Y. Kagawa, K Goto/ Materials Science and Engineering 4250(1998)285-290 Table I Properties of fibre and matrix Light Properties Sio, glass PMMA Strain Tensile strength(MPa) ≈70 Coefficient of thermal expansion (x CCD camera =6=1 Toshiba Ceramics, 9R2b Mitsubishi Rayon, Shinkolite* Fig. 2. Experimental set-up for direct observation. en[17]. Fig. I shows the shape and the dimensions of area was fixed to the specimen surface parallel to the the dCDC specimen. This specimen was chosen in fibre axis. The strain gage was mounted on the speci rder to have a slow matrix crack growth and to enable men surface at the crossing point of the fibre axis and direct observation of the crack front shape. All the the line 10 mm from the side end of the specimen.Ten surfaces of the specimen were mechanically ground and specimens were tested to obtain the common fracture polished to a 0 I um diamond paste finish. The nomina behaviour ibre volume fraction of the specimen was 0.02.A Fig. 2 shows the experimental set-up of the compres- hole(diameter 2r=8.0 mm) was drilled at the centre of sion splitting test. The crack growth behaviour during the DCDC specimen and natural pre-cracks were in- the test was directly observed from the direction per duced perpendicular to the fibre axis from the hole by pendicular to the crack plane by a CCD camera at bushing a sharp knife onto the edge of the hole. Fi- tached to a three-axes secure base. The crack front lly, two opposite-sided pre-cracks were carefully in erve duced in the DCDC specimen. The pre-crack lengths in light intensity [9]. The images from the CCD camera all the DCDC specimens were about 2 mm from the were stored in a video tape recorder at a rate of 30 hole edge. The ratio of the initial crack length from the images s. Some selected images during the compres- hole edge, co, to the hole radius, r, was co/r g 0.25, anc sion splitting test were picked up from the video record the distance between the initial crack tip to the fibre- ing to measure the total crack length, c. The definition matrix interface was x 4.0 mm. The DCDC specimen of the crack length used in this paper is shown in Fig of pure PMMA, hot-pressed under the same tempera- 3. The crack length, c, was defined as the length from he composite, was also prepare the hole edge to the crack front at the midsection of the for a comparison of the crack growth behaviour. The specimen through the thickness direction because the shape, dimensions and initial notch depth of the pure crack at the midsection of the specimen of pure PMMA PMMA specimen were completely identical to that of tended to grow faster than the crack at the surface the composite specimen A compression splitting test was performed at a defined as the crack front where it touches the interface constant crosshead speed of I mm min-I using ar The crack front curvature is also defined in Fig. 3: p Instron testing machine(Model 4204, Instron Corp, represents positive curvature,p represents negative USA)at 297 K in air. The specimen was sandwiched curvature, and when the crack front is straight p=oo between the rigid steel plates, and the load was applied perpendicular to the fibre axis. To measure the change of longitudinal strain near the fibre during the test, a 3. Results and discussion wire-wound strain gage with 2 x 5 mm- effective gage crosshead displacement relation of pure PMMA and dco points plotted 40 20 applied stress-crosshead displacement curve indicate ac the images selected to measure the crack length, c. Both near relation before the start growth from the pre-crack front. The stress for the Initial Crack Strain gage onset of non-linearity of the composite was larger than mm that of pure PMMa because of the reinforcing effect of Fig. 1. Shape and dimensions of the DCDC specimen the sio, fibre. After the onset of crack growth, the286 Y. Kagawa, K. Goto / Materials Science and Engineering A250 (1998) 285–290 Table 1 Properties of fibre and matrix Properties PMMAb SiO2 glassa Young’s modulus (GPa) 70 3.3 Poisson’s ratio 0.14 0.3 Tensile strength (MPa) :70 :78 Coefficient of thermal expansion (× 0.5 60 10−6 K−1 ) a Toshiba Ceramics, c9R2b Mitsubishi Rayon, cShinkolite® Fig. 2. Experimental set-up for direct observation. men [17]. Fig. 1 shows the shape and the dimensions of the DCDC specimen. This specimen was chosen in order to have a slow matrix crack growth and to enable direct observation of the crack front shape. All the surfaces of the specimen were mechanically ground and polished to a 0.1 mm diamond paste finish. The nominal fibre volume fraction of the specimen was :0.02. A hole (diameter 2r=8.0 mm) was drilled at the centre of the DCDC specimen and natural pre-cracks were in￾duced perpendicular to the fibre axis from the hole by pushing a sharp knife onto the edge of the hole. Fi￾nally, two opposite-sided pre-cracks were carefully in￾duced in the DCDC specimen. The pre-crack lengths in all the DCDC specimens were about 2 mm from the hole edge. The ratio of the initial crack length from the hole edge, c0, to the hole radius, r, was c0/r:0.25, and the distance between the initial crack tip to the fibre– matrix interface was :4.0 mm. The DCDC specimen of pure PMMA, hot-pressed under the same tempera￾ture and pressure as the composite, was also prepared for a comparison of the crack growth behaviour. The shape, dimensions and initial notch depth of the pure PMMA specimen were completely identical to that of the composite specimen. A compression splitting test was performed at a constant crosshead speed of 1 mm min−1 using an Instron testing machine (Model 4204, Instron Corp., USA) at 297 K in air. The specimen was sandwiched between the rigid steel plates, and the load was applied perpendicular to the fibre axis. To measure the change of longitudinal strain near the fibre during the test, a wire-wound strain gage with 2×5 mm2 effective gage area was fixed to the specimen surface parallel to the fibre axis. The strain gage was mounted on the speci￾men surface at the crossing point of the fibre axis and the line 10 mm from the side end of the specimen. Ten specimens were tested to obtain the common fracture behaviour. Fig. 2 shows the experimental set-up of the compres￾sion splitting test. The crack growth behaviour during the test was directly observed from the direction per￾pendicular to the crack plane by a CCD camera at￾tached to a three-axes secure base. The crack front shape was easily observed by the change of reflected light intensity [9]. The images from the CCD camera were stored in a video tape recorder at a rate of 30 images s−1 . Some selected images during the compres￾sion splitting test were picked up from the video record￾ing to measure the total crack length, c. The definition of the crack length used in this paper is shown in Fig. 3. The crack length, c, was defined as the length from the hole edge to the crack front at the midsection of the specimen through the thickness direction because the crack at the midsection of the specimen of pure PMMA tended to grow faster than the crack at the surface. After the crack front reaches the fibre, the crack tip was defined as the crack front where it touches the interface. The crack front curvature is also defined in Fig. 3: r+ represents positive curvature, r− represents negative curvature, and when the crack front is straight r=. 3. Results and discussion 3.1. Crack growth beha6iour Fig. 4(a) and (b) show the typical applied stress– crosshead displacement relation of pure PMMA and the composite, respectively. The points plotted in the applied stress–crosshead displacement curve indicate the images selected to measure the crack length, c. Both curves show a linear relation before the start of crack growth from the pre-crack front. The stress for the onset of non-linearity of the composite was larger than that of pure PMMA because of the reinforcing effect of Fig. 1. Shape and dimensions of the DCDC specimen. the SiO2 fibre. After the onset of crack growth, the