Technical report | Guide on the Effective Block Approach for the Fatigue Life Assessment of Metallic Structures

Abstract 

Simpler empirical characterisation of the mechanics of metal fatigue crack growth can offer an alternative to physics-based models, where the latter can be challenged by complex material behaviour of commercial metals when loaded by variable amplitude spectra. Such characterisation is anticipated to be more appropriate for cases where the loading consists of repeating blocks of spectra or other scenarios that result in near steady-state crack growth rates. This report examines the many types of characteristic block approaches available, and recommends a particular type that is considered suitable for fatigue life assessments, herein titled the Effective Block Approach, or EBA. Guidance is given on methods and implementation, and several case studies are reviewed where the EBA has been used to assess the life of RAAF airframes. In addition to computational efficiency, significant accuracy gains were achieved when the approach was underpinned by representative coupon tests. This capability is anticipated to give the RAAF more options to better assess and manage the safety, availability and cost of ownership of its air vehicles.

Executive Summary

Most RAAF aircraft are life-limited due to the metal fatigue failure mode. Predictive modelling is necessary for effective structural integrity management of these life limits. Mechanistic type approaches that attempt to model the physical and chemical behaviour of the metal fatigue mechanism can be difficult to develop to a sufficient level of reliability in a given timeframe. Variable amplitude loadings that cause retardation/acceleration effects are particularly difficult to simulate. This can significantly restrict the RAAF's options available for managing the structural integrity of its air vehicles. However, if the variable amplitude loading consists of regularly repeating blocks, or otherwise is considered to produce steady-state crack growth rates, then a characteristic block type modelling approach may offer an alternate path for conducting life assessments. This is anticipated to be particularly useful for full-scale test interpretation where the applied loading usually consists of repeating spectrum blocks.

Characteristic block type approaches consider the average crack growth rates for a spectrum block, and characterise these against the predominant influential parameters (e.g., crack size, stress or stress intensity factor). This report examines the many types of characteristic block approaches described in the literature, and highlights that the assumptions and efficacy of each can vary significantly. Guidance is given on distinguishing amongst them, and a recommendation is given for a particular type that is considered suitable for conducting fatigue life assessments, herein titled the Effective Block Approach, or EBA. This guide provides steps for using an EBA; considers the benefits and limitations; and reviews a number of case studies. It was found that, when the crack growth rates are underpinned by representative fatigue tests, an EBA can provide significant accuracy, efficiency and insights toward prediction of metal fatigue crack growth.

DSTO has significant full-scale, component and coupon test facilities that it uses to assess RAAF airframes. The efficient and accurate modelling of crack growth is a necessary capability for translating the test outputs into relevant life results for structural integrity management purposes. The EBA enables DSTO to continue to offer leading structural integrity advice, even when mechanics-based models are still being developed. Already, the EBA has been used to assess numerous airframes including Macchi wings, F/A-18 Hornet centre barrels and F-111 wings. This capability is anticipated to give the RAAF additional options to better assess and manage the safety, availability and cost of ownership of its air vehicles.