Week 3 Verification of Weld Joints

Objective:
Computational modelling and stress analysis of weld joints for their verification in a component and comparing the results for different weld materials.

Simulation parameters:
Mesh:
Since we want to verify the weld joints in the component, the mesh on the welds should be refined. The weld mesh is refined using body sizing and setting an element size of the refined mesh within 4mm and 1.8mm. The element size is varied because to ensure accurate results, a grid dependency test should be carried out. The varying weld mesh sizes are 4mm,3.5mm,3mm,2.5mm,2mm,1.9mm,1.8mm. Beyond 1.8mm, the model nodes surpass the academic version limit.

Boundary conditions:
A force load having the magnitude of 15000 N acts on the lower plate in the downward direction (Y axis).
The upper plate is supposed to be fixed to the wall and that is simulated using a fixed support which is applied to the bolt locations of the upper plate.

Analysis parameters:
The load of 15 kN is applied at once, i.e. in one loadstep only. To account for geometric and material non-linearities, large deflections are activated and thus the solution will undergo multiple equilibrium iterations to converge at the single loadstep.
For all the cases, the input parameter is the weld refined mesh element size and the output parameters are the maximum values of the equivalent stress, elastic strain, directional deformation in Y and the factor of safety for the VonMises failure criteria.

Case1:
In this case, it is assumed that the weld joints don’t use any flux material i.e. the weld material is made of the same material as the plates and the ribs which is stainless steel.

Mesh Convergence

For this case, mesh convergence fails till the element size of 1.8 mm as the difference in the results for varying element sizes is significant. Thus, for this case, the results at the most refined mesh possible will be considered i.e. weld mesh element size = 1.8mm as that will be the closest to the correct solution.

Stress:

Weld stress:

Strain:

Deformation:

Factor of safety:

Case2:
In this case, the weld material is made of aluminium alloy (high strength, wrought), and the rest of the plates and ribs are made of stainless steel.

Mesh Convergence:

For this case too, mesh convergence again fails till the element size of 1.8 mm as the difference in the results for varying element sizes is significant. Thus, for this case, the results at the most refined mesh possible will be considered i.e. weld mesh element size = 1.8mm as that will be the closest to the correct solution.

Stress:

Weld stress:

Strain:

Deformation:

Factor of safety

Case3:
In this case, brazing has been assumed to be done between the plates and ribs. To simulate brazing, the weld material has been set as cast bronze. The plates are made of stainless steel and the ribs are made of copper (wrought).

Mesh convergence:

For this case too, mesh convergence again fails till the element size of 1.8 mm as the difference in the results for varying element sizes is significant. Thus, for this case, the results at the most refined mesh possible will be considered i.e. weld mesh element size = 1.8mm as that will be the closest to the correct solution.

Stress:

Weld stress:

Strain:

Deformation:

Factor of safety

NOTE: For the above results, the factor of safety is calculated against the yield strength of the material on the basis of the maximum shear distortion energy density/VonMises criteria, Thus, the locations where the factor of safety goes below 1, there yielding of the material will occur.

Comparison of results:

For all the cases, the maximum stress occurs at one of the welded locations only. The highest stress is produced when the weld material is made of aluminium alloy and the lowest peak stress is obtained for the case of brazed connections which is logical because of all these materials, the aluminium alloy used has the highest yield strength and cast bronze has the lowest yield strength, and since faster yielding enables redistribution of stresses in the material, the peak stress obtaiend will be lower for materials with lower yield strength, thus expaling the values of the peak strengths obtained in this case.

For this loadcase, some of the brazing connections may fracture since the ultimate strength of cast bronze is 267MPa and the peak stress in these connections due to the load is 340.83 MPa.
We can use the probe tool to better gauge the connections which will break under this load. Any connections having VM stress higher than 267 MPa will break. 

Thus, brazing is not suitable for this type of load. 

For both case 1 and 2, the maximum stress occurs at a weld connection to the side of the hole on which the force is applied. The maximum elastic strain also occurs at the same locations as the maximum stresses, and since aluminium has a lower material stiffness (young's modulus) than stainless steel, the elastic strain and the deformation produced is more.