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Damage Inspection of Composite Structures
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Damage Inspection of Composite Structures

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Acoustics and Vibrations".

Deadline for manuscript submissions: closed (28 February 2019) | Viewed by 56581

Special Issue Editors

Dr. Nathalie Godin

E-Mail Website
Guest Editor
INSA of Lyon, MATEIS lab, UMR CNRS 5510, 69621 Villeurbanne, France
Interests: acoustic emission; damage mechanisms; mechanical behaviour; lifetime prediction; diagnostic and prognostic; modelling of AE in composite materials
Special Issues, Collections and Topics in MDPI journals
Dr. Athanasios Anastasopoulos

E-Mail Website
Guest Editor
Mitras Group Hellas ABEE, 14452 Athens, Greece
Interests: non-destructive testing (NDT); structural health monitoring; advanced signal processing by means of DSP, pattern recognition, and neural networks; acoustic emission; field testing; non-destructive inspection; composites

Special Issue Information

Dear Colleagues,

This special issue is dedicated to the damage detection and assessment that occurs in all kinds of composite materials and structures (polymer matrix composites, metal matrix composites, ceramics matrix composites, etc.). The present Special Issue intends to explore new directions in the field of non-destructive methods and prognostics applied to composite materials. Prognostics is a natural extension of the structural health monitoring (SHM). Indeed, the evaluation of the remaining useful lifetime is a key point from both safety and economic points of view. While the Issue is open to all interested researchers, several contributions come from the relevant session of the International Conference on Experimental Mechanics, ICEM18, 1-5 July 2018, Brussels.

The interests of this Special Issue include-but are not restricted to-the use of acoustic technology on composite materials and structures in various fields (aerospace, automobile, etc.). Experimental and numerical studies are welcome.

Topics of interest (among others) include:

  • Detection and identification of several damage mechanisms

  • Innovative methodologies in detection

  • Improvement in the localization of damage sources

  • Combination between several monitoring techniques

  • Diagnosis and prognostics methods of failures modes

  • Structural health monitoring (SHM)

  • Residual useful life prediction

Assoc. Prof. Dr. Nathalie Godin
Dr. Athanasios Anastasopoulos
Guest Editors

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Composite material

  • Damage evaluation

  • Lifetime prediction

  • Structural health monitoring (SHM)

  • Prognostic health management (PHM)

  • Non-destructive evaluation

  • Acoustic emission

  • Ultrasonic

  • Thermography

  • Fiber optics

  • Resistivity measurement

  • etc.

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Published Papers (13 papers)

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15 pages, 4255 KiB  
Article
Experimental Investigation on the Performance of Historical Squat Masonry Walls Strengthened by UHPC and Reinforced Polymer Mortar Layers
by Bin Peng, Sandong Wei, Libo Long, Qizhen Zheng, Yueqiang Ma and Leiyu Chen
Appl. Sci. 2019, 9(10), 2096; https://doi.org/10.3390/app9102096 - 21 May 2019
Cited by 11 | Viewed by 3321
Abstract
Strengthening historical brick masonry walls is important because these walls are major load-bearing members in many architectural heritages. However, historical brick masonry has low elastic modulus and low strength, historical masonry walls are prone to surface treatment or other structural intervention, and some [...] Read more.
Strengthening historical brick masonry walls is important because these walls are major load-bearing members in many architectural heritages. However, historical brick masonry has low elastic modulus and low strength, historical masonry walls are prone to surface treatment or other structural intervention, and some of the walls lack integrity. These characteristics make effective strengthening of historical masonry walls difficult. To address the issue, strengthening layers made up of ultra-high performance concrete (UHPC) are potentially useful. To investigate the strengthening effect of the UHPC layers, the authors constructed three squat walls using historical bricks and mortar collected from the rehabilitation site of a historical building, and strengthened two of the walls with a UHPC layer and a reinforced polymer mortar layer respectively. The three walls were broken down by horizontal cyclic force along with constant vertical compression, and then the unstrengthened one was strengthened in-situ by a UHPC layer and was tested again. The experimental results indicate that the UHPC layers significantly improved the in-plane shear resistance and cracking load of the squat walls, without decreasing the walls’ ultimate deformation. They effectively strengthened both moderately and severely damaged historical masonry walls, because the UHPC filled the existing damages and improved the integrity of the masonry substrate. In addition, the UHPC layers intervened the historical walls less than the reinforced polymer mortar layer. Therefore, the UHPC layers are efficient in strengthening historical squat masonry walls. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>The historical building and bricks: (<b>a</b>) The historical building under rehabilitation; (<b>b</b>) the bricks sampled from the historical building.</p> Full article ">Figure 2
<p>Profile of the historical wall specimens: (<b>a</b>) Front view; (<b>b</b>) right view of W1; (<b>c</b>) right view of W2 and W1S; (<b>d</b>) right view of W3.</p> Full article ">Figure 3
<p>Construction of the strengthening layers: (<b>a</b>) The UHPClayer; (<b>b</b>) The reinforced polymer mortar layer.</p> Full article ">Figure 4
<p>Test setup.</p> Full article ">Figure 5
<p>Lateral load cycles.</p> Full article ">Figure 6
<p>Lateral displacement of the walls: (<b>a</b>) W1; (<b>b</b>) W1S; (<b>c</b>) W2; (<b>d</b>) W3.</p> Full article ">Figure 7
<p>Failure modes of the walls: (<b>a</b>) Shear-compressive failure of the unstrengthened W1; (<b>b</b>) bed-joint slide of the strengthened walls.</p> Full article ">Figure 8
<p>In-plane shear resistance of the walls: (<b>a</b>) Skeleton curves; (<b>b</b>) resistance.</p> Full article ">Figure 9
<p>Stiffness degradation: (<b>a</b>) Stiffness degradation during the loading procedures; (<b>b</b>) stiffness at typical states.</p> Full article ">Figure 10
<p>Hysteretic loops of the walls: (<b>a</b>) W1; (<b>b</b>) W1S; (<b>c</b>) W2; (<b>d</b>) W3.</p> Full article ">Figure 11
<p>Energy dissipation coefficients (<span class="html-italic">E<sub>d</sub></span>) of the walls: (<b>a</b>) Calculation of the energy dissipation coefficient; (<b>b</b>) energy dissipation coefficients of the walls.</p> Full article ">Figure 12
<p>Percentage of performance rehabilitation and improvement of the strengthened walls comparing to the unstrengthened W1: (<b>a</b>) W1S; (<b>b</b>) W2; (<b>c</b>) W3.</p> Full article ">Figure 13
<p>Interaction mechanism between the UHPC layer and the masonry substrate.</p> Full article ">Figure 14
<p>The interface between the strengthening layers and the masonry substrate: (<b>a</b>) The coarse interface between the UHPC layer and the masonry substrate; (<b>b</b>) The smooth interface between the reinforced polymer mortar layer and the masonry substrate.</p> Full article ">Figure 15
<p>Strain of the strengthening layers: (<b>a</b>) Strain of the reinforcing bars of W3, the gauge position is illustrated in <a href="#applsci-09-02096-f003" class="html-fig">Figure 3</a>b; (<b>b</b>) Strain of the UHPC of W2, the gauge position is illustrated in <a href="#applsci-09-02096-f004" class="html-fig">Figure 4</a>.</p> Full article ">
17 pages, 3882 KiB  
Article
Guided Wave-Based Monitoring of Evolution of Fatigue Damage in Glass Fiber/Epoxy Composites
by Gang Yan, Xiang Lu and Jianfei Tang
Appl. Sci. 2019, 9(7), 1394; https://doi.org/10.3390/app9071394 - 3 Apr 2019
Cited by 9 | Viewed by 3134
Abstract
This paper presents an experimental study on detecting and monitoring of evolution of fatigue damage in composites under cyclic loads by using guided waves. Composite specimens fabricated by glass fiber/epoxy laminates and surface mounted with piezoelectric wafers are fatigued under tension–tension loads. A [...] Read more.
This paper presents an experimental study on detecting and monitoring of evolution of fatigue damage in composites under cyclic loads by using guided waves. Composite specimens fabricated by glass fiber/epoxy laminates and surface mounted with piezoelectric wafers are fatigued under tension–tension loads. A laser extensometer is used to obtain the degradation of longitudinal stiffness of the specimens under fatigue states to reflect the accumulation of internal fatigue damage. Meanwhile, at different fatigue cycles, one wafer acts as actuator to excite diagnostic guided waves, and the other acts as sensor to receive corresponding response waves. These guided wave signals are then processed by wavelet packet transform to extract characteristic features of energies in multiple frequency bands. A statistical multivariate outlier analysis is then performed to determine the existence of fatigue damage and to characterize their evolution using Mahalanobis squared distance. Experimental results have demonstrated the potential applicability and effectiveness of guided waves for continuous monitoring of fatigue damage in composite structures. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Figure 1

