High temperature aniso-thermal-mechanical analysis of superplastic forming tools

Deshpande, Aditya A. (2009) High temperature aniso-thermal-mechanical analysis of superplastic forming tools. PhD thesis, University of Nottingham.

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The main objective of the thesis is to establish a methodology to analyse the anisothermo-mechanical behaviour of a representative large industrial Superplastic Forming (SPF) tool made of XN40F material (40% Ni, 20% Cr, Balance Fe) to identify and evaluate different failure mechanisms to improve and predict the tool life. Sequentially coupled thermo-mechanical analyses under realistic loading conditions are developed within a general purpose non-linear Finite Element (FE) code, ABAQUS to predict and analyse the complex temperature-stress-strain cycles of the SPF tool.

The temperature dependent cyclic plasticity and creep material data is established for the tool material performing the multi-strain range isothermal cyclic tests and the stress relaxation tests for a range of temperatures. Various strain controlled thermomechanical fatigue-creep and stress controlled ratchetting tests are designed and performed based on the preliminary FE analyses of the tool. The strain controlled and the stress controlled representative tests are carried out to capture the most damaging phase of the SPF thermo-mechanical cycle. In addition to above tests, heat transfer tests are also carried out on the rectangular block of tool material to validate the employed heat transfer methodology.

Material constants are identified for different material behaviour models such as combined non-linear kinematic/isotropic hardening model for the cyclic plasticity, power law creep model for secondary creep and the two-layer viscoplastic model to address the combination of plasticity and creep. The identified constants are validated against the isothermal and thermo-mechanical fatigue tests. The FE modelling of the heat transfer tests using the calculated convective heat transfer coefficients and other thermal properties is carried out and the predicted thermal histories are compared with the experimental results.

The validated heat transfer methodology is employed to simulate the realistic thermal cycles of the SPF tool. In addition to thermal loading, the tool gravity and the clamping pressure to counteract the forming gas pressure are employed in the thermo-mechanical analyses of the tool. The tool platen contact is also modelled where the platen is considered as analytically rigid surface. Various thermo-mechanical analyses are performed to investigate the effect of different thermal cycles, heating and cooling rates and the batch sizes, i.e. number of parts formed in a forming campaign, on the tool damage.

Different strain and strain energy based thermo-mechanical fatigue life prediction methodologies are explored and evaluated using the isothermal and thermo-mechanical fatigue-creep lifing tests. The simple ductility exhaustion method is also developed to predict the ratchetting life of the specimen and the tool. The tool life predictions are performed employing the FE predicted stress-strain results into the identified stress-strain-life equations from the isothermal lifing tests. The predicted thermo-mechanical behaviour and tool lives are compared against the representative test and the industrial experience.

From all thermo-mechanical fatigue-creep and ratchetting test results and thermo-mechanical analyses of the tool, the fatigue-creep interaction is found to be the most important factor in the tool failure.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Leen, S.B.
Keywords: Superplastic forming (Metal-work), finite element method, thermal stresses, materials, thermal fatigue, service life (Engineering)
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 13088
Depositing User: EP, Services
Date Deposited: 12 Feb 2013 10:16
Last Modified: 21 Dec 2017 14:49
URI: https://eprints.nottingham.ac.uk/id/eprint/13088

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