Stress intensity factors for mixed-mode crack growth in imitation models under biaxial loading

Abstract

In this paper, a procedure to calculate stress intensity factors for imitation models of titanium alloys is proposed. Fatigue cracks are detected in a disk and blade “dovetail type” attachment in service. Based on the attachment dimensions and taking into account the biaxial loading conditions of the rotating compressor disk, two imitation model geometries of gas turbine engine compressor disks are developed. To accurately verify the biaxial loading conditions, the first imitation model of constant thickness is used. In order to completely reproduce the geometry of the compressor disk and the conditions of mixed mode crack growth, the second imitation model with a reduced cross section is proposed. Fatigue crack growth experiments of the imitation models are carried out at room temperature on a biaxial testing machine. Two different stress ratio values are applied several times to each imitation model in order to establish the experimental positions of the crack fronts. The elastic and plastic stress intensity factors used to represent the experimental results are computed using full-size 3D finite element analyses of the imitation models with surface quarter elliptical and throughthickness cracks. The use of the plastic stress intensity factor as a unified parameter for assessing the fracture resistance of materials and structures is supported. The advantages of using the computational and experimental results of imitation model II for verification and development of modern crack growth rates and lifetime prediction models are stated.