FATIGUE CRACK DETECTION BY ACOUSTIC EMISSION MONITORING IN THE COURSE OF LABORATORY STRENGTH TEST J.Běhal Aircraft Strength division Aeronautical Research and Test Institute Beranových 10, Prague, Czech Republic Abstract All-metal aircraft structure was inspected by acoustic emission method in the course of laboratory fatigue test. Acoustic emission method takes a part in modern range of non-destructive testing applications. The acoustic emission monitoring and appropriate analyses of experimental data are objects of research tasks today. The aim of this research is to formulate methodology, according which will be possible to reliably evaluate the material cracking. This study is derived on the base of laboratory experiments, where the service conditions are simulated on the aircraft frame and its model specimen. Introduction Non-Destructive Testing (NDT) is an integral part of modern industry. The safe-life philosophy secures an aircraft safety in service by a scatter factor related to experimental strength test. NDT is required in the course of full-scale fatigue test for establishing the fatigue life of the structure. Much more attention has been focused on NDT through damage tolerance approach. It can be define as the ability of aircraft structure to sustain expected limit loads in the presence of fatigue, corrosion or accidental damage until such damage is detected through inspections and repaired. The damage tolerance approach requires that damage should be detected before reaching the critical size under maintenance inspection schedule, so this approach forces periodic NDT inspection or permanent health monitoring of structural significant items in service. Aeronautical Research and Test Institute is the centre for research, development and testing in the Czech Republic. The Institute successfully fulfils orders from the Czech as well as foreign industries comprising both civil and military sectors. The Experimental Strength Department performs experimental task connected with certification strength tests of primary aircraft structure, their components and models. In order to meet present and future requirements for fatigue related NDT, there are studied choice of appropriate inspection method and technique inspection reliability development of inspection systems Airframe inspections are applied under difficult conditions, e.g., composites, steel/aluminium boundary, large curved surfaces, complex and unknown structures. Visual method, especially remote techniques, is the most common one, but its reliability is limited by human factor and surface conditions. Sophisticated instruments give better results and are used in selected points. Acoustic emission is permanently monitored and signs of fatigue cracking are checked in frequency domain. One of the advantages compared to other NDT methods is the possibility to observe damage processes during the entire load history without any disturbance to the specimen. Common problem for every inspection method is to difference between fatigue cracking and background noise, e.g. scratches for visual, geometric shape for ultrasound, material imperfection for eddy current, friction for acoustic emission.
Figure 1. Monitoring of acoustic emission A traditional approach is to combine statistical and empirical description, thereby establishing a statistical correlation between measurements and generalising the features. An alternative scheme is to train a neural network to extract the required material properties using a reference set of specimens and measurements. However, both of these approaches depend on acquiring a specific set of test data under controlled conditions. The data is then useful for estimating the properties of new samples whose treatment conditions and material quality are consistent with those of the reference set. Parallel to loading in the course of fatigue test, there is composed a health monitoring system suitable for service maintenance scheduling. Methods of non-destructive testing are used through crack detection and documentation of test specimen continuous degradation. Experience and validated practice are transfer from laboratory environment to service condition. Final surface treatment and outfit installation should be reflected for service usage. The different reliability and capability movement in the field condition are established by experimental test of full-equipped critical point model. Aluminium became an essential metal in aircraft industry. Although the integral structures using composites are validated, the most planes are made of metal today. Significant fraction of airframe structure consists of stiffened panels. The main loads are transferred through beam elements of the spars. According to fatigue point of view, the critical points of wing structure are present in the lower flange of main spar, Figure 2. Because of customer request a fatigue crack detection as well as possible, the periodic inspections by tradition techniques (visual, eddy current, ultrasound) were supported by acoustic emission monitoring. Figure 2. Inner area of airframe structure
Instrument Set-up Multi-channel monitoring system, Figure, is linked with PC. Piezoceramic sensors use external amplifiers. Frequency range varied between 100 and 600 khz. Figure. System of acoustic emission monitoring Full-scale Fatigue Test In the course of fatigue test of full-scale airframe, the most of measured emission events were not related to material cracking. There was plenty of friction between structure parts together and temporary buckling of the skin. There are several possibilities to detect the cracking by data analyses: localization through time differences of signal arrival shape of emission sample (duration, amplitude, ) frequency spectra of emission event In the example, the spar flange was a beam shape and the sensors were situated in the line. Lower flange of the wing spar was the critical element. The critical point of Al-alloy flange was hidden by another steel part near the wing attachment. Because of customer specification, it was not possible to dismount the row of bolted joints for inspection. The detection of fatigue crack as soon as possible was emphasised, due to later possibility of consideration the structure as damage tolerance. Structure conception and material differences did not allow inspections by traditional method of non-destructive testing, which generally required the direct access to the critical point. Acoustic emission sensors were placed along the spar flange and emission sources are localized linearly. The emissions of whole structure take effect in the post processing of measured data, Figure 4. Events in A place may be the respond of fatigue. The B place responds the rib joining in this area and an entrance of emission of remote sources.
