W.R. Whittington

Plasticity and Fracture Modeling/Experimental Study of a Porous Metal Under Various Strain Rates, Temperatures, and Stress States

A microstructure-based internal state variable (ISV) plasticity-damage model was used to model the mechanical behavior of a porous FC-0205 steel alloy that was procured via a powder metal (PM) process. Because the porosity was very high and the nearest neighbor distance (NND) for the pores was close, a new pore coalescence ISV equation was introduced that allows for enhanced pore growth from the concentrated pores. This coalescence equation effectively includes the local stress interaction within the interpore ligament distance between pores and is physically motivated with these highly porous powder metals. Monotonic tension, compression, and torsion tests were performed at various porosity levels and temperatures to obtain the set of plasticity and damage constants required for model calibration. Once the model calibration was achieved, then tension tests on two different notch radii Bridgman specimens were undertaken to study the damage-triaxiality dependence for model validation. Fracture surface analysis was performed using scanning electron microscopy (SEM) to quantify the pore sizes of the different specimens. The validated model was then used to predict the component performance of an automotive PM bearing cap. Although the microstructure-sensitive ISV model has been employed for this particular FC-0205 steel, the model is general enough to be applied to other metal alloys as well. [DOI: 10.1115/1.4025292]

Quantitative analysis of brain microstructure following mild blunt and blast trauma

We induced mild blunt and blast injuries in rats using a custom-built device and utilized in-house diffusion tensor imaging (DTI) software to reconstruct 3-D fiber tracts in brains before and after injury (1, 4, and 7 days). DTI measures such as fiber count, fiber length, and fractional anisotropy (FA) were selected to characterize axonal integrity. In-house image analysis software also showed changes in parameters including the area fraction (AF) and nearest neighbor distance (NND), which corresponded to variations in the microstructure of Hematoxylin and Eosin (H&E) brain sections. Both blunt and blast injuries produced lower fiber counts, but neither injury case significantly changed the fiber length. Compared to controls, blunt injury produced a lower FA, which may correspond to an early onset of diffuse axonal injury (DAI). However, blast injury generated a higher FA compared to controls. This increase in FA has been linked previously to various phenomena including edema, neuroplasticity, and even recovery. Subsequent image analysis revealed that both blunt and blast injuries produced a significantly higher AF and significantly lower NND, which correlated to voids formed by the reduced fluid retention within injured axons. In conclusion, DTI can detect subtle pathophysiological changes in axonal fiber structure after mild blunt and blast trauma. Our injury model and DTI method provide a practical basis for studying mild traumatic brain injury (mTBI) in a controllable manner and for tracking injury progression. Knowledge gained from our approach could lead to enhanced mTBI diagnoses, biofidelic constitutive brain models, and specialized pharmaceutical treatments.

Strain Rate and Stress-State Dependence of Gray Cast Iron

An investigation of the mechanical strain rate, inelastic behavior, and microstructural evolution under deformation for an as-cast pearlitic gray cast iron (GCI) is presented. A complex network of graphite, pearlite, steadite, and particle inclusions was stereologically quantified using standard techniques to identify the potential constituents that define the structure–property relationships, with the primary focus being strain rate sensitivity (SRS) of the stress–strain behavior. Volume fractions for pearlite, graphite, steadite, and particles were determined as 74%, 16%, 9%, and 1%, respectively. Secondary dendrite arm spacing (SDAS) was quantified as 22.50 lm 6 6.07 lm. Graphite flake lengths and widths were averaged as 199 lm 6 175 lm and 4.9 lm 6 2.3 lm, respectively. Particle inclusions comprised of manganese and sulfur with an average size of 13.5 lm 6 9.9 lm. The experimental data showed that as the strain rate increased from 10 3 to 10 s , the averaged strength increased 15–20%. As the stress state changed from torsion to tension to compression at a strain of 0.003 mm/mm, the stress asymmetry increased 470% and 670% for strain rates of 10 3 and 10 s , respectively. As the strain increased, the stress asymmetry differences increased further. Coalescence of cracks emanating from the graphite flake tips exacerbated the stress asymmetry differences. An internal state variable (ISV) plasticity-damage model that separately accounts for damage nucleation, growth, and coalescence was calibrated and used to give insight into the damage and work hardening relationship. [DOI: 10.1115/1.4035616]

A Coupled Experiment-finite Element Modeling Methodology for Assessing High Strain Rate Mechanical Response of Soft Biomaterials.

