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Title: Analysis of material deformation and fracture mechanism in incremental sheet forming by simplified testing methods
Author: Ai, Sheng
ISNI:       0000 0004 8509 3487
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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Incremental sheet forming (ISF) has seen its increasing applications in the sheet metal manufacturing industry in the last decades due to its unique characteristics of process flexibility and enhanced material formability. For single point incremental sheet forming (SPIF), its improved formability is attributed to the localized material deformation caused by a combination of tension, bending, shearing and cyclic loading. For double side incremental sheet forming (DSIF), the deployment of a rear tool introduces an additional compressive loading onto the material, which further strengthens the localized deformation. However, the complicated loading conditions and contact conditions prohibit a systematic and direct analysis of the factors affecting material formability in ISF, especially in DSIF. Furthermore, traditional testing methods have been proved incapable to explain the significant formability enhancement in ISF. In this research, to investigate the material deformation characteristics in ISF process, SPIF tests using material AA5251-H22 and AA6082-T6 were conducted. To explore damage accumulation in ISF, finite element (FE) damage modelling of ISF process using shear-modified GTN model with AA5251-H22 was developed. By tracing the evolution history of strain and shear-induced damage indicator of target elements in the FE simulation results, bending, shearing and localized deformation were confirmed in both SPIF and DSIF processes. However, comparing the forming depth of the conic parts obtained from the experiment and the FE model, it was found that shear-modified GTN model predicted premature fracture in ISF processes for material AA5251-H22, possibly because of the complicated loading conditions or the material type investigated. Material formability can be influenced by material properties, loading conditions as well as loading path. In order to investigate the effect of these three factors on material formability enhancement in ISF, two novel testing methods have been proposed in this research. To investigate the effect of individual deformation modes on the material deformation behaviour, a new testing method called Tension under Cyclic Bending and Compression (TCBC) test was developed. In this test, the complicated 3-dimensional geometrical constraints in ISF process was simplified into 2-dimensional. Moreover, the magnitude of the tension, bending, compression and cyclic loading can be explicitly varied by changing corresponding process parameters in the test. Testing rig was developed based on the proposed concept. Two materials, AA5251-H22 and AA6082-T6 were used in the test. By implementing the Taguchi Design of the Experiment, the significance and relative significance of these deformation modes were investigated. According to the statistical analysis of the experimental results, compression wasthe most significant factor affecting material formability in the test. Material AA5251-H22 showed higher sensitivity to the loading conditions, compared with AA6082-T6. Localized deformation and bending effect were also observed by tracing the strain and damage evolution of the elements during the test in the FE modelling of the test. Suppression of damage and postponement of fracture of the specimens in the TCBC tests were also observed, which can also be attributed to the localized material deformation. It was confirmed that that the TCBC test was an appropriate representation of the ISF process. To further investigate the effect of loading path onto the material deformation behaviour in ISF, another testing method called Biaxial Tension under Bending and Compression (BTBC) test was proposed. Testing rig was developed accordingly. In the BTBC test, instead of uniaxial tension, the cruciform specimen can be stretched in all four directions and the strain ratio of the two perpendicular directions can be varied. The effect of compression, bending and cyclic loading can also be adjusted. Deformation behaviour of the material aluminium alloy AA5251-H22 under two strain conditions, the plane strain and the equi-biaxial tension, were tested. Different combinations of the deformation modes were investigated in each test. The formability of the material in different tests was compared by measuring the distortion of circular grids inscribed onto the surface of the specimen. According to the experimental results and FE simulation results, the introduction of bending and compression contributed to material localized deformation. Material formability could be enhanced by the introduction of bending effect, which could be further improved by additional compression and the cyclic loading effect. The degree of formability improvement under different loading paths was different. Under plane strain condition, material formability can be progressively improved by introducing the deformation modes step by step. While under equi-biaxial tension condition, introducing compression and cyclic loading into bending did not necessarily lead to formability improvement. BTBC test provides more insight into the material deformation characterization of ISF process. Overall, in this research, the developed two simplified testing methods, the TCBC test and the BTBC test overcome the limitation of current commonly adopted material testing methods and the ISF test. The testing methods provide a fundamental explanation of the effect of material properties, loading conditions and loading path on the material deformation and fracture behavior in ISF.
Supervisor: Long, Hui Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available