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 defectinduced 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.
It is well known that the efficiency of the vibrothermographic non-destructive testing (NDT) technique can be enhanced by taking advantage of local defect resonance (LDR) frequencies. Recently, the classical out-of-plane local defect resonance was extended towards in-plane LDR for enhanced efficiency of vibrometric NDT. This paper further couples the concept of this in-plane LDR to vibrothermography, on the basis of the promising potential of in-plane LDRs to enhance the rubbing (tangential) interaction and viscoelastic damping of defects. Carbon fiber-reinforced composites (CFRPs) with barely visible impact damage (BVID) are inspected and the significant contribution of in-plane LDRs in vibrational heating is demonstrated. Moreover, it is shown that the defect thermal contrast induced by in-plane LDRs is so high that it allows for easy detection of BVID by live monitoring of infrared thermal images during a single broadband sweep excitation. Thermal and vibrational spectra of the inspected surface are studied and the dominant contribution of in-plane LDR in vibration-induced heating is demonstrated.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In this study, both linear and nonlinear vibrational defect imaging is performed for a cross-ply carbon fiber-reinforced polymer (CFRP) plate with artificial delaminations and for a quasi-isotropic CFRP with delaminations at the edge. The measured broadband chirp vibrational response is decomposed into different components: the linear response and the nonlinear response in terms of the higher harmonics. This decomposition is performed using the short-time Fourier transformation combined with bandpass filtering in the time-frequency domain. The linear and nonlinear vibrational response of the defect is analyzed by calculation of the defect-to-background ratio. Damage maps are created using band power calculation, which does not require any user-input nor prior information about the inspected sample. It is shown that the damage map resulting from the linear band power shows high sensitivity to shallow defects, while the damage map associated to the nonlinear band power shows a high sensitivity to both shallow and deep defects. Finally, a baseline-free framework is proposed for the detection and localization of out-of-sight damage. The damage is localized by source localization of the observed nonlinear wave components in the wavenumber domain.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A defect’s detectability in flash thermography is highly dependent on the applied post-processing methodology. The majority of the existing analysis techniques operate either on the time-temperature data or on the frequency-phase data. In this paper, we compare the efficiency of time- and frequency-domain analysis techniques in flash thermography for obtaining good defect detectability. Both single-bin and integrated-bin evaluation procedures are considered: dynamic thermal tomography and thermal signal area for the time-domain approach, and frequency domain tomography and adaptive spectral band integration for the frequency-domain approach. The techniques are applied on various carbon fiber reinforced polymer samples having a range of defect sizes and defect types. The advantages and drawbacks of the different post-processing techniques are evaluated and discussed. The best defect detectability is achieved using the integrated procedure in frequency domain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.