The residual stress state due to a spherical hard-body impact

The current study assesses the residual stresses and remnant damage caused by a spherical projectile impacting upon a flat surface. The immediate appl ication of this information is to the problem of foreign object damage (FOD ) associated with the ingestion of debris into an aircraft turbine engine a nd the subsequent reduction in component lifetime. The work is focused on t wo primary features: (i) the development of numerical models for the evalua tion of the deformation and stresses associated with the impact process and (ii) the use of spatially resolved residual stress measurements to verify experimentally the numerical analysis. As a first approximation, a quasi-st atic numerical model was developed by ignoring time-dependent effects (i.e. , strain-rate sensitivity, wave and inertia effects, etc.), where the effec ts of velocity were approximated by adjusting the depth and diameter of the resulting impact crater to match that of actual impact craters at the corr esponding velocity. The computed residual stresses and associated elastic s train gradients were compared to experimentally measured values, obtained u sing synchrotron X-ray diffraction (XRD) methods. This comparison indicated that the quasi-static numerical analysis was adequate for moderate impact conditions (velocity = 200 m/s, energy = 2.7 J); however, under more aggres sive conditions (velocity = 300 m/s, energy = 6.1 J), there was significant discrepancy between the numerical predictions and experimental measurement s. Such discrepancy may be attributed to several factors that can occur at higher impact velocities, including strain-rate sensitivity, microcrack for mation, and shear-band formation. A dynamic simulation, where the time-depe ndent effects of strain-rate sensitivity and elastic-wave interactions were approximated, provided results in closer agreement with the experimental d iffraction observations. (C) 2001 Published by Elsevier Science Ltd.