Interlaminar Mode I and Mode II Fracture Toughnesses in Ceramic Matrix Composites (CMCs)

Susceptibility to delamination is one of the major weaknesses of ceramic matrix composites (CMCs). Knowledge of the resistance of composite to interlaminar fracture is essential for life cycle prediction analyses of structural components. The current test method for Mode I-interlaminar fracture toughness, the double cantilevered beam (DCB), is not satisfactory for thin CMC specimens because the compliance of the cantilever arms yields a spurious energy release rate. For Mode II-interlaminar fracture toughness testing, the end notched flexure (ENF) test is the most popular, but this test is not a standardized test method as yet. CMC components are in heavy demand for parts subjected to high heat. However, elevated temperature creates more severe conditions for interlaminar fracture toughness testing. A need exists, therefore, for reliable Mode I- and Mode II-interlaminar fracture toughness test methods which are applicable to a wide range of CMC materials, allowing for quantification of Mode I and Mode II-fracture toughness and accounting for the effects due to complexity of ply architecture at room temperature and elevated temperature up to 1316C (2400F). We propose several alternative methods, and a means to evaluate them using finite element analysis and actual testing. We will refine and downselect the methods to standardize an efficient, accurate testing approach.