Material behaviour during crack propagation

According to equation (5.4), the critical stress of a pre-cracked specimen is given by 

There are differing definitions of stable and unstable crack propagation in the literature, based on the external load, as done here and in Gross [58], or on the energy balance [21]. 

The situation is analogous to that in plastic deformation The yield strength Rp characterises the stress at the onset of plastic deformation, the actual flow stress tf denotes the current stress during yielding. 

Sometimes, this transition between stable and unstable crack propagation at Ki is called Kic, for example in the standard astme561. In this book, Kic always denotes the value of the stress intensity factor at which crack propagation starts. 

As the figures 5.12(b) and 5.12(c) illustrate, an increasing crack-growth resistance Km does not guarantee stable crack propagation because the stress can still decrease. 

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Pressure on resources had materials science improving its products in leaps and bounds - not just metals but also the earlier polymers and natural fibres. The concept of fracture mechanics had been introduced during the First World War by the aeronautical engineer Alan Arnold Griffith (1893-1963) to explain brittle failures. The microscopic behaviour of materials, the way cracks propagated from surface flaws, led to the development of a particular palette of structural materials suited to severe conditions - elastic and ductile, with reserves of energy absorption when approaching failure point. 

The unloading-reloading loops in the load-deflection diagrams allowed to calculate the compliance of the beam during crack propagation. These results were compared with the outcomes of a FEM analysis with the STRUDL program, in which material behaviour is assumed to be fully elastic. A... 

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