SUMMARY
In the modeling of bolt connectors using beam elements, the inclusion of both bolt preload and thermal effects is crucial for ensuring accurate and reliable simulation results. Bolt preload, which represents the initial tension applied during assembly, significantly influences the load distribution and joint stiffness, while thermal effects account for changes in material properties and dimensions under varying temperatures, impacting the overall structural behavior. By incorporating these factors into beam element models, engineers can capture the essential mechanical behavior of bolt connections without resorting to computationally expensive full solid modeling. This approach not only streamlines the simulation process but also avoids excessive computational costs, enabling efficient and practical analysis for complex assemblies.
In this blog entry, the method is explained using a direct and simple example, where the different steps are shown in combination with the model description.
Model Creation :
In summary, the process involves creating beams in SpaceClaim/Discovery and establishing contacts to connect these beams to their corresponding holes, ensuring proper interaction and load transfer. This method aligns closely with the approach employed by Mechanical in the background.
Here some details how to create and define the model.
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Create the beams in SpaceClaim/ Discovery using a solid cylinder and then extracting the beam with Prepare->Extract.
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In this example, an additional beam has been added in the corner for further comparison.
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In Mechanical is possible to identify these beams. Please note the cross section properties, they are extracted from the original cylinder created in SpaceClaim. The 'Thermal Strain Effects' are activated by default.
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Create bonded contacts between the beam's end and bolt connection zones. Two contacts per beam are necessary.
Here is important to change the formulation to 'Beam', assign a material and a radius. This will allow to transfer heat energy through the beams. Defining this information will be useful to represent the bolt head and nut. Please consider you need to appropriately represent structural an thermal behavior by these values. For the second beam an additional contact between the beams end and solid corner is defined.
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After applying some thermal boundary conditions is possible to solve the transient heat transfer model. The free end of the corner beam has been defined as 10C. Note the temperature gradient at each beam.
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Isolating the LINK33 elements created in the thermal model in a user defined result, is possible to see how the connection beams transfer heat from the solid to the bolt line elements.
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Using this thermal solution, the load transfer to structural analysis is made in the usual way:
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The boundary conditions include a frictional contact between solid parts, a fixed support in the legs of the bottom solid and the free beam end. The bolt preload is included as well the load import from the third time step of the thermal analysis.
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Now the Structural model is solved considering both thermal and preload effects on the bolts:
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Because both ends of the corner beam are fixed by the structural boundary conditions, the stresses there are due only to the thermal load. This allows us to verify the thermal load on the beam element.
Finally, because creating all these beam and contacts can increase a lot the work to be done you can use the Object Generator tool to speed up this task.
https://blog.ozeninc.com/resources/improving-productivity-in-mechanical-with-object-generator
CONCLUSION
In conclusion, accurately modeling bolt connectors using beam elements requires careful consideration of both bolt preload and thermal effects to capture the critical factors influencing joint performance. This method not only provides a computationally efficient alternative to full solid modeling but also ensures reliable results for engineering analysis. To facilitate better understanding, an example model demonstrating this approach is available for download. This example serves as a practical reference, helping to illustrate the implementation and benefits of incorporating preload and thermal effects in bolt connector simulations.
Downloadable Resources
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