JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
Composite materials are currently used in many fields, some of which impose very strict reliability requirements in order to guarantee the service performance of mechanical components and the health and safety of individuals.
This means that testing methods that have been applied until now to metallic parts are also being applied these days to composite parts. Various sectors are using composites for integrated applications: the aerospace, boatbuilding and offshore oil industries, for example, are using non-destructive testing methods to guarantee the fitness for use of composite parts. These methods are applied both during and at the end of manufacturing, as well as in the course of maintenance and repair. The methods used are often derived from techniques used in the medical sphere, such as X-rays and ultrasound.
Other methods, including optical methods, have been adapted specifically for composites. This event provided the opportunity to bring together at the Palais des Congrès in Paris over 100 people representing a broad cross-section of the various research, industrial and enduser sectors, in order to exchange ideas on the topics of advanced applications and R&D in composites testing.
Summary of the presentations
Following opening addresses from Guy Lefebvre (Chairman of the French confederation for non-destructive testing) and Henri Walaszek of Cetim, Thomas Jollivet (Cetim) gave a presentation on industrial testing requirements.
He spoke about the specific characteristics of composite materials manufacturing and their consequences on the types of defects and damage. Hence, these materials may present manufacturing faults such as variations in thickness, porosity and fibre placement. For example, poor fibre placement generated a fatigue failure in a wind turbine blade.
This damage process is rendered complex by anisotropy and heterogeneity, characteristic of most composites. This means that fibres may tend to lose cohesion with the matrix, and cause micro-cracks to coalesce.
Non-destructive testing is all the more important in that in the event of an impact the damage caused is not always visible to the naked eye. A striking example is the catastrophic consequence of the presence of a small interlaminar fracture fault measuring just a few square centimetres, which caused the collapse of a 10-m-high composite structure.
Henri Walaszek (Cetim) presented "specific" and "global" NDT methods in relation to the different types of defects to be detected. The topics covered during the day were divided up as follows:
Acoustic methods are currently making great strides and now incorporate imaging techniques, making them easier to implement. Digital and microcomputing techniques now offer enhanced equipment portability for onsite applications. These techniques are often derived from those used in diagnostic ultrasound in the field of medicine, such as phased-array ultrasound.
Samuel Maillard (Cetim) presented phasedarray acoustic imaging applications for composite products in the boatbuilding industry. The test was carried out by passing a 5 MHz phased-array probe along the surface of the product. Two examples were discussed: abnormal resin distribution during the manufacturing process (Fig. 6) along with a mat delamination leading to a fault could be observed quickly and easily (Fig. 7). The technique also makes it possible to monitor damage by delamination of a fatigue specimen made of carbon-carbon composite This application requires physical contact being maintained between the part and the ultrasound probe via a coupling medium such as water when moving the probe over the surface of the product. Some flagship applications, which can be used in the workshop, are now free of any such constraint. Benjamin Campagne (EADS IW)
presented an application whereby the ultrasound is generated and detected by laser. A pulsed laser creates a mechanical shock on the surface of the product while an interferometric sensor decodes the Doppler effect produced by the ultrasound reflected from the surface of the product on the receptor laser beam (Fig. 9).
Such a system, called Luis 72 L, has been developed by Aérospatiale-Dassault and is now installed at the Technocampus 2 platform dedicated to composites research.
It should be noted that other test installations are now in use industrially, such as those developed by Lockheed Martin for inspecting the F22 American fighter jets.
Mr. Campagne pointed out that the interest of this technology lies in the possibility of testing complex shapes without having to adjust the probe to the contours of the product. An example test application in C-scan planar projection was given for a composite radome currently being developed (Fig. 10).
The acoustic testing methods for composites may also be applied to characterisation, i.e.: determining the mechanical characteristics of the materials.
The LMP Innovation Laboratory in Bordeaux is indeed carrying out studies on the use of ultrasound for detecting faults and for characterisation (determining the elastic constants of composites). Leading applications were presented by Professor Castaing, such as the use of air-coupling sensors to detect defects caused by impact, and artificial delamination testing via ultrasonic guided waves (Fig. 11).
The LMP lab is currently carrying out thesis work in collaboration with the Cetim foundation on the simulation of guided waves in composites. The lab has also demonstrated the value of guided waves for testing the adhesion of an aircraft repair patch (Fig. 12). Finally, LMP has developed a characterisation activity using guided waves in composites for the following areas:
Thermographic methods have greatly evolved over recent years. Pulsed locked-in thermography, for example, now offers considerably improved image resolution thanks to digital image processing. This processing is essentially based on the exploitation of surface temperature changes over time. Samuel Maillard (Cetim) gave a presentation on pulsed locked-in thermography applications for composite parts: water infiltration and crush damage were detected in a honeycomb composite, along with resin-poor areas in a boat hull.
For its part, the LMP Laboratory at the University of Bordeaux is studying the possibilities of using high-power ultrasound for the excitation of the test structures as a replacement for the thermal stressing that is usually employed.
