Structural Health Monitoring - Dissertation Example

?LITERATURE REVIEW This article seeks to establish the numerous methods used in analysing structural health and identify relevant damages to structures. The methods discussed are commonly utilised in analysing the presence of cracks, corrosion damage or dents in aluminium material structures. The available methods include wave based, Lamb waves, PZT and 3D laser vibrometer, utilised in ascertaining presence of damages within aluminium structures. These could be identified as the major methods used in structural monitoring of aluminium structures (Balageas, et al., 2006). Table of figures Figure 1 Evolution of materials used in structure construction (Balageas, et al., 2006) Figure 2 A schematic representation of coordinates and plate in lamb wave formation (Ryden, et al., 2004) Figure 3 propagation of Asymetric and Symetric lamb wave modes (NDT, 2012) Figure 4 Cross-section of a typical Contact transducer (Arnau & Vives, 2008) Figure 5 compact 3D laser vibrometer (Oliver, 2000) Structural Health Monitoring (SHM) Structural health monitoring can be defined as the process of implementing strategies aimed at identifying damages in engineering infrastructure. Damages, within the monitoring process, refer to changes occurring in the components and materials that could affect structural functioning adversely. The evaluation tools utilised ought to present non-destructive effects to the structures. In establishing the damage, past, present and future status of structures should be considered carefully (Farrar & Worden 2007). Changes with adverse effects to the functioning of the systems form the greatest concern referred as damages. The relevant changes should be quantifiable for classification as damages and their effects on the functioning of structures adverse. Motivations of SHM Structural health monitoring remains an essential process aimed at ensuring safety of engineering structures. Monitoring could be essential in identifying defects within structures, prompting appropriate corrective measure with the aim of rectifying the detected defects. This process might be carried out as part of routine maintenance procedures undertaken during the lifetime of engineering structures. Monitoring could therefore, precede planned maintenance procedures as it can assist in detecting changes within the structure that need rectification (Balageas et al. 2006). Necessary repairs to structures can be identified through monitoring before engaging the repairing teams. These processes can assist maintenance teams in identifying the required maintenance procedures for different projects. The adverse effects of damages could escalate to a point where structures become classified as unusable. Slow accumulation of damage could drive structures to this stage referred as failure. The process of implementing monitoring strategies involves making structural observations, over period, using standardised measurements and analysis. The monitoring process should provide analytical information regarding the future functioning of the structure in relation to the anticipated ageing associated with time (Farrar & Worden, 2007). Since ageing of structures remain inevitable, monitoring the ageing process become crucial in minimising the possibility of structures collapsing unexpectedly. Monitoring the ageing process provides engineers with information that can be utilised when performing routine maintenance on structures. This information can also be utilised when constructing new structures through identification of problems that might occur as observed in existing structures (Fassois & Sakellariou, 2007). The ageing process could impose significant changes into materials; changing their characteristics. Modifications to materials can, however, be undertaken artificially through addition of relevant components. The general trend of modifying the materials shows changes from simple, natural materials to complex, auto-adaptive materials as indicated in the figure below. These adaptations remain essential in increasing adaptability of materials to environmental conditions where materials are used. Figure 1 Evolution of materials used in structure construction (Balageas, et al., 2006) SHM evaluation SHM can be carried out on two major timelines – long-term and short-term. Short-term monitoring mainly occurs when checking the progress of projects. Long-term monitoring, however, remains the main method employed in structures intended to endure long durations of service. This monitoring aims at establishing the status of structures and remains fundamental during repairs to structures (Balageas, et al., 2006). Reports provided from such monitoring assist individuals in implementing essential repair programmes on engineering projects. Long-term monitoring can also provide information that could assist in making decisions, regarding structures’ capability to withstand extreme conditions like blasts and earthquakes. The capability of engineering structure to withstand extreme conditions relies heavily on the condition of the structure (Farrar & Worden, 2007). Information gathered during monitoring remains significant towards implementing essential changes to allow structures to survive the unexpected. SHM, Condition monitoring, non-destructive evaluation, statistical process control, and damage prognosis remain the fundamental disciplines for carrying out damage identification. Condition monitoring deals with identifying damages in rotating machinery. On the other hand, SHM deals mainly with fixed structures. These two disciplines, however, appear analogous to each other. While the