Abaqus 定义裂纹扩展方向 crack extension direction时的问题

Abaqus中定义crack时可以用Normal to crack plane来间接定义裂纹扩展方向(crack extension direction)，表达式为 q= t x n. q为crack extension direction, t为crack front的切线方向， n为定义的normal。 求教 t的方向是如何确定的？在建模时在什么地方可以定义？对于一个曲线上一个点，切线方向有两个。选择相反方向的 t的结果差别很大。
附上Abaqus Documentation中的片段：
Specifying the virtual crack extension direction

You must specify the direction of virtual crack extension at each crack tip in two dimensions or at each node along the crack line in three dimensions by specifying either the normal to the crack plane, , or the virtual crack extension direction, .

If the virtual crack extension direction is specified to point into the material (parallel to the crack faces), the J-integral values calculated will be positive. Negative J-integral values are obtained when the virtual crack extension direction is specified in the opposite direction.

Specifying the normal to the crack plane

The virtual crack extension direction can be defined by specifying the normal, , to the crack plane. In this case Abaqus/Standard will calculate a virtual crack extension direction, , that is orthogonal to the crack front tangent, , and the normal, . As shown in Figure 11.4.2–1, for a three-dimensional crack; for a two-dimensional crack, we simply have and . Specifying the normal implies that the crack plane is flat since only one value of can be given per contour integral.

Figure 11.4.2–1 Typical focused mesh for fracture mechanics evaluation.