Synchrotron Strain Mapping of the Residual Strain Distribution around Foreign Object Damage in Laser Shock Peened Ti-6AL-4V Alloy

Abstract

The current study investigates the effect of foreign object damage (FOD) on the pre-existing compressive residual stress field associated with laser shock peening (LSP) and its evolution upon combined LCF/HCF cycling. FOD was introduced onto an aerofoil-shaped specimen that had been previously LSP treated through ballistic impacts at angles of 0° and 45° to the leading edge. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0° impact. Significant through thickness compression was introduced parallel to the leading edge as a result of the LSP process. The residual strain distribution was mapped around the FOD notch by synchrotron X-ray radiation. The results show predominantly compressive stresses ahead of the notch, being greater for the 0 compared to 45 impact. No significant stress relaxation was observed after a combined (1000 HCF cycles superimposed on 1 LCF cycle) cycle.

Evolution of a laser shock peened residual stress field locally with foreign object damage and subsequent fatigue crack growth

“…Local to edge-on cube impact the residual stress is characterised by a peak compressive stress parallel to the leading edge immediately below the notch. Comparison of the stress fields introduced by leading edge FOD with [11,29] and without [25] prior LSP under similar conditions indicate that the stresses introduced at the notch tip are surprisingly similar. The effects of the prior laser peening are seen some distance further from the notch tip: both the tensile region around the crater and, most importantly, the tensile region lying some distance below the notch tip have been removed.…”

“…For thick Ti-6Al-4V components, the residual stresses introduced by laser peening are typically ~600 MPa near surface, and reduce linearly to a depth of around 2 mm whereupon balancing tensile stresses are encountered [9]. For thin (a few mm thick) aerofoil specimens, the compressive stresses typically extend through thickness provided there is sufficient material to restrain the lateral expansion of the peened region [10,11].…”

“…This broad pattern seems to occur irrespective of the impactor geometry, although the actual levels of stress vary. The effect of FOD impact on laser peened specimens has also been examined for both head on (0°) and 45° simulated impacts [11,29]. Local to edge-on cube impact the residual stress is characterised by a peak compressive stress parallel to the leading edge immediately below the notch.…”

A Critical Review of Laser Shock Peening of Aircraft Engine Components

“…They developed an FE model to study residual stresses generated by LSP. Preuss, Withers, and Zabeen et al [81][82][83][84] at the University of Manchester studied the effect on the residual stresses of FOD to a previously LSPed leading edge of the aerofoil-shaped specimen and found that different impacts angles of the foreign object caused different evolution of residual stress.…”