Design and analysis of the crack rail shear specimen for mode III interlaminar fracture

Abstract

The crack rail shear (CRS) specimen is a proposed test method to characterize the Mode III interlaminar fracture toughness of continuous-fiber-reinforced composite materials. The specimen utilizes the two-rail shear test fixture, and contains embedded Kapton film placed symmetrically about the midplane to provide starter cracks for subsequent characterization. Otherwise, specimen length and width are identical to the ASTM shear test specimen geometry. An analytical expression for the strain energy release rate is developed based on a strength of materials approach. The model illustrates the important material and geometric parameters of the test, and provides a simple data reduction scheme for experiments. A quasi three-dimensional, linear elastic finite element code, CCMQ3D, is employed to verify the pure Mode III fracture state and to determine admissible crack lengths. Deformation of the model shows that only the out-of-plane displacement is non-zero, indicating that a pure Mode III fracture state does indeed exist within the constraints of the Q3D assumption. Compliance and strain energy release rate predictions are in good agreement with the strength of materials model over the range of crack lengths, 0·15< a/ w<0·85. A fully three-dimensional, linear elastic finite element analysis of the CRS is employed to quantify the effect of finite length on the fracture state. Only intermediate crack lengths are investigated. Crack closure techniques are utilized to determine the components of the strain energy release rate present. Results indicate that a small boundary layer of mixed mode behavior exists at the free edges that diminishes to a pure Mode III fracture state. Compliance and strain energy release rate predictions by the 3D model show good agreement with the Q3D and strength of materials models.