Effect Of Padding On Flexural Response Of Sandwich Composites

Under flexural loading, premature indentation of face sheet is a major concern in sandwich-structured composites with a softer core. A composite material having sandwich structure is a special class of composite materials that is developed by adhesively bonding two thin but stiff skins to a lightweight but thick core. ASTM C393 standard recommends the use of a wider load pad under the loading pin to ensure distribution of concentrated load [21]. However, it does not address how padding influences the overall flexural behavior of sandwich composites. This investigation focuses on evaluating the effect of padding by comparing flexural tests of sandwich beams with and without padding. Rubber and Teflon materials are used as padding, which are adhesively bonded in the sandwich composite beams. Results show that flexural stiffness and peak load carried by the beams are not affected, but the failure modes can change significantly due to load-point padding. More significant core collapse (failure) in the padded case is the key difference in failure modes between the padded and unpadded cases. This change in failure mode requires careful attention, particularly, for the development of a proper test standard. We conducted finite element analysis, which clarifies the stress distribution in the face sheets and in the core due to padding as well as when padding is absent. Finite element analysis results show that greater volume of the core material is subjected to higher level of von Mises stresses, which occurs in the padded case, increasing the probability of core failure. High shear zones are also observed in the case of padding, and balsawood core cracking is consistent with these high shear zones. Results from this investigation shed light on current experimental methods and consequence of padding in altering failure modes and eventual load-deflection response in sandwich composites subjected to flexural loading.