July 2001 Multiple Cracking and Tensile behavior 1567 (a)64.0MPa (b)183.4MPa (c)2423MPa (d)301.4MPa (e)331.5MPa Fig. 4. Optical microphotographs of surface replica films showing matrix cracks in the longitudinal(0%)and transverse(90%)fiber bundles at different stress evels width at a stress equal to half the peak value, designated as dem Transverse Crac and the permanent strain, Eo, are plotted in Fig. 6 as a function of Matrix Crack the peak stress, G. The hysteresis loop width shows significant increase above 180 MPa, together with a saturation point of 300 MPa, which corresponds to the increasing matrix crack density On Predic Matrix Crack the other hand, the permanent strain increases for peak applied stresses above 65 MPa, and permanent strain increases are ob- served beyond the matrix crack density saturation point. (4 Thermal Expansion and Elastic Modulus of the Fiber The Cte of the UD composite was 4.07 X 10(K)and 4.01 x 10(K)for the longitudinal and transverse directions 0.16 0.14 lent Strain 050100150200250300350400 ○ Loop width,e Stress [MPa Fig. 5. Matrix crack densities in the longitudinal (0)fiber bundles, and transverse crack densities in the transverse (90)fiber bundles, as function of applied stress, a 89 cracks(indicated by the white arrows in Fig. 4(d)) are noted occur. Since the composite contains translaminar fiber bundles, e,: fibers, no delamination is observed at the interlaminar regions between the 0 and 90 fiber bundles. Thus, these cracks a002 interlansgested to propagate because of shear stresses in the (3) Hysteresis Measurements Peak Applied Stress [MPa] The presence of hysteresis is evident in Fig. 3 with permanent strains, implying a major contribution to the inelastic g. 6. Variation of permanent strain and maximum hysteresis loop width under loading/unloading testing Inction of peak applied stress, train from interfacial debonding and sliding. The hysteresis loop together with the predicted loopcracks (indicated by the white arrows in Fig. 4(d)) are noted to occur. Since the composite contains translaminar fiber bundles, i.e., z fibers, no delamination is observed at the interlaminar regions between the 0° and 90° fiber bundles. Thus, these cracks are suggested to propagate because of shear stresses in the interlaminar regions. (3) Hysteresis Measurements The presence of hysteresis is evident in Fig. 3 with appreciable permanent strains, implying a major contribution to the inelastic strain from interfacial debonding and sliding. The hysteresis loop width at a stress equal to half the peak value, designated as dεmax, and the permanent strain, ε0, are plotted in Fig. 6 as a function of the peak stress, s# p. The hysteresis loop width shows significant increase above 180 MPa, together with a saturation point of 300 MPa, which corresponds to the increasing matrix crack density. On the other hand, the permanent strain increases for peak applied stresses above 65 MPa, and permanent strain increases are ob￾served beyond the matrix crack density saturation point. (4) Thermal Expansion and Elastic Modulus of the Fiber and Matrix The CTE of the UD composite was 4.07 3 1026 (K21 ) and 4.01 3 1026 (K21 ) for the longitudinal and transverse directions, Fig. 4. Optical microphotographs of surface replica films showing matrix cracks in the longitudinal (0°) and transverse (90°) fiber bundles at different stress levels. Fig. 5. Matrix crack densities in the longitudinal (0°) fiber bundles, and transverse crack densities in the transverse (90°) fiber bundles, as a function of applied stress, s#. Fig. 6. Variation of permanent strain and maximum hysteresis loop width under loading/unloading testing as a function of peak applied stress, s# p, together with the predicted loop width for t 5 14 MPa. July 2001 Multiple Cracking and Tensile Behavior 1567