Criteria for unstable crack extension under displacement control

Cracks in components can extend under load or displacement control. Under displacement control usually the cracks extend first stable under decreasing load and afterwards unstably. The critical load for the onset of unstable cracks extension is dependent on the material, the geometry of the crack and of the component and on the compliance of the loading system. In this report general criteria for the relation between compliance and cracks size are developed. In particular the relations for linear-elastic fracture mechanics for flat and rising crack growth resistance curves, the plastic instability criterion and the J-integral methods are discussed. (orig.)