abaqus

一、特征长度
有两种解释：
定义一：The definition of the characteristic length is based on the element geometry: for beams and trusses we use the integration point length; for shell and planar elements we use the square root of the integration point area; for solid elements we use the cube root of the integration point volume. This definition of the characteristic length is used because the direction in which fracture occurs is not known in advance. Therefore, elements with large aspect ratios will have rather different behavior depending on the direction in which they crack: some mesh sensitivity remains because of this effect, and elements that have aspect ratios close to unity are recommended.
定义二：
The definition of the characteristic length depends on the element geometry and formulation: it is a typical length of a line across an element for a first-order element; it is half of the same typical length for a second-order element. For beams and trusses it is a characteristic length along the element axis. For membranes and shells it is a characteristic length in the reference surface. For axisymmetric elements it is a characteristic length in the r–z plane only. For cohesive elements it is equal to the constitutive thickness. This definition of the characteristic length is used because the direction in which fracture occurs is not known in advance. Therefore, elements with large aspect ratios will have rather different behavior depending on the direction in which they crack: some mesh sensitivity remains because of this effect, and elements that have aspect ratios close to unity are recommended.
通俗的汉语即为，画出的网格有很多条边，这些边用哪一个作为参与计算的长度呢？平面问题使用面积的平方根，三维使用体积的立方根，特殊的（参照定义二），轴对称使用r-z平面的特征长度，粘性元件，使用本构厚度（这里不太理解），梁和桁架，作为一维单元，使用单元轴的长度，壳使用参考表面的特征长度。
鉴于上述解释，要求网格划分中的网格尽量长度大小一致，均匀，避免狭长，过大过小，保证网格的长宽比为1.
网上（http://blog.sina.com.cn/s/blog_8fdfa8530102vm8n.html）提到做子程序开发时，有时候会用到单元特征长度，尤其是对于实体单元，子程序自带的特征长度在做断裂扩展时，是不能直接用的，需要自己根据裂纹扩展方向判断哪边为特征长度方向，然后将子程序自带的特征长度予以转化即可。例如，对于复合材料实体单元，厚度很小，面内损伤情况下，用体积的立方根作为单元特征长度误差就会交大，特征长度就要选取厚度方向
二、displacement at failure（失效位移）
http://130.149.89.49:2080/v6.14/books/usb/default.htm提到的计算方法为：
As discussed previously, once the damage initiation criterion has been reached, the effective plastic displacement(等效塑性应变), , is defined with the evolution equation

where L is the characteristic length of the element. is the equivalent plastic strain rate
The evolution of the damage variable with the relative plastic displacement can be specified in tabular, linear, or exponential form. Instantaneous failure will occur if the plastic displacement at failure（失效位移）, , is specified as 0; however, this choice is not recommended and should be used with care because it causes a sudden drop of the stress at the material point that can lead to dynamic instabilities.
三种表示方法：

（只介绍线性）Assume a linear evolution of the damage variable with effective plastic displacement
You can specify the effective plastic displacement, , at the point of failure (full degradation). Then, the damage variable increases according to

This definition ensures that when the effective plastic displacement reaches the value , the material stiffness will be fully degraded (d=1). The linear damage evolution law defines a truly linear stress-strain softening response only if the effective response of the material is perfectly plastic (constant yield stress) after damage initiation.
三、clearance dependency(接触间隙），heat generation(热产生)
选用表格后，表格数据必须从零间隙（闭合间隙）开始，并随着间隙的增加定义导热系数。您必须至少提供两对点。在最后一个数据点之后，热导率的值立即降为零，因此，当间隙大于对应于最后一个数据点的值时，则没有热导率。如果热导率也未定义为接触压力的函数，则对于所有压力，热导率将保持恒定在零间隙值。
热产生由两个参数控制，前者\eta表示摩擦中产生的能量有所少转化为热能，默认全部转化，第二个参数f表示产生的热能如何分配给接触对，f表示slave 分配到的热量。
四、contact - mechanical -normal behavior

constraint enforcement method 中的默认为使用“接触压力-过闭合关系强制实施约束”