A drone-based inspection system that can fly, hover, and navigate around structures to perform the inspection in an efficient/fast manner can considerably reduce inspection time. Active thermography is a well-known non-destructive testing method for inspection. However, using it on a drone is challenging due to the drone needing to carry an appropriate heat source, batteries or tethering system to power the heat source and to provide adequate flight time. This complicates the inspection process and can restrict the amount of thermal energy that can be applied to the inspected structure. Another challenge with drone-based active infrared thermography (DBAIT) is that, unlike traditional active thermography inspection in which, the source is either stationary or moving in a precisely controlled manner, the drone and the heat source are subjected to undesired dynamic motion. This paper presents the results of experiments performed to compare potential heat sources that can be retrofitted onboard a drone to conduct active thermographic inspection.
A drone-based inspection system that can move “freely” around an aircraft to perform the inspection of all the areas of interest in a fast and effective manner can have significant impact in reducing inspection time and cost. However, active thermography inspection using drone is challenging because the drone carrying the optical and thermal cameras is subjected to vibration and undesired motion. Since active thermography relies on the pixel temperature evolution over time, an unstable thermal video from a flying drone can cause error in the output results as any movement between the acquired images will affect the pixel position in the successive frames and thus disrupt the monitoring of the temperature evolution. This paper presents the outcome of experimental runs, where a commercially available drone equipped with both thermal and optical cameras was used to inspect a helicopter Main Rotor Blade (MRB) in a laboratory environment.
The growth of application using multi-material assemblies in the transportation industry has resulted in structural adhesive becoming increasingly used. These assemblies, in addition to be subjected to mechanical stresses, are also exposed to harsh environments throughout the life of vehicles with temperature variations, high humidity and exposure to de-icing salts and fluids. While such assemblies are tested for mechanical strength and fatigue resistance, it is also critical to identify the failure mode of adhesive bonds to ensure that proper actions are taken to prevent catastrophic failure. Despite the obvious need to qualify the adhesive failure modes, this task is typically relegated to a semi-quantitative analysis of cohesive/adhesive failure ratios based on the visual inspection of an experienced eye, which along with the use of adhesives of various colors, inevitably introduces variability in the qualifying process. Moreover, the characterization of adhesive performance typically involves analysing hundreds of coupons and while the general failure types are known: bulk of the adhesive (cohesive failure), substrate/adhesive interface (adhesive failure) and near-interface; quantification on each coupon suffers from inaccuracy and better means are needed. In this work, we introduce the use of pulsed thermography (PT) as a repeatable and objective solution to quantify failure modes of metal-adhesive-metal assemblies by harnessing the fundamental differences in thermal properties of the two materials. It is shown that the inspection performed in through transmission mode allows for the distinction of the various adhesion failures.
As part of on-going research into filament-based Material Extrusion (MEX; aka Fused Filament Fabrication) polymer additive manufacturing, a modified industrial printer equipped with an infrared (IR) window was used to perform temperature measurements using infrared thermography. The IR window allowed the measurements to be taken without interfering with the printing process; i.e. with the printer door closed, and with build plate and build volume heated to printing conditions. The IR camera, lens and IR window system was calibrated using a black body calibration source, and the effects of the IR window on the temperature measurements were determined. This paper covers the calibration process and steps required to obtained high accuracy temperature measurements, within 3°C of the printer’s build plate. In addition, examples of measurements obtained from prints of various polymers are presented.
Diagnosis and prognosis of failures for aircrafts’ integrity are some of the most important regular functionalities in complex and safety-critical aircraft structures. Further, development of failure diagnostic tools such as Non-Destructive Testing (NDT) techniques, in particular, for aircraft composite materials, has been seen as a subject of intensive research over the last decades. The need for diagnostic and prognostic tools for composite materials in aircraft applications rises and draws increasing attention. Yet, there is still an ongoing need for developing new failure diagnostic tools to respond to the rapid industrial development and complex machine design. Such tools will ease the early detection and isolation of developing defects and the prediction of damages propagation; thus allowing for early implementation of preventive maintenance and serve as a countermeasure to the potential of catastrophic failure. This paper provides a brief literature review of recent research on failure diagnosis of composite materials with an emphasis on the use of active thermography techniques in the aerospace industry. Furthermore, as the use of unmanned aerial vehicles (UAVs) for the remote inspection of large and/or difficult access areas has significantly grown in the last few years thanks to their flexibility of flight and to the possibility to carry one or several measuring sensors, the aim to use a UAV active thermography system for the inspection of large composite aeronautical structures in a continuous dynamic mode is proposed.
