Frequency-phase modulated thermal wave radar : stepping beyond state-of-the-art infrared thermography

Thermal wave radar is a state-of-the-art non-destructive testing method inspired by radio wave radar systems. The underlying principle of the technique is the application of a modulated excitation waveform by which the total energy of the response signal can be compressed in time-domain through cross-correlation. This leads to an enhanced depth resolution and increased signal to noise ratio in optical infrared thermography. Frequency sweep and Barker binary phase modulation are the two popular and widely researched excitation waveforms of the technique. In this research, a novel frequency-phase modulated waveform is introduced, which is designed for optimized performance of thermal wave radar

Multi-scale gapped smoothing algorithm for robust baseline-free damage detection in optical infrared thermography

Flash thermography is a promising technique to perform rapid non-destructive testing of composite materials. However, it is well known that several difficulties are inherently paired with this approach, such as non-uniform heating, measurement noise and lateral heat diffusion effects. Hence, advanced signal-processing techniques are indispensable in order to analyze the recorded dataset. One such processing technique is Gapped Smoothing Algorithm, which predicts a gapped pixel’s value in its sound state from a measurement in the defected state by evaluating only its neighboring pixels. However, the standard Gapped Smoothing Algorithm uses a fixed spatial gap size, which induces issues to detect variable defect sizes in a noisy dataset. In this paper, a Multi-Scale Gapped Smoothing Algorithm (MSGSA) is introduced as a baseline-free image processing technique and an extension to the standard Gapped Smoothing Algorithm. The MSGSA makes use of the evaluation of a wide range of spatial gap sizes so that defects of highly different dimensions are identified. Moreover, it is shown that a weighted combination of all assessed spatial gap sizes significantly improves the detectability of defects and results in an (almost) zero-reference background. The technique thus effectively suppresses the measurement noise and excitation non-uniformity. The efficiency of the MSGSA technique is evaluated and confirmed through numerical simulation and an experimental procedure of flash thermography on carbon fiber reinforced polymers with various defect sizes

Backside delamination detection in composites through local defect resonance induced nonlinear source behavior

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Introducing obliquely perforated phononic plates for enhanced bandgap efficiency

Porous phononic crystal plates (PhPs) that are produced by perpendicular perforation of a uniform plate have well-known characteristics in selective manipulation (filtration, resonation, and steering) of guided wave modes. This paper introduces novel designs of porous PhPs made by an oblique perforation angle. Such obliquely perforated PhPs (OPhPs) have a non-uniform through-the-thickness cross section, which strongly affects their interaction with various wave mode types and therefore their corresponding phononic properties. Modal band analysis is performed in unit-cell scale and variation of phononic bandgaps with respect to the perforation angle is studied within the first 10 modal branches. Unit-cells with arbitrary perforation profile as well as unit-cells with optimized topology for maximized bandgap of fundamental modes are investigated. It is observed that the oblique perforation has promising effects in enhancing the unidirectional and/or omnidirectional bandgap efficiency, depending on the topology and perforation angle of OPhP

Investigation to local defect resonance for non-destructive testing of composites

Local defect resonance (LDR) makes use of high frequency vibrations to get a localized resonant activation of a defective region. In this study, the LDR behavior of carbon fiber reinforced polymer (CFRP) coupons with three different types of damages is investigated using broadband measurements obtained with a scanning laser Doppler vibrometer (SLDV). First, the LDR response of flat bottom holes of different depths and sizes is evaluated using a signal-to-noise ratio. Next, results are obtained for ETFE inserts where the difference between (artificial) delaminations and inserts is outlined. At last, the vibrational response of a CFRP coupon with barely visible impact damage is investigated. This type of damage has a more complex structure, and it is shown that frequency band data (an alternative to the single frequency LDR) performs well in identifying such complex damage

Optimization and experimental validation of stiff porous phononic plates for widest complete bandgap of mixed fundamental guided wave modes

