Welding is the most common joining process for metals thanks to the many advantages it offers. It’s efficient and cost effective method of joining metals, permits considerable freedom in design and cab be mechanized. 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 1. Effect of stress concentration on fatigue life
Source: http://www.twi-global.com/technical-knowledge/job-knowledge/fatigue-testing-part-2-079/
Fig 2. Effect of increase in tensile strength on fatigue life
Source: http://www.twi-global.com/technical-knowledge/job-knowledge/fatigue-testing-part-2-079/
You don’t need to wait for fatigue testing to know how your product will perform under complex fatigue loads.
Simulation provides answers to questions where can material be saved and where must be extra added to achieve a specific service life.
It’s much cheaper to do the virtual fatigue test than real one.
By identifying and fixing potential issues with durability at the early design stage Your product is much more likely to past their tests as First-Time-Right.
The four most commonly used methods for fatigue life calculations of welded structures:
1. Nominal stress
2. Structural hot spot stress
3. Effective notch stress
4. Fracture mechanics approach
The selection of method depends on structure complexity, accuracy we want to achieve, availability of materials data and code requirements.
The nominal stress 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.
The structural hot spot stress method account for stress concentration effects caused by weld geometry but ignores the local notch effect of the weld toe. The method requires special techniques to calculate the hot spot stresses. 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 a distance of 1.0t. Depending on the weld detail category, specific S-N curves should be used.
The effective notch stress method takes 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, 1 mm is recommended by IIW, but for thinner plates, 0.05 mm should be used.
The fracture mechanics method for fatigue assessment of welded joints takes into account the fatigue behaviour of small cracks in a weld. A crack with specific size and location is modelled to calculate stress intensity factor (SIF). The fatigue crack growth rate and behaviour can be predicted accurately. 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 these methods to verify and optimise their products. Most of the commercial CAE software has a dedicated module to conduct fatigue analysis. By using a dedicated software for durability analysis like MSC Fatigue, nCode DesignLife or LMS Virtual.Lab, engineers can conduct 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 an experienced CAE engineer, the need of a fine mesh at weld toe is not an issue as sub-modelling technique an accurately predict stresses at any location of interest. Fig. 4 presents sub-model of the welded region in agricultural plow used for fatigue calculations.
Our engineers help many organisations to develop successful products. They also provide a reliable durability analysis of welded components to support design decisions .