JOURNAL OF MATERIALS SCIENCE 33 (1998) 3891 — 3898 Some considerations of asymmetric cracking in an applied-moment double cantilever beam C. H. HSUEH, E. Y. SUN, P. F. BECHER Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA E-mail: hsuehc@ornl.gov J. B. LI Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China In situ observations of crack propagation in applied-moment double cantilever beam specimens have been used to obtain the R-curve behaviour of Si 3 N 4 /50% BN—50% Al 2 O 3 laminated composites, in which the BN—Al 2 O 3 layers function as weak interphases. The crack plane and the crack direction were, respectively, normal and parallel to the plane of the laminated layers. During crack propagation, both delamination and crack deviation from the centreline of the specimen occurred. A deviated crack resulted in an uneven moment of inertia in the two beams of the specimen. For a non-laminated material, a deviated crack would become unstable, such that the crack would propagate towards the beam with the smaller moment of inertia. It was found in the present study that delamination in a laminated composite can stabilize the propagation of a deviated crack. The stabilization of a deviated crack with delamination was due to a decrease in the inequality in the moment of inertia of the two beams compared to that without delamination. 1998 Kluwer Academic Publishers 1. Introduction The applied-moment double cantilever beam (DCB) test has been used to characterize the fracture tough- ness of brittle materials [1—4]. For ceramic composites, the fracture toughness is often not a single-value parameter but can increase and asymp- totically approaches a plateau value as the crack ex- tends (i.e. the R-curve behaviour) [5—10]. In situ observations of crack propagation in an applied mo- ment DCB specimen have been used successfully to investigate the R-curve behaviour of whisker-rein- forced, self-reinforced, and intermetallic-bonded ce- ramic composites [2—4]. Recently, studies have been extended to laminated ceramic composites. However, due to the presence of weak interphases, the laminates exhibit both partial delamination and asymmetric cracking behaviour, to which the existing analyses for applied moment DCB tests cannot be readily applied. Also, compared to the propagation of a deviated crack in a non-laminated material, the propagation of a de- viated crack with delamination in a laminated com- posite is found to be more stable during the applied moment DCB test. The purpose of the present study was to analyse the fracture resistance of laminates exhibiting both partial delamination and asymmetric cracking during the ap- plied moment DCB test and to address the stability problem for the propagation of a deviated crack. First, experimental observations of partial delamination and asymmetric cracking in DCB specimens of laminated ceramic composites, which contain center- line grooves to guide the crack, are presented. Then, the traditional analysis of assuming an even distribu- tion of the applied moment between the two beams is used to analyse the fracture resistance of the com- posite with partial delamination and asymmetric cracking. Also, new theoretical analyses of assuming an uneven distribution of the applied moment are developed, and the criterion of minimum total elastic energy stored in the two beams during crack propaga- tion is adopted to define the distribution of the applied moments. Finally, the stability problem of the propa- gation of a deviated crack with or without delamina- tion is addressed. 2. Experimental procedure The laminated ceramic composite used in the present study consists of alternate layers of Si  N  (68 m thick) and 50 vol % BN—50 vol % Al  O  (30 m thick). The average grain diameter of Si  N  was 0.6 m, and BN had platelet-shaped grains, which were oriented more or less parallel to the layer plane (Fig. 1). The processing procedures of this laminate will be reported elsewhere [11]. The DCB specimen had the dimensions 9.88 mm wide2.32 mm thick 25 mm long. A 1.32 mm deep1.78 mm wide groove was machined at the centreline parallel to the 0022—2461  1998 Kluwer Academic Publishers 3891