Abstract
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 (∼103 s–1), to capture the higher strain rate response (∼105–106 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.
Highlights
Laser peening (LP) has emerged as a viable and effective surface treatment by which to introduce beneficial compressive stresses into the near surface regions of metallic components
We address the suitability of using a Johnson-Cook (JC) formulation based on lower strain rate material data (∼103 s–1) to capture the higher strain rate response (∼105–106 s–1) encountered during the LP process
In all cases the model predictions are reasonable representations of the measured responses, this can be largely attributed to the minimal strain rate dependence of these alloys at very low strain rates
Summary
Laser peening (LP) has emerged as a viable and effective surface treatment by which to introduce beneficial compressive stresses into the near surface regions of metallic components. Typical commercial applications employ a short wavelength (∼1 μm) Nd-glass or YAG laser with a short pulse duration (∼10–100 ns), high power intensity (∼1–10 GW/cm2), and high repetition rate (5–10 Hz) [16] An ablative medium, such as a black or aluminum tape, is often placed on the component surface prior to peening to boost the plasma formation. The magnitude of the shock pressure on the component surface depends upon the selected laser parameters, but in most cases is on the order of 1-5 GPa with a duration 2–6 times longer than the laser pulse [1] At these levels of impact, the resulting shock wave can induce strain rates in the components as high as 105– 106 s–1, which can significantly affect the yielding response [18] and the formation of residual stresses
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