Flat Bottom Holes Research Articles

Experimentally validated defect depth estimation using artificial neural network in pulsed thermography

Infrared thermography (IRT) proved to be a valuable technique in non-destructive testing due to its contactless nature, real-time application, and wide area coverage. However, the major hurdle to widespread application of IRT is quantification; more specifically quantifying the depth of defects from the acquired thermograms in low conductive host materials. In this paper, an artificial neural network (NN) is employed to detect defects depth in composite samples, coupled with a Pulsed Thermography PT setup, to complement prior work when NN algorithms were coupled to a line-scan thermography. Carbon Fiber Reinforced Polymer (CFRP) coupons with embedded and flat bottom holes defects were designed via 3D printing; to precisely control the defects morphology and depth. Firstly; the current study presents a proof of concept using a Multiphysics FEM simulation model of the inspection process, to generate the training sets of data, so that the developed NN is assessed in a deterministic (noise free) environment. Then, the proposed NN was further tested experimentally to validate its accuracy and performance. The accuracy of the developed NN for the synthetic data was more than 97% and for the experimental data was around 90%.

Gaussian windowed frequency modulated thermal wave imaging for non-destructive testing and evaluation of carbon fibre reinforced polymers

Infrared thermographic techniques show their potential usage for non-destructive testing and evaluation of various materials due to their inherent capabilities such as safe, full field, remote, qualitative and quantitative defect detection capabilities. In this paper, a Gaussian Weighted Frequency Modulated Thermal Wave Imaging approach is reported for detection of sub-surface defects in Carbon Fiber Reinforced Polymer (CFRP) sample for a given frequency modulated incident heat flux. Artificial flat bottom holes and metallic inclusions as subsurface defects are prepared for the experimental investigation. Matched filter algorithm is applied for detection of sub surface defects by correlation coefficient images and compared the detection capabilities with conventional frequency domain phase images. The effect of spectral reshaping on frequency modulated thermal wave imaging is investigated. The results of the experiments show spectral reshaping is the most suitable selection for enhancing inspection capability and obtaining the highest Signal to Noise Ratio (SNR) for a given CFRP material.

Experimental investigation on noise rejection capabilities of pulse compression favourable frequency‐modulated thermal wave imaging

Among the various non-destructive testing and evaluation methods, infrared thermography gained its importance due to its fast, whole field, remote and quantitative evaluation capabilities for inspection of various materials. Being an optimum technique in terms of usage of low peak power heat sources in a moderate experimentation time, frequency-modulated thermal wave imaging (FMTWI) plays a vital role in the infrared thermographic community. The noise rejection capabilities of the proposed pulse compression favourable FMTWI by using principal component analysis as a post-processing technique are highlighted. The proposed scheme has been tested on a mild-steel sample having sub-surface flat bottom hole defects located inside the test specimen at various depths. It is clear from the obtained results that the reconstructed pulse (main lobe) concentrates much of the imposed energy into a narrow duration, which enhances the defect detection sensitivity and resolution in order to visualise the sub-surface defects with higher signal-to-noise ratio.

Barker-coded modulation laser thermography for CFRP laminates delamination detection

A Barker-coded modulation thermal-wave imaging (BCTWI) with laser as an external stimulus source based on pulse compression technique is reported on carbon fiber reinforced polymer (CFRP) laminates delamination detection. CFRP samples with different size and depth flat bottom holes (FBHs) are prepared for providing simulated defects of CFRP interlayer delamination. Three kinds of thermal wave feature images (phase, delay time and peak) are formed and deeply studied on their capability of detectivity. Experimental results indicate that delay time and phase images are relatively suitable modality for low-power active thermography requirement due to insensitive to the various excitation power. BCTWI significantly improve SNR and defect depth resolution in comparison with linear frequency modulation thermography (LFMT).

An independent component analysis based approach for frequency modulated thermal wave imaging for subsurface defect detection in steel sample

Infrared thermography (IRT) is extensively used as non-destructive testing and evaluation (NDT&E) technique to inspect and characterize various solid materials and structures. In this paper, an emergent optical thermography NDT&E technique i.e. frequency modulated thermal wave imaging (FMTWI) has been used for the inspection of mild steel sample embedded with artificially constructed flat bottom circular holes of the same diameter at various depth. This article proposes an independent component analysis (ICA) to process the FMTWI image sequence for detecting the subsurface defects of mild steel sample. To evaluate the effectiveness of defect detection capability of the proposed method, the conventional data processing techniques viz. phase analysis, pulse compression and principal component analysis (PCA) have been compared with ICA. The signal-to-noise (SNR) has been considered to characterize and quantify the defect detectability and compared with conventional post-processing techniques to validate the efficiency of the proposed approach. The obtained results provide an insight into the robustness of the ICA approach for defect detection. Furthermore, an active contour model-based object detection technique has been employed for identification, localization, and extraction of the shape of the defects.

