Determining the constitutive response of polymeric materials as a function of temperature and strain rate

Compression stress-strain measurements have been conducted on two commercial high temperature thermoplastic polymers (polyamideimide (PAI) and partially crystalline polyetheretherketone (PEEK)) as a function of temperature (-55 °C to 300 °C) and strain rate (0.001 s$^{-1}$ to 2500 s$^{-1}$). A split-Hopkinson pressure bar (SHPB) was used to achieve strain rates of about 2500 s$^{-1}$ and conventional testing machines were used for strain rates from 0.001 s$^{-1}$ to 1 s$^{-1}$. A large variation in the mechanical response was observed over the range of temperatures tested for both polymers showing “yield" and “plastic" deformation below the glass transition temperature (T$_{\rm g}$). Above T$_{\rm g}$ both thermoplastic polymers exhibited increasing loading moduli with increasing strain rate or decreasing temperature. T$_{\rm g}$ appears to shift to higher temperatures as the strain rate increases. Below T$_{\rm g}$ both thermoplastic polymers exhibited a yield-type behavior followed by anelastic deformation. PEEK was seen to be more sensitive to strain rate and temperature than PAI. A range of different flow stress versus strain behaviors was observed at different temperatures and strain rates

Large-strain time-temperature equivalence in high density polyethylene for prediction of extreme deformation and damage

Time-temperature equivalence is a widely recognized property of many time-dependent material systems, where there is a clear predictive link relating the deformation response at a nominal temperature and a high strain-rate to an equivalent response at a depressed temperature and nominal strain-rate. It has been found that high-density polyethylene (HDPE) obeys a linear empirical formulation relating test temperature and strain-rate. This observation was extended to continuous stress-strain curves, such that material response measured in a load frame at large strains and low strain-rates (at depressed temperatures) could be translated into a temperature-dependent response at high strain-rates and validated against Taylor impact results. Time-temperature equivalence was used in conjuction with jump-rate compression tests to investigate isothermal response at high strain-rate while exluding adiabatic heating. The validated constitutive response was then applied to the analysis of Dynamic-Tensile-Extrusion of HDPE, a tensile analog to Taylor impact developed at LANL. The Dyn-Ten-Ext test results and FEA found that HDPE deformed smoothly after exiting the die, and after substantial drawing appeared to undergo a pressure-dependent shear damage mechanism at intermediate velocities, while it fragmented at high velocities. Dynamic-Tensile-Extrusion, properly coupled with a validated constitutive model, can successfully probe extreme tensile deformation and damage of polymers

Influence of energetic-driven “Taylor-Wave” shock-wave prestraining on the structure/property response of depleted uranium

The influence of shock prestraining, via direct energetic “Taylor-wave” (triangular wave) loading, on the post-shock structure/property behavior of depleted uranium (DU) was studied. Samples were shock prestrained within a “soft” shock recovery fixture composed of momemtum traps and a spall plate to assure 1-dimensional loading. The DU samples exhibit roughly a 30% increase in yield strength following shock prestraining to $\sim$45 GPa. The texture evolution in DU was quantified using electron-backscatter diffraction (EBSD). Detailed quantification of the substructure evolution following shock prestraining revealed high volume fractions of {130}, `{172}', and {112} deformation twins. The volume fraction of {130} twins was found to be $\sim$10x the volume fraction of `{172}' and {112} twins. Details of the twin system activation and volume fraction relative to the local Schmidt factor within grains are presented. The influence of HE-driven shock prestraining on the structure/property response of DU is compared and contrasted to that seen in 304SS and 316SS subjected to “Taylor-wave” shock prestraining

Influence of temperature, strain rate and thermal aging on the structure/property behavior of uranium 6 wt% Nb

A rigorous experimentation and validation program is being undertaken to create constitutive models that elucidate the fundamental mechanisms controlling plasticity in uranium-6 wt.% niobium alloys (U-6Nb). These models should accurately predict high-strain-rate large-strain plasticity, damage evolution and failure. The goal is a physically-based constitutive model that captures 1) an understanding of how strain rate, temperature, and aging affects the mechanical response of a material, and 2) an understanding of the operative deformation mechanisms. The stress-strain response of U-6Nb has been studied as a function of temperature, strain-rate, and thermal aging. U-6Nb specimens in a solution-treated and quenched condition (ST/Q) and after subsequent aging at 473K for 2 hours were studied. The constitutive behavior was evaluated over the range of strain rates from quasi-static (0.001 sec$^{ - 1})$ to dynamic ($\sim$2000 sec$^{ - 1})$ and temperatures ranging from 77 to 773K. The yield stress of U-6Nb was exhibited pronounced temperature sensitivity. The strain hardening rate is seen to be less sensitive to strain rate and temperature beyond plastic strains of 0.10. The yield strength of the aged material is less significantly affected by temperature and the work hardening rate shows adiabatic heating at lower strain rates (1/s)

