Fatigue fracture analysis and response plan design of automotive wiring harness

What is harness fatigue fracture?

The metal conductor (copper wire) of the wire harness undergoes sudden brittle fracture under alternating stress (caused by vibration or moving parts), although the stress value never exceeds the material strength limit. After a long period of repeated stress cycling, the conductor (vibration or transport body) undergoes sudden brittle fracture.

A vivid and intuitive life case

As shown in the figure, the tail of the mobile phone charging harness is used for a long time. Due to the twisting and bending stress on the interface tail harness during insertion and removal, the outer sheath first cracks (the outer sheath material is mostly TPE), and then the conductor copper wire will fatigue fracture if continued use.

So studying the bending resistance of charging wires becomes particularly important for improving the service life of charging harnesses

The following image shows the microscopic morphology of the fracture location of the conductor due to fatigue fracture. The fatigue port often occurs at the junction of hardness differences, as shown in the circle position of the charging wire in the figure above, and the fixed position of the fixing clip of the car wiring harness. The conductor will form dents in the long-term compression position, and there will be friction marks generated by mutual movement between the single wires of the conductor. Generally, there will be some oxidation at the fracture location of the conductor.

Tell me two special examples

The video analysis of ModeY's actual vehicle high-voltage wiring harness shows concerns about the fatigue fracture of its heat pump wiring harness. "One is that from the video, we can see that the heat pump has a rubber pad for shock absorption, and the shaking amount is quite large. We need to choose a higher vibration level plug-in. Due to the injection molding bracket that fixes the wiring harness and vibrates with the heat pump, the concern here is not very significant, The biggest concern is whether Tesla's design took into account the potential risk of fatigue fracture of the cable at the green box. The cable was specifically designed, and from the video, it can be seen that the heat pump has high vibration, and there is not much margin here, which may lead to the risk of fatigue fracture of the conductor.

Countermeasures for solving problems

1. Avoid copper wire oxidation
2. Control the twist pitch diameter ratio
3. Control the quality of insulation and sheath
4. Optimization of cable structure