FATIGUE CRACK GROWTH IN D16T A1‐ALLOY SHEET SUBJECTED TO BIAXIAL OUT‐OF‐PHASE LOADING

Abstract

Abstract— Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of D16T aluminium alloy under out‐of‐phase biaxial tension and tension‐compression. In the range of the biaxial load ratios λ from −0.5 to +0.5 and an R‐ratio of 0.3, fatigue striation formation took place beyond a crack growth rate near to 4 × 10−8 m/cycle. The striation spacing and the crack growth rate increase as the φ‐angle of the out‐of‐phase biaxial loads increases in the range of φ‐angles from 0° to 180°. The ratio between the increment of crack growth, da/dN, and the striation spacing, δ, is approximately 1 to 1 when da/dN is greater than 4 × 10−8 m/cycle. The relationship between the number of cycles from the beginning of a test up to the growth rate of 10−6 m/cycle (Nd), and the crack growth period, NP, from when the crack initiates up to the instant when that growth rate is reached, was determined for different λ ratios and φ angles. The value of Nd decreases as the φ angle is increased in the range from 0° to 1807deg;.Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method that involves an equivalent stress intensity factor Ke=KIF1(λ, R)F2(φ). The values of F2(φ) were determined.The calculated fatigue crack growth period, Nc, applicable up to and including the stage of fatigue striation formation (predicted by using both of the F1(λ, R) and F2(φ) functions) is correlated with the experimental data and the error is of the order of 15%.