Effectiveness of phased array focused ultrasound and active infrared thermography methods as a nondestructive testing of Ni-WC coating adhesion

ABSTRACTThe substrate/coating adhesion is a crucial parameter conditioning the quality of coating and its durability in service. For this reason, an inspection of the coating integrity, in particular, the presence of adhesion defects will be of great importance. The adhesion inspection is usually ensured by destructive methods, such as traction, interfacial indentation, four-point bending, testing scratch, etc. However, it is currently hampered by the absence of a satisfactory non-destructive method. Among the non-destructive testing technologies widely used in the industrial field, there are X-ray diffraction, ultrasonic inspection, and infrared thermography. In this paper, two methods are investigated: ultrasonic inspection, which becoming more efficient, especially with the emergence of phased array systems that allow to investigate different inspection angles and focusing depths, and the active infrared thermography. Experiments were performed on metallic coatings deposited on a mild steel substrate. Coatings were containing artificial defects (flat bottom holes with different diameters) at the interface and others were exempts of defects. Longitudinal waves with specific delay laws were generated through a phased array contact transducer (5 MHz of central frequency). Experimental results show that the ultrasonic method allows detecting and sizing defects with a diameter of 1 mm located in thick coatings.

Barker-Coded Thermal Wave Imaging for Non-Destructive Testing and Evaluation of Steel Material

Thermal non-destructive testing (TNDT) is one of the emerging inspection and evaluation techniques mostly used for subsurface defect detection in various industrial components. Besides the conventional thermography techniques (such as lock-in and pulse), recently introduced non-stationary thermal wave imaging (NSTWI) techniques gained its applicability in TNDT community due to their inherent testing capabilities such as improved sensitivity and enhanced resolution in inspecting and evaluating various solid materials for detecting subsurface defects. Barker-coded thermal wave imaging (BCTWI) is a one of the widely used NSTWI techniques, which facilitates the use of low peak power heat sources in moderate experimentation time in contrast to conventional TNDT techniques. In this paper, the pulse compression favorable NSTWI (BCTWI), the reconstructed pulsed (main lobe) data have been considered and processed using independent component analysis and named Barker-coded independent component thermography (BCICT). This proposed BCICT is implemented on a mild-steel sample to detect the artificially simulated flat bottom circular holes located at different depths inside it. The proposed technique extracts the sub-surface details such as flaws/defects hidden inside the sample by an unsupervised learning process, which helps in eliminating the manual interpretation of subsurface defects. The applicability of the proposed algorithm has been evaluated and validated experimentally with two different excitations schemes by considering the contrast and signal-to-noise ratio (SNR) as figure of merit. The results indicate that the BCICT technique offers higher contrast and SNR in comparison to conventional pulse-based TNDT technique.

Estimation of Damage Thickness in Fiber-Reinforced Composites using Pulsed Thermography

Nondestructive testing (NDT), including active thermography, has become an inevitable part of composite process and product verification, post manufacturing. However, there is no reliable NDT technique available to ensure the interlaminar bond integrity during composite laminate integration, bonding or repair where the presence of thin air gaps in the interface of dissimilar polymer composite materials would be detrimental to structural integrity. This paper introduces a novel approach attempting to quantify the damage thickness of composites (the thickness of air gaps inside composites) through a single-side inspection of pulsed thermography. The potential of this method is demonstrated by testing on three specimens with different types of defect, where the Pearson correlation coefficients of the thickness estimation for block defects and flat-bottom holes are 0.75 and 0.85, respectively. This approach will considerably enhance the degradation assessment performance of active thermography by extending damage measurement from currently two dimensions to three dimensions, and become an enabling technology on quality assurance of structural integrity.

