Tech Briefs

Strains have been found to be substantially proportional to crack lengths.

An experimental study of the mechanical behavior of cracked aluminum specimens repaired with composite-material patches has yielded findings that may eventually contribute to the development of more- effective composite patches and of techniques for predicting and detecting failures in composite-patched aluminum aircraft panels. Some prior studies have addressed various aspects of composite-patch repair of aluminum specimens, but until now, little attention has been given to such important aspects of mechanical behavior and properties as the relationships among stress, strain, and growth of cracks through-out the lifetimes of specimens. In this study, effects of initiation and growth of cracks on the residual strengths of the patched specimens were characterized. This study established a correlation among damage modes, residual strengths, and evolution of strain inside and outside the patched areas.

Aluminum Dog-Bone Specimens containing deliberately introduced defects were repaired with composite patches and then subjected to tensile tests.
In the experiments, tensile tests were performed on six 7075-T6 aluminum dog-bone specimens. A hole and widthwise notches were machined at the middle of each specimen, then widthwise starter cracks were grown from the notches (see figure). A rectangular patch made of a 16-ply composite of unidirectional (lengthwise) boron fibers in an epoxy matrix was centered at the hole and bonded to the aluminum by use of an adhesive film comprising a toughened adhesive in a polyester knit cloth carrier. Each specimen was instrumented with strain gauges at several locations inside and outside the patch area.