Online NDI concept for composite manufacturing process and approximations for the NDT of CFRP Materials Prior to the Autoclave Curing Process

Transcription

1 6th International Symposium on NDT in Aerospace, 12-14th November 2014, Madrid, Spain - More Info at Open Access Database Online NDI concept for composite manufacturing process and approximations for the NDT of CFRP Materials Prior to the Autoclave Curing Process Abstract Esmeralda CUEVAS 1, Covadonga GARCIA 1, Carlos DE MIGUEL-GIRALDO 2,Maria MORA 3 1 Aeronautical NDT Techniques, TECNATOM S.A., San Sebastián de los Reyes, Madrid, SPAIN, ecuevas@tecnatom.es 2 AIRBUS OPERATIONS SL, 3 FIDAMC This paper describes the methodology applied for the study and implementation of the Online NDT philosophy in composite materials manufacturing processes in the aeronautical industry. The two main objectives pursued with this working methodology are the early detection of any indications relating to the formation or presence of defects in the material, thus allowing for a quick response and avoidance of their systematic repetition in the rest of the elements in the series. Also allowed is the reduction or even the partial or complete elimination of the NDT tasks in production that are currently obligatory for obvious reason of certification. Considered here as part of the aeronautical components manufacturing process is the incorporation of nondestructive (NDT) inspection in a more integrated manner, forming part of the process itself in those stages in which a higher added value is ensured, thereby increasing its effectiveness and reducing waiting times or the need for repeated inspections. Three important lines of work were identified in a stage prior to this study. Firstly, the identification of production process parameters having a key influence on the formation or presence of defects. This first area focused on direct bagging processes and underlined the importance of monitoring the material prior to curing, the curing process in the autoclave and the final process of planing of the edges. Second was the exploration of the available technologies, providing the levels of reliability demanded by this industry and compatible with the entire production process, capable of suitably monitoring the parameters identified in the previous study. Third was an evaluation of the capacities and limitations of the different non-destructive tests considered in applications involving non-cured material: phased array ultrasonics, laser-generated ultrasonics, Air coupled ultrasonics and thermography. Laser-generated ultrasonics produced a partial curing process in the noncured materials inspected, while with phased array ultrasonics it was not possible to avoid contamination of the material; consequently, it was determined that the specific technologies for the inspection of non-cured composites were Air coupled ultrasonics and thermography. The developments described in this paper include those performed within the framework of the collaboration between TECNATOM and TARGET partners, and cover the automation and industrialisation of Air coupled ultrasonics and thermography inspections during the stage prior to curing, and the integration of technologies for monitoring prior to and during curing. The process of automation of the thermography and Air coupled ultrasonics techniques for the inspection of composites includes inspection module designs, the analysis of communication channels and the performance of tests on defined specimens allowing for the evaluation and validation of the technologies used. This paper describes the developments undertaken to introduce all these technologies in a more effective manufacturing process. Keywords: carbon fibre reinforced polymer (CFRP), laser-generated ultrasonics (LUS), Phased Array ultrasonics (UTPA), Air coupled ultrasonics (UTA), Thermography (TIR).