Ultrasonic flaw detection for two-phase Ti-6Al-4V based on secondary scattering

Ultrasonic inspection of two-phase Ti-6Al-4V materials is enhanced using a more complex microstructure model within the doubly-scattered response (DSR) limit. The new model is used to calculate the corresponding bounds of grain noise and these bounds are then used to define a time-dependent threshold for ultrasonic testing. A Ti-6Al-4V specimen with flat bottom holes is manufactured for ultrasonic measurements to verify the measurement sensitivity to sub-wavelength flaws. The experimental results show that the present method has obvious advantages over traditional methods associated with a fixed threshold as well as the time-dependent threshold based on the singly-scattered response (SSR). This work provides an effective tool for distinguishing sub-wavelength flaws from grain noise, thereby limiting both false positives and missed detections.

Detection of local defect resonance frequencies using bicoherence analysis

The nonlinear acoustic wave spectroscopy and defectometry (NAWSD) technique is found to be an efficient tool for monitoring intricate and complex defects in composite structures due to its better resolution and accuracy compared to the traditional methods. Local defect resonance (LDR) is one of such methods based on amplifying vibrations at the defect regions only, resulting in defect-selective and high resolution image. In the present work, numerical as well as experimental approach has been presented in order to detect the LDR frequency of aluminium plate having circular flat bottom hole (FBH) and delaminated glass fibre reinforced polymer composite (GFRP) plates. First, a wave propagation study using explicit dynamic analysis is performed followed by, bicoherence analysis to detect the LDR frequency numerically. The LDR frequency modes are also verified by steady state dynamic analysis. A set of experiments are then carried out using Gaussian white noise as well as chirp signal to determine the LDR frequencies from the Fast Fourier Transform (FFT) and bicoherence plots. In every cases, the bicoherence peaks corresponds to the second harmonic of LDR frequency. The bicoherence plots are found to be robust and effective tool for detection of LDR frequencies compared to FFT plots. Bicoherence analysis based LDR frequency determination technique can be further utilized in delamination and disbond imaging of composite structures.

Thermography is cool: Defect detection using liquid nitrogen as a stimulus

This paper explores cooling instead of heating for external stimulation in infrared thermography for NDT & E. Liquid nitrogen is poured on the surface of a steel sample with flat-bottom holes and a thermal camera captures the whole process. Pulsed thermography and lock-in thermography are also employed to obtain a good comparison. In image processing procedure, the principal component thermography method is performed on all thermal images. Moreover, the corresponding receiver operating characteristic curves are established and discussed. A comparison of the results indicates that the liquid nitrogen technique could be considered as a valid option for NDT & E.

A Quantitative Comparison among Different Algorithms for Defects Detection on Aluminum with the Pulsed Thermography Technique

Pulsed thermography is commonly used as a non-destructive technique for evaluating defects within materials and components. In the last few years, many algorithms have been developed with the aim to detect defects and different methods have been used for detecting their size and depth. However, only few works in the literature reported a comparison among the different algorithms in terms of the number of detected defects, the time spent in testing and analysis, and the quantitative evaluation of size and depth. In this work, starting from a pulsed thermographic test carried out on an aluminum specimen with twenty flat bottom holes of known nominal size and depth, different algorithms have been used with the aim to obtain a comparison among them in terms of signal to background contrast (SBC) and number of detected defects by analyzing different time intervals. Moreover, the correlation between SBC and the aspect ratio of the defects has been investigated. The algorithms used have been: Pulsed Phase Thermography (PPT), Slope, Correlation Coefficient (R2), Thermal Signal Reconstruction (TSR) and Principal Component Thermography (PCT). The results showed the advantages, disadvantages, and sensitivity of the various thermographic algorithms.

Lamb Wave Local Wavenumber Approach for Characterizing Flat Bottom Defects in an Isotropic Thin Plate

This paper aims to use the Lamb wave local wavenumber approach to characterize flat bottom defects (including circular flat bottom holes and a rectangular groove) in an isotropic thin plate. An air-coupled transducer (ACT) with a special incidence angle is used to actuate the fundamental anti-symmetric mode (A0). A laser Doppler vibrometer (LDV) is employed to measure the out-of-plane velocity over a target area. These signals are processed by the wavenumber domain filtering technique in order to remove any modes other than the A0 mode. The filtered signals are transformed back into the time-space domain. The space-frequency-wavenumber spectrum is then obtained by using three-dimensional fast Fourier transform (3D FFT) and a short space transform, which can retain the spatial information and reduce the magnitude of side lobes in the wavenumber domain. The average wavenumber is calculated, as a real signal usually contains a certain bandwidth instead of the singular frequency component. Both simulation results and experimental results demonstrate that the average wavenumber can be used not only to identify shape, location, and size of the damage, but also quantify the depth of the damage. In addition, the direction of an inclined rectangular groove is obtained by calculating the image moments under grayscale. This hybrid and non-contact system based on the local wavenumber approach can be provided with a high resolution.

