FatiguecrackmonitoringofrivetedaluminiumstrapjointsbyLambwave analysis and acoustic emission measurement techniques Se Âbastien Grondel a, * , Christophe Delebarre a , Jamal Assaad a , Jean-Pierre Dupuis b , Livier Reithler b a IEMN, UMR CNRS 8520, De Âpartement OAE, Universite  de Valenciennes et du Hainaut Cambresis, Le Mont Houy, 59313 Valenciennes Cedex 9, France b EADS, Centre commun de recherche, De Âpartement Mate Âriaux, 12 rue Pasteur-BP76-92152 Suresnes Cedex, France Received 18 January 2001; revised 27 March 2001; accepted 12 April 2001 Abstract Statistics show that fatigue crack development comes ®rst and foremost as a damage source in aerospace metallic structures. Currently, widespreadmethodsareavailabletoinspectthesestructures,buttheyarequitetime-consuming,costlyandrequirethestructuralsystemtobe idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectrictransducerscoupledontotheplates.Thedetecteddamageiscracksinjointresultingfromfatigueloading.Thecollectedsignals onthepiezoelectrictransducersareanalysedusingHilberttransformandtime±frequencyanalysis.Itisshownthatthe®nalinterpretationof Lambwaveanalysismayprovideameansofsizingthedefectsandfollowingthecrackdevelopment.Inadditiontothat,anacousticemission systemisusedjointlywiththeLambwaveanalysisinordertodiscussresultsanddamagedevelopment.Finally,itisdemonstratedthatboth methods can work together and the results obtained are in good agreement. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ultrasonic testing; Lamb waves; Acoustic emission; Fatigue 1. Introduction Aircraft metallic structures are usually inspected using traditional sensing techniques, such as visual inspection, radiography [1], eddy current testing [2], optical and ultrasonic methods [3,4]. These techniques are applied at discrete intervals and require the aircraft to be taken out of service. Since these processes can be very time consum- ing and expensive, it would be desirable to develop alternative procedures to perform continuous, in-service health and usage monitoring of aircraft structures. In face ofthisissue,thedevelopmentofastructuralin-builtdamage monitoring system assumes primary importance. Among the various techniques available, Lamb waves, which are waves propagating along plate-like structures, can offer a convenient method for a quick and continuous inspection of riveted structures. Indeed, the property of these guided waves to propagate at the same time over long distances and in the entire thickness of the structure allows their use for the monitoring of large areas with minimum number of transducers. Furthermore, if these transducers are permanently attached or even integrated into the structure, then with the appropriate telemetry and information processing, continuous monitoring of the structure becomes feasible. This type of monitoring seems particularly well suited to detect and locate defects, such as corrosion pitting or fatigue cracks. A vast amount of literature on ultrasonic works using Lamb waves for metallic plates is available, of which a small sampling is discussed in this paper. Worlton [5] was probably the ®rst who recognised the advantages of using Lamb waves for nondestructive plate tests. Then, Mans®eld [6], and Ball and Shewring [7] investigated the use of guided waves on a wide range of different strips and plates. Representative of the numerous studies of Lamb waves inspection is the work of Rokhlin [8], who studied lap shear joints both theoretically and experimentally and reported several studies [9,10] on the Lamb waves diffrac- tiononcracksordelaminations.Inthesameway,Roseetal. [11,12] have developed an overall, guided wave inspection technique for bond inspection, and this technique has proved to be successful to inspect tear straps and lap splice NDT&E International 35 (2002) 137±146 0963-8695/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S0963-8695(01)00027-5 www.elsevier.com/locate/ndteint * Correspondingauthor.Tel.: 133-3-27-51-12-39;fax: 133-3-27-51-11- 89. E-mail address: sebastien.grondel@univ-valenciennes.fr (S. Grondel).