Method and system for inspecting glass laminates for defects (e.g. wrinkles) using NIR or SWIR radiation, absorbent tracers, and image processing.

Abstract

The invention is a method and system aimed at detecting and analysing defects, and more specifically, wrinkle defects, in glass laminates using near-infrared (NIR) or short-wave infrared (SWIR) radiat

Description

This invention is aimed at builders of large composite structures such as wind turbine blades. In the manufacturing process for such blades layers of woven or stitched unidirectional or multiaxial glass fabric are applied into female shaped tools referred to as moulds. As the structures are large, sometimes in excess of 110m long, many layers of glass are required to accommodate the high loads, with laminates sometimes being in excess of 100m thick. This requires many layers of fabric. When all the fabric layers have been applied into the mould a plastic film is applied and sealed over the mould and a vacuum is applied to the complete laminate. A thermoset resin such as epoxy, vinyl ester, or polyester is then introduced via tubes laid into the assembly. The resin infuses the laminate stack and is allowed to cure, sometimes with the addition of heat to speed the process. The problem with the process is that defects can occur in the lay up, that are not apparent during the process, which result in the formation of fibre misalignment, or even in the worst case, wrinkles. These defects are sometimes evident in the final cured part and sometimes are not. If they are visible the laminate can be ground flat and work can proceed. If they are not, they are lost in the laminate. In both cases they will result in the mechanical properties of the structure being degraded, sometimes dangerously so. What is required, is a method of finding these defects ideally before the vacuum infusion process is complete. Measuring their size and then deciding on the best course of action to deal with them. Large defects will require removal and replacement of the fibres, with smaller defects it may be possible to allow them to remain in the structure. Current methods to detect and characterise wrinkles in composites fall into three main categories: 1. Optical detection, where the wrinkle can be seen and characterised. This can only happen when the wrinkle displacement is present at the surface, otherwise it cannot be detected. In the event that a displacement can be seen, the size and depth of the wrinkle cannot be determined without sectioning the composite. Optical techniques will struggle to detect misalignments of the fibres in the x, y and z axis deep in the laminate stack. Due to this, optical detection is usually used in conjunction with other methods, as described in EP3315961B1 - ‘Wrinkle characterization and performance prediction for composite structures using ultrasonic and optical inspection’. 2. Ultrasonic detection, which uses sound waves that reflect off internal interfaces. The time taken for these reflections to return to the detector can provide a digital crosssection to be examined. However, ultrasonic detection cannot tell what is behind one of these interfaces, and therefore cannot determine the size and shape of a wrinkle, just its presence under the surface, which often leads to a worst-case assumption of the wrinkle being present from the detected interface to the back of the composite. This can be combined with FEA analysis to predict sizes and impacts, but those will only be predictions, and often have to be worst-case scenarios. Ultrasonic inspection also requires the presence of a couplant, in order to receive a clear return signal. This can be difficult on a large structure, and may lead to contamination of the surface, which may require subsequent bonding. This is currently the most common method, and a number of related patents have been filed, such as DE102018006086A1 -‘Method for determining waves of laminate components of a rotor blade’ and US5554808A- ‘System and method for detecting ply wrinkling in a laminated composite’. 3. Infrared thermography, where a thermal pulse is applied, and the propagation of heat measured, the theory being that a wrinkle will disrupt regular heat transfer across the composite. The main weakness of this is the fact that in order to determine size and therefore effect of the wrinkle, it needs to be compared to a reference databank. This is highly inaccurate as the chances of having an exact match reference is low, meaning that assumptions have to be made to create estimates of size, rather than having reliable results specific to the given sample. This method is described in patent JP6876407B2 - ‘Infrared thermography method for wrinkle characterization in composite structures’. The current methods, listed above, can only be performed once the composite has finished cure, which means that in most cases it is too late to take a simple remedial action to fix the problem. The invention provides a method of such inspection for both the lay up prior to infusion of the resin, and also after infusion and cure. Timely repair of such defects even after resin cure can eliminate the cost of a blade failure in service which is an expensive event for any large onshore turbine and especially for a large off-shore turbine. Detailed Description of the Invention: Glass laminates are extensively used in various industries, including automotive, architecture, and aerospace, due to their strength, transparency, and aesthetic appeal. However, the presence of defects and specifically wrinkle defects in the applied layers of reinforcement materials in these laminates can compromise their structural integrity. The conventional methods for detecting wrinkle defects in glass laminates are time-consuming, subjective, and often prone to human error. The present invention addresses these limitations by utilizing NIR / SWIR radiation and corresponding NIR / SWIR absorbent tracers to enhance the visibility of wrinkle defects. The absorbent tracers, which are applied to the surface of the glass laminates, interact with the NIR radiation to produce a distinct contrast between the wrinkle defects and the surrounding material. This contrast consists of disrupted and altered tracer paths, whether the