High strain-rate material model validation for laser peening simulation

Finite element modeling can be a powerful tool for predicting residual stresses induced by laser peening; however the sign and magnitude of the stress predictions depend strongly on how the material model captures the high strain rate response. Although a Johnson-Cook formulation is often employed, its suitability for modeling phenomena at very high strain rates has not been rigorously evaluated. In this paper, we address the effectiveness of the Johnson-Cook model, with parameters developed from lower strain rate material data (∼10^3 s^–1), to capture the higher strain rate response (∼10^5–10^6 s^–1) encountered during the laser peening process. Published Johnson-Cook parameters extracted from split Hopkinson bar testing were used to predict the shock response of aluminum samples during high-impact flyer plate tests. Additional quasi-static and split Hopkinson bar tests were also conducted to study the model response in the lower strain rate regime. The overall objective of the research was to ascertain whether a material model based on conventional test data (quasi-static compression testing and split Hopkinson bar measurements) can credibly be used in FE simulations to predict laser peen-induced stresses

Relationships of tetragonal precipitate statistics with bulk properties in magnesia-partially stabilized zirconia

The toughness of magnesia-partially stabilized zirconia (Mg-PSZ) is controlled by metastable tetragonal precipitates that interact with crack tip stress fields. Understanding and controlling the precipitate size controls the resulting properties. The precipitate growth behaviour of Mg-PSZ (9·5 mol% MgO) samples was studied after an experimental regime of sintering, rapid quenching and isothermal aging at 1400°C and 1320°C. Average precipitate size on polished and etched samples was measured by SEM for each processing time and temperature. Precipitate sizes, precipitate population statistics, phase content of transformable tetragonal phase and fracture toughness are plotted and optimum precipitate sizes for maximum toughness are identified. Relationships between experimental results and martensitic theories are discussed. The precipitate population distributions did not follow those predicted by Lifshitz-Slyozov-Wagner-based theories

Pro‐ and Subeutectoid Behavior of the Tetragonal Phase in Magnesia‐Partially‐Stabilized Zirconia

Magnesia‐partially‐stabilized zirconia (Mg‐PSZ) is industrially important because of transformable metastable tetragonal precipitates which interact with and arrest cracks. This work addresses the precipitation of tetragonal phases at one composition, 9.5 mol% MgO, throughout a range of temperatures. High‐purity zirconia samples were sintered at 1700°C and rapidly quenched to heat‐treatment temperatures ranging from 1600° to 1100°C, then quenched to room temperature. Cooling rates through the tetragonal + MgO and the monoclinic + MgO two‐phase regions were found to affect the phase content. The kinetics of nonequilibrium phase transformation for high‐purity Mg‐PSZ are presented in terms of time‐temperature‐transformation diagrams. Copyrigh