event counts A B Figure 4. Acoustic emission localized in the lower flange of wing spar Model Test Specimen Draft of the model experiment is illustrated in the Figure 5. Al-alloy plate of 2 mm thickness is uniaxial loaded by harmonic cycling up to 100 MPa. Monitored emission is documented in the Figure 6. Crack growth in early stage of propagating as well as acoustic wave dispersion is observed. Common and characteristic features should be recognized for reliable identification of emission source. crack length [mm] 0 25 20 15 10 5 0 activity origin crack length about 1 mm 0 20 000 40 000 60 000 80 000 100 000 life [cycles] Figure 5. Technological sample and crack growth curve
Figure 6. Cumulative counts of acoustic emission events For the wave speed about c=4000 m/s and the frequency varied between 100 and 400 khz, the wave length is For object thickness about 2 mm, there are and the wave dispersion should be supposed. c 4 10 min 0,01[ m], f 400 10 max c 4 10 max 0,04[ m], f 100 10 min t 2mm min 10mm It seems to prove the inspection reliability, when material cracking is identified through signal features recognition, Figure 7. Neural network is useful instrument for the reliability quantification. 2400 1200 0 0 500 1000 1500 2000 2500-1200 -2400 15 10 5 0 200 400 max: 75 khz Figure 7. Analyses of acoustic emission events in frequency domain Discussion The task of acoustic emission monitoring is related to airframe inspection reliability. Specific airframe characteristics, strict demands for meeting the service safety, requirements resulted from service condition to support systems and emphases to service efficiency evoke to take care of areas, which hold some level of result uncertainty. Acoustic emission method take a
part in modern range of non-destructive testing applications and it appears to be perspective for next research in an aircraft industry. The acoustic emission monitoring and consequent analyses of experimental data are objects of several R&D projects today. The aim of next studies is concerning to find and formulate the decision criteria, according which will be possible to reliably evaluate if the material cracking is present or not. The methodology should be derived on the base of laboratory experiments where the material fatigue may be objectively described. Traditional non-destructive testing and monitoring methods all have specific drawbacks when applied to complex structures. However, future non-linear methods have great detection potential and are more sensitive to common and hidden defects. The outcome of the feasibility tests looks promising. The non-linear method may be implemented for detecting flaws in local relatively simple elements, because structure behaviour must be understudied for right signal interpretation. The Aeronautical Research and Test Institute is involved in national research programs, e.g. STRATECH, PROGRES and TANDEM, and European Health Monitoring of Aircraft by Nonlinear Elastic Wave Spectroscopy, where completely new system for non-linear NDT is developed. Aerospace industry is one of the most advanced and important fields of NDT applications. Tradition non-destructive inspection for structural defects is a vital component of maintenance. Non-destructive evaluation is opening the door to precise presentation of fatigue test results. Except quality inspection of a test specimen, non-destructive testing is widely used in the case of fatigue evaluation of aircraft structure. During the term of months or years, when the structure is loaded in laboratory, early detection of crack allows repairing just a little area and the fatigue test continues, as all the critical points of structure must be recognized. With aging aircraft the increase in damage detection is an important and difficult task. Also new is the issue of crack growth management. The effort to increase fatigue life of aircraft structures leads to an advanced design philosophy, which permits a fatigue crack being initiated during service. A schedule of structure inspections is established for aircraft in service and critical points are checked by NDT as maintenance task. Conclusion Experimental tasks are important for improving designer knowledge and choosing smaller scatter factor of proposed structure, decreasing structure weight and increasing service reliability. An optimization of design is the key tool especially in aircraft industry. Acoustic emission method is very useful tool for NDT inspection. The full-scale aircraft frames are loaded during simulation of service condition. Appropriate service maintenance for critical point is established and physical phenomena of inspection techniques are studied too. Acknowledgments This study was supported by the Ministry of Industry and Trade of the Czech Republic, project no. FT-TA/026. References 1. Převorovský, Z.: New NDI methods of aerial structures. rd International Conference Defektoskopie, Ostrava 200 2. Pazdera, L., Smutný, J., Prouzová, P.: Possibilities of utilisation of wavelet transformation at description of acoustic emission signals. rd International Conference Defektoskopie, Ostrava 200. Běhal, J.: Application of Non-destructive Testing to Investigation of Aircraft Structure Integrity, rd NDT in Progress, Praha 2005 4. Běhal, J.: Laboratory Strength Test and Acoustic Emission Monitoring. 4th YSESM, Castrocaro Terme 2005 5. Běhal, J.: Acoustic Emission Monitoring during Strength Test of Aircraft Structure, Czech Aerospace Proceedings 1/2005 6. Blaháček, M., Chlada, M., Převorovský, Z.: Acoustic emission source location based on signal features. 27th European Conference on Acoustic Emission, Cardiff 2006 7. Běhal, J.: Reliability of Fatigue Crack Detection by using Non-destructive Testing Methods. Proceedings of 2 nd Maintenance Management, Sorrento 2006