path deviates around or over a wrinkle. This enhanced contrast enables accurate detection and analysis of even subtle wrinkle defects. The NIR / SWIR radiation is selected as it has excellent penetration properties in the laminate transmitting well through glass and the other materials present in the stack - polyethylene film, polyethylene infusion media, peel ply (usually nylon or polyester or a combination of both nylon &polyester) and even cured epoxy resin. The tracers are specifically selected to have strong absorption in the NIR / SWIR frequency range. Not all tracers are suitable for this as they may be highly absorbing in the visible region but they may have low absorption in the NIR / SWIR region. Materials using carbon are useful in this respect as they absorb in both the visible and NIR / SWIR region. Thin carbon fibres can be used or lower cost fibres such as polyester that have been coated with a carbon based finish. The radiation wavelength selected is chosen to be one where there are no significant absorptions by the bulk of the materials in the laminate but there is a high absorption to the tracer yarns. The NIR region and the SWIR region are of great utility here. The NIR region is generally considered as being between 780 to 1400nm, and the SWIR region is 1400nm to 3000nm. It has been found that for glass fibres the range of 1500-2200nm is of interest and within that, the range of 1700 to 2000nm is most of interest. The invention is based on the principle that certain wavelengths of the electromagnetic spectrum penetrate glass fabrics and glass / resin laminates to a significant depth. The region of the electromagnetic spectrum used is between lOOOnm. to 2500nm. This encompasses the NIR Near infra-red 750nm-1400nm.and the SWIR (Short wave infra-red 1400nm-3000nm) regions. If fine tracers are incorporated into some or all the layers of fabric, these tracers can be made strongly absorbent to the NIR / SWIR radiation. The material of the tracers is not crucial, but it may be any suitable thermoplastic fibre, usually of polyester or nylon; or even carbon, basalt, aramid or pigmented glass fibre. By illumination of the laminate stack with the correct frequency of radiation and with a suitable detection device the tracers can be detected even if placed at depth in a glass laminate stack. The imaging can be carried out with the laser illuminating an area to be imaged and then an image is taken with the camera. Conversely, the laser can scan the surface of the laminate and exposures can be taken as it traverses the test area, with the camera adjusting its field of view as the laser progresses. Laser profiling technology, whereby the laser emitter and detector (camera) are mounted with the laser at a known angle a, can be post-processed such the detected reflection can be pinpointed in the x, y and z axes to create a 3-dimensional scan, further helping characterise the defect. The wrinkles that the invention is designed to detect are formed from a small error in the lay up of one ply. This can be deep in the laminate. As the materials are applied on top of it, the error is not apparent and as these laminates can be very thick (in excess of 50mm), many layers have to be applied at nominally 1mm thick each. After the fabrics have been applied some additional non-structural layers (known as disposables as they are discarded at the end of the process) are added, lightweight woven nylon, polyethylene mesh and finally a plastic film. When this is completed, a vacuum is applied to the complete stack. It is now that the layers are consolidated together. The defect that is within the laminate will trigger a wrinkle in the plies above, which will grow in size and extend in area. However, with all the “disposable” layers on top this defect is not easy to see and is easily missed in any inspection prior to infusion of the resin. The wrinkle formed will affect the tracers in a number of ways. If a wrinkle crosses a tracer the tracer line will be distorted and the regular pattern of the traces will be modified, with the tracers being drawn closer together closer when the laminate is imaged from above. Or in some cases two adjacent tracer lines will deviate together whilst remining at the same lateral distance apart. These are both patterns that can be seen in the x and y axes that indicate a change in the z axis and therefore presence of a defect. The tracers can be orientated in the 0-degree axis of the fabric (often referred to as the warp at any desired spacing, typically 100mm although closer at 25mm or wider at 300mm or more is possible. It is also possible to place the tracers perpendicular to the fabric roll direction, (often referred to as the weft direction) or any other orientation eg. + / - 45 degrees to the fabric roll direction, if the material is of a stitched construction. Alternatively, the tracers could replace some of the stitching fibres, therefore performing both tasks. They can be straight, or they can replace some of the stitched fibres if the reinforcement material is of the stitched variety; in that case they will end up with a zig zag appearance. The detection of the tracers can be achieved using a number of techniques. A simple camera operating in the SWIR / NIR region can capture the image. Any 3D camera working in the correct frequency could also be used. The benefit of the 3D camera is that it can accurately detect the position of the SWIR / NIR absorbent tracers in the z axis which makes for a more accurate detection of laminate imperfections (e.g. wrinkles). SWIR Vision Systems Durham NC, make such cameras, which can carry out laser profiling. These operates at frequencies of 400-2 lOOnm. It is possible to have tracers at multiple orientations in the same fabric. For example, tracers can be at both 0 and 90 degrees to the roll direction, this forming a grid like structure. Most weaving or stitching companies would be able to produce such reinforcements with tracers incorporated. A modification of the invention is to incorporate additives into the tracers that are fluorescent. These are designed to fluoresce (i.e. produce intense visible radiation when excited by a specific laser wavelength). If this approach is used a visible camera may be