The equivalence of EPS and EIFS based on the same crack growth life data

• Uses the Characteristic K approach to analyses a comprehensive set of AA7050-T7451 crack growth data.
• The crack growth rate was found to be a power law function with an exponent of approximately 2.
• The Effective Initial Flaw Size (EIFS) and the Equivalent Pre-crack Size (EPS) were considered.
• The two initiating discontinuity sizes descriptions were shown to be approximately the same.

This paper reviews variable amplitude (VA) fatigue crack growth rate descriptions and then reports on methods for establishing starting crack sizes in damage tolerance and durability analyses. The growth rate data were generated using 68 low stress concentration (Kt) test coupons made from 7050-T7451 aluminium alloy and subjected to five fighter aircraft VA stress histories (each stress history had either two or four stress scaling factors). The experimental da/dt data are described using a simple power law relationship where da/dt = C · Krefm. Four separate approaches for fitting the da/dt data to determine the derived constants C and m are evaluated. The fitted power law exponent (m) values for the individual spectrum da/dt behaviours are found to be slightly greater than 2.0.The paper summarizes the equivalent initial flaw size (EIFS) results (based on the fits from four da/dt fitting approaches) and compares the EIFS values to the corresponding equivalent pre-crack size (EPS) values based on a back extrapolation of the ln(a) vs. Flight Hour fit to 0.0 flight hours. It is noted that the EIFS and EPS values compare extremely well.