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Ultrasonic test protocol: MOST
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This page describes the protocol to be followed when performing ultrasonic testing for the MOST Research Group at Michigan Technological University. The ultrasonic testing can be used to find out the Poisson's ratioW of a material. You can also determine the Young's modulusW and the shear modulusW of a material.
A piezo electric material converts electrical signals to mechanical vibrations and vice versa. This property of piezo-electric materials can be used to measure the speed of sound in materials.
An electric current is sent to the piezo-crystal and the crystal vibrates. The current can be adjusted to produce a vibration that can cause ultrasonic sound waves. These waves propagate through the solid and are received back by the piezo-crystal. The piezo-crystal converts these waves back to electric signal and the time between two signal pulses can be measured.
The thickness can be measured using a caliper and velocity of sound is calculated by the formula:
Velocity = thickness*2/time = thickness*2*frequency
Poisson's ratio is the negative of the ratio of lateral (transverse) strain to axial (longitudinal) strain. The Poisson's ratio can be calculated using the ultrasonic test. The piezo-crystals can measure the velocity of ultrasonic sound in longitudinal and transverse directions. These velocities are then used to calculate the Poisson's ratio using the formula:
Where VT and VL are the transverse and longitudinal wave velocities respectively. E and G are the young's modulus and shear modulus respectively.
Test specimen requirements
The test specimen MUST have 2 flat surfaces on opposite sides of at-least 1 inch diameter (It can be any shape). The surfaces must be smooth.
Test lab and apparatus
Dr. Ed Laitila is in charge of the lab. Contact him before doing anything in the lab. The ultrasonic testing is done in room M&M U208. The apparatus consists of:
- Longitudinal Piezoelectric emitter/receiver (Green)
- Transverse Piezoelectric emitter/receiver
- Ultrasonic pulse emitter
- Buffer solutions
- Ultrasonic couplant D (for longitudinal piezo)
- Molasses (for transverse piezo)
The ultrasonic pulse emitter is already set to the required values. The default values are:
Do not change any properties unless you know what you are doing.
- Connect one Piezoelectric emitter/receiver to the pulse emitter.
- Apply some buffer solution to the specimen (a drop is enough).
- Press the emitter/receiver end on the specimen, (at the point where buffer solution is applied).
- Turn on the Ultrasonic pulse emitter.
- Obtain data
- Repeat for the other Piezoelectric emitter/receiver.
- Do the calculations as per the formulas given.
The software shows Pulses received vs Time(X axis). The time axis can be scaled as required, usually around 1-4 Microseconds. At a particular scale of time, the pulses received are clearly visible and the time between two pulses received can be measured. To measure the time between two pulses, pause the software (so that values do not change) and click at one pulse (peak of the pulse) and drag the mouse to the next pulse. The time and frequency will be shown near the bottom left side in the software. Note it down.
A rough diagram is shown below: