George Laird's blog

Doing a bit of Research on Brazing of Ceramics to Metals

I was doing a bit of online research on the FEA modeling of the brazing process. We have a client making high-tech ceramic to metal fixtures. The ceramic is joined to the metal substrate via a brazing process. Upon cooling to room temperature and depending upon the geometry and material selection, residual stresses can develop to the point of causing failure of the components. It is tricky and we have embarked on a modeling project to quantify the mechanical response of these fixtures. Well, back to the story line. During this search I stumbled across this old paper that I had co-authored with a client. The paper is titled "CFD Analysis of Automatic Test Equipment." It was a trek down memory lane to see these old CFD models and realize that we have been doing CFD consulting services for more than 20+ years. It was some sweet work on large test equipment that was cooled using both water and air loops. The paper shows how we used CFD global (machine) to local modeling (board-level) to arrive at accurately predicting chip junction temperatures. Looking back, yes the models were crude and the graphics a bit funky but what can we say - it was 2002 and it worked!

If you would like to read the paper, go ahead and click here

LS-DYNA: Observations on Material Modeling

This is the 4th in a series of informal articles about one engineer’s usage of LS-DYNA to solve a variety of non-crash simulation problems. The first was on LS-DYNA: Observations on Implicit Analysis, the second was on LS-DYNA: Observations on Composite Modeling, and the third was LS-DYNA: Observations on Explicit Meshing.

As a former metallurgist whose specialty was structure-property relationships, I have a keen appreciation for how materials deform under load. At the federal lab where I worked, we had a lot of mechanical test equipment where I could break, crush and impact all sorts of things. This experience grounded me in my appreciation of how difficult it is to simulate the mechanical response of materials using some sort of X-Y plot of stress versus strain.

LS-DYNA: Observations on Explicit Meshing

This is the 3rd in a series of informal articles about one engineer’s usage of LS-DYNA to solve a variety of non-crash simulation problems. The first was on LS-DYNA: Observations on Implicit Analysis, the second was on LS-DYNA: Observations on Composite Modeling and the fourth was LS-DYNA: Observations on Material Modeling.

I come from a background in implicit analysis where element quality can often be swept under the rug by the use of dense meshes and since models run quickly no one really cares about model size, i.e., ten million DOF. Whereas, what I enjoy about explicit is that it demands the upmost model preparation from the choice of element types to the creation of perfect quad and hex dominant meshes having the absolute minimum number of DOF.

What I have been noticing over the last couple of years in the explicit world is the creation of gigantic meshes that are justified by saying, “It runs just fine in four hours using 32 CPU-cores.” Although it runs, I wonder how much time was spent in debugging this beast, and also whether the mesh density was justified by experience or by the economy of using an off-shore meshing service.

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