The material inhomogeneity effect offers new possibilities for the development of novel damage-tolerant and fracture-resistant materials and components. The idea is to introduce intentional variations of material properties and/or residual stresses so that the crack driving force becomes low and a crack stops to grow.
Inspired by the composite architecture of especially fracture resistant biomaterials, the skeletons of deep-sea glass sponges, criteria have been developed for the optimum design of lamellar structures consisting of materials with spatial variations of the Young’s modulus (Kolednik et al. 2011, 2014). These theoretical studies demonstrate that the introduction of thin, compliant interlayers into a high-strength material can lead to a composite which is able to combine high stiffness, high strength and high fracture toughness. Hereby, the wavelength of the composite, i.e. the distance between the compliant interlayers, plays a crucial role.
Skeleton of the deep sea sponge Euplectella aspergillum (venus flower basket). Although the thin rods consist to 95% of brittle bio-glass, the structure is extremely fracture resistant. The tiny rods have a layered microstructure with very thin, soft protein layers between the glass layers.
In materials with constant Young’s modulus, spatial variations of the yield stress can also greatly improve the fracture resistance. The reason is that the crack driving force strongly decreases when the crack is located near the second interface of a soft interlayer. Numerical analyses enable us to derive optimum interlayer configurations, i.e. for a given bulk material, the yield stress and thickness of the soft interlayer are determined so that the crack driving force exhibits an absolute minimum (Sistaninia and Kolednik, 2014).
Part of this research has been funded by the Austrian COMET Competence Center Programme via the COMET K2 Center for Materials, Processing and Product Engineeringin Leoben. Cooperation partners Max Planck Institute of Colloids and Interfaces and Institute of Mechanics, Montanuniversität Leoben.
O. Kolednik, The yield-stress gradient effect in inhomogeneous materials. International Journal of Solids and Structures 37 (2000) 781-808.
P. Fratzl, H.S. Gupta, F.D. Fischer, O. Kolednik, Hindered crack propagation in materials with periodically varying Young's modulus – Lessons from biological materials. Advanced Materials 19 (2007) 2657-2661.
O. Kolednik, J. Predan, F.D. Fischer, P. Fratzl, Bio-inspired design criteria for damage-resistant materials with periodically varying microstructure. Advanced Functional Materials 21 (2011) 3634-3641.
O. Kolednik, J. Predan, F.D. Fischer, P. Fratzl, Improvements of strength and fracture resistance by spatial material property variations, Acta Materialia 68 (2014) 279-294.
M. Sistaninia, O. Kolednik, Effect of a single soft interlayer on the crack driving force, Engineering Fracture Mechanics 130 (2014) 21–41.