Finding flaws in. composites

Transcription

1 flaws in composites From ultrasonics, through infrared and thermographic techniques, to more exotic, developmental methods, non-destructive testing (NDT) of composites can take many forms. George Marsh reports on some of the principles and equipment involved in this vital area. Ultrasonic C-scan remains the NDT technique most widely used for composites inspection. It can show up a wide range of manufacturing and in-service defects. This C-scan of a calibration panel was produced during a project to standardize ultrasonic inspection of polymer matrix composites. (Picture COURBS~ of NPL.) M ost composite boat owners will recall a surveyor assessing hull condition by tapping it with coins. A bright ringing tone means good laminate, while any dull, muffled sound could spell trouble. This principle was mechanized by Rolls-Royce in the UK in its Tapometer, an instrument comprising a tapping head and a microprocessor-based control unit. Although spin-off company Rolls-Royce MatEval no longer exists, high-tech tapping was also developed by Boeing whose technology was later incorporated in the Tap Hammer from Wichitech Industries. Changes of greater than 10% in response to the tap pulse, as presented on a display, indicate possible flaws. Today NDT of composites ranges from tapping (still used for revealing the presence of flaws if not their nature), to true rocket science - as in imaging faults by tracking the paths of neutrons through rocket motor materials. Well up at the leading edge are the latest scanning systems for both production and in- field inspection that can build meaningful two or three-dimensional (3D) images at speed, without physical contact, reliably, economically and with only modest operator skill. Ultrasound Tap testing works because defects and delaminations impede the flow of acoustic energy. This effect is more pronounced with ultrasonics where air, contained within voids and other flaws, is likewise an effective barrier to ultrasound. A piezoelectric transducer provides impulses whilst a separate trans- ducer senses the returned energy. Where access allows, a transducer is placed at each side of the test sample so that a through-transmission test can be carried out. But in cases where only one side is accessible, both transducers are located on that side and a pulse echo technique is used. According to Robert Philips, technical director of UK company Ultrasonic Sciences, composites require higher trans- ducer powers than metals, along with sensitive logarithmic receivers able to pick up faint returns. Low frequencies, from say 400 khz to 1 MHz penetrate best, but for high resolution and definition freq- uencies of up to 5 MHz are used. Discon- tinuities become evident either in an A- scan trace displayed on an oscilloscope, or in a two-dimensional (2D) plan view gen- erated from a line-by-line scan (C-scan). Ultrasound has become the most ubiquitous composite NDT technique. Ultrasound has become the most ubiquitous composite NDT technique, able to detect faults ranging from voids, porosity, disbonds and delaminations, through resin cracking plus broken and misaligned fibres, to inclusions. It is non-hazardous, often portable, requires no sample prep- aration and, with the latest transducers and signal processing, can detect quite small imperfections. A drawback has been the need for contact between the test probe and material surface, usually via a liquid couplant such as water or glycerine. Ultrasonic Sciences markets systems relying on squirter probes that provide the coupling fluid and these have been 42 REINFORCEDplastics D e c e m b e r /02/$ - see front matter Elsevier Science Ltd. All rights reserved.

