Directional-to-isotropic transitions in metal/matrix composite joints

The mechanical behavior of transition joints between continuous-fiber metal/matrix composites and non-reinforced or discontinuously reinforced metallic sub-elements is discussed. The broader themes are (i) the role of joint geometry relative to the strength of the tensile and shear interfaces and (ii) the effect of particle reinforcement of the isotropic sub-element. The study focuses on the deformation phenomena at the directional-isotropic transition and the relevant interfaces. Model joints were manufactured by pressure infiltration of molten Al–4.5%Mg into suitable preforms, a technique that minimizes gross interfacial defects and the ensuing variability of joint strength. In general, joint strength increases with insertion ratio owing primarily to the contribution of shear at the lateral interfaces. Experimental results combined with finite-element analysis highlight the critical roles of plastic constraint and stress concentrations arising from the geometry and property mismatch. The plastic constraint can operate in two size scales, one associated with the macroscopic dimensions of the transition, and the other with the reinforcing particles, when present. Both effects increase the load-carrying ability of the joint — notably through the interface formed by the termination of the continuous fibers — with attendant implications for joint design. The mechanisms that initiate failure are discussed in the context of the stress and strain profiles near this interface in combination with data from other joint configurations.