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Technical Review 1 - Bolted Joints and Contact

Bolted Joint Separation, Slip and Fatigue - FE Representation of Preload

Perhaps the most basic characteristic of a bolted joint is that the preload in the bolts varies as a result of changes in the external load applied to the joint. The value of preload in operation is important to prevent slip, separation or fatigue failure of the joint.

The required preload is achieved by tightening the nut which essentially puts in an amount of strain in the active part of the bolt, between the head and the nut. As a result of changes in operating load, this initial strain will vary to maintain force equilibrium between the bolt itself and the surrounding material (the abutment) on which the external load is acting.

Representing this in FEA is not straightforward since any initial strain specified in an ordinary element will remain fixed as the load is applied. If the bolt is represented as a 2 node beam, then the problem can be solved through the application of a temperature differential to the beam's nodes. This imposes a prescribed strain and providing the abutment as well as the means by which the external load is applied to it are represented well, then this initial strain will react to changes in the external load and correctly represent the behaviour of the bolt.

The figure shows a 'slice' through a bolted joint represented in this way. This is a fairly complex joint, with misalignment. The model was created to show how the stresses in the abutment were affected by both misalignment and the application of external load.

Use of Beam Elements to Predict Slip

In contrast to the relatively complex contact model described above, beam elements can be used to connect shells or solid elements in a joint. Although this gives misleading stresses in the region of the beams, the load transfer across the joint, as well as the distribution along it, can be represented using this technique. Consequently, this approach is used in FE models to provide bolt shear forces and predict the occurrence of bolt slip; a precondition for fatigue failure of the bolt or fretting in the joint.

Conrod Bolts

There are inertial loads on a conrod arising from the very high linear acceleration of the piston and rotation of the crankshaft, as well as the reciprocating movement of the conrod itself. It is important to ensure that the big end bolt loads retain sufficient preload and do not stretch, possibly allowing separation of the cap from the rod itself.

An alternative method to that described above can be used to represent a preloaded bolt: the ABAQUS™ bolt element has been used successfully in this situation.

Normally, bolted joints don't fail in the abutment. However, due to space restrictions around a big end bearing, the amount of abutment material is less than for a conventional joint and stresses in the abutment may need to be considered. The effect of the bolt preload also causes a radial load, via the thread helix angle; this can be represented by using rings of rod elements between the bolt elements and the bolt hole inclined at the helix angle such that, as the bolt preload is applied, the rods tend to expand the bolt hole.

Modelling Contact

Interaction between mating parts can be represented in many FE packages, using various algorithms and graphic systems. Only certain element formulations are compatible with contact analyses and the analyst must ensure that a compatible element type is specified. Runs can be carried out using incompatible elements, but convergence will never be achieved.

Modelling contact requires an iterative solution, using steps in 'pseudo-time'. For conventional implicit codes, these are usually conditionally unstable and so it is up to the user to control the run parameters to ensure appropriate convergence and accuracy.

The contact elements themselves (often called 'gap' elements) are usually 2 stage springs, which ideally have an infinitely high stiffness when the gap is closed and zero stiffness when it is open. In practice these values have to be similar enough to the other entries in the stiffness matrix to avoid ill conditioning and so have to have less extreme values if the solution is to converge. Adjusting these stiffness values may be important in obtaining a reliable result.

Problems with multiply connected bodies often arise as a result of spurious rigid body movements of the various parts. In general, all of these have to be removed to get the solution to converge.

Contact in a Conrod

Conrods consist of several parts. Contact between some or all of these will have to be represented. This would include:

• Big end cap to main body

• The big end shells to each other

• Big end bores

• Small end shells to the small end bore

• Bolt heads to the big end cap