- Error Reduction for Region-of-Interest Computed Tomography
With conventional CT, the object being examined must fit within the detector’s field of view (FOV). However, when using region of interest (ROI) CT, which is used to image objects that are too large to fit within the maximum FOV for a system or to apply high geometric magnification to a specific area of an object, the projection data is truncated. Trying to directly reconstruct the projection data with common FBP type algorithms causes imaging errors caused by the missing data and truncation artefacts. These combined errors can mask significant information about the object and stop accurate density measurements or volume segregation taking place.
To solve this, our experts tested two data completion methods – a cosine extension method and an estimation from model method – to approximate the missing portion of the projection data prior to reconstruction. The tests were developed for the inspection of carbon-fibre-reinforced-plastic (CFRP) panels, which, despite being large in size require a high level of magnification to image any defects.
For this project, TWI’s experts fabricated a 50x50x5mm CFRP panel to include five holes drilled into the surface as an image quality indicator (IQI).
- Improving the Integrity Management Process for Bonded Hoses
Also investigating the use of CT was a joint industry project designed to validate the use of x-ray tomography/gamma source digital detection inspection methods for the inspection of bonded hoses, in the presence and absence of service fluids in the hose bore. The TWI team explored the use of radiographic modelling to develop and optimise inspection regimes, providing a critical assessment of the potential impacts of minimum detectable flaw size on the integrity of bonded hoses. This work aimed to enable greater confidence for asset management while recommending viable, proven in-situ inspection procedures and tool specifications.
- Development of Quantitative Radiographic Tomography Technology for the In-Situ Inspection of Welded Austenitic Safety-Critical Pipework in the Nuclear Power Generation and Petrochemical Industries
The publicly-funded ‘TOMOWELD’ project was created to find a solution for the inspection of critical pipework for the nuclear and petrochemical industries. This pipework was typically made from austenitic stainless steel due to its corrosion and fracture resistance. However, defects can form where these pipes are welded together. If they are not located, these defects can grow and lead to mechanical failure in the pipelines. At the time of the project, film radiography was commonly used because it was not significantly affected by the grain structure. However, the technique was limited by long exposure times and the results showing a 2D image of a 3D object. The TOMOWELD project partners worked to develop a robust, mobile X-ray tomographic system for the accurate inspection of austenitic steel welds at the sensitivity levels required by the nuclear industry. The aim of the project was to overcome inspection limitations with the provision of 3D information of internal structures, allowing for cross-sectional analysis and dimensional measurement.
- Automated Inline Inspection and Quality Control of Net-Shape Power Metallurgy Components using Microfocus Three-Dimensional X-Ray Computed Tomography Imaging (Additive Manufacturing)
Another publicly-funded project, QualiNET, addressed the manufacture of components through powder metallurgy (PM) processes and other advanced net-shape techniques, such as powder injection moulding (PIM) and additive manufacturing by laser sintering. PM is suited to high-volume production, but the use of increasingly finer powders for refined microstructures can mean that any defects could have a significant impact on production output, including loss of material and efficiency as well as potential in-service failure. The QualiNET project worked to develop a microfocus, 3D X-ray computed tomography system for the automatic quality control of PM components.
- SWAK Project: Determining Aerospace Composite Bond Quality
CT was also instrumental in the SWAK project, which was created to address the issue of ‘kissing bonds’ in the aerospace industry. As the industry moved towards adhesive joints as an alternative to riveting, bolting or welding, there came a need to address the issue of
Kissing bond defects, also called zero-volume dis-bonds between adhesive and adherend. These defects appear to show solid-to-solid contact but there is no tensile strength or volume at the interface between the adhesive and the adherend. But, because the bond appears to have been made, these defects are difficult to locate using NDT techniques. These defective bonds can occur during joint manufacture because of poorly prepared adherents and compromise the joint strength.
TWI worked alongside Brunel University London and GMI Aero SAS on this project, where different NDT technologies were tested for the locating of kissing bond and other hard to find structural defects. Computed tomography was one of the technologies shown to be successful for the detection of contaminated kissing bonds.
- Development of a Standard Reference Geometry for XCT
The team at our facility in Port Talbot, Wales designed and manufactured a standard reference geometry for use with X-ray computed tomography (XCT) for the European Space Agency (ESA). At the time of the project there were limited standard reference geometries for use within XCT, so the aim was to allow for image quality to be verified based upon a series of criteria and constraints, informed by a literature review. Taking account of the literature review findings, and the requirements from ESA, the potential geometries were subjected to a set of weighted evaluation criteria, which resulted in the final design (Figure 1). The final design was based around a step cylinder comprised of aluminium, with removable internal cylindrical pins that are raised within the part to represent different penetrable thicknesses. To assess the image quality, micrometre scale line features were placed in a circular pattern around the removable pins along with hemispherical patterns to represent porosity. Figure 2 shows an XCT image of the manufactured step cylinder.
- Failure Investigation of Welded Moulded Plastic Components
TWI used CT to support automotive industry goals to identify and develop new materials and processes to drive powertrain efficiency. With lightweighting in parts requiring the replacement of conventional metals with lighter polymers and hybrid materials, engine components require the use of advanced engineering thermoplastic materials that are moulded into complex 3D structures and then joined together. The quality of the finished parts is reliant on both the material and the joint design for specific welding processes. Verifying the weld quality had been an ongoing challenge for both manufacturers and suppliers, which was borne out by premature in-service failures being reported by automakers.
