Fatigue Crack Propagation Of A7N01Aluminum Alloy Under Mixed-mode Loading And Its Numerical Simulation

Fatigue is an important failure form for the load-carrying structures. The majoritystudies of fatigue crack growth are usually performed under mode I loading condition.However, in many practical cases, cracks are not always normal to the maximumprincipal stress, and a mixed mode (combined modes I and II) condition prevails at thetip of such cracks. For mixed mode loading, crack growth behavior is more complex.In the dissertation, the method combined with the experiment and theory predictionis adopted. The experiments of a fatigue crack under mixed mode loading are carriedout with A7N01P-T4aluminum alloy. The compact tension shear (CTS) specimens anda mixed mode loading device are used. The effects of loading angle on crack growthrate and crack growth angle are analyzed. Fatigue propagation accumulation damagenear the crack tip is well evaluated by the using of Jiang-Sehitoglu cyclic plasticitymodel and Jiang multi-axial fatigue damage criterion.Non-welded specimens are tested under mixed mode loading condition. In order toobtain stress intensity factors both KIand KII, a numerical analysis is performed. Thegrowth direction of the cracks for different loading angles follows the maximum stresscriterion, and keeps unchanged during the test. The SIF KIincreases with the crackpropagation, and the changing of crack plane doesn’t influence its value. Once the crackgrowth direction changes, KIImeets a decrease and keeps far less than KI. Equivalentstress intensity factor Kvproposed by Tanaka is employed. The crack growth rate is inaccordance with the Paris’ law.Mixed mode loading tests for welded specimens demonstrates that, the interfacebetween the weld metal and based metal can affect the crack propagation path. Cracktends to extend along the interface at low loading angle, while at high loading angle, itis always propagating along its original direction. However, lack of fusion deterioratesthe strength of the interface, which makes the crack go through the interface even athigh loading angle. The Paris’ law is inadequate to describe the crack growth rate forwelded specimen in this case, especially when the crack propagates across the interface.Material constants in Jiang-Sehitoglu cyclic plasticity and Jiang multi-axial fatiguemodel are established by a monotonic tension experiment. Based on implicit stressintegration algorithm, the constitutive model is implemented into finite elementprogram MSC.MARC by user-subroutine Hypela2.f. The numerical scheme wasverified with simple boundary condition. It is showed that, there is good agreementbetween the simulated data and experimental results.Combined Jiang-Sehitoglu cyclic plasticity model with Jiang multi-axial fatigue damage criterion, the damage near the crack tip is obtained. Crack growth direction isdetermined by comparing the damage on different material planes, and the results fitbetter with experiments than the maximum stress criterion. The crack growth rate isevaluated by integrating the damage in a specific direction near the crack tip, which is inline with the experimental data.