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Residual stresses in a stress lattice: experiments and finite element simulations
Combitech AB, P.O. Box 15042, 580 15 Linköping.
Department of Mechanical Engineering Jönköping University P.O. Box 1026, 551 11 Jönköping.
Department of Mechanical Engineering Jönköping University P.O. Box 1026, 551 11 Jönköping.
2009 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 209, no 9, 4320-4328 p.Article in journal (Refereed) Published
Abstract [en]

In this work, residual stresses in a stress lattice are studied. The residual stresses are both measured and simulated. The stress lattice is casted of low alloyed grey cast iron. In fact, nine similar lattices are casted and measured. The geometry of the lattice consists of three sections in parallel. The diameter of the two outer sections are thinner than the section in the middle. When the stress lattice cools down, this difference in geometry yields that the outer sections start to solidify and contract before the section in the middle. Finally, an equilibrium state, with tensile stresses in the middle and compressive stresses in the outer sections, is reached. The thermo-mechanical simulation of the experiments is performed by using Abaqus. The thermo-mechanical solidification is assumed to be uncoupled. First a thermal analysis, where the lattice is cooled down to room temperature, is performed. Latent heat is included in the analysis by letting the fraction of solid be a linear function of the temperature in the mushy zone. After the thermal analysis a quasi-static mechanical analysis is performed where the temperature history is considered to be the external force. A rate-independent J2-plasticity model with isotropic hardening is considered, where the material data depend on the temperature. Tensile tests are performed at room temperature, 200 °C, 400 °C, 600 ° C and 800 ° C in order to evaluate the Young’s modulus, the yield strength and the hardening accurate. In addition, the thermal expansion coefficient is evaluated for temperatures between room temperature and 1000 °C. The state of residual stresses is measured by cutting the midsection or the outer section. The corresponding elastic spring-back reveals the state of residual stresses. The measured stresses are compared to the numerical simulations. The simulations show good agreement with the results from the experiments.

Place, publisher, year, edition, pages
Elsevier , 2009. Vol. 209, no 9, 4320-4328 p.
Keyword [en]
Residual stresses; Stress lattice; Grey iron; Finite elements; Thermomechanics; Casting
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-72345DOI: 10.1016/j.jmatprotec.2008.11.025OAI: oai:DiVA.org:liu-72345DiVA: diva2:459233
Available from: 2011-11-25 Created: 2011-11-25 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Robustness Analysis of Residual Stresses in Castings
Open this publication in new window or tab >>Robustness Analysis of Residual Stresses in Castings
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about robustness analysis of residual stresses in castings. This topic includes the analysis of residual stresses in castings and the robustness analysis itself, both covered in the thesis.

Residual stresses are important when designing casted components. For instance, the residual stress state after casting might affect the fatigue life, facilitate crack propagation and cause spring-back related problems when a casted component is machined or used. Examples of components where such problems are recognized are stamping dies and brake discs, both considered in the thesis. Residual stresses in castings are simulated by finite element analysis in this thesis. A sequential un-coupled approach is used where a thermal analysis of the solidification and cooling generates a temperature history. Then a quasi-static structural analysis is performed, driven by the temperature history. During the structural analysis residual stresses are developed due to different cooling rates in combination with plasticity. For comparison, measurements of residual stresses in castings have also been performed. The agreement between analyses and measurements is satisfactory.

In a residual stress analysis there are several random variables such as process, geometrical and material parameters. Usually those random variables are assumed to be deterministic and their nominal values are used. It can be beneficial to include the variation of the random variables in analysis of residual stresses. For that purpose robustness analysis of the residual stresses are performed in this thesis. In some of the appended papers the robustness is evaluated with respect to variation in e.g. Young’s modulus, yield strength and hardening, thermal expansion coefficient, geometric dimensions and time in mould of the casting. The robustness analyses are performed by using metamodels as surrogates to the finite element model, due to the computational expensiveness of the residual stress analyses. Conventional regression models, Kriging approximations and an optimal polynomial regression model, proposed in one of the appended papers, are metamodels used in the thesis. When a metamodel is established the choice of the design of experiments can be crucial. The generation of the design of experiments is also investigated in the thesis. For instance, a hybrid method constituted by a genetic algorithm and sequential linear programming is proposed for the generation of optimal design of experiments. A-, D-, I- and S-optimal design of experiments are generated by the developed  hybrid method. Those design of experiments as well as Latin  Hypercube sampled design of experiments are used throughout the thesis. Since residual stress analysis, robustness analysis and metamodeling are considered in the thesis, more or less all parts required to perform robustness analysis of residual stresses in castings are covered.

Results in the thesis show that the level of residual stresses in castings can be high due to the casting process. Thus, crack development and spring-back related problems might be influenced by those stresses. Results also show that the level of residual stresses can be very dependent on the variation in certain random variables such as the thickness of the casting, hardening and Young’s modulus. Therefore, it can be of importance to include the variations of the random variables in order to accurately predict the residual stresses when designing castings.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 44 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1415
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72354 (URN)978-91-7393-002-4 (ISBN)
Public defence
2012-01-20, E1405, Tekniska högskolan, Jönköping, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2011-11-25 Created: 2011-11-25 Last updated: 2012-04-02Bibliographically approved

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