Effect of loading frequency on the strain of MI-SiC/SiC composites

Abstract

In this work, we tried to answer the question: what is the role loading frequency plays in strain accumulation during time-dependent tests of Melt-infiltrated SiC/SiC CMCs? Time-dependent tests were conducted at room temperature, 815°C as a possible temperature for the phenomenon of pesting, and 1204°C as an oxidizing temperature. Stress levels were selected from tensile stress–strain curves to range from 110.4 MPa where no major cracks could be detected to 220.8 MPa at the end of the knee region at a point of possible matrix crack saturation. Loading frequencies of 30 Hz and 1 Hz fatigue, and 2-h dwell-fatigue were investigated. Residual tensile strength tests of samples that did not fail during time-dependent experiments were carried out. Initial observations lead us to divide the question above to two parts, one addressing the value of the strain to failure and the other addressing the steady-state strain rates. Strain to failure during residual tests added to strain accumulated during time-dependent tests for 22 samples tested at different stresses, load frequencies, and temperatures had a consistent average of 0.005 with a small standard deviation. This could mean that there is a reasonable possibility that this material has a finite “strain capacity.” Strain versus time curves of all time-dependent tests were successfully fitted using a single 3-parameter equation while accounting for the effect of temperature and stress-level, to a reasonable extent. Such observation, despite of some of the variability in the data, could mean that loading frequency have a small, or negligible, effect on the strain-rate.