Showing papers by "Nik Rajic published in 2016"
TL;DR: In this article, the elastic properties in a numerical Lamb wave model are optimized to achieve good correlation between model predictions and experimental observations, where the material properties are determined under a Lamb wave propagation regime, the strain rates and amplitudes are consistent with the intended modelling application.
20 citations
TL;DR: In this paper, a distributed fiber optic strain measurement system (ODiSI B) was used for full-scale fatigue tests of ex-service F/A-18 Hornet center fuselages, in support of the Royal Australian Air Force's structural integrity management programs for its frontline fighter fleet.
Abstract: The Defence Science and Technology Group has been conducting full-scale fatigue tests of ex-service F/A-18 Hornet centre fuselages, in support of the Royal Australian Air Force’s structural integrity management programs for its frontline fighter fleet for over 12 years. Historically, conventional electrical resistance foil strain gauges have been used extensively on these tests to monitor and record the structural response to loading; however, there are limitations with these in terms of cost, installation times and physical complexity. Developments in commercially available, distributed fibre optic strain measurement systems presents the opportunity to overcome these limitations, as demonstrated on the most recent centre fuselage test article. Based on Rayleigh scattering, the system (‘ODiSI B’ by Luna Innovations) was trialled which allowed comparisons of strain response, spatial resolution and noise levels with conventional foil gauges. Comparisons were also made of the full-field strain mapping capability of the system with full-field stress mapping by thermoelastic stress analysis. Furthermore, the distributed fibre optics demonstrated their potential to detect crack propagation on a coupon with induced crack growth.
15 citations
TL;DR: In this paper, the fundamental limits to modal decomposition resolution and bandwidth that exist for such sensors are explored and potential sources of noise and distortion encountered due to limitations of the sensor fabrication and interrogation methods are also discussed.
Abstract: Lamb-wave based structural health monitoring (SHM) approaches are typically constrained to operate below the first cut-off frequency to simplify the interpretation of the wave field in the time-domain. However from a diagnostic perspective, it is desirable to unlock the additional information encoded in the higher-order Lamb wave spectrum. Wave-mode decomposition is necessary for the extraction of useful information from multi-modal acoustic wave fields, which requires spatially dense sampling over the field. The instrument of choice for this task is the laser Doppler vibrometer, which is capable of producing detailed spectral decompositions. However vibrometry is not suited to in-situ measurement for SHM. Fibre Bragg gratings (FBGs) are capable of sensing Lamb waves and detection of higher order modes using FBGs has been previously demonstrated. The ability to multiplex multiple short-length gratings along a single fibre to create an FBG array gives rise to an in-situ sensor with sufficiently dense spatial sampling of an acoustic wave field to perform useful wave-mode decomposition. This paper explores some of the fundamental limits to modal decomposition resolution and bandwidth that exist for such sensors. Potential sources of noise and distortion encountered due to limitations of the sensor fabrication and interrogation methods are also discussed. In addition, modal decomposition of Lamb waves with frequencies up to 1.25 MHz is demonstrated in a laboratory experiment using an array of sixteen ~1 mm long gratings bonded to an aluminium plate. At least four modes are distinguishable in the resulting spectral decomposition.
5 citations
TL;DR: In this paper, three ultrasonic methods for determining the material properties of a film adhesive were investigated. And the authors found that the elastic modulus required to accurately model Lamb waves in this adhesive is approximately 60% higher than the value determined by conventional quasi-static testing.
4 citations
TL;DR: The AUSAM+ module as discussed by the authors is a compact device for AU excitation and interrogation, which has the footprint of a typical current generation smart phone and provides autonomous control of four send and receive piezoelectric elements, which can operate in pitch-catch or pulse-echo modes.
Abstract: Structural health monitoring (SHM) systems using structurally-integrated sensors potentially allow the ability to inspect for damage in aircraft structures on-demand and could provide a basis for the development of condition-based maintenance approaches for airframes. These systems potentially offer both substantial cost savings and performance improvements over conventional nondestructive inspection (NDI). Acousto-ultrasonics (AU), using structurallyintegrated piezoelectric transducers, offers a promising basis for broad-field damage detection in aircraft structures. For these systems to be successfully applied in the field the hardware for AU excitation and interrogation needs to be easy to use, compact, portable, light and, electrically and mechanically robust. Highly flexible and inexpensive instrumentation for basic background laboratory investigations is also required to allow researchers to tackle the numerous scientific and engineering issues associated with AU based SHM. The Australian Defence Science and Technology Group (DST Group) has developed the Acousto Ultrasonic Structural health monitoring Array Module (AUSAM + ), a compact device for AU excitation and interrogation. The module, which has the footprint of a typical current generation smart phone, provides autonomous control of four send and receive piezoelectric elements, which can operate in pitch-catch or pulse-echo modes and can undertake electro-mechanical impedance measurements for transducer and structural diagnostics. Modules are designed to operate synchronously with other units, via an optical link, to accommodate larger transducer arrays. The module also caters for fibre optic sensing of acoustic waves with four intensity-based optical inputs. Temperature and electrical resistance strain gauge inputs as well as external triggering functionality are also provided. The development of a Matlab hardware object allows users to easily access the full hardware functionality of the device and provides enormous flexibility for the creation of custom interfaces. This paper discusses the impetus for the concept, and outlines key aspects of the hardware design and the module capabilities. The efficacy of the system is demonstrated through the results of first-of-class testing, as well as laboratory AU studies on a flat plate using an array of piezoelectric elements.
3 citations
TL;DR: In this article, the authors presented a summary of progress toward the development of an autonomous stress-imaging capability based on an uncooled thermal camera core significantly smaller than any infrared imaging device previously applied to TSA.
Abstract: The historical reliance of thermoelastic stress analysis on cooled infrared detection has created significant cost and practical impediments to the widespread use of this powerful full-field stress measurement technique. The emergence of low-cost microbolometers as a practical alternative has allowed for an expansion of the traditional role of thermoelastic stress analysis, and raises the possibility that it may in future become a viable structural health monitoring modality. Experimental results are shown to confirm that high resolution stress imagery can be obtained from an uncooled thermal camera core significantly smaller than any infrared imaging device previously applied to TSA. The paper provides a summary of progress toward the development of an autonomous stress-imaging capability based on this core.