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Guided Wave Based Delamination Detection for Composite Structure Health Monitoring

S Saravanan*, N.Q.Guo, and B.S.WongSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue 50,

Singapore 639798sara0013@ntu.edu.sg

ABSTRACTStructural health monitoring (SHM) refers to the process of implementing damage detection and monitoring strategy for aerospace, civil and mechanical engineering infrastructures. Guided waves (GW), which have been used in NDT of plate-like structures, can be used for active monitoring and interrogating the health of the structures. Due to long range

propagation with less attenuation, GW requires less number of sensors or low sensor density for monitoring. Preliminarywork in exploring the potential of GW for active SHM in composites is discussed here. In particular, the influence of thefiber orientation on the GW velocity is studied , and the interaction of the GW with delamination is examined, whichdemonstrates the appreciable sensitivity.

1. IntroductionComposite materials are widely used in aerospace

and infrastructure applications due to its highspecific stiffness and strength. Impact, lightning,fatigue, material degradation and unanticipateddiscrete loading may cause delaminations incomposites. Delamination is a great threat for composite structures which may lead to catastrophicfailure if not detected early and repaired in time.With increase in industrial usage of composites, theconventional NDE methods are insufficient to cater for the needs of long range, quick global inspection,in service inspection or on-flight monitoring of structures. Guided waves (GW) have the potential toovercome this limitation and insufficiency. Guidedwaves are the elastic waves that propagate within the

boundaries of the structures, for example, the Lambwaves in traction free plates [1]. Lamb wave baseddamage detection approach has the ability to inspectlarge size plate- like structures quickly, and toinspect the entire cross sectional area of the structure[2]. Lamb wave can travel over a long distance evenin a material with high attenuation ratio thus a broadarea can be examined quickly, thus time can besaved. It has excellent sensitivity to multiple defectswith high precision of identification [3].

2. Guided wave for delamination detection Technical Challenge

Lamb wave can propagate in symmetric or anti-symmetric modes. The Lamb wave seems to be amore viable method for quick scanning of large areaand due to its exclusive responses to differentdamage status. But more complex scattering andattenuating phenomena due to the heterogeneous andanisotropic nature of composite complicates theinterpretation of Lamb wave signals, anddistinguishing such signals remains problematic.These waves are dispersive, i.e. the velocity of

propagation varies with frequency further worsenthe signal interpretation. Unlike isotropic materiallike aluminium, the composite materials havedifferent velocity of travel in different fiber directions. The fundamental symmetric mode or S0mode is preferred for delamination detection due to

non-dispersive and less attenuation. Due to thecomplexity in propagation and multi modal

presence, modelling is essential to understand the basic propagation characteristic in compositestructure.

FE ModelingSince no analytical solutions exist for interaction

of Lamb wave with delamination, FE modelling iscarried out to study and understand the interaction of Lamb wave with delamination for better detection.The 2D FE model is solved using PZFLEX software[Weidlinger Associates Inc.]. The plane strain modelhas a size of 300mm in length and 1mm in thicknessof 8 layer cross ply [(0,90) 2]s laminate. Eachunidirectional CFRP layer has a thickness of 0.125mm with material properties E 11: 126.6 GPa, E 33: 8.7GPa, G 13: 3.7 GPa, 13: 0.32, 23: 0.5, and : 1605kg/m3 [2]. GW is in the frequency range of severalhundred kHz range or above, and have shortwavelengths. To obtain a high spatial resolution, thenumerical model must have sufficiently smallelement lengths. However, using a small elementsize would result in a large number of elements,leading to an increase in computational time. Four node quadrilateral elements with a size of 0.5 x0.125 mm are used. Even though the elements arefairly coarse, the results are found satisfactory for the low frequency range being investigated.Delamination of size D (varies 4, 8, 16, 20 mm) isintroduced in the? mid of laminate by nodeseparation between layer 7 and 8 from bottom. A 5cycle sinusoidal wave in frequency range of 100kHzto 1000kHz weighted by a half sine window isapplied at the left end of the laminate as pressureload to generate the S0 mode Lamb wave in thelaminates.

3. Results and DiscussionFig.1 shows the dispersion curve of cross ply

laminate. The S0 mode is non-dispersive and it isquite evident from its flat slope and it travels faster,while the A0 is more dispersive in the investigatedfrequency range. The S0 velocity in this frequencyrange is found to be 6325 m/s which match wellwith the experiment result. The variation of

mailto:sara0013@ntu.edu.sgmailto:sara0013@ntu.edu.sg

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propagation velocity in different fiber orientation isshown in Fig.2. Unlike aluminum (whose velocity is5400m/s), the S0 velocity is 8897m/s in thelongitudinal direction (0 0) and varies or reducesasymptotically to 2605m/s in the transverse direction(90 0). This shows the wave travel s faster along the

fiber than that across the fiber. Furthermore, itimplies that most of the wave energy propagates inthe fiber direction. This significantly affects thedelamination response signal when the transducer and delamination is positioned in a different fiber orientation.

Fig.1 Dispersion curve of 8 layer cross ply laminate

Fig. 2 Guided wave velocity in various fiber angles

Figs.3 and 4 show s the responses of the guided waveinteraction with a 20 mm delamination. It can beobserved that the incident S0 mode is modeconverted by the presence of delamination to A0mode. The incident S0 energy is breaking into four

parts: S0 reflection, mode-converted A0 reflection,S0 transmission and mode-converted A0transmission. The breakup of energy in variousmodes depends up on the size of the delamination.On increasing the delamination size, the S0 and A0reflection increase while both transmissions arereduced as shown in Fig.5. In conventionalultrasonic testing the sensitivity or smallest defectcan be detected is /2 or half wavelength [4]. Guidedwave behaves similarly or has better sensitivity indetecting delamination compared to conventionalultrasonic.

Fig.3 Response monitored in left side

Fig.4 Response monitored in right side

Fig.5 Sensitivity of Guided wave

4. ConclusionThe potential of GW for delamination detection

in SHM was investigated. It is observed that guidedwaves have a better sensitivity in findingdelamination. Further studies need to be carried outto find the influence of fiber orientation in thedelamination response.

References[1] T . Kundu, 2004, Ultrasonic Nondestructive

Evaluation: Engineering and Biological Material Characterization , CRC Press LLC.[2] N. Guo, P. Cawley, 1993, Journal of Acoustical Society of America , 94, 2240-2246.[3] Z. Su, et al., 2006, Journal of Sound & Vibration ,

295, 753-780.[4] J.Krautkramer and H.Krautkramer, 1990,Ultrasonic Testing of Materials , Springer-Verlag.

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