4842 LIU et al.: LAMINATED COMPOSITE sonic coupling) indicated that ultrasonic techniques mal residual stresses. Note that each ply is under a have the potential to characterize anisotropic damage biaxial residual stress state [9]. A laser-ultrasonic (LU) approach has sub- Tensile testing was performed with a screw-driven sequently been developed and used to monitor the Instron 4200 machine at a crosshead speed of 0.03 anisotropic damage evolution in a unidirectional mm/min. The axial strain was measured with an Nicalon" SiC fiber-reinforced calcium aluminosilic- extensometer(I in. gage length). The specimen was ate( CAS/SiC)composite in situ during uniaxial ten- loaded to a pre-set stress level, held at that level for sile loading in its fiber direction [10]. This laser-ultra- laser-ultrasonic measurements, and the stress then sonic technique revealed that extensive transverse reduced to 10 MPa. During the loading and unloading softening of the elastic moduli accompanied degra- process, acoustic emission events were also recorded dation of the axial elastic modulus. this was believed Details of the lu measurement and acoustic emission to be caused by fiber-matrix debonding near the inter- recording techniques are identical to those reported section of fibers with transverse matrix cracks. Here, previously [101 we apply this LU method to investigate anisotropic damage accumulation in a0°90°cros- ply CAs/s composite loaded in uniaxial tension 3, STRESS-STRAIN BEHAVIOR AND VISUAL In this study, anisotropic damage in 0%/90%cross- OBSERVATIONS OF DAMAGE EVOLUTION ply samples was characterized using ultrasonic velo- 3.L. Stress-strain behavior cities measured in situ within two principal planes The anisotropic damage behavior is represented by a A typical loading/unl stress-strain curve for deterioration of the elastic stiffness constants Cu. C2 a CAS/SiC cross-ply d the corresponding and C33(determined from the ultrasonic velocities) acoustic emission(A Its are shown in Fig. 2 in the three principal directions. In addition, acoustic The deviation of the stress-strain curve from the lin emission sensing, intermittent surface replica charac- ear regime occurred at around 50 MPa, and correlated finesse ns and loading/unloading hysteresis curve with an acceleration of AE at about the same stress ere acquired Correlations between the elastic level. A detailed correlation between AE events, the deterioration and tropic damage crack initiation stress and the residual stress will be accumulation are discussed and compared with exist- discussed in Section 5 ing views of damage progression in unidirectional 3.2. Matrix cracking evolution CAS/SIC composites. Figures 3-5 are optical micrographs of surface rep- licas exhibiting matrix cracks at different stages of 2. EXPERIMENTAL APPROACH loading. At low stress levels(<75 MPa) as shown in Fig 3(a) and(b). some(marked) matrix cracks in the Laminated 0%/90 CAS/SIC composite material was 90 ply extended across the entire layer, however a provided by Corning, Inc.(Corning, NY). The majority of the cracks were rather short and confined material was composed of 16 plies with a ply thick- within the ply boundary by fiber bridgment. The met- ness of about 170 um. The material properties have allograph clearly indicated that, in contrast to th been presented elsewhere [8, 11]. The dimensions of viewpoint adopted by many modeling studies, matrix the tensile specimens were -150 mmX10 mmx cracks, once initiated, do not always span an entire .7 mm. The sample ends were bonded to low-modu- ply. Instead, the initiation of new matrix cracks and lus fiberglass tabs for gripping. The edges of the the growth of pre-existing cracks(to span an entire specimens were polished before testing so that acetate ply) progressed simultaneously. For example, Fig replicas could be taken at various stages of loading 4(aHc)reveals a series of surface cracks at three dif- for crack detection and monitoring of its progression. ferent stress levels. A newly initiated crack in the 90o The coordinate system used to characterize both ply at the cross-ply boundary at 93 MPa [marker I the fiber architecture and the test configuration is in Fig. 4(a)l had partially propagated across the ply illustrated in Fig. 1, where, for clarity, only four plies after the stress was increased to 106 MPa [Fig. 4(b) near the middle plane are shown. Direction I was the At a higher stress of 120 MPa, this crack had spread oading direction while 3 was the laminate thickness across the entire ply [Fig. 4(c). Figure 4(c)also direction. Continuous SiC fibers were aligned along shows an example of a matrix crack(marker 2)that oth the 1-and 2-directions. Since the load was was initiated at the high stress level applied in the 1-direction, the plies with fibers in this Figure 5 presents a higher magnification of a sur- rection were the 0 plies. Double 90 layers were face replica that shows cracks deflecting around th present at the center of the lay-up, and an even num- fibers in the 90 layer leaving a debonded interface ber of 0%and 90 plies existed in the sample Conse- The matrix cracks in the 90 plies exhibited both juently, the 0/90 cross-ply used for the tests is a straight and curved paths, depending on the local symmetric laminate, and there should be no coupling arrangement of fibers within the ply. The curved between stretching and bending [2]. Also shown sche- cracks( Fig 4, marker 1) appeared to have extended matically in Fig. I are the directions of laminate ther- more slowly than the straight cracks [see cracks in4842 LIU et al.: LAMINATED COMPOSITE