Toughening and repairability of high-strength CFRP by interlayer resin structure control

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Interlaminar fracture toughness under mode I and II loadings was investigated for unidirectional CF/epoxy laminates with ionomer interleaf. Crack path of interlaminar fracture is often arrested within the interlayer resin rich region. In the present study, the toughened interlayer resin was expanded to the interlayer/base lamina interphase. Since the crack path was located inside the toughened region, excellent interlaminar properties are expected by this new toughening concept. Using the high bonding properties of ionomer, the repairability of delaminated composites was also tried. The results are summarized as follows :(1) Control of interlayer resin structureUnidirectional CF/epoxy laminates was molded with ionomer interleaf of the thickess of 12 to 200 mm. EPMA analysis revealed that interphases were formed between the ionomer layer and the base CF/epoxy laminas. These interphases contain a mixture of ionomer, epoxy and carbon fibers.(2)Interlaminar fracture toughnessThe fracture t … More oughness of ionomer interleaved CF/epoxy laminates was much higher than that of base CF/epoxy laminates both under mode I and II loadings. For mode I loading, the high level of the toughness was kept constant with the crack growth.(3)Ionomer thickness effectMode I interlaminar toughness initially increased with the increase of ionomer interleaf, and then leveled off. For mode II loading, the toughness continuously increased with the ionomer thickness, and reached 9 to 10 kJ/m^2, which is one of the highest among already reported results.(4)Mode I delamination fatigueHigh crack growth resistance was kept without respect to the increment of the crack length because the crack path was at the interface of the ionomer/interphase or inside the interphase region, arrested by fibers.(5)RepairabilityThe delaminated specimen was hot-pressed again, and the interlaminar toughness change was investigated. Although hot-pressing without additional inomer film gave poor results, the repair with ionomer film brought the toughness comparable to the virgin laminates. Less