Comparison of fatigue limit strength of Ti–6Al–4V in tension and torsion after real and simulated foreign object damage

Abstract

Flat samples of Ti–6Al–4V forged plate material were subjected to ballistic impact from 3.18 mm diameter steel spheres at velocities of 200 or 300 m/s and subsequently fatigue tested in either tension or torsion to determine their fatigue limit strength corresponding to 10 6 cycles. Pendulum and quasi-static indentations were also produced at the same depths as the ballistic indents for comparison. Fatigue notch factors, k f, were compared with FEM computed elastic stress concentration factors, k t, at the appropriate locations where stresses were maximum. Factors contributing to the effect of the ballistic impact and pendulum and quasi-static indents included geometry of notch, residual stresses induced by the indentation procedure, microstructural damage, stress gradients away from notch hot spots, and possible strengthening mechanisms arising from local deformation at and near the impact site. Stress relief annealing was used to isolate the effects of residual stresses in half of the samples. Microstructural analysis was used to identify mechanisms of damage and fatigue initiation sites. Among the major findings, stress relieved specimens were generally found to have higher fatigue strengths than those not relieved, indicating that tensile residual stress fields were produced near the ultimate failure locations during the indenting process. Ballistic impact was found to be more damaging than either quasi-static or pendulum impacts. Finally, stress relieved specimens in several cases showed little or no reduction in fatigue strength, even when local values of k t were substantial, indicating some type of strengthening mechanism developed during indentation.