Papers by Author: Wing Kong Chiu

Abstract: This work presents a computational investigation into the scattering of edge guided waves travelling by a notch. To establish a good understanding of this scattering phenomenon, the analysis was conductedon a range of length scales. The finite element analysis indicate that the edge guided surface waves are scattered by the presence of a notch which resulted in a SH₀-like appearance wave radiating into the medium. This can be mistaken as a mode conversion of the fundamental lamb modes or even a source at notch tips. The phenomenon becomes harder to notice at higher frequency as increasing the frequency decreases the speed and both the bulk and surface waves travel at identical speeds. A clear understanding of this interaction furthers our knowledge in one of the most prominent interaction in the study of acoustic waves for structural health monitoring.

Abstract: The scattering of a fundamental symmetric wave mode by a notch on the blind side of weep hole is described in this paper. It will report on findings obtained from computational simulations to determine the effect and interaction of the impinging waves with the defect on the open hole located on the blind side of the incident wave. The finite element simulation results showed mode conversions of fundamental modes, leaky edge waves on the circumferential surface and source-like diffractions radiating from the tip of the notch and hole. These findings highlight the potential of applying this wave phenomenon to quantify defect located hard-to-inspect areas by positioning actuator and sensor in accessible regions of metallic structures and is relevant to the development and improvement of current techniques in non-destructive inspection of metallic structures

Abstract: The ability to optimise structures requires a thorough understanding of the loadsthat they are subjected to. Many composite materials in use today are subjectedto complex loading patterns that exhibit multi-axial stress and straincharacteristics. It is not sufficient to model material data for thesestructures based purely on uniaxial information. Unlike the well understoodfailure criteria of materials when subjected to uniaxial loads; biaxial failureenvelope has not been defined with sufficient experimental data particularly inthe tensile load region. There has not been any standard specimen geometrydefined for biaxial testing. Also the effective area when subjected to loadingis not as easily known as is the case in uniaxial loading. Thus biaxial testspose a greater level of difficulty when establishing failure stresses for thematerial. The authors look at establishing a specimen geometry that is suitablefor use preliminarily in isotropic materials. These specimen geometries must beable to ensure failure at the anticipated gauge region. Investigation into thefirst quadrant of the biaxial failure envelope under tension-tension is lookedat with insight into matrix failure as the dominant mode of failure. Numerical resultsand preliminary experimental results for FM355 epoxy specimens are presentedand compared with existing failure models such as Von Mises criterion and thefailure criterion used in Strain Invariant Failure Theory.

Abstract: A review of some of the various fatigue models introduced over the years for both metallic materials, in particular aluminium alloys followed by fatigue and durability concerns associated with composite materials. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal structures to composite structures. With this brings the added concern of certifying new components as the damage mechanisms and failure modes in metals differ significantly than composite materials such as carbon fiber reinforced polymers (CFRP). The certification philosophy for composites must meet the same structural integrity, safety and durability requirements as that of metals. Hence this is where the challenge now lies. Substantial work has been conducted in the reparability of composite structures through bonding using various adherend thicknesses and joint types and has been shown to have higher durability than mechanically fastened repairs for thin adherends however these are currently unacceptable repair methods as they cannot be certified. Repairs are designed on the basis that the repair efficiency can be predicted and should be designed conservatively with respect to the various failure modes and include the surrounding structure.

Abstract: The use of composite materials as a replacement for commonly used metals such as aluminium and steel are increasing in the engineering industry, particularly in the aerospace sector. The move towards light weight and high stiffness structures that have good fatigue durability and corrosion resistance has led to the rapid move from metal to composites. This change allows for further flexibility in design and fabrication of various components and joints. There are three main categories of joints used in composite materials – mechanically fastened joints, adhesively bonded joints and the combination of the two called hybrid joints. In order to adequately understand the effectiveness of these joints, substantial testing and validation is required, particularly in the use of hybrid joints for real life applications. Static testing, load distribution and parametric studies of hybrid joints have been investigated by various researchers; however further work is still required in understanding the durability and fatigue of hybrid joints and ensuring that both the adhesive and mechanical fasteners can work together effectively in producing an optimum joint. Mechanical fastening alone in composite laminates is not a preferred joining method as they create high stress concentrations around the fastener holes. Adhesive bonding although has numerous benefits it is difficult to detect the bond defect particularly in cases where weak bonds can occur during applications and it is sensitive towards the environmental conditions. Thus hybrid joints are seen arguably as being more effective in joining composite components together and offer greater residual strength. Hence the performance, strength and long-term durability of these joints need to be further investigated and be applied to practical situations whilst assisting in repair certification.

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: 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: 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 describes a technique, quantifying the differences in waveforms between damaged and undamaged states to measure damage severity. The technique has been verified experimentally and demonstrated promise in detecting and quantifying subsurface defects. Finite element analysis (FEA) has been performed alongside as a valid model may avoid time consuming and expensive experiments. Testing is first performed on a simple flat plate and a realistic wing skin structure with geometry such as stiffening ribs. This revealed geometry will alter the regions of scatter which leads queries over optimal sensor location. It was also discovered actuator location may not necessarily be intuitive. The comparisons made here show that implementation of an ISHM system on real world subjects is feasible despite added complexity.