Giving confidence in performance
Defects and irregularities in the materials of safety-critical components and large structures like bridges, tunnels and power plants can compromise structural integrity, reduce lifetime and increase the likelihood of failure. Ultimately, this could lead to devastating consequences for the environment or seriously threaten the safety of people.
Our work provides customers with the confidence they need when designing, building and maintaining structures. We help to monitor structural health and integrity and extend the lifetime of structures, even in the most hazardous environments.
Non-destructive testing (NDT) provides a means of identifying damage and irregularities in materials and is often the only means of obtaining information about the current 'health' of a structure. It can be used to detect and size in-service and manufacturing defects. NDT is used for on-line production quality control through to in-service inspection, structural health monitoring and life management.
NPL has extensive NDT and surface analysis instrumentation and expertise. We are able to support industry in materials characterisation, product development and long-term performance assessment and damage monitoring. NPL aims to promote the advancement of metrology underpinning non-destructive testing, condition monitoring and diagnostic engineering (including structural health monitoring) for design and quality assurance purposes.
Ultrasound
NPL offers a range of ultrasonic NDT methods for the characterisation of materials and detection of defects.
Contact probes
A range of contact probes are available in the range of 1 to 30Mhz in both longitudinal and shear modes. These can be used to assess material properties or locate defects in a component in the same manner as is used in industry. The system consists of handheld probes that are coupled to the sample with a water based couplant and can be 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.
- 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.
Ultrasonic C-scan
NPL uses an ultrasonic C-scan water tank for scanning large parts in immersion. It has a resolution of 1mm and is capable of scanning an area 650 mm x 650 mm. It is capable of carrying out flat, rotational and programmable surface following scans with probes in the frequency range of 0.5 to 50MHz.
This technique is used to detect, measure and characterise a wide range of manufacturing and in-service defects in materials particularly composites and is routinely used in the aerospace industry. We have developed procedures which are designed to place the use of C-scan techniques on a sound and traceable basis.
- 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 needs to be able to be immersed in water.
Scanning Acoustic Microscopy
To complete the range of ultrasonic methods available for advanced engineering materials there is a Scanning Acoustic Microscope (SAM). This has as its smallest a 5 µm spatial sampling, and a 120 µm beamwidth. This also utilises immersion to take very detailed scans of samples without some of the problems associated with larger diameter transducers. The scanning bed is much smaller than the C-scan tank so is only appropriate for smaller samples but can be used to find porosity and other hard to detect defects in these samples as well as providing the opportunity to ‘slice’ through the data in time and visualise the position of defects in a sample.
- 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 limits on size due to the size of the water bath and needs to be able to be immersed in water.
X-ray computed tomography (XCT) and radiography
X-ray analysis of an object, either in 2D or reconstructing images in 3D allows the measurement of the geometry of external (surface) and internal features without the need to disassemble a component. Localised differences in attenuation under x-ray illumination provide a cross-sectional picture of the density of a component from which variations caused by pores or inclusions can be identified. It is useful for complex components where it might be difficult to use other techniques but there are limits in the resolution and penetration of the x-rays for different materials.
Within the advanced engineering materials group there is a 2D X ray cabinet with a 110 kV source with digital scanning bed providing 83 µm resolution for samples up to around 200 x 200mm. X ray CT facilities and stronger sources are available in other groups around the laboratory.
- 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 on metals and denser materials due to greater contrast. Very dense metals become challenging.
Thermography
The way in which heat propagates through objects can be used to detect sub-surface features and some types of failures without needing to cross-section the objects under study. The object can be heated in a variety of ways, and an infra-red camera is used to image the heat distribution and its propagation into the surface. One of the features that makes this technique so attractive is that it can operate close to room temperature and is totally non-contact.
NPL has flash thermographic, long pulse and lock in thermographic equipment available for materials testing and defect detection. Each of these different methods has its own respective advantages and drawbacks for defect detection such that the appropriate method can be selected in each inspection scenario. Typically, these techniques are sensitive to near surface defects such as delaminations and see most benefit in materials where the propagation of heat within the sample is relatively slow.
