Testing > TMF Testing

Thermo-mechanical fatigue testing, or, simply TMF testing is a fatigue test performed under more complex conditions.  A normal fatigue test applies a cyclic test stress at a constant temperature.  A TMF test applies not only a cyclic stress, but also a cyclic temperature.  This means the stress goes from minimum to maximum, whilst the temperature also goes from a minimum to a maximum.  Now this sounds only a little bit more complex than a normal fatigue test, but it’s a lot more complex.

Thinking about it, we need to think about timing.  In a jet engine, stress and temperature change throughout a flight meaning the maximum stress a component may experience can happen at the highest, lowest or intermediate temperature.  But it is also important whether the component is heating up or cooling down.  And, if the stress is increasing or decreasing.  This makes TMF a very complex thing to study.

The two most studied types of TMF are called in-phase and out-of-phase.  This is where the maximum and minimum temperature and stress happen either at the same time or at the completely opposite.  Check out the following animations to see what we mean.

But this isn’t the only way stress and temperature can combine.  Actually there is a lot of ways temperature and stress can combine which impact how long a component will last.

When we look at a TMF test, it is difficult to measure the temperature.  Usually we would use thermo-couples which are little wires that are stuck to the sample.  This becomes a problem as the sample is stretching and the temperature is changing, meaning the surface of the sample keeps expanding and contracting (making it hard to stick anything to it).  At the ISM we have done work on using thermography or thermal imagery to measure temperature.  You can see how this works in the following animation which shows you how the furnace, extensometer, and thermal camera are set up for a TMF test.