MATERIAL E EGEERNG SEVIER Materials Science and Engineering A250(1998)285-290 Direct observation and modelling of the crack -fibre interaction process in continuous fibre-reinforced ceramics: model experiments Yutaka Kagawa*, Ken Got Institute of Industrial Science, The University of Tokyo, 7-22-1 Roppongi, Minato ku, Tokyo 106-8558, Japan Abstract The effect of the matrix-fibre interface bonding and debonding condition on the crack growth behaviour in a fibre-reinforced ceramic matrix composite was studied using a model glass fibre-reinforced PMMA matrix composite. The crack growth process rom a centre notch is monitored using a compression splitting test. From direct observation three characteristic stages can b identified in the crack growth process of the composite, namely elastic constraint(stage I), matrix crack bowing(stage In) and crack bridging(stage III). Partial interface debonding occurs at the end of stage I and cylindrical interface debonding occurs at the end of stage Il. The crack growth rate is accelerated just after the onset of interface partial debonding and this indicates that a debonded interface reduces the crack growth resistance. The partial interface debonding which occurs before fibre breaking plays an important role on the crack growth mechanism. C 1998 Elsevier Science S.A. All rights reserved Keywords: Crack deflection; Crack growth rate: Interface debonding 1. Introduction reported [9-13]. However, details of the 3D crack -fibre interaction processes have not yet been considered for Characteristic mechanical properties of fibre-rein- incorporation in the studies. Thus, it seems important forced ceramics arise from the development of a mi- to understand details of these stages of crack growth crofracture process. Recent experiments on a direct behaviour. In this paper, attention has been focused on observation of the fracture behaviour of continuous a detailed observation of crack-fibre interaction and its fibre-reinforced ceramic matrix composites revealed effect on the crack growth rate in continuous fibre-rein- that matrix cumulative microfracture occurs during the forced ceramic matrix composite, using a Sio, fibre-re- linear deformation range of the stress-strain curve. inforced PMMA model composite Evidence of matrix microcracking in Sic fibre-rein forced glass matrix composites was reported and theo retical considerations were also made [1-6]. The authors recently reported that the onset of early stage 2. Material and experimental procedure matrix cracking in the unidirectional Sic (NicalonTM) A commercially available pure Sio, fibre(diame- fibre-reinforced borosilicate glass composite occurs at ter =2.4 mm: 9R2, Toshiba Ceramics, Tokyo, Japan) an applied stress of x 90-100 MPa and that the exten sion of matrix microcracking was strongly related to was used as reinforcement and a commercially available matrix crack-fibre interactions [6-8]. The interaction PMMA plate(thickness=5.0 mm; ShinkoliteA, Mit process was divided into three stages, namely(i)elastic subishi Rayon, Tokyo, Japan) was used as matrix constraint,(1i) crack bowing and (iii) crack bridging Properties of the fibre and matrix are listed in Table 1 [9-11]. The effect of the elastic constraint and the [14, 15]. Two fibres aligned parallel to each other with spacing of a 2.0 mm were sandwiched between PMMA associated crack bowing on the fracture resistance of plates and subsequently hot-pressed in ambient air.The the unidirectional fibre-reinforced ceramics has been hot pressing temperature and pressure were 473 K and 2.5 MPa, respectively. Corresponding author. Tel. +81 3 34026231; fax: +81 3 The hot-pressed composite was mechanically cut into 34026350; e-mail: kagawa @iis. u-tokyo ac jp a double cleavage drilled compression(DCDC) speci- 0921-5093/98/S19.00 C 1998 Elsevier Science S.A. All rights reserved. PIS0921-5093098)00603·0Materials Science and Engineering A250 (1998) 285–290 Direct observation and modelling of the crack–fibre interaction process in continuous fibre-reinforced ceramics: model experiments Yutaka Kagawa *, Ken Goto Institute of Industrial Science, The Uni6ersity of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan Abstract The effect of the matrix–fibre interface bonding and debonding condition on the crack growth behaviour in a fibre-reinforced ceramic matrix composite was studied using a model glass fibre-reinforced PMMA matrix composite. The crack growth process from a centre notch is monitored using a compression splitting test. From direct observation three characteristic stages can be identified in the crack growth process of the composite, namely elastic constraint (stage I), matrix crack bowing (stage II) and crack bridging (stage III). Partial interface debonding occurs at the end of stage I and cylindrical interface debonding occurs at the end of stage II. The crack growth rate is accelerated just after the onset of interface partial debonding and this indicates that a debonded interface reduces the crack growth resistance. The partial interface debonding which occurs before fibre breaking plays an important role on the crack growth mechanism. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Crack deflection; Crack growth rate; Interface debonding 1. Introduction Characteristic mechanical properties of fibre-rein￾forced ceramics arise from the development of a mi￾crofracture process. Recent experiments on a direct observation of the fracture behaviour of continuous fibre-reinforced ceramic matrix composites revealed that matrix cumulative microfracture occurs during the linear deformation range of the stress–strain curve. Evidence of matrix microcracking in SiC fibre-rein￾forced glass matrix composites was reported and theo￾retical considerations were also made [1–6]. The authors recently reported that the onset of early stage matrix cracking in the unidirectional SiC (Nicalon™) fibre-reinforced borosilicate glass composite occurs at an applied stress of :90–100 MPa and that the exten￾sion of matrix microcracking was strongly related to matrix crack–fibre interactions [6–8]. The interaction process was divided into three stages, namely (i) elastic constraint, (ii) crack bowing and (iii) crack bridging [9–11]. The effect of the elastic constraint and the associated crack bowing on the fracture resistance of the unidirectional fibre-reinforced ceramics has been reported [9–13]. However, details of the 3D crack–fibre interaction processes have not yet been considered for incorporation in the studies. Thus, it seems important to understand details of these stages of crack growth behaviour. In this paper, attention has been focused on a detailed observation of crack–fibre interaction and its effect on the crack growth rate in continuous fibre-rein￾forced ceramic matrix composite, using a SiO2 fibre-re￾inforced PMMA model composite. 2. Material and experimental procedure A commercially available pure SiO2 fibre (diame￾ter=2.4 mm; 9R2, Toshiba Ceramics, Tokyo, Japan) was used as reinforcement and a commercially available PMMA plate (thickness=5.0 mm; Shinkolite®A, Mit￾subishi Rayon, Tokyo, Japan) was used as matrix. Properties of the fibre and matrix are listed in Table 1 [14,15]. Two fibres aligned parallel to each other with spacing of :2.0 mm were sandwiched between PMMA plates and subsequently hot-pressed in ambient air. The hot pressing temperature and pressure were 473 K and 2.5 MPa, respectively. The hot-pressed composite was mechanically cut into a double cleavage drilled compression (DCDC) speci- * Corresponding author. Tel.: +81 3 34026231; fax: +81 3 34026350; e-mail: kagawa@iis.u-tokyo.ac.jp 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S09 21- 5093(98)00603 - 0