2 included in multi-axis scanning systems used for testing composites. Automating the use of such systems is a challenging task. In one of Ultrasonic Science s more computerized devices, for example, not only are squirter probes mounted on a scanning frame programmed to follow complex-curvature profiles, but receiver gain and other parameters are adjusted dynamically to take account of the characteristics of the scanned material. Intelligent C-scan image construction and interpretation helps reduce the need for high expertise from a human operator. Engineers have tried to circumvent the need for liquid coupling, particularly where water ingress could be deleterious as with honeycomb structure. Transducers incorporating solid couplants such as lead niobates, synthetic rubbers and certain hydrophylic polymers include specialized examples such as roller probes for bond testing. These still, however, require contact. Avoiding contact altogether has been the holy grail and at a lecture given at the 8th European Conference on Non-Destructive Testing held in Barcelona this summer, Bernhard R. Tittmann, Schell Professor of Engineering at Penn State College of Engineering, welcomed the emergence of viable transducers that can couple in air rather than in liquid. Citing ultrasound advantages of relative economy, ability to detect internal flaws, rapid interpretation of results and suitability for computerization, he believes that air coupling is significantly extending the utility of this NDT form. Although the first air-coupled ultrasound transducers appeared in the 1970s sensitive devices had to await pioneering work in the US during the 80s. Progress has been made in three ways. One approach is to place an acoustic impedance transition layer, of soft polymer or compressed fibre composition for instance, in front of the piezoelectric element. This solution is adopted in powerful ceramic-based transducers that work in tone bursts rather than single In the through-transmission liquid-coupled mode, ultrasound is transmitted from a transducer on one side of the part through a water jet into the part. This JO-axis system from Ultrasonic Sciences provides simultaneous control in all axes to keep both water jets aligned and normal to the surface. (Picture courtesy of Ultrasonic Sciences Ltd.) pulses and are now operating with both portable and fixed NDT installations. Slow area scanning rates associated with raster-like C-scan have been accelerated by operating multiple probes in parallel. The second approach (somewhat eclipsed by the first in practice) is to trans- fer impulse energy electrostatically, capac- itive transducers having shown their ability to propagate ultrasound frequen- cies of up to 5 MHz across an air gap. A third method uses lasers to excite the required ultrasonic mode. One such device, the Laser Ultrasonic Inspection System (LUIS) from Canadian company UltraOptec, marries laser excitation to a pulse-echo technique. Producing C-scan views, the system operates without direct contact or a coupling medium. An obliquely incident beam is used to scan curved surfaces while software constructs a 3D representation of the scanned sam- ple. LUIS is available in three versions, the largest of which is gantry-mounted and can scan sizeable structures, on aircraft for instance, automatically. Laser pulse ultra- sonic systems exist that incorporate Fourier spectral analysis sample discontinuities to detect cracks, and other flaws. Though pulse ultrasonics are in the majority, a technique variation in which energy is applied in continuous waves has advantages that the presence and nature of defects can be revealed by the patterns set up when waves reflected from the boundaries of sub-surface dis- continuities interfere with those at the surface. These interference patterns can be analysed with computer assistance. Different types of ultrasonic waves behave in different ways. Normal bulk waves incident at 90 are frequently used but offsetting the probes and sending a shear wave between them can produce better results in certain laminates. Signals directed at a critical oblique become angle plate or Lamb waves, typically propagating considerable distances with- in the thickness of the material and increasing the area that can be scanned between any pair of transducers. This can be useful for curved surfaces such as those in pultruded pipes. Rayleigh (sur- face) waves can detect surface anomalies that might indicate underlying flaws. In one interesting technique, nor- mal bulk waves are used to evaluate either applied stress or modulus. In uni- directional carbon/epoxy composite, for instance, waves applied across the fibre axis are deviated by an amount which depends on the material s stiffness. Measuring this deviation can therefore establish the modulus. Examination of December 2002 REINFORCEDplastics 43

3 interferometric patterns at the material s surface can yield the same information. Researchers at NPL in the UK have harnessed the effect in ultrasonic devices monitoring the level of cure in a composite during commercial processing. The technique can also yield insights into temperature, pressure and thickness. Since penetration and scattering losses depend on frequency, another refinement is to use multiple test frequencies. QMI Inc in the USA has been using frequencies as low as 50 khz to test more complex composite materials, though spot resolution at this figure degrades to 8-10 mm, compared with l-2 mm at a more normal 400 khz. NPL s Dr Graham Sims says one factor still constraining full adoption of scanning ultrasonics as a universal NDT tool for composites, is a lack of established comparison yardsticks and reference standards. In the absence of generally-accepted calibration methods it has been difficult to compare like with like. His team recently launched new initiatives