Scattering of Lamb Waves from Typical Structure Defects in Plates - Research Paper Example

? Scattering of lamb waves from typical structure defects in plates July 9, Scattering of lamb waves from typical structure defects in plates Executive summary Structural health monitoring is a process of periodic detection of damages on structure’s components, as well as development and implementation of plans for remedying of the detected damages. The procedure aims at locating damages, determines their severity, and has several advantages, such as timely and accurate results for efficient management of structures. Its implementation through visual inspection, traditional non-destructive evaluation methods, and remote monitoring has widely been used but the techniques are expensive and labour intensive. Lamb wave based SHM however relies on wave propagation, offers characteristics of efficiency and convenience and this study seeks to investigate its application. The study is significant to structural health monitoring and involved stakeholders to monitoring processes because it seeks to validate a convenient and cheaper monitoring approach towards higher survivability of structures. The study aims at investigating feasibility of application of 3D laser vibrometer in conjunction with Lamb wave technique. This will incorporate literature review, implementation of experimental and specimen design, measurement of scattering waves on used structures, and analysis of results. Numerical approach was used to investigate robustness of the lamb wave model. A validated simulation study was done using ANSYS program to study propagation and scattering of waves and an experiment to investigate propagation and scattering of waves was done on aluminium specimens with the aid of 3D vibrometer. The simulation and the experiment identified effects of a blind hole on wave propagation and scattering. Validation of the simulation demonstrated point differences in propagation and scattering relative to position of a blind hole. The study achieved its objective and established that application of 3D laser vibrometer in conjunction with Lamb wave technique is feasible for structural health monitoring. Introduction Background information Structural health monitoring defines a process of damage detection and formation of a plan or strategy for remedial engineering. Its significance has grown among engineers because of its ability to generate timely and accurate data on health and functionality of structures, properties that allows it to ameliorate maintainability and safety concerns (Staszewski et al. 2004). The monitoring approach detects anomalies, specifies anomalies’ exact location, and evaluates damage extent towards corrective measures, a scope that offers economic, and safety advantages. Other applications of SHM include mitigation of uncertainty, planning for schedule activities, and test of hypothesis. There are three theoretical SHN techniques, visual inspection, traditional non-destructive evaluation methods, and remote monitoring. Visual inspection involves inspection by experienced and trained personnel while traditional non-destructive evaluation applies wave propagation approaches for defect detection. The approaches are however expensive, labour intensive and complicated while remote monitoring is automated and autonomous, only requiring attention on critical conditions (Thomas et al. 2009). Lamb waves for remote monitoring, for instance, only rely on wave propagation but instrumentation and interpretation needs hinder its efficiency (Franco et al. 2008). Lamb-wave based SHM efficient and convenient for detecting metallic structure cracks and delamination and disbanding of composites (Ong and Chiu 2012). Significance The study is significant to structural health monitoring as it proposes a cheap and convenient monitoring strategy that will facilitate regular inspection for damage detection and remedies. This scope extends the study’s significance to stakeholders to monitoring processes because validating the lamb wave based method will ensure the stakeholders’ interests in structure’s safety and survivability. Objectives and aims The project’s core objective is to conduct a feasibility study on the application of 3D laser vibrometer in conjunction with lamb wave technique. It pursues the following aims in order to realize this objective. 