An experimental investigation on the effect of load ratio and temperature on fatigue crack growth of AA7075-T651

Abstract

This study presents the fracture mechanics based synergistic effect of load ratio (R = 0.1 to 0.7) and temperature (25 to 200°C) on fatigue crack growth in aerospace grade AA7075-T651. Quarter compact tension specimens under constant amplitude fatigue loading revealed that fatigue life decreased significantly (by 65%) for a temperature increase from 25 to 200°C at R = 0.1. However, increasing R from 0.1 to 0.7 at 200°C extended life several fold (26 times). A transition in the crack growth mechanism was observed from ΔK dominated behavior at ambient conditions to Kmax controlled propagation due to synergistic effect of elevated temperatures and high R values. Succinct and close-spaced striations were observed on fractographic examination of fractured surfaces for specimens tested at 25°C. However, the striations were blurred and widely spaced (a 25-fold increase in striation spacing compared to ambient temperature) for specimens tested at elevated temperatures. While ΔK was observed as the primary crack driving force for combinations of room temperature and low values of R, however, ΔKeff and Kmax dominated the crack growth behavior for high R and elevated temperatures. The use of a single-parameter model accurately predicted crack growth rates at low R (R = 0.5) across all temperature values used in this investigation. The two-parameter model improved the correlation of high R Paris-regime crack-growth, while deviations observed in the near-threshold region. This modeling approach provides with quantitative framework for predicting da/dN under complex thermomechanical conditions.