What can I detect with it? Voids, delaminations, layer thicknesses
What materials can I use it for? Most useful in composite and other poor conductors of heat as the heat propagation can be captured. Less effective in metallic materials.
Laser Shearography
Laser shearography is a non-destructive testing method that measures the rate of change out of plane displacement of a surface due to an applied stress. This is achieved via monitoring the surface using a camera along with a generated laser pattern. The stress can either be caused by direct loading of the specimen or can be introduced in a controlled way by the equipment using either heating lamps or a vacuum hood. It is a large area testing method that measures over all the area that the camera observes and is therefore non-contact.
The method is used to find delaminations and voids within the near surface of a sample and as such sees most benefit in application to composite materials. It does not have a large depth of penetration only reaching up to 5mm into a material generally.
- What can I detect with it? Voids and delaminations.
- What materials can I use it for? Generally most effective in composite materials.
Microwave scanning
An x-y scanning table is available for large area scanning of non-conductive samples for the detection of voids, delaminations and water ingress using microwaves. Using our scanning table, the point measurements can be built up into an image of the part and any anomalous areas highlighted. There is also limited capability in testing parts around their curvature.
A range of different frequency probes are available that will have a different wavelength and therefore be sensitive to different size defects. Having a wavelength of a similar scale to the feature of interest is the most effective and NPL has 10.5 GHz (λ ~ 3 cm), 24.1 GHz (λ ~ 1.3 cm), 34.0 GHz (λ ~ 0.9 cm) probes to cater for a range of defect sizes.
- 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 and provide information on the internal structure of the sample. Can be used to locate conductive material through the reflection of the microwave energy.
Eddy current testing
Standard eddy current testing equipment is available for conductivity testing and non-conductive layer thickness assessment. Capability includes 60 kHz Conductivity testing probes and 0.5-1 MHz and 4-6 MHz pencil probes for precise application and detection of cracks. This equipment can be used to assess coating thickness, conductivity of materials and for detecting surface cracks in conductive, usually metallic materials. This is representative of the equipment used in the aerospace industry for crack detection and can be very useful for detecting small cracks in electrically conductive samples.
- 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 only or the thickness of non-conductive layers through increased lift off (decreased strength of eddy currents).
Eddy current/magnetic induction thickness gauging
Standard equipment is available for the testing of coating layer thickness on metallic or ferromagnetic substrates based on the principles of eddy currents or magnetic induction. Commonly used for applications such as the determination of paint thickness, very thin layers can be measured.
A range of probes are available for different thickness ranges as well as duplex coating measurements. This equipment can be used to assess the quality of coating thickness on materials and is representative of the equipment used in industry for this purpose.
- 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.
Digital image correlation (DIC)
DIC is a general term for comparing images but at NPL it is used to describe an innovative non-contact optical technique for measuring strain and displacement of both macro and microstructures. It offers a versatile analytical tool that is now being used extensively in experimental mechanics in a diverse range of applications. We have recently expanded the method to include applications in the civil engineering and nuclear industries, performing NDE on buildings and tunnels. We have also developed algorithms to automatically detect defect signatures in a single pass in some cases, alleviating the need for multiple images.
Additional materials testing methods
Resonant Ultrasound Spectroscopy (RUS)
Used to determine elastic engineering properties of anisotropic materials by using an ultrasonic frequency sweep to detect peaks in resonant frequencies. Contact method using whole body resonance of a small volume with well-defined geometry.
- What can I detect with it? Elastic engineering properties of anisotropic materials.
- What materials can I use it for? Non lossy materials, metals or ceramics.
Impact excitation
Manual or electromagnetic striking of a sample with the use of a microphone to record the response to the impact. Fast Fourier Transform analysis of these acoustic signals is carried out to either determine the elastic engineering properties of regular geometry isotropic materials or as an instrumented tap test for the non-destructive testing of components searching for areas that sound different due to the presence of a defect.
- What can I detect with it? Elastic engineering properties of regular materials or areas of difference within a component that may be due to the presence of defects.
- What materials can I use it for? Regular geometry isotropic materials (for elastic properties) or tap test in various materials.
The broad array of techniques available for the testing of advanced engineering materials allows NPL to provide confidence in the selection of the most appropriate technique for a specific inspection scenario.
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