接触笔记
1、Defining contact pairs in ABAQUS/Standard
After the selection of contact pair surfaces, three key factors must be determined when creating a contact formulation:
⑴ the contact discretization; ⑵ the tracking approach; and
⑶ the assignment of “master” and “slave” roles to the respective surfaces.
1.1 the contact discretization
ABAQUS/Standard offers two contact discretization options: a traditional “node-to-surface” discretization and a true “surface-to-surface” discretization. 1.1.1 Node-to-surface contact discretization
Traditional node-to-surface discretization has the following characteristics:
⑴ The slave nodes are constrained not to penetrate into the master surface; however, the nodes of the master surface can, in principle, penetrate into the slave surface
⑵ The contact direction is based on the normal of the master surface.
⑶ The only information needed for the slave surface is the location and surface area associated with each node; The direction of the slave surface normal and slave surface curvature are not relevant.Thus, the slave surface can be defined as a group of nodes—a node-based surface.
⑷ Node-to-surface discretization is available even if a node-based surface is not used in the contact pair definition
Fig.1 Node-to-surface contact discretization
1.1.2 Surface-to-surface contact discretization To optimize stress accuracy, surface-to-surface discretization considers the shape of both the slave and master surfaces in the region of contact constraints. Surface-to-surface discretization has the following key characteristics: ⑴ Contact conditions are enforced in an average sense over the slave surface, rather than at discrete points (such as at slave nodes, as in the case of node-to-surface discretization). Therefore, some penetration may be observed at individual nodes; however, large, undetected penetrations of master nodes into the slave surface do not occur with this discretization. ⑵ Surface-to-surface discretization is not applicable if a node-based surface is used in the
contact pair definition. 在某一个迭代步中,面对面的接触计算成本一般较点对面的接触的计算成本高,但多数情况下这个成本不会高很多,只有在下列情况下才会让计算成本急剧增大: ⑴ 模型的绝大部分区域被包含于接触中; ⑵ 当主动面比从属面网格划分还要精细时; ⑶ Multiple layers of shells are involved in contact, such that the master surface of one contact pair acts as the slave surface of another contact pair. 尽管如此,但点对面的接触需要花费更多的迭代步才能达到数值稳定,从某种意义上来说,在一个分析步中,无法判定到底是用点对面接触还是面对面接触计算成本低.
1.2 Contact tracking approaches
In ABAQUS/Standard there are two tracking approaches to account for the relative motion of the two surfaces forming a contact pair in mechanical contact simulations:
⑴ The finite-sliding tracking approach ⑵ The small-sliding tracking approach 1.3 Fundamental choices affecting the contact formulation
Your choice of contact discretization and tracking approach have considerable impact on an analysis. In addition to the qualities already discussed, certain combinations of discretizations and tracking approaches have their own characteristics and limitations associated with them. These characteristics are summarized in Table 1. You should also consider the solution costs associated with the various contact formulations
Table 1 Comparison of contact formulation characteristics
1.4 选择主动面和从属面的几个原则
⑴ Analytical rigid surfaces and rigid-element-based surfaces must always be the master surface. ⑵ A node-based surface can act only as a slave surface and always uses node-to-surface contact. ⑶ Slave surfaces must always be attached to deformable bodies or deformable bodies defined as rigid. ⑷ Both surfaces in a contact pair cannot be rigid surfaces with the exception of deformable surfaces defined as rigid
一般来说,当定义两个基于单元的面作为解除对作用面时,当存在一个较小的面和一个较大的面时，一般将较小的面定义为从属面。当两个面大小接近时，选取较“硬”的面或单元划分笔记粗糙的面作为主动面。值得注意的是，“硬”的面不一定是材料弹性模量大的材料，比如当一个薄金属片和一个橡胶材料接触时，此时就应该将薄金属片所属的面定义为从属面。当两个面区域接近，“硬度”也接近时，此时往往需要反复尝试才能得到较好的结果。 与点对面接触相比，面对面接触中主动面和从属面的选取，对计算结果的影响并不是很大。但是，当错误的将网格粗糙的面定义为从属面时，此时也许会引起计算成本的急剧增加。
1.5定义接触对
为了定义一个接触对，必须指定一对接触面或者一个自接触的面，一个contact formulation。每一个接触对可以定义不同的作用面性质。 1.5.1 Defining contact between two separate surfaces When a contact pair contains two surfaces, the master and slave surfaces are not allowed to include any of the same nodes and you must choose which surface will be the slave and which will be the master. ABAQUS/Standard定义接触默认采用的是有限滑移、点对面接触。如果定义的是小滑移，默认的也吃采用点对面接触。
