Experimental and numerical investigations of deformation and fracture mechanisms in double-sided incremental forming

Peng, Wenxuan (2022) Experimental and numerical investigations of deformation and fracture mechanisms in double-sided incremental forming. PhD thesis, University of Nottingham.

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Incremental sheet forming (ISF) is a novel sheet forming technology, which has had a rapid development in the last two decades. ISF and the Single Point Incremental Sheet Forming (SPIF) processes have considerable advantages of low cost, high flexibility and adaptability in manufacturing small batch and customized products for automotive, medical and aerospace applications. The general ISF process only requires the use of a Computer Numerical Control (CNC) milling machine to control a semi-spherical tool traveling along a pre-defined toolpath to form a blank sheet on a dedicated fixture. Through continuous deformation by layers, the blank workpiece is incrementally deformed to the desired shape. However, there are shortcomings in terms of formability and accuracy in SPIF process as reported in literature. As a result, several variants are proposed and developed to overcome these SPIF limitations.

Double-Sided Incremental Forming (DSIF) is a new variant of advanced ISF processes, developed from a flexible die concept, by using an additional tool to follow the main or master tool movement on the other side of the formed sheet to provide local support. The additional tool support improves the deformation stability and helps to achieve improved formability and accuracy as compared to SPIF whilst it keeps the same benefits of conventional ISF processes.

This thesis reports work on the deformation and fracture mechanisms of SPIF and DSIF processes through experimental investigation and Finite Element (FE) simulation. The accurate predictions were achieved by a modification of shear mechanisms in damage modelling, as well as the correct estimation for machine deflection during experiments.

The methodologies were initially validated in using two bespoke testing methods, Tension under Cyclic Bending (TCB) and Tension under Cyclic Bending and Compression (TCBC) tests as they resemble the local bending and tension features of SPIF and DSIF processes. Experimental validation was carried out by using a bespoke TCB and TCBC test rig with grade 1 pure Ti samples. The additional compression in TCBC captures the key features of formability improvement in DSIF process. The analytical model and FE results showed the bending effect induced the local deformation feature, where the local stress state was the key to fracture occurrence.

To predict fracture under TCB and TCBC conditions, a Lode angle parameter and stress triaxiality based new shear modified Gurson-Tvergaard-Needleman (GTN) damage model was developed and implemented through ABAQUS VUMAT user subroutine in FE simulations. As validated in TCB and TCBC simulations, an accurate prediction to the elongation-to-fracture with an error less than 10% was achieved. The results suggest the relatively high stress triaxiality due to pure tension in reverse-bending stage is the determinative factor causing fracture initiation at the surface contacting with the bending roller.

In SPIF and DSIF testing of AA5052 material with a pneumatic support tool, a particular case of formability reduction was observed due to an “over-bending” phenomenon. This proactive compensation approach could result in tool misplaced when the machine deflections occur. To quantify the effect of machine deflections in ISF processing, a general approach based on Homogeneous Transformation Matrix (HTM) was proposed to simulate the tool positioning errors during practical SPIF and DSIF tests of grade 1 pure Ti. The comparison between the part profiles from experimental and FE results indicated a significant improvement of accuracy in geometry prediction.

By implementing the developed machine stiffness estimation approach and the modified GTN model in FE simulations of SPIF and DSIF processes with grade 1 pure Ti sheets, the formed geometry and fracture depth can be correctly predicted. For the first time, the cyclic local bending was identified as a common deformation feature in both TCB/ TCBC and SPIF/ DSIF. The additional compression reduces the shear and delays the damage initiation and hence improves the formability improvement in TCBC and DSIF as compared to their counterparts of TCB and SPIF respectively. However, three-dimensional bending in SPIF and DSIF increases stress triaxiality that moves the location of fracture initiation to the surface of non-contact region to the master tool. In conclusion, local bending and additional support tool induces different stress states by combinations of tension, shear and compression, results in the discrepancies between SPIF and DSIF in terms of damage evolution and process formability being achieved.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Ou, Hengan
Becker, Adib
Keywords: Incremental sheet forming, Cyclic bending under tension and compression, GTN model, Lode angle parameter
Subjects: T Technology > TS Manufactures
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 68319
Depositing User: Peng, Wenxuan
Date Deposited: 15 Mar 2022 04:40
Last Modified: 15 Mar 2022 04:40
URI: http://eprints.nottingham.ac.uk/id/eprint/68319

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