9. Interfaces and Interphases

Several types of bonds may exist at the juncture between adjacent phases in contact. On the microscale of many composite materials, most desired is a perfect bond along a sharp spatial boundary S of vanishing thickness. It guarantees that both traction and displacement vectors remain continuous on S. Contact between phase surfaces may also involve presence of one or more interphases, thin bonded layers of additional homogeneous phases introduced, for example, as coatings on particles or fibers, or as products of an interfacial chemical reaction. During composites manufacture and/or loading, an interface is expected to transmit certain tractions between adjacent constituents. When the resolved tensile and/or shear stress reaches a high magnitude, the interface may become imperfect by allowing partial or complete decohesion, a displacement jump, possibly accompanied by a distribution of ‘adhesive’ tractions. In an opposite situation, a high compressive stress may cause radial cracking in one of the phases in contact, or in the surrounding matrix. While magnitudes of interface tractions determine material propensity to distributed damage, the work required by either decohesion or radial cracking must be provided by release of potential energy, which is proportional to phase volume Chap. 5. Therefore, small inhomogeneities are less likely sources of damage than large ones.