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No description available.

Mechanical deformation of a metal is accompanied by the dissipation of energy in the form of heat as a result of thermodynamically irreversible processes occurring at the microscale. This is applicable to deformation in both elastic and plastic regime so long as the thermodynamically irreversible processes are activated. It follows that there is a possibility of identifying the condition of the material microstructure by evaluating the heat dissipated during deformation. In the thesis, the continuous temperature rise due to the heat dissipation in a material under cyclic loading is obtained using an infrared (IR) detector. Most metals dissipate a very small amount of heat in their elastic range (few mK.s-1). As a consequence, the temperature change is usually below the thermal resolution of the infrared detector used. To enable an accurate measurement to be made, the experiments were conducted in a specially designed setup which eliminated parasitic heat sources. Spatial averaging was used to improve the signal to noise ratio and the dissipative heat source was extracted from the thermal data using the thermomechanical heat diffusion equation. The spatial averaging technique successfully provided a consistent detection threshold of just under 1 mK.s-1. To demonstrate the effectiveness of the enhanced thermal resolution, the effect of material microstructure on the dissipative heat source was studied in 316L stainless steel. Different microstructures were produced by heat treating strip specimens to give a homogeneous field of observation over a large area of the IR detector. Monotonic tensile test and microhardness test were performed on each of the specimen to establish the change in properties resulting from the different microstructures. Micrographs were produced, which showed that the grain size only increased at the highest temperature, for the other heat treatment any difference in the dissipation would be mostly as a result of change in dislocation density. At equivalent stress levels, the microstructure had a significant effect on the dissipative heat source. In the vicinity of welds, material microstructures are inhomogeneous over relatively small areas. To capture possible spatial heterogeneities in the heat source, a 3D (2D in space and time) least square estimation of the temperature evolution was performed in place of the spatial averaging technique. The temperature data were fitted over a small window which is swept throughout the entire data set resulting in a spatial map of the dissipative heat source. The method was verified using a 'hole-inplate' specimen under tensile cyclic loading which has an inhomogeneous stress field and hence dissipation. The approach was then used on the data collected from a welded 316L specimen exhibiting an inhomogeneous microstructure but tested in nominally uniaxial stress. It was shown that the region corresponding to the base metal had the highest dissipation followed by a gradual decrease in the heat source across the heat affected zone. The heat source increases subsequently as the centre of the fusion zone is approached. A modified procedure employing higher spatial resolution focusing on the fusion zone also revealed differences in the heat source between different weld passes. Strain measurements made on identical specimens using digital image correlation verified the material properties local to the weld. The work in the thesis clearly demonstrates that the dissipative heat sources associated with microstructural behaviour in welds can be identified successfully using IR thermography.

The main focus in this work has been to improve the understanding of how the monitored structure affects the performance of guided wave acoustic emission systems. This was to address poor performance of an Airbus acoustic emission system when it was used to monitor a complex section of an aircraft wing during a fatigue test. To do this the whole acoustic emission system was modelled. The focus of the modelling effort was in two parts. The first was to define a suitable source for a fatigue crack in aluminium to use as an input to the model. This was found from the literature and compared with results from Airbus tests. The second part was to develop an approach to model the guided wave propagation in large structures. This led to the development of empirical transmission models that could be created with reduced effort compared to other transmission modelling techniques. These transmission models were deliberately conservative in their prediction of amplitude to ensure they could safely be used to determine which transducers would detect acoustic emission events at different locations. The whole system model could then be used to determine acoustic emission system performance for different scenarios. By varying the structure in the model its influence on system outputs such as detection and location of acoustic emission events could be demonstrated. Therefore a tool has been created to aid the future development and deployment of acoustic emission systems. There are two other major achievements in this thesis. The first is the development of an efficient method to collect guided wave data over large areas using a design of experiments based technique. The second is an analysis of results from a long term active guided wave structural health monitoring experiment. Understanding this behaviour is necessary for the further deployment of these systems.