To help provide confidence in welded moulded plastic components for the automotive industry, our experts undertook an assessment of destructive and non-destructive examination methods for weld integrity. Tests were conducted on a vapour separator (Figure 3), that had been welded using linear vibration welding (LVW). This is one of the most widely used welding techniques for thermoplastics for under-the-bonnet applications. Inspection showed tell-tale signs of poor weld (Figures 4a, 4b, 4c), with the computed tomography (CT) scans providing the most revealing flaws. This was despite the component passing general QA tests and burst tests.
CT images and videos were able to show evidence of poor welding procedures (Figures 5-7), even where there was no access to the welded locations of the component, so that their quality could not be directly compared to the design rules. While the research showed that all current investigative techniques, (e.g. visual inspection, microtoming and burst test) remained valid, CT was able to identify issues including non-uniform weld flash around the perimeter of the weld, and inconsistencies between the weld-flash and the weld-gauge of the tongue and groove joint, which did not pass the criteria set by the automotive company’s design rules.
- Additive Reinforced Friction Stir Welds
Although CT is a non-destructive testing (NDT) technique, there have been instances when TWI has used it within a destructive testing framework. One such project applied X-ray microscope tomography to characterise the homogeneity of friction stir welds that had been reinforced using additive nanoparticles. Silicon carbide particles had been added to two 4mm aluminium plates prior to friction stir welding. A sample was then prepared by cutting off a cylinder from the FSW specimen in order to allow a fine enough resolution for the detection of very small features and to equalise the X-ray penetration through the sample (thickness= constant diameter). The cut off sample can be seen in Figure 9, and the position of the sample in relation to the source and the detector in Figure 10. Because the sample had been cut, the computed tomography cannot be classified as an NDT technique.
The results (such as those shown in Figures 11-12) demonstrated the capability of X-ray microscope tomography to show particles against the aluminium alloy base in 3D as well as the detection of voids in the weld, with the X-ray microscope tomography system having the potential to locate particles down to 1μm.
- Refurbishment of Railway Axles
Our experts combined CT with expertise in laser cladding to test a potential solution to high levels of potentially unnecessary scrappage in the rail industry. Strict safety regulations led to high scrappage rates for railway axles that had suffered even minor wear or corrosion. To address this this costly material wastage, TWI joined an industry consortium for the ReLASE project, which aimed to develop a cost-effective and highly durable coating-based method for repairing worn railway axles. Laser cladding (Figure 13) was investigated as a solution for repair the of axle wheel seat areas. Coupon samples (Figure 14) were produced with a range of coating compositions that could then be compared using XCT imagery (Figures 15-16) to assess coating quality, defect characterisation and down-selection of coupons for fatigue testing. The results of the testing informed the development of an equivalent, but more portable, ultrasonic inspection technique as well as allowing for the optimisation of the coating process for the selected powders to minimise defects such as cracking, voiding, porosity, and lack of fusion. Additional fatigue testing compared the mechanical performance of cladded coupons with coupons of parent material. The project used both non-destructive and destructive testing methods to demonstrate the successful application of laser-engineered coatings to rail axles as a means of cost-effective refurbishment.
- Digital Volume Correlation (DVC) Analysis of Aluminium Foam
https://www.twi-global.com/media-and-events/insights/digital-volume-correlation-dvc-analysis-of-aluminium-foam
Cellular materials such as cork, bamboo and sponge are widespread in nature. Their unique combination of properties have led them to be replicated with synthetic cellular materials using polymers, metals, glasses, ceramics and carbon with the aim of mimicking their behaviour. These multifunctional materials promise low weight, low thermal conductivity, high specific surface area, ability to undergo large deformations at relatively low and constant stresses without rebounding, making them particularly attractive for lightweight structures, fluid flow control, porous electrodes, energy absorption, thermal insulation, acoustic damping, and vibration damping.
XCT has been shown as a successful method for performing NDT on materials containing cellular structures for the qualitative and quantitative inspection of components and intricate structures with 3D characterisation. Previous studies had used XCT to investigate aluminium (AI) foam, providing information on cross-section shape, cell size, and ligament area distribution. However, these studies only investigated the foam in an undeformed state. TWI addressed this gap by creating a project using XCT and digital volume correlation (DVC) to assess AI foam behaviour under mechanical or thermal load.
TWI used water jet cutting to form a cylindrical specimen of AI open-cell foam that was then subjected to quasi-static compression. The compression was interrupted at intervals (Figure 17) so that the specimen, which was held at compressive load, could be examined with XCT. The
Acquired datasets were analysed to identify damage micro-mechanisms, and to perform a digital volume correlation (DVC) study using both local digital volume correlation analysis (LADVC) and global digital volume correlation analysis (GADVC).
XCT was able to provide information about cell wall microstructure, including the presence of microscopic features such as foreign particles, precipitates and of micro-pores (Figure 18). Such features could lead to a non-homogeneous stress/strain distribution. Figures 19-23 show the 3D rendering of the Al foam during the in-situ compression experiments, which revealed four main stages of material behaviour. This study exploited the use of complementary methods, including time-lapse imaging by XCT, in-situ experiments and DVC analyses to provide insights into Al foam behaviour when subjected to compression. The same approach can be used to study different cellular structures.
- Development of Offset X-Ray Computed Tomography Inspection
TWI’s work with CT not only assists industry with general and specific challenges but also involves advancing the process itself. One such core research programme (CRP) project addressed the limitation of XCT where the size of component that it can handle is restricted by the field of view of the digital X-ray detector.
TWI developed offset CT as a technique that allows larger components to be inspected, thereby extending the capabilities of existing equipment. Larger components referred to those objects whose width, even at the lowest magnification, does not fit within the field of view of the digital detector. Before this research, offset CT had been used in academia, but only to a limited extent by industry. TWI’s work developed sufficient understanding of the process to implement and deploy the inspection method.