Long pulse thermography investigations of basalt fiber reinforced composite

The long pulse thermography is a suitable technique for detection and quantification of defects in low thermal conducting materials, especially for composites. Basalt fiber reinforced epoxy composite is studied for determining defect depths using long pulse thermography. Flat bottom holes with specified depths and sizes are made in the composite for simulating delamination defects. Thermal images of the specimen are studied to obtain thermal contrast and characteristic peak time. A relation between the slope peak time and defect depth by peak derivative method is obtained analytically for long-pulsed thermography. Defect sizing is determined from the full- width- half- maximum approach. Experimental and numerical results are presented and analyzed.

The Maximum Entropy Method in Ultrasonic Non-Destructive Testing-Increasing the Resolution, Image Noise Reduction and Echo Acquisition Rate.

The use of linear methods, for example, the Combined Synthetic Aperture Focusing Technique (C–SAFT), does not allow one to obtain images with high resolution and low noise, especially structural noise in all cases. Non-linear methods should improve the quality of the reconstructed image. Several examples of the application of the maximum entropy (ME) method for ultrasonic echo processing in order to reconstruct the image of reflectors with Rayleigh super-resolution and a high signal-to-noise ratio are considered in the article. The use of the complex phase-shifted Barker code signal as a probe pulse and the compression of measured echoes by the ME method made it possible to increase the signal-to-noise ratio by more than 20 dB for the image of a flat-bottom hole with a diameter of 1 mm in a model experiment. A modification of the ME method for restoring the reflector image by the time-of-flight diffraction (TOFD) method is considered, taking into account the change of the echo signal shape, depending on the depth of the reflector. Using the ME method, 2.5D-images of models of dangling cracks in a pipeline with a diameter of 800 mm were obtained, which make it possible to determine their dimensions. In the object with structural noise, using the ME method, it was possible to increase the signal-to-noise ratio of the reflector image by more than 12 dB. To accelerate the acquisition of echoes in the dual scan mode, it is proposed to use code division multiple access (CDMA) technology based on simultaneous emission by all elements of the array of pseudo-orthogonal signals. The model experiment showed the effectiveness of applying the ME method.

Optical excitation fractional Fourier transform (FrFT) based enhanced thermal-wave radar imaging (TWRI)

In this paper, we demonstrated a novel thermal-wave radar imaging approach with use of a dual-directional (down then up) chirp (or linear frequency modulation, LFM) modulated laser as an external excitation source and signal processing by Fractional Fourier transform (FrFT), which can enhance the defect detectability and extend the depth-resolution dynamic range. The thermal-wave signal was reconstructed by use of dimensionless normalization scaling (DNS) method, and furthermore, it explored the centralized feature of energy spectral density in FrFT domain. The amplitude and phase angle at the peak energy density in FrFT domain were extracted to form the corresponding image and used for the defect detection and identification. The experiments were carried over a carbon fiber reinforced polymer (CFRP) specimen with the artificial flat bottom holes (FBHs) to validate the defect detection capability using FrFT based enhanced TWRI compared to the FFT based TWRI or conventional lock-in thermography (LIT) by taking the defect signal to noise ratio (SNR) into account.

