Key points for fatigue strength design of chassis suspension bracket mold

Key points for fatigue strength design of chassis suspension bracket mold
The fatigue life target of suspension brackets is usually on the order of 10 ^ 6 cycles, and the mold side can contribute to rounded transition, density, and residual stress control.
Fatigue failure starts from stress concentration
The stress concentration factor can reach 2-3 times at the sharp corners and section changes on the bracket. The nominal stress is not high, and the local area has approached the yield strength of the material.
The first action in mold design is to make all interior corners rounded. Rounded corners below R3 are difficult to form stably on castings, and it is recommended not to be smaller than R3.
For every 1mm increase in fillet size, the local stress concentration factor can decrease by 10% -15%. But excessive rounded corners will increase weight and require balance.
Density determines the location of crack initiation
The fatigue strength of the shrinkage zone may only be 50% -60% of that of the dense zone. Cracks often originate from the edge of shrinkage, rather than at the point of maximum stress.
Suspension brackets are often cast using low-pressure or gravity casting. The density of low-voltage components is more uniform, and it is recommended that the X-ray judgment of key areas should not be lower than level 2.
Cold or iron bars should be arranged at thick and large sections. Controlling the solidification time difference within 3 seconds can reduce axial shrinkage.
Residual stress will offset the design margin
Uneven cooling of castings can leave residual tensile stress. The superposition of surface tensile stress and working stress significantly reduces the fatigue life.
The temperature for stress relief annealing is usually set at 180-220 ℃ and held for 2-4 hours. Excessive temperature can affect the strengthening effect of T6 treatment.
Xinfeng Mold will incorporate the cooling sequence into the design input of the suspension bracket mold, and reduce the temperature gradient at the thickness boundary through partitioned waterways.
Three experiences in structural design
Firstly, avoid reinforcing ribs that cross each other. The intersection forms a hot spot, which is both a source of shrinkage and a point of stress concentration.
Secondly, the thickness of the reinforcement should be 0.6-0.8 times the wall thickness. Thick tendons will form new hot nodes, while thin tendons will make filling difficult.
Thirdly, leave sufficient protrusions around the installation holes. The diameter of the protrusion is generally not less than 1.8 times the aperture, otherwise the pre tightening force of the bolt will cause local deformation.
Verification method
The bench fatigue test is the most direct verification. Load according to the load spectrum and record the number of cycles when cracks appear.
The test piece should be extracted from the mass production state and cannot be replaced by manual trial production pieces. The difference in density between the two can lead to incomparable results.
CAE fatigue analysis can be used as a screening tool in the design phase, but material parameters need to be calibrated with measured data to avoid significant errors.
Common Misconceptions
Misconception one is to only perform static strength verification. Cases of fatigue failure despite passing static strength standards are not uncommon in chassis components.
The second misconception is to enlarge all rounded corners. An increase in rounded corners can increase weight and may also alter the path of force transmission.
Misconception three is neglecting the surface quality of castings. For every doubling of surface roughness Ra, the fatigue strength may decrease by 5% -10%.
Frequently Asked Questions
Q: What is the fatigue target for suspension brackets?
Answer: The common 6th power cycle of 10 is executed according to the load spectrum.
Question: What is the minimum rounded corner?
Answer: It is recommended not to be smaller than R3, as it is difficult to form stably if it is too small.
Question: What is the appropriate thickness for the tendon?
Answer: Generally, it is 0.6-0.8 times the wall thickness.
Question: What is the stress relief temperature?
Answer: Keep at 180-220 ℃ for 2-4 hours.
Q: What are the levels of X-ray judgment?
Answer: It is recommended that the key area should not be lower than level 2.
Q: Can fatigue tests be conducted on trial parts?
Answer: Not recommended, as there is a significant difference in density compared to mass-produced parts.
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