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.

中文翻译：

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热循环下复合夹层结构的性能

研究了热致微裂纹对空间级层压夹芯板力学性能的影响。开发了一种使用液氮的简单非接触装置，使材料经受 -170℃ 的低温，冷却速率为 24℃/min。然后将样品暴露在烘箱内150℃的高温下。通过在循环过程中对横截面进行显微镜观察来监测微裂纹的形成和扩展。在多次循环后进行平面拉伸试验。量化微裂纹后，在循环次数和平面机械强度之间建立相关性。据观察，裂纹形成在大约 40 次循环时饱和，避免了进行更多热循环的需要。在层压板的自由边缘和中间都观察到微裂纹形成。中间的裂纹密度相对小于自由边缘的裂纹密度。将非接触式冷却的结果与直接氮气接触冷却的样品进行比较。显微检查和平面测试显示了接触式和非接触式冷却样品之间的差异。非接触冷却样品的平面拉伸强度降低了 15%，而接触冷却样品的粘合强度降低了约 30%。进行 3D 有限元分析以定性地确定应力集中的位置，这些位置可能是裂纹形成的位置。在模型和实验结果之间观察到良好的一致性。显微检查和平面测试显示了接触式和非接触式冷却样品之间的差异。非接触冷却样品的平面拉伸强度降低了 15%，而接触冷却样品的粘合强度降低了约 30%。进行 3D 有限元分析以定性识别应力集中的位置，这些位置可能是裂纹形成的位置。在模型和实验结果之间观察到良好的一致性。显微检查和平面测试显示了接触式和非接触式冷却样品之间的差异。非接触冷却样品的平面拉伸强度降低了 15%，而接触冷却样品的粘合强度降低了约 30%。进行 3D 有限元分析以定性地确定应力集中的位置，这些位置可能是裂纹形成的位置。在模型和实验结果之间观察到良好的一致性。进行 3D 有限元分析以定性地确定应力集中的位置，这些位置可能是裂纹形成的位置。在模型和实验结果之间观察到良好的一致性。进行 3D 有限元分析以定性地确定应力集中的位置，这些位置可能是裂纹形成的位置。在模型和实验结果之间观察到良好的一致性。