SUMMARYThis paper highlights the potential of integrating SHM concepts into an external fixator with the aim of using it to assess the healing of a fractured femur. A finite element analysis was first performed on the fixated femur to understand the response of the fixated femur in its fractured and healed state. The underlying mechanics will be used to establish a suitable monitoring strategy for assessment of healing. The results will be supported by a series of experiments using a fixated saw-bone femur, which has been cut to simulate fracture. The cut was filled with epoxy, and the curing of this epoxy was used to simulate healing of the fractured region. The findings lead to an actuation and sensing protocol able to determine the state of union of a fixated long bone. This methodology was tested further by using modelling clay to approximate the mass added by soft tissue surrounding the bone. These results illustrate that the integration of SHM and orthopaedic concepts can provide a quantitative measure that can be used in conjunction with existing techniques for the monitoring of the state of healing of the fracture. This represents a significant potential of reducing or eliminating the qualitative healing assessment.

A B S T R A C T A series of experimental investigations are performed to show the potential of integrating sensing elements into an external fixation for healing assessment of a fracture femur. This investigation is supported by finite element analyses that highlight the fundamental structural dynamics, which enable a stiffness-based healing assessment methodology. A saw-bone femur externally fixated with a Hoffman II will be subjected to an artificial fracture. The healing of the fractured femur is simulated with the curing of epoxy applied to the fractured region. The finite element analyses results will help determine the useful modes for assessing the state of healing of the fracture that can be attributed to the changes in the stiffness of the fixated structure. The findings will be tested against a set of experiments to show how the stiffness-related quantities can be delineated from the dynamic response. The results reported will show distinct changes to the frequency response functions as the epoxy cures demonstrating the potential of integrating sensors onto an external fixation for healing and fracture union assessment.Keywords bone fracture; bone healing assessment; structural health monitoring. N O M E N C L A T U R E(x,y,z) = Cartesian coordinate system ρ = density E = Young's modulus ν = Poisson's ratio f = frequency Z tf ( f ) = transfer function v force (t) = voltage time series from force transducer attached to impactor (load cell) v PVDF (t) = voltage time series from polyvinylidene fluoride sensor (film sensor) vˆf orce f ð Þ = voltage amplitude in frequency domain from force transducer attached to impactor vˆP VDF f ð Þ = voltage amplitude in frequency domain from polyvinylidene fluoride sensor ε(t) = strain time series ε s (f) = strain spectrum i(t) = input force time series Iˆf ð Þ = input force spectrum K force = force constant K PVDF = strain constant

Currently, structures are designed without structural health monitoring (SHM) in mind. It is proposed that SHM should be addressed at the design stage of new structures. This paper explores the benefit which can be gained from such considerations. The scope encompasses Lamb-wave-based SHM and a given fatigue critical location (FCL). Optimization is performed using specialised ray tracing. A case study is carried out using a specimen that simulates a hard-to-inspect region in a fuel vent hole in wings structures of aircraft. This work will report on the potential use of the focussing of stress wave to improve detectability of defect in this hard-to-inspect location. Following optimization, results are produced numerically and experimentally. The results revealed sensitivity to damage is nearly doubled while minimum detectable damage size is significantly decreased. As a result, this study brings together an assortment of specialised tools to form a workflow ready for implementation.

Lap joints are widely used across many critical structures such as aircraft and bridges. Lamb waves have long been proposed to monitor lap joints against defects such as disbonds. However, there are many challenges which must be answered to make use of Lamb wave technology. Frequency selection is often overlooked, and many authors will select a single frequency without knowing if other frequencies will result in better sensitivity. Another challenge is the features (mode conversion, attenuation, reflection) associated with damage are also inherent in a lap joint. This sharing of features can lead to confusion (false positive/negative) depending on the chosen damage detection strategy. Furthermore, almost all proposed methods require a baseline reading of the structure in its flawless state. Relying on a baseline reading can result in false positives due to shifts in sensor outputs caused by ageing and inconsistent environmental conditions. Instead of a baseline, this article proposes a technique which uses strategically positioned sensors to detect Lamb wave modes generated only in the presence of a disbond. The technique is first developed using a numerical study and then verified with an experimental study. Several frequencies are trialled and detailed in this article which shed light on the ideal frequency selection when using this method.

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.