Abstract

Composite structures (panels, antenna, brackets / fittings) are often cured in an autoclave to acquire the required space grade quality. These days the industry is focusing on out of autoclave manufacturing methods which leads to more voids inside the laminate compared to those manufactured in the autoclave. In this work, the influence of voids on the micro-crack formation, under thermal cycles and environmental conditions, were investigated and analyzed. Thermal cycle experiments were performed using liquid nitrogen and oven followed by microscopic observation of the polished cross-section of the 900 layered plies. Cracks were monitored, counted, and measured in void and void free areas. Void content was characterized using a microscope and the images obtained were analyzed using the ImageJ software to compare crack density and area. It was observed that the micro-cracks were formed both around the voids and in void free areas. It was noticed that as the number of thermal cycles is increased, the number of micro-cracks around the voids increased much faster as compared to the void free areas. It was witnessed that most of micro-cracks propagated in the transverse direction. Also the formation of microcracks around the void depended on the void size, shape, and its location. Further, the Interlaminar Shear Strength (ILSS) of the samples were also measured. The results indicated that ILSS reduced as the number of thermal cycle increased, since the micro crack density also increased. One Moisture cycle was introduced as a possible way to detect microcracks at the early stage of thermal cycling. Experimental results showed that more microcracks will be appeared after moisture cycling. This
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can be attributed to the reduction of matrix stiffness due to the moisture absorption and also the process conditions which includes nitrogen submersion and heating above 100 0C. The finite element method was used to simulate the experimental process. Micro, meso and macro models were created with respect to original samples, voids and positions so as to calculate the stress distribution and its concentration. It was observed that the simulated results are in good agreement with the experimental results. Therefore, from the experimental studies and FEM analysis it is possible to predict the location of micro-crack formation which further depends on the void shape, its location and size. Hence, these data can be used in the prediction of material and structure service life.