Designing Interfaces
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the toughness of structures with appropriately oriented interfaces can be increased. The problem of a crack terminating at right angles on an interface between two dissimilar but isotropic média was solved by Swenson and Rau2 and others.3-4 Hère, we présent the results for the problem of a crack impinging on an interface between orthotropic and anisotropic média with the principal material axes parallel and perpendicular to the interface. From hère we develop the crack deflection criterion applicable to spécifie cases such as a coated Pitch-55 carbon fiber.5 The results are condensed into a design chart that allow determining a maximum permissible interface strength for a given set of elastic constants for the coating and reinforcement if the strength of the reinforcement is also given. This treatment tells us that low modulus coatings allow us to design higher interface strengths into the composites. A higher interface strength can be translated into higher transverse strengths. This micromechanics analysis coupled with synthesis techniques and direct measurements of interface strengths or intrinsic work of fracture values allow us to design, synthesize, and evaluate interfaces.67 In short, with thèse tools we can engineer the material from the inside out. In composite Systems, it is often easier to measure the work of fracture. Gupta's article, "An Evaluation of the Interface Tensile Strength-Toughness Relationship," treats the relationship between the cohesive strength of the interface and the intrinsic toughness of the interface through the use of the universal bonding corrélations.8-9 This work points to the importance of segregants on the grain boundary or bi-material interfaces and of how they may control the intrinsic work of fracture and hence the cohesive strength of the interface, (which can be measured directly for model laminate Systems by the laser spallation technique described in the lead article). The thermodynamic properties of the liquid metal/ceramic interface is of inter-
est to the materials processor and is the subject of the article by K.C. Russell, S-Y. Oh, and A. Figueredo. Silicon carbide particles are stirred into Al-Si alloys in the primary métal matrix composite commercially produced by Duralcan. Since thèse particles are not readily wetted by the matrix, work must be done on the System to force wetting. This work is termed the work of immersion and is equal to the différence between the wetted and the unwetted surface énergies per unit surface area [W, = S(ysl - y„)]. The work of immersion of a reinforcement preform can be determined dynamically from pressure infiltration experiments10 in which the threshold pressure for infiltration is observed, or by sessile drop experiments in which contact angles are measured. A knowledge of the energetics of surfaces profoundly affects the processing économies of this class of materials. The work of adhésion can also be deduced from thèse measurements and related to the intrinsic work of fracture of interfaces. Russell et al. review both the expé
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