The work described in this thesis investigates the effects that scaling has on key structural integrity concepts, namely, stress fields, stress intensity factors, and the J-integral. Scaled models are an attractive concept in scenarios where full scale testing is not possible, and they are used extensively in other engineering fields. Little research into the practical applications of scaling in a structural integrity context exist however, which provided the motivation for the work. Scaling laws for these three structural integrity parameters are developed, such that the load can be scaled, along with the geometry, while maintaining the parameter of interest. Two sets of experiments and their results are described, which consist of simple aluminium beams in four point bend configurations, to verify the scaling laws for stress fields and stress intensity factors, and to highlight practical issues surrounding scaling in real life. The scaling laws themselves do not take into account the effect of scaling on the other parameters. As each parameter follows a different law, and as all the parameters are capable of contributing to failure, it is shown that the scaling laws are not capable of describing the behaviour of a component for a complete range of scale factors. By extrapolating results, and with the use of failure assessment diagrams to visualise this, it is possible to see that depending on the geometry, material properties, and loading regime, there will come a point with which the failure mechanisms will change. There are certain conditions however, in which scaling is an appropriate and useful tool. For specimens where fracture occurs, if the small scale yielding conditions at the crack tip are maintained across the sizes, then scaled models can be reliably used to produce a model that accurately replicates the fracture conditions, and from which results from the scaled model can be transferred across to the full size. For the small scale yielding conditions to be maintained, the limitation will be on how small the scaled model can be made. Similarly for models where failure is due to the global stress field, scaling can be used provided this remains the dominant contributor to failure. Where there are stress concentrating features, care must be taken if the scaled model is larger in sizes than the original specimen, as this can tend towards small scale yielding conditions, and consequently a change in failure mechanism. Where these conditions are met however, then scaled models may confidently be used to replicate and further investigate the failure conditions of the original specimens. The case studies considered throughout the development of the scaling laws, and in the experiments, are relatively straightforward, and while representative of test specimens used in materials testing, they are not accurate representations of real components. A complex case study is finally considered, which relates the results and findings from the work to a real component, and subject to realistic constraints and boundary conditions. The case study consists of a parametric finite element study, which aims to replicate failure criteria in a scaled down component. The resultant models obtained are able to meet this criteria, however in do- ing so the geometry is altered, and drifts from what might be considered true scaling. No “all encompassing” scaling law is derived to describe how to produce the scaled component, and prior knowledge of the stress state is required for the parametric study. The methodology is deemed useful, however, for scenarios where full scale modelling is not possible, yet physical validation of the modelling methods are required.

This dissertation considers the fracture instability of nuclear graphite, specifically of isotropic Gilsocarbon, grade IM1-24, which acts as a structural component and neutron moderator within reactors. The presence of cracks within this graphite informs its behaviour and necessitates a study of fracture properties and instability. Amongst the factors studied, a major finding was that the size effect was the most prevalent. Two aspects of instability were also examined: the crack driving force or energy release rate and the fracture resistance or the incremental work of fracture. The conditions between the extremes of load control and displacement control affecting the energy release rate were studied, based on the compliance of the surrounding components or additional elastic material, generally known as elastic follow-up. The effects of elastic follow-up and specimen geometry on fracture instability was investigated in an idealised model. Two sets of experiments were presented to quantify the effect and to validate the idealised benchmark study. No measurable differences were exhibited at the equivalent degrees of elastic follow-up achieved in the experimental work. Additionally, the effects of load multiaxiality on the fracture of graphite were investigated. Despite the influence of load multiaxiality on fracture stress of graphite, there was little effect in post-peak fracture behaviour indicating the lack of influence on fracture stability. Moreover, to evaluate fracture resistance, this work investigated the crack growth resistance curves, KR and R. To produce these curves, a considerable number of experiments of cyclic load and unload, with crack propagation, is presented. Different sized compact tension specimens were tested, to investigate the size effect typically exhibited in quasi-brittle materials which describes the fracture behaviour of IM1-24. The rising KR and R-curve behaviour observed in all sizes, especially in the more distinct initial fracture stages of KR, can be attributed to the formation of a bridging zone in the wake of the propagating crack. A mismatch between the scaling of the fracture process zone and the specimens was also exhibited, evident from the considerable differences in apparent toughness KQ as well as the linear elastic contributions to the work of fracture. The results indicated that the fracture stability of IM1-24 graphite is only marginally affected by elastic follow-up, whilst size effect is a more prominent contributor.

Suppression of undesirable vibrations is critical to ensure good dynamic performance of engineering structures. Vibration suppression problems can be categorised in terms of vibration behaviour, such as transient, self-excited or steady-state vibrations. Passive vibration suppression methods are widely adopted due to their inherent advantages, such as simplicity and reliability. Additionally, the introduction of the inerter has fundamentally expanded the achievable performance of passive vibration suppressors. Appropriate methodologies for the design of optimal passive vibration suppressors are needed to tackle particular problems. This thesis develops systematic methodologies to design both configurations and physical arrangements of passive vibration suppression devices, solving a variety of vibration problems. Also, this work demonstrates the effectiveness of inerter-based devices in each case. A transient vibration suppression problem is studied first, and considers stable shimmy oscillations in an aircraft main landing gear (MLG). Both frequency-domain and time-domain approaches are adopted with the frequency-domain method considered less effective since the system mode shapes vary significantly when certain devices are added. Several beneficial inerter-based configurations are identified with the proposed time-domain methodology. A second example of a transient vibration problem includes an initial impact excitation; here the aircraft landing touch-down process. Following the approach established, beneficial shock strut configurations have been identified. However, it has been found that the amount of energy dissipated is unsatisfactory. To address this, an additional constraint on energy dissipation is considered, leading to an absorber with double-stage stiffness being proposed. The instability of self-excited vibrations can be avoided with suitable vibration suppressors. To this end, a design methodology of selecting the device parameter values is proposed. The nonlinear MLG shimmy phenomenon is studied here. A bifurcation study is performed to investigate the effects of the shimmy-suppression devices on the MLG dynamics. It shows that the utilisation of a specific proposed spring-damper configuration results in improved robustness against varying aircraft operation conditions over a traditional shimmy damper. The benefits of two inerter-based configurations are also demonstrated by enhanced device robustness. The steady-state vibration suppression of a hydraulic engine mount has also been analysed, and a design approach for determining the optimal physical device arrangement has been developed. It enables optimisations of all possible networks with predetermined number and types of fluid passageways, through which the fluid can pass between two chambers in the engine mount. To improve the performances over a wide range of frequencies, linear optimal designs are identified whilst the manufacturing limitations are also considered. Furthermore, optimal parametric design is conducted considering nonlinear behaviour in fluid passageways.