Figure 1
<p>Experimental setup: (<b>a</b>) glass fiber/epoxy GF/EP specimen and (<b>b</b>) fatigue test and monitoring systems.</p> Full article ">Figure 2
<p>Longitudinal stiffness degradation with fatigue cycles: (<b>a</b>) specimen S_A and (<b>b</b>) specimen S_B.</p> Full article ">Figure 2 Cont.
<p>Longitudinal stiffness degradation with fatigue cycles: (<b>a</b>) specimen S_A and (<b>b</b>) specimen S_B.</p> Full article ">Figure 3
<p>Diagnostic excitation with center frequency of 250 kHz: (<b>a</b>) time trace and (<b>b</b>) frequency spectrum.</p> Full article ">Figure 4
<p>Response wave signals after different fatigue cycles for specimen S_A: (<b>a</b>) six signals and (<b>b</b>) zoomed signals.</p> Full article ">Figure 5
<p>Response wave signals after different fatigue cycles for specimen S_B: (<b>a</b>) six signals and (<b>b</b>) zoomed signals.</p> Full article ">Figure 5 Cont.
<p>Response wave signals after different fatigue cycles for specimen S_B: (<b>a</b>) six signals and (<b>b</b>) zoomed signals.</p> Full article ">Figure 6
<p>Wavelet packet (WP) component energies for the baseline wave signal: (<b>a</b>) specimen S_A and (<b>b</b>) specimen S_B.</p> Full article ">Figure 6 Cont.
<p>Wavelet packet (WP) component energies for the baseline wave signal: (<b>a</b>) specimen S_A and (<b>b</b>) specimen S_B.</p> Full article ">Figure 7
<p>Variations of WP component energies of wave signals with fatigue cycles: (<b>a</b>) specimen S_A and (<b>b</b>) specimen S_B.</p> Full article ">Figure 8
<p>Contaminated baseline signals with different levels for specimen S_A: (<b>a</b>) Level 1 contamination and (<b>b</b>) Level 2 contamination.</p> Full article ">Figure 9
<p>Outlier analysis results for specimen S_A with Level 1 contamination: (<b>a</b>) probability density function (PDF) of baseline Mahalanobis squared distance (MSD) and (<b>b</b>) MSDs for different fatigue states.</p> Full article ">Figure 10
<p>Outlier analysis results for specimen S_A with Level 2 contamination: (<b>a</b>) PDF of baseline MSD and (<b>b</b>) MSDs for different fatigue states.</p> Full article ">Figure 10 Cont.
<p>Outlier analysis results for specimen S_A with Level 2 contamination: (<b>a</b>) PDF of baseline MSD and (<b>b</b>) MSDs for different fatigue states.</p> Full article ">Figure 11
<p>Outlier analysis results for specimen S_B with Level 1 contamination: (<b>a</b>) PDF of baseline MSD and (<b>b</b>) MSDs for different fatigue states.</p> Full article ">Figure 12
<p>Outlier analysis results for specimen S_B with Level 2 contamination: (<b>a</b>) PDF of baseline MSD and (<b>b</b>) MSDs for different fatigue states.</p> Full article ">
12 pages, 3256 KiB  
Article
An Omnidirectional Near-Field Comprehensive Damage Detection Method for Composite Structures
by Zhiling Wang, Zhenwei Xiao, Yonglin Li and Yudong Jiang
Appl. Sci. 2019, 9(3), 567; https://doi.org/10.3390/app9030567 - 8 Feb 2019
Cited by 5 | Viewed by 2410
Abstract
As one of the active structural health monitoring methods based on the Lamb wave, the ultrasonic phased-array damage detection method can provide information such as damage location and range more intuitively, which is why this method is a research hotspot in the field [...] Read more.
As one of the active structural health monitoring methods based on the Lamb wave, the ultrasonic phased-array damage detection method can provide information such as damage location and range more intuitively, which is why this method is a research hotspot in the field of Lamb wave-based damage monitoring. However, the ultrasonic phased-array damage detection method intended for the far field is not applicable to near-field damage monitoring. In addition, the traditional one-dimensional piezoelectric phased-array damage imaging method suffers from a blind area in the near field, and the data collection time of its angle scanning is relatively long. In view of these problems, this paper proposes an omnidirectional damage imaging monitoring method, combining the near-field sampling phased-array damage monitoring algorithm and the two-dimensional phased-array. The proposed method is verified by experiments using complex composite materials, and the results obtained show that the proposed omnidirectional near-field sampling phased-array damage imaging method is suitable for real-time damage detection in complex composite materials. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Figure 1

