FRACTOGRAPHIC ANALYSES OF FATIGUE CRACK GROWTH IN D16T ALLOY SUBJECTED TO BIAXIAL CYCLIC LOADS AT VARIOUS R‐RATIOS

Abstract

Abstract Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of D16T aluminium alloy under biaxial tension and tension‐compression. In the range of the biaxial load ratios λ from ‐ 1 to +1.5, and in the range of R‐ratios 0.05 to 0.8, fatigue striation formation took place over a crack growth rate near to 4×10−8 m/cycle. The striation spacing and the crack growth rate decrease as the ratios λ and R increase. The ratio between the increment of crack growth, da/dN, and the striation spacing, δ, is approximately 1:1 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 2.14×10−7 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 and R‐ratios. The value of Nd increases as the stress ratio, λ, is increased.Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method involving an equivalent stress intensity factor Ke, =K1,F(λ, Rs). The values of F(λ, R) for the growth rate (F(λ, R)s) and for the striation spacing (F(λ, Rs) were determined and compared.The fatigue crack growth period, Ntp, applicable to the stage of fatigue striation formation, (predicted by using both of the F(λ, R) values) is correlated with the experimental data and the error is of the order of 15%.