2 1. Introduction The work described in this paper was funded by National Project named TARGET. TARGET is an ambitious project where a consortium made up of 14 industrial partners led by Airbus Operations was set up, the experience and capacities of these partners being complementary and allowing the approach and scope of the technical objectives of the project to be established. The four-year project ( ) receives a subsidy from the sub-programme for support for national strategic technical research consortia (Consorcios Estratégicos Nacionales de Investigación Técnica - CENIT), awarded by the Centre for Industrial Technology Development (CDTI) in The general objective of the TARGET project is the research and development of new smart and environmentally sustainable technologies for the generation of structures in composite materials. In particular, the research centres on materials, both new and already existing, and processes allowing the use of large autoclaves to be done away with, establishing the knowledge base required for the development of new machinery and concepts for the automation of the aforementioned processes. Composite NDI Community has identified Online Non-destructive inspection as a concept that conveniently developed and implemented can provide additional benefits to composite manufacturing process. In this paper is presented what is aimed with this concept and also some essential aspects to develop the proper implementation of NDI technologies into composite production plants. The reliability and maturity of the monitoring technologies, their compatibility with production environment as a whole and clear evidences of the benefits and added values to the process are some of the essential requirements to be demonstrated in order to start with the integration of the technologies in production environment. TECNATOM s participation centres on the area of non-destructive testing inspections, a critical activity in the processes of producing these materials. The company also carries out investigations of the introduction of different non-destructive inspection techniques in the different phases of manufacturing, and participates in the study for optimisation of the manufacturing processes and the study of applications of these materials and processes. One of the activities of the TARGET project is a study of the inspection of composite materials prior to the curing stage. This includes the design, preparation and fine-tuning of the inspection techniques. Also part of this activity are the tests performed using the specimens manufactured, optimisation of the inspection systems as regards hardware and software resources, signal processing and optimisation algorithms, etc., the generation of data files and the correlation of data with the defectology considered, with the collaboration of the Scientific Research Council (CSIC) and the Polytechnic University of Valencia (UPV). The final objective for TECNATOM is to draw conclusions regarding the applicability of the different non-destructive techniques considered for the inspection of non-cured materials and their future integration with machines. 2. Online NDI concept and development phases: example on direct bagging process Online Non-destructive inspection is based on the inspection during manufacturing time and no requiring the completion of the part. It is identified as a concept that conveniently developed and implemented can provide important benefits to composite manufacturing process. The main objectives of this approach are: a) The detection of possible indications during the manufacturing process anticipating the presence of defects and therefore enabling a sooner actuation to mitigate problems and avoiding systematic repetitions

3 b) The reduction of Non Destructive Inspections tasks that today are mandatorily performed in production of Composite Parts. c) The reduction of cycle time by the integration of inspection & manufacturing The integration of Online inspection technologies into composite manufacturing process necessarily requires of a previous and detailed development plan that must include the following aspects: 1. Deployment and interconnection among each of the composite production phases. 2. Identification of the key parameters of the manufacturing processes and their relation with possible defectology in the parts. This task necessarily based on the experience of the process and justified by historial records is essential for the success of this new inspection concept. 3. Capture requirements for the monitoring technologies in production environment. 4. First selection of possibles technologies to monitor the key parameters of the process. Evaluation of the cost/time of the technologies taking into account their current technology readiness level for these applications. 5. Preparation of technology development plan to ensure and demonstrate their maturity (performance and reliability) and complete compatibility when working inside the production environment. 6. Phase of integration, acquisition and tests of the developed measurement technologies into the manufacturing process. 7. Study and cross-correlation between results obtained with online measurement technologies and conventional NDI. This phase requires a parallel working time and aims to demonstrate the identical quality assurance level for the composite structures when following any of these approaches. 8. Verification and validation of the performance and benefits of online monitoringt technologies in real manufacturing conditions. As an example, figure 1 show graphically the result of the list of key parameters identified in the analysis performed to the specified composite process of direct bagging and also the corresponding candidate monitoring technologies being investigated.

4 Figure 1. Online NDI key parameters in direct bagging and possibles technologies 3. Non-destructive tests considered Assessing energy efficiency and reducing production costs in the industry is currently part of the aeronautical strategy. The intention is to simplify and reduce in the future the inspection task subsequent to curing, investigating in situ advanced methods and techniques for application prior to curing, without an autoclave. In addition to increasing the aforementioned energy efficiency, investigating ways of introducing non-destructive inspection prior to curing of the materials ensures the quality of the part model and helps to rule out whatever defects might be generated during the manufacturing phase. The non-destructive assessment methods considered to be most adequate and reliable in the case of non-cured materials are LUS (laser-generated ultrasonics), UTA (Air coupled ultrasonics), conventional ultrasonics, UTPA (phased array ultrasonics), wheel and TIR (thermography), which will be described below. 3.1 Parts considered for the study The specimens used for the validation and fine-tuning of the inspection methods considered have different compaction and defectology features. The parts inspected (C1, NC1, NC2, NC3 and I1), manufactured by FIDAMC, are each formed by 30 fabric layers, the difference between them being the different intermediate compacting processes they undergo throughout manufacturing. The following figure shows the manufacturing processes of each, the yellow indicating the moment at which intermediate compacting occurs, and the red the points where inclusions are introduced. Also indicated in this figure are the tests performed in each of the manufacturing stages.