Figure 1
<p>The near-field wavefront diagram of a one-dimensional array.</p> Full article ">Figure 2
<p>The principle of sampling near-field phased-array ultrasound imaging.</p> Full article ">Figure 3
<p>Flowchart of the near-field sampling ultrasonic phased-array damage imaging monitoring process.</p> Full article ">Figure 4
<p>The fuel tank structure.</p> Full article ">Figure 5
<p>The experimental setup. (<b>a</b>) The experimental system (<b>b</b>) The cross-array and its numbered components.</p> Full article ">Figure 6
<p>Schematic diagram of the piezoelectric sensor and energizing sensing channel.</p> Full article ">Figure 7
<p>The propagation of the signal in the 90°-direction.</p> Full article ">Figure 8
<p>The tank speed curve.</p> Full article ">Figure 9
<p>The locations of the piezoelectric elements and single damages.</p> Full article ">Figure 10
<p>The excitation signal and the sensor signal.</p> Full article ">Figure 11
<p>The sensor and damage scattering signals.</p> Full article ">Figure 12
<p>Damage imaging. (<b>a</b>) Angle-time-amplitude damage imaging (<b>b</b>) Cartesian damage imaging.</p> Full article ">Figure 13
<p>Damage imaging after the enhancement of the cross-array image.</p> Full article ">
17 pages, 8371 KiB  
Article
Advanced NDT Methods and Data Processing on Industrial CFRP Components
by Vito Dattoma, Francesco Willem Panella, Alessandra Pirinu and Andrea Saponaro
Appl. Sci. 2019, 9(3), 393; https://doi.org/10.3390/app9030393 - 24 Jan 2019
Cited by 20 | Viewed by 4009
Abstract
In this work, enhanced thermal data processing is developed with experimental procedures, improving visualization algorithm for sub-surface defect detection on industrial composites. These materials are prone to successful infrared nondestructive investigation analyses, since defects are easily characterized by temperature response under thermal pulses [...] Read more.
In this work, enhanced thermal data processing is developed with experimental procedures, improving visualization algorithm for sub-surface defect detection on industrial composites. These materials are prone to successful infrared nondestructive investigation analyses, since defects are easily characterized by temperature response under thermal pulses with reliable results. Better defect characterization is achieved analyzing data with refined processing and experimental procedures, providing detailed contrasts maps where defects are better distinguished. Thermal data are analyzed for different CFRP specimens with artificial defects and experimental procedures are verified on real structural aeronautical component with internal anomalies due to impact simulation. A better computation method is found to be useful for simultaneous defect detection by means of automatic mapping of absolute contrast, optimized to identify defect boundaries. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Figure 1

