Small punch creep testing: Review on modelling and data interpretation

Abstract

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.

A study on the evolution of the contact angle of small punch creep test of ductile materials

“…Li and Šturm identified a correlation between the strain at the contact boundary and the central displacement [11,17,21,22]. Li and Šturm's third order polynomial relation, equation (9) …”

“…Furthermore, the specimen is assumed to uniquely experience a membrane stretching deformation, while the strain variation in the through-thickness direction is neglected [11,15]. During small punch creep test, the specimen deformation is caused by bending prior to membrane stretching, therefore, the work presented in this paper is also aimed to investigate the applicability of Chakrabarty's theory to SPCT data interpretation [7,8,11,25]. The study was carried out by investigating the evolution of the contact angle, θ 0 , throughout the test.…”

“…However, Chakrabarty's theory of membrane stretch forming over a rigid hemispherical punch head, reported in ref. [15], is able to provide an analytical tool for the interpretation of small punch creep test data [10,11,15]. In Chakrabarty's study large plastic deformations are taken into account and the geometry and the loading conditions partly reflect those encountered in the SPCT [20].…”

“…The specimen is clamped by a load of 500 N applied to the holder reference point. Since the specimen exhibits large deformations and since its shape significantly changes during the creep analysis, the non-linear geometry formulation has been adopted [8,11,29,30]. …”

“…Indeed, the interaction of several non-linearities, such as large deformations, large strains, non-linear material behaviour and non-linear contact interactions between the specimen and the punch, induces a very complex multi-axial stress field in the specimen which also evolves in time. This affects the SPCT fracture mechanism [2,7] and introduces several challenges into the identification of a robust correlation to convert SPCT data into respective standard uniaxial creep test data [7,[9][10][11]. Another major concern is the non-repeatability of the testing method, since the experimental results depend on the set up geometry [1, 4,12,13].…”