Effect of Material Property and Hole Cold Expansion Rate on Fatigue Crack Initiation and Propagation on 2024-T351 Aluminum Alloy Holes

Aluminum alloys exhibit excellent strength to weight ratio and other useful mechanical properties desirable in aircraft industries. It is a common practice to use riveted holes to join these parts together. Cold expansion is a highly popular technique of extending fatigue crack initiation and propagation life of these holes. Earlier, various studies on fatigue crack growth and life estimation of aluminum alloys were conducted. Studies included the variation of crack growth life with changes in stress ratio, changes of fatigue life in comparison with plain holes with cold expanded holes, environmental effects, low and high cycle fatigue behavior, etc. Some of the studies were also conducted on pre-cracked cases along with cases considered for short edge margin holes. But in this parametric study, a numerical investigation is carried out to find the variation of fatigue crack initiation and propagation life in variation with material property specifically of different yield strengths with varying the rate of hole cold expansion. Here, 2024-T351 aluminum alloy is chosen as the test material. The design of the numerical setup consists of nine different cases by varying three different yield strengths from different manufacturing lots along with three different rates of cold expansion. For the combinations of highest yield strength and the highest percentage of cold expansion, fatigue crack initiation delays and at the same time crack growth rate delays significantly. For the combinations of lowest yield strength and the lowest percentage of cold expansion, both crack initiation life and crack growth life become shorter compared with highest ones. The maximum strain appears at the mandrel entry side of the hole when the model geometry is pulled with tensile load. From this strain pattern, 0.2mm entry hole corner type crack is defined as the initial crack size. When comparing with the baseline model, it is found that the fatigue lifespan for baseline is significantly shorter in terms of maximum allowable crack size with applied cyclic stress range between 206 MPa to 10.3 MPa. The baseline model fails only with a through-thickness crack size of 0.7mm while for the cases of parametric study, model even with the lowest yield strength and lowest cold expansion percentage, doesn’t fail below a crack size 3mm. It is observed that the variation of material yield strength can contribute in a notable manner when accompanied with different rate of hole cold expansion. This study can be useful to applications where variation of material strength exists with a lot to lot and at the same time improved fatigue crack growth rate prediction is desired from cold expanded holes.