Figure 1
<p>(<b>a</b>) Layout of artificial defects and position. (<b>b</b>) Exploded Assembly of Plate I, identical for Plate II.</p> Full article ">Figure 2
<p>(<b>a</b>) CFRP multi stringer multi-stringer component’s geometry and (<b>b</b>) and artificial damage geometry.</p> Full article ">Figure 3
<p>(<b>a</b>) Top view of the thermographic setup for CFRP plates’ acquisition and (<b>b</b>) experimental setup.</p> Full article ">Figure 4
<p>(<b>a</b>) CATIA assembly view of new PLA bracket tool and (<b>b</b>) experimental setup for ultrasonic C-scan controls around simulated impact zone of aeronautical component.</p> Full article ">Figure 5
<p>(<b>a</b>) Lateral view of the thermographic setup for multi-stringer component’s acquisition and (<b>b</b>) experimental setup.</p> Full article ">Figure 6
<p>(<b>a</b>) Map of isotherm at the beginning of the cooling phase for real component and (<b>b</b>) raw thermal image for the central damaged zone of real component using heating time of 7 s.</p> Full article ">Figure 7
<p>(<b>a</b>) Map of isotherms at the beginning of the cooling phase for artificial Plate I and (<b>b</b>) defect presence example on raw thermal image, using heating time of 20 s.</p> Full article ">Figure 8
<p>(<b>a</b>) Thermal profiles and (<b>b</b>) absolute contrast example for defect-1 (Ø 25 mm) and intact area in Plate I, using heating time of 20 s.</p> Full article ">Figure 9
<p>(<b>a</b>) Thermal profiles and (<b>b</b>) absolute contrast example for defect-2 (Ø 10 mm) and intact area in Plate I, using heating time of 20 s.</p> Full article ">Figure 10
<p>(<b>a</b>) Thermal profiles on multi-stringer component on left side of impact and (<b>b</b>) absolute contrast curve for delaminations with respect to intact area, using heating time of 7 s.</p> Full article ">Figure 11
<p>(<b>a</b>) Thermal profiles on multi-stringer component on right side of impact and (<b>b</b>) absolute contrast curve for delaminations with respect to intact area, using heating time of 7 s.</p> Full article ">Figure 12
<p>Raw thermal image of Plate I with circular reference zones and defect’s relative Absolute contrast examples.</p> Full article ">Figure 13
<p>(<b>a</b>) Example of comparison between raw thermal map and (<b>b</b>) contrast map of CFRP Plate I/Side B.</p> Full article ">Figure 14
<p>Comparison between raw thermal map (<b>a</b>) and (<b>b</b>) contrast Image of CFRP Plate I on Side A obtained after 39.4 cooling seconds after heating pulse of 30 s.</p> Full article ">Figure 15
<p>Comparison between raw thermal map (<b>a</b>) and contrast image (<b>b</b>) of CFRP Plate II on Side A obtained after 5 cooling seconds after heating pulse of 15 s.</p> Full article ">Figure 16
<p>A-scan (<b>left</b>) and S-scan (<b>right</b>) of non-damaged zone around impact simulation cut.</p> Full article ">Figure 17
<p>C-scan inspection over the multi-stringer component’s surface on damaged zone along the artificial cut and an example of A-scan and S-scan on right side delamination.</p> Full article ">Figure 18
<p>A-scan and S-scan of two damaged zones on the left side (<b>a</b>) and the right side (<b>b</b>) of impact simulation cut of multi-stringer component.</p> Full article ">Figure 19
<p>Contrast map of multi-stringer component using heating pulse of 3 s (<b>a</b>) and 10 s (<b>b</b>).</p> Full article ">Figure 20
<p>Contrast map of multi-stringer component using heating pulse of 7 s.</p> Full article ">
13 pages, 4503 KiB  
Article
Detection and Characterization of Debonding Defects in Aeronautical Honeycomb Sandwich Composites Using Noncontact Air-Coupled Ultrasonic Testing Technique
by Honggang Li and Zhenggan Zhou
Appl. Sci. 2019, 9(2), 283; https://doi.org/10.3390/app9020283 - 15 Jan 2019
Cited by 24 | Viewed by 5299
Abstract
The finite models of honeycomb sandwich composite with intact and embedded debonding defects are constructed. The sound pressure in fluid domain and the stress strain problem in solid domain are related by acoustic-structure coupling method, which visually shows the propagation process and modal [...] Read more.
The finite models of honeycomb sandwich composite with intact and embedded debonding defects are constructed. The sound pressure in fluid domain and the stress strain problem in solid domain are related by acoustic-structure coupling method, which visually shows the propagation process and modal characteristics of the acoustic wave inside the honeycomb sandwich composite. The simulation results show that the transmission longitudinal wave T1 (transmission initial wave) can effectively characterize debonding defects of honeycomb sandwich composite. However, in the actual detection of honeycomb sandwich composite, there are some problems, such as poor Signal-to-noise ratio (SNR) of received signal, incognizable transmission initial wave. In order to solve these problems, this paper proposes to apply polyphase coded pulse compression technique to air-coupled ultrasonic testing system. The actual test results show that the SNR of received signal is effectively improved, the transmission initial wave can be effectively identified, and the compressed signal has a good response to debonding defect. The air-coupled ultrasonic testing C scan result of honeycomb sandwich composite verifies the rationality and correctness of the theoretical simulation and signal processing technique, which promotes industrial application of air-coupled ultrasonic testing technique in the aerospace field. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>The diagram of honeycomb sandwich composite.</p> Full article ">Figure 2
