Growth Characteristics of a Fatigue Crack Approaching and Growing Beneath an Adhesively Bonded Doubler

Abstract

The title problem was investigated experimentally and numerically. Aluminum adherends and two adhesives, AF-55 and AF-127, were utilized in the investigation. Each specimen had a crack introduced at an undoubled edge. Fatigue cycles with a fixed average remote-stress maximum of 103.4 MPa at a stress ratio of 0.1 were applied to grow the initial crack up to and beneath the doubler. Crack-growth rates were measured to assess the inhibiting effect of the doubler, and ultrasonic scans were made to determine debond-zone shapes. After each crack had grown approximately halfway beneath the doubler, a secondary crack initiated at the edge of the doubler, precipitating rupture of the specimen. A finite-element model of the specimen employed a singularity element to represent the crack-tip neighborhood and shear springs to represent the adhesive layer. Springs were released in the model to simulate local debonding. The stress-intensity factor was computed as a function of crack length for linear and nonlinear representations of the two adhesives used in the experimental program.