| Popis: |
Metal Matrix Composites (MMC) and Intermetallic Matrix Composites (IMC) have been identified as potential material candidates for advanced aerospace applications. They are especially attractive for high temperature applications which require a low density material that maintains its structural integrity at elevated temperatures. High temperature fatigue resistance plays an important role in determining the structural integrity of a material. There are several fundamental issues that surface when considering high temperature fatigue response of MMC's and IMC's. Among them are test technique, failure criterion and life prediction. This study attempts to examine the relevance of these concepts as they pertain to an IMC material, specifically unidirectional SiC fiber reinforced titanium aluminide. As a part of this study, a series of strain- and load-controlled fatigue tests were conducted on unidirectional SiC/Ti-24Al-11Nb (atomic %) composite at 425 and 815°C. Several damage mechanism regimes were identified by using a strain-based representation of the data, Talreja's fatigue life diagram concept. Results from these tests were then used to address issues of test control modes, definition of failure and testing tech niques. Finally, a strain-based life prediction method was proposed for an intermetallic matrix composite (IMC) under tensile cyclic loadings at elevated temperatures. Styled after the "Universal Slopes" method, the model utilizes the composite's tensile properties to estimate life. Factors such as fiber volume ratio (V f), number of plys and temperature dependence are implicitly incorporated into the model through these properties. The model parameters were determined by using fatigue data at temperatures of 425 and 815°C. Fatigue life data from two independent sources were used to verify the model at temperatures of 650 and 760°C. Cross-ply life data from specimens with ply lay-ups of (0/90) 2s and (0/±45/90) 2s at 760°C were also predicted. Correlation between experimental and predicted lives was found to be very reasonable. |
