Structural Health Monitoring By Vibration Measurement with Non-contact Laser Excitation

Transcription

1 Structural Health Monitoring By Vibration Measurement with Non-contact Laser Excitation Feblil HUDA Candidate for the degree of Doctoral Supervisor: Prof. Itsuro Kajiwara Division of Human Mechanical System and Design Introduction Damage identification as a part of structural health monitoring (SHM) plays an important role to make sure the current condition of structures. In general, there are two main steps of diagnostic procedure in damage identification: the experimental measurement and the data processing. The diagnostic methods largely studied and applied are those based on vibration measurements, since they allow non-destructive evaluation on investigated structures. The usual approach in damage detection based on vibration measurement consists in the determination of dynamic characteristics changes due to the presence of the damage. The dynamic characteristics of a structure can be assessed from the frequency response functions (FRFs) which are obtained by applying an excitation force to the structure and measuring the excitation force and vibration response. Some problems and inconvenient are found in these methods, that usually involving part directly mounted and contacted to the surface of tested parts for applying excitation force, can possibly contaminate tested parts and add safety constraints such as the need for additional inspection, endanger measurement operators if the measurements last on narrow operational area and even do not give the possibility of conducting the measurement in narrow area. These conditions make non-contact method become important. In order to solve the problems in vibration excitation for damage detection, an integrated non-contact damage detection system based on vibration is important to be developed. The measurements must be employing of non-contact structural excitation testing with high reproducibility, enable to extract measurement in high frequency and enable working in small area. Noncontact vibration excitation is known well using laser system which may fulfill the requirements. Laser Excitation In this dissertation, the vibration testing and health monitoring system based on an impulse response excited by laser is proposed to detect damage on structures. This idea is built based on the applicability of laser in giving excitation to structures and the needs to have high reproducibility non-contact excitation system on structure. The source of excitations is proposed in two methods: laser ablation [1] and laser-induced breakdown [2]. Laser ablation The process of the laser ablation is presented in Fig. 1. When a laser beam is irradiated on a metal surface, it will be absorbed by the metal, and the atoms absorbing the laser light release ions. The absorption of the energy in the laser beam by metal will also generate high temperature plasma, and large quantities of particles are then released (in the form of a plume) from the metal (Fig. 1(a)). Momentum is then generated when a mass Δm released at a velocity v from the metal, represented by Δmv, and this expresses the laser-induced impulse. Fig. 1 Process of the laser ablation [1] To generate a larger excitation force on the structure, a water droplet is placed on the metal surface during laser ablation as shown in Fig. 1(b). The total resulting momentum is Δmv+ΔMV, resulting in a larger impulse with than without the water droplet. This impulse constitutes the excitation force on the structure. The input characteristics different between laser excitation and impulse hammer can be seen in Fig. 2. Fig. 2 shows that the excitation by laser occurred in really short time, while the excitation by impulse hammer shows curvy excitation force. This is a fact that ideal impulse force to metal structure could be sourced from non-contact excitation by laser ablation. Load [N] Laser Atoms etc Time [s] :Laser Fig. 2 Different characteristics between laser excitation and impulse hammer [1] Laser-induced breakdown Plume ( m) (a) Laser ablation without water droplet Water droplet Laser Atoms etc. :Impulse hammer The laser-induced breakdown (LIB) refers to the formation of plasma through the cascade process caused by electrons emitted from atoms and molecules that have absorbed multiple photons through a multi-photon v v V Vapor ( M) Plume ( m) (b) Laser ablation with water droplet

