CHAPTER 7 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
7.2 Suggestions for future work
7.2.4 Further development of the apparatus for testing under non-linear strain paths 156
The effect of the non-proportionality of strain path on the determination of an FLD has not been investigated. The current biaxial testing apparatus can be used for biaxial tests under linear strain paths. The design of the apparatus could be modified in order to conduct formability tests subjected to non-linear strain paths, which means that the ratio of minor strain to major strain can be varied during deformation. Obtained results from this formability tests would be useful for validation of damage accumulation models.
7.2.5 Accuracy evaluation of materials modelling
Damage evolution is usually associated with stress state and strain path. By taking these two factors into account, stress-based and the principal strain-based viscoplastic-damage models enable good prediction of material failure for evaluating formability of alloys under hot stamping conditions. The accuracy of predicted forming limits of AA6082 can be further validated through performing a real forming test experimentally under HFQ conditions.
The anisotropic behaviour of the material and the dependence of strain path have not been taken into account in these two models. The anisotropy of alloys at elevated temperature can be further investigated experimentally by biaxial tensile tests. Since strain path is usually non-linear in an industrial forming process, material modelling for the prediction of an FLD needs to be proposed for non-linear strain paths. In another way, a stress-based FLD which is strain path independent can also be determined at elevated temperatures.
157
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LIST OF PUBLICATIONS
1. Shao, Z., Q. Bai, and J. Lin, A Novel experimental design to obtain forming limit diagram of aluminium alloys for solution heat treatment, forming and in-die quenching process. Key Engineering Materials, 2014. 622-623: p. 241-248.
2. Shao, Z., et al., Planar test system, in UK Patent 2015: UK (GB 1514084.1).
3. Shao, Z., et al., Analysis on experimental techniques for generating FLD at elevated temperatures, in International Conference on Hot Stamping of UHSS2015.
4. Shao, Z., et al., Experimental investigation of forming limit curves and deformation features in warm forming of an aluminium alloy. Journal of Engineering Manufacture, 2016 (In Press).
5. Shao, Z., et al., Development of a new biaxial testing system for generating forming limit diagram under hot stamping conditions. Experimental Mechanics, Under Review.
6. Shao, Z., et al., Design and optimisation of cruciform specimens for the evaluation of sheet metal formability under hot stamping conditions. Waiting for Submission.
165
APPENDIX THE RELATION OF APPARATUS
GEOMETRY TO STRAIN AND STRAIN RATE IN A CRUCIFORM SPECIMEN
A1. Geometrical relationship in the designed biaxial mechanism
The invented apparatus can convert an input of uniaxial force into an output of bi-axial forces.
The core parts of the apparatus, as shown in Figure A.1, comprise an input rotatable member, a plurality of rigid connection means, a drive shaft and an output rotatable member. The rotation of the input rotatable member around the axis of rotation rotates the drive shaft, thereby in turn rotating the output rotatable member to which it is coupled around the axis of rotation. In order to investigate the geometrical relationship in the designed biaxial mechanism, the input rotatable member and the output rotatable member are presented in one figure, as shown in Figure A.2.
Figure A.1 Core parts of the apparatus
(1- Input rotatable member, 2- Rigid connection means, 3- Drive shaft, 4- Output rotatable member, 6- Carriages with specimen on top, 7- Guide rails)
4 2
3 1 5
6
7
166
Figure A.2 Geometrical relationship in the biaxial mechanism
In Figure A.2, R1 is the radius of the input rotatable member and L1 is the length of the rod connecting it to a jaw of the Gleeble. S1 is the length from the initial location of the Gleeble jaw to the centre of rotation. The radius of the output rotatable member is R2 and L2 is the length of the rod connecting it to the gripping point on the specimen arm. The initial distance from this gripping point to the centre of rotation is S2. When the input rotatable member rotates an angle of 𝜃1 and travels a distance of H1, the output rotatable member rotates
In Figure A.2, R1 is the radius of the input rotatable member and L1 is the length of the rod connecting it to a jaw of the Gleeble. S1 is the length from the initial location of the Gleeble jaw to the centre of rotation. The radius of the output rotatable member is R2 and L2 is the length of the rod connecting it to the gripping point on the specimen arm. The initial distance from this gripping point to the centre of rotation is S2. When the input rotatable member rotates an angle of 𝜃1 and travels a distance of H1, the output rotatable member rotates