EVALUATION OF SMALL CRACK GROWTH RATE OF ALUMINUM ALLOYS UNDER DIFFERENT STRESS GRADIENTS

The conventional method for the evaluation of small crack growth rate in both anannealed and an age-hardened aluminum alloy is investigated under a high nominal stress level.The small crack growth rate can be expressed as de / dN = Cσⁿe (σ:the nominal stressamplitude, e: crack length) for different types of loading and materials, the constants C and n differ in these conditions. However, the crack growth rate in both cases can generally beexpressed as de / dN=C¹ε_ρⁿ¹e( ε_ρ: the plastic strain amplitude), where C' and n' are constant inboth types of loading. Consequently, fatigue lives in both mpes of loading can be predicted usingthe former expression, and the cyclic stress-strain curve in each type of loading.