Abstract

A spacecraft during its operation, can experience temperature variations as high as ± 185 °C. Materials used in its structure such as carbon fiber reinforced polymer (CFRP) solid laminates and sandwich structures with honeycomb core are sensitive to the hostile space environment. One of the most common types of defect is microcracking. This thesis aims to study the effect of thermally induced microcracks on the mechanical property of the composite honeycomb sandwich structure. Composite honeycomb sandwich structure with Kevlar core and facesheet made of carbon fiber/ cyanate ester resin was studied. Sandwich material made of the different core and facesheet thickness were examined to study the thickness effect. To expose the samples to cryogenic temperature, the samples were submerged in Liquid Nitrogen (LN2). To get it elevated temperature, samples were placed in a convection oven. Two different experimental setups for cryogenic conditioning was used, one with accelerated cooling and the other with a slower rate of cooling. This was done to study the effect of the cooling rate on the formation of microcracks. Longitudinal microcracks were primarily observed between the facesheet and core interphase. Microscopic observation was done on two perpendicular polished cross-sections of the samples (ribbon and transverse ribbon directions). Microcracks were quantified using parameters such as crack density and crack length. Flatwise tensile mechanical test was performed on the samples to study the effect of microcracks. Good correlation was made between the crack area and the mechanical strength with the increase in thermal cycles. It is observed that the crack formation get saturated after a number of cycles, avoiding the need to conduct more thermal cycles. Microcrack formation both at the free edge and middle of laminate was observed. The crack density at the middle was comparatively less than the ones found on the free edges. Results for non-contact cooling and direct nitrogen contact cooling were compared. Microscopic inspection and flatwise test show significant difference between contact and non-contact cooled samples. The effect of thermal cycling on the different core and facesheet thicknesses for the same material system was compared. Samples with thicker core seemed to be more sensitive to microcracking. 3D finite element analysis (FEA) was conducted on the sandwich structure geometry to predict the experimental observations. The FEA results are in good agreement with the experimental findings.