εðtÞ¼ Uob Uib Ls ¼ 2Cb Ls ð t 0 εrðτÞdτ (59.2) ε ðtÞ¼ Vob Vib Ls ¼ 2Cb Ls εrðtÞ (59.3) Where As and Ls are the cross-sectional area and the length of the testing region of the specimen. Cb is the one-dimensional elastic stress wave velocity in the input/output bar material, Ab the cross-section area and Eb Young’s modulus of the input/output bar, Uib andUob the displacements of the specimen/input bar interface and the specimen/output bar interface and Vib and Vob are the corresponding interface velocities. DIC is used to assess the full field strain in the dynamic tensile specimen. Therefore, a fine speckle pattern is sprayed randomly on the specimen surface. Images with a resolution of 704 208 pixels and a frame rate of 37,500 fps are recorded with a Photron FASTCAMSA-1 high speed camera. 59.3 Results and Discussion Several specimens of each material are tested. Figure 59.4 is the true stress–true strain curves of based UTS800 and based UTS700 steels. Figures 59.5 and 59.6 show the true stress–true strain curves of welded UTS800 and welded UTS700 steels, respectively. From these Figures, it is seen that the results of tests are very repeatable. In Fig. 59.7, Stress–strain curves of all four investigated steels are compared together. It is seen in Fig. 59.7, the stress–strain curves of based UTS800 almost overlap the ones of welded UTS800, except there is a little scatter at the end of the curves. This means the dynamic mechanical behaviour of welded UTS800 is nearly the same as the based UTS800. On the other hand, the stress–strain curves of based UTS700 intersect the ones of welded UTS700, this causes the flow stress of welded UTS700 is higher in small strain (<~17 %) but lower in large strain (>~17 %) than based UTS700. When comparing to UTS700, flow stress of UTS800 is higher both in based and welded materials. The fracture strains of welded UTS800 and welded UTS700 are scattered, this indicates that the scattered fracture strain have been caused by the uncertainty of welded process, because this scatter is not pronounced in based UTS800 and based UTS700. Especially noteworthy is the yield properties of materials have also been affected by the process of welded, the clear yield peak, observed at the onset of plastic deformation in the based materials, disappears or weakens in the welded materials with the bake hardening treatment during the process of welded, this phenomenon was also been found in TRIP steel by Slycken et al. [11]. To avoid the stress oscillations in initial plastic range, the stress corresponding to 4 % offset strain, σ0.04, is used to evaluate the yield property of materials. Figure 59.8 shows the tensile strengthσb andσ0.04 of all four investigated steels. It is 0.4 0.0 0.1 0.2 0.3 0.5 0 200 400 600 800 1000 True-stress(MPa) True-strain Material, Strain-rate Base-UTS800-1#,1190/s Base-UTS800-2#,1280/s Base-UTS800-3#,1310/s Base-UTS800-4#,1230/s Base-UTS700-1#,1350/s Base-UTS700-2#,1300/s Base-UTS700-3#,1260/s Base-UTS700-4#,1270/s Base-UTS700-5#,1270/s Fig. 59.4 Stress–strain curves of based UTS800 and based UTS700 steels 59 Dynamic Tensile Testing of Based and Welded Automotive Steel 491
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