一、特征長度
有兩種解釋：
定義一：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.
定義二：
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.
通俗的漢語即為，畫出的網格有很多條邊，這些邊用哪一個作為參與計算的長度呢？平面問題使用面積的平方根，三維使用體積的立方根，特殊的（參照定義二），軸對稱使用r-z平面的特征長度，粘性元件，使用本構厚度（這里不太理解），梁和桁架，作為一維單元，使用單元軸的長度，殼使用參考表面的特征長度。
鑒于上述解釋，要求網格劃分中的網格盡量長度大小一致，均勻，避免狹長，過大過小，保證網格的長寬比為1.
網上（http://blog.sina.com.cn/s/blog_8fdfa8530102vm8n.html）提到做子程序開發時，有時候會用到單元特征長度，尤其是對于實體單元，子程序自帶的特征長度在做斷裂擴展時，是不能直接用的，需要自己根據裂紋擴展方向判斷哪邊為特征長度方向，然后將子程序自帶的特征長度予以轉化即可。例如，對于復合材料實體單元，厚度很小，面內損傷情況下，用體積的立方根作為單元特征長度誤差就會交大，特征長度就要選取厚度方向
二、displacement at failure（失效位移）
http://130.149.89.49:2080/v6.14/books/usb/default.htm提到的計算方法為：
As discussed previously, once the damage initiation criterion has been reached, the effective plastic displacement(等效塑性應變), , 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.
三種表示方法：

（只介紹線性）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(接觸間隙），heat generation(熱產生)
選用表格后，表格數據必須從零間隙（閉合間隙）開始，并隨著間隙的增加定義導熱系數。您必須至少提供兩對點。在最后一個數據點之后，熱導率的值立即降為零，因此，當間隙大于對應于最后一個數據點的值時，則沒有熱導率。如果熱導率也未定義為接觸壓力的函數，則對于所有壓力，熱導率將保持恒定在零間隙值。
熱產生由兩個參數控制，前者\eta表示摩擦中產生的能量有所少轉化為熱能，默認全部轉化，第二個參數f表示產生的熱能如何分配給接觸對，f表示slave 分配到的熱量。
四、contact - mechanical -normal behavior

constraint enforcement method 中的默認為使用“接觸壓力-過閉合關系強制實施約束”

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