used. The tracers can be placed in every ply of the laminate, however in most laminates this is not necessary to detect wrinkles. Wrinkle formed in one layer will form wrinkles in the plies laid up on top that will tend to grow as each additional ply is placed on top. This means that in laminate 50mm thick it may be sufficient to have between one and 3 plies with the tracer, in the laminate stack. In order to aid the identification of the tracer plies different spacings or orientations of tracers can be used. The image analysis can be of two basic types. Measurement of the tracers’ relative positions, (spacing) and orientation to detect any misalignment due to the presence of a wrinkle or other layup defect. If a confocal camera is used which is configured to have a narrow depth of field it can be focused on the tracer position and can measure the position of the tracer in the laminate stack in the z-axis. This is also capable of detect wrinkles as the z- axis position of the traces will deviate in the area of the wrinkle or defect. It is apparent to anybody skilled in the art that other methods of adding tracers are possible without the addition of new fibres. The method can work with any system of introducing detectable lines, for example painting fluorescent lines on different layers, applying a NIR / SWIR reflective ink or any other way in which internal deformations can be detected and measured using lasers to detect an internal reference. Additional features The system could easily be used to also detect the position and state of cure of the resin front during the infusion process. Here a frequency of the NIR / SWIR would be selected that was strongly absorbing for the mixed resin, with a resulting spectrum having key information regarding the cure of the resin (i.e. The amine bands of the epoxy curing agent, or the epoxy bands of the epoxy resin in the case of an epoxy resin). It is also possible, by taking the reflected NIR / SWIR signal and converting it into a spectra with a hyperspectral camera, to build a multivariant analysis model to perform a number of analyses. These analyses could be, but not restricted to, for mix ratio, state of cure, void content and fibre volume fraction. The various components to the invention are. Fabrics with tracers incorporated into their structure. These can easily be built into the fabric structure during the weaving or stitching stage of their construction. They can be of any composition, but they must be absorbent in the NIR / SWIR frequency. Illumination with the correct wavelength is required. Lasers are ideal for such illumination. These can be tuned to the ideal wavelength that will give as good as possible penetration of the glass whilst also give as high as possible absorption in the tracers. Typical frequency would be in the 1500 to 2000nm range. Optogama make such lasers. The additional advantage of using this wavelength is that these lasers are deemed “eye safe” as they have a frequency is excess of 1400nm. NIR / SWIR Camera. A specialized setup is employed, comprising a bit-sensitive camera capable of capturing high-resolution images with enhanced sensitivity to NIR / SWIR radiation. The camera is positioned to capture images of the glass laminates illuminated by the NIR / SWIR radiation source. The captured images are then processed using image processing algorithms specifically designed to identify and analyse wrinkle defects. These algorithms employ edge detection, pattern recognition, and machine learning techniques to differentiate between wrinkle defects and other surface irregularities. SWIR Durham NC. USA make such cameras. Software. The software-based image processing module provides real-time analysis of the captured images, generating reports on the presence, location, and severity of wrinkle defects. This enables operators to take immediate corrective actions, minimizing production errors and ensuring the production of high-quality glass laminates. Stemmer GmbH make such software, Stemmer Common Vision Blox. Advantages of the Invention: The method and system described herein offer several advantages over existing techniques for inspecting glass laminates for wrinkle defects: 1. Enhanced Detection: The use of high penetration properties of the NIR / SWIR radiation in glass fabrics and NIR / SWIR absorbent tracers significantly improves the visibility of wrinkle defects, enabling their accurate detection with thick laminates in excess of 50mm thick. 2. Rapid and Objective Analysis: The software-based image processing module analyses the captured images in real-time, providing objective and consistent analysis of wrinkle defects, eliminating human error and subjectivity. 3. Cost-effective: The proposed system can be integrated into existing glass manufacturing processes with minimal modifications, making it a cost-effective solution for quality control. 4. Increased Productivity: With real-time analysis and immediate reporting, operators can take prompt actions to rectify defects, minimizing production downtime and increasing overall productivity. Detectors. A detector that will operate in this range is required. These can be a modified visible light camera with a detector that is tuned to operate in the NIR range. The optics of such a camera can be glass, as glass is transparent to the bands that are being used. Suitable campers are supplied by Edmund Optics. Software. The images from the detection device (camera) can be viewed by a quality technician, or the image can be processed using image analysis software. Such software would identify the position of the tracers, their straightness, the width between them in each layer, and any discontinuities in them. Wrinkles will result in deviations in the tracer path and also a shortening in the distance between adjacent tracers. Suitable software is supplied by Stemmer Imaging The scope of the presently claimed invention is defined by the following claims. The claims set out below should not be construed to require that the steps within be performed in the given order. Nor should they be construed to exclude two or more steps or portions of said steps being performed concurrently or to exclude any portions of two or more steps being performed alternatingly.

