Key Engineering Materials Vol. 558

Abstract: Monitoring the healing of long bones has been studied extensively to reduce the period of encumbrance and unnecessary pain for patients suffering from fractured bones. This is more critical for unstable fractures in the pelvis as the patients can bedridden for up to 12 weeks to allow proper healing to take place. Current methods employed to monitor long bone healing are insufficient for applications in the pelvis as the human pelvis presents a significant change in geometry which demands a different approach. This paper explores an approach where vibration analysis is used to provide in-situ monitoring of a healing fracture in a human pelvis. Experimental tests were conducted on 4^th generation synthetic pelvises instrumented with an array of PZT sensors. The synthetic pelvises were cut at the sacrum to simulate a fractured pelvis followed by the application of araldite epoxy to simulate healing by allowing the epoxy to cure. Measurements were collected from the sensor array over the curing period to obtain the transfer functions (TFs) for various excitations. An impact hammer was utilised to obtain powerful broadband excitations while the PZT sensors were used to detect the response in the synthetic pelvis as a results of these excitation signals. A comparison of TF against cure time (healed amount) indicates the presence of a significant relationship with the stiffness recovery of the epoxy at the cut of the synthetic model.

Abstract: This paper reports on findings that extend previous work for the purpose of in-situ structural health monitoring of defects on the blind side of open holes using plate waves. A series of computational studies is presented to understand how and why the ultrasonic scattered wave field can be detected on the accessible surface. The uniqueness of these findings is that the length-scale of the defect and the incident waves are comparable. The combination of the experimental-computational-analytical approach gives rise to new insights and guidance for the quantification of defects located in hard-to-inspect regions of future unitised metallic and composite structures. The outcomes advance the knowledge base of inspection of hard-to-access regions with actuators and sensors placed in easily accessible locations.

Abstract: The incorporation of in situ structural health monitoring is currently an after-thought used to address critical areas identified in testing or service. This paper reports on a series of analytical/experimental work seeking to demonstrate the implementation of in situ structural health monitoring (iSHM) at the design stage of critical structures. This work is intended for the design of future generation aircraft. The work presented describes a systematic redesign scheme based on Lamb wave technology. The results demonstrate a strong possibility that such a system is effective and feasible and comes at a tolerable cost to the structure. To demonstrate the efficacy of this proposed design scheme, a series of experimental results will be presented using the fatigue critical location of structure representing the lower wing skin of an aircraft structure as a test case.

Abstract: Health monitoring of civil infrastructure systems has recently emerged as a powerful tool for condition assessment of infrastructure performance. With the widespread use of modern telecommunication technologies, structures could be monitored periodically from a central station located several kilometres away from the field. This remote capability allows immediate damage detection, so that necessary actions are taken to reduce the risk. Optical fiber sensors offer a relatively new technology for monitoring the performance of spatially distributed structures such as pipelines. In this regards, several commercially available strain and temperature sensing equipment such as discrete FBGs (Fibre Bragg Gratings) and fully distributed sensing techniques such as Raman DTS (distributed temperature sensor) and Brillouin Optical Time Domain Reflectometry (BOTDR) typically offer sensing lengths of the order of 100 km's. Distributed fiber optic sensing offers the ability to measure temperatures and/or strains at thousands of points along a single fiber. In this paper, the authors will give a brief overview of these optical fiber technologies, outline potential applications of these technologies for geotechnical engineering applications and experience in utilising BOTDR in water pipeline monitoring application.

Abstract: Piezoelectric materials such as lead-zirconate-titanate (PZT), lead-metaniobate, and piezo-composites are the materials of choice for acoustic imaging in medical diagnosis as well as underwater ultrasonic microphones and underwater sonar. PZT materials have the advantage of having high electro-mechanical coupling, low internal losses and excellent environmental durability. Nonetheless, in order to improve energy transmission the high acoustic impedance of piezoelectric ceramics needs to be matched to the lower acoustic impedance of biological tissues and water. For actuators resonated in their thickness mode, energy transmission can be improved by means of intermediate layers of material of carefully selected thicknesses and acoustic properties. Sometimes a backing layer is also added to the back of the actuator to damp the acoustic back-wave. The process of making these types of transducers is generally costly due to the nature of the manufacturing process and the required level of accuracy. This paper describes an inexpensive method of manufacture low-cost, low-impedance, piezoelectric transducers. The fundamental physical principles behind this new type of sensor-actuator, as well as various examples of imaging low-impedance targets using a prototype of this newly developed sensor-actuator system will be presented.

Abstract: Acoustic Emission (AE) monitoring is an effective tool for prioritising more conventional non-destructive inspections of above ground tanks, either process or storage vessels. Results obtained from AE monitoring may be assessed against an existing data bank permitting plant management to prioritise maintenance to tanks with high active corrosion rates or leaks. AE results may be used to extend inspection periods of tanks without leaks or active corrosion.This paper briefly covers the theory of AE leak detection and corrosion monitoring including some background research into the difference between AE from corrosion and leaks. It looks at the equipment used and provides a summary of tank test results conducted by ATTAR and other overseas organisations as well as reviewing AE tank inspection economics.