sonic coupling) indicated that ultrasonic techniques have the potential to characterize anisotropic damage [9]. A laser-ultrasonic (LU) approach has sub￾sequently been developed and used to monitor the anisotropic damage evolution in a unidirectional Nicalon SiC fiber-reinforced calcium aluminosilic￾ate (CAS/SiC) composite in situ during uniaxial ten￾sile loading in its fiber direction [10]. This laser-ultra￾sonic technique revealed that extensive transverse softening of the elastic moduli accompanied degra￾dation of the axial elastic modulus. This was believed to be caused by fiber–matrix debonding near the inter￾section of fibers with transverse matrix cracks. Here, we apply this LU method to investigate anisotropic damage accumulation in a 0°/90° cross-ply CAS/SiC composite loaded in uniaxial tension. In this study, anisotropic damage in 0°/90° cross￾ply samples was characterized using ultrasonic velo￾cities measured in situ within two principal planes. The anisotropic damage behavior is represented by a deterioration of the elastic stiffness constants C11, C22 and C33 (determined from the ultrasonic velocities) in the three principal directions. In addition, acoustic emission sensing, intermittent surface replica charac￾terizations and loading/unloading hysteresis curve data were acquired. Correlations between the elastic stiffness deterioration and anisotropic damage accumulation are discussed and compared with exist￾ing views of damage progression in unidirectional CAS/SiC composites. 2. EXPERIMENTAL APPROACH Laminated 0°/90°CAS/SiC composite material was provided by Corning, Inc. (Corning, NY). The material was composed of 16 plies with a ply thick￾ness of about 170 µm. The material properties have been presented elsewhere [8, 11]. The dimensions of the tensile specimens were |150 mm310 mm3 2.7 mm. The sample ends were bonded to low-modu￾lus fiberglass tabs for gripping. The edges of the specimens were polished before testing so that acetate replicas could be taken at various stages of loading for crack detection and monitoring of its progression. The coordinate system used to characterize both the fiber architecture and the test configuration is illustrated in Fig. 1, where, for clarity, only four plies near the middle plane are shown. Direction 1 was the loading direction while 3 was the laminate thickness direction. Continuous SiC fibers were aligned along both the 1- and 2-directions. Since the load was applied in the 1-direction, the plies with fibers in this direction were the 0° plies. Double 90° layers were present at the center of the lay-up, and an even num￾ber of 0° and 90° plies existed in the sample. Conse￾quently, the 0°/90° cross-ply used for the tests is a symmetric laminate, and there should be no coupling between stretching and bending [2]. Also shown sche￾matically in Fig. 1 are the directions of laminate ther￾mal residual stresses. Note that each ply is under a biaxial residual stress state. Tensile testing was performed with a screw-driven Instron 4200 machine at a crosshead speed of 0.03 mm/min. The axial strain was measured with an extensometer (1 in. gage length). The specimen was loaded to a pre-set stress level, held at that level for laser-ultrasonic measurements, and the stress then reduced to 10 MPa. During the loading and unloading process, acoustic emission events were also recorded. Details of the LU measurement and acoustic emission recording techniques are identical to those reported previously [10]. 3. STRESS–STRAIN BEHAVIOR AND VISUAL OBSERVATIONS OF DAMAGE EVOLUTION 3.1. Stress–strain behavior A typical loading/unloading stress–strain curve for a CAS/SiC cross-ply sample and the corresponding acoustic emission (AE) events are shown in Fig. 2. The deviation of the stress–strain curve from the lin￾ear regime occurred at around 50 MPa, and correlated with an acceleration of AE at about the same stress level. A detailed correlation between AE events, the crack initiation stress and the residual stress will be discussed in Section 5. 3.2. Matrix cracking evolution Figures 3–5 are optical micrographs of surface rep￾licas exhibiting matrix cracks at different stages of loading. At low stress levels (,75 MPa) as shown in Fig. 3(a) and (b), some (marked) matrix cracks in the 90° ply extended across the entire layer; however a majority of the cracks were rather short and confined within the ply boundary by fiber bridgment. The met￾allography clearly indicated that, in contrast to the viewpoint adopted by many modeling studies, matrix cracks, once initiated, do not always span an entire ply. Instead, the initiation of new matrix cracks and the growth of pre-existing cracks (to span an entire ply) progressed simultaneously. For example, Fig. 4(a)–(c) reveals a series of surface cracks at three dif￾ferent stress levels. A newly initiated crack in the 90° ply at the cross-ply boundary at 93 MPa [marker 1 in Fig. 4(a)] had partially propagated across the ply after the stress was increased to 106 MPa [Fig. 4(b)]. At a higher stress of 120 MPa, this crack had spread across the entire ply [Fig. 4(c)]. Figure 4(c) also shows an example of a matrix crack (marker 2) that was initiated at the high stress level. Figure 5 presents a higher magnification of a sur￾face replica that shows cracks deflecting around the fibers in the 90° layer leaving a debonded interface. The matrix cracks in the 90° plies exhibited both straight and curved paths, depending on the local arrangement of fibers within the ply. The curved cracks (Fig. 4, marker 1) appeared to have extended more slowly than the straight cracks [see cracks in