Damping is an important material property for dynamically loaded structures, such as aircraft, vehicles, and rotary or reciprocating machinery. Low damping may increase vibration amplitudes which can cause excessive dynamic loading and can detrimentally affect a structures performance. Accurate prediction of the dynamic loads is necessary to avoid catastrophic failure especially during resonant vibrations. In this work, a MEMS (microelectro-mechanical sensor) accelerometer system is constructed to measure vibration over a wide frequency range (from 0-3000 Hz). Measurements of the damping ratio taken with MEMS accelerometers were compared to similar measurements using IEPE (integrated electronics piezo-electric) accelerometers and the experimental results validated against a thermoelastic model and published studies. The damping measured using IEPE accelerometers was larger due to spurious damping effects attributed to the much larger cables. The low spurious damping of the MEMS accelerometers allows for the accurate determination of the damping of a material over a wide frequency range. The MEMS accelerometer setup used in this work offers a method to accurately measure the damping of dynamically loaded, lightweight structures, where attaching traditional accelerometers can cause significant error in the measured damping or where non-contact measurement may be unsuitable.

The high bandwidth available in the terahertz domain is promising for non-destructive evaluation (NDE) and inspection applications. Although 3D-printing offers great flexibility and fast design iteration when developing quasi-optical components for such applications, common 3D-printing materials tend to suffer from high dielectric loss at terahertz frequencies, exacerbating signal-to-noise ratio issues that arise due to the low output power of typical terahertz sources. Cyclic Olefin Copolymer (COC) has favourable properties at terahertz frequencies, and has proven useful in developing custom quasi-optical components. In this work, we present a variety of 3D-printed lenses used in NDE and inspection applications.

Integrating scrap rubber particles as fillers into polymer matrix composites offers a cost effective and environmentally sustainable pathway to manage tyre waste through the creation of value-added products. This research explores the low-velocity impact (LVI) response and compression after impact (CAI) properties of rubberised foam-core glass fibre-reinforced epoxy (GFRE) sandwich composites. Syntactic foam cores integrated with rubber particles were manufactured using vacuum-assisted resin transfer moulding (VARTM). The compression properties of rubberised foam core, vital for resisting impact damage during LVI, were examined. Results show more than 40% reduction in compression strength and modulus of the syntactic foam upon the inclusion of 33 wt.% rubber particles. The LVI response and residual compression properties of rubberised foam-core composites were also evaluated. Rubberised foam cores caused a marginal reduction in the peak impact force and led to approximately 60% reduction in the delamination area. The pre-impact compression strength was unaffected by rubber particles within the core as the GFRE face sheets carried most of the compression load. Post-impact compression strength was slightly higher in rubberised foam-core composites due to reduced delamination. Digital Image Correlation (DIC) analysis tracking of the strain evolution during CAI experiments revealed the stress-raising effect of the impact damaged region. This study showcases sustainable scrap tyre management through the inclusion of rubber particles into foam-core composites without substantially reducing in-plane compression properties before or after low-velocity impact.

Ultrasonic pulse-echo C-scanning is routinely used to detect and quantify damage in composite structures. Ultrasonic inspection can be performed using portable equipment to identify the presence of barely-visible impact damage in fibre-reinforced composite laminates. A known limitation of ultrasonic inspection is the acoustic shadowing of deeply buried delamination damage by shallow delaminations, preventing complete inspection of a structural volume. Shadowing leads to an under-representation of the size and extent of delamination damage in impacted composite samples. Residual strength assessment of damage structures can be performed using high fidelity Finite Element (FE) models; however, accurate strength prediction is dependent on an accurate representation of the internal damage state. An impacted 32 ply quasi-isotropic UD composite sample was used to assess the feasibility of coupling ultrasonic inspection data with high fidelity FE models. A reconstruction method, based on the layup of the laminate, was used to recover information that could not be measured using planar ultrasonic inspection due to shadowing effects. The enhanced damage state was then used as an initial input for FE models and the residual strength predicted using the well-established Rx-FEM formulation that can account for the complex interaction of delaminations, matrix cracks and fibre failure. The predicted post-impact strengths were within 10% of the experimental mean. The methodology presented herein demonstrates a capability to predict the performance of damaged composite structures using in-field ultrasonic inspection data to support aircraft sustainment.

A means by which the strength of composite aircraft structures can degrade significantly in-service is via low-velocity impact induced while in maintenance (e.g., tool drop). Discrete damage modeling (DDM) has been used successfully for post-impact compressive strength. The current study considered the post-impact fatigue durability via DDM for matrix failure and continuum damage mechanics (CDM) for progressive fiber failure, and validated using advanced experimental techniques. Specifically, 1) new methods for mapping the damage from nondestructive inspection, 2) new techniques for tracking the development of damage experimentally, and 3) new methods for predicting remaining life of the damaged composite under compression-compression loading are proposed. The new models proposed predicted the development of subcritical damage and damage development relatively accurately once the inclusion of progressive fiber failure was introduced. The study presents new experimental data in sufficient detail to verify new methodologies proposed using DDM in combination with CDM for predicting the remaining fatigue life following a low-velocity impact.

