Thermal and mechanical fatigue of 6061 Al - P100 Gr metal matrix composite
Hansen, Robert C.
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Aluminum-Graphite composite material was tested under thermal and mechanical cyclic loads. Initial thermal cycling experiments of a diffusion bonded, 11 ply composite showed crack nucleation at regions of poor liquid metal infiltration in the fiber tows when heated from room temperature (RT) to 540 C after one cycle. Similar experiments from RT to 100 C did not show similar damage after 120 cycles. Dilatometry experiments showed existence of thermal residual strain after the first heating cycle between (a) RT to 100 C and (b) RT to 540 C. Thermal strain hysteresis was observed in both temperature ranges. These effects have been explained using matrix time dependent plasticity and residual stress concepts. It was found that residual stress existing at RT after fabrication, due to differences in coefficient of thermal expansion of aluminum and graphite, seriously effected subsequent strain behavior. Transmission electron microscopy revealed poor wetting of graphite fibers due to the lack of standard titanium boride coatings used in these composite materials. Two different sample configurations, along with the monolithic matrix material, were tested in the range of 65 to 95 percent of their respective ultimate bending strengths (UBS). Crack propagation studies, along with S-N curves, are also presented.
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