In many situations where the characterisation of the mechanical behaviour of a specific material is required, source material for manufacture of conventional test specimens may be at a premium. Examples include the validation of new alloys for use in the power industry, the description of the heat affected zone (HAZ) of weldments 1 or performing a remnant life study on an in service component (such as steam pipe work used extensively in the power generation industry). The potential for a limit in sample material has necessitated the development of small specimen designs and associated test methods, particularly for the determination of the creep behaviour of a sample material. The small punch creep test (SPCT) has the potential to characterise the full uniaxial creep curve (as the specimen is taken to fracture). It is for this reason that the small punch creep test has attracted much interest from the research community. Owing to the complex deformation mechanism interactions experienced in the small punch creep test, interpretation of the results has received attention from many authors since its application was proposed by Parker et al. in the 1990s 2 (based on small punch plasticity test by Manahan et al. in the 1980s 3 – 5 ). In this review paper, several methods for the interpretation of small punch creep test (SPCT) data are reported and compared, together with examples of their application. Considerations for finite element (FE) modelling of small punch creep tests are highlighted and critiqued. Recommendations for potential areas of future research are also presented based on the authors’ investigation into published literature and research.

To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret. This paper presents a novel numerical model able to generate results that match the experimental findings. For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 • C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen. Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data. The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model. Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected. The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a 'state of the art' creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture. The new modelling method will

The small punch creep testing technique is able to provide creep properties from a very small amount of material. However, a universal and robust technique, to convert small punch creep testing results to corresponding uniaxial creep test data, has still not been established. In addition, the experimental output can be affected by several sources of uncertainty, such as friction between the components of the test rig and the specimen, and inaccuracies in the geometry of the experimental set-up and the testing procedures. This article reports the results of three-dimensional elastic/creep finite element analyses of small punch creep testing, taking into account geometrical inaccuracies in the initial punch position and the loading direction. The results of the calculations show that the initial position of the punch and the loading direction can considerably affect the variation in the specimen's central deflection with time and the final time to failure. The minimum displacement rate was found to decrease when the punch moves away from the centre of the specimen and when the angle between the loading direction and the axis of the test rig increases. The time to failure increases when the punch deviates from the perfect axi-symmetric configuration. The effects of the direction of the load increase as the initial distance of the punch from the centre of the specimen increases. Analytical correlations, corresponding to the inaccuracies investigated, are also proposed.

This paper shows the results of finite element (FE) analyses of Small Punch Creep Testing (SPCT) of a P91 steel at 600°C using two different approaches to model the friction between the specimen and the punch. The numerical results obtained by using the "classical" Coulomb friction model (i.e. constant friction coefficient) have been compared with those obtained by a more modern formulation, which takes into account the effects of local loading conditions, i.e. the contact pressure, between the contacting bodies (the small disc specimen and the punch) on the coefficient of friction. The aim of the work is to investigate the effects of the friction formulation used for the calculations on the numerical results representing the output of the test, i.e. the variation of the punch displacement versus time and the time to rupture.The calculations, carried out for various load levels, showed that the friction coefficient is not constant at all positions on the contacting surface between the punch and the specimen during the deformation process. The maximum value for the coefficient of friction is reached at the contact edge, which is a very important region in the specimen, because this is the position at which most of the creep deformation occurs.As expected, the displacement versus time curve (that is usually the only output obtained from experimental SPCTs) is affected by friction formulation which is used, as this directly influences the stress and strain fields in the specimen.

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