Abstract
The relationship between rupture stress and largest crack length has been studied for the intercrystalline creep fracture of a 20Cr–35Ni alloy. The creep testing was carried out as uniaxial constant load testing at 700° C,with nominal stresses ranging from 78·5 to 196 N/mm2, and with rupture times from 3·5 to 535h. The true rupture stress, σ, was determined from the strain rate at the onset of the final fracture; this was possible since for the present type of alloy the true stress in the tertiary stage is related to the strain rate by Norton's Law. The largest crack length, c, was determined by measurements on the ruptured specimens using light microscopy.A linear regression analysis showed that σ was related to c by the expression σc n=const., with n=0·55±0·07 (95% confidence interval).Hence, the Griffith criterion, έ = const., is a good description of the relationship between σ and c. The effective crack area at the start of rupture was also determined and was found to be approximately inversely proportional to the rupture stress.
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