The impression creep Monkman Grant relationship

Impression creep testing is an established small-scale testing technique in which the indentation rate of a rectangular indenter can be converted into equivalent creep strain rate. It is a versatile technique in that, once a stable indentation rate is established, either the stress, temperature, or both, can be varied to provide data under multiple test conditions on the same specimen. It does not however produce a specimen failure. In order to overcome this limitation, use can be made of an empirical relationship between the creep strain rate obtained in the impression test and the rupture life obtained in a conventional uniaxial creep test at the same stress and temperature. This relationship, termed the Impression Monkman Grant relationship, has been applied successfully to grade 91 steel where it has been shown that rupture life predicted from impression testing is in good agreement with actual rupture life obtained by conventional uniaxial testing. The relationship has proved particularly useful for plant application in situations where mis-heat treated grade 91 pipework with lower than expected creep strength has been encountered, requiring an estimate of creep strength to justify continued operation in service.