2 process when a laser beam is focused in a gas. A portion of this plasma energy is transformed to a shock wave, which is the source of the sound generated by LIB, which has the potency to be acoustic excitation force. This is one of the high power laser effects. The LIB threshold in air is I W/m 2. When the laser's local intensity is smaller than this threshold, a convex lens can be used for focusing the laser beam, thereby reducing the spot radius. By passing the laser beam through the convex lens which focuses the beam, the local intensity of the laser beam reaches to or above the minimum LIB threshold of W/m 2 [3-5]. The process to achieve acoustic excitation by LIB is presented in Fig. 3. From the figure, it can be seen that Nd: YAG pulse laser is used to produce laser beam, and laser beam is then passed to convex lens. The LIB occurs in the distance of focal length of the convex lens from the convex lens position. Fig. 3 Process to achieve acoustic excitation by LIB Fig. 4 shows times response sound pressure generated by LIB. The ten measured waveforms of the sound pressure are plotted. It is shown on the figure that the point sound source generated by LIB on that configuration has high reproducibility and can generate ideal impulse excitation, occurs in short time. Sound pressure [Pa] Time [ms] Fig. 4 Time responses of sound pressure generated by LIB [2] The laser excitation can apply an ideal impulse excitation, making it possible to use the Fourier transform of the output response for the evaluation of the vibration characteristics of the system as the frequency response function. The Fourier transforms of time histories of an input force f(t) and output response x(t) into the frequency domain are described by ( ) ( ) (1) ( ) ( ) (2) The output response x(t) can be expressed by the convolution integral with an impulse response function h(t) of the system: ( ) ( ) ( ) (3) The Fourier transform of Eq.(3) results in ( ) ( ) ( ) (4) where, ( ) is the frequency response function of the system. When f(t) is an impulse force, ideally the Dirac delta function, the Fourier transform of f(t) becomes ( ), and Eq.(4) is expressed by ( ) ( ) (5) The result is that the Fourier transform of the response x(t) becomes the frequency response function. Non-contact laser excitation and damage identification In damage identification using vibration approach, the ideal excitation method is required to ensure the accuracy, reliability and easiness of measurement. In applying non-contact laser excitation to damage identification, the mechanism of laser excitation needs to be carefully considered, so that the excitation method not only gives the easiness in measurement, but also enhances the quality of measurement. Non-contact laser excitation by laser ablation is a type of excitation that is usually applied to metal structures. Besides giving ideal impulse excitation, this excitation method has strength point in measuring high frequencies vibration, but laser ablation causes a small damage on measured structure, so this excitation method will be appropriate to be applied to metal structures that have no problem with the effect of small damage caused by ablation. It will be suitable to be combined with damage identification on bolted joint structure. Acoustic excitation by LIB is non-contact excitation which gives possibility to conduct measurement in really narrow area; this kind of excitation causes no damage. So, this kind excitation is suitable to be combined with damage detection on extremely light and flexible structure. Acoustic excitation by LIB will be applied and combined with damage identification method to detect damage in membrane structures. Bolt Loosening Analysis and Diagnosis by Non-contact Laser Excitation Vibration Test Detecting a loosening of bolted joints is important for ensuring the function of structure or subsystem. Vibration measurements approach for bolt loosening detection usually use contact excitation or hammering method to excite a structure to get the frequency response, requiring trained technicians over a long period. The reproducibility of hammering method by technicians is low; the measurement of dynamics characteristics requires high reproducibility. This issue led researchers to find other excitations method with higher degree of reproducibility. Vibration measurement systems using laser excitation can guarantee an extremely high degree of reproducibility of measurement [1]. This research concerns about the use of laser excitation for detecting the bolt loosening. Finite element model of bolted joint The finite element analysis software ANSYS 14.0 has been used to model bolted joint with pretension force and mating part contact. The model is constructed in form of cantilever, where one of the flanges is fixed, as shown in Fig. 5. The SOLID186 element of ANSYS is used to construct physical model of bolted joint. The total number of nodes and elements are and 5412 respectively. The pretension force is given by using pretension element PRETS179.

