Damage critical curve

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. 

In similarly prepared specimens, which appear to be damaged (such as the lower curve for monolithic NbaSn in Easton and Schwall [ ] or the curve for specimen 1 in Ekin [% a peak critical current did not exist. If it is assumed, however, that these damaged specimens had a tensile prestrain in them, they appear as a logical extension of the curve in Fig. 9, with their Ic starting at a degraded value on the tensile side of the abscissa. 

In order that the material may follow the curve in Fig. 6.23b, it is necessaiy for the critical fibre volume to be exceeded. The critical fibre volume is defined as the volume of fibres which, after matrix cracking will cany the load which the composite sustained before cracking. In such a way, a brittle material in which one cracking surface could be formed with low energy, transforms into the pseudo-plastic body, which can cany momentaiy the overioad with no visible damage. Therefore, the... 

One can see that there is a critical strain Eq below which the material does not damage. The value of Eq is a function of the polymer-solvent couple. In simplest cases, for moderately polar polymers such as poly(2,6-dimethyl oxyphenylene) (PPO) (Bernier and Kambour, 1968), Eq depends on the solvent solubility parameter as shown in Fig. 12.6. The dashed zone corresponds to values of Ec determined in air. In the case of polar polymers, e g. poly(methyl methacrylate) (PMMA), the behaviour can be more complex the curve Eq = A s) can exhibit several minima. 

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