Experimental and numerical study of elastic-plastic mixed-mode fracture

The elastic-plastic fracture behavior of stable crack initiation, growth, and direction for 2024-T3 and 2XXX aluminum sheet alloys under influence of mixed-mode I/II loading has been experimentally and numerically investigated. The experimental study was done using 100 mm wide tensile dogbone type specimens with a centrally located sharp fatigue crack slanted at an angle. By varying the crack orientation angle distinct levels of mixed-mode were considered and investigated. The influence of specimen thickness and the 2a/W ratio were also studied for {dollar}beta{dollar} at 45 degrees. The Geometric Moire method and close-up video imaging were employed on all testing. From the Geometric Moire method, full field deformation information in terms of displacement around crack tip were obtained. With close-up video image process, the crack tip opening displacement (CTOD), crack tip opening angle (CTOA), and crack extension angle were obtained during the crack initiation and the extension.; A two dimensional finite element analysis, using ABAQUS, was used to obtain fracture parameters. Two different types of finite element models were developed. One model (global model) represents the full specimen with the same geometry as the one used in the experiment. The second model (local model) represents a local area, 30 mm by 25 mm, around the crack tip. The global model is used for crack initiation study and the local model for crack extension. Load at initiation was used as an input for the global model whereas the u and v displacements from geometric moire fringes were used for the local model.; The experimental results show that the fracture parameter CTOA value starts high at early crack extension and gradually approaches to a constant value at longer crack extension. The constant value of CTOA (measured at the distance, equal to half of the thickness of the fracture specimen, behind the crack tip) is 6.5{dollar}spcirc{lcub}-{rcub}7spcirc{dollar} for 2024-T3 and 8{dollar}spcirc{dollar} for 2XXX alloy. The constant value of CTOA was shown to be independent of the initial crack orientation angle at all mixed-mode levels of loading. However, mode-mixity influence the CTOA values at the initial crack extension. In addition, a higher mode II component was found to reduce CTOA values at initial crack extension. Both experimental and numerical results show that the initiation of crack extension under mixed-mode loading conditions occurs at lower load when compared with the load required for mode I initiation. Also, the measured and computed CTOA from the hybrid experimental-numerical model (local model) agreed well during crack propagation and the CTOA magnitude was found to be constant.; Thickness was found to influence the CTOA values both at early crack extensions and during the stable crack growth. Specimens with higher thickness indicated lower fracture toughness and lower CTOA. The projected crack length, 2c/W, did not influence the CTOA during the stable crack growth.; All the experimental results indicated the higher fracture toughness of 2XXX aluminum sheet as compared to 2024-T3 material. The constant CTOA value for 2XXX was shown to be 25% higher than that of 2024-T3.