abaqus系列之接觸.doc

接觸筆記1、Defining contact pairs in ABAQUS/StandardAfter the selection of contact pair surfaces, three key factors must be determined when creating a contact formulation: the contact discretization; the tracking approach; and the assignment of “master” and “slave” roles to the respective surfaces.1.1 the contact discretizationABAQUS/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 discretizationTraditional node-to-surface discretization has the following characteristics: 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 The contact direction is based on the normal of the master surface. 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 nodesa node-based surface. Node-to-surface discretization is available even if a node-based surface is not used in the contact pair definitionFig.1 Node-to-surface contact discretization1.1.2 Surface-to-surface contact discretizationTo 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: 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. Surface-to-surface discretization is not applicable if a node-based surface is used in the contact pair definition.在某一個迭代步中,面對面的接觸計算成本一般較點對面的接觸的計算成本高,但多數(shù)情況下這個成本不會高很多,只有在下列情況下才會讓計算成本急劇增大: 模型的絕大部分區(qū)域被包含于接觸中; 當(dāng)主動面比從屬面網(wǎng)格劃分還要精細(xì)時; 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.盡管如此,但點對面的接觸需要花費更多的迭代步才能達(dá)到數(shù)值穩(wěn)定,從某種意義上來說,在一個分析步中,無法判定到底是用點對面接觸還是面對面接觸計算成本低.1.2 Contact tracking approachesIn 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: The finite-sliding tracking approach The small-sliding tracking approach1.3 Fundamental choices affecting the contact formulationYour 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 formulationsTable 1 Comparison of contact formulation characteristics1.4 選擇主動面和從屬面的幾個原則 Analytical rigid surfaces and rigid-element-based surfaces must always be the master surface. A node-based surface can act only as a slave surface and always uses node-to-surface contact. Slave surfaces must always be attached to deformable bodies or deformable bodies defined as rigid. Both surfaces in a contact pair cannot be rigid surfaces with the exception of deformable surfaces defined as rigid一般來說,當(dāng)定義兩個基于單元的面作為解除對作用面時,當(dāng)存在一個較小的面和一個較大的面時，一般將較小的面定義為從屬面。當(dāng)兩個面大小接近時，選取較“硬”的面或單元劃分筆記粗糙的面作為主動面。值得注意的是，“硬”的面不一定是材料彈性模量大的材料，比如當(dāng)一個薄金屬片和一個橡膠材料接觸時，此時就應(yīng)該將薄金屬片所屬的面定義為從屬面。當(dāng)兩個面區(qū)域接近，“硬度”也接近時，此時往往需要反復(fù)嘗試才能得到較好的結(jié)果。與點對面接觸相比，面對面接觸中主動面和從屬面的選取，對計算結(jié)果的影響并不是很大。但是，當(dāng)錯誤的將網(wǎng)格粗糙的面定義為從屬面時，此時也許會引起計算成本的急劇增加。1.5定義接觸對為了定義一個接觸對，必須指定一對接觸面或者一個自接觸的面，一個contact formulation。每一個接觸對可以定義不同的作用面性質(zhì)。1.5.1 Defining contact between two separate surfacesWhen 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定義接觸默認(rèn)采用的是有限滑移、點對面接觸。如果定義的是小滑移，默認(rèn)的也吃采用點對面接觸。1.5.2 用對稱的主從接觸對提高接觸模擬精度對于點對面接觸，主動面上的節(jié)點很容易penetrate到從屬面上去，此時，提高從屬面上單元的網(wǎng)格劃分精度，有助于減少這樣的刺入，提高運行速度。才外，讓兩個面都是基于單元定義時，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.命令語句如下： *CONTACT PAIR, INTERACTION=interaction_property_namesurface_1, surface_2surface_2, surface_11.5.2.1 對稱主從接觸結(jié)果的解釋對于單一的主從接觸，輸出結(jié)果僅對從屬面輸出。而對稱主從接觸，每一個面都是從屬面，均輸出計算結(jié)果。問題在于，兩個從屬面上的接觸壓力并不是相對獨立的，也并不一定相等，總的接觸壓力為兩個面上的接觸壓力之和。1.6給接觸對賦予接觸面定義命令語句如下：*CONTACT PAIR, INTERACTION=interaction_property_name*SURFACE INTERACTION, NAME=interaction_property_name1.7 選擇接觸面除了小滑移、面對面接觸之外，主動面必須為單一面。三維梁單元、桁架單元，不能用來作為主動面，但卻可以定義為從屬面。二維梁單元、桁架單元可以定義為主動面或從屬面。