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During design phase you will know how structure performs under cycling loading without prototyping and testing.
THE ADVANTAGE OF EFFECTIVE NOTCH STRESS METHOD OVER THE HOT SPORT STRESS METHOD
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.
FEA FATIGUE CALCULATIONS OF WELDED STRUCTURES IN PRACTICE
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 fracture mechanics method
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 effective notch stress method
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.
The structural hot spot stress method
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.
The nominal stress fatigue calculation method
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.
COMPLEX STRUCTURE REQUIRES FEA FATIGUE ANALYSIS METHOD
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
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Fig.1 Effect of stress concentration on fatigue life
FATIGUE CALCULATION SERVICES OF WELDED STRUCTURES
Gain knowledge of your weldment durability during design phase with our FEA fatigue life assessment services. You can design lightweight structure for expected service life without over-engineering it, utilize our virtual durability FEA testing lab.
Fatigue Strength Calculation of Welded Joint
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).
Fig.2 . Effect of increase in tensile strength on fatigue life
FATIGUE LIFE PREDICTION METHODS OF WELDED JOINTS
The four most commonly used methods for fatigue life calculations of welded structures:
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The selection of method depends on structure complexity, accuracy we want to achieve, availability of materials data and code requirements.
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WHAT YOU GAIN WITH FEA FATIGUE ANALYSIS SERVICES
You will avoid overengineering of weldments resulting in lightweight design
Be ahead of the competition by offering lightweight but durable products.
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