3 Frequency responses with different tightening torques got from experiments in vertical and horizontal direction are shown in Fig. 7. The resonance peaks in frequency responses are shifted to lower frequency and the shifting is significant in high frequency, while in low frequency the response seems not to change so much. The Frequency response of simple one-bolt joint in normal condition is presented in Fig. 8. Fig. 5 Simple bolted joint model In this model, the tightening torque will be applied to the bolt in form of pretension force. The relationship between tightening torque and occurred pretension is described by d (6) where F, T, K, and d are pretension force, tightening toque, torque coefficient, and nominal diameter of bolt respectively. The contact modeling is presented by surface-surface contact elements, which is a pair of contact element CONTA174 and target element TARGE170. There are three types contact used in this model: bonded, frictional, and no separation. In order to determine the contact type on contact between flanges, the region, where the stresses due to pretension that produced clamping force are predominant, is set to be bonded each other. That region in normal condition looks like a conical shape and covers a range between 25 o α 33 o, suggested by Osgood [6]. The bolt head and the nut are also assumed to be glued to the flanges due to the clamping force. Some pretension forces for normal and loosening condition will be applied to the bolt to get the stress data on contact between flanges. By applying conical shape suggested by Osgood, the values of stress on outer radius of conical shape from normal pretension force are taken as the benchmark and limit for bonded area, and the lower stress areas will be frictional and no separation. A series of static structural analysis and frequency response analysis by pre-stress modal analysis are then conducted. Verification of laser excitation vibration measurement result by finite element analisis Vibration test by using laser excitation is conducted for a simple model of bolted joint, and the results are then validated by finite element analysis. The tightening torques are given by digital torque wrench for normal condition (24.5 Nm) and loose conditions (less than 24.5 Nm). A vibration testing arrangement using a high power pulse laser is shown in Fig. 6. (a) (b) Fig. 7 Frequency response of simple one-bolt joint by experiment (a) in vertical (b) in horizontal (a) (b) Figure 8 Frequency response of simple one-bolt joint in normal condition (a) in vertical (b) in horizontal The frequency responses in normal condition by simulation and experiment show a good agreement. They are not only having the same peaks, but also having the same tendency. This good agreement can be seen both in vertical and horizontal directions of test. Fig. 9 shows the frequency response of simple one-bolt joint in vertical direction with loose conditions, and all of simulation-experiment comparison results show the same tendency. Fig. 6 Vibration testing arrangement using the high power pulse laser Fig. 9 Frequency response of simple one-bolt joint in vertical with loose condition and tightening torques of (a) 20 Nm, (b) 18 Nm, (c) 15 Nm, (d) 10 Nm

4 Bolt loosening detection approach The Recognition Taguchi (RT) method is a statistical evaluation method used to detect the loose bolt in this research. RT method is a type of MT (Mahalanobis Taguchi) method used in the field of quality engineering as a pattern recognition method. In this method, data is measured a multiple number of times under identical conditions, and a unit space for normal condition is defined. For unknown data (loose condition), distances in relation to unit spaces are calculated and compared to determine damage index (DI), which defined as D Damage Index D (7) Where D is the Mahalanobis distance for unknown data, and D is the covariance of the Mahalanobis distances for normal condition. If DI > 1, the system is under damage (in this case undergoes loosening). If DI 1, the system is normal. In order to demonstrate loose bolt detection on bolted joint, a six-bolt joint model is constructed; several damages are applied to the joint by giving different tightening torques on certain bolt position. The list of damage cases can be seen in Table 1. Table 1 the list of damage cases Condition Tightening Torque (Nm) Bolt 1 Bolt 2 Bolt 3 Bolt 4 Bolt 5 Bolt 6 Normal Damage Damage Damage Damage Damage Damage Laser excitation vibration measurement system is applied to six-bolt joint. The model, bolt numbering, excitation point, and measurement points can be seen in Fig. 10. The excitations by laser are given in a point, and measurements of the responses are conducted in six points which represent the measurement of response on each bolt. Fig. 10 The excitation and measurement points position for bolt loosening detection Result The measurements were conducted first for normal condition to get the unit space that was constructed from ten sets of power spectrum data. The measurements for several damage conditions were then conducted to get the only one set power spectrum data for each case of damage. By comparing damage data with the unit space in the same range of interest, the damage index would be gotten. The damage index on the bolt in every case of damage by simulation and experiment data can be seen in Table 2 and Table 3 respectively. Table 2 Damage index based on simulation result Post. Bolt Bolt Bolt Bolt Bolt Bolt Case Damage Damage Damage Damage Damage Damage Table 3 Damage index based on experiment result Post. Bolt Bolt Bolt Bolt Bolt Bolt Case Damage Damage Damage Damage Damage Damage Table 2 and Table 3 show that most of DI is greater than 1, which means that the damage condition (loosening) of the joint almost can be detected in every