Eberhard Moser (Dantec) presented the advantages of shearography for composites testing. Shearography enables the rapid testing of large surfaces by measuring surface deformations of the material when subjected to stress. This stress may be mechanical or thermal. An example result is the testing of a surface measuring 800 x 800 mm, lasting only minutes. The materials tested were CFRP, GLARE, honeycomb, laminated components and foams. Shearography can detect delamination, impact damage in aerospace maintenance, interlaminar fractures and internal corrosion.
The technique is highly portable and operational on-site. Remarkable examples were presented such as the inspection of an Awacs dome (Fig. 13), the inspection of helicopter rotor blades and the inspection of wind turbine blades (Fig. 14). This technology is highly suited to robotisation, such as deployed for the inspection of composite components on Ariane V (Fig. 15).
Acoustic emission and data processing Nathalie Godin of the MATEIS laboratory at INSA presented the lab’s work on the use of acoustic emission to characterise damage and to forecast the life expectancy of structures. She presented a methodology for classifying and processing acoustic emission files. This methodology is used to detect signals linked to damage mechanisms, measure the criticality of the different mechanisms and define the critical mechanisms.
The use of classifiers for predicting the residual lifespan remains subject to lab research.
The advantage of acoustic emission is the possibility of monitoring the entire tested structure thanks to the propagation of sound waves in the component. There are applications now appearing which consist in integrating all kinds of sensors in the structures concerned in order to test and monitor the structural condition throughout the structure’s lifetime. This approach is commonly designated "structural health monitoring", or "SHM".
Mr. Skawinski ( Ecole des Mines de Douai) reviewed the different SHM methods. The most frequently used sensors are optical fibres, with piezoelectric sensors sometimes being employed. They are incorporated into the product or on its surface. They may be used for testing during the manufacturing of the composite, measuring internal temperatures, monitoring the resin flow front and measuring residual stress. They may also be used for rigidity testing prior to commissioning.
Optical fibre sensors enable strain and temperature measurements and the detection of damage via acoustic emission. Their use is limited by the fragility of the fibres and the connection requirements.
An original application developed in conjunction with Cetim consists in using carbon fibres that are integrated in the structure. An example was given for monitoring damage. An application using 0.25 mm piezoelectric strips as ultrasound emitters and receivers for pinpointing impact damage on a fuel tank was mentioned.
Pierre Ferdinand (CEA) then presented optical fibre SHM applications for the railway sector. Two types of sensors can be used:
Some spectacular applications were mentioned, such as:
The latter instrumentation may ultimately be used for inspecting the status of the pantographs of locomotives using the line. In the current context of the opening of the railway market to the competition, this may be of serious economic interest.
J.B. Perrin (Spectroscan) presented an innovative application derived from his own work. Here, the X-ray inspection consists in what his company has dubbed "radiosynthetic" microtomography and is based on the principle of “adaptive” X-ray acquisition depending on the shape and design of the object. The authors point out that this offers major time savings when compared to "traditional" X-ray tomography, thanks to this "adaptation".
The focal depth is 0.1γm. 3D images are produced, allowing the detection and volume quantification of component porosity (Fig. 17).
Automation in NDT
The generalisation of robotisation in industry has now spread to NDT. Robots are of particular interest when it comes to the automatic inspection of parts with complex and variable shapes using ultrasound instrumentation that is awkward to handle.
Such is the case for the water nozzles/jets used for acoustic coupling in the acoustic transparency testing of large panels. The use of robots in NDT enables automatic inspection of large, curved panels (Fig. 18) and provides an alternative to the traditional automatic water jet C-scans that use digital control mechanisms that are far less flexible.
R&D work is being carried out in both public and private laboratories in association with both large companies and SMEs that are often grouped together in competitive clusters. Clusters involved in composites R&D include Astech, Aerospace Valley, EMC2, Elastropole, Plastopolis and Pegase.
Highlighted in particular was the Technocampus platform in Nantes, which carries out research into composites and their non-destructive testing, and which serves as a hub for aerospace, naval and local stakeholder initiatives.
Importance of standardisation
Nathalie Geslin (AFNOR) presented the aspects of non-destructive testing as pertaining to standardisation. The industrial need has been identified for the integration of "composite product" specifics within European standardisation. US standardisation, by comparison, is far more advanced on this subject, and the call was put out for volunteers to help out on the AFNOR committees. Robert Levy, on behalf of COFREND, reminded those present of the importance of participating in the working parties of this association, which carry out pre-normative work in many areas of nondestructive testing. He also evoked the existence of a support system for French small and medium-sized enterprises involved in the standardisation procedure. This system is implemented by COFREND.
This overview brought to light the dynamism in the field of non-destructive testing for composites, and in particular the innovative technological practices this involves, offering testing that is faster, more accurate and more traceable. Improving the non-destructive testing of processes still requires upstream R&D, and the commitment of the laboratories involved in this can give nothing but cause for optimism.