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Condition monitoring of rolling element bearing is vital for condition-based maintenance (CBM) in many industries. A key obstacle at present is the ability to accurately quantify the severity of the bearing faults, which is commonly measured in terms of the bearing defect size. Limitations of previous studies in the area include: (i) most accelerometer-based approaches were developed for artificial bearing faults instead of naturally developed spalls, and (ii) a systematic comparison between accelerometers and alternative measurements is not available. Therefore, this thesis aims at obtaining effective methods to estimate and track the growth of bearing spalls. This has been achieved by both advancing the processing of accelerometer signals and exploiting the capabilities of alternative measurements. Firstly, a novel approach based on accelerometers is proposed, which utilises natural frequency perturbations to estimate spall size. By comparing it with the well-established existing methods, it was found that all methods are effective for artificial spalls, but only the newly proposed approach is successful for naturally developed faults. Then, three alternative measurements (acoustic emission, instantaneous angular speed, and radial load) are investigated and benchmarked against acceleration on UNSW’s bearing test rig. It was found that radial load was far superior in fault-size estimation comparing to all other sensors, and achieved more precise results than accelerometers with less complex processing. This was justified considering radial load as a proxy for radial displacement, whose potential was recently suggested by theoretical studies. To confirm this, in the last part of this work, actual displacement sensors (proximity probes) were installed on the bearing test rig and a larger gearbox facility. Both experiments demonstrated that the proposed displacement approach can effectively estimate the size of natural spalls, with very limited signal processing required. This thesis has therefore provided three significant novel contributions to the field of bearing fault severity assessment: (i) the development of a new acceleration-based approach, effective on natural spalls for the first time, (ii) the collection and analysis of a new and comprehensive database of alternative measurements, obtained on naturally developed spalls, (iii) the discovery of the superior effectiveness of direct displacement measurements.

The use of aluminium alloy members in building construction has increased worldwide in recent years, especially in areas with adverse conditions such as severe industrial and marine environments. Aluminium members can be used in the structural systems of residential, industrial and commercial buildings as purlins, floor joists, rafters, decks and wall studs. corrosion resistance, lightweight nature and high strength-to-weight ratio are some of the important advantages of aluminium members. Despite of these advantages, the elastic modulus of aluminium is only one-third that of steel, making aluminium members highly susceptible to various failure modes including bearing failure. This bearing failure, commonly known as web crippling, is one of the critical failure modes of thin-walled members including lipped channel sections that are unstiffened against flexural loads. To date, only a limited amount of research studies has been conducted to investigate the web crippling failure phenomenon for aluminium structural members, and no research has been carried out on the web crippling behaviour of roll-formed aluminium lipped channel sections. Hence, this research aims to conduct an experimental study using the new American Iron and Steel Institute (AISI) S909 test procedures and associated numerical studies for lipped channel sections under four loading conditions: End-Two- Flange (ETF), Interior-Two-Flange (ITF), End-One-Flange (EOF) and Interior-One- Flange (IOF). This research also considers both unfastened (flanges not connected to the support) and fastened (flanges connected to the support) conditions to cover all possible restrain scenarios in the real-world applications. Following extensive experimental tests, a series of finite element analyses (FEA) were performed to develop precise numerical models of all the tested aluminium lipped channel sections. The general-purpose software ABAQUS, with quasi-static explicit solver, was employed for this study since contact and large deformations were involved. The models were validated using the experimental results and a good agreement was achieved in terms of ultimate failure strengths, load-deflection responses and failure modes. Subsequently, the validated models were used as a basis to conduct a detailed parametric study to investigate a range of commonly used aluminium lipped channel sections with varying thicknesses, heights, inside bent radii, bearing lengths and aluminium alloy grades. The results acquired from this research were compared with the nominal web crippling strengths predicted by the currently available design rules recommended by the Australian, European and American Standards. As a result of the comparisons, weaknesses in the current design standards were identified and suitable modifications were made based on the experimental and numerical results to accurately predict the web crippling capacities of aluminium lipped channel sections. Further, a suitable Direct Strength Method (DSM)-based design approach was developed in this study and associated equations were established to accurately predict the elastic bucking and plastic loads of aluminium lipped channel sections under web crippling action.

Primary aerospace composite structures are commonly assembled with bolted joints due to their ability to transfer high loads and ease of assembly. However, when bolted joints are used beyond their originally intended design life, joint strength can be significantly reduced due to the accumulation of internal damage, necessitating frequent inspections. Furthermore, internal damage in composites (delamination, matrix cracks) can continue to propagate without visual indications, thus nondestructive testing is required. As a result, maintenance can become very costly, particularly for aircraft that are in-service beyond their designed life expectancy. By establishing a comprehensive understanding of damage propagation behavior, engineers can determine which damage modes to inspect for and reduce inspection frequency. This research aims to support and improve maintenance operations, fleet management, and aircraft design practice by investigating delamination initiations and propagations in the vicinity of fastener holes within fiber-reinforced composite materials. Static and fatigue bearing were performed using novel test methods developed as part of this research for countersunk fastener joints: the modified countersunk double lap shear (DLS), single lap shear (SLS), and semi-circular notch (SCN) test configurations. DLS and SLS static and fatigue experimental test results were compared to study joint configuration, laminate stacking sequence, and loading condition effect on bearing damage initiation and growth under both static and fatigue loading. From static and fatigue tests, it was observed that major bearing damage accumulates in the straight shank region of the countersunk hole indicating most of the bearing load is carried by the straight shank region. Fatigue bearing test data showed that when the bolted hole elongates, stiffness decreases and internal delamination damage area growth becomes detectable through C-scan. Stated in reverse, if no measurable hole elongation is found, significant delamination is not expected. Complex damage morphology forms in this region, emanating from the loaded bearing face, and creating large wedge-shaped regions that drive delamination propagation with additional loading cycles. Additionally, optical microscopy observations indicated that pin bending might have affected bearing damage growth. In order to understand the effect of pin bending, a custom designed semi-circular notched experiment was performed on the countersunk hole geometry and compared to the DLS static experiments. Results indicated that the pin bending had no strong effect on the bearing failure morphology for the selected diameter to thickness ratio. Finite element analysis using Virtual Crack Closure Technique (VCCT) and Hashin failure criteria in Abaqus was used to further understand the internal stress state of the specimen configurations and to investigate the rate of delamination growth and arrest in the SCN and DLS configurations. Results from FEA were used to more comprehensively understand the observations from static and fatigue experiments and to verify hypotheses formulated to explain these observations.