This is often also known as Modal Testing. It is a method of testing that allows us to calculate the natural frequencies (modes), modal masses, modal damping ratios and mode shapes of a test structure. This is commonly done using either impact hammer testing or shaker testing. Here we are going to deal only with impact hammer testing.
In theory, we would impact the structure under test with a perfect impulse. This would be of infinitely short duration. This would result in a constant amplitude in the frequency domain. Of course, in real life, such an impulse is not possible. Instead, we have a known contact time. The duration of this time is directly linked to the frequency content of the force applied. In hammer impact testing (modal testing) we use a special hammer with a load cell in its tip to measure the force of the impact.
So, to test the structure we need to use the instrumented hammer to generate our impulse and then measure the response. This needs to be done at several points on the structure. There are several ways we can instrument this:
- Place accelerometers at many positions on the structure and impact once. This is very time efficient as it requires a single impact and data capture. In reality, we would perform a few impacts in order to perform averaging, but it is still a very quick test. However, this would require a large investment in transducers and an equally large investment in a measurement system with enough channels to simultaneously record all of our responses.
- We could fix a single accelerometer to one position and then impact our structure at several locations. This is known as a ‘roving hammer’ test. This uses the least resources, but takes longer as we have to make several measurements. This is the most common form of hammer impact testing.
- Finally, we could impact our structure at a fixed position and move a single accelerometer around several positions. This is known as a ‘roving accelerometer’ test. While this is still efficient in terms of transducers and measurement system, it is least efficient in terms of time since moving an accelerometer is time consuming. This method is usually used in situations where space considerations make it possible to fix accelerometers, but there is not enough space to use a hammer.
Once the instrumentation aspects of the test are decided, one needs to consider the software that will assist with the capture of the data and produce the results. A good test system will:
- Lead a user through all the steps necessary to configure the test and prepare the measurement system
- Step through the measurement process
- Provide clear feedback at each stage. For instance, use audio feedback so that the user can concentrate on performing the test without having to refer to the computer screen at each stage
You can see an example of software used to setup and carry out a hammer test (Roving Hammer type) in the following video by James Wren…
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