Interactive modelling and experiment roles in understanding the creep behaviour of metal

Session

Microscopy to Modelling

Authors

Professor Mahmoud Mostafavi (4), Dr Ranggi Ramadhan (4), Dr Abdullah Al-Mamun (2), Mr James Ball (3), Dr Eralp Demir (5), Dr David Collins (3), Dr Dylan Agius (4), Professor David Knowles (1)

Affiliations

1. Henry Royce Institute
2. University of Bangor
3. University of Birmingham
4. University of Bristol
5. University of Oxford

Keywords

Cyclic Creep, Crystal Plasticity, Synchrotron, 3DXRD

Abstract text

Stainless steel type 316 and its variations (316H, 316L, 316L(N)) are ubiquitous in thermal power plants including nuclear thanks to its high ductility, corrosion resistance, and relatively good creep behaviour. As such, the high temperature mechanical behaviour of stainless steel has been well researched. The issue, however, is that the complex loading sequence of a power plant combined with its very long operational time make laboratory tests with similar conditions impossible. The purpose of this work is to develop a physics-based model which allows the design engineers to predict the behaviour of a component under a realistic cyclic long term high temperature loading condition. To develop a physics-based model, having high fidelity experimental results is key. Laboratory techniques such as high-resolution electron back-scatter diffraction can provide a wealth of information on the strain distribution at grain level on the surface of the material which can be complemented by length scale appropriate simulation techniques such as crystal plasticity finite element modelling. Critically, the damage induced by cyclic creep damage occurs inside the material thus not measurable non-destructively via the surface measurement techniques. While through thickness non-destructive laboratory experiments such as X-ray tomography can be helpful in characterisation of damage development in its latter stages within the material, quantification of the early stages of deformation which can be key in later damage formation is less straightforward in laboratory space. 

This work reports the deformation of grains at high temperature within stainless steel type 316 using synchrotron X-ray diffraction at Diamond Light Source, I12. Monochromatic pencil beam of 0.7 x 0.7 mm2 was used to monitor the elastic strain relaxation of sample heated up to 650oC under displacement controlled cyclic creep loading. Loads at high temperature were applied on the specimens using an Electro-Thermal Mechanical Testing (ETMT). The combination of relatively large average grain size of the material (∼150 micrometre) and the small gauge volume resulted in spotty diffraction patterns. This allowed individual grains to be tracked and their average grain measured using a three-dimensional X-ray diffraction technique developed by the university of Birmingham and Diamond Light Source [1]. The experimental parameters such as loads and cycles and hold durations were designed using a crystal plasticity model previously developed by the authors [2]. In return, the experimental results such as the strain relaxation rate were used to validate and verify the accuracy of the model at a grain level. This is an extension to the validation of the model previously published which compares the behaviour of grain families rather than individual grains [3]. 

The main outcome of the work is a crystal plasticity finite element model, validated at grain level, capable of qualifying the interaction of prior cyclic loading on the creep stress relaxation (which leads to damage formation) of material at high temperature. The model will be used to inform the safety assessment procedures for integrity of high temperature power plants reducing the need for complex and lengthy experiments. 

References

 

[1]      Ball JAD, Kareer A, Magdysyuk O V, Michalik S, Vrettou A, Parkes N, et al. Implementing and evaluating far-field 3D X-ray diffraction at the I12 JEEP beamline, Diamond Light Source. J Synchrotron Radiat 2022;29:1043–53. doi:10.1107/S1600577522004088.

[2]      Agius D, Mamun A, Simpson CA, Truman CE, Wang YQ, Mostafavi M, et al. Microstructure-based crystal plasticity modelling of stress relaxation from forward creep. Int J Solids Struct n.d.

[3]      Mamun A Al, Agius D, Simpson CA, Reinhard C, Truman C, Mostafavi M, et al. The effects of internal stresses on the creep deformation investigated using in-situ synchrotron diffraction and crystal plasticity modelling. Int J Solids Struct 2021;229:111127. doi:https://doi.org/10.1016/j.ijsolstr.2021.111127.