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Internal Strain Monitoring of Vacuum Bag Only Process Using Optical Fiber Sensors



Vacuum bag only (VBO) cure is one of out-of-autoclave (OOA) methods that are attractive alternatives to the conventional autoclave-based cure method. Though prepreg sheets are used for the VBO method, only vacuum pressure is applied. Thus, products can be fabricated without any size constraints, and the manufacturing cost is significantly reduced. However, low applied pressure leads to higher void content in VBO products than that in autoclave cured ones, degrading their quality. Several researchers investigated void emergence and fiber impregnation. Although important insights have been obtained in these works, no researches addressed the residual strain which also affects part quality. This study measured strain development during the VBO cure using fiber Bragg grating (FBG) sensors which is one of the optical fiber sensors. Specimens were fabricated with two cure processes: one was the manufacturer’s recommended cure cycle (MRCC) and the other was a modified cure cycle that was proposed in this study to reduce the residual strain. Cross sectional observation was also conducted to evaluate the void content. The results revealed that the residual strain is 1300 με smaller in the modified cure than the MRCC even though the void contents are the same. It implies that the internal strain state should be considered in addition to the void content to achieve high quality parts. Then, a tensile test was performed to evaluate the effect of the residual strain on the strength. Cross ply specimens fabricated with the two cure cycles were tested. The modified cured specimen showed 450 με higher strength than the MRCC cured specimen, showing the effectiveness of the modified cure cycle.

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