Determination of Material Constants for Creep Damage Models Using Small Two-Bar and Notched Specimens

The work presented in this paper forms part of the research related to the development of small specimen creep testing techniques, which can be used when only small volumes of materials are available. Commonly used small creep test specimen types such as the impression and small ring creep tests can be only used to determine the minimum creep strain rate data. In this paper, two novel small-sized creep test specimens are described: (i) the recently developed small two-bar specimen, which is suitable for use in obtaining both uniaxial creep strain rate and creep rupture life data, and (ii) the newly developed small notched specimen, which can be used to determine the multiaxial stress state parameter. The two specimen types have been used to determine a full set of material constants for Norton model, Kachanov and Liu-Murakami creep damage models. Conversion relationships have been obtained based on the reference stress method in conjunction with the finite elements analyses and have been used to convert the two-bar specimen data to the corresponding uniaxial data. Two P91 power plant steels have been used to assess the accuracy of the two testing methods, (i) a weak P91 (Bar-257) steel at 650°C and (ii) a normal P91 (as received) steel at 600°C. The correlation between the data obtained from the two small specimens testing techniques and the corresponding uniaxial and Bridgeman specimens tests is excellent. The major advantages of the two novel small specimens testing techniques, over some existing small specimen creep testing techniques, are also highlighted in this paper.