Debonding Defects Research Articles

Nondestructive Evaluation of Rubber Composites Using Terahertz Time Domain Spectroscopy

In recent years, with the progress of science, more and more detection methods are being used in various fields. However, the nondestructive testing of nonmetallic materials still needs further study. In this paper, an analysis of the time domain characteristics of rubber materials using terahertz detection technology was carried out, obtaining different defect rubber material spectral characteristics as well as imaging results and data. The results show that the THz spectrum imaging technique can detect the thickness of rubber material in the 0.1 ~ 4.0 terahertz band, and the image is clear and the resolution high. Meanwhile the time domain waveform obtained is sensitive to the debonding defects of the sample rubber, is suitable for judging the overall performance of the rubber inner defect detection, and can provide the scientific basis for rubber material performance.

Dominance of debonding defect of CFST on PZT sensor response considering the meso-scale structure of concrete with multi-scale simulation

Abstract Piezoelectric-lead-zirconate-titanate(PZT)-based interface debonding defects detection for concrete filled steel tubulars (CFSTs) has been proposed and validated through experiments, and numerical study on its mechanism has been carried out recently by assuming that concrete material is homogenous. However, concrete is composed of coarse and fine aggregates, mortar and interface transition zones (ITZs) and even initial defects and is a typical nonhomogeneous material and its mesoscale structure might affect the wave propagation in the concrete core of CFST members. Therefore, it is significantly important to further investigate the influence of mesoscale structure of concrete on the stress wave propagation and the response of embedded PZT sensor for the interface debonding detection. In this study, multi-physical numerical simulation on the wave propagation and embedded PZT sensor response of rectangular CFST members with numerical concrete core considering the randomness in circular aggregate distribution, and coupled with surface-mounted PZT actuator and embedded PZT sensor is carried out. The effect of randomness in the circular aggregates distribution and the existence of ITZs are discussed. Both a local stress wave propagation behavior including transmission, reflection, and diffraction at the interface between concrete core and steel tube under a pulse signal excitation and a global wave field in the cross-section of the rectangular CFST models without and with interface debonding defects under sweep frequency excitation are simulated. The sensitivity of an evaluation index based on wavelet packet analysis on the embedded PZT sensor response on the variation of mesoscale parameters of concrete core without and with different interface debonding defects under sweep frequency voltage signal is investigated in details. The results show that the effect of the interface debondings on the embedded PZT measurement is dominant when compared to the meso-scale structures of concrete core. This study verified the feasibility of the PZT based debonding detection for rectangular CFST members even the meso-scale structure of concrete core is considered.

Ultrasonic guided wave-based detection of composite insulator debonding

Based on the theory of ultrasonic guided wave propagation, a simplified model of bilayer composite insulator was constructed, and the dispersion curves of longitudinal (L) and torsional (T) guided wave modes were obtained. To address the problem of composite insulator debonding, the propagation velocity and energy decay of longitudinal and torsional ultrasonic guided waves in the insulator bilayer model were studied, and a method for detecting the axial and circumferential debonding of composite insulator was proposed based on the ultrasonic guided wave attenuation characteristics. On the basis of the theoretical analysis, experiments were conducted to detect the debonding of composite insulator. The experimental results showed that: the same guided wave modes propagated at the same speed on the silicone rubber and rod, but differed in energy distribution; as the guided waves attenuated excessively during propagation in the silicone rubber, only segmented detection was possible for long composite insulator; when there was debonding within the detection range, the energy attenuation of guided waves would increase. Debonding size would also influence the attenuation coefficient, therefore, we could determine whether there was debonding defects as well as the size of defects by detecting the propagation attenuation coefficient of guided waves. L-mode guided waves could detect whether there was debonding defects, but could not determine the size of debonding. T mode guided waves could be used to estimate the size of debonding defects.

