y Kagawa K aterials Science and Engineering 4250(1998)285-290 Crack growth direction Crack Front Crack length Crack front Curvature radius Fig 3. Schematic drawing of the definition of the crack length. c, and crack curvature, p. curves of both pure PMMA and the composite showed from the hole edge perpendicular to the main crack in a non-linear deformation up to the total fracture of the both the pure PMMa and the composite specimens. specimen Then the crack proceeded rapidly and the specimen was Fig 5(a) and(b)show the fracture appearance of the completely separated into two pieces. Note that multi pure PMMA and the composite, respectively. Under ple fibre fracture was observed(Fig. 5(b)) because of loading, the cracks in pure PMMA and the PMMA he smaller fracture strain of the sio, fibre than that of matrix of the composite began to grow from the pre- the PMMA matrix crack tip and proceeded straight, and the crac A typical example of the crack growth process of the was parallel to the loading direction. After satu composite obtained by the direct observation is shown f the applied load(when the crosshead had in Fig. 6, and a schematic drawing of the crack growth over x 4 mm), the other, opposite-sided, crack initiated behaviour is shown in Fig. 7. The white dotted line in Fig 6 indicates the observed matrix crack front. Con idering the change of the crack front shape caused by interaction with the fibre, the crack growth process of the composite was divided into three characteristic stages, i.e., elastic constraint stage (stage I),matrix 000 crack bowing stage(stage II)and fibre bridging stage (stage IIl). At stage I, the crack front had positive curvature,pt, and the crack front shape was identical to that in pure PMMA. As the matrix crack front at the midsection of the specimen reached a l mm from the fibre, the crack decelerated only in front of the fibre Crosshead displacement /mm and the curvature became negative, i.e. p, Finally the crack front became a mixture of positive curvature, p+, and negative curvature, p The thermally induced stresses generated in the ma ix are tensile in the fibre axial direction, g? sive in the radial direction al and tensile in the circumference direction, a B. The thermal stresses in the matrix are estimated by following ref. [16 and the result is shown in Fig. 8. Note that in this reference a assumed to be constant with radial distance while the hermal stresses aT and al. decrease with the inverse of the square of the radial distance. The tensile stress in Crosshead displacement/mm the fibre axial direction accelerates the crack growth Fig. 4. Gross applied stress -crosshead displacement curve of both [12], and thus, thermal stress cannot be the reason for pure PMMA (a) and the composite(b) the observed crack deceleration The deceleration of theY. Kagawa, K. Goto / Materials Science and Engineering A250 (1998) 285–290 287 Fig. 3. Schematic drawing of the definition of the crack length, c, and crack curvature, r. curves of both pure PMMA and the composite showed a non-linear deformation up to the total fracture of the specimen. Fig. 5(a) and (b) show the fracture appearance of the pure PMMA and the composite, respectively. Under loading, the cracks in pure PMMA and the PMMA matrix of the composite began to grow from the pre￾crack tip and proceeded straight, and the crack plane was parallel to the loading direction. After saturation of the applied load (when the crosshead had moved over :4 mm), the other, opposite-sided, crack initiated from the hole edge perpendicular to the main crack in both the pure PMMA and the composite specimens. Then the crack proceeded rapidly and the specimen was completely separated into two pieces. Note that multi￾ple fibre fracture was observed (Fig. 5(b)) because of the smaller fracture strain of the SiO2 fibre than that of the PMMA matrix. A typical example of the crack growth process of the composite obtained by the direct observation is shown in Fig. 6, and a schematic drawing of the crack growth behaviour is shown in Fig. 7. The white dotted line in Fig. 6 indicates the observed matrix crack front. Con￾sidering the change of the crack front shape caused by interaction with the fibre, the crack growth process of the composite was divided into three characteristic stages, i.e., elastic constraint stage (stage I), matrix crack bowing stage (stage II) and fibre bridging stage (stage III). At stage I, the crack front had positive curvature, r+, and the crack front shape was identical to that in pure PMMA. As the matrix crack front at the midsection of the specimen reached :1 mm from the fibre, the crack decelerated only in front of the fibre and the curvature became negative, i.e. r−,. Finally, the crack front became a mixture of positive curvature, r+, and negative curvature, r−. The thermally induced stresses generated in the ma￾trix are tensile in the fibre axial direction, sz T, compres￾sive in the radial direction, sr T, and tensile in the circumference direction, su T. The thermal stresses in the matrix are estimated by following ref. [16] and the result is shown in Fig. 8. Note that in this reference sz T is assumed to be constant with radial distance while the thermal stresses sr T and su T, decrease with the inverse of the square of the radial distance. The tensile stress in the fibre axial direction accelerates the crack growth [12], and thus, thermal stress cannot be the reason for the observed crack deceleration. The deceleration of the Fig. 4. Gross applied stress–crosshead displacement curve of both pure PMMA (a) and the composite (b)