Piezoelectric lead zirconate titanate (PZT) sensors are widely used in various structural health monitoring (SHM) applications, where data acquired by the PZT sensors are used for damage detection. Any failure of the PZT sensors will have a detrimental effect in the ability of SHM systems to detect damage. Therefore, detecting faulty PZT sensor is critical to reduce any false-calls associated with malfunctioning sensor to ensure proper functionality of SHM systems. This paper proposes a self-diagnostic method to monitoring the health of PZT sensors using the electro-mechanical impedance (EMI) data in two steps. In the first detection step, the onedimensional convolutional autoencoder (1D-CAE) is employed to obtain the reconstruction error as anomaly scores from the raw EMI data. Hence, the faulty PZT sensors can be detected by comparing the anomaly score with a pre-defined threshold. In the second diagnostic step, the data feature is first extracted with the 1D-CAE. The extracted feature is then fed into a multilayer perceptron (MLP) classifier to classify the fault type of the PZT sensor. The proposed method was validated through experiments, where typical in-service induced damages such as impact, environmental effect, sensor breakage localized high temperature heating, etc. were introduced. The results demonstrate the effectiveness of the proposed method for both detection and diagnosis of various types of PZT sensor damage.
Non-Destructive Evaluation (NDE) trials were performed on aramid and ultra-high molecular-weight polyethylene (UHMWPE) based armor systems. Pulsed thermography, continuous heating, and lock-in thermography were investigated for various types of damage. It is shown that the infrared thermography results vary significantly based on the material and thickness of the armor system, and only certain types of damage can be confidently identified. While the pulsed thermography performed in reflection mode was the fastest and provided the strongest indication signal for some types of damage, deeper damage on thicker armor system needed to be performed in transmission mode. Due to inherent material properties variations in these armor systems, the infrared images were affected by non-uniformity. In addition, due to low thermal conductivity, the inspections were sporadically affected by non-uniform heating. Approaches are presented to address the non-uniform heating issue affecting the inspection of those low thermal conductivity materials.
In this paper, thermographic inspections, ultrasonic C-scan and terahertz imaging were used to detect damages caused by impacts in natural, non-natural and hybrid composites. In particular, different hybrid structures were used. In some samples, numerical simulations were performed to predict the damage. A comparison of the results based on experimental and simulated experiments were afterwards conducted with the aim to explore the inspection capability of each technique.
Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Thermographic approaches including microvibrothermography, microlaser line thermography, and microlaser spot thermography on the basis of pulsed and lock-in techniques were proposed. These techniques are used to detect the submillimeter porosities in a stitched T-joint carbon fiber reinforced polymer composite specimen. X-ray microcomputed tomography was used to validate the thermographic results. Finally an experimental comparison of microlaser excitation thermography and microultrasonic excitation thermography was conducted.
Stitching is used to reduce dry-core and reinforce T-joint structure. However, it might cause new types of flaws, especially submillimeter flaws. In this paper, new approaches including micro-VT, lock-in micro-LLT and micro-LST based on both lock-in and pulse methods are used to detect submillimeter flaws in stitched CFRP. A comparison of laser excitation thermography and micro-VT on micro-porosities is conducted. Micro-CT is used to validate the infrared results. Then, a finite element analysis (FEA) is performed. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The model is validated for the experimental results. Finally a comprehensive experimental and simulation comparison of micro-LLT and micro-LST based on both lock-in and pulse methods is conducted.
Stitching is used to reduce dry-core (incomplete infusion of T-joint core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Microscopic inspection, ultrasonic c-scan, pulsed thermography, vibrothermography, and laser spot thermography are used to investigate the internal flaws in a stitched T-joint carbon fiber-reinforced polymer (CFRP) matrix composites. Then, a new microlaser line thermography is proposed. Microcomputed tomography (microCT) is used to validate the infrared results. A comparison between microlaser line thermography and microCT is performed. It was concluded that microlaser line thermography can detect the internal submillimeter defects. However, the depth and size of the defects can affect the detection results. The microporosities with a diameter of less than 54 μm are not detected in the microlaser line thermography results. Microlaser line thermography can detect the microporosity (a diameter of 0.162 mm) from a depth of 90 μm. However, it cannot detect the internal microporosity (a diameter of 0.216 mm) from a depth of 0.18 mm. The potential causes are given. Finally, a comparative study is conducted.
