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The introduction of thin interlayers with low yield stress can greatly improve the strength and the fracture toughness of inherently brittle materials. The reason is that the spatial yield stress variation affects the crack driving force, which strongly decreases when the crack tip is located in the interlayer region, near the boundary to the hard matrix material. This can lead to crack arrest. The material inhomogeneity effect appears without previous delamination of the interlayer. The decisive parameters influencing the effect are the interlayer spacing (the wavelength of the yield stress variation), the interlayer thickness and the yield stress ratio between interlayer and matrix. Based on numerical simulations with the configurational forces concept, it is demonstrated how the architectural parameters of the multilayer must be chosen in order to enhance the fracture stress and the fracture toughness of the material. An iterative procedure is proposed to find the optimum configuration. It is found that the optimum wavelength is inversely proportional to the square of the applied stress. A similar relation is given for composites with spatial variations in Young's modulus.

A material with spatial variation in the elastic modulus E can have a much higher apparent fracture resistance and fracture stress than a comparable homogeneous material. The effect occurs due to the strong decrease of the crack driving force, which leads to crack arrest when the crack tip is located in the region with low elastic modulus. From the results of exemplary numerical studies and simple fracture mechanical considerations, models are derived in order to predict the fracture stress and fracture toughness of the inhomogeneous materials. It is shown that high values of fracture stress and fracture toughness can be reached if the amplitude of the E variation is high enough to provide crack arrest and the wavelength of the E variation is small. The beneficial effect of material property variations also occurs if the width of the compliant region is very thin and the loss in stiffness of the structure is almost negligible. The concept is applicable for various types of composite materials; examples are presented.

A procedure to predict the fracture toughness of a ceramic multilayer composite made of different phases is presented. The procedure requires experiments to measure the intrinsic fracture toughness of the phases and to determine the required material data. The numerical modeling includes a conventional finite element stress analysis and the calculation of the crack driving force based on the concept of configurational (material) forces. The procedure yields the fracture toughness of the composite as a function of the crack length. A bending bar consisting of layers made of alumina and an alumina–zirconia composite is investigated. The bar has a crack perpendicular to the interfaces. The spatial variations of both the thermal residual stresses and the elastic modulus induce shielding and anti-shielding effects on the crack, which are quantified. The numerically predicted fracture toughness is compared with the experimental values.

An Al6061-20%Al 2 O 3 powder metallurgy (PM) metal matrix composite (MMC) with a strongly clustered particle distribution is subjected to equal channel angular pressing (ECAP) at a temperature of 370 °C. The evolution of the homogeneity of the particle distribution in the material during ECAP is investigated by the quadrat method. The model proposed by Tan and Zhang [Mater Sci Eng 1998;244:80] for estimating the critical particle size which is required for a homogeneous particle distribution in PM MMCs is extended to the case of a combination of extrusion and ECAP. The applicability of the model to predict a homogeneity of the particle distribution after extrusion and ECAP is discussed. It is shown that ECAP leads to an increase of the uniformity of the particle distribution and the fracture toughness.

For a nitrogen alloyed ferritic-austenitic duplex steel X3CrMnNiMoN 25 6 4 the crack growth resistance is recorded in terms of the J -integral, J , and the energy dissipation rate, D . Due to the anisotropic microstructure of elongated ferrite and austenite domains, very different resistance curves appear for three different crack plane orientations. Metallographic and stereophotogrammetric studies lead to a model which is able to explain these curves. It is found that the geometrical arrangement of the (brittle) ferrite and the (ductile) austenite domains is the main influence factor on the crack growth resistance (and not the volumetric fraction of ferrite and austenite). Since no standard J IC -values can be determined, a procedure is proposed to evaluate an alternative measure of the fracture initiation toughness, i.e. “quasi-initiation” J -integral values.