Design considerations for rotating laminated metal-matrix-composite shafts

Abstract

A logical step in the application of fiber-reinforced composites is to take advantage of their light-weight/high-strength potential and replace traditional monolithic shaft designs with composite materials. In the case of aircraft engine shafts where the high-temperature environment excludes the use of most traditional materials, a high-strength titanium alloy is recommended. The focus of this paper is the evaluation of Ti-21S monolithic and SiC/Ti-21S composite shafts under various loadings and damage conditions. This is accomplished through the development of a fully inelastic axisymmetric generalized plane-strain model that incorporates the generalized method of cells (GMC) micromechanics model (that includes the capability of fiber/matrix debond), as well as the effects from a rotational body force. The inelastic analysis is performed through the method of successive elastic solutions. In this study, the elastic response of homogeneous and variously oriented fiber-reinforced shafts are compared in order to demonstrate the possible advantages of composite design. The greatest differences are observed in the circumferential stress and strain in the circumferentially wound (90°) shaft, with savings of as much as 11% in circumferential stress and 53% in circumferential strain compared to that of a similarly loaded homogeneous shaft. For the composite shafts subject to both inelastic effects and debond, the 90° strong and weak bond shafts offer the only consistent improvement over traditional monolithic designs. The interaction between any other fiber orientation and inelasticity/debond damage requires careful consideration of individual designs to create a reliable shaft. This paper demonstrates the feasibility of using lighter-weight/stronger composite shafts, as well as the complexity of the design problem, along with the careful consideration of practicality that must be taken prior to the abandonment of traditional monolithic designs in highly critical applications.