<p>The honeycomb sandwich composite model simplification process. (<b>b</b>) is the partial enlargement of the honeycomb composite material of (<b>a</b>); (<b>c</b>) is a cross section of the honeycomb core of (<b>b</b>).</p> Full article ">Figure 3
<p>The two-dimensional model of honeycomb sandwich composite.</p> Full article ">Figure 4
<p>The two dimensional model of honeycomb sandwich composite embedded with debonding defect.</p> Full article ">Figure 5
<p>Honeycomb sandwich composite sample and defects layout.</p> Full article ">Figure 6
<p>Schematic diagram of experimental setup; DAQ: Data Acquisition; PC: Personal Computer.</p> Full article ">Figure 7
<p>The simulation result of sound field distribution with 19.32μs in honeycomb sandwich composite.</p> Full article ">Figure 8
<p>The sound field distribution of a intact honeycomb sandwich composite model and a honeycomb sandwich composite model with embedded debonding defect at 20.01 μs; (<b>a</b>) The sound field distribution of an intact honeycomb sandwich composite model at 20.01 μs; (<b>b</b>) The sound field distribution of a honeycomb sandwich composite model with an embedded debonding defect at 20.01 μs. R: reflected longitudinal wave; El: edge longitudinal wave; T1: transmitted longitudinal wave; T2: transmitted longitudinal wave; LL: leakage Lamb wave; L: Lamb wave.</p> Full article ">Figure 9
<p>A-wave signal obtained from the computational simulation of the intact model and the embedded defect model. (<b>a</b>) corresponds to the intact model, and (<b>b</b>) corresponds to the embedded defect model.</p> Full article ">Figure 10
<p>The original signal for detecting honeycomb sandwich composite material.</p> Full article ">Figure 11
<p>The P3 polyphase coded pulse compression signals are collected separately from the defect area and the intact area. (<b>a</b>) is the compressed signal collected in the intact area, and (<b>b</b>) is the compressed signal collected in the defect area.</p> Full article ">Figure 12
<p>The C-scan result of honeycomb sandwich composite.</p> Full article ">
16 pages, 6301 KiB  
Article
Interface Characterization within a Nuclear Fuel Plate
by James Smith, Clark Scott, Brad Benefiel and Barry Rabin
Appl. Sci. 2019, 9(2), 249; https://doi.org/10.3390/app9020249 - 11 Jan 2019
Cited by 4 | Viewed by 3451
Abstract
To predict the performance of nuclear fuels and materials, irradiated fuel plates must be characterized efficiently and accurately in highly radioactive environments. The characterization must take place remotely and work in settings largely inhospitable to modern digital instrumentation. Characterization techniques based on non-contacting [...] Read more.
To predict the performance of nuclear fuels and materials, irradiated fuel plates must be characterized efficiently and accurately in highly radioactive environments. The characterization must take place remotely and work in settings largely inhospitable to modern digital instrumentation. Characterization techniques based on non-contacting laser sensing methods enable remote operation in a robust manner within a hot-cell environment. Laser characterization instrumentation can offer high spatial resolution and remain effective for scanning large areas. A laser shock (LS) system is currently being developed as a post-irradiation examination (PIE) technique in the hot fuel examination facility (HFEF) at the Idaho National Laboratory (INL). The laser shock technique will characterize material properties and failure loads/mechanisms in various composite components and materials such as plate fuel and next-generation fuel forms in high radiation areas. The laser shock-technique induces large amplitude shock waves to mechanically characterize interfaces such as the fuel–clad bond. As part of the laser shock system, a laser-based ultrasonic C-scan system will be used to detect and characterize debonding caused by the application of the laser shock. The laser shock system has been used to characterize the resulting bond strength within plate fuels which have been fabricated using different fabrication processes. The results of this study will be to select the fabrication process that provides the strongest interface. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Figure 1