1.5.2 用对称的主从接触对提高接触模拟精度 对于点对面接触，主动面上的节点很容易penetrate到从属面上去，此时，提高从属面上单元的网格划分精度，有助于减少这样的刺入，提高运行速度。才外，让两个面都是基于单元定义时，可以用symmetric master-slave method。To use this method, define two contact pairs using the same two surfaces, but switch the roles of master and slave surface for the two contact pairs. This method causes ABAQUS/Standard to treat each surface as a master surface and, thus, involves additional computational expense because contact searches must be conducted twice for the same contact pair. The increased accuracy provided by this method must be compared to the additional computational cost. All of the contact formulations are available for symmetric master-slave contact pairs, and can be applied using the same options discussed above. 命令语句如下： *CONTACT PAIR, INTERACTION=interaction_property_name
surface_1, surface_2 surface_2, surface_1
1.5.2.1 对称主从接触结果的解释 对于单一的主从接触，输出结果仅对从属面输出。而对称主从接触，每一个面都是从属面，均输出计算结果。问题在于，两个从属面上的接触压力并不是相对独立的，也并不一定相等，总的接触压力为两个面上的接触压力之和。
1.6给接触对赋予接触面定义
命令语句如下：
*CONTACT PAIR, INTERACTION=interaction_property_name *SURFACE INTERACTION, NAME=interaction_property_name
1.7 选择接触面
除了小滑移、面对面接触之外，主动面必须为单一面。
三维梁单元、桁架单元，不能用来作为主动面，但却可以定义为从属面。二维梁单元、桁架单元可以定义为主动面或从属面。 Edge-based surfaces on three-dimensional shell elements cannot be used in a contact analysis in ABAQUS/Standard.
1.8 结果输出
You can write the contact surface variables associated with the interaction of contact pairs to the ABAQUS/Standard data (.dat), results (.fil), and output database (.odb) files. All contact pair results are given at the constraint points of the slave surface. The constraint points correspond to the slave nodes except in the case of finite-sliding, surface-to-surface contact, in which case each slave facet contains multiple constraint points. You can: ⑴ request output associated with a given contact pair;
⑵ request output associated with a given slave surface, including contributions from all of the contact pairs to which the slave surface belongs; and
⑶ limit the output by specifying a node set containing a subset of the nodes on the slave surface except in the case of finite-sliding, surface-to-surface contact. 下面为常用的接触输出语句：
*CONTACT PRINT, SLAVE=SURFNAME, MASTER=SURFNAME, NSET=NODESET 以下为输出到.dat文件的结果形式：
对于结果的解释： ⑴ This output request creates a table of output variables in the printed data (.dat) file. Each row of the table corresponds to a slave node in node set SNODES. The first column of the table identifies the slave node for that row. Because this is a mechanical contact simulation, the second column specifies the contact status at the slave node. Since the contact property definition includes frictional properties, the contact status may be open (OP), closed and sticking tangentially (ST), or closed and sliding tangentially (SL). The remaining columns contain the surface variables requested. In this example the default variables—contact pressure, contact opening, frictional shear stress, and relative tangential slip—were requested. ⑵ The OP status indicates that the slave node is not in contact with the master surface. In the sample output above, node 101 is open and, consequently, the contact pressure variable CPRESS is zero. The COPEN variable reports that this node is 0.66 length units away from the master surface. ⑶ The ST status indicates that the slave node is in contact with the master surface and is “sticking.” The frictional shear stress acting at the node is below the critical shear stress , where p is the value of contact pressure shown under CPRESS. In the sample output above, node 102 is sticking since the frictional shear stress CSHEAR1 is below the critical value of 2.64 (0.4 × 6.59). The CSLIP1 variable is the total accumulated (integrated) slip at the slave node. The negative magnitude of CSLIP1 indicates that the node has moved in the negative first slip direction on BSURF. Accumulated slip and slip directions are discussed in more detail below in “Output of tangential motion of the surfaces.”