EXAMINING THE STRUCTURE OF MILL ROLLS METAL WITH LASER SURFACING FOR THEIR QUALITY CONTROL

The authors have investigated microstructure and crystal struc­ture of the steel samples of 9Kh2MF and 8Kh3SMFA steel with la­ser surfacing. The samples were taken from working shafts of reverse rolling mill in conditions of OJSC «Uralmashzavod». Brewing surface cracks in rolls with the use of laser is considered as an effective method of parts restoring in small-scale production. The research was carried out to control the quality of steel products with laser surfacing. Quality control of working rolls of rolling mills with laser surfacing is aimed at identifying the defects of metallurgical origin (nonmetallic inclu­sions, discontinuities, regions with heterogeneity of chemical compo­sition) in zones of surfacing and thermal influence and is performed by ultrasonic method. Metallographic study of the microstructure and crystal structure of steel samples with laser surfacing was necessary to develop an ultrasonic testing technique. The main way to detect de­fects of metallurgical origin in steels is scanning electron microscopy with functions of micro-X-ray spectral analysis (EDS-analysis) and diffraction of backscattered electrons (EBSD-analysis). The metallo­graphic study was carried out using a scanning electron microscope Carl Zeiss AURIGA CrossBeam equipped with analytical systems for studying the elemental surface composition by X-ray spectral analysis (EDS) and the crystal structure of the surface by diffraction of back­scattered electrons (EBSD). As a result of metallographic examination, steel-laser welded samples taken from the working rollers of the re­verse rolling mill were found to have defects of metallurgical origin along the surfacing boundary. The size of microinhomogeneities for 9Kh2MF steel is 10 – 50 μm; the elemental composition includes Mn, Si and O. The size of microinhomogeneities for 8Kh3SMFA steel is 1 – 3 μm, and the elemental composition includes Mn, Cr and Mo. It was established that metal on melting is less textured and has more homogeneous acoustic characteristics than base metal, it must be taken into account at ultrasonic quality control of steel products with laser surfacing. At ultrasonic inspection of laser-surfaced working rolls, we recommend setting the signal fixation level with reflectivity equivalent to the flat-bottom hole diameter of 1.5 mm.

Detection of Flat-Bottom Holes in Conductive Composites Using Active Microwave Thermography

Active microwave thermography (AMT) is an integrated nondestructive testing (NDT) technique that utilizes a microwave-based thermal excitation and subsequent thermal measurement. AMT has shown potential for applications in the transportation, infrastructure, and aerospace industries. This paper investigates the potential of AMT for detection of defects referred to as flat-bottom holes (FBHs) in composites with high electrical conductivity such as carbon fiber-based composites. Specifically, FBHs of different dimensions machined in a carbon fiber reinforced polymer (CFRP) composite sheet are considered. Simulation and measurement results illustrate the potential for AMT as a NDT tool for inspection of CFRP structures. In addition, a dimensional analysis of detectable defects is provided including a radius-to-depth ratio threshold for successful detection.

Image processing based quantitative damage evaluation in composites with long pulse thermography

Pulsed thermography is a contactless and rapid non-destructive evaluation (NDE) technique that is widely used for the inspection of fibre reinforced plastic composites. However, pulsed thermography uses expensive and specialist equipment such high-energy flash lamps to generate heat into the sample, so that alternative thermal stimulation sources are needed. Long pulse thermography was recently developed as a cost-effective solution to enhance the defect detectability in composites by generating step-pulse heat into the test sample with inexpensive quartz halogen lamps and measuring the thermal response during the material cooling down. This paper provides a quantitative comparison of long pulse thermography with traditional pulsed thermography and step heating thermography in carbon fibre and glass fibre composites with flat-bottomed holes located at various depths. The three thermographic methods are processed with advanced thermal image algorithms such as absolute thermal contrast, thermographic signal reconstruction, phase Fourier analysis and principal component analysis in order to reduce thermal image artefacts. Experimental tests have shown that principal component analysis applied to long pulse thermography provides accurate imaging results over traditional pulsed thermography and step heating thermography. Hence, this inspection technique can be considered as an efficient and cost-effective thermographic method for low thermal conductivity and low thermal response rate materials.

Assessment of uncertainty in damage evaluation by ultrasonic testing of composite structures

Ultrasonic testing (UT) is a commonly used non-destructive testing method of composite materials. UT enables determining a geometry of internal damage, its surface area and depth location. However, measurement uncertainty should be taken into consideration when interpreting UT results, which follows from various factors including the test material parameters and selection of the operating parameters, such as the scanning method and applied transducer’s characteristics. Moreover, uncertainty in the flaw size assessment is caused by signal or image processing methods applied to the obtained ultrasonic scans, which may return incorrect results. The article presents an overview of factors influencing on the measurement uncertainty depending on variable UT parameters, with reference to appropriate standards, as well as post-processing of the obtained scans. The experimental studies aimed at investigation on the measurement errors were performed on composite specimens made of CFRP with flat-bottom holes of various diameters using the Pulse-Echo and the Phased Array UT methods.