other disciplines major in identification of damages, damage prognosis becomes essential once damage is identified (Fassois & Sakellariou 2007). Engineers utilise the prognosis in determining the remaining life of a system containing identifiable damages. Application of the prognosis appears in estimating the remaining lifespan of a system containing damages. The intensity of the damages is used in making these estimations. Challenges of SHM During the undertaking of structural health monitoring engineers face surmountable challenges in getting useful information. One fundamental challenge lies within the fact that ageing remains an inevitable element that could alter a system’s functioning (Bently & Hatch 2003). The challenge posed here is that of differentiating functional problems resulting from ageing and damage. While the effects might be similar, ageing does not constitute damage. Another fundamental challenge remains that of accelerated damage. Monitoring normally occurs routinely, within specified timeframes. Accelerated damage might occur between the monitoring sessions causing immense structural destruction. Accelerated damage might not be anticipated by the people constructing the structure and this necessitates the need for SHM, a system that could detect effects of accelerated damage (Calomfirescu, 2008). Another major challenge for SHM comes when utilising monitoring sensors. The challenge lies in providing or developing sensory devices which cannot get damaged. The sensor devices remain prone to damage and their failure could be catastrophic. Failure by these devices to detect damage could mislead engineer in believing that the structures’ health remains perfect. The inability to get failure-free sensor devices means that physical observation and analysing of the structure remains the only sure method of performing Structural Health Monitoring. Sensors can however be utilised when performing physical monitoring of structures as they can help reduce the monitoring duration while providing accurate observations. Installation of sensors into the structures during construction can be overcome through observation using sensory devices. WAVE –BASED The wave based method utilises the use of waves with specified frequencies in identifying defects within plates. Utilisation of this method creates a simulation of the structures being analysed and engineers can be able to make estimations of any expected negative features. The wave based analysis is commonly applied at the designing stages of engineering structures as it remains helpful in making future predictions (Hal et al. 2003). This method continues to be extensively utilised in designing stages of structures as it allows designing engineers to create simulations of the actual desired structures, intended for construction. Steady state deformations in structures immensely improve the utilisation of this method in the development of engineering structures. This application, however, remains largely limited to low-frequency applications as the accuracy in measurement decreases in higher frequencies (Chen, 2007). Evaluation of the wave based method Non-exact shape functions become expressed in field variables within the elements domains. The method falls short of efficient prediction formula in mid-level frequencies, and deterministic prediction methods are utilised in these circumstances. This causes engineers to subdivide construction into several sub-domains aiming at eliminating the challenges posed by mid-level frequencies. This method provides relatively small matrices, which consequently result in increased computational efficiency when utilising this method (Hal, et al., 2003). The increased computational efficiency can be utilised when dealing with mid-level frequencies in wave based methods. The accuracy of this method, however, remains limited to relatively small models and computation. The estimation and establishment of damages using this method highly depends on threshold frequencies of materials. The frequencies determine the maximum thickness of materials that can be tested. Wave based methods can only test aluminium materials of up to 0.02m thickness. When designing aluminium structures with thickness exceeding 0.02m, several plates can be merged to create the desired thickness (Chen 2007). This allows engineers to maintain the maximum thickness testable using wave-based methods. Longitudinal waves remain the commonly applied waves in the testing of aluminium based structures. This limitation of thickness in materials can be overcome using aluminium alloys that introduce physical changes to the material, while maintaining some essential characteristics like malleability. Wave based method can be applied when coupling 3 aluminium plates within structures. The accuracy of wave-based methods under these circumstances remains higher than that of finite elements. In double planes, estimations can be easily made using the wave-based methods. This, however, poses the danger of making surmountable approximation errors on the final result, compromising prediction accuracy (Hal et al. 2003). These predictions, however, remain similar to those of finite elements. Differences occur at 157Hz where wave-based method cannot accurately predict the approximation. This discrepancy could be attributed to the high frequencies which prove difficult for wave-based method approximations. Accurate prediction at such high frequencies could entail bending of linear-dependent wave functions, which could present adverse effects to the final results (Sotiropoulos, 2001). The inability to approximate the values correctly is attributed to the necessity