Abstract: This paper presents a dynamically reconfigurable multivariable Micro-Electro-Mechanical Systems (MEMS) sensor array, capable of reconfiguration in real time, to meet the sensing demands of unattended systems operating in highly variable environments, with an emphasis on maintaining operation of these systems in the presence of structural damage. This array is comprised of multiple instances of identical sensors which can be dynamically reconfigured to target a variety of measurands including acceleration, rotational rate, magnetic fields, temperature, air pressure and density. A simulated environment is used to illustrate how the array can be dynamically reconfigured to respond to variations in several of these parameters. Also shown are simulations that demonstrate the ability of such a sensor array to continue operation in the presence of structural damage.

Abstract: This paper reports on the multiphysics modelling of a bi-axial vibration energy harvesting (VEH) approach, with experimental validation of the model predictions. The authors have developed a harvester able to generate voltage under bi-axial vibrations. The harvesting approach is based on a magnetoelectric (ME) transducer that is positioned between a fixed magnet and oscillating ball bearing, which steers a changing magnetic field through the transducer to generate a voltage. The transducer combines magnetostrictive and piezoelectric properties to convert magnetic potential into electrical energy. Analytical modelling of this phenomenon is difficult due to the highly coupled nature of this interaction, so Comsol multiphysics software is used to make predictions of output using the finite element method (FEM). A peak open-circuit harvester voltage of 39.4 V is predicted for a ball bearing oscillating with 4.5 mm amplitude, agreeing reasonably well with measured harvester voltage of approximately 35 V. The modelling is applied to a two-dimensional representation of the system, which is shown to be sufficient for a basic understanding of the highly coupled nature of interactions, and a basis for optimising the magnetoelectric vibration energy harvesting approach.

Abstract: This paper investigates the optimisation of wire-coil transducers for a recently described strongly nonlinear electromagnetic (EM) vibration energy harvester, by coupling previously derived dynamics of the mechanical system with finite element analysis (FEA) to determine the harvesters EM response. The harvester is implemented in a permanent-magnet/ball-bearing arrangement, where vibrations in a host structure induce oscillations of the ball-bearing. The movement of the bearing changes the magnetic flux in a circular pancake wire-coil, inducing an electromotive force (EMF) in the coil and hence a voltage in the harvester circuit. A quintic-modified Duffing equation is applied to predict frequency-displacement relations for the nonlinear dynamics of the harvester. Faradays Law of Induction is implemented with quasi-static FEA modelling of the magnetic field and linked to the dynamics of the system to develop a numeric model for voltage predictions. The issue of back-EMF and damping is also investigated. A fully integrated mechanical-electromagnetic model is shown to compare well to the quasi-static numerical model. The output characteristics of the prototype harvester are then compared with the numerical model. An optimal coil height of 2 mm is predicted, and demonstrated experimentally to produce 20.3 mW from a 12 Hz, 500 milli-g host vibration. Further investigation of coil inner radius and outer radius yields a predicted resistive load power transfer increase of 18% with the optimal coil geometry.

Abstract: The application of Lamb waves to damage and/or defect detection in structures is typicallyconfined to lower frequencies in regimes where only the lower order modes propagate in order to simplifyinterpretation of the scattered wave-fields. Operation at higher frequencies offers the potentialto extend the sensitivity and diagnostic capability of this technique, however there are technical challengesassociated with the measurement and interpretation of this data. Recent work by the authorshas demonstrated the ability of fibre Bragg gratings (FBGs) to measure wave-fields at frequencies inexcess of 2 MHz [1]. However, when this work was extended to other thinner plate specimens it wasfound that at these higher frequencies, the cyanoacrylate adhesive (M-Bond 200) used to attach theFBG sensors to the plate was significantly affecting the propagation of the waves. Laser vibrometrywas used to characterise the wave-field in the region surrounding the adhesive and it was found that theself-adhesive retro-reflective tape applied to aid with this measurement was also affecting the wavefieldin the higher frequency regime. This paper reports on an experimental study into the influence ofboth of these materials on the propagating wave-field. Three different lengths of retro-reflective tapewere placed in the path of Lamb waves propagating in an aluminium plate and laser vibrometry wasused to measure the wave-field upstream and downstream of the tape for a range of different excitationfrequencies. The same experiment was conducted using small footprint cyanoacrylate film samplesof different thickness. The results show that both of these surface-mount materials attenuate, diffractand scatter the incoming waves as well as introducing a phase lag. The degree of influence of thesurface layer appears to be a function of its material properties, the frequency of the incoming waveand the thickness and footprint of the surface layer relative to the base material thickness. Althoughfurther work is required to characterise the relative influence of each of these variables, investigationsto date show that for the measurement of Lamb Waves on thin structures, careful considerationshould be given to the thickness and footprint of the adhesive layer and sensor, particularly in the highfrequency regime, so as to minimise their effect on the measurement.