Current research projects
Fundamental models for creep
The role of cell structures and basic prediction of creep strain curves
Grain boundary sliding, formation and growth of creep cavities
Prediction and extrapolation of creep rupture
Fundamental models for creep
Research leader: Rolf Sandström.
Previous PhD students: Arash Hosseinzadeh Delandar, Fangfei Sui, Jing Zhang
Keywords: Creep, secondary stage, precipitation hardening, solid solution hardening, stress exponent, copper, creep resistant steels
Project period: 2015-
Project description: In the past creep deformation was almost invariably described with the help of empirical or semi-empirical models involving a number of adjustable parameters. This meant that it was difficult or impossible to use the models to identify the mechanisms involved and to make predictions. However, starting with a basic dislocation model, it has been possible to develop formulae for the secondary creep rate of pure metals. They have been verified to be valid over a wide range of temperatures and stresses and can handle creep rates over many orders of magnitude. The stress exponent can vary from 3 to 50. To describe alloys as well, additional models have been developed. By assuming that the critical factor is the time it takes for dislocations to climb across particles, precipitation hardening can be described quantitatively. Models for solid solution hardening that is due to the interaction between dislocations and the Cottrell atmosphers of solutes have been formulated. The interaction gives rise to a back stress and an increased activation energy for creep.
Publications
- Sandström, R., Zhang J., Modeling the Creep of Nickel, Journal of Engineering Materials and Technology, 143 (2021)
- Sui, F., Sandström R., Creep strength contribution due to precipitation hardening in copper–cobalt alloys, J Mater Sci, 54 (2019) 1819-1830
- Sui, F., Sandström R., Wu R., Creep tests on notched specimens of copper, J Nucl Mater, 509 (2018) 62-72
- Spigarelli, S., Sandström R., Basic creep modelling of aluminium, Materials Science and Engineering: A, 711 (2018) 343-349
- Delandar, A.H., Sandström R., Korzhavyi P., The role of glide during creep of copper at low temperatures, Metals, 8 (2018)
- Zhang, J., Sandström R., Influence of W in solid solution on the creep rate of nickel, in: American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP, 2018
- Sandström, R., Fundamental models for the creep of metals, in: T. Tanski (Ed.) Creep, inTech, 2017
- Sui, F., Sandström R., Slow strain rate tensile tests on notched specimens of copper, Materials Science and Engineering: A, 663 (2016) 108-115
Source of funding: SKB
The role of cell structures and basic prediction of creep strain curves
Research leader: Rolf Sandström.
Previous PhD student and Scientist: Junjing He
Keywords: Creep curves, primary creep, tertiary creep, cell structure, subgrain, cold work
Project period: 2014-
Project description: Primary creep has successfully been modelled assumining that the high creep rate is due to a low dislocation density in the primary stage. Typically the so called f(phi)-model is obeyed, i.e. log of the creep rate descreases linearly with log of the creep strain. Cold work can have a dramatic effect on the creep strength. Copper gives a dramatic example since the rupture time can increase up to six orders of magnitude including the influence of the shape of the creep strain curves. This has been possible to model quantitatively taking the substructure into account. In spite of the existence of numerous empirical models, the understanding of the controlling mechanisms behind tertiary creep has been modest. For pure metals the balance between the applied stress and a back stress due to the substructure plays an important role. When the teriary stage is reached, the increase in the back stress can no longer keep up with increase in the true stress and the creep rate increases. For many types of steels, log of the creep rate is linear in the strain, which is referred to as the W(Omega)-model. Although this behaviour has known for 45 years, it is still not fully explained.
Publications
- Sandström, R., He J.-J., Prediction of creep ductility for austenitic stainless steels and copper, Mater High Temp, (2022) 1-9.
- Sandström, R., Formation of Cells and Subgrains and Its Influence on Properties, Metals, 12 (2022
- Sandström, R., Sui F., Modeling of tertiary creep in copper at 215 and 250 °c, Journal of Engineering Materials and Technology, Transactions of the ASME, 143 (2021)
- Sandström, R., Formation of a dislocation back stress during creep of copper at low temperatures, Materials Science and Engineering A, 700 (2017) 622-630
- Sandström, R., The role of cell structure during creep of cold worked copper, Materials Science and Engineering: A, 674 (2016) 318-327
- Wu, R., Pettersson N., Martinsson Å., Sandström R., Cell structure in cold worked and creep deformed phosphorus alloyed copper, Mater Charact, 90 (2014) 21-30
- Sandström, R., Creep strength in austenitic stainless steels, in: ECCC2014 3rd International ECCC Conference, Rome, 2014
- Sandström, R., Creep strength of austenitic stainless steels for boiler applications, in: Coal Power Plant Materials and Life Assessment: Developments and Applications, 2014, pp. 127-146
Source of funding: SKB
Grain boundary sliding, formation and growth of creep cavities
Research leader: Rolf Sandström.
