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Improvement of Bending Strength of Carbon Fiber/Thermoplastic Epoxy Composites
—Effects of Molecular Weight of Epoxy on Carbon Fiber/Matrix Interfacial Strength and Connection of Cracks in Matrix

DOI: 10.4236/ojcm.2017.74014, PP. 207-217

Keywords: Thermoplastic Epoxy Composites, Weight-Average Molecular Weight, Interfacial Shear Strength, Crack Propagation, Crack Connecting Probability

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Abstract:

The bending strength of carbon?fiber/thermoplastic epoxy composites?(CF/TP-EP Compo.)?had?bi-linear increasewith increase of weight-average molecular weight (Mw) of matrix. The transition in the bending strength appeared at around 55k of Mw (k?means 103). SEM observation of fractured surface of CF/TP-EP Compo. showed that the fracture mode changed from interfacial failure to fiber breakage dominated failure. The smooth surface of carbon fibers appeared at lower Mw than 55k while some resin remained on the fibers indicating good adhesion between carbon fiber and matrix at higher Mw than 55k. The interfacial shear strength between carbon fiber and matrix bi-linearly increased with an increase of Mw similarly to the bending strength of the composite, measured by the micro droplet test. The dynamic loss tanδ?of the matrix measured at 2Hz also showed a bi-linear relationship with respect to Mw having a knee point at Mw = 55k. The connection probability of two cracks introduced oneach side of specimens also confirmed that the interfacial strength between carbon fiber and matrix is the key for the mechanical performance of CF/TP-EP Compo. in bending.

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