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The four most commonly used methods for fatigue life calculations of welded structures:
The selection of method depends on structure complexity, accuracy we want to achieve, availability of materials data and code requirements.
From static strength perspective good weld can be as strong as the base metal, however fatigue strength is much lower (Fig. 1) and does not increase with the use of higher strength material (Fig. 2).
It is quite simple to determine stress level for simply supported cantilever beam, much harder for complex structure like trailer frame (reinforcements, holes, sudden geometry change, impact of bolts pretension). Numerical analysis can accurately capture impact of all mentioned factors. Knowing location of stress concentrations and level of stress makes it possible to reduce it during design phase before prototyping.
Fig. 3. Stress contour plot on semitrailer frame
is the most common method and the majority of the design codes include it. For simple structures like building steel frames it gives satisfactory results with a minimum calculation effort.
account for stress concentration effects caused by weld geometry ignores however the local notch effect of the weld toe. Method requires special techniques to calculate the hot spot stress. The value is calculated by linear extrapolation of surfaces stresses at two points, first at a distance of 0.4t (t being the plate thickness) away from weld toe and second at 1.0t. Depends on weld detail category the specific S-N curves should be used.
taking into account notch effect of weld root and toe radii. The actual radius at weld toe is replaced by an effective notch root radius. For structural steel and plate thickness larger than 5 mm root radius of 1 mm is recommended by IIW, for thinner plates 0.05 mm should be used.
for fatigue assessment of welded joints takes into account the fatigue behavior of small cracks in a weld. A crack with specific size and location is modelled to calculate stress intensity factor (SIF). This method is used when crack has been detected or is expected to occur after fabrication or in-services. It allows to plan the preventative inspection at the right time to monitor crack size.
The most common methods used for fatigue analysis of complex welded structures by Finite Element Analysis technique are structural hot spot and effective notch stress method. Many companies from automotive, heavy machinery or aerospace industries utilize those methods to verify and optimise their products. Using dedicated software for durability analysis like MSC Fatigue, engineers can conduct efficiently a detailed fatigue assessment.
The stress results using effective notch stress method are mesh independent and require only one S-N curve to calculate fatigue life. For experience CAE engineer the need of a fine mesh at weld toe is not an issue, using submodeling technique an accurate stresses can be derived at any location of interest. Fig. 4 presents submodel of welded region in agricultural plow used for fatigue calculations.
During design phase you will know how structure performs under cycling loading without prototyping and testing.
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