A time-frequency analysis algorithm for ultrasonic waves generating from a debonding defect by using empirical wavelet transform

Time-frequency distributions (TFDs) are key indicators for inspecting debonding defects using ultrasonic nondestructive testing (NDT) or nondestructive examination (NDE). A novel algorithm combining empirical wavelet transform (EWT), Hilbert transform (HT) and short time Fourier transform (STFT) is presented to obtain the TFD of ultrasonic testing waves. First, we decompose the signal into mono-components by means of EWT to satisfy the mono-component requirement of HT. Next, correlation coefficients between the empirical modes and the decomposed signal are computed to extract valuable components. Further, the instantaneous amplitude (IA) of the extracted valuable components are calculated using HT. Finally, using the computed IAs, the peak time representing the arrival time of the component is obtained. To calculate the corresponding frequency, a method based on STFT is applied on the valuable components. The principle of the proposed method is illustrated by a numerical signal. The effectiveness of the proposed method in extracting the TFD of ultrasonic testing wave is validated by an experimental analysis of the ultrasonic NDT used for the detection of debonding defect in a composite construction. In comparison with STFT, smoothed pseudo Wigner-Ville distribution (SPWVD), and empirical mode decomposition (EMD), a higher accuracy of the TFD can be obtained using the proposed algorithm.

Multi-physical field guided wave simulation for circular concrete-filled steel tubes coupled with piezoelectric patches considering debonding defects

The active interface debonding detection for concrete-filled steel tubes (CFSTs) using Piezoelectric Lead Zirconate Titanate (PZT) patches has been proposed and experimentally studied in recent years. In order to further investigate the guided wave propagation and the response of embedded PZT sensor, transient dynamic analysis on multi-physical field coupling models composed of surface-mounted PZT actuator, embedded PZT sensor and a circular CFST specimen considering the coupling effect between the PZT patches and CFST member and the influence of interface debonding defects is carried out numerically. The guided wave propagation within the CFST-PZT coupling system without and with interfacial debonding defects under single period impulse signal is simulated and compared. The voltage response time history of the embedded PZT sensor in coupling models under both sinusoidal and sweep frequency sinusoidal excitations are simulated as an alternative solver. Numerical results demonstrate that interface debonding leads to changes in guided wave propagation path, traveling time and the PZT sensor response. The sensitivity of a quantitative evaluation index called as the normalized wavelet packet energy of the output voltage of embedded PZT sensor to the initiation of interface debonding defect and its length and depth is illustrated.

Monitoring degradation in concrete filled steel tubular sections using guided waves

Concrete filled steel tubes are extensively applied in engineering structures due to their resistance to high tensile and compressive load and convenience in construction. But one major flaw, their vulnerability to environmental attack, can severely reduce the strength and life of these structures. Degradation due to corrosion of steel confining the concrete is one of the major durability problems faced by civil engineers to maintain these structures. The problem accelerates as inner surface of steel tube is in contact with concrete which serves as electrolyte. If it remains unnoticed, it further accelerates and can be catastrophic. This paper discusses a non-destructive degradation monitoring technique for early detection corrosion in steel tubes in CFST members. Due to corrosion, damage in the form of debonding and pitting occurs in steel sections. Guided ultrasonic waves have been used as a feasible and attractive solution for the detection and monitoring of corrosion damages in CFST sections. Guided waves have been utilized to monitor the effect of notch and debond defects in concrete filled steel tubes simulating pitting and delamination of steel tubes from surrounding concrete caused by corrosion. Pulse transmission has been used to monitor the healthy and simulated damaged specimens. A methodology is developed and successfully applied for the monitoring of concrete filled steel tubular sections undergoing accelerated chloride corrosion. The ultrasonic signals efficiently narrate the state of steel tube undergoing corrosion.

