Mike F Lopez

Influence of Shock Prestraining and Grain Size on the Dynamic‐Tensile‐Extrusion Response of Copper: Experiments and Simulation

The mechanical behavior of, and damage evolution in high‐purity Cu is influenced by strain rate, temperature, stress state, grain size, and shock prestraining. The effects of grain size on the tensile mechanical response of high‐purity Cu have been probed and are correlated with the evolution of the substructure. The dynamic extrusion response of shock prestrained Cu demonstrates the significant influence of grain size on the large‐strain dynamic tensile ductility of high‐purity copper. Eulerian hydrocode simulations utilizing the Mechanical Threshold Stress constitutive model were performed to provide insight into the dynamic extrusion process. Quantitative comparisons between the predicted and measured deformation topologies and extrusion rates are presented.

Influence of Temperature and Strain Rate on the Compressive Behavior of PMMA and Polycarbonate Polymers

Compression stress‐strain measurements have been made on commercial polymethylmethacrylate (PMMA) and polycarbonate (PC) polymers as a function of temperature (−197C to 220C) and strain rate. A split‐Hopkinson‐pressure bar (SHPB) was used to achieve strain rates of about 2500 s−1 and a servo‐hydraulic tester was used for lower strain rate testing (0.001 to 5 s−1). The mechanical response of these transparent polymers is quite different. The strength of PC is weakly dependent on strain rate, only moderately dependent on temperature, and remains ductile to −197C. In contrast, the strength of PMMA is linearly dependent on temperature and strongly dependent on strain rate. Significantly, PMMA develops cracking and fails in compression with little ductility (7–8% total strain) at either low strain rates and very low temperatures (−197C) or at high strain rates and temperatures very near ambient.

Influence of Microstructural Anisotropy on the Spallation of 1080 Eutectoid Steel

While the influence of crystallographic texture on elastic and plastic constitutive response has seen extensive investigation in recent years, the influence of texture on the dynamic fracture of engineering materials remains less extensively explored. In particular, the influence of anisotropy, both textural and morphological, on the spallation behavior of materials remains poorly quantified. In this study, the spallation response of 1080‐steel has been studied as a function of microstructural morphological anisotropy. In this study the influence of elongated MnS stringers, resident within a crystallographically isotropic eutectoid steel, on the spallation response of 1080 steel was investigated. The spallation response of a fully‐pearlitic 1080 steel loaded to 5 GPa was found to be dominated by the heterogeneous nucleation of damage normal and orthogonal to the MnS stringers. Delamination between the matrix pearlitic microstructure and the MnS stringers was seen to correlate to a significantly lower pull...

Influence of Shock‐Wave Profile Shape (“Taylor‐Wave” versus Square‐Topped) on the Shock‐Hardening and Spallation Response of 316L Stainless Steel

While much has been learned over the past five decades concerning shock hardening and the spallation response of materials shock‐loaded using “square‐topped” shock profiles, achieved via flyer plate loading, considerably less quantitative information is known concerning direct in‐contact HE‐driven or triangular‐wave loading profile shock prestraining on metals and alloys. In this paper the influence of shock‐wave profile, using both “square‐topped” and triangular‐wave pulses, on the shock hardening and spallation response of 316L stainless steel is presented. The shock hardening in 316L SS, using a triangular‐shaped pulse and square‐topped pulse (pulse duration of 0.75 μsec) to a peak shock pressure of 6.6 GPa was found to be reasonably similar. Square‐wave loading at 6.6 GPa is observed to result in incipient spallation in 316L SS while triangular‐wave loading to an equivalent peak stress is quantified to exhibit no wave‐profile “pull‐back” nor damage evolution.

Substructure Evolution in Energetic‐Driven Spherically Shock‐Loaded Copper

Post‐shock‐recovered metallurgical analysis of solid metal spheres shock loaded via spherical energetic(HE) loading provides a unique opportunity to quantify the substructure evolution in a material subjected to converging Taylor‐wave (triangular‐shock pulse) loading. In this paper detailed quantitative metallographic, orientation‐imaging microscopy (OIM), and texture analysis is presented characterizing the gradient in substructure generated in Cu subjected to a spherical HE shock loading pulse at VNIIEF. The substructure in the recovered sphere is seen to include: 1) a spherical cavity generated in the center of the sphere due to shock‐wave convergence and release, displaying ductile dimpled failure and no evidence of melting, 2) a gradient in deformation (slip and deformation twins) from the center outward to the surface, and 3) numerous shear cracks and/or spall planes. The substructure evolution is discussed relative to that previously observed in Cu shock prestrained via either 1‐D triangular‐shaped...

The role of interfaces on dynamic damage in two phase metals

For ductile metals, the process of dynamic fracture during shock loading is thought to occur through nucleation of voids, void growth, and then coalescence that leads to material failure. Particularly for high purity metals, it has been observed by numerous investigators that voids appear to heterogeneously nucleate at grain boundaries. However, for materials of engineering significance, those with inclusions, second phase particles, or chemical banding it is less clear what the role of grain boundaries versus other types of interfaces in the metal will be on nucleation of damage. To approach this problem in a step-wise fashion two materials have been investigated: high purity copper, and copper with 1% lead. These materials have been shock loaded at 1.4 GPa and soft recovered. In-situ VISAR and post mortem metallography reveals significantly less damage in the metals with no lead. The role of lead at grain boundary triple points and its behavior during shock loading will be discussed.

INFLUENCE OF MICROSTRUCTURE ON THE BAUSCHINGER EFFECT AND THE SHOCK HARDENING IN 1080 HIGH‐CARBON STEEL

The importance of a microstructurally‐controlled Bauschinger component to defect storage during the shock loading process has been shown to be correlated to both quasi‐elastic release effects and reduced shock hardening in materials. In the current study shock recovery experiments have been conducted on a high‐carbon 1080 steel as a function of three microstructural states; pearlitic, partially‐spheriodized, and where the cementite has been fully spheriodized. The 1080 steel in the pearlitic condition is shown to exhibit a significant Bauschinger effect while the fully spheriodized microstructure is observed to display significantly higher shock hardening when shock prestrained to an equivalent 12.8 GPa. The shock hardening response of 1080 steel is discussed in terms of the micromechanisms controlling defect generation and storage during shock loading in materials and the importance of the Bauschinger effect on modeling shock hardening in some materials.