Abstract: Stiffness degradation formulas are given for a composite laminate with constrained cracking.Because the formulas are expressed as a function ofcrack density,the relation between crack density and stresses in the damage zone is constructed so as to predict the stiffness degrada- tion directly from the stress distribution in damage zone. The generalization of the stiffness deg- radation formulas is made for any angle-plied lamina by rotating the coordinate axes twice.

Key words: composite laminate, matrix cracking, crack density, stiffness degradation, angle-plied lamina