Wave propagation simulation and its wavelet package analysis for debonding detection of circular CFST members

In order to investigate the interface debonding defects detection mechanism between steel tube and concrete core of concrete-filled steel tubes (CFSTs), multi-physical fields coupling finite element models constituted of a surface mounted Piezoceramic Lead Zirconate Titanate (PZT) actuator, an embedded PZT sensor and a circular cross section of CFST column are established. The stress wave initiation and propagation induced by the PZT actuator under sinusoidal and sweep frequency excitations are simulated with a two dimensional (2D) plain strain analysis and the difference of stress wave fields close to the interface debonding defect and within the cross section of the CFST members without and with debonding defects are compared in time domain. The linearity and stability of the embedded PZT response under sinusoidal signals with different frequencies and amplitudes are validated. The relationship between the amplitudes of stress wave and the measurement distances in a healthy CFST cross section is also studied. Meanwhile, the responses of PZT sensor under both sinusoidal and sweep frequency excitations are compared and the influence of debonding defect depth and length on the output voltage is also illustrated. The results show the output voltage signal amplitude and head wave arriving time are affected significantly by debonding defects. Moreover, the measurement of PZT sensor is sensitive to the initiation of interface debonding defects. Furthermore, wavelet packet analysis on the voltage signal under sweep frequency excitations is carried out and a normalized wavelet packet energy index (NWPEI) is defined to identify the interfacial debonding. The value of NWPEI attenuates with the increase in the dimension of debonding defects. The results help understand the debonding defects detection mechanism for circular CFST members with PZT technique.

NUMERICAL SIMULATION ON STRESS WAVE PROPAGATION OF STEEL-CONCRETE COMPOSITE STRUCTURES WITH INTERFACE DEBONDING BY SPECTRAL ELEMENT METHOD

To investigate the elastic stress wave propagation in steel-concrete composite structures, a 2 dimensional spectral element model for steel-concrete composite sections was established and the elastic stress wave propagation in the hybrid section with and without interface debonding defects was simulated under single point excitation. The interface debonding was simulated by removing concrete elements between steel tube and concrete core. The wave field and displacement responses of different nodes between the models with and without defects were compared to explain the effect of interface debonding on elastic stress wave propagation. Finally, the displacement responses of different numerical models with different levels of interface debonding were compared to investigate the relationship between the displacement response and the interface debonding defects. Spectral element method (SEM) provides an efficient way for the stress wave simulation of steel-concrete composite structures, which is helpful for the study on the mechanism for interface debonding detection of steel-concrete composite structures based on stress wave measurement.

A Time-Frequency Research for Ultrasonic Guided Wave Generated from the Debonding Based on a Novel Time-Frequency Analysis Technique

Carbon fibre composites have a promising application in the future of the vehicle, because of their high strength and light weight. Debonding is a major defect of the carbon fibre composite. The time-frequency analysis is fundamental to identify the defect on ultrasonic nondestructive evaluation and testing. In order to obtain the instantaneous frequency and the peak time of modes of the ultrasonic guided wave, an algorithm based on the Smoothed Pseudo Wigner-Ville distribution and the peak-track algorithm is presented. In the algorithm, a masking step is proposed, which can guarantee that the peak-track algorithm can automatically exact the instantaneous frequency and the instantaneous amplitude of different modes on the Smoothed Pseudo Wigner-Ville distribution. An experiment for detecting the debonding for a type of carbon fibre composite is done. The presented algorithm is employed on the experimental signals. The processed result of experimental signals reveals that the defect can stimulate new modes, and there is a quantitative relationship between the defect size and the frequency of the new mode. The presented technique provides a valuable way to detect the presentence, calculate the size, and locate the position of the debonding defect.

