Comparison of Short and Long Fatigue Crack Growth in 7075‐t6 Aluminum

Abstract

The fatigue crack growth rates of physically-short cracks (0.5 < a < 1 .O mm), intermediate cracks (1 < a < 2 mm) and long cracks (7 < a < 25 mm) were compared using SEN type tensile specimens in 7075-T6 aluminum alloy with load ratios, R, of 0.05, -1 and 0.5 under constant amplitude testing at room temperature. It was found that the short cracks grew much faster than long cracks based on applied AK with da/dN < m/cycle. Even the intermediate cracks grew faster than the long cracks below 10-7m/cycle. The transition crack lengths where similitude with A K existed was between 1 and 2 mm. Mean stress effects were similar for R = 0.05 and -1, but R = 0.5 caused increased crack growth rates. The above differences are partially attributed to crack closure effects. Based upon plastic zone sizes, LEFM was justifiable with all the experiments.

Physically short crack propagation in metals during high cycle fatigue

“…This other short crack series was drawn from the paper by Bu and Stephens, Ref. [17]. The present model reproduces a slower propagation rate, especially near the threshold, in agreement with the experimental results.…”

“…However, data reported in Ref. [17] was not completely coherent with the short crack model here obtained, because PSC higher propagation rate than LC was extended to very large crack size. Fig.5(c) shows short crack propagation data for Ti-6Al-4V (R = −1).…”

“…Eqs.1 and 2 agree in terms of asymptotes: they give rise to the same threshold and the same crack propagation rate at high driving force ∆ K ≫ ∆ K th but in the intermediate region they differ significantly, since [17,18]. (c) Ti-6Al-4V (R = −1), Ref.…”

“…In particular, short crack data reported in Ref. [17], showed a higher rate than long cracks, even at quite large crack size, in the order of millimeters. As discussed above, Eq.11 allows for PSC higher propagation rate than LC even for a crack larger than d 2 .…”

“…Eq.2 was fitted to the data, drawn from the literature, for the materials mentioned above, and it is reported in Fig.1 where it is clearly effective in describing the near threshold propagation (Eq.2 is termed as "Klesnil-Lukas" in Fig.1 [16] 7075-T6 -1 4.0 3.14 1.1 × 10 −11 [17,18] Ti-6Al-4V -1 5.6 4.26 6.7 × 10 −14 [19] Ti-6Al-4V 0.1 4.2 4.05 9.0 × 10 −13 [20] Table 2 Eq.2 fitting parameters for considered materials.…”