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Simulation | Add a Little Heat

What else would I be doing on vacation than working with materials in Autodesk Simulation Multiphysics (formerly Algor)? I was testing a microwave oven heating tray, using various materials including 0.5 inch thick high-density polyethylene (HDPE). In the process I ran through a couple interesting things that I thought I’d show you:

  • How fast this went
  • How easy it is to add heat to a static simulation
  • A consideration about the linear material model in static analysis

The entire setup and analysis takes less than 15 minutes, including the meshing. Let’s take a look.

Setup

The analysis is quite simple. Static stress load atop the tray in a linear material model,  however I wanted to add the resulting temperature loads to the component. A little study indicated that raw HDPE suffers little temperature change due to the processes involved in microwave oven cooking. (What we are not discussing is the potential problems of material transfer during the cooking process). The surface temperature is however affect by the cooking food.

The tray was exported directly from Autodesk Inventor to Simulation using the Add-in tool.

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Constraints

After the part was meshed, I constrained the outer edges as if the part is supported on a curved surface, as many plates will be. The constraints allowed rotation and translation towards the interior region, and fixed against lateral translation and rotation.

Loads

0.22 kg of mass was divided among the middle 1000 nodes of the upper surface using the nodal lumped masses and a rectangular selection set pulled through the upper cross section. This functionality makes applying a placed load quite easy in static stress situations, and permits applied gravity to do it’s part.

imageWhy so many elements in this part? I needed the bending stress evaluated well, which required 3 elements of cross section in the bending features, and resulted in some small surface meshing.

Standard gravity was also applied to the Y axis.

Mass load results

The results are consistent with what we’d expect from a simple load. Somewhere near 4 mm of sag deflection. Now that we have a expected results without the heat applied, lets add that factor into the analysis.

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Surface Temperature

A static surface temperature of 200°C  was applied across the entire upper surface of the component. That is quite high, but I wanted to know just how much change I could expect at the upper range of the temperature. Since the microwave does not heat the part, but instead the food on top of the part, the temperature is only factored against the upper surface. The remaining surfaces of the upper portion of the tray were subjected to half the heat in order to simulate some radiated heat from the food.image

The Analysis Parameters

This is very important. In the analysis parameters is a section to indicate the magnitudes related to various static factors that may or may not be applied. Make sure you evaluate these parameters to ensure that all your factors are being applied in the study. Here’s what I mean:

By default, a static stress analysis has all the load magnitudes set to 0, except the pressure and gravity. You must change the thermal load to 1 in order for these factors to be taken in consideration. Just right click the analysis scenario header, and pick edit.

Deflection results of Mass and Thermal loads

Here we see 1.5 mm vertical deflection with the mass and thermal loads applied. Without the mass considered, the tray deflects 5 mm vertically.

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Below is the temperatures applied to each node. The specific heat capacity of HDPE is 2000 J/Kg-K.

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Throughout the analyses, the safety factor of the material never breaches 1.5. At this point I’m more concerned with material transfer than the failure of the design.

More Considerations

There are possibly more ideal methods of applying thermal loads to stress analyses. Some areas to investigate are  performing steady state heat transfer analysis, and then apply the results to a static stress analysis. This will account for the energy emitted in a far more accurate application of the load.. Another thing that needs to be adjusted to this analysis is to move to non-linear material models. This will permit the study to continue into the non-elastic regions of this plastic material, as the heat transfer may likely take us.

We’ll cover more on this soon. Keep checking back.