I used 2.4 MHz transducers to measure the longitudinal and transversal sound velocities in low density polyethylene (LDPE) with a plate thickness of 8 mm. The longitudinal sound velocity was measured to be about 2 km/s, however I was unable to measure the transversal sound velocity (no reflected pulse was obtained). Is this due to high damping of the transversal sound waves in this material? How can I measure sound waves better in polymer samples?
Decrease plate thickness, change frequency of sound?
I suppose some resasons for disappearance of the reflected signal:
1. no emission
transmitter does not provide any deflection in the boundary plane of incoming signal
- try to change the position of the source on the incoming side
2. no propagation - the transversal perturbation reveals a high decrement, beacuse the frequency lies close to a peak in the absorption spectrum of the material:
first try to detect the transmitted signal on the opposite side;
if it's the case:
- try to change the frequency;
- try to use another signal shape instead of harmonic sinusoudal
(e.g.single step, rectangle etc.) and ensure, there is a propagation at all
3. no reflection from the opposite boundary
- try to change the bondary condition, for example replace the free
surface by the bounded one (glued to another material)
please tell me about Your results, thank You!
I suppose you decrease a lot the shear modulus G of LDPE due to dissipation.
1. use lower amplitude in source,
and / or
1. use lower Temperature, if possible.
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