3D Carbon fiber polymer matrix composites (3D CF PMCs) are increasingly used for aircraft construction due to their exceptional stiffness and strength-to-mass ratios. However, defects are common in the 3D combining areas and are challenging to inspect. In this paper, Stitching is used to decrease these defects, but causes some new types of defects. Infrared NDT (non-destructive testing) and ultrasound NDT are used. In particular, a micro-laser line thermography technique (micro-LLT) and a micro-laser spot thermography (micro-LST) with locked-in technique are used to detect the micro-defects. In addition, a comparative study is conducted by using pulsed thermography (PT), vibrothermography (VT). In order to confirm the types of the defects, microscopic inspection is carried out before NDT work, after sectioning and polishing a small part of the sample..
Thermo-mechanical fatigue (TMF) tests and strain to crack (SC) tests at elevated temperature are important aspects to the total fatigue life for many engineering applications. During a TMF test, crack inspections are commonly done in a disruptive manner using an acetate replication method; and post-test crack evaluations are done using both optical and scanning electronic microscopy methods. Similarly, inspections during a typical SC test are also performed in a disruptive manner. This paper demonstrates that infrared imaging can be used as an in-situ inspection approach to detect crack during TMF and SC tests at high temperature. It is also demonstrated that this technique allows for the reduction or elimination of the need for downtime that is typically required for disruptive inspection. The results obtained by induction thermography are compared to those obtained via traditional methods and post-test evaluation. The induction thermography inspections were carried out at several temperatures and showed that the temperature used during the test does not influence the crack detection capability. It is demonstrated that induction thermography can detect cracks smaller than 500 μm and has potential for monitoring and generating a crack growth curve.
Water ingress in honeycomb structures is of great concern for the civil and military aerospace industries. Pressure and
temperature variations during take-off and landing produce considerable stress on aircraft structures, promoting moisture
ingress (by diffusion through fibers or by direct ingress through voids, cracks or unsealed joints) into the core. The
presence of water (or other fluids such as kerosene, hydraulic fluid and de-icing agents) in any of its forms (gas vapor,
liquid or ice) promotes corrosion, cell breakage, and induce composite layer delaminations and skin disbonds. In this
study, testing specimens were produced from unserviceable parts from military aircraft. In order to simulate atmospheric
conditions during landing, selected core areas were filled with measured quantities of water and then frozen in a cold
chamber. The specimens were then removed from the chamber and monitored for over 20 minutes as they warm up
using a cooled high-resolution infrared camera. Results have shown that detection and quantification of water ingress on
honeycomb sandwich structures by passive infrared thermography is possible using a HD mid-wave infrared cameras for
volumes of water as low as 0.2 ml and from a distance as far as 20 m from the target.
The quantification of pitting corrosion in terms of material or metal loss is required for the understanding of pipe
condition. One approach to accurately map pitting corrosion is with a high-resolution laser scanner. However,
this process is time consuming and requires the removal of the pipe segment and sandblasting of its surface.
In this study, thermography is considered for the field testing. We investigated the potential of quantifying
pitting corrosion with thermography technique. A cleaned pipe was inspected with the pulsed thermography (PT) technique. Extracted signal features were used to characterize metal loss. The algorithms to process PT inspection data and extract signal features to characterize the pitting corrosion are presented in this paper.
In this study, a mechanical shaker operating at low frequencies is demonstrated to be a viable excitation source for
vibrothermography. Additionally, a low-cost transduction approach based on commercially available piezoelectric
materials (PZT) is investigated. These PZT transducers are assessed for their excitation efficiency to allow crack
detection in metallic structures. Cracks as small as 1 mm are detected using the mechanical shaker regardless of a beam
structure orientation or crack location. Although the low-cost
PZT-based transduction approach had sufficient excitation
power to generate vibration, localized heat generation was not observed at crack locations.
