Deformation and Failure Characteristics of Center-Notched Unidirectional Boron/Aluminum at Room and Elevated Temperatures

Abstract

The global and local deformation characteristics of center-notched unidirectional B/Al composites were examined both in the elastic and inelastic regions of the load-displacement curves. The global deformation was measured at room temperature by applying the conventional compliance gage, while the local deformation was measured by means of the interferometric displacement gage (IDG) technique at room and elevated temperatures. The effects of notch length and test temperature on the deformation characteristics and on damage initiation and progression were determined. The local compliance calibration curves, obtained with the IDG technique, were found to be highly sensitive to notch length. With increasing load, significant nonlinearity in the load-crack-opening displacement (COD) and jumps in COD have been observed, resulting from the crack-tip damage progression in the form of fiber breakage, matrix inelastic (plastic) shear deformation, and matrix cracking. The results obtained with the IDG technique were qualitatively correlated with high magnification (× 150) visual observations in rela-time via a closed circuit television. The IDG technique could be easily applied at elevated temperatures. Results from the IDG indicate that significant changes in the deformation characteristics occur at temperatures above 204°C (400°F). The experimental global and local compliance calibration curves were compared with predictions and an excellent agreement has been established. The predictions of COD were made by applying an existing analytical model which assumes the existence of a zone of longitudinal inelastic shear deformation emanating from the notch-tip. Good agreement between prediction and experiment could be established. The comparison provided a simple procedure to evaluate the in situ matrix shear yield stress. The experimental results indicate that the analytical modeling for the inelastic deformation of the subject material should incorporate both the matrix strain hardening and the mechanism of sequential failure.