Development of non-destructive small specimen creep testing techniques

Having knowledge of the current creep strength of service-aged components in high temperature installations such as nuclear power stations, oil refineries and chemical plants is essential for their safe and economic operation. Obtaining this knowledge may involve the use of small material samples. These small samples may be removed from weld regions or from component surfaces. Improving small specimens creep testing techniques, whereby a reliable uniaxial minimum strain rate and rupture data can be obtained, has been a major engineering concern for the last 20 years or so. This thesis includes the development of the small ring creep testing specimen in order to allow the ring specimen to be manufactured and tested with various shapes and geometries. The shape and size of the available small material samples normally dictates the ring shape, e.g., circular or elliptical. However, changing the ring shape leads to a change in the conversion factors, which are used to convert the ring data to the corresponding uniaxial data. Therefore, the effects of the ring geometry with different thicknesses, on the conversion factors, are described in this work. The finite element analyses have been used to assess the effects of shear deformation on the ring behaviour and also to determine the optimum ring ii geometry. Nickel base Superalloy 738 steel at 800oC and (Bar-257) P91 steel at 650oC have been used to validate the testing method. Two new small sized creep test specimens are also described in this thesis, i.e., (i) a small (Two-bar) specimen, which is suitable for use in obtaining the uniaxial MSR and creep rupture data and (ii) a small notched specimen which is suitable for obtaining the multiaxial stress state parameter. The specimen testing techniques, modeling, loading and manufacturing are described for both specimen types in this work. Finite element analyses have been used to assess the effects of the two-bar specimen (TBS) dimensions on the conversion factors, the failure time, the minimum strain rate, and to determine optimum dimension ratio ranges for the specimen. The two-bar specimen and the small notched specimen have been used to obtain a full set of material constants for two high temperature materials, i.e., (i) typical (as received) P91steel at 600oC and (ii) weak (Bar-257) P91 steel at 650oC. The results show remarkably good agreement between the data obtained from the two new small specimen testing techniques and the data obtained from corresponding uniaxial tests. The major advantages of the small ring specimen, the two-bar specimen and the small notched specimen testing techniques, over the existing small specimens testing techniques, are also included.