Factors in the Compressive Strength of Composite Sandwich Panels with Thin Facesheets

Abstract

An experimental investigation was conducted to investigate the contribution of various factors to the compressive strength of composite sandwich panels with thin facesheets. The main factors investigated were the laminate configuration and the inclusion of notches, the latter to provide information on the damage tolerance of such configurations. Tests were conducted on sandwich panels with 25.4 mm thick Nomex honeycomb core and secondarily-bonded facesheets made from AWl 93PW/3501-6 graphite/epoxy fabric. Two different facesheet configurations, both 0.35 mm thick and consisting of two plies of the fabric, were investigated: (0/90) and (45). Unnotched specimens and specimens with holes ranging from 3.18 mm to 9.53 mm in diameter were tested monotonically to failure in compression. In a number of applications of thin graphite/epoxy facesheets in sandwich configurations, it has been reported that the existence of facesheet wrinkling played a significant role in the response and failure of these panels. This includes a reduction in the failure load from that anticipated. The possibility of the existence of face wrinkling and the factors involved were investigated by placing strain gages on both facesheets in a number of locations. This included strain gages within the region of influence of the hole for these specimens. Specimen failure was seen to be a result of compressive failure of the facesheets. Evidence of any local instability of the facesheets possibly indicative of wrinkling was observed in the strain response for only one particular configuration. However, the failure mechanisms did not indicate any instability in all cases. The failure mechanism was dependent on the facesheet configuration as was response to the presence of notches. The (0/90) facesheet configuration showed an almost linear stress-strain curve up to failure. Failure occurred with a neat fracture perpendicular to the applied load. This case showed typical sensitivity to the presence of the notches for hole diameters larger than 3.18 mm and was adequately correlated using available models for composite laminates. For the (45) facesheet configuration, a highly stress-strain behavior was observed much as in tension tests. Damage patterns at failure were characterized by fractures developing along the +45 and -45O directions and at the interply