to bend wave functions, consequently limiting their ability to give accurate prediction of structural positions. Challenges of wave based method The challenge caused by mid-frequencies and high-frequencies can be solved through application of the deterministic, numerical approach based on Trefftz approach. The wave based method commonly applies wave functions utilised in expanding the pressure function, aimed at correctly predicting outcome in mid-frequencies. The ideology of utilising wave functions effectively reduces “numerical stress” resulting from numerical approximation (Chen 2007). The formulation of this method provides engineers with a perfect formula for solving steady-state acoustic problems, prevalent in structural designing process. While this method remains an approximation method, the correctness and accuracy could be tested using numerical simulation. A commercial package called COMSOL, utilising finite elements is used for testing purposes. LAMB WAVES Lamb waves could be explained as elastic waves, with particle motion that lies directionally perpendicular to the plate, and are present in solid plates. The sets of lamb waves present in plates contain velocities dependent of wavelengths and plate thicknesses (Su & Ye 2009). Increased research, since the discovery of these waves, has seen the engineering profession utilise lamb waves in numerous non-destructive testing procedures. The different lamb waves remain constrained to the elasticity of surfaces guiding them. The properties of the surfaces have immense impact on the characteristics displayed by lamb waves (Lu, 2006). These waves are guided by the boundaries of materials propagating them. This characteristic remains fundamental towards the utilisation of these waves in understanding structural geometry. Evaluation of lamb wave Lamb waves result from multiple reflections and change of mode in longitudinal waves and shear waves on plate surfaces (Calomfirescu 2008). The figure below illustrates the formation of lamb waves with particle displacement represented by x and y directions in the illustration. Figure 2 A schematic representation of coordinates and plate in lamb wave formation (Ryden, et al., 2004) Velocity dispersion appears commonly in lamb waves due to the elasticity of materials carrying the waves, and material density. This knowledge remains essential in the study of the characteristics of lamb waves (Calomfirescu, 2008). Physically, the plate thickness to wavelength ratios remains a fundamental parameter in understanding the wave behaviour exhibited. Determination of effective stiffness and wave velocity depends on the thickness-wavelength ratio. While this knowledge might be available, computation and calculation of the results requires numerical methods, hence becoming cumbersome and tiring. With the introduction of computers, this problem appears to have been solved and the application of lamb waves in non-destructive structural testing immensely improved. Lamb waves could be utilised in different structural analysis including assessing damages and characterising flaws. When assessing changes in material attributes acousto-ultra sonic testing procedure is applied in providing detailed information regarding imminent damages in engineering structures (Sotiropoulos 2001). Lamb waves have the capability to irradiate large plates and propagate through substantial distances, consistently. This makes them best suited for these tests when performing acousto-ultra sonic testing. Ultrasonic testing utilises the higher-order modes while acoustic testing commonly uses the zero-modes. Proper study and understanding of the wave characteristics remains essential in delivering accurate results regarding lamb wave tests. Skilful utilisation of this knowledge could deliver highly accurate analysis of structures. When testing aluminium materials lamb wave oscillations increase at regions with notable crack damages (Lu 2006). Damages can be evaluated through comparison of subsequent data in relation undamaged conditions of aluminium plates. Any area presenting increased oscillation could indicate presence of crack damage. However, since lamb waves depend on differences in oscillations, detecting corrosion sports and dents in aluminium structures remains difficult. The waves remain contained within the material and unless there is a break, the waves continue flowing through. Classification of Lamb waves Lamb waves can be classified according to existing frequencies available. Zero-order modes are those that contain frequencies recorded at zero. These waves are extremely important since this category can exist within the entire frequency spectrum available. The prevalent characteristics of these waves, however, keep changing at different frequencies. Zero-order models can be classified into symmetrical (S0) and antisymetric (a0) orders (Calomfirescu, 2008). Symmetrical order is characterised by waves travelling at plate velocity in low-frequency, with notable velocity changes with frequency increase. In these waves, aluminium plates stretch in the propagation direction while contracting in thickness direction. This mode is also called extensional mode because of the stretching and compressing effects (Lu, 2006). The antisymetric mode appears highly dispersive at low frequencies, with velocities being proportional to frequency square roots. As the frequencies increase, these relationships start breaking down with the velocities converging towards Rayleigh velocity. A plate thickness of more than three wavelengths could result in regeneration of the waves and becoming Raleigh waves. The propagation of these modes occurs