Composite materials are known for their high stiffness and strength at lower weight as compared to conventional structural materials. Recently, there has been a growing interest in finding the new ways to decrease delamination failure, which is a life limiting factor of laminated composites. This review paper emphasizes on the effects of different reinforcement structures on mode I fracture toughness and possible ways to improve fracture toughness. A brief description on intrinsic and extrinsic mechanisms of crack growth has been discussed along with the earlier investigations and recent developments for mode I fracture toughness testing. Factors that affect the fracture toughness are also discussed. A brief knowledge of mode I fracture toughness of traditional and advanced fiber-reinforced composites is given, which could help researchers to understand fracture behaviors of composites and thus, it can help engineers to design composites with higher interlaminar strength.

This paper reports the mode I interlaminar fracture toughness and fracture mechanisms of two-dimensional (2D) plain woven composite and three-dimensional (3D) angle-interlock woven composite. The fracture toughness behaviors were tested with double cantilever beam method at the different loading rates from 0.5 to 100 mm/min. Critical strain energy release rate was calculated to compare the difference between the 2D and the 3D woven composites. The fractographs were photographed with scanned electronic microscopy and optical microscopy to show the fracture morphologies. We found that the 3D angle-interlock woven composite has high fracture toughness than that of 2D woven composite. The binder yarns resist the crack initiation and propagation to increase the fracture toughness. While the lower in-plane stiffness of the 3D woven composites should be considered fully for designing the 3D woven composites.

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Composite materials have found widespread applications in the automotive, aerospace, and building industries. Several components are joined together for these applications, by some temporary or permanent bonding approach. The increased use of different materials and their combinations such as composites makes the whole joining process something to be thoroughly considered before continuing. Several aspects need to be studied before spending significant time and financial resources. Considering these challenges in this paper we have provided a review of the investigations that have been made on fiber-reinforced composite joints. The level of development in various types of joints and joining techniques such as mechanical bonding, adhesive bonding, and fusion bonding along with their advantages and disadvantages is given. Several parameters affecting the performance of composite joints such as joint configuration, material selection and properties, geometric parameters, dominating failure modes, and environmental factors are described briefly. To verify the performance of composite joints, guidance on joint testing is given (both destructive and non-destructive).

Fracture behaviors of three-dimensional braided composites could be designed from braided preform structures. Here, we compare the fracture behaviors of three-dimensional four direction (3D4d) and five directional (3D5d) braided composites under impact loading. A drop weight impact tester combined with a high-speed photography system was employed to characterize the progressive damages and fracture behaviors, which include crack growth and crack mouth opening displacement. The two-dimensional digital image correlation method was utilized for evaluating deformation contours and strain field. A finite element analyses model was established to reveal impact damage evolution, crack growth, stress distribution, and energy absorption at microstructure level. We found that axial yarns had great effect on the crack propagation paths. The stress at the notch bottom caused yarn breakage and then resulted in a sharply growing crack. The axial yarns in the 3D5d braided composites impede the progress of cracks under transverse impact. The 3D5d braided composites have smaller crack mouth opening displacement than that of the 3D4d composites. The braided preform is the main energy absorption component. With the same volume fraction, axial yarns exhibit the highest energy absorption. Due to the existence of axial yarns, the resistance to the fracture and crack growth of the 3D5d braided composites is better than the 3D4d braided composites.