To consider required joint life, you determine the fatigue strength of weld joints. The joint strength can be divided into three distinctive areas according to the number of cycles:
- Area of low-cycle strength (for N ≤ 10 3 cycles approximately) - joint strength is roughly constant, independent on cycles, static calculation is sufficient for check of the joint.
- Area of timed strength (for 10 4 ≤ N ≤ 10 6 cycles approximately) - joint strength decreases when the cycle number increases.
- Area of permanent strength (for N> 10 6 cycles approximately) - joint strength is again roughly constant, the endurance limit se or te is used for determining the fatigue strength of joint.
Dependence of joint fatigue strength σ A or τ A on the number of cycles N for symmetrical reversed loading is displayed in the following image:
If the weld joint is calculated on the condition of finite life in the area of timed strength, the finite-life fatigue limit σ f or τ f must be known for determining the joint strength. An empirical formula σ f = 10 Y N X is used for calculation of this limit if the endurance limit se is known. Exponents are calculated according to:
while:
|
σ' U ≈ 0.9 S U |
- for reversed bending |
|
σ' U ≈ 0.75 S U |
- for traction - pressure |
|
σ' U ≈ 0.72 S U |
- for reversed torsion (shear) |