This paper investigates the use of finite element to model frictional heating based vibrothermography for the detection of
fatigue cracks in steel specimens. First, a finite element modal analysis is carried out to predict the optimal excitation
frequencies. Some thermographic experiments using an infrared camera are carried out to help updating a coupled
thermo-mechanical model built to simulate the thermographic inspection process and to explain the heat generation and
transfer related to it. Experimental investigations also confirmed that the technique is able to detect cracks as short as 0.1
mm. The developed model is able to simulate the thermographic inspection process with a maximum error of 2.13 % on
the temperature distribution. The Fourier transform applied to numerical data reveals that the temperature evolution at
the crack face changes according to the excitation frequency and is modulated due to the nonlinearity induced by the
crack. The model also serves to confirm that the test is non-destructive since the calculated stress at the crack tip is less
than the specimen material's yield stress.
The primary objective of this study was to demonstrate the effectiveness of various strain measurement techniques at
detecting the disbonding of a composite repair patch and then using this information to validate a new capacitance based
disbond detection technique. The instrumented repair patch was parametrically designed with the help of Finite Element
Analysis (FEA) software to have a stress concentration at its tip. This stress concentration was designed to produce a
disbond during fatigue testing, without the need for the introduction of any foreign material to create an artificial disbond
condition. The aluminum substrate was grit blasted and the instrumented patch was bonded using FM®73 adhesive, and
was cured following the recommendations of the manufacturer. The geometric characteristics of the patch followed
standard repair guidelines for such variables as material selection, taper angles and loading conditions, with the
exception of the area designed for premature disbond. All test specimens were inspected using non-destructive testing
technique (ultrasound pulse echo) to guarantee that no disbonding had occurred during curing of the specimen. The
specimens were placed under fatigue loading to induce a disbond condition between the aluminum substrate and the
patch. The specimens were cyclically loaded and strain gauges bonded to strategic locations on the aluminum and
composite patch surface to be able to measure changes in surface strains as the disbond progressed. A Digital Image
Correlation (DIC) system was also used to measure full field strains over the gauge length of the coupon. The DIC
results were compared with the strain gauge data and were used to provide a qualitative measure of the load transfer in
the bonded specimen, which clearly demonstrated the change in surface strain that occurred as the composite patch
disbonded from the aluminum substrate. Thermoelastic Stress Analysis (TSA) was also used to measure surface strains
on the composite patch. Thermoelastic stress analysis proved to be the most sensitive technique for experimentally
monitoring the disbond process in real time. Failure analysis of the specimens using optical microscope techniques was
performed to determine the type of failure between the patch and the substrate. The results of this work will serve to test
the different types of sensors available for the design and manufacturing of a "Smart Patch" for aircraft structure
applications.
Inspection of aerospace components has always been a challenge. Infrared thermography has demonstrated to be a useful tool for this matter. In this paper, we offer a comparative study involving three active techniques: pulsed thermography, lock-in thermography and vibrothermography. Some of these techniques have proven to be more effective than others for a specific type of system. We compare the experimental results from these three techniques as applied to two typical aerospace parts: honeycomb structures and Glare. The later is perhaps the most challenging of all as will be pointed out. Some insights are provided regarding the most suitable technique for a number of typical situations.
There is a growing international interest in thermal inspection systems for asset life assessment and management of defense platforms. The efficacy of flash thermography is generally enhanced by applying image processing algorithms to the observations of raw temperature. Improving the defect signal to noise ratio (SNR) is of primary interest to reduce false calls and allow for easier interpretation of a thermal inspection image. Several factors affecting defect SNR were studied such as data compression and reconstruction using principal component analysis and time window processing.
The fusion of data from Edge of Light(EOL) and eddy current inspections of aircraft lap joints is investigated in this study. The pillowing deformation caused by corrosion products is estimated by the EOL technique first. Eddy current (ET) techniques, e.g. multi-frequency eddy current testing (MF-ET) and pulsed eddy current testing (P-ET), can provide depth-sensitive inspections of fuselage joints. The objective of this study is to investigate how the testing results obtained from the two different methods correlate to each other and what kind of complementary information is available in each
result. This work contains two steps. First, the EOL inspection is quantified through a calibration process where a laser displacement sensor is used to provide the reference. The EOL estimation is for the total material loss while the eddy current or pulsed eddy current testing is employed to provide the complementary information on the remaining thickness.
Second, the ET data are fused with the principle component analysis method and the results are calibrated by a calibration experiment. Finally, the bottom layer corrosion is estimated through the subtraction of EOL and ET results. The preliminary results are presented in this paper.
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