A means by which the strength of composite aircraft structures can degrade significantly in-service is via low velocity impact induced while in maintenance (e.g. tool drop). Recently, improvements in the fidelity of non-destructive characterization and physics-based modelling have made possible significantly improved strength and durability assessment. The post-impact residual strength and durability was predicted using damage mapping along with the widely known Rx-FEM and the CB2ATA frameworks that permit discrete and discrete damage informed continuum damage modelling. The results showed that the complexity of the damage mechanics means that the solution is sensitive to a number of input parameters that need to be accurately obtained. This includes accurate mapping of the delamination and lamina compression strength. The studies herein in combination with the development of non-destructive evaluation methods seek to provide the foundation for a capability to predict accurately the performance of damaged and degraded composite structures for airworthiness decision making.

Polymer-matrix composite (PMC) materials used for a wide variety of engineering applications are often manufactured as hybrid carbon- and glass-fibre-reinforced laminates to exploit the respective properties of each material. From the exterior, hybrid PMCs are visually deceptively similar to monolithic laminates, but just as their mechanical, electrical and thermal properties differ, so their acoustic properties vary due to hybridisation. In this article, the mechanical and structural properties of hybrid PMC laminates pertinent to acoustic measurements and that affect sound propagation, such as: material density, stiffness, and laminate layup sequence, and their influence on ultrasonic defect detection, are explained via a detailed analysis of the ultrasonic waveforms, experimental inspection data, and analytical modelling.

A model is presented to determine the change in mutual impedance between a driver and pickup coil due to eddy-current interaction with a crack in a conductive two-layer plate system where the plates are separated by a non-conducting layer of constant thickness. Here, the impedance change of interest arises from a through-thickness crack of finite length located in the hidden lower plate. The model assumes that both conducting plates are electromagnetically thin and is therefore valid for large electromagnetic skin-depth. The model predictions are compared with experimental measurements using a pair of cylindrical air-cored coils arranged to simulate an eddy-current sliding probe. Agreement between the calculated and measured impedance change is obtained over the frequency range 10 Hz–3 kHz for which the plates can be treated as electromagnetically thin, and reasonable qualitative agreement was obtained at higher frequencies. A method for computing the induced current streamlines in the presence of the crack is also described and visualisation of the computed streamlines is used to assist in the interpretation of the impedance change for different probe orientations. These results provide a valuable initial insight into the interaction between a buried crack in a multilayer structure and the electromagnetic fields induced and detected by eddy-current sliding probes.

Nondestructive evaluation of compressed cracks is a major challenge. A quantitative study of the effect of crack tip closure on the pulse-echo ultrasonic sizing of delaminations in fibre-reinforced polymer-matrix composites (FRP) is presented. In particular, this study focuses on the interaction of ultrasound with a closed crack or kissing disbond, and their effect on the ultrasonic inspectability of FRP laminates consisting of carbon and glass plies. The compression of laminar cracks in these two different laminate types is clearly detectable via both pulse-echo and through-transmission ultrasonic measurements, but the reflected ultrasonic pulses in the two material types exhibit markedly different behaviour. The glass-fibre laminates show a drop in the reflected signal for crack openings up to approximately half the crack growth load, whereas the corresponding carbon-fibre laminates show the expected increase in the reflected signal as the crack opens. The origins of the observed effect of crack closure on the reflection and transmission of ultrasound are analysed in detail to ascertain possible mechanisms responsible for these effects.

Supersonic particle deposition (SPD) is an additive manufacturing technology with demonstrated potential for the repair of corrosion damage in metallic aircraft components. Repairs via SPD are applied with the aim of restoring damaged components to at least the equivalent structural properties of their original design. A successful SPD repair must have sufficient cohesive strength to prevent further cracking under fatigue loading, and the quality of the repair must be able to be monitored nondestructively on a periodic basis during ongoing operation of the repaired aircraft. In this study, we demonstrate the capability of two nondestructive inspection (NDI) techniques, thermoelastic stress analysis and angle-beam ultrasonic inspection, to both assess the health of an SPD coating and to detect and characterise representative fatigue crack growth beneath it. The results obtained via NDI are explained with reference to the microstructural features of the SPD coating and the substrate, as analysed via optical microscopy. (C) 2017 Published by Elsevier Ltd.

