Prediction of Notched Strength of 3D Carbon Carbon Materials Under Tension with Small Off-Axis Angle,Part II

Abstract

Under tension with small off-axis angle, the notch in a plane specimen of 3D carbon carbon material propagates self-similarly at first. Then, the specimen is sheared to rupture along the direction of the fiber bundles in some of the layers and perpendicular to the fiber bundles in the remaining layers, as the number of broken fiber bundles reaches a critical value r_c. The failure is catastrophic. No additional load is needed to break more fiber bundles after the first fiber bundle in front of notch tip is broken. The observation suggests that the notched strength of 3D carbon carbon specimen is dictated by the tensile strength of fiber bundle at notch tip, as in the case of simple tension. The difference lies only in that the fiber bundle in this case is subjected to shear stresses besides tensile stresses and the shear stress substantially reduces its tensile strength. Experimental mea surements of tensile and shear strength of fiber bundles under combined tensile and shear stresses were performed. The stress field near the notch tip was computed using the finite element method based on a global-local concept. Finally, the notched strength of 3D car bon carbon specimen was calculated and ratios of r_c/n (n is the total number of fiber bundles in the width direction) were also predicted. The predictions match the experimen tal results very well.