⑷ The SL status indicates that the slave node is in contact with the master surface and it is sliding—the frictional shear stress is at the critical shear stress ==. In the sample output above, node 103 is sliding, and the frictional shear stress CSHEAR1 is equal to the friction limit 1.73 (0.4 × 4.32). ⑸ In the absence of frictional properties when a slave node is in contact with the master surface, its status reads CL for “closed.”
2、Modeling contact interference fits in ABAQUS/Standard
2.1 Resolving excessive initial overclosures
3、ABAQUS/Standard中接触模拟的常见错误
3.1 解决初始接触的错误
3.1.1 消除初始的过盈接触和张开 当两个不同Part的面接触时，由于单元网格不一致，很可能会在两个面之间留下小的gap或penetration。默认的，ABAQUS/Standard会将初始penetration当成interference fits，并会相应地在接触一开始的时候处理掉，见“Modeling contact interference fits in ABAQUS/Standard,” Section 29.2.4. 计算中必须通过调整从属面位置来提高接触模拟精度，以保证接触刚开始时计算中没有penetration。当初始的clearance或overclosure与单元典型尺寸相比较小时，在小滑移接触模拟中，你可以精确的指定clearance或overclosure，以消除初始的过盈接触和张开，见“Adjusting the surfaces in a contact pair” in “Adjusting initial surface positions and specifying initial clearances in ABAQUS/Standard contact pairs,” Section 29.2.5. 3.1.2 消除刚体位移 动力分析中刚体位移不会引起数值奇异问题，但在静力问题中，当一个体没有给予足够的约束时，将会引起刚体位移，从何会引起数值奇异问题和大位移（“Numerical singularity” warning messages and very large displacements） 可以通过指定该体的边界条件，或用弹簧或阻尼器将该体接地，以消除刚体位移。 如果不能通过上述办法消除刚体位移，ABAQUS/Standard还会提供一些工具，可以在接触模拟过程中，自动的解决刚体稳定性问题。见“Automatic stabilization of rigid body motions in contact problems” in “Adjusting contact controls in ABAQUS/Standard,” Section 29.2.12.
3.1.3 解决过大的interference fits ABAQUS/Standard interprets initial overclosures as interference fits, which it tries to resolve in the first increment of a step. If the initial overclosures are an unintended result of mesh discretization, you should use one of the methods discussed above to remove the overclosures. In some cases the interference fit may be intended but too large for ABAQUS/Standard to resolve in a single increment. In this situation you should redefine the interference fit to allow resolution of the overclosures over multiple increments. See “Modeling contact interference fits in ABAQUS/Standard,” Section 29.2.4, for more information.
3.2 低精度表面（Poorly defined surfaces）
粗糙的网格、不合适的单元和过度扭曲的表面形状，均可导致接触计算的中止。 3.2.1 主控面定义重复节点 三维有限滑移接触分析中，应避免用相同坐标的节点定义不同的surface，这样的定义容易引起接缝或裂缝（seam or crack）。 虽然从CAE默认的视角来看，这个定义的面仍然是一个连续有笑的面，但当接触计算开始时，从属面上的点很可能滑落到这个seam or crack中去，使得从属面上的某些节点被黏附在主控面后面，从而引起计算的中止。类似的情况也会出现在finite-sliding, surface-to-surface contact。
3.2.2 避免沿表面边界的接触问题 有限滑移接触分析中，主控面必须定义得足够大，以至于可计算接触分析中所用可能存在的位移。如果主控面定义不恰当，从属面上的从属点很可能在迭代计算过程中滑落到主控面后面，引起振颤问题（chattering）。 当计算过程中出现振颤问题时，msg文件中会有一个或几个从属点不停的循环出现闭合和张开，此时，可在关键词*Contact Pair中实用参数Extension Zone来扩大主控面的尺寸，具体见“Extending master surfaces and slide lines,” Section 29.2.8.
