LIU et al.: LAMINATED COMPOSITE 4843 9o°ply 0°ply addd Fig. 1. Coordinates and layer residual stress states for the 0/90 cross-ply laminate. Loading direction is along the I-direction became more prevalent than in the 90 ply because of the overlapping of matrix cracks from the 90% plies on either side of the 0o ply. The penetration of pre existing cracks into neighboring 0 plies was accompanied by the initiation of new matrix cracks in the 90 ply. This again contrasts with a previous report that matrix cracks in the 90 ply were saturated a when the stress had reached 80 MPa [3]. It has also been noted that the ultimate tensile strength of five composite samples tested here ran 130 to 160 MPa, while in previous studies the failure stress for this composite was reported from 120 MPa Strain (%) [12] to 220 MPa 13, 13]. These differences may be attributed to sample variations [14] Fig. 2. Loading/unloading stress-strain curve of CAS/SiC Metallographic observations of surface replicas cross-ply laminate and the recorded acoustic emission events. taken after unloading and failure are consistent with the notion that matrix cracks initiated in the 90 ply the top 90 ply in Fig 3(b)]. Although previous stud- near the ply boundary, then gradually extended into es have also indicated that the matrix cracks initiated the 90 ply. As shown in Fig. 7(taken after sampl first in the 90 plies [3, 12], the observations of crack failure), matrix cracks in the central double 90 layer growth presented here are not in total agreement with are arrested near ply boundaries. This is believed to all of the prior investigations. For example, Beyerle be due to uneven crack openings in the 90 ply: crack et al. [3] suggested that matrix cracks in the 90 ply openings near the ply boundary(initiation site)were ahvays spanned the entire ply once initiated, whereas generally larger than those at the ply center for a Mall and Kim [12] believed that matrix cracks given stress level. Additionally, fibers in the 0 plies occurred in a random manner and that no single crack exert frictional forces upon crack closure during w across an entire ply unloading. which could also contribute to a lower The average crack density was determined as a extent of crack closure in the 90o ear the ply function of stress from the surface replicas and is boundary. This is further illustrated by comparing presented in Fig. 6. Crack initiation stress was taken Fig. 8(a), taken at a stress of 137 MPa, with Fig. 8(b) as 50 MPa, which was estimated from the stress- taken after unloading to 10 MPa from 150 MPa. It strain and AE measurement. At stress levels can be seen that crack openings near the central part below -95 MPa the crack density in the weaker 90 of the 90 ply crack(shown by arrow)are signifi ply was higher than in the 0 ply. As the applied cantly reduced after unloading. Since there was no stress increased, the 90 ply cracks then began to pen- obvious increase in the reloading modulus compared etrate into the 0 ply. By the time the stress had with the unloading modulus, what has been seen in reached above 95 MPa, matrix cracks in the 0 ply the central region of 90 ply after unloading [FigLIU et al.: LAMINATED COMPOSITE 4843 Fig. 1. Coordinates and layer residual stress states for the 0°/90° cross-ply laminate. Loading direction is along the 1-direction. Fig. 2. Loading/unloading stress–strain curve of CAS/SiC cross-ply laminate and the recorded acoustic emission events. the top 90° ply in Fig. 3(b)]. Although previous stud￾ies have also indicated that the matrix cracks initiated first in the 90° plies [3, 12], the observations of crack growth presented here are not in total agreement with all of the prior investigations. For example, Beyerle et al. [3] suggested that matrix cracks in the 90° ply always spanned the entire ply once initiated, whereas Mall and Kim [12] believed that matrix cracks occurred in a random manner and that no single crack grew across an entire ply. The average crack density was determined as a function of stress from the surface replicas and is presented in Fig. 6. Crack initiation stress was taken as 50 MPa, which was estimated from the stress– strain curve and AE measurement. At stress levels below |95 MPa the crack density in the weaker 90° ply was higher than in the 0° ply. As the applied stress increased, the 90° ply cracks then began to pen￾etrate into the 0° ply. By the time the stress had reached above 95 MPa, matrix cracks in the 0° ply became more prevalent than in the 90° ply because of the overlapping of matrix cracks from the 90° plies on either side of the 0° ply. The penetration of pre￾existing cracks into neighboring 0° plies was accompanied by the initiation of new matrix cracks in the 90° ply. This again contrasts with a previous report that matrix cracks in the 90° ply were saturated when the stress had reached 80 MPa [3]. It has also been noted that the ultimate tensile strength of five 0°/90° composite samples tested here ranged from 130 to 160 MPa, while in previous studies the failure stress for this composite was reported from 120 MPa [12] to 220 MPa [3, 13]. These differences may be attributed to sample variations [14]. Metallographic observations of surface replicas taken after unloading and failure are consistent with the notion that matrix cracks initiated in the 90° ply near the ply boundary, then gradually extended into the 90° ply. As shown in Fig. 7 (taken after sample failure), matrix cracks in the central double 90° layer are arrested near ply boundaries. This is believed to be due to uneven crack openings in the 90° ply: crack openings near the ply boundary (initiation site) were generally larger than those at the ply center for a given stress level. Additionally, fibers in the 0° plies exert frictional forces upon crack closure during unloading, which could also contribute to a lower extent of crack closure in the 90° ply near the ply boundary. This is further illustrated by comparing Fig. 8(a), taken at a stress of 137 MPa, with Fig. 8(b) taken after unloading to 10 MPa from 150 MPa. It can be seen that crack openings near the central part of the 90° ply crack (shown by arrow) are signifi- cantly reduced after unloading. Since there was no obvious increase in the reloading modulus compared with the unloading modulus, what has been seen in the central region of 90° ply after unloading [Fig