This study offers a combined experimental and finite element (FE) simulation approach for examining the mechanical behavior of soft biomaterials (e.g. brain, liver, tendon, fat, etc.) when exposed to high strain rates. This study utilized a Split-Hopkinson Pressure Bar (SHPB) to generate strain rates of 100-1,500 sec(-1). The SHPB employed a striker bar consisting of a viscoelastic material (polycarbonate). A sample of the biomaterial was obtained shortly postmortem and prepared for SHPB testing. The specimen was interposed between the incident and transmitted bars, and the pneumatic components of the SHPB were activated to drive the striker bar toward the incident bar. The resulting impact generated a compressive stress wave (i.e. incident wave) that traveled through the incident bar. When the compressive stress wave reached the end of the incident bar, a portion continued forward through the sample and transmitted bar (i.e. transmitted wave) while another portion reversed through the incident bar as a tensile wave (i.e. reflected wave). These waves were measured using strain gages mounted on the incident and transmitted bars. The true stress-strain behavior of the sample was determined from equations based on wave propagation and dynamic force equilibrium. The experimental stress-strain response was three dimensional in nature because the specimen bulged. As such, the hydrostatic stress (first invariant) was used to generate the stress-strain response. In order to extract the uniaxial (one-dimensional) mechanical response of the tissue, an iterative coupled optimization was performed using experimental results and Finite Element Analysis (FEA), which contained an Internal State Variable (ISV) material model used for the tissue. The ISV material model used in the FE simulations of the experimental setup was iteratively calibrated (i.e. optimized) to the experimental data such that the experiment and FEA strain gage values and first invariant of stresses were in good agreement.

Effects of Strain Rate on Mechanical Properties and Fracture Mechanisms in a Dual Phase Steel

Strain rate sensitivity of sheet steels affects their formability and crashworthiness. This contribution reports strain rate sensitivity and effects of strain rate on fracture micro-mechanisms in a commercial dual phase sheet steel (DP590). Uniaxial tensile tests were performed at strain rates of 10−4/s, 1/s and 3200/s to characterize effects of strain rate on ultimate tensile strength and ductility. Fracture surfaces of the tested specimens were quantitatively characterized using stereological techniques to understand the fracture micro-mechanisms. Obtained data indicates that the basic fracture micro-mechanism remains the same with respect to strain rate but strain partitioning in ferrite and martensite is a strong function of strain rate.

Application of a Microstructure-Based ISV Plasticity Damage Model to Study Penetration Mechanics of Metals and Validation through Penetration Study of Aluminum

A developed microstructure-based internal state variable (ISV) plasticity damage model is for the first time used for simulating penetration mechanics of aluminum to find out its penetration properties. The ISV damage model tries to explain the interplay between physics at different length scales that governs the failure and damage mechanisms of materials by linking the macroscopic failure and damage behavior of the materials with their micromechanical performance, such as void nucleation, growth, and coalescence. Within the continuum modeling framework, microstructural features of materials are represented using a set of ISVs, and rate equations are employed to depict damage history and evolution of the materials. For experimental calibration of this damage model, compression, tension, and torsion straining conditions are considered to distinguish damage evolutions under different stress states. To demonstrate the reliability of the presented ISV model, that model is applied for studying penetration mechanics of aluminum and the numerical results are validated by comparing with simulation results yielded from the Johnson-Cook model as well as analytical results calculated from an existing theoretical model.

