The influence of fiber orientation on the crack propagation behavior was studied with single edgenotched specimens which were cut from an injection-molded plate of short-fiber reinforced plastics of polyphenylenesulphide (PPS) reinforced with 30wt% carbon fibers. Specimens were cut at five fiber angles relative to the molding direction, i.e. ??= 0° (MD), 22.5°, 45°, 67.5°, 90° (TD). Fracture mechanics parameters derived based on anisotropic elasticity were used as a crack driving force. Macroscopic crack propagation path was nearly perpendicular to the loading axis for the cases of MD and TD. For the other fiber angles, the crack path was inclined because the crack tended to propagate along inclined fibers. For mode I crack propagation in MD and TD, the resistance to crack propagation is improved by fiber reinforcement, when the rate is correlated to the range of stress intensity factor. The crack propagation rate, da/dN, was slowest for MD and fastest for TD. For each material, the crack propagation rate is higher for larger R ratio. The effect of R ratio on da/dN diminished in the relation between da/dN and the range of energy release rate, ?GI. Difference among MD, TD and matrix resin becomes small when da/dN correlated to a parameter corresponding the crack-tip radius, H?GI, where H is compliance parameter. Fatigue cracks propagated under mixed loading of mode I and II for the fiber angles other than 0° and 90°. The data of the crack propagation rate correlated to the range of total energy release rate, ?Gtotal, lie between the relations obtained for MD and TD. All data of crack propagation tend to merge a single relation when the rate is correlated to the range of total energy release rate divided by Young’s modulus.
The effect of test temperature on the fatigue crack propagation behavior was studied with center-notched specimens of PPS (polyphenylene sulfide) reinforced with 30 wt% short carbon fibers. Specimens were cut from injection-molded plates with 1 mm thickness at three angles of the loading axis relative to the molding flow direction, i.e. θ = 0° (MD), 45°, 90° (TD). Crack propagation tests were conducted under the stress ratio of 0.1 at four temperatures below and above the glass transition temperature T g = 363 K:room temperature (RT = 298 K), 343 K, 373 K and 403 K. The macroscopic crack path was nearly perpendicular to the loading axis for MD and TD at all temperatures. The crack growth direction of 45° plates was inclined against the loading axis, and the inclined angle was decreased with increasing temperature. In the relation between the crack propagation rate, da/dN, and the stress intensity factor range, ΔK, da/dN was slowest for MD, and increased with increasing fiber angle at all temperatures. The crack propagation rate of each fiber angle was nearly the same at RT and 345K, and increased greatly at temperatures of 373 K and 403K above T g . When da/dN was correlated to the J-integral range, ΔJ, the relations for different fiber angles came closer at each temperature, and also for each fiber angle the influence of test temperature on da/dN was decreased. The inelastic deformation of the matrix was mainly responsible for the acceleration of crack propagation seen in da/dN vs ΔK relation at high temperatures.
The internal stress in crystalline thermoplastics, polyphenylene sulphide (PPS), reinforced by carbon fibers of 30 mass% was measured by the diffraction method using synchrotron with energy of 12.3 keV. The stress in the matrix was determined by the sin 2 method with side-inclination optics of transmitted X-ray diffractions. Using skin-layer strips cut parallel, perpendicular and 45° to the molding direction of the injection molded plates, the matrix stress was measured under the uniaxial applied stress. The matrix stresses in the fiber direction, , were determined. The coefficients determined by the transmission method are fairly close to the reported values determined by the reflection method. The experimental values were at least qualitatively agreed with the prediction derived based on micromechanics. The quantitative difference between experiment and prediction is mainly due to the neglect of the distribution of fiber orientations in the micromechanics prediction. Tensile residual stresses were measured in the matrix in the transmission method and were larger in the fiber direction than in the parallel direction. These residual stresses were caused by the mismatch of the thermal expansion coefficient between matrix and fibers.
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