On plastics and composites the name of the material is not enough: two laminates with the same designation can respond to ultrasound differently. Feasibility is verified on the sample.
- ✓ High attenuation: low frequency and high energy
- ✓ Acoustic velocity much lower than in metals
- ✓ In laminates every interface between plies generates an echo
- ✓ Preliminary trial on a sample of known thickness
Plastics, rubbers and composite materials share a characteristic that makes them difficult for ultrasonic measurement: they absorb acoustic energy far more than metals do. In polymers the absorption is viscoelastic in nature, that is, part of the wave’s energy is converted into heat as the pulse travels; in composites there is the added scattering produced by the contrast between fibre and matrix, which deflects a share of the beam at every interface.
The propagation velocity changes too, being appreciably lower in these materials than in steel. The practical consequence is that an instrument configured on a metal returns, on the same part in plastic, a meaningless value: the conversion between time of flight and thickness uses the wrong coefficient. The correct velocity must therefore be set case by case, and determined experimentally.
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In laminates a third element appears. A composite is made of stacked plies, and every interface between plies reflects part of the pulse: the signal returning to the probe contains not one echo but a sequence of closely spaced echoes. Telling the one corresponding to the far wall from the others requires seeing the shape of the signal, not just the displayed figure: this is why on these materials the A-Scan function substantially changes the reliability of the inspection.
What makes the measurement difficult
| Factor | Effect and countermeasure |
|---|---|
| Viscoelastic absorption | Weak signal: low frequency and high transmission energy |
| Reduced acoustic velocity | Mandatory setting on a sample, never table values for metals |
| Layered structure | Multiple interface echoes: reading the signal in A-Scan is required |
| Fillers and porosity | Additional scattering, variable even for the same material designation |
| Contact surface | Couplant compatible with the polymer and removable without damage |
The trial on the sample is the only valid verification
No data sheet can guarantee in advance that a measurement will succeed on a composite or on a filled plastic. Formulation, fibre percentage and orientation, porosity and temperature alter the acoustic behaviour even within the same declared family: two apparently identical parts can give different outcomes.
The reliable verification consists in trying on a real sample, ideally of known thickness measured mechanically. That trial yields two pieces of information: whether the backwall echo is usable and which propagation velocity to set. This is the way of working we prefer: assessing feasibility together before the purchase.
Recommended instruments
| Product | Why it is relevant |
|---|---|
| Dakota PMX4-DL | High penetration for strongly attenuating plastics, rubbers and composites |
| Dakota PMX2-DL | Precision measurement on readily accessible non-metallic materials |
| Transducers for precision thickness gauges | Single crystal and delay line probes for thin thicknesses |
| Ultrasonic couplants | Stable, removable coupling on polymer surfaces |
Frequently asked questions
In many cases yes, but it depends on the individual material. Fibre, matrix, porosity and number of plies change the acoustic response: feasibility is established only by trying on a real sample.
Almost always because the propagation velocity of a metal is set. In polymers the velocity is much lower: without setting on the real material, the conversion between time of flight and thickness is wrong.
It is strongly recommended. In a laminate every interface between plies generates an echo: seeing the signal makes it possible to recognise which one corresponds to the far wall and to avoid readings locked onto intermediate reflections.
Back to Materials and difficult cases or write to us from the Contact page.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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