Non-destructive testing (NDT) services

Contact ultrasonic testing

• What can I detect with it? Material/layer thickness, ultrasonic velocity, wall thinning, voids, porosity, delaminations, cracks.
• What materials can I use it for? Any type of material, but surface condition and ability to use a couplant is a consideration.

A range of handheld contact probes ranging from 1 to 30 MHz are available in both longitudinal and shear modes, which can be used to assess material properties or detect defects in a component. The probes are coupled to the sample using a water-based couplant, and manually scanned to test every point of interest. Alternatively, they can be used in a through transmission manner to determine the speed of sound within a material. Lower frequency 54 kHz probes are also available, which can be similarly used to assess ultrasonic pulse velocity and cracking in concrete. These probes are ideal when the samples of interest cannot be immersed in water, or when it is necessary to mimic in situ inspection conditions. 

Ultrasonic C-scan for inspection of large parts 

• What can I detect with it? Material/layer thickness, ultrasonic velocity, wall thinning, voids, porosity, delaminations, cracks.
• What materials can I use it for? Any type of material that can be immersed in water.

Ultrasonic C-scan is used to detect, measure and characterise a wide range of manufacturing and in-service defects in composite materials, and is routinely used in the aerospace industry. 

Large parts are immersed in a water tank and scanned with 1 mm resolution over a 650 x 650 mm area. Probes ranging from 0.5 to 50 MHz in frequency are used to enable flat, rotational and custom surface-following scans that accommodate difficult geometries.

Scanning acoustic microscopy (SAM)

• What can I detect with it? Material/layer thickness, ultrasonic velocity, wall thinning, voids, porosity, delaminations, cracks.
• What materials can I use it for? Any type of material that can be immersed in water, but there are size limits due to the dimensions of the water bath.

SAM completes NPL’s range of ultrasonic methods for advanced engineering materials. SAM also uses water immersion to provide coupling, and can scan at much higher temporal and spatial resolutions than our C-­scan equipment. It uses focused probes with a much smaller active area, allowing the detection of smaller defects than conventional ultrasonics. However, SAM is limited in terms of both overall sample size – due to the scanning bed size – and thickness, due to the penetration power of the probes.

NPL’s scanning acoustic microscope offers minimum spatial sampling of 5 µm, a 120 µm beamwidth, and can perform scans at frequencies up to 75 MHz. The system can perform tomographic acoustic microscopy imaging, providing the opportunity to ‘slice’ through the imaging data in a manner akin to X-ray CT – albeit not in 3D – to visualise the position of defects in a sample. This is suitable for identifying porosity and other hard to detect defects in any type of material.

• What can I detect with it? Voids, cracking, porosity, wall thickness.
• What materials can I use it for? Any type of material, but particularly effective for metals and denser materials where the contrast is greater. Very dense metals become challenging.

X-ray analysis of an object – here with 2D images – allows measurement of the geometry of external (surface) and internal features of components without disassembly. Localised differences in attenuation under X-ray illumination provide a cross-sectional picture of the density of a component, which enables the identification of variations caused by pores or inclusions. It is useful for the analysis of complex components where other techniques may be difficult to apply. However, there are limits in terms of resolution and X-ray penetration for different materials.    

The Advanced Engineering Materials (AEM) group at NPL has a 2D X-ray cabinet with a 110 kV source and a digital scanning bed, providing 83 µm resolution for samples up to approximately 200 x 200 mm in size. 

• What can I detect with it? Voids, delaminations, layer thicknesses.
• What materials can I use it for? Most useful for composite materials and other poor conductors of heat. Less effective in metallic materials.

The way heat propagates through objects can be used to detect sub-surface features and some types of failures without cross-sectioning the sample. Objects can be heated in a variety of ways, and heat distribution and propagation into the surface is then imaged using an infrared camera. This non-contact technique can be performed at temperatures close to ambient.

NPL offers flash, long pulse and lock-in thermography services for materials testing and defect detection, with the choice of method depending on the particular inspection scenario. Typically, these techniques are sensitive to near-surface defects, such as delaminations, and are most beneficial in materials with low thermal conduction, allowing heat propagation to be captured.

• What can I detect with it? Voids and delaminations.
• What materials can I use it for? Most effective in composite materials.

There are several types of laser shearography systems available, which vary in their complexity and capabilities. The technique is typically used to identify delaminations and voids near to the surface of a sample, and is most beneficial for composite materials.

The equipment at NPL measures the rate of change of the out-of-plane surface displacement caused by applied stress without damaging the material, either by direct loading of the specimen or through controlled methods such as heating lamps or a vacuum hood. This non-contact method uses a camera and an applied laser speckle pattern to compare the changes. It is effective for detecting delamination, voids and disbonds in both sandwich and honeycomb structures, but depth penetration can be limited in simple composite panels, and it is less effective for metallic samples.

• What can I detect with it? Voids, delaminations, water ingress.
• What materials can I use it for? Only materials that are not electrically conductive will be penetrated by microwaves to provide information on the internal structure of the sample. Can be used to locate encapsulated conductive material through the reflection of the microwave energy.

Microwave imaging exploits the dielectric properties of materials, as only those that are not electrically conductive can be penetrated by microwaves. It can provide information on the internal structures of samples, as well as check for voids, delamination and water ingress. Microwave scanning can also be used to locate conductive materials within a sample as they reflect microwaves, creating detectable signal variations. 

An XY scanning table enables indexed recording of point measurements that can be used to build an image of the sample, highlighting any anomalous areas. Microwave scanning is most effective at a wavelength of a similar scale to the feature of interest; 10.5 GHz (λ ~3 cm), 24.1 GHz (λ ~1.3 cm) and 34.0 GHz (λ ~0.9 cm) probes are available at NPL, to cater for a range of defect sizes.

• What can I detect with it? Surface cracks in metallics and other conductors, coating layer thicknesses.
• What materials can I use it for? Conductive materials, or the thickness of non-conductive layers through increased lift off (decreased strength of eddy currents).

Eddy current testing uses electromagnetic induction to detect surface and near-surface defects in conductive materials by measuring changes in induced electrical currents. Standard eddy current testing can help to assess the thickness of non-conductive layers on conductive substrates, and to detect surface cracks in high conductivity – usually metallic – materials. NPL’s capabilities include 0.5-1 and 4-6 MHz pencil probes similar to those used in the aerospace industry for detection of small cracks in electrically conductive samples, and conductivity testing with 60 kHz probes.

• What can I detect with it? Measure coating layer thicknesses.
• What materials can I use it for? Non-conductive paint thickness on conductive or ferromagnetic substrates.

Eddy current and magnetic induction principles can be used to determine non-conductive coating layer thickness on conductive – usually metallic – or ferromagnetic substrates. It can measure very thin layers, and is commonly used for applications such as the determination of paint thickness. Various probes are available at NPL to assess the quality of coating thicknesses across different ranges, as well as for duplex coating measurements.