This thesis is concerned with the application of porous treatments as a means of flow and aerodynamic noise reduction. An extensive experimental investigation is undertaken to study the effects of flow interaction with porous media, in particular in the context of the manipulation of flow over a flat plate and past the blunt trailing edges. Comprehensive boundary layer and wake measurements have been carried out for a long flat plate with solid and porous blunt trailing edges. Unsteady velocity and surface pressure measurements have also been performed to gain an in-depth understanding of the changes to the energy–frequency content and coherence of the boundary layer and wake structures as a result of the flow interaction with a porous treatment. The interaction of the flow with the porous substrate was found to significantly alter the energy cascade within the boundary layer. Results have shown that permeable treatments can effectively delay the vortex shedding and stabilize the flow over the blunt edge via mechanisms involving flow penetration into the porous medium and discharge into the near-wake region. It has also been shown that the porous treatment can effectively destroy the spanwise coherence of the boundary layer structures and suppress the velocity and pressure coherence, particularly at the vortex shedding frequency. The flow–porous scrubbing and its effects on the near-wall and large coherent structures have also been studied. The emergence of a quasi-periodic recirculating flow field inside highly permeable surface treatments has also been investigated. This study has identified several important mechanisms concerning the application of porous treatments and paves the way for further investigation into the interaction of the porous media with different flow fields and development of tailored porous treatments for improving the aerodynamic and aeroacoustic performance of different aero- and hydro-components.

No description available.

Robotic involvement is envisaged for exploration of human-inaccessible areas such as planetary space, confined and unstructured environments, and radioactive places. An exploration mission usually includes multiple tasks that are difficult or even impossible to finish using a single robot. Modular robots aim to solve this problem by providing a robotic system wherein robotic modules can be reconfigured to accomplish diverse tasks. In this work, research is undertaken on the design, manufacturing and control of a modular robotic system consisting of straight extending modules. Each robotic module of the modular robot can be actively controlled or can respond passively to external forces. The modular elements can be connected simply for ease of manual reconfiguration. A new connectivity strategy for building modular robotic structures using rigid connector nodes, active and passive modular elements is investigated. Comparisons of the new connectivity and a conventional connectivity using compliant connector nodes are made with respect to kinematics, locomotion and deformation of some robotic structures. Modular units including a prismatic actuator, a rigid connector node and a passive revolute joint are then designed, manufactured and tested. More modular elements are further replicated for building modular robotic structures leading to a final prototype system with eight prismatic actuators, four rigid connector nodes and four passive revolute joints. Each prismatic actuator is equipped with a locking mechanism and possesses three different working states: it can either be actuated, locked or passive. The three-state prismatic actuator is self-contained with its own computation, communication, actuation and sensing capabilities. A proportional-integral-derivative (PID) controller is implemented to control the position of the prismatic actuator. The actuation and locking forces of the prismatic actuator are experimentally evaluated. The prismatic actuator can vertically lift an external load of 29.4 N. The locking force of the mechanical locker is 78.6 N, enabling the actuator to be capable of vertically supporting a weight of about 2.5 kg in the locked state. The minimum force required to passively move the prismatic actuator is also measured as 8.34 N. The performance of the PID controller, three states and state transitions of the prismatic actuator are then validated by a series of physical experiments. Experimental results demonstrate that the maximum absolute value of the displacement error is to be 0.175 mm in the actuated state, and state transitions between actuated, locked and passive states are physically achievable. Moreover, state transitions of two and multiple prismatic actuators are also realized resorting to communications between the prismatic actuators. As a high-level control strategy, a central pattern generator (CPG) neural network is first applied to modular robotic structures composed of the fabricated robotic modules. Physical experiments show that the modular robotic structures achieve a worm-like locomotion gait through the coordination of their actuators' movements, substantiating the feasibility and effectiveness of the mechanical design and control strategy. Modular robotic structures with greater number of elements are constructed in a physics-based robot simulator. A generalized CPG neural network and a role-based control method are developed for controlling these simulated modular robots. Computer simulations are then conducted to further demonstrate locomotion capability of modular robotic structures composed of three-state prismatic actuators. Simulation results show that the generalized CPG method is scalable to a broad range of robotic structures with different number of modules. The three-state prismatic actuator can be applied to releasing physical constraints of a robotic structure during task execution and achieving a walking pattern by using state transitions.