3.2.3 面单元网格粗糙 如果面单元网格太粗糙，将会发生以下几个问题： ⑴ 主控面刺入从属面过多 在点对面接触中，当从属面单元网格过于粗糙，以至于主控面会很严重的刺入到从属面
中去时，将会引起错误，此时应细化从属面单元网格。 对于面对面接触，虽然这种接触模式会有效抵制主控面刺入到从属面中，但当从属面网格比主控面还要粗糙时，此时的计算成本将会变得非常大。
3.3 接触模拟中的过多迭代
ABAQUS/Standard提供了一些方法，可以调整接触模拟迭代过程，以提高计算效率，并不影响计算精度。
3.3.1 Converting severe discontinuity iterations in weakly determined contact conditions ABAQUS/Standard对规则平稳的迭代和严重不连续迭代加以区分。最普遍的严重不连续现象有张开－闭合转变和静止－滑移的摩擦面行为转变。 在两种情形下，默认的算法会导致收敛问题或过多的小增量步。第一种情况是接触定义不明确。例如，在冲压问题中，冲头与薄片在边界上发生接触，但冲头中心处定义并不明确。典型的，该点处于接触状态时，接触压力会很小；当该点没有处于接触状态时，其张开距离也会很小。这就会引起振颤问题。 第二种情况是大接触问题，即一个模型中存在很多接触点（或接触对）。在此种情况下，ABAQUS会经过多次迭代来判定其初始接触条件（因为每一次迭代会有不同的接触点其接触状态发生变化，或张开、或闭合）。默认的情况下，
3.3.2 Controlling the increment size based on penetration distance in unconverged iterations
多数接触迭代计算过程中，如果penetration超过了指定的距离hcrit，ABAQUS/Standard就会放弃当前增量步，而尝试一个更小的增量步。对于有限滑移、面对面接触和几何线性小滑移分析，并没有严格的hcrit距离规定。
默认情况下，hcrit等于通过单元表面几何体的半径。在下面几类情形中，有必要修改hcrit⑴ 主控面高度扭曲。如图，在迭代过程中，从属面节点b有可能落入到主控面，相当距离值：
于从属节点penetration主控面，overclosure 即h，小于hcrit。此时迭代过程会尝试将节
点b移动到主控面上的投影c点处。为了避免此类迭代，可尝试指定一个较小的hcrit，强迫ABAQUS/Standard放弃当前操作，而尝试一个较小的增量步。
⑵ a node-based surface在接触面中存在时，ABAQUS/Standard将会无法计算得到一个合理的hcrit。当存在其它面接触队时， ABAQUS/Standard采用从属面上单元的average dimension作为hcrit。如果没有其它接触对存在，ABAQUS/Standard将采用整体模型的a characteristic element dimension。
⑶ 从属面形状变化剧烈
⑷ 软接触中允许较大的penetration。 命令语句如下：
*CONTACT PAIR, HCRIT=hcrit
4、Extending master surfaces and slide lines
延伸主控面或滑移线，可以： ⑴ 在有限滑移问题中，可以避免从属面节点落入或滑道主控面背后； ⑵ 在小滑移问题中，当主控面与从属面没有相接时，可以保证从属面节点能在主控面上找到一个投影点； ⑶ 能避免一些接触模型中数值计算问题； ⑷ 不能用来代替正确的接触； ⑸ 不能用来减少接触面下的单元数目（surface based on element） ⑹ 仅适用于点对面接触（node-to-surface discretization）
For node-to-surface contact you can specify the size of the extension zone, e, as a fraction of the end segment or facet edge length (see Figure 29.2.8–2). If e is set to zero, ABAQUS will not extend the ends. The value given must lie between 0.0 and 0.2. The default value is 0.1 for node-to-surface
contact; surface extensions are not available for surface-to-surface contact。
命令语句如下： *CONTACT PAIR, SMALL SLIDING, EXTENSION ZONE=e
5、调整接触控制
接触控制在ABAQUS/Standard中：
⑴ 不能用于修改多数接触问题的默认接触属性设置； ⑵ 可用于计算过程中未提供有效收敛控制的接触算法； ⑶ 可用于未建立有效接触模拟过程中。
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