Nik Rajic

Thermoelastic stress analysis with a compact low-cost microbolometer system

This article describes the development and validation of a novel thermoelastic stress analysis (TSA) system based on a low-cost microbolometer device. The use of a microbolometer for a highly synchronous and delicate temperature measurement breaks a longstanding and exclusive reliance on high performance, cooled photon detectors for thermoelastic applications. It is shown that despite markedly inferior noise equivalent temperature detectivity and dynamic response specifications, microbolometers are capable of achieving comparable levels of stress measurement performance. The practical implications for experimental stress analysis are significant. Microbolometers are relatively low in capital cost, small in size, have good tolerance to shock and vibration and consume less power than their photon counterparts, attributes that confer enormous practical advantages. It is argued that the emergence of TSA systems that are more affordable and better suited to in-service application could help to promote a much broader use of this powerful technique in applications across the life cycle of high value civil, maritime and aerospace assets, from the validation of finite element modelling for design to in-service structural integrity assessment. A full-scale fatigue test of a flight-critical aircraft structural component is employed as a case study to demonstrate important aspects of the capability. Future directions in the development and application of low-cost miniaturised systems are also discussed.

Reduced length fibre Bragg gratings for high frequency acoustic sensing

In-fibre Bragg gratings (FBGs) are now well established for applications in acoustic sensing. The upper frequency response limit of the Bragg grating is determined by its gauge length, which has typically been limited to about 1 mm for commercially available Type 1 gratings. This paper investigates the effect of FBG gauge length on frequency response for sensing of acoustic waves. The investigation shows that the ratio of wavelength to FBG length must be at least 8.8 in order to reliably resolve the strain response without significant gain roll-off. Bragg gratings with a gauge length of 200 µm have been fabricated and their capacity to measure low amplitude high frequency acoustic strain fields in excess of 2 MHz is experimentally demonstrated. The ultimate goal of this work is to enhance the sensitivity of acoustic damage detection techniques by extending the frequency range over which acoustic waves may be reliably measured using FBGs.

Integrated piezoceramic transducers for imaging damage in composite laminates

This paper presents a two-phase imaging methodology to characterise damage in composite laminates utilising Lamb waves generated by integrated piezoceramic transducers. The proposed methodology uses the transducers to sequentially scan the composite laminates before and after the presence of damage by transmitting and receiving Lamb wave pulses. In phase one the damage localisation image is reconstructed by analysing the cross-correlation of the wavelet extracted information from scatter signals with the excitation pulse for each transducer pair. A potential damage area is then reconstructed by superimposing the image observed from each actuator and sensor signal path. In phase two Lamb wave diffraction tomography is used to reconstruct an image quantifying size and shape of the damage based on the same set of measurement data and identified damage location in phase one. The two-phase imaging approach together with the modified diffraction tomography reconstruction algorithm enables a significant reduction of the required number of transducers without the need to know the damage location in advance. Numerical and experimental results are presented to demonstrate the efficiency, accuracy and sensitivity of the proposed methodology.

Detection of Disbond Growth in a Cyclically Loaded Bonded Composite Repair Patch Using Surface-mounted Piezoceramic Elements

This paper reports on an experimental study in which an array of surface-mounted lead zirconate titanate elements (PZT) are used for the in situ detection of disbond growth in a bonded composite repair patch. Two techniques are used to track the evolution of disbond growth: the transfer function method and the electromechanical impedance method. Both techniques were found to provide a reliable and robust basis for the detection of disbond growth. The results also demonstrate the importance of transducer placement relative to the disbond location as a factor in the sensitivity to disbond growth.

A performance comparison between cooled and uncooled infrared detectors for thermoelastic stress analysis

This study compares the stress-measurement sensitivity of several commercially available vanadium-oxide microbolometers to a scientific grade cooled indium antimonide imager. The devices were tested under similar conditions on the same mechanically-loaded subjects; one a uniaxially loaded plate containing a circular hole and the other a representative aircraft wing-skin coupon. The microbolometers are shown to consistently outperform the cooled imager for scan durations of 1500 load cycles or more despite having noise equivalent temperature detectivities (NETD) that were inferior by factors of between approximately 2 and 6. This finding is significant in two respects: it suggests that an NETD specification has only limited value as a sensitivity metric for thermoelastic stress analysis and secondly it confirms that microbolometers are able to furnish high-fidelity stress measurements of a type traditionally associated with cooled infrared imagers which are generally more costly to acquire and more cumbersome to use.

