Damage Tolerance of Titanium Alloy Rotorcraft Components: Advantages and Challenges

Abstract

A damage tolerance (DT) testing method is necessary to support DT design and certification and to simplify flight test monitoring of titanium yoke flexure components. A building block testing program was conducted. While the validity of the basic approach was confirmed by the testing results, a number of aspects have been revealed that are of primary importance to designing and certifying titanium yoke flexure components in particular, and any titanium rotorcraft component in general. In common practice, the through-crack data are used as material properties for crack growth analysis, although the majority of crack growth in rotorcraft components is observed to occur in part-through mode. The application of through-crack data to surface crack of a titanium part needs to be evaluated when taking the surface conditions into account. This is because the crack growth behavior of titanium components was found to be sensitive to microstructure changes and residual stresses that are induced during machining and surface treatment. Underestimating these factors and using through-crack data to predict the part-through crack growth can lead to an incorrect conclusion for a titanium component. Furthermore, unevenness of part-through crack growth between depth and surface growth needs to be considered differently for the basic material in terms of material form and heat treatment, and for the components with machined and shot-peened surfaces. This paper presents considerations supported by testing that lead to improvements in testing and analysis of rotorcraft titanium components and will allow for better use of this material for rotorcraft components.