Thermographic stepwise assessment of impact damage in sandwich panels

Metal-polymer-metal sandwiches can find promising applications in the automotive field thanks to their lightweight and formability. The paper focuses on the effect of low velocity impacts on the residual mechanical behavior. Experimental stepwise tests are run on undamaged and impacted specimens with different combinations of thickness and grade for the outer steel skins and the inner polymeric core. Surface temperature evolution is thermally monitored during the tests with the aim to characterize the induced damage and to identify a parameter able to quantify the residual strength of the panel. Several approaches have been considered. The analysis of the thermal amplitude trend with the lock-in thermography evidences a variation in the thermal behavior of the specimens, defining a corresponding damage stress σD. We found a 20% σDdifference between undamaged and damaged specimens. Moreover, impacted specimens experience a temperature and stress concentration at the impact area dependent on the indentation. Based on these results, we evidence the possibility to relate impact indentation with the damage stress estimated by thermography and with the stress concentration factor induced by the impact. Therefore, thermography is a useful and valid tool for post-impact damage detection, monitoring and quantification of these multi-layer sandwich materials.