Attachments
An Attachment is a set of boundaries, edges, or points on a flexible or rigid component used to connect it to another flexible or rigid component through a joint or spring. An attachment can be defined on the boundaries of a solid element, on the edges of a shell element, and on the points of a beam element.
When an attachment is connected to a flexible body, you can use two different formulations: rigid or flexible.
In the rigid attachment formulation, all selected boundaries or edges behave as if they were connected to a common rigid body. This may cause an unrealistic stiffening and local stress concentrations.
In the flexible version, the boundaries are allowed to deform, and the rigid body constraints are enforced only in an average sense.
The attachment formulation is similar to that of a rigid connector. In the rigid case, the only degrees of freedom needed to represent this assembly are the ones needed to describe the movement of a rigid body. In 2D this is just two in-plane translations, and the rotation around the out-of-plane axis.
In 3D the situation is more complex. Six degrees of freedom are necessary, usually selected as three translations and three parameters for the rotation. For finite rotations any choice of three rotation parameters is singular at some specific set of angles. For this reason, a four-parameter quaternion representation is used.
Some extra degrees of freedom are added for each attachment where the flexible formulation is used.
When an attachment is defined on a rigid component, it does not create any degrees of freedom of its own and directly picks the degrees of freedom of the rigid component.
The formulation of the attachment feature is same as the rigid connector formulation, which is discussed in Rigid Connector in the Structural Mechanics Module User’s Guide.
Some useful information about the attachment feature:
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The attachment can be defined either on a flexible or on a rigid component. It is not possible to select the boundaries from both types of components in a single attachment. The reason is the different formulation of attachment for flexible or rigid components.
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The attachment’s center of rotation is the centroid of its selected boundaries, edges, or points. In a joint, it is possible to select the attachment center of rotation as the center of the joint.
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If the attachment is connected to a flexible domain, the forces and moments on an attachment with a boundary selection are computed by summing the reaction forces on the selected boundaries. There is an option to use these forces and moments to evaluate the joint forces.
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For an attachment in the Shell interface, the selection consists of edges. The forces and moments on an attachment are then computed by summing the reaction forces and reaction moments on the selected edges. There is an option to use these forces and moments to evaluate the joint forces.
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For an attachment in the Beam interface, the selection consists of points. The forces and moments on an attachment are then computed by summing the reaction forces and reaction moments on the selected points. There is an option to use these forces and moments to evaluate the joint forces.
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When an attachment is defined in the Shell or Beam interface, the normal force cannot be defined through Attachment reaction forces in the Friction subnode of a joint.
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In a joint you select attachments to establish its connection with flexible, or optionally, rigid bodies. This is accomplished by setting up a relation between the source attachment and destination attachment degrees of freedom. Thus, the motion of an attachment is governed by the joints where this attachment is used.
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An attachment which is not referenced by any joint acts as an unconstrained rigid connector.
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The Multibody Dynamics Interface
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The Solid Mechanics Interface, The Shell and Plate Interfaces, and The Beam Interface in the Structural Mechanics Module User’s Guide.
 
The links to sections described in external guides, such as the Structural Mechanics Module User’s Guide, do not work in the PDF, only from the on-line help in COMSOL Multiphysics.