LIU et al.: LAMINATED COMPOSITE 4845 200m Fig. 5. Optical micrograph showing that the matrix crack propagation(stress 106 MPa)in the 90 layer deflects around fibers, fiber ends in 90 plies are visible due to debonding 2—0o900 90o ply Fg.6. Average matrix crack densities in0°90° plies,,0°and 90 plies as a function of stresses 200 same area at different loading stages: (a)93 MPa, arrow I indi- cates short matrix cracks initiated at 90 plies near ply bound- ries; (b)106 MPa, showing the crack growth within a 90 ply (arrow 1);(c)120 MPa, crack I spans the 90 ply, arrow 2 shows a newly initiated matrix crack a=(Cl+ Css)sin20+(C33+ Css)cos0 b= C55 Cu sin28+ C33 cos20(4)LIU et al.: LAMINATED COMPOSITE 4845 Fig. 4. Optical micrographs of surface replicas taken in the same area at different loading stages: (a) 93 MPa, arrow 1 indi￾cates short matrix cracks initiated at 90° plies near ply bound￾aries; (b) 106 MPa, showing the crack growth within a 90° ply (arrow 1); (c) 120 MPa, crack 1 spans the 90° ply, arrow 2 shows a newly initiated matrix crack. a 5 (C11 1 C55) sin2 q 1 (C33 1 C55) cos2 q, (3) b 5 C55 1 C11 sin2 q 1 C33 cos2 q (4) and Fig. 5. Optical micrograph showing that the matrix crack propagation (stress 106 MPa) in the 90° layer deflects around fibers; fiber ends in 90° plies are visible due to debonding. Fig. 6. Average matrix crack densities in 0°/90° plies, 0° and 90° plies as a function of stresses. Fig. 7. Surface cracks after failure, where cracks in the 90° ply are only visible near the ply boundary