The contribution of the clamping force in the technique used in this study to stretch-bend low carbon steel samples was investigated to support the subsequent changes in the microstructure and properties of the formed material with regard to parent material. Although plastic deformation by cold working is known to induce texture or preferred orientation to the grains of a formed material while decreasing its ductility and increasing the strength, as well as inducing residual stress, it is not known how the different directions (rolling, transverse and oblique/45°) of the sheet steel will respond to this stretch bending technique. The first part of the research work involved a thorough literature review on sheet metal forming processes and their effect on the formed material with interest on the above mentioned directions of the sheet. It became clear from the literature overview that cold working of a material will induce strain-hardening which varies with the magnitude of cold work, resulting in changes in the strength and ductility of the material. Besides, when plastic deformation is not uniform (e.g., tensile and compressive) throughout the entire cross section of the formed part, residual stresses remain in the material with the grains been elongated along the direction of the maximum strain. The main parameters that were considered and controlled in this study are as follows: strain experienced / stress induced into the form sample, the sample direction, the stroke length and the clamping torque, the generated radius of curvature. The chapters that follow the literature review, deal with the set-up of the different equipment used in this study, the specimen preparation as well as the recording, the calculation and interpretation of the results. It was found that the stress magnitude that generated the different radii of curvature (120 mm, 150 mm and 185 mm) was between 1 percent & 13 percent higher than the parent material’s yield strength with the lower stress been associated to the smaller radius of curvature and the higher stress to the higher radius. The stress induced into the sample during forming was not only proportional to the stroke length but also to the distance between the punch’s tip and the sample and the sample to the die’s nadir. The clamping torque adopted was restricted to the manual capacity of the operator who used a preset torque wrench to fasten the plate sample into the jig. Plate samples of low carbon steel were cut to angles of 0°, 45°, and 90° to the rolling direction of the sheet material and stretch-bent on a single-action mechanical press to 120 mm, 150 mm, and 185 mm radii of curvature. The preliminary results indicate that stretch-bent samples had increased hardness to the parent plate, in particular below the surface layers up to around 1.1 mm depth. Since there is a well established relationship between hardness, yield and tensile strengths for steel, the yield and tensile strengths of the formed material were estimated using the Nobre et al [34] incremental relation, which relates the linearity between relative increments of hardness and yield strength. Changes were not noticeable at the microstructural level of the formed samples. Meanwhile, samples from which higher plastic deformation stress values were calculated were not those absorbing higher impact energy when Charpy specimen cut from plate and stretch-bent samples were tested. The maximum relieved residual stress in the parent material was predominantly compressive and represents in magnitude approximately 12 percent (average for the three directions) of its original yield strength. In the stretch-bent samples, the relieved residual stress was compressive in the outer curved section with a magnitude about 50 percent of the parent material yield strength and tensile in the inner curved section with a magnitude approximately 25 percent of the parent material yield strength.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nmmu/vital:9627 |
| Date | January 2012 |
| Creators | Victor, Ngea Njoume |
| Publisher | Nelson Mandela Metropolitan University, Faculty of Engineering, the Built Environment and Information Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MTech |
| Format | xxx, 191 leaves, pdf |
| Rights | Nelson Mandela Metropolitan University |
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