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Anyone who has come across the dilemma of sizing welds has wondered if FEA can help? So why is that no software on the market claims to have perfected this art so pertinent to manufacturing industry.
FEA with all its calculative functionalities has certain whims that need to be precisely catered to. FEA prefers all parts are gravity cast blanks with polished surfaces free of any imperfections. Moreover, FEA assumes all structures are heat treated to remove any residual stresses. The manufacturing process in itself should be numerically controlled. All these assumptions are obvious to produce consistent and predictable part to part performance.
Weldments on the other hand never have any sort of consistency. They vary due to change in temperature, chemistry of weld beads, chemistry of base materials, weld geometry, residual stresses etc. Hence many consider sizing welds an art and often end up with under-sized or over sized welds. Anyone who has actually attempted in sizing welds using FEA has observed several “hot spots” or stress singularities at the transition from weld bead to geometry. One obvious question then arises are these stresses controlling? What values have to be taken into consideration?
Throat Shear Method
The Throat shear method eliminates the need to estimate initial weld size. This method accounts for allowable strength per unit length in a weld based on weld type, and electrode used. These standard allowables are based on physical tests and hence account for variability. Moreover obtaining forces and moments from geometry are easy compared to stresses. Loads are less sensitive to local geometry and mesh independent. So if you mesh a component with three different mesh sizes, reaction loads from all three would be same, unlike stresses, which would vary.
This methodology does not require you to model weld geometry if using shell geometry. For solid models, there is some extra work required. One can simply build mesh with welded parts intersecting.
In the areas you need to size welds, place split lines to force evenly spaced vertices along the weld seam. Solve using draft quality elements for boundary conditions applied. Extract forces at the vertices; convert them into per unit loads. Compute weld size by dividing resultant load by allowable strength. Allowable can be obtained using standards AWS D 1.1. Force values are easily available if “Compute Free Body Forces” option is checked under study properties. FBD forces are available for vertices, edges, faces in Solid works 2007.
I am sure this sneak preview using this methodology has raised tons of questions. For more in depth discussion on the topic refer welds presentation on COSMOS companion series.
www.solidworks.com/companion
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