CATERTALIA Pergamon Acta mater.48(20004841-4849 www.elsevier.com/locate/actamat ANISOTROPIC DAMAGE EVOLUTION IN A%/90 LAMINATED CERAMIC-MATRIX COMPOSITE Y.-M. LIUT, T E. MITCHELL and H N G. WADLEY+2 Center for Materials Science, Mail Stop K- 765, Los Alamos National Laboratory, Los Alamos, NM 87545, USA and Department of Materials Science and Engineering, School of Engineering and Applied cience, University of Virginia, Charlottesville, VA 22903, USA Received 22 February 2000: accepted 2 August 2000) Abstract-Anisotropic damage evolution in a 0/90 laminated Nicalon"SiC fiber-reinforced calcium alumi- nosilicate(CAS)glass-ceramic composite during uniaxial tensile deformation has been investigated using a variety of non-invasive characterization techniques. The elastic constant reduction in the three principal direc- tions was measured from in situ laser-generated ultrasonic velocity measurements in various sound prop gation directions. They indicate that, in addition to a large drop in elastic stiffness in the loading directio he constants characterizing the nominal elastic stiffness transverse to the loading direction were al degraded Surface replicas taken intermittently during loading revealed that transverse softening of the elastic stiffness was associated with fiber/matrix interface damage mainly in the 0 plies, while the large softenin of the elastic stiffness in the loading direction was the result of multiple matrix cracking in both the 0o 90 plies. While the ultrasonic data allowed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identified the crack initiation stress the 90 plies and correlated it to macroscopically observable deviations of the stress-strain curve fr lies t behavior. These matrix cracks were found to have initiated preferentially in the weak 9 90/0 ply boundaries. 2000 Acta Metallurgica Inc. Published by elsevier Science Ltd. All Keywords: Acoustic, Non-destructive testing, Multilayers; Ceramics; Composites 1 INTRODUCTION loaded with the tensile axis aligned parallel to the 0o Laminated composites sometimes have significant ply direction, it is generally believed that transverse advantages over unidirectional composites for multi- matrix cracks first appear in the weaker 90 plies[3]) axially loaded structural applications because the These small cracks are thought to extend laterally to orientation,thickness and stacking sequence of the span the entire 90 layer and only then penetrate into fiber-reinforced laminae can be varied to satisfy the neighboring 5]. Finite element analyses needed stiffness and strength requirements in differ- have indicated that the degradation of the elastic stiff- ent loading directions [1, 2). However, ensuring the ness constants of cross-ply laminates is related to the acking,fiber/matrix interface debonding and inter- ure damage evolution in situ and to confirm this view- oly delamination are more complicated than in the point. Conventionally, damage evolution is charac- case of a unidirectional composite loaded in one terized by an analysis of the uniaxial stress-strain direction. A detailed understanding of the anisotropic behavior [3, 7, 8]. In this approach, the effective aspects of damage evolution is therefore important for Young's modulus in the loading direction at various both designing improved laminated composites and deformation stages is determined from utilizing them more reliably in structural applications. loading/unloading curves and compared with metallo When a brittle-matrix 0/90 cross-ply laminate is graphic observations of the crack density. The recog nition of anisotropic damage and the assessment of its mechanical significance have been limited, in part, 四cmg Intel Corporation, Assembly Tech- by the absence of effective experimental method CH5-263,5000 West Chandler Boul- ologies for its measurement respondence should be addressed. Fax: Recent ultrasonic studies on an sic/sic bi-direc 1359-6454100/520.00@ 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved PI:S1359-6454(00)00286-XActa mater. 48 (2000) 4841–4849 www.elsevier.com/locate/actamat ANISOTROPIC DAMAGE EVOLUTION IN A 0°/90° LAMINATED CERAMIC-MATRIX COMPOSITE Y. -M. LIU†1 , T. E. MITCHELL1 and H. N. G. WADLEY†2 1 Center for Materials Science, Mail Stop K-765, Los Alamos National Laboratory, Los Alamos, NM 87545, USA and 2 Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903, USA ( Received 22 February 2000; accepted 2 August 2000 ) Abstract—Anisotropic damage evolution in a 0°/90° laminated Nicalon SiC fiber-reinforced calcium alumi￾nosilicate (CAS) glass–ceramic composite during uniaxial tensile deformation has been investigated using a variety of non-invasive characterization techniques. The elastic constant reduction in the three principal direc￾tions was measured from in situ laser-generated ultrasonic velocity measurements in various sound propa￾gation directions. They indicate that, in addition to a large drop in elastic stiffness in the loading direction, the constants characterizing the nominal elastic stiffness transverse to the loading direction were also degraded. Surface replicas taken intermittently during loading revealed that transverse softening of the elastic stiffness was associated with fiber/matrix interface damage mainly in the 0° plies, while the large softening of the elastic stiffness in the loading direction was the result of multiple matrix cracking in both the 0° and 90° plies. While the ultrasonic data allowed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identified the crack initiation stress in the 90° plies and correlated it to macroscopically observable deviations of the stress–strain curve from linear elastic behavior. These matrix cracks were found to have initiated preferentially in the weak 90° plies near the 90°/0° ply boundaries.  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Acoustic; Non-destructive testing; Multilayers; Ceramics; Composites 1. INTRODUCTION Laminated composites sometimes have significant advantages over unidirectional composites for multi￾axially loaded structural applications because the orientation, thickness and stacking sequence of the fiber-reinforced laminae can be varied to satisfy the needed stiffness and strength requirements in differ￾ent loading directions [1, 2]. However, ensuring the durability of brittle-matrix laminates can be problem￾atic because damage modes such as transverse ply cracking, fiber/matrix interface debonding and inter￾ply delamination are more complicated than in the case of a unidirectional composite loaded in one direction. A detailed understanding of the anisotropic aspects of damage evolution is therefore important for both designing improved laminated composites and utilizing them more reliably in structural applications. When a brittle-matrix 0°/90° cross-ply laminate is * Present address: Intel Corporation, Assembly Tech￾nology Development, CH5-263, 5000 West Chandler Boul￾evard, Chandler, AZ 85226, USA. † To whom all correspondence should be addressed. Fax: 1804 924 3032. 1359-6454/00/$20.00  2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S13 59-6454(00)00286-X loaded with the tensile axis aligned parallel to the 0° ply direction, it is generally believed that transverse matrix cracks first appear in the weaker 90° plies [3]. These small cracks are thought to extend laterally to span the entire 90° layer and only then penetrate into neighboring 0° plies [3–5]. Finite element analyses have indicated that the degradation of the elastic stiff￾ness constants of cross-ply laminates is related to the transverse matrix crack density [1, 5, 6], but relatively little experimental work has been conducted to meas￾ure damage evolution in situ and to confirm this view￾point. Conventionally, damage evolution is charac￾terized by an analysis of the uniaxial stress–strain behavior [3, 7, 8]. In this approach, the effective Young’s modulus in the loading direction at various deformation stages is determined from loading/unloading curves and compared with metallo￾graphic observations of the crack density. The recog￾nition of anisotropic damage and the assessment of its mechanical significance have been limited, in part, by the absence of effective experimental method￾ologies for its measurement. Recent ultrasonic studies on an SiC/SiC bi-direc￾tional composite immersed in a water bath (for ultra-