TABLE OF CONTENTS Conventional fiber-reinforced polymer (FRP) laminates have a layered two-dimensional construction. The lack of reinforcement in the through-thickness or z-direction results in the laminates having low interlaminar strength and fracture resistance.
Laminated two-dimensional composites are not suitable for applications where through-thickness stresses may exceed the (low) tensile strength of the matrix (or matrix/fiber bond) and in addition, to provide sufficient residual strength after anticipated impact events, two-dimensional laminates must be made thicker than required for meeting strength requirements. The resulting penalties of increased cost and structural weight provide impetus for the development of more damage-resistant and tolerant composite materials and structures.
Considerable improvements in damage resistance can be achieved by using tougher thermoset or thermoplastic matrices together with optimized fiber/matrix bond strength. However, this approach can involve significant costs, and the improvements that can be obtained are limited. There are also limits to the acceptable fiber/matrix bond strength because high bond strength can lead to increased notch-sensitivity.
An alternative and potentially more effective means of increasing damage resistance and through-thickness strength is to develop a fiber architecture in which a proportion of fibers in the composite are oriented in the z-direction. This fiber architecture can be obtained, for example, by three-dimensional weaving or three-dimensional braiding. A much simpler approach is to apply z-direction reinforcement to a conventional two-dimensional fiber configuration by stitching; however, this does not provide all of the benefits of a full three-dimensional architecture.
In all of these approaches, a three-dimensional preform is first produced and is...
