Analysis of the Application of a Nanosecond Laser Pulse for Dynamic Hardness Tests Under Ultra-High Strain Rates
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Experimental and numerical tests of surface plastic deformation generated under different strain rates were performed. Deformations were introduced by both classical Brinell and laser pulse hardness tests. An Nd:YAG laser with a wavelength of 1064 nm and a laser pulse time length of 10 ns was used to generated a shock wave to induce local plastic deformation on the material surface. The laser pulse induces a repeatable plastic deformation of a surface without thermal effects on the surfaces. Based on imprint geometry, the dynamic hardness of materials was evaluated at a strain rate of the order 107 s-1. Numerical analyses carried out included quasi-static and dynamic Brinell hardness tests and laser pulse interactions with materials. The Rusinek-Klepaczko constitutive model applied in the calculations allows the prediction of the mechanical characteristics at a strain range strain range from 10 to 4 s-1 to 107 s-1. Numerical and experimental results from the surface plastic deformations show close agreement.
KeywordsLaser pulse Dynamic hardness Plastic deformations Metals
Experimental and numerical tests of surface plastic deformation were generated under different strain rates. Deformations were introduced by classical Brinell’s or laser pulse hardness tests . The Nd:YAG laser with a wavelength of 1064 nm and a laser pulse time length of 10 ns was used to generated a shock wave in order to induce local plastic deformation of surface layer. The laser pulse induces a repeatable plastic deformation without thermal effects on the surfaces. Based on imprint geometry, the dynamic hardness has been evaluated at a strain rate of the order 107 s−1. The numerical analyses have included quasi-static and dynamic Brinell hardness tests as well as laser pulse interactions with materials. The Rusinek-Klepaczko constitutive model (R-K) , applied in the calculations, has enabled to predict mechanical characteristics at a strain range from 10−4 s−1 to 107 s−1. The umerical and experimental results of surface plastic deformations have shown good agreement.
Three different materials were selected for the analysis: commercial pure aluminium, copper and stainless steel (AISI304). Mechanical properties under quasi-static loading conditions were determined at room temperature within a range of strain rates from 10−4 s−1 to 10−2 s−1 using a servo-hydraulic testing machine . At high strain rate loading conditions, the split Hopkinson bar methodology  and the miniaturized direct impact compression test method were applied  to obtain strain rates ranging from 4 × 104 s−1 to 8 × 104 s−1. The Brinell hardness was determined using a 2.5 mm radius steel ball and 613 N load according to EN-ISO 6506-1:2005.
The hardness was defined as a specific work of plastic deformation. This approach was applied for both the typical hardness measurements using an indenter and the determination of the dynamic hardness by laser pulse. The force was estimated on the basis of the pressure generated as a result of the laser pulse interaction with material .
Numerical Analysis Results
To simulate the history of the plasma pulse pressure evolution with time, P(t) data, introduced by Peyre,  were applied. The bottom specimen surface was constrained, whereas pressure generated due to laser action was applied to the central region of the opposite surface (Fig. 1(b)).
A comparison between experimental results and numerical predictions of the HDL hardness is shown in Fig. 5(b). It may be observed that the numerically obtained values are overestimated in comparison to the experimental ones.
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