1- To review and evaluate past research in SHM including the tools and techniques involved; 2- To design an experimental rig and specimens; 3- To conduct measurements scattering of Lamb waves on the structures; 4- To analyse the results with the aid of 3D finite element software This thesis is organized into four chapters. Chapter 1, the introduction, discusses background and significance of SHM and the use of lamb wave together with the study’s aims and objectives. Chapter 2 is the literature review and offer operational definitions. Chapter 3 is the methodology, explain rationale for research design, and describes data collection and data analysis procedures and techniques while chapter four analyzes and discusses the study’s results. Literature review The literature review analyzes methods for analyzing structural health and identifying damages such as cracks, corrosion spots, and dents in aluminium plates. Structural health monitoring Structural health monitoring defines the process of implementing strategies for identifying quantifiable damages in engineering structures. The damages are changes in material components that threaten functionality and monitoring should be free from further adverse effects and should consider history and future of the subject structure (Farrar & Worden 2007). Structural health monitoring motivation Monitoring is a motivational stage to engaging repair team because it identifies the need for remedy and allows for repair planning (Balageas et al. 2006). Monitoring also predicts structure’s lives and offers information towards maintenance (Farrar & Worden 2007; Fassois&Sakellariou 2007). Structural health monitoring evaluation Evaluation can be long term or short term, aims at establishing structures’ conditions toward repairs, and helps in implementation of repair programs (Balageas, et al., 2006; Farrar & Worden, 2007). Evaluation is also significant to forecasting that incorporates damage prognosis concept (Fassois & Sakellariou 2007). Challenges of Structural health monitoring Structural health monitoring however faces diversified challenges such as inevitability of structures’ aging, accelerated damage, susceptibility of sensory devices to damage, and inability to get failure-free sensor devices. Wave based methods Wave based methods utilize waves to detect defects and remains helpful in making future predictions (Hal et al. 2003). It is suitable for designing stages of structures but limited low frequency applications for accuracy concerns (Chen, 2007). Evaluation of wave based methods Efficiency of wave based methods are limited to short level frequencies but sub dividing constructions into many domains overcome this challenge (Hal, et al., 2003). The method is further limited by to small models and computations. The method also relies on the relationship between frequency and plate thickness but thickness can be adjusted by combining plates (Chen 2007). Thickness limitation also exists but is manageable through application of aluminium alloys. Though wave based methods can be applied in structures with higher accuracy than in finite elements, this approach is susceptible to more errors, especially beyond 157 Hz and even bending of wave functions is not a solution (Sotiropoulos, 2001). Challenges of wave based methods Mid and high frequencies offers the major challenge to the method but application of the deterministic, numerical approach based on Trefftz approach can manage them (Chen 2007). Lamb waves Lamb wave are considered elastic and have been used for non-destructive testing procedures (Su & Ye 2009; Lu 2006). Lamb waves: Introduction Multiple reflections and mode changes cause lamb waves as shown bellow (Calomfirescu 2008). Figure 1:A schematic representation of coordinates and plate in Lamb wave formation (Ryden, et al., 2004) Involved computations in lamb waves require numerical methods but computer applications have maintained its popularity (Calomfirescu 2008). Applicable structural analysis and ultra sound procedures also empower the method assessment, detection, and prediction of damages (Sotiropoulos 2001). Changes in wave oscillation allow for detection of damages but face challenges in detection of corrosion sports and dents in aluminium structures (Lu 2006). Classification of lamb waves Lamb waves can be zero-order mode or higher order lamb wave. Zero-order modes have a frequency of zero and cover the entire frequency spectrum. The waves’ characteristics changes with frequency and can further be classified as either symmetric (S0) or asymmetric (A0) (Calomfirescu, 2008). Symmetric model waves travel at plate velocity and low frequencies, the velocities changes with frequencies (Lu, 2006). Asymmetric mode waves are however dispersive with disproportionate relationship between velocity and frequency. The following figure shows preparative modes of the wave types. Figure 2: Propagation of asymmetric and symmetric Lamb wave modes (NDT, 2012) Higher-order lamb waves occur with zero order waves at high frequencies and are observable (Su & Ye, 2009). Challenges in lamb wave technology Challenges such as irregularly shaped structures and generation of various wave modes are common with the lamb waves (Calomfirescu 2008; Su & Ye 2009). Piezoelectric transducers (PZTs) Piezoelectric transducers apply piezoelectricity principle that accumulates in particular solids and involves electric charge from pressure (Arnau&Vives 2008). Evaluation of