Research on a laser ultrasound method for testing the quality of a nuclear radiation protection structure

Laser ultrasonics has been investigated for inspecting the quality of a nuclear radiation protection structure. A possibility is proposed to improve the signal to noise ratio (SNR) of a laser ultrasonic inspection system. Then, a nuclear radiation protection structure composed of an AISI 1045 steel sheet connected with a lead alloy sheet by using an epoxy resin adhesive was manufactured with simulated defects. A non-contact laser ultrasonic inspection system, where the measured signals were filtered using a wavelet threshold de-noising method, was established to conduct a series of experiments. The proposed signal processing method can significantly improve the SNR of measured laser ultrasound signals on a rough solid surface. Compared with the SNR of original ultrasonic signals measured in transmission and reflection, the SNR of processed transmitted and reflected signals is improved by 13.8 and 16.6 dB, respectively. Moreover, laser ultrasonic C-scans based on the transmission and pulse-echo method can detect the simulated de-bonding defects, and the relative deviation between the measured sizes and design values is below 9%. Therefore, the laser ultrasonic method combined with effective signal processing can achieve the quantitative characterization of de-bonding defects in nuclear radiation protection structures.

Nondestructive examination of polymethacrylimide composite structures with terahertz time-domain spectroscopy

THz reflective time domain spectroscopy (THz-RTDS) has been considered as an effective method to detect hidden objects with potential for supplementing other NDE technologies for foam composite adhesive structure debonding defects. PMI (Polymethacrylimide) is a heat-resistant foam material, with the highest strength and stiffness to weight ratio. It is widely used in various parts of airplanes, especially the wing leading edge and rudder, landing gear doors, wing-body/wingtip fairings and so on. We analyzed the features of adhesive debonding defect based mainly on the variation of the time-domain wave form and compared with the inclusion defect. The quantification of degrees of adhesive debonding can be readily achieved with THz-RTDS images based on the delay of the wave front and the main reflection time-domain waveforms. Typical accuracy of about 100 μm was achieved.

Assembly of semi-analytical models to address linear buckling and vibration of stiffened composite panels with debonding defect

The substitution of conventional mechanical fasteners by adhesive joints has been advocated by the aircraft and aerospace industries due to the weight saving potential. Flaws such as debonding of the adhesive layer between the skin and the stiffener may greatly affect the structural behavior of composite panels. Within this context, this work presents a semi-analytical approach for the numerical investigation on the effects of skin-stiffener bonding flaw size on the vibration and linear buckling behavior of T-stiffened composite panels. Skin and stiffener have been modeled using an assembly of curved and flat panel components, with each domain approximated using a set of hierarchical polynomial functions. A penalty-based approach has been used to assemble the various domains and to model the debonded region between the stiffener flange base and the plate. This approach ensures full compatibility in terms of displacements and rotations between the stiffener’s base top face and the panel bottom face allowing to model different skin/stiffener debonding lengths. The results obtained using the proposed semi-analytical models have been compared and verified against numerical predictions based on finite element analyses.

Numerical study on the mechanism of active interfacial debonding detection for rectangular CFSTs based on wavelet packet analysis with piezoceramics

In recent years, Piezoelectric Lead Zirconate Titanate (PZT) based active interfacial debonding defect detection approach for concrete-filled steel tubular (CFST) columns has been proposed and validated experimentally. In order to investigate the mechanism of the PZT based interfacial debonding detection approach, a multi-physics coupling finite element model (FEM) composed of surface-mounted PZT actuator, embedded PZT sensor and a rectangular CFST column is constructed to numerically simulate the stress wave propagation induced by the surface-mounted PZT actuator under different excitation signals with different frequency and amplitude. The measurements of the embedded PZT sensor in concrete core of the CFST columns with different interfacial debonding defect lengths and depths are determined numerically with transient dynamic analysis. The linearity between the PZT response and the input amplitude, the effect of different frequency and measurement distance are discussed and the stress wave fields of CFST members without and with interface debonding defects are compared. Then, the response of the embedded PZT in concrete core is analyzed with wavelet packet analysis. The root mean square deviation (RMSD) of wavelet packet energy spectrum of the PZT measurement is employed as an evaluation index for the interfacial debonding detection. The results showed that the defined index under continuous sinusoidal and sweep frequency signals changes with the interfacial defects length and depth and is capable of effectively identifying the interfacial debonding defect between the concrete core and the steel tubular. Moreover, the index under sweep frequency signal is more sensitive to the interfacial debonding. The simulation results indicate that the interfacial debonding defect leads to the changes in the propagation path, travel time and the magnitude of stress waves. The simulation results meet the findings from the previous experimental study by the authors and help understand the mechanism of interfacial debonding defect detection for CFSTs using PZT technology.