Interaction of High Frequency Lamb Waves with Surface-Mount Sensor Adhesives

The application of Lamb waves to damage and/or defect detection in structures is typicallyconfined to lower frequencies in regimes where only the lower order modes propagate in order to simplifyinterpretation of the scattered wave-fields. Operation at higher frequencies offers the potentialto extend the sensitivity and diagnostic capability of this technique, however there are technical challengesassociated with the measurement and interpretation of this data. Recent work by the authorshas demonstrated the ability of fibre Bragg gratings (FBGs) to measure wave-fields at frequencies inexcess of 2 MHz [1]. However, when this work was extended to other thinner plate specimens it wasfound that at these higher frequencies, the cyanoacrylate adhesive (M-Bond 200) used to attach theFBG sensors to the plate was significantly affecting the propagation of the waves. Laser vibrometrywas used to characterise the wave-field in the region surrounding the adhesive and it was found that theself-adhesive retro-reflective tape applied to aid with this measurement was also affecting the wavefieldin the higher frequency regime. This paper reports on an experimental study into the influence ofboth of these materials on the propagating wave-field. Three different lengths of retro-reflective tapewere placed in the path of Lamb waves propagating in an aluminium plate and laser vibrometry wasused to measure the wave-field upstream and downstream of the tape for a range of different excitationfrequencies. The same experiment was conducted using small footprint cyanoacrylate film samplesof different thickness. The results show that both of these surface-mount materials attenuate, diffractand scatter the incoming waves as well as introducing a phase lag. The degree of influence of thesurface layer appears to be a function of its material properties, the frequency of the incoming waveand the thickness and footprint of the surface layer relative to the base material thickness. Althoughfurther work is required to characterise the relative influence of each of these variables, investigationsto date show that for the measurement of Lamb Waves on thin structures, careful considerationshould be given to the thickness and footprint of the adhesive layer and sensor, particularly in the highfrequency regime, so as to minimise their effect on the measurement.

Inspections of Helicopter Composite Airframe Structures using Conventional and Emerging Nondestructive Testing Methods

This paper presents nondestructive testing (NDT) results and analysis from the inspection of composite specimens representing typical helicopter parts. The specimens include monolithic laminates produced from carbon fiber reinforced plastic (CFRP), Nomex honeycomb core sandwich panels with CFRP skins, and CFRP frame-skin joint panels. External protection layers comprising copper mesh and fiberglass were also included in the specimens. These panels were fabricated with a wide range of defects to simulate helicopter in-service damage including delamination and skin-core disbond along with barely visible impact damage. The study aims to assess a number of conventional and emerging NDT techniques suitable for rapid in situ and off-site inspection of helicopter composite structures. The techniques considered are flash and sonic thermography, radiography, and different ultrasonic inspection modes including pulse-echo, through-transmission, and phased array. These techniques are compared on their ability to detect and characterize the fabricated defects.

A strategy for achieving improved piezoceramic transducer durability under high structural loading

This paper describes a strategy that is shown to be effective in reducing load transfer into surface-bonded piezoceramic transducer elements and could provide a practical means of improving the mechanical durability of these elements in structural health monitoring applications. The concept relies on the introduction of local out-of-plane or secondary bending in the structural host through a structural augmentation called the stand-off element (SoE). Analytical and numerical modelling is supplemented with experimental evidence to show that large reductions in transferred load can be achieved with the insertion of a SoE, without adding significantly to the transducer profile or impairing acoustic transduction efficiency, important factors in structural health monitoring.

Advanced Instrumentation for Acousto-Ultrasonic Based Structural Health Monitoring

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.

Thermoelastic stress analysis and structural health monitoring: An emerging nexus

Recent innovations in thermal-detector technology are shown to foster the development of an in situ structural health monitoring capability based on thermoelastic stress analysis, a powerful full-field stress-measurement technique that offers an attractive set of diagnostic and prognostic capabilities, some of which are not found in any of the established structural health monitoring modalities. After reviewing these capabilities, the article examines an experimental case study involving the monitoring of an F/A-18 airframe structure subject to full-scale fatigue testing under representative flight spectrum loading spanning nearly one aircraft life time. The case study confirms the viability of the concept and also highlights some of its primary limitations.