Piezoelectric transducers The system detects pressure and energy changes in structural monitoring (Moheimani & Fleming 2010). It converts energy to ultimate mechanical energy that relays changes for detection (Safari & Akdogan 2008). Frequency and thickness are the core parameter in the system that can be ultrasonic or contact and generated frequency from the system is propotionsl to thickness of involved structure (Arnau&Vives 2008). The following figure shows a transducer. Figure 3:Cross-section of a typical contact transducer (Arnau&Vives, 2008) 3D Laser Vibrometer This is a non-contact instrument for sensing vibrations in structures with a wide range measuring potential (Sharma 2009). Characteristics of vibrometer The equipment has optical sensor with independent sensors whose output beams are inclined at 12 degrees. Each sensor also has a lens at an angle (Bently& Hatch, 2003). The vibrometer also has channels for generating analogue velocity outputs and three velocity-decoding modules (Oliver, 2000). Advantages of 3D Laser Vibrometer The system’s advantages includes in multidimensional scope, high efficiency level, and minimal noise (Sharma, 2009). 3D Finite Element Analysis This is a flexible and precise way of predicting structure’s performance through application of software such as ANSYS and software for FEM, and is comprehensive development processes of all products (Long, Jinliang&Qiding, 1995; Strang& Fix, 1973). The technique may also adopt varies lamb wave actuation methods (Gopalakrishnan, Ruzzene&Hanagud, 2011; International Modal Analysis Conference &Proulx, 2011 Raghavan&Cesnik, 2007). Methodology The study applied numerical simulation and empirical investigation. Numerical simulation Numerical simulation involved modelling, meshing, simulation set up, post processing, and validation and verification of results. The 3D FE method was used to simulate propagation and scattering of waves through aluminium hole specimen. Geometry and meshing was then done using ANSYS Workbench 14. Figure 4: A mesh FE simulation The numerical simulation was then solves and the plate modelled using an 8-noded 3D reduced integration element. The transducer was located at about 395 mm from the through hole centre and exited signal modulated. A piezoelectric transducer then used to excite light waves. Figure 5: Overview of the simulation model A wave was then applied to the simulation model at the end of 0.0001 s and the detected signal is shown bellow. Figure 5: The Lamb wave signal Plate mid thickness location was then monitored for detection of the different waves. ANSYS was used to control time and and propagations captured. Experimental investigation The experimental investigation was implemented in four steps. Test specimens were created, some with blind hole and some without. Lamb wave was then generated by a PZT that was attached on the specimen surface and lamb wave sensing done by using 3D alignments. Data analysis followed this. Analysis and discussion Numerical simulation results Results from the simulation showed different patterns for propagated wave after interaction with the blind hole as shown in the following images. (a) (b) Figure 6 :Lamb wave scattering at (a) 44.451 µs and (b) 52.639 µs (a) (b) Figure 7: Scattering of theLamb wave the blind hole at (a) 63.167 µs and (b) 73.695 µs Experimental results Snapshots from experimental set up are shown bellow. (a) (b) (c) (d) (b) Figure 9: Snap shots of velocity field in X-direction (in-plane) at: (a) 23.83µs (b) 30.08 µs (c) 39.45 µs and (d) 54.69µs Figure 104:Snap shots of velocity field in Z-direction (out-of-plane) at (a) 23.83µs (b) 30.08 µs (c) 39.45 µs and (d) 54.69µs (a) (b) (c) (b) Figure 115: RMS velocity fields in (a) X, (b) Y and (c) Z directions Discussion The images identify uniform propagation before interaction with the blind hole but the pattern changes upon interaction with the blind hole. A similar change with respect to time is noted in scattering of wave that is significant after interaction with the blind hole. Wave measurement around the blind whole ascertains this difference and confirms effectiveness of the method in detecting the blind hole and its validity for application as a benchmark for validating experimental results. Wave patterns in the experimental set up also registered differences after interaction with the blind hole. The set up therefore detected the blind hole and similarity in its results to the simulation means that both methods are effective in detecting the blind hole. Conclusion The study aimed a investigating robustness of the Lamb based damage detection method. It applied a validated numerical simulation method ascertained robustness of the experimental wave based method. The study therefore achieved its objective and conclude that wave based methods is effective in structural damage detection in monitoring. 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