13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Measurement of Internal Stress and Internal Resistance Resulting from Creep of Type 316H Stainless Steel Bo Chen1,*, David J. Smith1, Peter E.J. Flewitt2, 3, Shu Yan Zhang4 1 Department of Mechanical Engineering, University of Bristol, Bristol BS8 1TR, UK 2 Interface Analysis Centre, University of Bristol, 121 St. Michael’s Hill, Bristol BS2 8BS, UK 3 H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK 4 ISIS, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK * Corresponding author: b.chen@bristol.ac.uk Abstract Descriptions of high temperature creep deformation often use the concept of the effective stress, which includes the presence of the internal stress. Many experimental techniques have been applied to measure the internal stress induced by creep deformation. However, there is still a debate about the validity of the measured values. This is partly because the distinction between internal stress and material internal resistance is unclear. In this paper, neutron diffraction measurements, undertaken using the spallation source at the Rutherford Appleton Laboratory, UK, are combined with in-situ loading to investigate the internal state of a Type 316H stainless steel. By undertaking measurements of the lattice strain for different grain families, before, during and after mechanical loading, the internal stress and internal resistance induced by prior creep were determined. The results show that these two parameters are important measures of the internal state, each changing during creep and influencing creep deformation rate. Additionally, internal stress is shown to be dependent on specific crystallographic planes of each grain family. The results are discussed with respect to the underlying mechanisms of creep deformation in stainless steel. Keywords Internal Stress, Internal Resistance, Creep, Neutron Diffraction, Austenitic Stainless Steel 1. Introduction Materials may deform by one of several different mechanisms depending upon the applied stress and temperature. It is convenient to present these mechanisms in the form of a deformation mechanism map [1]. More importantly, the use of engineering polycrystalline materials over the operational service life (typically >105 hours) produces a potential to change the initial microstructure, which can affect the controlling deformation mechanisms in creep [2, 3]. It has been recognised by Biberger and Gibeling [4] that creep deformation rate, cε&, relies on the state of the microstructure, temperature and applied stress. Thus, the deformation rate is described by a kinetic equation of the form: 1 2 垐 ( , , , ) c a n f ε σ Τ σ σ σ = ⋅⋅⋅⋅ & (1) where aσ is the applied stress, T is the temperature, 1ˆσ, 2ˆσ and ˆnσ represent a series of known and unknown internal state parameters which characterise the current state of the material. In addition, creep deformation leads to changes to the internal state parameters. Two of these parameters are considered here: (i) the creation of internal strains arising from strain incompatibility, for example due to the different creep deformation rates of individual grains in polycrystalline materials; (ii) a change in the material resistance. The first is called as internal stress and the second is called as internal resistance, as described by Chen et al. [5]. Many experimental techniques have been developed to provide a quantitative measure of these two terms, such as the widely accepted stress dip test technique [6], analysis of asymmetric X-ray diffraction peak profiles [7], dislocation density and geometrical arrangement obtained using transmission electron microscopy (TEM) [8]. In addition, creep deformation models, incorporating the term internal stress or internal resistance, have been developed to improve the prediction in creep deformation rate. These include models proposed by Estrin and Mecking [9],
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