Edge-based surfaces on three-dimensional shell elements cannot be used in a contact analysis in ABAQUS/Standard.1.8 結(jié)果輸出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: request output associated with a given contact pair; request output associated with a given slave surface, including contributions from all of the contact pairs to which the slave surface belongs; and 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.下面為常用的接觸輸出語句：*CONTACT PRINT, SLAVE=SURFNAME, MASTER=SURFNAME, NSET=NODESET以下為輸出到.dat文件的結(jié)果形式：對于結(jié)果的解釋： 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 variablescontact pressure, contact opening, frictional shear stress, and relative tangential slipwere requested. 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. 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.” The SL status indicates that the slave node is in contact with the master surface and it is slidingthe 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). 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/Standard2.1 Resolving excessive initial overclosures3、ABAQUS/Standard中接觸模擬的常見錯誤3.1 解決初始接觸的錯誤3.1.1 消除初始的過盈接觸和張開當(dāng)兩個不同Part的面接觸時，由于單元網(wǎng)格不一致，很可能會在兩個面之間留下小的gap或penetration。默認(rèn)的，ABAQUS/Standard會將初始penetration當(dāng)成interference fits，并會相應(yīng)地在接觸一開始的時候處理掉，見“Modeling contact interference fits in ABAQUS/Standard,” Section 29.2.4.計算中必須通過調(diào)整從屬面位置來提高接觸模擬精度，以保證接觸剛開始時計算中沒有penetration。當(dāng)初始的clearance或overclosure與單元典型尺寸相比較小時，在小滑移接觸模擬中，你可以精確的指定clearance或overclosure，以消除初始的過盈接觸和張開，見“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 消除剛體位移動力分析中剛體位移不會引起數(shù)值奇異問題，但在靜力問題中，當(dāng)一個體沒有給予足夠的約束時，將會引起剛體位移，從何會引起數(shù)值奇異問題和大位移（“Numerical singularity” warning messages and very large displacements）可以通過指定該體的邊界條件，或用彈簧或阻尼器將該體接地，以消除剛體位移。如果不能通過上述辦法消除剛體位移，ABAQUS/Standard還會提供一些工具，可以在接觸模擬過程中，自動的解決剛體穩(wěn)定性問題。見“Automatic stabilization of rigid body motions in contact problems” in “Adjusting contact controls in ABAQUS/Standard,” Section 29.2.12.3.1.3 解決過大的interference fitsABAQUS/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）粗糙的網(wǎng)格、不合適的單元和過度扭曲的表面形狀，均可導(dǎo)致接觸計算的中止。3.2.1 主控面定義重復(fù)節(jié)點三維有限滑移接觸分析中，應(yīng)避免用相同坐標(biāo)的節(jié)點定義不同的surface，這樣的定義容易引起接縫或裂縫（seam or crack）。雖然從CAE默認(rèn)的視角來看，這個定義的面仍然是一個連續(xù)有笑的面，但當(dāng)接觸計算開始時，從屬面上的點很可能滑落到這個seam or crack中去，使得從屬面上的某些節(jié)點被黏附在主控面后面，從而引起計算的中止。類似的情況也會出現(xiàn)在finite-sliding, surface-to-surface contact。3.2.2 避免沿表面邊界的接觸問題有限滑移接觸分析中，主控面必須定義得足夠大，以至于可計算接觸分析中所用可能存在的位移。如果主控面定義不恰當(dāng)，從屬面上的從屬點很可能在迭代計算過程中滑落到主控面后面，引起振顫問題（chattering）。當(dāng)計算過程中出現(xiàn)振顫問題時，msg文件中會有一個或幾個從屬點不停的循環(huán)出現(xiàn)閉合和張開，此時，可在關(guān)鍵詞*Contact Pair中實用參數(shù)Extension Zone來擴(kuò)大主控面的尺寸，具體見“Extending master surfaces and slide lines,” Section 29.2.8.3.2.3 面單元網(wǎng)格粗糙如果面單元網(wǎng)格太粗糙，將會發(fā)生以下幾個問題： 主控面刺入從屬面過多在點對面接觸中，當(dāng)從屬面單元網(wǎng)格過于粗糙，以至于主控面會很嚴(yán)重的刺入到從屬面中去時，將會引起錯誤，此時應(yīng)細(xì)化從屬面單元網(wǎng)格。對于面對面接觸，雖然這種接觸模式會有效抵制主控面刺入到從屬面中，但當(dāng)從屬面網(wǎng)格比主控面還要粗糙時，此時的計算成本將會變得非常大。3.3 接觸模擬中的過多迭代ABAQUS/Standard提供了一些方法，可以調(diào)整接觸模擬迭代過程，以提高計算效率，并不影響計算精度。3.3.1 Converting severe discontinuity iterations in weakly determined contact conditionsABAQUS/Standard對規(guī)則平穩(wěn)的迭代和嚴(yán)重不連續(xù)迭代加以區(qū)分。最普遍的嚴(yán)重不連續(xù)現(xiàn)象有張開閉合轉(zhuǎn)變和靜止滑移的摩擦面行為轉(zhuǎn)變。在兩種情形下，默認(rèn)的算法會導(dǎo)致收斂問題或過多的小增量步。第一種情況是接觸定義不明確。例如，在沖壓問題中，沖頭與薄片在邊界上發(fā)生接觸，但沖頭中心處定義并不明確。典型的，該點處于接觸狀態(tài)時，接觸壓力會很??；當(dāng)該點沒有處于接觸狀態(tài)時，其張開距離也會很小。這就會引起振顫問題。第二種情況是大接觸問題，即一個模型中存在很多接觸點（或接觸對）。在此種情況下，ABAQUS會經(jīng)過多次迭代來判定其初始接觸條件（因為每一次迭代會有不同的接觸點其接觸狀態(tài)發(fā)生變化，或張開、或閉合）。默認(rèn)的情況下，3.3.2 Controlling the increment size based on penetration distance in unconverged iterations多數(shù)接觸迭代計算過程中，如果penetratio