Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, which is comparable to the thermal environment experienced by spacecraft structures. After each thermal cycle, coupons were inspected for microcracks under an optical microscope at the cross section. The microcracks were then quantified using parameters like crack length and crack density with increase in the number of cycles. Flatwise tensile test was conducted on the coupons after every 10 thermal cycles, up to 60 cycles, to make a correlation between crack density and mechanical strength. It was observed that by increasing the number of thermal cycles, the crack density increases and the flatwise tensile strength decreases up to a specific number of cycles. Finite element analysis was performed to predict the possible location of microcracks formation and compared with experimental observation. Good correlation was observed.
Sandwich panels made of polymeric composite materials used in hostile thermal fatigue environments are prone to microcracking due to internal stresses. The impact of micro-cracking as a result of thermal fatigue is more severe in sandwich structures as opposed to solid laminates.Four sandwich panel configurations are studied. They are quasi-isotropic panels made of polymeric carbon fiber reinforced skin bonded by adhesive to honeycomb Kevlar cores.Different facesheet and core thicknesses are investigated. Samples are subjected to thermal cycles from -195°C to 150°C. Microscopic inspection is performed at the sample cross-section for a number of cycles to observe the location and density of cracks. It is observed that cracks are formed mainly at the adhesive/composite interface. Also, microcracks are formed more in the core ribbon direction compared to the core transverse direction. For all samples, after 40 thermal cycles, the total crack length becomes saturated and remains almost constant and no more damage happens. To study the effect of microcracks on mechanical property, flatwise tensile test was performed at room temperature. By increasing the number of cycles, the crack area increases and the flatwise strength decreases. Experimental data indicates that the samples with higher core to facesheet thickness ratio has higher microcrack lengths and lower flatwise strength. Therefore, sandwich panels with thinner facesheet and thicker core are more susceptible to the damage if subjected to the thermal fatigue.
Effect of thermally induced microcracks on mechanical performance of a space grade laminated sandwich panel is investigated. A simple non-contact setup using liquid nitrogen is developed to subject the material to low temperature of −170℃ with cooling rate of 24℃/min. Then the samples are exposed to the elevated temperature of 150℃ inside oven. Microcracks formation and propagation are monitored through microscopic observation of cross-section during the cycling. Flatwise tensile test is performed after a number of cycles. A correlation is made between number of cycles and flatwise mechanical strength after quantifying the microcracks. It is observed that the crack formation gets saturated at about 40 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 are compared with samples from direct nitrogen contact cooling. Microscopic inspection and flatwise test show differences between contact and non-contact cooled samples. Flatwise tensile strength for non-contact cooled samples shows 15% reduction, while the contact cooled samples have about 30% decrease in bond strength. A 3D finite element analysis is conducted to qualitatively identify the location of stress concentration which can be possible sites of crack formation. Good agreement is observed between the model and experimental results.
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