Required minimum safety factor of weld joint during static loading n s represents a ratio of allowable stress and the yield strength of joint material n S = S Y / σ Al or n S = S Y / τ Al .
Required safety of weld joint is affected by method and quality of weld design (shape and machining of the weld surface, weld reinforcement, weld homogeneity, penetrations, and so on), operation conditions, requirements for joint reliability, and potential threat to human life at weld breaking. Many other effects must be considered when setting its value.
Calculation procedures do not consider possible sudden brittle fractures and change of material mechanical values due to the temperature and residual stress. Only the nominal stress is set by the calculation in a certain section for given loading. Stress concentrations and internal stresses are not considered. Consider all these facts when you are setting a required minimum joint safety.
Used calculation method must be considered when specifying the safety factor. Both calculation methods of statically loaded welds solve the weld safety in a different manner.
Standard calculation procedure
The allowable weld loading is compared directly with the calculated normal, shear, or resulting reduced stress to find whether the weld is satisfactory. This comparison is made according to the weld type and design and the way of loading. It is obvious that the required weld safety depends on the type and direction of stress that arises in the weld joint. It is also necessary to specify different safety factors for different type, shape, and design of welds and for various load combinations. Informative values of recommended minimum safety factor values for different weld types are presented in the following table.
|
Weld type, loading |
n S |
|
Butt welds loaded with traction |
1.6 ... 2.2 |
|
Butt welds loaded with bend |
1.5 ... 2.0 |
|
Butt welds loaded with shear |
2.0 ... 3.0 |
|
Butt welds loaded with loading |
1.4 ... 2.7 |
|
Fillet welds loaded in the plane of joining the part |
2.0 ... 3.0 |
|
Fillet welds loaded spatially |
1.4 ... 2.7 |
|
Plug and groove welds |
2.0 ... 3.0 |
|
Plug (resistant) welds loaded with shear |
1.6 ... 2.2 |
|
Plug (resistant) welds loaded with tearing |
2.5 ... 3.3 |
Method of comparative stresses
Compares allowable weld loading with the auxiliary comparative stress. This comparative stress is obtained from calculated partial stresses by means of conversion factors of weld joint. The effects of different types of stresses, which arise in weld joint, to safety of weld joint, are considered in the safety factor. You need only one common safety factor for any type, shape, and design of weld and arbitrary combination of loading. The recommended minimum value of safety factor is given in the n S =<1.25...2> range.