Composite Swingarm: Layup Design and Failure Analysis

The problem

Swingarms must be stiff and strong while carrying hub-motor or chain loads and motor torque reaction. Composites can reduce mass, but layup design, failure prediction and joints to metal parts are hard. Students need a rigorous method that links layups to stiffness and failure, and is honest about what shell first-ply failure analysis can and cannot predict.

Objectives

• Define swingarm load cases including vertical, braking, lateral and motor torque reaction.
• Build a shell model with composite layups, ply orientations and metal inserts.
• Check stiffness, first-ply failure with Tsai-Wu and other criteria, and buckling.
• Compare mass and stiffness with an aluminium baseline and validate with tests.

What you need

Key concepts

Layup

The stacking sequence of plies with their materials, thicknesses and fibre angles, for example 0, ±45 and 90 degrees.

Classical laminate theory

A method to compute laminate stiffness and ply stresses from ply properties and the stacking sequence, useful to check simple sections by hand.

First-ply failure

The load at which the first ply meets a failure criterion such as Tsai-Wu. It is a conservative design point but does not predict delamination or final failure.

Metal insert

A metal part bonded or co-moulded into the composite to take bearing and bolt loads at the pivot and axle.

Step-by-step guide

1. 1

   Define loads and baseline

   Take an example e-scooter or electric motorcycle swingarm with a stated rear wheel load and motor torque. Define load cases: vertical bump, braking, lateral cornering and motor torque reaction for a hub motor or chain pull. Model an aluminium baseline swingarm and run static studies for stiffness and stress so the composite has a target. Label all load factors as assumptions.

   Checkpoint: A load table and baseline results for mass, vertical and torsional stiffness and minimum factor of safety.

2. 2

   Gather material data

   Collect ply properties for a candidate carbon or glass fibre prepreg or fabric: E1, E2, G12, Poisson's ratio and strengths in tension, compression and shear, from supplier data. Plan coupon tests to check key values, such as tension and in-plane shear. Enter the properties in a custom orthotropic material in SOLIDWORKS and note the source of each value. Use conservative compression strengths if the data are uncertain.

   Checkpoint: A material table with sources and a custom material set up in SOLIDWORKS.

3. 3

   Build the shell model

   Model the swingarm outer surfaces with Boundary or Lofted surfaces, then split faces into regions where layups will change: pivot, arms and axle. Model metal inserts at pivot and axle as solids. Mesh composite regions as shells and connect them to inserts with bonded contact. Check shell normals point consistently. Name each face region clearly so layups can be traced in reports. Keep insert geometry simple at first and add detail only where stresses demand it.

   Checkpoint: A shell model with defined layup regions, solid inserts and consistent normals that meshes cleanly.

4. 4

   Define layups

   For each region, define composite shell layups with ply material, thickness and angle relative to a reference direction set per face. Start with a quasi-isotropic layup, then a layup with more 0-degree plies along the arms and ±45 plies for torsion. Check ply orientation plots. Check symmetry and balance to avoid unwanted coupling. Record each layup in a table with ply count, angles and total thickness. Make sure the reference direction is consistent across neighbouring faces.

   Checkpoint: Layups defined for every region with orientation plots confirming fibre directions.

5. 5

   Check with laminate theory

   For a simple arm section, compute laminate stiffness and ply stresses with classical laminate theory in MATLAB, Python or Excel under the bending and torsion from your loads. Compare with the shell model at a section away from inserts. Differences larger than a stated margin point to orientation or property errors. Use the same material data in both methods so the comparison checks the model setup rather than the inputs. Repeat for a torsion-only case.

   Checkpoint: Laminate theory and shell results agree within a stated margin at the check section.

6. 6

   Run static and failure analysis

   Run static studies for each load case and plot displacement, ply stresses and the failure criterion, such as Tsai-Wu, ply by ply. Find the first-ply failure load factor in each case. Check stress in the inserts and at the bonded interface. Run a mesh convergence check at the pivot region. Compare Tsai-Wu with maximum stress and Tsai-Hill to see whether they agree on the critical ply and location. Record the governing load case for each region.

   Checkpoint: First-ply failure factors above the target in every case, with convergence at the pivot shown.

7. 7

   Check stiffness and buckling

   Compute vertical and torsional stiffness and compare with the aluminium baseline. Run a buckling study on the composite model under the braking and lateral cases to check thin walls. Run a frequency study to check the first modes. Change layups to raise stiffness or buckling factor where needed. Compare frequencies with the aluminium baseline to see whether the lighter swingarm moves any mode into the motor or road excitation range. Record buckling mode shapes for the report.

   Checkpoint: Stiffness at least equal to the baseline and buckling factors above the target.

8. 8

   Optimise the layup

   Compare layup variants with configurations: ply count, angle mix and local reinforcement at the pivot. Record mass, stiffness and first-ply failure factor for each. Plot mass against torsional stiffness and choose a layup meeting all targets with the lowest mass. Keep ply drop-offs gradual for manufacturing. Check that the chosen layup still meets every load case, not only the governing one. Keep the rejected layups in the report for comparison, with the reason each was rejected.

   Checkpoint: A layup comparison table and a chosen design with mass saving against the baseline.

9. 9

   Validate with tests

   Run coupon tests to check ply properties and update the model. Make the swingarm, or a representative arm section with an insert, and test it on a static rig for vertical and torsional stiffness, then to failure if safe. Compare stiffness and failure location with the model. Note that delamination and insert debonding are not predicted by first-ply failure in shells.

   Checkpoint: Measured stiffness compared with the model within a stated margin, and failure mode compared with predictions.

10. 10

    Report

    Report loads, baseline, material data, layups, laminate checks, failure and buckling results, optimisation, tests and limits. Discuss delamination, insert bonding, manufacturing variation and fatigue as open issues. Recommend further work such as progressive damage or cohesive interface analysis in another tool, and fatigue tests. Include the layup table, material sources and test data in appendices. State which predictions were validated and which were not. Be clear that the work does not certify the swingarm for road use.

    Checkpoint: A report with validated results and clearly stated limits.

Testing & validation

• Baseline aluminium swingarm results for comparison.
• Laminate theory check against the shell model.
• Mesh convergence at the pivot.
• Coupon and rig tests compared with predictions.

Expected results

A composite swingarm design with layups, stiffness, first-ply failure and buckling results, a mass saving compared with aluminium, and validation against coupon and rig tests.

Take it further

• Add progressive damage or cohesive interface analysis in another tool.
• Run fatigue tests on the arm section.
• Study a natural fibre composite for sustainability.

Viva and review questions

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