A significant amount of uniaxial tensile tests has been carried out using Gleeble systems to investigate the viscoplastic deformation of boron steel (22MnB5) under hot stamping conditions. However, due to heat loss through the end clamps, a temperature gradient in the reduced parallel section of dog-bone shaped specimens is inevitable. In the work reported in this paper, the effect of temperature gradient on measured outcomes is examined. Uniaxial tensile tests on 1.5 mm thick boron steel specimens are carried out, under hot stamping conditions and strain fields are quantified using the digital image correlation (DIC) technique. The effect of gauge length on the properties of boron steel, as calculated from observed test results, is determined. Compared with the test at room temperature, a bell-shaped strain distribution occurs within the gauge length even before the appearance of the maximum load. Also, average strain within the gauge length, especially in the later stages, changes with gauge length within the investigated range, and thus, different engineering stress-strain curves and fracture strains are determined. In addition, normalized strain rate is significantly dependent on gauge length, which results in over 16% difference among the computed flow stresses by using a unified constitutive model. The characterized properties of the material are dependent on gauge length and thus, a testing standard for measuring thermal-mechanical data of materials by using a Gleeble need to be defined.

中文翻译：

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热冲压条件下硼钢单轴拉伸试验中非均质应变场的测量与分析

使用 Gleeble 系统进行了大量单轴拉伸试验，以研究热冲压条件下硼钢 (22MnB5) 的粘塑性变形。然而，由于通过端夹的热量损失，狗骨形试样的减小平行截面中的温度梯度是不可避免的。在本文报告的工作中，检查了温度梯度对测量结果的影响。在热冲压条件下对 1.5 毫米厚的硼钢试样进行单轴拉伸试验，并使用数字图像相关 (DIC) 技术量化应变场。根据观察到的测试结果计算出标距对硼钢性能的影响。与常温试验相比，甚至在出现最大载荷之前，在标距内就会出现钟形应变分布。此外，标距内的平均应变，特别是在后期，随着研究范围内标距的变化而变化，从而确定不同的工程应力应变曲线和断裂应变。此外，归一化应变率显着依赖于标距，这导致使用统一本构模型计算出的流动应力之间存在超过 16% 的差异。材料的特性取决于标距长度，因此需要定义使用 Gleeble 测量材料热机械数据的测试标准。在研究范围内随标距的变化，从而确定不同的工程应力应变曲线和断裂应变。此外，归一化应变率显着依赖于标距，这导致使用统一本构模型计算出的流动应力之间存在超过 16% 的差异。材料的特性取决于标距长度，因此需要定义使用 Gleeble 测量材料热机械数据的测试标准。在研究范围内随标距的变化，从而确定不同的工程应力应变曲线和断裂应变。此外，归一化应变率显着依赖于标距，这导致使用统一本构模型计算出的流动应力之间存在超过 16% 的差异。材料的特性取决于标距长度，因此需要定义使用 Gleeble 测量材料热机械数据的测试标准。