Loss of functional and structural fatigue performance in Nitinol due to laser shock peening

Laser shock peening of austenitic Nitinol using a picosecond laser with 35 mJ laser energy resulted in compressive residual stresses in the surface. However structural fatigue experiments in stress-controlled mode revealed a reduction in fatigue life from 1, 86, 672 cycles in the unpeened condition to 89, 318 cycles in the peened condition. Functional fatigue experiments conducted in strain-controlled mode revealed a difficulty in complete strain recovery of the peened samples when compared to the unpeened samples. Differential scanning calorimetry experiments revealed a shift in the As and Af transformation temperatures to higher values in the peened sample. The deterioration in the structural and functional characteristics of the peened Nitinol was attributed to the enhancement in dislocation density brought about by the peening process, albeit enhancement in surface roughness from 0.25 mu m to 0.31 mu m could also be contributing towards the poor structural fatigue performance. This work reveals that for functional materials such as Nitinol, laser shock peening may not necessarily contribute to enhancement in fatigue performance. We also show in the work that PVC black tape is better than black paint as an ablation layer for generating compressive residual stresses during laser shock peening of Nitinol.