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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.8, #6)
Loading Rate Dependence of Mode II Fracture Behavior in Interleaved Carbon Fibre/Epoxy Composite Laminates by Wei Jiang; F. F. Y. Tsang; S. C. Tjong; R. K. Y. Li; J. K. Kim; Yiu-Wing Mai (pp. 361-369).
The mode II delamination fracture toughness (G IIC ) for interleaved and non-interleaved CFRP were measured at loading rates ranging from 0.2 to 125 mm/min. It was observed that interleaving could enhance G IIC for CFRP significantly. Furthermore, G IIC was observed to be independent of the loading rate for the non-interleaved CFRP. For the interleaved CFRP, the loading rate has a strong influence on G IIC , and the loading rate dependence can be explained in terms of the fracture surface morphologies.
Keywords: CFRP; mode II fracture; interleaving; loading rate dependence; delamination fracture
Properties of Weft Knitted Composites Affected by Preform Stretching by Goran Demboski; Gordana Bogoeva-Gaceva (pp. 371-384).
The influence of preform stretching on mechanical properties of weft knitted PP/glass fibre composites was estimated. Stretching of Rib 1:1 weft knitted fabric was performed prior to consolidation in a wale or course direction and biaxially. Preform stretching increased tensile and bending strength of composite as a result of additional orientation of the reinforcing fibres in the stretching direction. Stretching in a wale direction was shown to be most effective due to the specific inherent structure of Rib 1:1 fabric. The impact toughness of composites was not significantly changed as a result of decreased preform elasticity in the direction of stretching.
Keywords: glass fibre/polypropylene composites; weft knitted preforms; stretching; mechanical properties
Fatigue Propagation of Fibre-Bridged Cracks in Unidirectional Polymer-Matrix Composites by E. K. Gamstedt; S. Östlund (pp. 385-410).
In order to improve the fatigue resistance of polymer-matrix composites by materials design, or to conceive micromechanics based models for life predictions, the underlying micromechanisms must be understood. Experimental investigations have revealed fibre-bridged cracking as a toughening micromechanism that retards further fatigue crack growth in a unidirectional 0° carbon-fibre-reinforced epoxy. The bridging fibres exert a closing traction on the crack surfaces, thereby reducing the driving force for crack growth. In this study, the growth of bridged cracks has been quantified by a surface replication technique. The da/dN–ΔK curve defined in terms of nominal stress-intensity factors shows a crack retarding behaviour. The crack growth curve can be replotted in terms of the effective stress-intensity factor where the contribution of the cohesive crack surface forces from the bridging fibres are taken into account. This curve falls somewhat closer to that of the neat matrix material, but the difference is still considerable, and it shows a decelerating propagation. Therefore, there must be other active toughening mechanisms besides fibre bridging, that slow the crack propagation down, and account for the fatigue resistant behaviour of the tested material. Ways by microstructural design to promote the fatigue resistant mechanisms are discussed.
Keywords: fatigue; fibre bridging; crack propagation; polymer matrix composite (PMC); Paris law
Effect of Crack Propagation Directions on the Interlaminar Fracture Toughness of Carbon/Epoxy Composite Materials by J. H. Hwang; C. S. Lee; W. Hwang (pp. 411-433).
The interlaminar fracture toughness of carbon/epoxy composite materials has been studied under tensile and flexural loading using width-tapered double cantilever beam (WTDCB) and end-notched flexure (ENF) specimens. This study experimentally examines the effect of various interfacial ply orientations, α (0°, 45° and 90°) and crack propagation directions, θ (0°, 15°, 30° and 45°) in terms of the critical strain energy release rate. Twelve differently layered laminates were investigated. The fracture energy is deduced from the data according to the compliance method and beam theory. Beam theory is used to analyze the effect of crack propagation direction. The geometry and lay-up sequence of specimens are designed to probe various conditions such as skewness parameter and beam volume. Results show that fiber bridging occurred due to non-midplane crack propagation; this causes the difference in fracture energy calculated by both methods. For the construction of safer and more reliable composite structures, we obtain the optimal stacking sequence from the initial fracture energy in each mode.
Keywords: fracture toughness; interfacial ply orientation; crack propagation direction; delamination; carbon/epoxy composites; width tapered double cantilever beam (WTDCB); end notched flexure (ENF); crack-tip splitting