Study on intelligent recognition detection technology of debond defects for ceramic matrix composites based on terahertz time domain spectroscopy.

With the wide use of high-temperature-resistant ceramic matrix composites (CMCs) in aviation and space flight, it is important to detect the quality of the bonding. This paper used terahertz (THz) time-domain spectroscopy nondestructive testing technology to inspect the bonding defects of the CMC. This paper puts forward a method-extraction method, which is applied to make samples to simulate the bonding defect of CMC by embedding polytetrafluoroethylene (PTFE) sheets with 0.12 mm thickness into the adhesive layer and extracting it after curing and presetting the bonding defects. On the basis of the classical and analytical algorithms, such as the maximum in time-domain and power spectrum integration, through further study in the THz spectral characteristics of bonding samples for CMC, we specifically introduce the upper debond coefficient, lower debond coefficient, average absorption coefficient for the frequency domain, centroid coefficient for the frequency domain, and other characteristics. By optimizing the THz detection characteristics set, as a sample, we adopt the neural network intelligent recognition algorithm to detect the upper and lower debond defects in samples and realize the intelligent identification for CMC debond defects.

Nondestructive Evaluation of Friction Stir-Welded Aluminum Alloy to Coated Steel Sheet Lap Joint

Dissimilar lap joints of aluminum sheet (AA 6061) of 2 mm thickness and zinc-coated steel sheet of 1 mm thickness were produced by friction stir welding with different combinations of rotational speed and travel speed. Ultrasonic C- and B-scanning, and radiography have been used in a complementary manner for detection of volumetric (cavity and flash) and planar (de bond) defects as the defects are in micron level. Advanced ultrasonic C-scanning did not provide any idea about the defects, whereas B-scanning cross-sectional image showed an exclusive overview of the micron-level defects. A digital x-ray radiography methodology is proposed for quality assessment of the dissimilar welds which provide three-fold increase in signal-to-noise ratio with improved defect detection sensitivity. The present study clearly shows that the weld tool rotational speed and travel speed have a decisive role on the quality of the joints obtained by the friction stir welding process. The suitability of the proposed NDE techniques to evaluate the joint integrity of dissimilar FSW joints is thus established.

Investitation of mobile ultrasonic propagation imager as a tool for composite wind blade quality control

According to the statistical reports, almost 50% of wind blade failures are due to manufacturing defects. Therefore, reliable nondestructive testing (NDT) methods are required to create a standard for blade manufacturing quality control that reduces maintenance costs and catastrophic accidents. Applying conventional NDT methods is currently impossible without major modifications; this would also be time-consuming because wind turbine blades are large, thick and heterogeneous composite structures.This study proposes an in-process quality control (IPQC) method based on an ultrasonic propagation imaging (UPI) system. The UPI system is based on laser-induced ultrasound and has the capabilities of noncontact, fast, curved structural and real-time inspection results. These capacities have the potential to be applied to IPQC in blade manufacturing and assembly in a Smart Blade Factory. To provide a proof-of-concept for Smart Blade Factory based on the UPI system as an IPQC tool, a 750-kW actual blade section was tested, and the inspection results were presented based on a multiple-defect visualization platform. Debonding, inclusion and void defects in a wind blade made of glass fiber reinforced plastic and PVC grid panels were studied. The inspection results proved high feasibility for the application of the UPI system to IPQC in blade manufacturing.

