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

项目概述：材料在拉力试验中的真实力学响应被记录为真应力与真应变的关系图。当横截面积发生局部减少（颈缩）时，这仅在不稳定点（即UTS）之前有效。颈缩的发生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。记住，材料的流动应力很大程度上依赖于应力状态，因此必须引入校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样轴线的拉伸分量，将取决于颈部的几何形状。可以进行修正；最常用的是圆柱形试样通过布里奇曼方程[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。为了校准材料损伤和断裂模型（例如修改的Armstrong-Zerilli），需要使用现场监测技术来测量这些变量。量化缩颈的实验技术的发展将允许在张力不稳定点之外建立材料行为。数字图像相关（DIC）和其他先进的光学方法（如Dunnett et al.[2]中基于棱镜的宏观摄影和图像处理）的使用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颈缩参数（即颈曲率和截面半径）直至失效，涵盖各种测试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合并并关联到本构材料模型中，以便通过设计和理论物理模拟加以利用。在许多经验和半物理模型中，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情况的能力，例如变形过程中应变率发生变化或由于绝热加热而不可避免地发生温度变化。因此，这项工作将利用机械阈值应力（MTS）模型，该模型使用机械阈值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”。一个工作模型建立参数的许多金属涵盖FCC， HPC和BCC晶体结构将得到确认。

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.