Study of Fatigue Crack Growth in Al-Mg-Si Alloys Using a Predictive Model under Positive and Negative Load Ratios

Abstract

The study of fatigue crack growth (FCG) is aimed at residual life estimations in order to apply the damage tolerant criterion. Usual approaches are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics,ΔK, as the governing driving force for crack propagation. An alternative approach is the use of predictive theoretical schemes arising from damage mechanics. Although they havent achieved a reliability level high enough to be used in design, predictive models may be important in some situations like material selection. In the present work, a predictive FCG method based on the cumulative damage of volume elements along the crack path is employed. The development of the work includes considerations about the stress distribution in the cracked body and the stress-life and strain-life relations used in the computational procedure. A previously developed analytical expression for the stress distribution ahead of the crack in a finite width plate, based on the numerical analysis performed by the Finite Element Method, is used in the predictive method. The stress field is determined for both upper and lower limits of cyclic loadings. The fatigue crack growth behavior of three Al-Mg-Si alloys: AA 6005, AA 6351 and AA 6063, tempered and aged for the T6 condition, was analysed for positive and negative R-ratios. In order to check the model results, constant amplitude FCG tests with load ratios ±0,5 were carried out in M(T) specimens. The experimental results, compared to the computational simulations, show that it is possible to obtain predictions of FCG behaviour for both positive and negative load ratios.

Fatigue Crack Growth in an Al-Mg-Si Alloy under Negative Load Ratio

“…In order to assess the stress distribution ahead of the crack, analytical equations specific to specimens with central cracks were proposed from numerical results obtained by the Finite Element Method for cyclic loading in a previous work [9] and successfully tested for R < 0 for AA6005 alloy in [10]. The proposed method allowed one to estimate the effect of normalized crack length and the load ratio R on the effective stress amplitude ahead of the crack, and indicate a dependence of the cyclic plastic zone size on R. Fig.…”

On the fatigue crack growth behaviour of selective laser melting fabricated Inconel 625: Effects of build orientation and stress ratio

“…The stacking direction of the SLM process was along Z‐axis. The FCG data that performed at stress ratios of −0.5, 0.1 and 0.5 were collected to check the scope of the model for negative stress ratio conditions. In Figures A and B, the FCG rate versus Δ K and Δ κ # of the above materials is separately provided.…”

“…The FCG rate points of 7075‐T7451 aluminium alloy (SENT specimens), MIM 316 L stainless steel (SENB specimens), SLM Ti6Al4V alloy (CT specimens) and Al‐mg‐Si alloy (M(T) specimen) were associated with (a) SIF range, Δ K and (B) effective driving force factor range, Δ κ # [Colour figure can be viewed at wileyonlinelibrary.com]…”