Determination Of Parameters In The Localised Decrease In Cross Sectional Area In Tensile Specimens Using DIC

使用 DIC 确定拉伸试样横截面积局部减少的参数

• 批准号: 1842954
• 金额: --
• 依托单位: Imperial College London
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-1842954
• 关键词: Determination; Parameters; Localised; Decrease; Cross

Project Summary: The true mechanical response of a material subjected to a tension test is recorded as a plot of true-stress against true-strain. This is only valid up to the instability point (i.e. UTS) when a localised decrease in the cross-sectional area occurs (necking). The onset of necking is accompanied by the establishment of a tri-axial state of stress in the neck: the uni-axial state is destroyed by the geometrical irregularity. Bearing in mind that the flow stress of a material is strongly dependent on the state of stress, a correction has to be introduced to convert the tri-axial flow stress into a uni-axial stress. The tri-axial stresses in the neck, which include the tensile component perpendicular to the axis of the specimen generated by the boundaries of the neck, will depend on the geometry of the neck. A correction can be applied; the most commonly used is that for cylindrical specimens via the Bridgeman equation [1]. This requires a continuous monitoring of the radius of curvature of the neck, and the radius of the cross section at the thinnest part of the neck. In-situ monitoring techniques are required to measure these variables for the purpose of calibrating material damage and fracture models (e.g. modified Armstrong-Zerilli). The development of experimental techniques to quantify necking in-situ will allow material behaviour to be established beyond the instability point in tension. The use of Digital Image Correlation (DIC) and other advanced optical methods (e.g. prism-based macrophotography and image processing as in Dunnett et al. [2]) will enable a technique to be developed that could continuously monitor the necking parameters (i.e. curvature of the neck and the radius of the cross section) over the entire stress-strain curve to failure, covering the quasi-static to dynamic strain rate range at a variety of test temperatures. Incorporating and relating this additional information to constitutive material models for utilisation by design and theoretical physics simulations is then required. It is common with many empirical and semi-physical models to use strain as a "state" parameter. In general, strain is not appropriate as a state parameter as it limits the models capability of handling situations where a path change occurs, as in the case of a strain rate change or inevitably temperature change due to adiabatic heating during deformation. The work will therefore make use of the Mechanical Threshold Stress (MTS) model, which uses the mechanical threshold stress as an internal state parameter to characterise the "structure" of a material at every deformation state. A working model establishing parameters for a number of metals covering the FCC, HPC and BCC crystal structures will be confirmed.

项目摘要：材料经过拉伸测试后的真实机械响应被记录为真应力与真应变的曲线图。当横截面面积发生局部减小(缩颈)时，这仅在失稳点(即UTS)之前有效。颈缩的开始伴随着颈部三轴应力状态的建立：单轴状态被几何不规则性破坏。考虑到材料的流动应力强烈地依赖于应力状态，必须引入修正以将三轴流动应力转换为单轴应力。颈部的三轴应力，包括由颈部边界产生的垂直于试件轴线的拉伸分量，将取决于颈部的几何形状。可以应用修正；最常用的是通过Bridgeman方程对圆柱形试件进行修正[1]。这需要持续监测颈部的曲率半径，以及颈部最薄部分的横截面半径。需要现场监测技术来测量这些变量，以便校准材料损伤和断裂模型(例如，修正的阿姆斯特朗-泽里利模型)。原位定量颈缩的实验技术的发展将使材料的行为能够在拉伸不稳定点之外建立起来。使用数字图像相关(DIC)和其他先进的光学方法(如Dunnett等人的基于棱镜的微距摄影和图像处理)。[2])将使一种技术得以开发，该技术可以在整个应力-应变曲线上连续监测直至失效的颈缩参数(即颈曲率和横截面半径)，覆盖在各种试验温度下的准静态到动态应变率范围。然后，需要将这些附加信息与构造材料模型结合起来，以供设计和理论物理模拟使用。在许多经验模型和半物理模型中，使用应变作为“状态”参数是很常见的。一般来说，应变不适合作为状态参数，因为它限制了模型处理路径变化的能力，例如应变速率变化或变形过程中绝热加热导致的不可避免的温度变化。因此，这项工作将利用机械门槛应力(MTS)模型，该模型使用机械门槛应力作为内部状态参数来表征材料在每种变形状态下的“结构”。将确定一些金属的工作模型，以确定涵盖FCC、HPC和BCC晶体结构的参数。