Conventional check procedures at fatigue loaded weld joints (based on the ultimate or yield strength of material) do not provide sufficient guarantee of safe joint design, so the fatigue strength of joints is used to check fatigue loaded joints.
1. Specifying an endurance limit
In the first step the calculation determines the endurance limit at constant strength σ e or τ e for the specified type, design, loading, and material of weld joint.
2. Specifying finite-life fatigue limit
The finite-life fatigue limit σ f or τ f is calculated for the specified joint life that is in the (N< 10 6 cycles) range of timed strength. Calculation continues with this finite-life fatigue limit.
3. Calculation of parameters of particular fatigue loadings
Mean values for given upper and lower cycle loadings are calculated their mean values according to the following formulas. It is done for all specified loadings.
4. Effect of strokes
If strokes effect the joint besides the fatigue loading, their influence must be included into the calculation. This calculation is implemented by using the dynamic stroke factor in the formula for determining the maximum calculated loading:
F max = F m + η F a or M max = M m + η M a
5. Calculation of actuating stress in the weld joint
Mean cycle stress σ m or τ m and upper cycle stress σ h or τ h are calculated for the specified mean cycle loading F m , M m and maximum calculated loading F max , M max with the formulas used in static calculation. These stresses are used for calculation of cycle amplitude according to the formula:
σ a = σ h - σ m or τ a = τ h - τ m
6. Specifying fatigue strength of joint
For calculated stress and known endurance limit, the resulting strength of fatigue joint is easily determined according to the selected fatigue curve. The procedure of fatigue strength determination for both normal and shear stresses is obvious from the following pictures.
Haigh charts of normal stress for different σ a / σ m ratio (modified Godman fatigue curve is used):
Haigh charts of normal stress for different τ a / τ m ratio (modified Godman fatigue curve is used):
7. Joint check
In the last step the program calculates the joint safety factor n C = σ A / σ a and compares it with the required safety degree. For convenient weld joint, the condition n f ≤ n C must be satisfied.