5 Figure 2. Tests proposed on each specimen The lay up describes the orientation of each of the fabrics incorporated to manufacture the specimen. The only specimen with defects is number I1, which has defects in fabric layers 2 (defects 14 and 38), 15 (defects 16 and 40) and 28 (defects 18 and 42). On completion of manufacturing of the parts, and their inspection in different stages of the noncured materials process, the parts are cured in accordance with a known autoclave cycle. 3.2 Equipments NDT 1. Laser-generated ultrasonics (LUS) The laser-generated ultrasonics technique uses an Nd generator laser: YAG, that emits infrared radiation with a wavelength of 1064nm (model Ultra50 (GRM) by Quantel), and an interferometry detector, in this case also based on a laser (the bandwidth of the detector is 1-10MHz). Specifically, our experimental LUS equipment is made up of a laser with fibre coupling for the generation of ultrasounds in the material, a detector with fibre coupling that sends and receives the sample laser signal, and computer controlled translation axes for scanning. 2. Air coupled ultrasonics Various types of pulser / receivers are used in our study, plus an oscilloscope (Tektronix) and several pairs of transducers with frequencies of 0.25MHz and 0.65Mhz, as well as an aluminium mould for test performance. This method allows inspections to be addressed without a coupling medium, the ultrasounds being propagated through the air, as the name of the method suggests. 3. Conventional ultrasonics The tests are performed using electronic equipment (Multi2000 Pocket 16x64: Phased Array electronics with 16 full parallel channels and 64 multiplexed channels), shoes of different materials and thicknesses (16-30mm), several types of coupling media and transducers of different characteristics: PA, conventional, wheels... and frequencies of between 0.185MHz and up to 5MHz. 4. Thermography The operating equipment consists of the Thermacam S640 infrared camera (FLIR), with a 640x480 long wave detector, 24º lens and thermal sensitivity of 30mK at 30ºC, and a programmable 1500W programmable halogen spotlight that controls the duration of the pulses.

6 The camera is connected to a computer by firewire in order to allow for the direct acquisition of images at 50 Hz, and the operating software is Thermacam Researcher. 3.3 NDT Test and Results 1. Laser-generated ultrasonics When a laser pulse impinges on the surface of a material, a part of the electromagnetic energy of the laser source is absorbed by it and converted into heat. This generation mechanism leads to rapid thermal expansion, which in turn generates ultrasonic waves in the body of the material (thermoelastic regime). Controlling the energy density on the surface of the sample (spot size) without damaging it is a premise in LUS systems. The tests performed consisted of generating a line-shaped spot (4 mm) thanks to an optical system of flat-convex lenses. The light sent by the generating laser achieves temperatures that are sufficiently high as to undergo combustion processes after a few milliseconds of exposure to the detector. By placing an optical system in the emission path of the laser, the detector converts this signal into a discontinuous emission synchronised with the emitting laser, in order to take full advantage of the signal generated by the latter, exposing the material to the energy of the detector for as little time as possible. The results show that the heat of the ultrasound generation and detection laser is counterproductive in the case of non-cured CFRP composites, since it leads to their partial curing. 2. Air coupled ultrasonics Two different techniques were tested: normal incidence transmission (longitudinal waves) and pitch and catch from a single face (Lamb wave), as shown in the following figures. Two transducers are used in transmission, placed on opposite sides of the sample (see Figure 2) and two transducers on the same face of the sample and with oblique incidence for pitch and catch (see Figure 4 UTA Pitch and Catch), where the receiving transducer picks up the emission from the film of composite material when the Lamb wave travels through it. Transductor Emisor Transductor receptor Reflexión especular Molde de aluminio (12 mm) Aire Transductor acoplado al aire Onda de Lamb Figure 3 UTA transmission Figure 4 UTA Pitch and Catch The results show that wide band (>25%) and low frequency (<0.5 MHz) transducers are required to avoid the problem of the very high attenuation of non-cured CFRP. Likewise, a very high sensitivity (< -30 db) is required in order to have a sufficiently high signal level transmitted. The applicability of the technique is demonstrated in non-cured materials with different degrees of compacting between fabric layers, since the results obtained using the transmission technique show that it is possible to identify, isolate and register the signal transmitted through the material and differentiate it from the multiple reflections that might appear (Figure 5 UTA transmission, signal transmitted in non-cured CFRP with different degrees of compacting). The presence of defects in the material causes a loss of the energy transmitted, which is seen as a

