D.K. Francis

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.

Finite Element Model for Plymouth Tube Processing using Internal State Variables

The complete manufacturing process of cold drawn welded steel tubing was simulated using a history dependent internal state variable plasticity-damage model. In order to implement the history dependent model, the Internal State Variables (ISV) Plasticity-Damage (DMG) model that has been developed at Sandia National Labs and Mississippi State University was employed for a low carbon steel AISI 1010, which is the alloy used by the partnering tube manufacturing company. Even though the model is performed at the structural scale, the history variables (internal state variables) which are driven by microstructural changes—e.g., dislocation density and grain size—were tracked through the whole sequences, and the history model captured real thermal and mechanical behaviors of the target material. Once the internal state variables were extracted from the simulation, the model showed critical deformation histories that one could not notice with the naked eye, and must therefore be considered for future design of the tubing process.