Crack growth measurements in boron/aluminum composite laminates and related analysis

This study was an evaluation of the fracture of boron/aluminum composite laminates. Evaluation was carried out in terms of K_max, R-curves and J-integral.

The boron/aluminum laminates used were 0°/90°/0°/90°/0°/90°/0°/90°/0°/90°/0°/90°/0° and 0°/90°/0°/-45°/+45°/90°/0°. The boron fibers were 0.0056 inches in diameter and 6061 aluminum alloy was used for the matrix. The material was fabricated using diffusion bonding and was approximately 0.1 inches thick.

Four widths of specimens were utilized in the study. These were two, four and eight inch wide compact tension specimens and one wide center notched specimens. Specimens were machined using electric discharge machining.

Two direct and two indirect methods were used to measure crack growth in the specimens. The direct method utilized crack propagation gages and the optical measurement of the crack front from photographs. The indirect methods were based on the change in the potential measured across the plane of the crack and the compliance change of the specimens. Compliance measurements were obtained from load versus load-point displacements and load versus crack opening displacements.

The K_max values varied with specimen width. similar effect was noted in the results obtained from the J-integral analysis. The width effect in both analysis techniques was due to plastic deformation of the matrix material. The plastic deformation caused notch tip blunting to the extent that neither the K_max or J-integral were able to describe the failure of the materials. It was concluded that all the specimens failed by net section yielding with only a small stress concentration present.