average strain of 0.5 was sufficient to produce adiabatic shear bands in this steel at an average strain rate of 18,000 s−1. Nanohardness variations found across the adiabatic shear band are thought to be caused by the fragmentation and spheroidization of the Fe3C and the overall deformation and work hardening of the pearlitic microstructure. The cracks formed at the termination of the adiabatic shear band caused the sample to fracture in a ductile mode.

Micromechanics of spall and damage in tantalum

The authors conducted a series of plate impact experiments using an 80-mm launcher to study dynamic void initiation, linkup, and spall in tantalum. The tests ranged in peak shock pressures so that the effect of peak pressure on the transition from void initiation, incipient spall, and full spall could be studied. Wave profiles were measured using a velocity interferometry system (VISAR), and targets were recovered using {open_quotes}soft{close_quotes} recovery techniques. The authors utilized scanning electron microscopy, metallographic cross-sections, and plateau etching techniques to obtain quantitative information concerning damage evolution in tantalum under spall conditions. The data (wave profiles and micrographs) are analyzed in terms of a new theory and model of dynamic damage cluster growth.

Dynamic mechanical response and thermal expansion of ceramic particle reinforced aluminium 6061 matrix composites

Abstract The mechanical response of silicon carbide (SiC) particle and boron carbide (B4C) particle reinforced aluminium 6061 alloy was studied under quasi-static and dynamic loading conditions, using an Instron universal testing machine and split Hopkinson pressure bar respectively. The stiffness and yield strength of the B4C and SiC particle composites were significantly enhanced as compared to the unreinforced alloy. The strain hardening behaviour of the SiC particle reinforced composites was not significantly different as compared to that of the unreinforced alloy, at either low or high strain rates. On the other hand, the strength and strain hardening of the B4C particle reinforced composites was significantly greater than that of the unreinforced alloy, at both low and high strain rates. Differences in the mechanical behaviour of the two composites was attributed to the differences in the strength of the reinforcing particles and bonding between the B4C and SiC particles and the matrix alloy. Therma...

Modeling of ferrite structure after deformation in the two-phase region

Abstract This study presents some modeling aspects of microstructure development under intercritical (austenite–ferrite two-phase region) forming conditions in microalloyed and low carbon steels. The employed integrated computer model effectively links the advanced finite-element approach simulating metal flow and heat transfer during hot plastic deformation with the submodels describing complex microstructure development. It is proposed that a correct quantitative description of the microstructure allows the selection of processing parameters to control the microstructure inhomogeneity. The effective strain accumulated in the ferrite phase and temperature distributions were combined with the empirical formulas that enable calculation of ferrite grain size with separation between the two mechanisms of refinement process, i.e., transformation from hardened austenite and ferrite recrystallization. The predicted ferrite structures show reasonable agreement with those obtained in various experiments. Hence, the distribution of the mechanical properties will naturally be predicted based on the initial process conditions and using the proposed modeling procedure.

High strain-rate-induced cleavage fracture in mild carbon steel

The brittle and/or cleavage fracture behavior of 1008 carbon steel was characterized under dynamic spall loading conditions. The roles of grain size, carbide morphology, and high strain-rate plastic deformation on the spall-induced cleavage fracture were quantitatively estimated following the analysis of Linet al.

A comparison of shear localization susceptibility in U-0.75 wt pct Ti and W-Ni-Fe during high strain rate deformation

A comparison of shear instability in U-Tivs W alloy during high strain rate deformation is presented. Experimental quasi-static and dynamic deformation data are used to formulate the constitutive description based on the mechanical threshold stress model (MTS). The MTS model is used to predict the deformation behavior of U-Ti and W-alloy beyond the conventionally achievable experimental capabilities. We suggest that uranium alloys are more prone to catastrophic-localized deformation (adiabatic shearing) due to the existence of a soft high-temperature phase, which is reached by uranium alloys undergoing large strains at high strain rate of deformation.

Void Coalescence Model for Ductile Damage

A model for void coalescence for high strain rate ductile damage in metals is presented. The basic mechanism is void linking through an instability in the intervoid ligament. The formation probability of void clusters is calculated, as a function of cluster size, imposed stress, and strain. A wave speed limiting is applied to the cluster size enhancement of cluster growth. Due to lack of space, model formulas are merely described and not derived.

Quantitative description of damage evolution in ductile fracture of tantalum

Dynamic ductile fracture has been studied through incipient spallation experiments on two grades of tantalum. A commercially pure Ta material incipiently spalled at 252 m/s, a highly pure Ta material incipiently spalled at 246 m/s, and a highly pure Ta material preshocked at 250 m/s and incipiently spalled at 246 m/s were used. Microstructural parameters of the fracture process such as porosity, void-size distributions, and void aspect ratios have been quantified using image analysis and optical profilometry techniques. The commercially pure Ta, the highly pure Ta preshocked prior to spall, and the annealed high-purity Ta exhibited 27, 16.6, and 5.5 pct porosity, respectively. The void-size distribution observed in all three tests was adequately represented by either a log-normal or a linear combination of a log-normal and a Weibull distribution function. At least 80 pct of the aspect ratios observed in all three tests were adequately represented by a gamma distribution function.

Experimental quantitative damage measurements and void growth model predictions in the spallation of tantalum

Flyer plate experiments were performed on tantalum under incipient spallation loading conditions. The back surface velocities were recorded using a VISAR and the samples were soft-recovered. The resulting microstructural damage was quantified using image analysis and optical profilometry. This data was used to calibrate a void growth model of ductile fracture. Comparisons between experimental measurements and the calibrated model predictions for two different shock pressures are made. The results show that the VISAR pullback signal or “spall strength” is not a valid measure of dynamic failure resistance (energy) for the case of very clean high Peierls energy materials. The void growth model cannot adequately simulate the experimental tests slightly outside the calibration condition.

The Effect of Material Cleanliness on Dynamic Damage Evolution in 10100 Cu

Two different plates of 10100 Cu (> 99.99 % Cu) were incipiently failed using flyer plate impact in a gas gun equipped with soft recovery. One plate has a grain size of 53 μm and the other, 63 μm. The cleanliness of the plates was assessed using the cold crucible melt technique. The smaller grain sized material has a number density of second phase particles about four times higher than the other material, but with a smaller mean size. The size distribution of second phase particles is well represented by a log normal distribution. The Stepanov corrected spall strength, as measured by a VISAR, of larger grain sized material is about 50 % higher than that of the other material under incipient loading conditions. The difference in spall strength between the materials decreases with increasing shock pressure. Void nucleation plays a dominant role in describing the resulting porosity distribution in the incipiently failed samples. The cause of the difference in spall strength between the plates is attributed t...