The influence of the fibre-polymer matrix interfacial bond strength on the explosive blast response of carbon fibre laminates is experimentally investigated. Air blast tests were performed on laminate target panels containing woven carbon fabric treated with a fibre-sizing agent that promoted either weak or strong bonding with the polymer matrix. The laminates were subjected to increasing shock wave impulse loads by increasing the mass or reducing the stand-off distance of the plastic explosive charge. High-speed digital image correlation photography (DIC) during explosive blast testing revealed the dynamic deformation responses and surface strains of the laminate targets were not influenced significantly by the type of fibre-sizing agent, except at high shock wave impulse levels. However, non-destructive testing using ultrasonics and X-ray computed tomography revealed that the initiation and growth of blast-induced damage (e.g. delamination cracks, matrix cracks, fibre fracture) was dependent to the interfacial bond strength. Damage occurred at a high shock impulse level in the laminate with strong fibre-matrix interfacial bonding, and this is attributed to higher flexural and interlaminar fracture toughness properties compared to the composite with weak interfacial bonding. The post-blast mechanical properties were also higher for the laminate with strong interfacial bonding. This study reveals that the fibre-sizing agent has a substantial influence on the explosive blast resistance and post-blast properties of laminates, and it is essential that an agent having high chemical compatibility with the polymer matrix is used to promote strong bonding.

Detection and characterisation of defects and damage that arise in marine composites is imperative, to ensure the safe and reliable operation of marine vessels and structures. This chapter discusses the types of defects occurring in marine composites, and introduces the concept of nondestructive testing (NDT) to inspect structures for flaws without reducing their future usefulness. The methodologies of point inspections, wide-area NDT and structural health monitoring are discussed, and examples are given of the performance of a range of techniques that fall within these categories.

Marine composite structures subject to dynamic loading typically incorporate more than one material type, and consist of laminate sections up to hundreds of millimetres in thickness. These solid hybrid laminates exhibit different behaviour in static and fatigue loading from thin aerospace composite laminates and sandwich structures. There is therefore a need to better understand the likely damage and degradation mechanisms that will occur in these thick structures and to concurrently develop nondestructive evaluation (NDE) technology to meet the consequent inspection problems. In this paper we present details of an ongoing fatigue program on marine composite blades. The challenges for ultrasonic NDE of thick composites, and emerging inspection methods using state-of-the-art inspection systems and analysis tools will be discussed.

: Since its introduction in the 1970s, magnetic rubber testing (MRT) has been used successfully to inspect critical high-strength steel aerospace components for surface-breaking fatigue cracks. In the residual-field variant of MRT, inspections are performed following the application and subsequent removal of a magnetic field, relying on the remanent magnetisation of the component to produce crack indications. For certain geometries, residual-field MRT is capable of reliable detection of cracks as small as 0.43 mm (0.017 inch) in surface length. However, for other conditions of specimen geometry and magnet configuration, it was observed that crack indications could be shorter or even absent when using residual-field MRT compared with active-field inspections. This report presents the results of an experimental and theoretical study into the formation of residual-field MRT indications and the rationale for verification of adequate field strength. It is proposed that a combination of inadequate magnetisation, shallow crack depth and the presence of a reversed tangential magnetic field at the surface of the specimen contribute to these observed anomalies in residual-field MRT compared with active-field MRT. The results of a related investigation into active-field MRT are presented in a companion report.

Nondestructive evaluation (NDE) research on composite materials has been ongoing for several decades, during which time their use has expanded significantly in the aerospace, marine, petrochemical, energy, construction and transport sectors. Initially, many composites were employed as fairings or reinforcements, but they are being increasingly used in primary and secondary load-bearing structures, where a mechanical failure has significantly greater safety implications. This increased scope has resulted in composite structures of significant thickness and complexity. Despite this, there has not been a corresponding increase in research pertinent to the detection and characterisation of defects in thick structures, apart from a brief period of interest by the NDE community in the early 1990s. This review critically assesses advances reported in the NDE of thick-section composites (structures of thickness above 15 mm are considered for the purposes of this review), and identifies future research opportunities to overcome the limitations of existing technologies.

: The results of a benchmark experiment for transient eddy-current nondestructive evaluation are reported. The benchmark configuration corresponds to the canonical geometry of an air-cored probe coil positioned above a conductive plate containing a long back-face slot. The coil is excited by an exponentially-damped step function current and measurements are made of the change in the transient magnetic field due to the slot. The aim of the work is to provide experimental data for validation of theoretical models under development within the wider NDE community and to provide a common geometry against which the performance of a range of such models can be compared. A further aim of the work is to stimulate the ongoing development of quantitative methods in transient eddy-current research.

This paper overviews some recent S&T innovations in smart materials and structures at the Australian Defence Science and Technology Organisation (DSTO) under a Corporate Enabling Research Program (CERP) on Signatures, Materials and Energy. The CERP program includes development and transitioning of technology across the maritime, air and land domains, with the major focus of the smart materials program component being to increase the safety, availability and maintainability of Defence assets. Three specific examples are provided of the smart materials and structures program, ranging across the spectrum of technology readiness from new concept phase to technology transitioning, viz.: (i) Advances in smart sensing for prognostics-based platform management; (ii) Fabrication of nanostructured and ultrafine grained materials through top-down severe plastic deformation processing of bulk materials; (iii) Innovative application of carbon nanotubes/conducting polymers as artificial muscles for low-power propulsion and control of small autonomous underwater systems. In each case, the DSTO effort is underpinned by strong university or industry linkages to deliver challenging interdisciplinary S&T.