7 drop in the amplitude of the transmitted signal. The UTA results for pitch and catch reveal a low signal amplitude and a very poor signal to noise ratio for non-cured CFRP (Figure 6 UTA pitch and catch, signal transmitted in non-cured CFRP with different degrees of compacting ) I1 8 telas compactada 0 8 telas compactadas NC3 10 telas compactada -1 Amplitud (V) -2 8 telas compactadas + 1 sin compactar Amplitud (V) -0.2 C1 30 telas compactada -3 8 telas compactadas + 2 sin compactar telas compactadas + 4 y compactar -0.4 Onda de Lamb Reflexión especular Tiempo (s) x Tiempo (s) x 10-4 Figure 5 UTA transmission, signal transmitted in noncured CFRP with different degrees of compacting Figure 6 UTA pitch and catch, signal transmitted in non-cured CFRP with different degrees of compacting 3. Conventional ultrasonics The main limitation affecting the inspection of non-cured material is contamination. Different inspection strategies are addressed without using liquid coupling media, but this prevents the transmission of the ultrasounds between the part and the transducer, which is a very important conditioning factor. The results obtained (Figure 8 ACQ with wheel allow this technique to be ruled out, since the ultrasounds are incapable of passing through the vacuum bag that is essential to prevent contamination during the manufacturing process. Delimitación aproximada de la zona Figure 7 Specimen C1 with 16 layers. Figure 8 ACQ with wheel Figure 9 ACQ with PA 4. Thermography In the tests, the camera and the halogen projector are placed on a plane parallel to the specimen to be inspected, with suitable focusing, with a view to achieving a maximum percentage of inspection and a uniform beam of light. Various sequences are analysed with different pulse durations and at different distances, in order to be able to compare the different degrees of compacting in subsequent analysis. The experiences focused on a specific area of the part with different degrees of compacting and oriented towards the defects in specimen I1 (see Figure 1). As the TIR results show, depending on the increase in temperature on the ordinates axis and the number of frames on the abscissa, higher degrees of compacting imply greater heat transfer for each of the specimens in fabric layer 30 (see Figure 9).

8 Tª 5,5 3,5 1,5 Tela 30 C1 NC1 NC2 NC3-0, Frames Figure 10 TIR graph/compacting of fabric layers The excitation times at which defects are detected in the inspections of specimen I1 are 0.8 to 3 seconds, and the results show that inclusions are detected up to the placing of the fourth or fifth fabric layer, counting from the one in which the defects are positioned (see Figure 11 TIR ACQ in specimen I1 (defects14 and 38)) ACQ in layer 2 ACQ in layer 4 ACQ in layer 8 Figure 11 TIR ACQ in specimen I1 (defects14 and 38) 4. Conclusions Online NDI is identified as a potential methodology to improve the control quality of composite manufacturing process. A working procedure has been established for the identification, development and integration of online technologies into the manufacturing process in order to get the expected benefits. From the results obtained to date, it may be concluded that, the two NDT candidates for the inspection, prior to the autoclave curing process, are infrared thermography and Air coupled ultrasonics, in their two configurations: pitch and catch and transmission. The objective of these techniques is the detectability of defects during the manufacturing process, when the composite material has not yet been cured. As regards laser-generated ultrasonics, an in-depth study of different alternatives for non-cured components should be considered, since it is necessary to keep the temperatures very low in order to avoid curing of the material inspected. In principle, the local increase in temperature due to the effects of generation and detection rules out the possibility of using this technique for the inspection of non-cured composite materials. For their part, conventional ultrasonics using multiple versions of the pulse-echo technique (PA, wheel, conventional ) do not allow an optimum ultrasonic response to be obtained, for which reason this method is ruled out for the inspection of non-cured material. Other technologies as identified in table figure 1 are currently in development process.