to address these issues. These include a sample composite panel containing a number of standard flaws. Hotting up as competitor A family of techniques that is emerging to challenge ultrasonics in scanning applications is thermography. Using an infrared (IR) camera to image an area rather than a succession of points in a line-by-line C-scan promises faster material evaluation in volume applications. Airlines, for instance, can manually scan composite control surfaces on aircraft while they are still warm after flying, to check for disbonds or other damage. Although thermal systems have so far been less sensitive than their ultrasonic counterparts to defects that are small and deep, advantages of no contact or coupling medium, together with faster imaging, should more than compensate. As with ultrasonic NDT, thermography can be based on the application of either transient impulses or waves, these techniques being already available in equipment from companies such as Thermal Wave Imaging, SecondWave Systems Corp, NDC Infrared and Netzsch Geratebau. Engineering Detroit-based Thermal Wave Imag- ing, for instance, offers both hand portable and trolley-mounted imaging IR systems. Claiming that impulse thermo- graphy is both faster and requires less user skill than established radiography, ultrasound and eddy current techniques a spokesperson suggested that, For the first time this puts a powerful NDT tool in the hands of operators who are not NDT specialists. Both systems use a high-speed thermal imaging camera to scan the subject mate- rial, a heat pulse having first been applied with a xenon flash source to raise surface temperature by a few C. The more sophisticated EchoTherm is well estab- lished in the automotive industry (eg Ford, General Motors, Daimler Chrysler, Nissan), in aerospace (Boeing, Lockheed- Martin, Northrop Grumman, NASA) as well as for industrial inspection of devices ranging from printed circuit boards and solar cells to pipelines. ThermoScope is smaller and more portable. employ state-of-the-art Both systems analysis to present results to the operator quickly, enabling production processes to be adjusted if necessary. MOSAIQ software uses synthetic reconstruction to present the systems thermal images with unusual clarity, and multiple images can be evaluated simul- taneously. Able to detect flaws of mini- mum size about equal to their depth, the Thermal Wave systems have proved able to find delaminations, disbonds, porous areas, impact damage and moisture penetration of honeycomb. In a technique variation, timeresolved infrared (TRIR) relies on the application of a long heating pulse of modest power rather than the more usual brief optical flash. According to Jane Spicer, research scientist at the Applied Physics Laboratory of Johns Hopkins University, monitoring the way surface temperature develops over a longer time enables the depth of a defect and its thermal characteristics to be determined in a single measurement, without the need for a second calibration measurement made in part of the composite known to be sound. It also avoids the sample damage that can occur with short, intense pulses. Spicer and her colleagues have also tried heating samples with microwaves rather than conventional optical sources. This can be particularly revealing when used with optically opaque but microwave-transparent materials that may contain localized absorbing regions, such as composites holding entrapped water. Microwave TRIR can also detect conducting fibres of carbon or metal in dielectric materials. This raises prospects for a potential non-contact monitoring method in which small fibres could be used as embedded sensors which, when excited by a microwave source, could be remotely interrogated using infrared. The relative insensitivity of conventional impulse thermography to deep flaws can be overcome with lock-in phase thermography, a technique in which heat is applied to the specimen in the form of a sinusoidal wave. Part of the wave which penetrates into the material is reflected from internal defects, and these reflections interfere with the surface wave. Changes in phase and amplitude in the surface interference patterns enable defect characteristics to be determined. Small defects down to 1 mm in size can produce sufficient phase difference at the surface to be detected. Because sensitivity is also frequency dependant, it pays to tune the system before use. Best hope for an in-process thermographic technique to rival ultrasonics for image detail and depth is probably thermal tomography. Here the part is scanned in slices and high computing power is used to assemble the resulting 3D image. Given appropriate algorithms and selection of frequencies known to provide best contrast, the system can be highly effective. One expert who agrees firmly that a measure of future NDT systems for evaluating composite or other layered 44 REINFORCEDplastics December 2002

4 structures must be their ability to deliver defect maps that are instantly meaningful to quality inspectors, is Professor Vladimir Vavilov from Tomsk University. Vavilov, who has become something of an evangelist for thermal NDT and has for years sought to foster international cooperation in its development, outlines all the above techniques, with others, in short courses he gives for bodies such as the International Society for Optical Engineering. Future NDT systems for evaluating composite structures must be able to The QMI Sonda-007 air-coupled ultrasonic inspection system from Sonatest. deliver defect maps that are meaningful to quality inspectors. Computerized tomography (CT) was first developed for radiography where, used with advanced digital imaging software, it has freed X-ray systems from the need for film and greatly sped up imaging. To overcome the need to move the X-ray source to scan an