High temperature/high frequency fatigue crack growth in titanium metal matrix composites
Date of Original Version
An experimental study was conducted to examine the effects of high temperature/high frequency loading on the crack growth behavior in titanium composites. Fatigue crack growth tests were performed on the unidirectional SM1240/TIMETAL-21S composite in air environment at three different temperatures: 24, 500, and 650°C with a loading frequency of 10 Hz and a stress ratio of 0.1. Tests were also carried out in vacuum and on aged specimens. In all the tests, the crack length was measured continuously while the crack opening displacement (COD) was measured at discrete positions along the crack length. Results show that, at all temperatures, the crack progresses in two sequential stages; a stage in which the crack growth rate decreases as the crack length increases followed by a stage characterized by an accelerated crack growth rate. The first stage corresponds to a crack-bridging condition and is the focus of the analysis in this paper. In additional to COD measurements, the microstructural features associated with the fracture process during this stage was examined by scanning electron microscopy with emphasis being placed on the matrix crack morphology, debonding location, temperature, and environmental influences on the fiber coatings. An analytical procedure, employing concepts of the theory of fracture mechanics and micromechanics analysis of the stress state along bridging fibers, was applied in order to calculate the fiber-bridging stress, the frictional shear stress, and the effective stress intensity factor. Results of this work suggest that the fiber-bridging stress and the frictional shear stress are variables along the crack length and are test temperature dependents. Furthermore, it was concluded that both the crack-tip opening displacement and the effective stress intensity factor can be used as measurements of the crack-tip driving force in the crack-bridging stage. Based on the experimental and analytical results collected in this study, a bridging damage mechanism under the loading condition specified here is suggested. This mechanism argues that the effect of temperature on the high frequency crack growth process is established through the temperature-related modification of the frictional shear stress and the resulting adjustment of the internal stress state in the debonded regions of the bridging fibers.
ASTM Special Technical Publication
Zheng, Dian, and Hamouda Ghonem. "High temperature/high frequency fatigue crack growth in titanium metal matrix composites." ASTM Special Technical Publication 1253, (1996): 137-163. doi:10.1520/stp18222s.