Previous PhD student and Scientist: Junjing He
Keywords: Cavitation, nucleation, constrained growth , cyclic loading, austenitic stainless steels, ferrtic-bainitic steel
Project period: 2015-
Project description: The first step was to formulate a quantative model for grain boundary sliding (GBS). This is essential since GBS is believed to the main mechanism for nucleation of creep cavities.With the help of the GBS result, a basic model for the formation of the creep cavities has been developed. With this model quantative predictions can be made (for the first time) and they have been been applied successfully to austenitic stainless steels and copper. Models for constrained growth of cavities have been improved using FEM modelling. Creep-fatigie interactions during cyclic loading have been covered. Results for 1Cr0.5Mo have been possible to describe.
Publications
- Sandström, R., Basic Creep-Fatigue Models Considering Cavitation, Transactions of the Indian National Academy of Engineering, 7 (2022) 583-591.
- Sandström, R., He J., Survey of Creep Cavitation in fcc Metals, in: W.B. Tomasz Tanski (Ed.) Study of Grain Boundary Character, inTech, 2017, pp. 19-42.
- He, J., Sandström R., Creep cavity growth models for austenitic stainless steels, Materials Science and Engineering: A, 674 (2016) 328-334.
- He, J., Sandström R., Modelling grain boundary sliding during creep of austenitic stainless steels, J Mater Sci, 51 (2016) 2926-2934
- Sandström, R., Wu R., Hagström J., Grain boundary sliding in copper and its relation to cavity formation during creep, Materials Science and Engineering: A, 651 (2016) 259-268
- He J., Sandström R., Growth of creep cavities in austenitic stainless steels, in: 8th European Stainless Steel & Duplex Stainless Steel Conference, ASMET, Graz, 2015, pp. 561-570.
Source of funding: EU
Prediction and extrapolation of creep rupture
Research leader: Rolf Sandström.
Previous PhD student and Scientist: Junjing He
Keywords: Creep, brittle rupture, ductile rupture, cavitation, austenitic stainless steels,
Project period: 2016-
Project description: With the help of the Calphad approach, the formation and development of particles can be modelled. It was demonstrated in the MacPlus project that even quite complex behaviour can be handled, for example six different types of particles could be descrinbed for HR3C. Combing these results with the basic models for precipitation hardening and solid solution hardening as well as for cavitation, both ductile and brittle rupture can be predicted. This has successfully been demonstrated for a number of austenitic stainless steels.
Although these basic models are available for prediction and extrapolation of creep rupture data, they are not used extensively. A problem with the traditional statistical methods for extrapolation that are typically based on a time-temperature parameter (TTP), has been the absence of methods for estimating the error. Such methods have now been formulated. An important aspect is that if constraints on the first and second derivatives are included there is a pronounced improvement in the quality of the result. Attempts have also been made to use neural networks to predict creep rupture. This is quite feasible. However, it is quite essential to take into account the constraints in the same way as for the TTP analysis to obtain meaningful results. If the reults from neural networks are used for extrapolation of results, it is likely that constraints must be introduced in many applications. This can be done either by programming them in the neural network code or just by carefully checking the results to ensure that the constraints are fulfilled.
Publications
- Sandström, R., He J.-J., Error estimates in extrapolation of creep rupture data and its application to an austenitic stainless steel, Mater High Temp, 39 (2022) 181-191
- He, J., Sandström R., Creep rupture prediction using constrained neural networks with error estimates, Mater High Temp, (2022) 1-13
- Sandström, R., He J., Error Estimates in Extrapolation of Creep Rupture Data: Applied to an Austenitic Stainless Steel, ASME 2021 Pressure Vessels & Piping Conference, (2021)
- He, J., Sandström R., Application of Fundamental Models for Creep Rupture Prediction of Sanicro 25 (23Cr25NiWCoCu), Crystals, 9 (2019)
- He, J., Sandström R., Basic modelling of creep rupture in austenitic stainless steels, Theoretical and Applied Fracture Mechanics, 89 (2017) 139-146.
- He, J., Sandström R., Brittle rupture of austenitic stainless steels due to creep cavitation, in: Procedia Structural Integrity, 2016, pp. 863-870.
Source of funding: EU
