Abstract:

The fatigue crack growth threshold defines the stress intensity level, ΔK, below which a crack will not propagate. Recent research has shown some of the threshold data generated based on ISO and ASTM testing standards has some limitations, influencing the data in unforeseen ways. One of these limitations refers to the steady-state condition of threshold value. In the present study in view of the limitation mentioned an attempt has been made to determine the fatigue threshold value by regression analysis. In the present study Al7075 alloy is subjected to heat treatment such as annealing and cryogenic treatment and tested for fatigue crack growth (FCG) as per ASTM E-647 standards.Servohydraulic controlled fatigue crack growth testing machine was used for load ratio of 0.1 and frequency of 30Hz at constant amplitude cycle. The experimental results show an improvement in crack resistance for cryotreated Al7075 alloy. Annealed Al7075 showed improved stiffness compared to cryo-treated specimen. Microstructure analysis revealed changes in microstructure of specimens, which influence the mechanical characteristics of the material.

Key Words: ISO, ASTM, cryogenic treatment, annealing, fatigue crack growth (FCG).