Not all materials are easy to measure with the ultrasonic technique. Cast irons, composites, thick plastics, coarse-grained alloys, glass fibre, elastomers show attenuation, scattering and grain noise that can make measurement unstable or impossible with a standard instrument and probe. This guide summarises the technical strategies for obtaining reliable results even on the most difficult materials, using dedicated Dakota NDT instruments and probes.
What makes a material “difficult”
A material is considered difficult for ultrasonic measurement when it shows one or more of the following characteristics:
- High attenuation: the ultrasonic signal rapidly loses energy inside the material, the return echo is weak or absent
- Coarse crystalline grain: inclusions or grain inhomogeneities (>0.1 mm) generate spurious reflections (grain noise) that mask the back wall echo
- Anisotropy: the speed of sound varies with direction (typical of fibres and composites)
- High acoustic impedance: difficult coupling between probe and material
- Non-standard temperatures: the speed of sound varies with temperature
- Irregular or rough surfaces: they disturb the acoustic contact
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Typically difficult materials
- Grey cast iron: lamellar graphite scatters the signal, variable velocity 5000-5600 m/s
- Ductile cast iron: better than grey but still attenuating
- High-alloy steels: complex microstructure (Inconel, Hastelloy, duplex)
- Bronzes and brasses: coarse grain in castings
- Glass fibre composites (GFRP): strong attenuation, variable velocity
- Carbon fibre composites (CFRP): pronounced anisotropy, requires dedicated probes
- Thick plastics (HDPE, PP, PTFE): low velocity 2000-2400 m/s and attenuation increasing with thickness
- Elastomers and rubbers: hard to measure beyond a few millimetres
- Cement, concrete, granular materials: not measurable with the standard technique
Strategy 1: choose the right instrument
Dakota NDT offers instruments specifically designed for difficult materials:
- Dakota PMX4-DL (High Penetration Thickness Gauge): designed specifically for materials with high attenuation, coarse grain and substantial thicknesses
- CMX series (CMX1-DL/CMX3-DL): they offer TDG (Time Dependent Gain) built into the transducer list for attenuating materials
- FX series (FX70-DL/FX81-DL): flaw detectors with automatic TCG (Time Corrected Gain) and adjustable damping for noisy materials
- For cast iron, prefer instruments with A-Scan to interpret the signal (CMX1-DL or higher)
Strategy 2: choose the right probe
- Lower frequencies (1-2 MHz): they penetrate attenuating materials better, at the cost of lower resolution
- Larger diameter probes: they collect more return energy and are more stable on rough surfaces
- Probes with delay line: useful for measuring thin thicknesses and for more stable coupling
- Dedicated high-temperature probes for measurements on hot materials (up to 500 °C on some models)
- Dual element probes: better than single element on corroded and irregular surfaces
Strategy 3: effective acoustic coupling
- Quality couplant gel: choose a viscosity suited to the material and to the position (vertical vs horizontal)
- Surface preparation: remove loose rust, scale and non-adhering paint; rough surfaces may require lapping with fine abrasive paper
- Constant pressure on the probe, without excess: too much pressure does not improve the signal and damages the probe
- At high temperatures: use dedicated high-temperature gels (silicone, glycerine, polyethylene glycol)
- On small curved surfaces: check that the probe is in full contact, otherwise change the probe radius
Strategy 4: calibration and measurement parameters
- Calibration on identical material: whenever possible, always calibrate on a sample of the same material and the same batch as the part to be measured
- Check the speed of sound: it can vary significantly between different cast iron castings or steel heats
- Velocity Mode (VM): on the CX4, CX8-DL and higher models it allows the real velocity to be determined on a part of known thickness
- Damping: set to high values (on the CMX series: 50-1500 ohm selectable) to reduce ringing on noisy materials
- Gate positioning: on flaw detectors, set the gates manually to exclude the initial grain noise
Strategy 5: signal validation with A-Scan
On difficult materials it is always recommended to check the signal in A-Scan mode. The graphic interpretation reveals:
- Whether the back wall echo is really the one being measured or the instrument is triggering on grain noise
- Whether there are intermediate echoes from flaws, inclusions or delaminations
- Whether the signal-to-noise ratio is acceptable (at least 3:1)
- Whether the echo shape is clean or scattered (an indicator of heterogeneous grain)
The Dakota instruments with A-Scan are the CMX1-DL, CMX2-DL, CMX3-DL, CMX10-DL, MX2-DL, PMX (the whole Precision series) and the FX series of flaw detectors.
For an assessment of your specific application and an on-site demonstration on difficult materials, contact our technical department.