3D composites are composites manufactured with fiber preforms arranged in complex, 3D shapes. The preform can be woven, braided, or stitched. Then, a resin is applied to the preform to create the composite. Generally, 3D composites are used in the most highly engineered and technical applications where complex mechanical stresses are encountered and precise physical properties are required. Because the fibers are arranged in all directions, they can react to complex stresses and strains in a way that is not possible with other types of composites.
3D woven composites are those where the preform is a single piece of relatively thick fabric which can be formed to its final shape without layering. This type of preform is similar to the 2D weaving process, and allows for the production of fabrics up to 10 cm thick with a fiber volume fraction of around 50%. To create even thicker preforms, fibers can be woven from one layer to another and then back again to lock the two layers together in a process known as angle interlocking. This process is advantageous because of the finished material’s resistance to delamination. In general, 3D woven composites are especially useful when the material will be subject to out-of-plane loading because of the high resistance to delamination. They also are extremely formable and porous, meaning that they can take shape more quickly and easily.
In contrast, 3D braided composites are formed from an extension of the 2D braiding process. Braided composites have many advantages over filament wound composites because they are in general tougher and have a higher fatigue strength. Additionally, when a third set of axial yarns (triaxial braids) is introduced to the braiding process, the braid is much more stable and is nearly isotropic.
And finally, 3D composites can be produced simply and cheaply through stitching in the through thickness with a high strength thread. Basically, a high strength yarn (such as glass, carbon, or Kevlar) is sewn through an uncured laminate. This process produces a composite with a variety of improved in-plane mechanical properties such as tensile and compressive strengths. Alternatively, Z-pinning is a method introduced in the late 1980s that involves embedding cured reinforcement fibers into a thermoplastic foam which is then placed on top of a dry fabric and vacuum bagged. Then, the foam collapses as the temperature and pressure increase, and the fibers slowly are pushed into the dry fabric. This method provides a higher stiffness through the thickness of the finished part.
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