Equivalent initial flaw size testing and analysis of transport aircraft skin splices

Abstract

The equivalent initial flaw size (EIFS) concept was developed nearly 30 years ago in an attempt to account for the initial quality, both manufacturing and material properties, of a structural detail prone to fatigue cracking. Widespread use of this concept has been limited due to the large amount of test data required to develop a reliable EIFS distribution. In this effort, an EIFS distribution was determined for four types of flat, production like transport aircraft fuselage skin joints loaded by remote tension. Two crack growth prediction codes, AFGROW and FASTRAN, were used to not only develop the EIFS but also to compare the crack growth algorithms in each code. The EIFS calculations are prone to compounding errors in the crack growth analysis due to the changing stress intensity factor solutions and stress fields as the crack gets longer. Thus, only including EIFS calculations for mechanically small cracks, crack lengths less than 1.27 mm, results in a mean EIFS of 18.0 μm with a standard deviation of 3.78 μm.

The EIFS distribution for anodized and pre‐corroded 7010‐T7651 under constant amplitude loading

“…The EIFS approach models a defect in a material as if it were a crack that, given the same loading conditions, would produce the same fatigue life 1,6,9,10,17 Figure 1. illustrates this concept.…”

“…However, this depends on accurate crack detection, which can be problematic 18 . Fawaz recently calculated an EIFS distribution for fuselage skin joints by fitting a crack growth rule to marker band observations and back projecting to zero cycles 6 . This method is quite accurate due to the large amount of data used to determine each EIFS value but it cannot be used for a generalized model.…”

“…The EIFS approach models a defect in a material as if it were a crack that, given the same loading conditions, would produce the same fatigue life. 1,6,9,10,17 Figure 1 illustrates this concept. Both the actual crack, which initiates from a corrosion pit, and its equivalent EIFS-initiated crack produce the same fatigue life.…”

“…The formulation of mechanistic models for predicting corrosion growth 4,5 The creation of models and method to predict and manage the impact of existing corrosion damage on structural integrity 6–11 Holistic models combining all of the above (e.g.…”

“…The creation of models and method to predict and manage the impact of existing corrosion damage on structural integrity 6–11 …”