A biaxial fatigue model for axial-torsion strain cycling is proposed which includes elastic and plastic strain life regimes at elevated temperatures. The biaxial fatigue model is based on the concept that the transition from the plastic strain region to the elastic strain region on the bilinear fatigue life curve occurs at a cycle at which the elastic and plastic strain components of the total applied strain are equal. This transition corresponds to the intersection of the elastic and plastic strain lifelines of either a uniaxial or a torsional strain cycling condition. The transition point can combine the fatigue strength and fracture ductility coefficients of the material. The von Mises yield criterion was modified by using the Davis and Connelly triaxiality factor (TF) to account for the effects of the stress state and strain rate on the fracture ductility at elevated temperatures, which decreases as TF increases. Experimental data by the authors as well other available data were analyzed on the basis of the proposed model.

The low-cycle fatigue (LCF) behavior of Waspaloy was studied in uniaxial and torsional loading for two heat treatments at 24 and 649°C. It was shown that for both heat treatments deformation and failure mechanisms were independent of stress state at 24°C. Deformation occurred by precipitate shearing and the formation of intense shear bands for the coarse-grain/small-precipitate (CG-SP) condition and by precipitate looping and loosely defined shear bands for the fine-grain/large-precipitate (FG-LP) condition. Failure in both microstructures was associated with the formation of shear cracks. At 649°C deformation and failure mechanisms for the FG-LP condition were independent of stress state, and the mechanisms were similar to those observed at 24°C. For the CG-SP condition, the situation was different. Failure occurred on principal planes when tested in torsion and on shear planes when tested in uniaxial tension. The mechanism transition is interpreted in terms of deformation mode and microstructural instability. In general, there was a more pronounced decrease in life with increasing temperature for uniaxial specimens than for torsional specimens. This result is interpreted in terms of an environmental interaction that is accelerated by a comparatively significant dilatational strain component.

Titanium matrix composites (TMCs) are being evaluated as structural materials for elevated temperature applications in future generation hypersonic vehicles. In such applications, TMC components are subjected to complex thermomechanical loading profiles at various elevated temperatures. Therefore, thermomechanical fatigue (TMF) testing, using a simulated mission profile, is essential for evaluation and development of life prediction methodologies. The objective of the research presented in this paper was to evaluate the TMF response of the [0/90]2s SCS-6/TIMETAL-21S subjected to a generic hypersonic flight profile and its portions with a temperature ranging from −130 to 816°C. It was found that the composite modulus, prior to rapid degradation, had consistent values for all the profiles tested. The accumulated minimum strain was also found to be the same for all the profiles tested. A micromechanics-based analysis was used to predict the stress-strain response of the laminate and of the constituents in each ply during thermomechanical loading conditions by using only constituent properties as input. The fiber was modeled as elastic with transverse orthotropic and temperature-dependent properties. The matrix was modeled using a thermoviscoplastic constitutive relationship. In the analysis, the composite modulus degradation was assumed to result from matrix cracking and was modeled by reducing the matrix modulus. Fatigue lives of the composite subjected to the complex generic hypersonic flight profiles were well correlated using the predicted stress in 0° fibers.

No abstract

No abstract