as indicated in the figure below Figure 3 propagation of Asymetric and Symetric lamb wave modes (NDT, 2012) Higher-order modes of lamb waves occur with the increase in frequencies, but the zero-order waves never disappear. These orders occur when the resonant frequency of specified plates is reached. Their occurrence and existence remains subject to resonant frequency as they occur at resonant frequency, and exist at frequencies above. Being conditional waves, they can be clearly observed under favourable conditions but slight changes to the conditions causes overlapping, making the waves undistinguishable (Su & Ye, 2009). Once they occur, these waves don’t have upper frequency limitations. This characteristic makes these waves highly significant in the analysis of thick plates. These waves could be utilised in huge steel structures due to their lack of upper frequency limitations, which render most waves useless at high frequencies. Challenges of utilising lamb waves The dispersion characteristic of lamb waves enables them to be utilised in analysing numerous materials, because dispersion is not affected by the interface medium. The geometry of structures being studied however should be very compact. The challenge in utilising lamb waves occurs in irregular shaped structures where dispersion becomes affected by the structural shape (Calomfirescu 2008). This limitation, however, can be utilised in characterising individual flaws in structures being tested. Flaws in structural systems normally scatter the impinging waves prompting detection of the existing flaws. The generation of different wave modes remains a major challenge in the non-destructive structural testing process using lamb waves. Since these waves generate themselves at certain frequencies, creating conducive environment for the generation of some waves becomes a surmountable challenge (Su & Ye, 2009). For these waves to give accurate results, the conditions of their existence must be properly monitored, the challenge at this stage becomes controlling the occurrence of other waves. While maintaining the existence conditions for a specified higher-order mode, these conditions might become conducive for the generation of another wave. Overlapping could occur and effectively limit the capability to clearly observe certain wave. PIEZOELECTRIC TRANSDUCER PZT Piezoelectric Transducer utilise the principle of piezoelectricity – electric charge which accumulates in certain solids. This electric charge results from pressure applied on these solids containing the charge. Piezoelectric effect can be defined as electromechanical interaction occurring in electrical and mechanical states of crystalline materials (Arnau & Vives 2008). This effect is reversible in these materials exhibiting the characteristics, and the mechanical energy can be converted to electrical energy and vice versa. The conversion of energy using transducers can take various forms as they instruments are not limited to electromechanical energy. Evaluation of Piezoelectric transducers PZT The systems utilised in PZT contain sensory devices attached to the systems. The major function performed by the sensors involves the detection of pressure changes (mechanical energy), and convert this to electrical energy, which gets displayed in a gauge within the system (Moheimani & Fleming 2010). In structural monitoring the transducers used perform both detection and action creation regarding energy changes. These transducers convert mechanical energy into electrical and vice versa. When the systems sense pressure differences within structural components, a gauge normally moves, indicating presence of differences. The components of transducers that provide actions are called actuators piezoelectric transducers have the capability to effectively convert electrical energy into sound. During engineering structural monitoring, the transducers utilised are piezoelectric models. These have the capacity to convert electrical energy into sound. When the transducer sensors detect mechanical energy, this effect is relayed instantly to the actuator in the form of electrical energy. Being piezoelectric in nature, the transducer converts electric energy into sound. This process enables the system to produce some alert sound upon detection of mechanical energy (Safari & Akdogan 2008). The mechanical energy recorded normally results from changes in structural pressure at different points. The point where these changes become notable can therefore be examined using other elements to determine the nature of pressure. The active transducer element’s thickness is determined according to desired transducer frequency. These two parameters remain inversely proportional, with high frequencies requiring thinner active elements. Frequencies emitted by transducers entirely depend on backing material used in manufacturing the transducer (Arnau & Vives, 2008). Figure 4 Cross-section of a typical Contact transducer (Arnau & Vives, 2008) Ultra sonic transducers might be preferred over contact transducers when testing relatively thin structural materials like aluminium. These transducers produce high accuracy in thin materials within structures as opposed to thick structures constructed using other materials like steel or concrete. Types of transducers 1. Contact transducers – as the name suggests these transducers are utilised in area where direct contact with the structure being analysed could be established. Figure 4 shows a cut-away section of a typical contact transducer. 2. Ultrasonic transducers – these are utilised in situations where analysis must be contacted from a distance. This could be attributed to the production of sound which can be utilised in conducting analysis from long distances (Wang & Tang 2008). 