USAF Characteristic K Approach: A Robust Tool for Predicting Fatigue Crack Growth under Various Underload Spectra

Area of Science:

• Materials Science
• Mechanical Engineering
• Fracture Mechanics

Background:

• Evaluating component durability for limited life replacement requires accurate fracture mechanics tools.
• Understanding the impact of underloads on crack propagation is crucial for predicting component lifespan.

Purpose of the Study:

• To assess the predictive accuracy of the USAF 'Characteristic K' method combined with the Hartman-Schijve adaptation for small crack growth under various underload spectra.
• To validate this combined approach using experimental data for aluminum alloy AA7050-T7451.

Main Methods:

• Utilized the USAF 'Characteristic K' method and the Hartman-Schijve adaptation of the NASGRO crack growth formula.
• Analyzed published da/dN versus ΔK small crack growth data for five distinct underload spectra.
• Compared predicted crack growth curves with experimentally measured curves.

Main Results:

• The combined USAF 'Characteristic K' and Hartman-Schijve method demonstrated reasonable accuracy in predicting the impact of underloads on small crack propagation.
• Good agreement was observed between predicted and measured crack growth curves across all five examined underload spectra.
• This study is the first to highlight the efficacy of this specific combination for predicting small crack growth under diverse underload conditions.

Conclusions:

• The USAF 'Characteristic K' approach, when integrated with the Hartman-Schijve equation, provides a reliable tool for predicting small crack growth under various underload conditions.
• This predictive capability is valuable for assessing the durability and optimizing the design of components intended for limited life replacement.
• Further research can build upon these findings to refine predictive models in linear elastic fracture mechanics.