Welds

FINITE ELEMENT BASE FATIGUE CALCULATION OF COMPLEX WELDED STRUCTURES

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/

WHY COMPANIES USE FINITE ELEMENT BASED FATIGUE CALCULATION

Saves time

You don’t need to wait for fatigue testing to know how your product will perform under complex fatigue loads.

Gives valuable insight

Simulation provides answers on questions where can material be saved and where must be extra added to achieve a specific service life.

Lower cost

It’s much cheaper to do the virtual fatigue test than real one.

Increase confidence

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.

FATIGUE LIFE PREDICTION METHODS OF WELDED JOINTS

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 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 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 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 those method to verify and optimise their products. Most of the commercial CAE software has dedicated module to conduct fatigue analysis. Using dedicated software for durability analysis like MSC Fatigue, nCode DesignLife or LMS Virtual.Lab engineers can conduct efficiently a detailed fatigue assessment.

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.

BEFORE YOU DO ANY FATIGUE CALCULATION

Make sure you use correct loads and boundary conditions

Changing loads by 10% may affect fatigue life by 100% so make sure you have proper loading history. If you are using FE simulation check boundary conditions to avoid over-constraining your model.

Check stress values

If you are not experience CAE engineers refine the mesh to check your results are accurate enough. If you are using Solid Works or Autodesk Inventor use automatic mesh refinement or conduct stress convergence study. Avoid sharp corners by modelling fillets instead.

Use correct fatigue material properties

Make sure you use correct S-N curve for your structural detail category according to specific code. If applicable consider also S-N curve stress ration and effect of mean stress.

VKB TECH ENGINEERING EXPERTISE IN FATIGUE ANALYSIS OF WELDED STRUCTURES

Our engineers help many organisations to developed successful products. They provide a reliable durability analysis of welded components to support design decisions to companies.