Parametric Analysis of Interlaminar Toughness of Unidirectional Carbon Fiber and Woven Carbon Fabric Composites

The literature review shows that carbon fiber reinforced composites have been widely used in a variety of structural applications in the aerospace, automotive and civil industry [1-19]. High specific modulus (stiffness to weight ratio) and intralaminar tensile fracture toughness [20] are possible the main reasons in the widespread use of these composites. Moreover, the intralaminar tensile fracture toughness is relevant not only to material qualification for the design of composite aerostructures, but also to the definition of the softening laws used in the computational models for predicting the behavior of composite structures [20]. Laminated composite materials made of brittle matrices are susceptible to interlaminar cracks (interlaminar mode of fracture -delamination) and to propagation of that cracks also. Especially low-velocity impact damage and micro-cracks formed during manufacturing, service, or maintenance cause to delamination in laminated composite materials. It is interesting to note that, the performance evaluation of the advanced reinforcing fibers such as carbon [21,22], and epoxy resins [23–26], in final composite is necessary for their safe application, especially for the manufacturing of large light weight structures[3, 5-7].

For the composites of interest, the delamination process is typically brittle. Cracks in the form of delaminations and disbonds are the most common failure modes observed in composite structures [27].One approach to solve this problem involves the use of three-dimensional woven and braided reinforcements [28-32]. Fiber stitching [33] or architected adhesives [34] are also alternative methods for solving such problems. It should be mentioned that interlaminar fracture resistance [35, 36] remains a weakness of polymer composites. Such property indicates the amount of stress required to propagate a pre-existing thin crack. On the other hand, damage tolerance is the desired basic property for various structures depending upon the end application [37]. A more systematic and theoretical analysis is required for fracture toughness characterization of composites which is still on the way of growth as compared to metals.

Several theories have been proposed to composites, some focusing on the fracture toughness associated with fibre-dominated tensile failure [38-42], others on the fracture characteristics of composite laminates and developed a fracture criterion which showed that the critical stress intensity factor for fibre failure is a material constant [43], as well as the tensile intralaminar fracture toughness of woven composite laminates [44]. Woven fiber reinforcement is typically used in applications where multidirectional laminates are required (ship hull). It should be mentioned that woven fabric composites exhibit relatively unstable crack growth compared to unidirectional laminates [45, 46]. Unstable crack growth in woven fabric composites can be observed as the crack jumps between transverse tows. Woven fabrics tend to have heavier tows (e.g., higher filament count) than unidirectional reinforcement. Another parameter that affects the fracture toughness is the width of the specimen which does not affect the fracture toughness of unidirectional composites [47]. Moreover, for the case of woven composites, further investigation is needed to determine size effects of woven fabric composites on fracture toughness.