object, systems for directing the beam have been developed. For example Digiray Corp of San Francisco, USA, effects a scan by moving the beam in a raster pattern. Digiray originally developed its technology under contract to NASA Langley, and NASA has used the system for evaluating airframe structures on a number of advanced aircraft, while Boeing and composite material manufacturers are among commercial customers. The system is said to be particularly good in evaluating bonded joints and has proven several times more sensitive than ultrasound in finding moisture in honeycomb sandwich parts. Other radiographic systems, in which information is deduced from the way waves are refracted, reflected or diffracted, also have scanning potential. X-ray refraction topography, for example, has proved able to image fibre/matrix debonding resulting from mechanical and thermal cycling, while diffraction topography can reveal local fibre orienta- tion of carbon fibre reinforced plastics (CFRP) and polymer chain orientation in injection moulding materials. Radio- graphy does, though, require extensive health and safety precautions so that its use is best reserved for applications requiring deep volumetric imaging. Fringe Outside these mainstream composite NDT techniques are others at present on the fringe. Shearography, for instance, is a method in which a laser is used to sense tiny physical displacements of test samples subjected to small stresses applied mechanically or by using vibra- tion, pressure, vacuum, heat or micro- wave energy. A vacuum shearography system from Laser Technology Inc of Pennsylvania is used to scan the fila- ment-wound fuselage of Raytheon s Premier 1 corporate jet, placed in a large vacuum tank. Any delamination, dis- bond, crushed core, defective core splice or other quality snag is revealed in a test said to be faster than a comparable sonic system could achieve. ultra- In a holographic variant of this tech- nique, two lasers are used to measure dis- placement with extreme sensitivity. As this method is extremely vibration prone, its field use is often impractical. Nevertheless, equipment is available. At least one company, US-based Karl Stetson Associates, offers a system that can be operated in both shearographic and holographic modes. Laser holographics feature in Electronic Speckle Pattern Interferometry (ESPI), a full field strain measurement method based on the measurement of submicrometer displacements of surfaces in the three directions. Especially useful for assessing thin or compliant specimens, it can also detect small sub-surface nonuniformities, especially when augmented by linear carrier fringes. InterScience Inc and others have been developing the technique for assessing composites for fatigue, heat and other damage. NPL is another organization seeking to develop this non-contact scanning method and its traceability. An NPL team has successfully applied Germany-based Ettemeyer s Q300 EPSI to characterise microcracks in glass-epoxy laminates and to establish the Young s modulus of the deteriorated laminate to a higher accuracy than that provided by extensometers or strain gauges. Other developments have included the study of mixed-mode delamination and friction at debond interfaces in composite laminates. Brunel University in the UK and others are working on sensing minute December2002 REINFORCEDplastics 45

5 sound signatures emitted from materials under stress, in the acoustic emission technique. Acoustography, a method in which sound waves are directed at the material, is capable of much higher area scan rates than conventional ultrasonics. In eddy current analysis, current flow is sensitive to structural changes in carbon fibres, which are conductive, and scans can show individual tows several layers down, revealing any impact damage. Returning finally to rocket science, novel Photon Induced Positron Annihilation (PIPA) technology has proved able to detect early signs of fatigue and composites embrittlement which could lead to cracking. A photon beam from a linear wave accelerator impacts the sample causing the formation of positrons, electrons and neutrons. Positrons and electrons, which are oppositely charged, annihilate each other releasing energy as gamma radiation. The precise nature of this radiation depends on where exactly it originated, so that analysing the gamma ray patterns can yield hidden defect signatures. PIPA can image to significant depths, almost 8 cm in some tests. It is more precise in identifying subsurface defects than all NDT technologies except X-ray. But, as with radiography, shielding is needed and the cost of equipment on the market, like that from Positron Systems Inc, can be daunting. Even so, along with acoustography and acoustic emission, PIPA may yet win a place in the NDT spectrum though ultrasonic, radiographic and increasingly thermographic testing, most likely used in various combinations, are likely to remain the composites NDT mainstream for some years to come. W Digiray Corp; tel: +I l site: 510; web Karl Stetson Associates; tel: +l ; website: k.stetson@pcholo.com; Laser Technology Inc; tel: +l ; website: NPL; tel.: ; website: www. npl.co.uk; NDC Infrared Engineering; tel: +I ; website: Netzsch Geratebau GmbH; tel: ; website: Positron Systems Inc; tel: l 923; website: www,positronsystems.com Qualiv Material Inspection Inc (QMI); tel: +I ; website: wwwqmi-inc. corn; SecondWave Systems Corp; tel: +I ; website: corn; Sonatest plc; tel: +44-l ; site: web- Thermal Wave Imaging Inc; tel: +l ; website: Strain (%) Top: an ESPl deformation measurement system. Boffom: full field axial displacement images of CI laminate section with a hole in it. (Diagram mwfesy of NPL.) Ultrasonic Sciences; tel: ; website: UltraOptec Inc; tel: +l ; website: Wichitech Industries Inc; tel: +l ; website: 46 REINFORCEDplastics December 2 002