Phononic crystal plates (PhPs) have promising application in manipulation of guided waves for design of low-loss acoustic devices and built-in acoustic metamaterial lenses in plate structures. The prominent feature of phononic crystals is the existence of frequency bandgaps over which the waves are stopped, or are resonated and guided within appropriate defects. Therefore, maximized bandgaps of PhPs are desirable to enhance their phononic controllability. Porous PhPs produced through perforation of a uniform background plate, in which the porous interfaces act as strong reflectors of wave energy, are relatively easy to produce. However, the research in optimization of porous PhPs and experimental validation of achieved topologies has been very limited and particularly focused on bandgaps of flexural (asymmetric) wave modes. In this paper, porous PhPs are optimized through an efficient multiobjective genetic algorithm for widest complete bandgap of mixed fundamental guided wave modes (symmetric and asymmetric) and maximized stiffness. The Pareto front of optimization is analyzed and variation of bandgap efficiency with respect to stiffness is presented for various optimized topologies. Selected optimized topologies from the stiff and compliant regimes of Pareto front are manufactured by water-jetting an aluminum plate and their promising bandgap efficiency is experimentally observed. An optimized Pareto topology is also chosen and manufactured by laser cutting a Plexiglas (PMMA) plate, and its performance in self-collimation and focusing of guided waves is verified as compared to calculated dispersion properties

Efficient detection of production defects in a CFRP aircraft component by means of flash infrared thermography

Flash thermography is a promising technique to perform quick and non-contact non-destructive testing of composite materials. However, several limitations such as non-uniform heating and lateral heat diffusion complicate the accurateness of this technique. This paper presents an experimental case study of flash thermography for the non-destructive testing of a CFRP component of an aircraft with production defects. Three post-processing techniques, namely Differential Absolute Contrast (DAC), Pulsed Phase Thermography (PPT) and Principal Component Thermography (PCT), are applied to the recorded dataset. A qualitative comparison between these processing techniques is performed based on their defect enhancement capabilities in a component with industrial complexity

Performance of frequency and/or phase modulated excitation waveforms for optical infrared thermography of CFRPs through thermal wave radar : a simulation study

Following the developments in pulse compression techniques for increased range resolution and higher signal to noise ratio of radio wave radar systems, the concept of thermal wave radar (TWR) was introduced for enhanced depth resolvability in optical infrared thermography. However, considering the highly dispersive and overly damped behavior of heat wave, it is essential to systematically address both the opportunities and the limitations of the approach. In this regard, this paper is dedicated to a detailed analysis of the performance of TWR in inspection of carbon fiber reinforced polymers (CFRPs) through frequency and/or phase modulation of the excitation waveform. In addition to analogue frequency modulated (sweep) and discrete phase modulated (Barker binary coded) waveforms, a new discrete frequency-phase modulated (FPM) excitation waveform is introduced. All waveforms are formulated based on a central frequency so that their performance can be fairly compared to each other and to lock-in thermography at the same frequency. Depth resolvability of the waveforms, in terms of phase and lag of TWR, is firstly analyzed by an analytical solution to the 1D heat wave problem, and further by 3D finite element analysis which takes into account the anisotropic heat diffusivity of CFRPs, the non-uniform heating induced by the optical source and the measurement noise. The spectrum of the defect-induced phase contrast is calculated and, in view of that, the critical influence of the chosen central frequency and the laminate’s thickness on the performance of TWR is discussed. Various central frequencies are examined and the outstanding performance of TWR at relatively high excitation frequencies is highlighted, particularly when approaching the so-called blind frequency of a defect

Enhanced detectability of barely visible impact damage in CFRPs : vibrothermography of in-plane local defect resonances

This paper demonstrates the enhanced detectability of barely visible impact damage in CFRPs through low power vibrothermography of in-plane local defect resonances (LDR). In-plane LDR (LDRxy), with a higher cut-off frequency than out-of-plane LDR (LDRz), generally enhances the rubbing interaction and viscoelastic damping of defects and leads to higher vibration-induced heating. The most prominent LDRz and LDRxy frequencies of an impacted CFRP are extracted from its vibrational spectra under a broadband sweep excitation, measured by a 3D infrared laser Doppler vibrometer. The sample is then inspected through lock-in vibrothermography at the extracted LDR frequencies and the distintively higher detectability of LDRxy compared to LDRz is evidenced. Moreover, it is observed that the thermal contrast induced by LDRxy is so high, that it allows for easy detection of impact damage by live monitoring of infrared thermal images during a single broadband sweep vibration excitation