Fracture mechanics in aircraft failure analysis: Uses and limitations

Fracture mechanics is generally accepted as essential to the design of metallic aircraft structures, notably for predicting and validating the fatigue crack growth and residual strength properties. Applications of Linear Elastic Fracture Mechanics (LEFM) began in the early 1970s, and would be expected by now to have reached a mature status. Hence one might also expect that LEFM methods can often be used for analysing service fatigue failures. However, the DSTO’s and NLR’s experience is that only non-LEFM analyses are directly useful for determining service failure fatigue crack growth (FCG) behaviour. LEFM analyses can assist in analysing FCG in full-scale, component and specimen tests, and these analyses can be used – within limits – to predict some aspects of the FCG behaviour of in-service structures and components. The DSTO has developed a hybrid method, the Effective Block Approach (EBA) using both LEFM and non-LEFM analyses, from full-scale testing. The EBA enables a wide range of FCG predictions for high performance aircraft.

► LEFM and non-LEFM fatigue crack growth analysis methods.
► Quantitative Fractography (QF) essential.
► Only non-LEFM analyses (with QF) directly applicable to service failures.
► LEFM based analyses unreliable without calibration by QF data.