Interrupted Quasi-static and Dynamic Tensile Experiments of Fully Annealed 301 Stainless Steel

This research examined the evolving microstructure of quasi-static and dynamically loaded fully annealed metastable 301 austenitic stainless steel (SS). Experiments were performed to an interrupted strain value of 20% and to failure using a tension Kolsky bar (1000/s) and an electromechanical load frame (0.001/s). Electron Backscatter Diffraction (EBSD) identified the microstructural evolution from the as-received condition to the 20% strain level for the high and low rate interrupted samples. This material achieved over 60% elongation to failure with increasing strength as strain rate increased, as expected. Fractography analysis using SEM showed particles in the microstructure and a ductile failure mode. The 301 SS exhibited a greater amount of phase transformation from parent austenite to α’-martensite at the dynamic strain rate when compared to the quasi-static strain rate during the interrupted experiments. This result is indicative of the increased propensity for austenite to α’-martensite phase transformations at the high strain rate.

Robust Intermediate Strain Rate Experimentation Using the Serpentine Transmitted Bar

The stress-strain behavior of a material at intermediate strain rates (between 5/s and 500/s) is important for characterizing dynamic deformation events. A material’s mechanical behavior can be strain rate dependent; calibrating constitutive models at actual strain rates of interest are essential for high fidelity simulations. Strain rates below 5/s are easily accomplished with conventional electro-mechanical or servo-hydraulic load frames. Strain rates above 500/s are typically performed with the split Kolsky/Hopkinson pressure bar (SHPB) and other devices depending upon the strain rate. The intermediate strain rate regime is a difficult test regime in which researchers have tried to extend the use of specially instrumented servo-hydraulic load frames or very long Hopkinson bars. We describe a novel design of a serpentine Hopkinson transmitted bar that allows for accurate and robust load acquisition in the intermediate strain rate regime. This design produces repeatable stress-strain results without the stress oscillations typical of a specially instrumented servo-hydraulic load frame and produces data for a longer test time than a conventional Kolsky/Hopkinson bar of the same length.

Structure-property responses of bio-inspired synthetic foams at low and high strain rates

Abstract Various aluminum foams were fabricated with a structure comparable to the Terrapene carolina (box turtle) shell hierarchy as a synthetic means of attaining the lightweight, yet impact-resistive, nature of the biological counterpart. Each foam was constructed from a single aluminum alloy but with different morphologies and foam densities. By borrowing from the sophistication of biological design, the aluminum foams were shown to exhibit robust mechanical performance. High strain rate experimentation, via split Hopkinson pressure bar, was utilized to reveal the strain rate sensitivity of the foams as well as a metric to compare impact performance. The structure-property relations, necessary for accurate material modeling, were also characterized by way of optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and nanoindentation tests. The robust varying mechanical performance was attributed to the biologically inspired materials design.

Corrosion‐Stress Relaxation Effects on Tensile Properties of an AZ61 Magnesium Alloy

One way to increase gas mileage 50% by 2050, a goal of the U.S. government, is to reduce the weight of the vehicle, using lightweight alloys such as magnesium. AZXX Mg alloys have been investigated for this purpose, but are still highly susceptible to stress corrosion cracking. The presence of pits and hydrogen embrittled sections concentrate the stress, leading to cracking and failure. In order to determine the interaction between tensile properties and corrosion behavior, two saltwater environments were used to examine the effects of chloride ion exposure on the corrosion of an extruded AZ61 alloy held under constant strain (approximately 80% of tensile yield strength) over 60 hours. The effects of constant strain on the surface corrosive behavior and the tensile strength were determined at various intervals. The stress-strain relationship minimally decreased over time for the salt spray environment, while large changes were seen in the stress-strain relationship for the immersion environment. In addition, there was a minimal decrease in stress over 60 hours of the AZ61 alloy in the salt spray environment but a 20% decrease in stress over 60 hours in the immersion environment. The differences between stress-strain relationships were attributed to a decrease in surface area of the samples due to the continuous presence of water for the immersion environment, which resulted in a decrease in the ability to withstand applied stress. The formation of pits due to chloride ions for both environments concentrated the applied stress, leading to a decrease in the elongation to failure for both environments.