Tales of the lab: 4 Memories of junior years

Oscilloscopes! I haven’t used oscilloscopes since what I feel is a century.

In reality, maybe 4 years.

What I mostly remember from the basic electronics we learned in the Classes Préparatoires after High School is that the most important button of all is the “autoset” button. Incredible how all oscilloscopes around the world are from the same brand and look the same. This makes life easier. Same units too.

It is actually quite easy to make it work properly, with a little of common sense, and the precious help from google.

My goal is to measure the mechanical properties of the ceramics I produced, more precisely, the elastic and shear moduli, to know how hard it is to deform them. Ceramic materials usually do not deform in tension when you try to stretch them, which is the common procedure for measuring these moduli. So I want to use not a mechanical method to access these properties, but instead use a technique based on ultrasonic waves propagation.

The first positive point of this measurement is that the set up I want to use is located in the basement, thus it is very peaceful and quiet. The second positive point and not the least! is that is it non-destructive.

Ceramics can be tested, not in tension, but in compression, which results in the failure of the specimen. Similar tests are performed on materials from any family, polymers, metals, gels… Mechanical testing by mechanical failure, strain, compression, bending… usually damages the tested sample.

When the material is very precious because you spent hours if not days for preparation, using such a destructive method will require a tremendous amount of work, or an unreliable level of statistic data if only a few specimen can be tested.

But using ultrasounds still require some efforts, in particular in terms of preparation. For example, having a sample with flat parallel surfaces in essential.

Then, the measurement per se is conducted as follows. First, I need to apply a gel on the ceramic, exactly like during echography.

Second, I connect piezoelectric transducers to a pulse generator and select the frequency written on the transducer to tune the generator.

Third, I place the transducer on the gelled surface of the sample, press to make sure that the contact is good and that there are no bubbles.

Finally, I turn the generator on. It will send a pulse, and I will record on the oscilloscope both the initial pulse and the signal coming back to the transducer after its propagation through the thickness of the sample.

The basic physics behind the measurement is that the pulse sent by the transducer as a pulse is a wave. This wave can travel through the material after crossing the gel. Arriving at the other surface of the sample, it is reflected and sent back to where it came from. This is the pulse echo method.

Only, due to its propagation in the sample, the wave might have deformed and get attenuated. For example, there might have been many reflections along the way of the propagation of the wave, inside the material. This can make difficult the analysis of the signal that came back. A simple bubble, a crack, an in-homogeneity, will have an effect. Also, if you, on purpose, placed some potential reflectors inside the material… Researchers are so masochistic.