Crack-extension resistance

Fracture toughness (plane strain) K c The crack extension resistance under conditions of crack tip plane strain. 

The stabihty of crack extension in such materials depends on the rate of change of the appHed driving force to that of the fracture resistance, equation 4. 

For an increment of crack extension, A A is represented by a movement from B to D on the load versus deflection curve, the energy consumed may be represented by the area Attjj. Hence the crack growth resistance may be expressed as... 

Sulphur concretes appear well suited for use in environments corrosive to Portland cement concretes. The extensive work by the U.S. Bureau of Mines shows their material performs admirably in such environments. When used as a lining, the initial stress-strain behaviour will allow the material to adapt to the main structural element and relieve internal stresses without cracking. Corrosion resistance will be maintained by the material thereafter, even though the stress strain behaviour alters. On its own, the material retains sufficient strength to withstand typical loads involved in this type of application (eg. liquid container). Sudicrete has not been tested much in this area, although laboratory tests show similar promise. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

Table II lists the Kq values, K values based on maximum load and initial crack length, and KR values based on maximum load and instantaneous crack length at that load. KR values are much higher than KQ and Ki values. This indicates that the crack growth resistance of the material increases as the crack extension occurs in the material. The instantaneous crack lengths were obtained using the following procedure.

The representation of the Jr values as a function of the crack extensions Aa defines i he resistance curve J-Aa. 

In [3], it was noticed that DCB specimens with a higher bending stiffness required a longer crack extension before the steady-state crack growth resistance was attained. In [ I], the concept of a bridging law was introduced to characterise the f -curve for DCB specimens. 

A conventional fracture mechanics strength parameter indicating the resistance of a material to crack extension. 

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