Project Leader: Otmar Kolednik

The material inhomogeneity effect has been tested experimentally and has given results as predicted by the numerical analyses with the configurational force concept. Inspired by the composite architecture of especially fracture resistant biomaterials, multilayers have been produced, consisting of a high-strength aluminium alloy Al7075 as bulk material. Two types of composites were investigated: In the first, the aluminium sheets are separated by polymer layers, so that both the Young’s modulus and the yield strength of the constituents are different. In the second type, layers of pure aluminium are introduced by roll bonding; in this case only the yield strength differs.

Al7075-multilayers with interlayers made of polymer (left) and pure aluminium (right) exhibit a tremendous increase in fracture toughness. The crack becomes ineffective after crack arrest in the interlayer, and the specimen behaves like in a tensile test. In both composites, crack arrest originates purely from the material inhomogeneity effect and happens without interface delamination. Delamination starts later during load increase, caused by lateral contraction of the bulk material.

Both multilayers exhibit a very strong increase in fracture resistance compared to the homogeneous bulk material. The composite with polymer interlayers shows also a very strong increase in fatigue resistance, whereas the improvement is not so strong in the composite with aluminium interlayers.

Important publications

R. Pippan, K. Flechsig, Fatigue crack propagation behavior in the vicinity of an interface between materials with different yield stressesMaterials Science and Engineering A283(2000) 225-233.

J. Zechner, O. Kolednik, Paper multilayer with a fracture toughness of steelJournal of Materials Science 48 (2013) 5180–5187.

J. Zechner, O. Kolednik, Fracture resistance of aluminum multilayer compositesEngineering Fracture Mechanics 110 (2013) 489–500.

O. Kolednik, J. Predan, F.D. Fischer, P. Fratzl, Improvements of strength and fracture resistance by spatial material property variationsActa Materialia 68 (2014) 279-294.

M. Sistaninia, O. Kolednik, Effect of a single soft interlayer on the crack driving forceEngineering Fracture Mechanics 130 (2014) 21–41.