position of measurement, and the highest value of damage index in the same damage case represents the position of the loose bolt. These results correspond to the scenario of some cases of loosening designed in this research. Damage Detection in Membrane Structures Using Non-contact Laser Excitation and Wavelet Transformation The vibration of membranes has been investigated with different types of non-contact excitation and vibration measurement methods, such as horn-acoustical excitation and capacitance displacement sensor, speakers-acoustical excitation and laser vibrometer, laser excitation by laser ablation and laser Doppler vibrometer (LDV), but these measurement methods do not fulfill the requirements of ideal non-contact measurements method for membrane. The ideal measurement method for membrane should be nondestructive, allow conducting experiment in small area, attach no sensors/exciters to objective structure which causes the changes of mass and stiffness, and give ideal point-excitation to the structure. Vibration test using laser-induced breakdown (LIB) is non-contact and nondestructive excitation method which involves no any attachments to objective structures, and offers the possibilities of applying acoustical excitation on a point with some distance between exciter and membrane structure. This research concerns about the use of laser excitation by LIB for detecting damage in membrane structure. Vibration testing system using LIB In this research, an excitation to a membrane structure is applied in the form of acoustic excitation achieved by generating an ideal point sound source at specific location via LIB which offers the possibility of conducting experiments with acoustic excitation in limited space. To measure the output response, two LDVs are set to obtain the responses of the membrane structure. These measurements enable to extract the mode shapes of membrane. A spectrum analyzer (A/D; NI-4472B, Software; Catec CAT-System) is used for

5 analysing the Fourier spectrum of the structure. The vibration testing set-up is shown in Fig. 11. Fig.11 Vibration testing set-up for the membrane structure The membrane material is Kapton, manufactured by Du Pont-Toray Co., Ltd. Owing to its good resistance to temperature; Kapton is often used in actual space applications. The membrane here is 200 mm 200 mm in size and 50 µm thick which is clamped on four sides by metal clamps and kept stretched by three 700 gram masses and one side is fixed. The masses and the fixed side are connected to the metal clamps by steel wires. FEA investigation of damage detection on the membrane structure The finite element analysis software ANSYS 14.0 is used to conduct the pre-stress modal analysis to generate the mode shapes of normal and damaged membrane structures. The SHELL181 element of ANSYS is used to construct the physical model of the membrane, and the SOLID186 element is employed to model the clamps at the corners of the membrane. The membrane is uniformly divided into approximately 10,000 2 mm 2 mm four node membrane elements. The finite element model of the membrane structure is shown in Fig. 12. using iso-surface concept, the position of damage can be recognized. Application of damage detection To verify and to examine the applicability the damage detection approach using 2-D CWT by simulation, three different single-damage conditions were induced in the membrane finite element models. The details of the three damage cases are listed in Table 4. These damages are induced to membrane by simulation and experiment. Table 4 Details of the damage induced on the membrane Damage type Damage size (mm) Closest measurement point (x,y) in mm L-cut (120,120) L-tear (140,100) I-cut (80,100) The process of analysis for detecting the damages can be explained by using Fig. 13. The mode shape extracted from simulation is interpolated to get more data points of mode shape which then transformed by 2- D CWT to get wavelet coefficient. The boundary treatment is applied to move very high value on the corners and the edge. The position of damage can be recognized by applying iso-surface concept to wavelet coefficient. The same procedure is applied to vibration mode shape extracted from vibration measurement by laser excitation. The comparison between simulation and experimental results is presented in Table 5. Fig. 12 Finite element model of the membrane structure Two-dimensional continuous wavelet transformation for damage detection in membrane structure The two-dimensional continuous wavelet transformation (2-D CWT) considered in this study is based on the formulation by Antoine [7], and wavelet computations are performed using MATLAB and the YAWTb toolbox. The procedure to detect the position of damage in the membrane structure is adopted from the procedure suggested by Fan [8], using the 2-D CWT derivative Gauss (Dergauss2d) and iso-surface concept, which was used successfully for detecting damage on plates using mode shape data. The mode shape data is transformed by 2-D CWT with several scales and by Fig. 13 Process of the analysis for detecting the L-cut damage on the membrane by simulation data (a) first principal mode shape (41.78 Hz), (b) interpolated first principal mode shape, (c) wavelet coefficient, (d) wavelet coefficient with boundary treatment, (e) 3-D view of isosurface, (f) top view of iso-surface. Table 5 shows that the damages on membrane could be detected well by using proposed method. The position of L-cut and L-tear damages could be detected using first principal mode shape, but I-cut damage could not be detected using first principal mode shape, thus the higher order mode shape (in the frequency of Hz by simulation and 851 Hz by simulation) was employed to detect the position of damage. The simulation and experimental results are in good agreement.

