How to conduct bending fatigue and radial fatigue tests on wheel hubs
The wheel hub needs to go through both bending and radial fatigue, according to ECE R124 or customer specifications; What molds can do is to maintain density and rounded corners.
The difference between the two experiments
Bending fatigue simulates the bending moment during turning, with the load applied to the rim edge and the spokes subjected to alternating bending stress.
Radial fatigue simulation of vertical load is applied through a drum to assess the overall strength of the wheel rim and spokes.
The number of cycles required for the two experiments is different, and the specific values are determined according to the standards and wheel specifications.
Where does failure usually occur
Cracks caused by bending fatigue often originate from the transition fillet between the spoke root and the hub. This is a stress concentration area and also a high-risk area for shrinkage and loosening.
Radial fatigue cracks often occur at the connection between the wheel rim and spoke, or in the weld area of the wheel rim.
Airtight failure often occurs before fracture. Monitoring changes in air pressure during the experiment can detect cracks in advance.
Two variables that can be controlled by the mold
The first is rounded corners. The R angle is too small, and the stress concentration factor increases. It is recommended that the fillet at the root of the spoke be no less than R3.
The second is density. There are areas where shrinkage occurs, and the fatigue strength may decrease to about half of the dense zone, where cracks often begin.
Xinfeng Mold will focus on the design of the compensating channel at the root of the spoke on the wheel hub mold, and add some cooling if necessary.
The influence of materials and heat treatment
The fatigue strength of aluminum alloy is sensitive to heat treatment. Improper solid solution and aging parameters in T6 treatment can lead to a decrease in strength and plasticity.
The grain size of castings also affects fatigue. The grain size is coarse, and the resistance to crack propagation decreases.
Heat treatment deformation can lead to uneven wall thickness, local stress increase, and indirectly affect fatigue life.
Preparation before the experiment
The test samples should come from normal production batches, and the density of manually produced parts is not representative.
Perform size and airtightness testing before the experiment, and record baseline data. Abnormalities occurred during the experiment, making it easier to compare.
The experimental equipment must meet the standard requirements. Insufficient rigidity of the tooling will result in distorted load distribution.
Analysis path after failure
First, look at the location of the crack origin. The root of the spokes is often due to insufficient shrinkage, while the rim is often due to wall thickness or material issues.
Perform another fracture analysis. A ductile fracture indicates plastic fracture, while a cleavage or transgranular fracture indicates material or process abnormalities.
Finally review the mold and process parameters. Check the dimensions, mold temperature, and pouring temperature one by one.
The coordination method of Xinfeng mold
Xinfeng Mold will require customers to provide fatigue test results for the wheel hub mold project, in order to review the shrinkage and fillet design.
The value of this closed loop for subsequent wheel development is higher than that of a single pass test.
The load spectrum of fatigue testing should correspond to the actual working conditions of the vehicle, and passing the laboratory test does not mean that the vehicle is safe for loading.
The failed parts of the experiment should retain the fracture surface, and subsequent process improvements need to rely on it to locate the cause.
The bolts of the test fixture should be checked once every round, as looseness can distort the load distribution.
Frequently Asked Questions
Q: What is the difference between the two experiments?
Answer: Bending simulates turning moment, and radial simulates vertical load.
Q: Where do cracks usually start?
Answer: The rounded corner at the root of the spoke is connected to the wheel rim.
Question: What is the minimum rounded corner?
Answer: It is recommended that the root of the spoke should not be less than R3.
Q: What is the impact of density?
Answer: The fatigue strength of the shrinkage zone may be only half of that of the dense zone.
Q: Can we use trial parts for testing?
Answer: Not recommended, density and mass production parts are not the same thing.
Question: What should I check first after a failure?
Answer: First, look at the location of the crack origin, and then perform fracture analysis.
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