W.H. Ong

Enhancement of Lamb wave–based in situ structural health monitoring through guided local geometry changes

The incorporation of in situ structural health monitoring is currently an afterthought used to address critical areas identified in testing or service. This article investigates the use of local geometry changes to improve detectability of damage in the form of a line crack at a given fatigue critical location. A methodology encompassing especially developed Lamb wave tools is presented, which may allow the implementation of in situ structural health monitoring at the design stage of critical structures. A systematic redesign will be presented based on Lamb wave technology, which has been enabled by high-speed ray-trace modelling. By modelling propagation of wavefronts as rays, the high cost of iterating designs in finite element or experimental methods can be overcome. Implementing the ray-trace modelling, optimized design changes lead to improved damage sensitivity. Damage sensitivity has been quantified through well-used scatter/difference techniques. This was followed by analysing the strength and stiffness of the redesigned structures to reveal any consequential changes. To demonstrate the efficacy of this proposed methodology, a case study is presented where the fatigue critical location of structure representing the lower wing skin of an aircraft structure is analysed.

Numerical modelling of scattered Lamb waves through varied damage size in challenging geometry

Lamb waves have been accepted as a potential technology for damage detection. However, industry has not adopted the technology due to the fundamental nature of current studies. This article attempts to bring together several areas of knowledge to make a step towards adoption. Commonly, readings from damaged specimens are compared with baseline readings to establish damage. A quantitative approach (scatter calculation) is taken in this article to reveal areas or sensitivity and possible quantification of damage. This method is applied to displacement fields collected through laser vibrometry and numerical simulations. The numerical simulations are performed alongside to assess their usefulness and feasibility when paired with scatter calculations. Analysis of two specimens is presented here: an idealised flat plate and representative aircraft lower wing skin are progressively cut to investigate their response to damage. Despite similar crack-actuator placement in both specimens, the damage-sensitive areas are significantly impacted by geometry changes. This leads to a shift in optimal location of sensors and corresponding prospects of damage quantification.

Determination of the State of Strain of Large Floating Covers Using Unmanned Aerial Vehicle (UAV) Aided Photogrammetry

Floating covers used in waste water treatment plants are one of the many structures formed with membrane materials. These structures are usually large and can spread over an area measuring 470 m × 170 m. The aim of this paper is to describe recent work to develop an innovative and effective approach for structural health monitoring (SHM) of such large membrane-like infrastructure. This paper will propose a potentially cost-effective non-contact approach for full-field strain and stress mapping using an unmanned aerial vehicle (UAV) mounted with a digital camera and a global positioning system (GPS) tracker. The aim is to use the images acquired by the UAV to define the geometry of the floating cover using photogrammetry. In this manner, any changes in the geometry of the floating cover due to forces acting beneath resulting from its deployment and usage can be determined. The time-scale for these changes is in terms of weeks and months. The change in the geometry can be implemented as input conditions to a finite element model (FEM) for stress prediction. This will facilitate the determination of the state of distress of the floating cover. This paper investigates the possibility of using data recorded from a UAV to predict the strain level and assess the health of such structures. An investigation was first conducted on a laboratory sized membrane structure instrumented with strain gauges for comparison against strains, which were computed from 3D scans of the membrane geometry. Upon validating the technique in the laboratory, it was applied to a more realistic scenario: an outdoor test membrane structure and capable UAV were constructed to see if the shape of the membrane could be computed. The membrane displacements were then used to calculate the membrane stress and strain, state demonstrating a new way to perform structural health monitoring on membrane structures.

Three-dimensional laser scanning vibrometry as a visualisation tool for structural health monitoring research

Abstract The application of three-dimensional scanning laser vibrometry for visualising stress wave propagation in a specimen with a non-surface penetrating defect is presented in this paper. The aim of this paper is to demonstrate the use of this measurement technique to achieve a good insight of the propagation of the stress wave over a given region on a test specimen. An investigation into the interaction of the various wave modes with the defect in the structure can be enhanced with this knowledge. The ability to acquire the time series at an array of measurement points in a given region on the structure provide for an excellent means for visualising the propagation of the incident stress and how they interact with the defect present. In addition to the visualisation aspect, the data acquired can also be transformed to assess the modal content of the various wave modes present within the structure. For the purpose of this paper, a flat aluminium test plate with a non-surface penetrating defect will be used. The application of this form of non-contact measurement technique for the stress wave visualisation work will be illustrated. It will be shown that this measurement technique can be used to establish the modal content and the time-development of the incident and the scattered wave modes in the test specimen.

Numerical Simulation of Vibrational Methods for the Healing Assessment of an Internally Fixated Femur

Current bone healing monitoring techniques (X-ray, CT, manual manipulation) are subjective and qualitative. A quantitative monitoring technique for an internally fixated femur based on vibrational techniques is investigated. An intramedullary (IM) nail fixated femur will be at the focus of the investigation. The femur and fixation were modelled and their structural dynamics were solved using finite element analysis. This allowed visualisation of various mode shapes and identification of those most sensitive to healing. It was found that torsional modes were most sensitive to the stiffness change associated with healing. This data was then used as a guide for the placement of simulated accelerometer sensors and the simulated excitation point to predict the cross spectrum data that would be obtained during an experiment. Upon analysis of this data the torsional mode suggests that a relationship can be drawn between the natural frequency of the torsion mode and the state of health

Healing Assessment of Fractured Femur: Orthopaedic SHM

Bone fractures are fixated to facilitate healing. The fixations used are broadly classified as internal and external fixation. External fixations are often used as a temporary means to stabilise the fracture prior to the application of the internal fixation. However, in the case of injuries suffered during combat, external fixations have been reported to be used as definitive treatments. When fixated, the healing process of the fractured region can take several months depending on the extent of the fracture. The ability to ascertain union at the fracture region is crucial to the patient’s return to normal duties. The assessment techniques available include X-ray and CTscans. This paper presents a set of findings to demonstrate the utilisation of the construction of an external fixation to integrate structural health monitoring concepts to facilitate the establishment of the state of union of a fractured femur. The paper will firstly provide a brief review of healing and union of fractured bones, the types of fixation used and the current methods of assessing healing and bone union. A set of experimental results will be presented to demonstrate the potential of integrating structural health monitoring concept in the overall clinical management of fractures. doi: 10.12783/SHM2015/178

Designing for Lamb Wave Based In Situ Structural Health Monitoring

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.

Damage Monitoring in Realistic Structures Using Lamb Waves

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.