Damage Tolerance Analysis of a Helicopter Component

Fatigue critical helicopter components are, in general, subjected to complex high frequency dynamic loading. Due to these high frequencies small flaws growing at relatively slow rates can propagate to failure in a short period of time. Consequently, the demonstration of damage tolerance for those components must include the analysis of near-threshold crack propagation, i.e. growth in the low-to-mid stress intensity factor range (ΔK) regime. This paper presents a fatigue crack growth analysis of a helicopter airframe component, which was used as part of a round-robin study into helicopter fatigue, using a non-similitude based crack growth law, termed the ‘generalised Frost-Dugdale law’. The component geometry, material data and complex loading spectrum were distributed to the helicopter structural analysts worldwide with the objective of assessing the capability of current structural analytical tools to predict fatigue crack growth and thus incorporate damage tolerance into helicopter design. The generalised Frost-Dugdale model utilised companion fatigue coupon data tested under a similar spectrum to derive the necessary two input parameters, which along with a finite element computed width correction factor, lead to good agreement between the prediction and the component’s experimentally derived crack growth. The example presented in this paper demonstrates how the helicopter industry (amongst others) can accurately apply damage tolerance design principles both in the initial design as well as to ensure continued airworthiness