Static and fatigue behaviour of unidirectional composites in compression

In this research, the test methods and specimen configurations are studied for both static and fatigue characterization of composite materials in compression, using carbon/epoxy as an example. A new test method was developed to allow for both static and compression fatigue testing. Finite element analyses were performed to analyze the effect of Teflon inserts between the tabs and specimen on the stress distribution in the specimen. The test uses a conventional material testing machine equipped with hydraulic grips. No fixture was needed to test the specimen, which is an important advantage especially in fatigue testing, where the fixture itself can be damaged. The plasma projected wedges of the hydraulic grips provide good friction between the wedges and the specimen. No anti-buckling device was needed because of the short gage length used. The use of Teflon inserts effectively reduces the stress concentration at the end of the tabs in both static and fatigue loading. Experimental results of the new test method for static loading of unidirectional AS4/3501-6 are discussed. The new approach results in comparable static compression strength, and failure predominantly in the middle of the gage length. Then, fatigue testing results are presented for unidirectional AS4/3501-6. To completely characterize this material under compression fatigue loading, life curves with different stress ratios (R = $ rm sigma sb{min}/ sigma sb{max}$) are produced in the form of constant life diagrams. The residual strength and stiffness were measured by static compression of the specimen after compressive cyclic loading and removal before final failure. These results are presented in the form of strength and modulus degradation curves. The compression-compression fatigue test results are part of a complete fatigue progressive damage theory developed at McGill University. (Abstract shortened by UMI.