Using FEA to Supplement IR

By

Jack M. Kleinfeld, P.E.

Kleinfeld Technical Services, Inc.

Finite Element Analysis or FEA is a method of performing calculations of complex systems that would be difficult to achieve in other ways. An example of its application is in heat transfer analysis. A sample application of interest to thermographers is presented here, where the internal temperatures of a simulated and simplified electrical component are calculated based on its external temperatures, a knowledge of its construction, and the assumption that the heating takes place at the contact surface in the device.

Many other applications of FEA for IR exist. For example, it can be used to estimate the ground temperature over a buried hot pipe to determine whether IR will be able to detect the pipe's location. It can be used to estimate the time dependent response of a wall to solar heating and air temperature changes, so that the thermographer knows the best time to examine the wall for proper construction.

The electrical component modeled here is a contact inside a sealed enclosure. The thermographer can see the exterior of the enclosure and the connection points, but cannot see the actual contact, which, in this case, is the source of heating. Here is what the component looks like:

There is not much to see! A cut-away cross section shows the internal parts.

The two copper strips each have a contact block mounted on them that is the assumed source of the heating. (The location of the contact is marked.) The outside (blue) surfaces and the ends of the copper strips are visible to the thermographer. It was assumed that a temperature of about 90F (actually got 91.4F) on these external surfaces was measured, with the ambient air at 80F, for a 10F rise. The thermographer observed that neither the connection points nor the wires leading away from the component appeared to be the source of the heating. It appeared that the heat was internal. The most logical place for the heat source is the contacts between the two strips. Either they are dirty, pitted, corroded, or there is not enough contact pressure between them. In any event, they are the cause of the problem.

The issue facing the thermographer and the owner of this component is: "What is the temperature of the contacts, given that there is a 10F rise on the OUTSIDE? Have the contacts overheated to the point of annealing? Is the interior so hot that the component is dangerous?"

Those questions have been addressed by using FEA to analyze the heat transfer in the component. The results are specific to the construction, geometry, materials, and conditions of the particular component. DO NOT use them for anything else!

This figure is the exterior of the component showing the temperatures calculated on it for the outside. It would serve as a check against the thermographer's actual results.

The temperature color scale runs from 91.4 to 96.9F. However, the exterior does not exceed 92F. This is essentially what the thermographer got to see, and to worry about.

This figure is the interior of the component.

The temperature scale is the same. The contact points and the areas surrounding them are clearly seen as the hottest places and have about a 17F rise over ambient.

Whether the 17F rise is critical or not is irrelevant to this example. It is significantly different than the 10F rise seen on the outside. For different materials, geometry, etc. it would be another value. BUT, it would be a value that could be calculated so that a decision could be made about the condition of the component based on information!

We would be glad to discuss this type of application with you as it relates to your work and what we could provide you. The use of FEA can make thermographers more efficient, more effective, more useful, and more profitable, by saving them time and wasted trips, and by extending the information that they can provide.

Look at the other examples currently on the Kleinfeld Technical Services, Inc. website, at www.KleinfeldTechnical.com.

You can reach us by e-mail at JKEngineer@KleinfeldTechnical.com .

or by phone at:

718-884-6644

866-884-6644 toll free

2001 Kleinfeld Technical Services, Inc.