3D LASER VIBROMETER The 3d laser vibrometer is non-contact instrument used in sensing vibrations within structures. This vibrometer measures all three linear velocity components of vibrating structures from a single structural location. The machine can easily analyse different vibrating areas of a single vibrating structure. This vibrometer provides a wide range of measuring capability making it perfect for advanced application monitoring (Sharma 2009). The system comprises of different levels of vibrating controlling elements and an optical sensor for sensing movements and vibrations. These elements remain focused to a single point that is approximately 300mm from the main lens of the equipment. Characteristics of the vibrometer The optical sensor of the equipment comprises of independent sensors, each acting as a component of the optical sensor. These sensors are fitted with output beam inclines at 12 degree angles. This angular inclination enables the sensors to converge enough light, essential in presenting high-quality measurements of the desired vibrating components. Each sensor also contains a lens fitted at an angular inclination of 120 degrees from the plane. The narrow angles of the sensors remain sufficient in allowing beams passing through into environmental testing chambers inside the equipment (Bently & Hatch, 2003). The focal point of the equipment could be lengthened through narrowing the angular inclination of the sensors. Controller unit of the vibrometer has upper and lower sections which contain the basic vibrometer components. The lower chamber houses three laser modules and a power supply unit, while the upper one contains a signal processor with three channels (Sharma 2009). These vibrometer channels perform the functions of generating the analogue velocity outputs required for making fundamental calculations. The system also contains three velocity decoding modules and input modules which perform the function of demodulating Doppler signals, once received (Oliver, 2000). The bandwidth of applications determines the velocity decoding and input modules necessary in undertaking the project. The equipment is fitted with range selection knob utilised in controlling the velocity of the channels. The figure below represents a typical 3d laser vibrometer. Figure 5 compact 3D laser vibrometer (Oliver, 2000) Advantages of 3D laser vibrometer This system remains highly advantageous compared to other systems performing similar functions. Being three dimensional, the system provides measurements in various dimensional vibration velocities simultaneously. This enables quick assessment of structures as orthogonal vibration velocities are measured in an instance, and the system has no mass loading. When using this system, the numerous geometrical calculations are delivered in real-time as calculation occurs simultaneously with the information gathering process (Oliver 2000). Due to the numerous sensors fitted into the system, the system becomes highly sensitive and does not require reflectivity enhancement for accurate results. The range of frequencies applicable within the system is wide, hence the system could be utilised in wide variety of applications. The efficiency of this system remains notably high compared to other systems because it combines several technological options. The system provides three different velocity ranges that enable engineers to get high resolution identifications. This effectively increases the accuracy of the information that people retrieve using the 3d laser vibrometer technology (Sharma, 2009). Though the efficiency of the system remains high, its utilisation in immobile structures fails to produce results of equal accuracy. The system measures vibrations produced by engineering structures, hence structures without vibrations could be difficult to monitor using the system. This system produces minimum noises increasing its extensive utilisation in various structural monitoring processes. Reduced noise level make this method the preferred choice in conducting evaluations of structures located in areas with noise control. References Arnau, A & Vives, A 2008, Piezoelectric Transducers and Applications, New York, Springer. Balageas, D, Fritzen, C & Guemes, A 2006, Structural Health Monitoring,Wlitshire, ISTE Ltd. Bently, D & Hatch, C 2003, Fundamentals of rotating machinery diagnostics, New York, ASME Press. Calomfirescu, M 2008, Lamb Waves for Structural Health Monitoring in Viscoelastic Composite Materials, Berlin, Logos. Chen, Y 2007, Vibrational and Guided Wave Based Approaches for Quantitative NDE of Adhesive Composite Joints, Lincoln, University of Nebraska. Farrar, C & Worden, K 2007, ‘An introduction to structural health monitoring’, Philosophical transactions of the royal society, Vol. 365, pp. 303-316. Fassois, S & Sakellariou, J 2007, ‘Time series methods for fault detection and identification in vibrating structures’, Philosophical Transactrions of the Royal Society, vol. 365, no. 1851, p. 411–448. 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Sharma, V 2009, Laser Doppler Vibrometer for Efficient Structural Health Monitoring, Ann Arbor, ProQuest LLC. Sotiropoulos, D 2001, Iutam Symposium on Mechanical Waves for Composite Structures Characterization, Dordrecht, Kluwer Academic Publishers. Su, Z & Ye, L 2009, Identification of damage using Lamb waves : from Fundamentals to Applications, Berlin, Springer. Wang, K & Tang, J 2008, Adaptive Structural Systems with Piezoelectric Transducer Circuitry, New York, Springer. Read More