INSIGHTS INTO FRACTURE MECHANICS FOR A TI/SIC FIBRE COMPOSITE BY 4D X-RAY TOMOGRAPHY

High resolution X-ray tomography and diffraction have been used to study the evolution of a fatigue crack in a Ti-6AL-4V/35vo1% SCS-6 SiC monofilament composite. The crack has been grown through the matrix normal to the direction of the fibres. In addition to information about the path taken by the crack front through the matrix as it successively by-passes rows of bridging fibres, a number of important features of the crack front have been quantified for direct comparison with analytical and numerical fracture mechanics models. Tomography has revealed a crack that is open at K-min, and which opens further by only a few microns at K-max. Diffraction has shown that even at K-max only a relatively small region behind, and in front of, the crack has a tensile stress in the fibres. The tensile stresses at K-max in the fibres bridging the crack range from around 600 MPa at the crack tip towards 1000 MPa at the notch and rise only slowly with crack growth. Fibre/matrix interfacial shear stresses up to 60 MPa have been observed at the boundary of the fibre sliding zone. The bridging stresses have been used to determine the crack tip shielding, shielding the crack tip to about a half of the nominal stress intensity at K-max. At K-min compressive stresses in the bridging fibres tend to hold the crack open, imparting a negative crack tip shielding and reducing the range of stress intensity experienced. The effective stress intensity at the crack tip evaluated from the change in crack opening displacement (measured by tomography) is in reasonable agreement with that derived from the crack tip bridging stresses (measured by diffraction). Both measures indicate that the crack tip stress intensity range increases only slowly with crack length despite large increases in that nominally applied because of the increasing number of bridging fibres. Furthermore the effective R ratio is significantly greater than the nominally applied value (0.1).