Structure/property (constitutive and dynamic strength/damage) characterization of additively manufactured 316L SS

For additive manufacturing (AM), the certification and qualification paradigm needs to evolve as there exists no “ASTM-type” additive manufacturing certified process or AM-material produced specifications. Accordingly, utilization of AM materials to meet engineering applications requires quantification of the constitutive properties of these evolving materials in comparison to conventionally-manufactured metals and alloys. Cylinders of 316L SS were produced using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1kW Yb-fiber laser. The microstructure of the AM-316L SS is detailed in both the as-built condition and following heat-treatments designed to obtain full recrystallization. The constitutive behavior as a function of strain rate and temperature is presented and compared to that of nominal annealed wrought 316L SS plate. The dynamic damage evolution and failure response of all three materials was probed using flyer-plate impact driven spallation experiments at a peak stress of 4.5 GPa to examine incipient spallation response. The spall strength of AM-produced 316L SS was found to be very similar for the peak shock stress studied to that of annealed wrought or AM-316L SS following recrystallization. The damage evolution as a function of microstructure was characterized using optical metallography

Structure/property (constitutive and dynamic strength/damage) characterization of additively manufactured 316L SS

For additive manufacturing (AM), the certification and qualification paradigm needs to evolve as there exists no “ASTM-type” additive manufacturing certified process or AM-material produced specifications. Accordingly, utilization of AM materials to meet engineering applications requires quantification of the constitutive properties of these evolving materials in comparison to conventionally-manufactured metals and alloys. Cylinders of 316L SS were produced using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1kW Yb-fiber laser. The microstructure of the AM-316L SS is detailed in both the as-built condition and following heat-treatments designed to obtain full recrystallization. The constitutive behavior as a function of strain rate and temperature is presented and compared to that of nominal annealed wrought 316L SS plate. The dynamic damage evolution and failure response of all three materials was probed using flyer-plate impact driven spallation experiments at a peak stress of 4.5 GPa to examine incipient spallation response. The spall strength of AM-produced 316L SS was found to be very similar for the peak shock stress studied to that of annealed wrought or AM-316L SS following recrystallization. The damage evolution as a function of microstructure was characterized using optical metallography

The mechanical response of a Uranium-Niobium alloy: A comparison of cast versus wrought processing

A rigorous experimentation and validation program is being undertaken to develop “process aware” constitutive models that elucidate the fundamental mechanisms controlling plasticity in uranium-6 wt.% niobium alloys (U-6Nb). The first alloy is a “wrought” material produced, by processing a cast ingot via forging and rolling into plate. The second material investigated is a direct cast U-6Nb alloy. The purpose of the investigation is to determine the principal differences, or more importantly, similarities, between the two materials due to processing. It is well known that parameters like grain size, impurity size and chemistry affect the deformation and failure characteristics of materials. Metallography conducted on these materials revealed that the microstructures are quite different. Characterization techniques including tension, compression, and shear testing were performed to quantify the principal differences between the materials as a function of stress state. Dynamic characterization using a split Hopkinson pressure bar in conjunction with Taylor impact testing was conducted to derive and thereafter validate constitutive material models. The primary differences between the materials will be described and predictions about material behavior will be made

Characterization of shocked beryllium

While numerous studies have investigated the low-strain-rate constitutive response of beryllium, the combined influence of high strain rate and temperature on the mechanical behavior and microstructure of beryllium has received limited attention over the last 40 years. In the current work, high strain rate tests were conducted using both explosive drive and a gas gun to accelerate the material. Prior studies have focused on tensile loading behavior, or limited conditions of dynamic strain rate and/or temperature. Two constitutive strength (plasticity) models, the Preston-Tonks-Wallace (PTW) and Mechanical Threshold Stress (MTS) models, were calibrated using common quasi-static and Hopkinson bar data. However, simulations with the two models give noticeably different results when compared with the measured experimental wave profiles. The experimental results indicate that, even if fractured by the initial shock loading, the Be remains sufficiently intact to support a shear stress following partial release and subsequent shock re-loading. Additional “arrested” drive shots were designed and tested to minimize the reflected tensile pulse in the sample. These tests were done to both validate the model and to put large shock induced compressive loads into the beryllium sample