Interlaminar fracture is a primary damage mode in laminated composites and. may occur due to interlaminar defects induced by improper use, faulty maintenance, and foreign object impact damage. In engineering situations, the majority of interlaminar fracture in service may be developed under mixed mode deformations. It is necessary to evaluate mixed mode interlaminar fracture toughness in laminated composites since it gives information on estimating the resistance to interlaminar fracture under mixed mode deformations.Although several types of specimens for the mixed mode interlaminar fracture test have been suggested in the literature [1][2][3][4][5][6], each method has limitations in determining mixed mode interlaminar fracture toughness in laminated composites. The CLS (cracked lap shear ) specimen has been adopted for the mixed mode interlaminar fracture test by Wilkins et al. [1]. They evaluated total energy release rate by the critical load at fracture and the specimen compliance. Finite element analysis was employed to decompose the mode I and mode II component of the CLS specimen. The mode I contribution to the CLS specimen was 25 percent of the total energy release rate. Different mixed mode ratios may be obtained by changing the thickness ratios of the shear ply and strap ply. However, a wide range of mixed mode ratios can not be obtained from the CLS specimen. The antisymmetric test fixture with the composite fracture specimen has been used to investigate the interlaminar fracture behavior under mixed mode deformations by Hong and Yoon [2]. The composite fracture specimen is bonded to the antisymmetric test fixture and may be removed after testing so that the antisymmetric test fixture is reusable. While this method obtains a wide range of mixed mode ratios by varying the applied loading angle and reveals unstable crack growth consistently under mixed mode deformation, it is difficult to achieve a natural starter crack due to the large thickness of the composite fracture specimen. The MMF (mixed mode flexure) specimen has been used for mixed mode interlaminar fracture tests by Russell and Street [4]. The geometry of the MMF specimen is similar to the ENF (end notched flexure) specimen for the mode II test except that the load is applied to the upper arm at the delamination end. Russell and Street obtained the contribution of the mode I component to total energy release rate by the linear beam theory. While this type of specimen may be applicable to obtain different mixed mode ratios by changing the thickness ratios of the upper and lower part on the delamination plane, it is an inconvenient method for the evaluation of mixed mode interlaminar fracture Int Journ of Fracture 43 (1990)
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