Thermo-mechanical stress analysis in platelet reinforced composites with bonded and debonded platelet end

An analytical model has been performed to analyze the stress transfer in platelet reinforced composite subjected to both tensile loading and residual thermal stresses. Two sets of the matrix/platelet displacement solutions, which were called respectively as the far-field solution and the transient solution, were exactly derived based on the theory of elasticity. These two sets of the displacement solutions were then superposed to obtain simplified analytical expressions for the matrix/platelet stress field components. The main difference with the previous works here were that the thermal residual stresses were considered in this article. The analytical results obtained here are then validated by the FEM modeling. Interestingly, good agreements are found between the analytical and numerical predictions. Another superiority of the proposed analytical model was in its capability of solving problems with platelet/matrix debonding defect.

A novel image enhancement algorithm based on stationary wavelet transform for infrared thermography to the de-bonding defect in solid rocket motors

In this paper, a multi-scale image enhancement algorithm based on low-passing filtering and nonlinear transformation is proposed for infrared testing image of the de-bonding defect in solid propellant rocket motors. Infrared testing images with high-level noise and low contrast are foundations for identifying defects and calculating the defects size. In order to improve quality of the infrared image, according to distribution properties of the detection image, within framework of stationary wavelet transform, the approximation coefficients at suitable decomposition level is processed by index low-passing filtering by using Fourier transform, after that, the nonlinear transformation is applied to further process the figure to improve the picture contrast. To verify validity of the algorithm, the image enhancement algorithm is applied to infrared testing pictures of two specimens with de-bonding defect. Therein, one specimen is made of a type of high-strength steel, and the other is a type of carbon fiber composite. As the result shown, in the images processed by the image enhancement algorithm presented in the paper, most of noises are eliminated, and contrast between defect areas and normal area is improved greatly; in addition, by using the binary picture of the processed figure, the continuous defect edges can be extracted, all of which show the validity of the algorithm. The paper provides a well-performing image enhancement algorithm for the infrared thermography.

Quality evaluation of rubber-to-metal bonded structures based on shearography

The quality of bonding is often a concern in rubber-to-metal bonded parts with regard to the integrity and stability of the structure. In this study, shearography has been used to detect the out-of-plane deformation of the defects caused by thermal and vacuum stresses. A sample of the vulcanized styrene-butadiene rubber bonded to steel with known artificial voids was prepared and during bonding process a region was glued with thick adhesives. The rubber surface of the sample was inspected in thermal radiation and vacuum modes respectively with a self-designed shearography device. Meanwhile, a numerical simulation was conducted to predict the out-of-plane deformation of the rubber surface in these two stress modes. Results from the numerical simulation and the experiments indicated that the debonding defects could be inspected in both loading modes. In the thermal radiation mode the region with thick adhesives could be identified successfully. This study provides a guideline for quality control of rubber-to-metal structures using an optical method.

광 적외선열화상을 이용한 풍력 블레이드의 결함 크기 정량화 연구

Abstract A wind turbine blade is an important component in wind-power generation, and is generally exposed toharsh environmental conditions. Ultrasonic inspection is mainly used to inspect such blades, but it has beendifficult to quantify defect sizes in complicated composite structures. Recently, active infrared thermography hasbeen widely studied for inspecting composite structures, in which thermal energy is applied to an object, and aninfrared camera detects the energy emitted from it. In this paper, a calibration method for active optical lock-inthermography is proposed to quantify the size. Inclusion, debonding and wrinkle defects, created in a wind bladefor 100 kW wind power generation, were all successfully detected using this method. In particular, a 50.0φ mmdebonding defect was sized with 98.0% accuracy. Keywords: Wind Turbine Blade, Optical Infrared Thermography, Lock-in Method, Quantitative Defects Detection[Received: December 18, 2014, Revised: February 3, 2015, Accepted: February 17, 2015] *과 학기술연합 대원교(UST) ** ,항공기시스템공학 한국표준과학연구원 안전측정센터 Corresponding Author: Safety MeasurementCenter, Korea Research Institute of Standards and Science, Daejeon 305-340, Korea (E-mail: mychoi@kriss.re.kr)