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Effect of lay-up orientation on fracture toughness of CFRP laminates

Yavuz, Burak Ogun
Çöker, Demirkan
Parnas, Kemal Levend
Delamination in advanced composites, which causes reduction in the laminate strength, has been a continuing problem. Since it is relatively easier to manufacture, the general practice for the fracture toughness is to conduct tests with 0//0 interface and use these experimental values in the analyses. However, the fracture toughness values are known to vary for different angle interfaces. This happens to be a significant issue in the case of impact. When interlaminar properties of only 0//0 interface is used in all interfaces of the cohesive zone analyses, results generally do not match with experiment ones. In order to improve these analyses, a meaningful array of interface properties for experimental fracture toughness values must be known. For example, a delamination in a 0//0 interface can propagate in any direction. Moreover, delaminations are not in 0//0 interface. In general; it can be in any interface including 0//90, 0//45 etc. There is program created for determining the interlaminar properties for delamination of different interfaces by utilizing double cantilever beam (DCB), end-notched flexure (ENF) and curved beam strength (CBS) experiments. In this part of the program, Mode I fracture toughness values of 0//0, 45//45 interfaces are considered in the experimental program. Mode I experiment is conducted according to ASTM D5528 (Ref. 2). The program will continue with Mode I and Mode II fracture toughness values, and interlaminar strength values of 0//0, 25//25, 45//45, 65//65, 90//90, 25//-25, 45//-45, 65//-65 interfaces. These results provide a practical material tool covering various range of interfaces of fracture toughness for certain CFRP and GFRP unidirectional laminates. It will hopefully decrease the problems in computational studies which generally use only 0//0 interface properties for all interfaces.