6 Table 5 Comparison of damage detection between simulation and experimental results Damage type Simulation result Experimental result L-cut L-tear Fig. 15 Process of the analysis for detecting I-cut and square-hole damage on the membrane by experimental data (a) mode shape (799.1 Hz), (b) wavelet coefficient with boundary treatment, (c) 3-D view of iso-surface, (d) top view of iso-surface I-cut In order to make sure applicability the proposed approach to detect more damage, I-cut and square-hole damages are introduced to the membrane. The positions of damage on the membrane can be seen in Fig. 14. Because these damages cannot be detected well using first principal mode shape, the higher order mode is considered to detect these two damages on the membrane. Conclusion In this dissertation, two methods of non-contact excitation by laser were elaborated. These excitation methods have high reproducibility, can apply ideal impulse excitation to structure, have the big potency to be implemented to vibration testing without measuring the input, and really powerful in extracting high frequency measurement, cause the easiness of implementing them to structural health monitoring (SHM). These have been proofed by the application of non-contact laser excitation by laser ablation in detecting bolt loosening and by the application of noncontact laser excitation by laser induced breakdown in detecting damage in membrane structure. Fig. 14 Membrane with I-cut and square-hole damages Vibration testing using laser excitation is conducted on the membrane. Some vibration mode shapes at high frequency got from experiment are checked to find the mode shape which contains dominant peak of both types of damages. It is found at natural frequency of Hz, the mode shape contains dominant peaks which show the local mode of damage. The analysis process for detecting these two damages is presented in Fig. 15. The position of the two damages can be detected well. References [1] I. Kajiwara, and N. Hosoya, Vibration testing based on impulse response excited by laser ablation, Journal of Sound and Vibration, 330, , [2] N. Hosoya, M. Nagata, I. Kajiwara, Acoustic testing in a very small space based on a point sound source generated by laser-induced breakdown: stabilization of plasma formation, Journal of Sound and Vibration, 332, , [3] Q. Qin, K. Attenborough, Characteristics and application of laser-generated acoustic shock waves in air, Applied Acoustics, 65, , [4] M. Oksanen, J Hietanen, Photo acoustic breakdown sound source in air, Journal of Ultrasonics, 32, , [5] V.B. Georgiev, V.V. Krylov, Q. Qin, K. Attenborough, Generation of flexural waves in plates by laser-initiated air borne shock waves, Journal of Sound and Vibration, 330, , [6] Shigley, J.E., [Mechanical Engineering Design Eight Edition], McGraw-Hill, USA, , (2006). [7] J.P. Antoine, R. Murenzi, P. Vandergheynst, Twodimensional wavelets and their relatives, Cambridge University Press, Cambridge, 2004 [8] W. Fan, P. Qiao, A 2-D continuous wavelet transform of mode shape data for damage detection of plate structures, International Journal of Solids and Structures 46 (2009)