Claims

1. A method of inspection of laminates and dry laminate stacks prior to resin infusing by the use of NIR ( SWIR) radiation and tracers within the reinforcement materials, as following:

2. NIR / SWIR absorbent tracers are incorporated into the laminate prior to resin introduction.

3. These tracers can be detected by NIR / SWIR lasers and cameras.

4. The radiation can be in the range 800nm-2500nm5. The radiation is most preferably 1900nm6. The glass reinforcement fibres can be woven or stitched construction7. The tracers can be polyester, dyed or pigmented with any colour that is absorbent in the required NIR / SWIR band chosen.

8. The analysis can be carried out on the reinforcement stack in its dry state before the resin is introduced, or after the resin has been introduced with or without full cure of the resin.

9. The resin can be epoxy, vinyl ester, or polyester or any other suitable thermoset10. The images generated can be manually viewed or may be processed with a suitable image processing software11. The image processing software can detect and measure any change in the tracers, and from that, detect the presence of a defect, and characterise it.

12. Corrective steps can then be taken, depending on the users preference.

13. The tracers could have fluorescent materials added and use a laser tuned to excite the fluorescent additive into generating a high intensity response in the optical region of the electromagnetic spectrum.

14. The system could also be used to detect the position, mix ratio and state of cure of the resin system used during the infusion process.

15. The system could also be used to detect the presence of air bubbles and voids within the laminate.