Multiaxial Response of Metal Matrix Composites at Elevated Temperature

Principal Investigator:
Cliff Lissenden (Engineering Science and Mechanics)

Researchers in the Composites Manufacturing Technology Center (CMTC) at Penn State University, in collaboration with researchers at the NASA Lewis Research Center, are investigating the multiaxial response of metal matrix composites (MMC) at elevated temperature. MMC are candidate materials for use in advanced aero-propulsion systems. Significant improvements to engine performance, associated with decreased weight and improved properties at elevated temperature, are anticipated if MMC can replace conventional monolithic materials. Many structural elements, including impellers, shafts, and rotors, are subjected to multiaxial loads during operation. However, to date very few multiaxial experimental results are available. Researchers at Penn State and NASA Lewis are conducting axial-torsional experiments on thin-walled MMC tubular specimens at elevated temperature in NASA Lewis' biaxial laboratory. The picture above and to the left shows the experimental set-up. Tests are conducted in a servo-hydraulic axial-torsional test machine, with an induction heating system used to provide a uniform temperature in the gage length. Strains are measured with microlevel accuracy by an axial-torsional extensometer.

Research is currently focused on deformation under multiaxial loading. One objective is to experimentally determine surfaces of constant inelasticity. To the left a Surface of Constant Inelastic Strain Rate (SCISR) in modified axial-torsional stress space is shown for stainless steel at 650C, illustrating the viability of the experimental procedure. However, MMC deform inelastically due to matrix viscoplasticity as well as due to the development and evolution of internal damage, most notably interfacial debonding. Thus, experimental results on MMC will indicate the effects of damage, and its interaction with matrix viscoplasticity. Once initial inelastic surfaces have been defined, preloading will be applied and subsequent inelastic surfaces defined to enable the construction of a hardening law for MMC.

Theoretical modelling is ongoing in conjunction with the experimental investigation. Both finite element and analytical micromechanical models are being used to predict multiaxial response of MMC. To the left, predicted yield surfaces are shown in axial-torsional stress space for silicon carbide/titanium (SiC/Ti) in both the undamaged and damaged conditions. These predictions were obtained using the methods of cells micromechanical model. Damage is considered by including interfacial constitutive relations that allow the fiber to debond from the matrix. The unified viscoplastic theory of Bodner and Partom is employed for the matrix. Other viscoplastic models will be considered. The experimental results are being used to guide the construction of an acceptable model.