A few days ago, my friend Chris Thompson, founder and owner of Appian Way Technologies, took a look at my model of the safety latch and suggested the following changes in order to improve the simulation study.

First of all, he added small fillets to the areas of the latch where stress concentrations are expected, at the “root” of the latch. He also “cut” the model in half, in order to take advantage of its symmetry through the use of symmetry constraints, which can be found among the Advanced Fixtures available in SolidWorks Simulation.

The symmetry fixtures will simulate the half of the latch that was cut from the model. Having this fixture in place will prevent any displacements across the plane of symmetry, but allow displacements on the plane of symmetry. The idea behind this is to reduce the number of equations necessary, as well as the solving time. In order to use this constraint, right click on Fixtures, and select Advanced Fixtures, Symmetry. He selected the left planar face of the latch to define the plane of symmetry, as you can see in the following image.

Chris also talked to me about the possibility of improving results by any of two options: manually refining the mesh and using mesh controls, or making use of the h-adaptive solution method, which is available only for static analysis and solid elements. Why is this going to improve results? Well, simply because any solution obtained through FEA will depend on our choices for discretization (a.k.a. meshing). Different choices for meshes will also cause different discretization errors, and we can estimate these errors by making systematic (planned and gradual) changes to the mesh and analyzing the impact of such changes in the results of our study. This is often called a convergence process. The way we can do this is by simply starting with a study that uses an average element size mesh, and then, in subsequent studies, gradually refine the global mesh (reduce the size of the elements), while keeping an eye on any changes in stress and strain in the whole model or in areas of interest (in this case the fillets). We’ll know the process is converging when any further refinement of the mesh produces insignificant changes in the magnitude of the results. This can be a long and tedious process.

Further manual refinement consists of applying mesh controls to the areas of interest in the model. Basically, mesh controls allow us to refine the mesh locally, only in those areas of interest where we expect high concentration of stress, while the rest of the model is meshed using a much larger element size, thus reducing the number of equations and time needed to solve the study, at least when compared to global mesh refining. Mesh controls can be applied to edges, vertices, faces or entire components of assemblies, and they need to be applied before meshing the entire model.  The way to apply mesh controls is by right clicking on the mesh icon in the Simulation Study tree and select Apply Mesh Control.

Here in this image you can appreciate the way Chris applied a mesh control to that couple of fillets. He selected the two faces and used an element size of 0.029 in and a Ratio of 1.5.  This Ratio parameter simply specifies the ratio between element sizes in consecutive transitional layers when going from the global mesh element size to the local mesh element size. A Ratio of 1.5 is usually default.

Chris also applied mesh controls to the curved face of the cutout you see on the bottom of the latch, where stresses also concentrate, and to that edge on the tip of the latch, that he created by means of a split line, and used to define the Use Reference Geometry Advanced Fixture that I applied in the original study to make sure the latch had that 5 mm displacement, remember?

He then meshed the rest of the model using the default mesh element size. Notice in this image the transition between mesh element sizes in different areas of the model.

So that’s the manual way to do it, but this refinement process can also be automated, by using the h-adaptive Solution Method. By the way, the “h” refers to the size of the element, so the convergence process through mesh refinement is actually called “h convergence process”, since the size of the elements is gradually reduced.

To make use of the h-adaptive solution method right click on the name of the study in the Simulation Study tree and select Properties, then select the Adaptive tab, and under Adaptive method option select h-adaptive.  You have a few options to choose from here.  From the help document, “Target Accuracy sets the accuracy level for the strain energy norm in the model, which is not the same as stress accuracy level.” A default value of 98% means that the convergence process will stop if the difference in the strain energy norm between two loops drops below 2%. Accuracy Bias instructs the solver how to concentrate on getting stress results: Local (all the way to the left) will cause the solver to concentrate on getting accurate peak stress results for those very localized areas with high strain energy errors (the fillets) by highly refining the mesh in those areas, while Global (all the way to the right) will cause the solver to ignore high, localized strain energy errors and concentrate on getting accurate overall stress results for the whole model.  The maximum number of loops will tell the solver how many times to repeat the process of mesh refinement. Looping will end when Target Accuracy is achieved or when the maximum number of loops is reached. If Mesh Coarsening is selected, it simply means that during the mesh refining process our original mesh can actually be made coarser in some areas of the model, as the solver sees fit. This way the mesh will be refined only where needed.

This is the mesh that my friend Chris achieved for the latch by using the h-adaptive solution method with default values and a maximum number of loops of 3.

As my friend pointed out to me, the h-adaptive method is useful not only to save us from the tedious process of manual mesh refinement, but also for those times when we’re not exactly sure where the areas of high concentration of stresses will be.

Thanks, Chris!