Figure 1
<p>The plate fuel specimen, geometries, plasma constraining mechanism and back surface velocity detection for laser shock testing is shown.</p> Full article ">Figure 2
<p>The plate fuel specimen and geometries for laser shock testing is shown.</p> Full article ">Figure 3
<p>Ultrasonic C-scan images depicting bonded interfaces that let sound travel through the fuel plate. The hot isostatic pressing (HIP) process appears to have healed the coating blisters on foil 21E-2B1. Also note the presence of possible surface indentations and/or un-bonded areas indicated by the lack of transmission (dark spots).</p> Full article ">Figure 4
<p>Example of clad-clad plate with locations for shock testing indicated.</p> Full article ">Figure 5
<p>Example of fuel–cladding test fuel plate with locations for shock test indicated.</p> Full article ">Figure 6
<p>Diagrams showing the plate geometry and nomenclature used for laser shock testing. (<b>A</b>) is the cross-sectional figure, and (<b>B</b>) shows the experimental geometry.</p> Full article ">Figure 7
<p>This graph shows the measured maximum surface velocity measured in the various Los Alamos National Laboratory (LANL) interface samples with varying levels of contamination.</p> Full article ">Figure 8
<p>Results from bulge testing performed at LANL indicating that contaminated plates resulted in higher interfacial fracture energy compared to the plate made using the typical cleaning process [<a href="#B10-applsci-09-00249" class="html-bibr">10</a>].</p> Full article ">Figure 9
<p>Representative images of the colorized debond locations in LANL plates caused by laser shock testing are shown. Note: The images are not to scale.</p> Full article ">Figure 10
<p>This graph shows the measured maximum surface velocity measured in the various Pacific Northwest National Laboratory (PNNL) interface samples with variable process parameters. The fuel plates contain the corresponding fuel foils: 21C-2B, 21D-2B, 21E-2B1, 21D-1.</p> Full article ">Figure 11
<p>Representative images of the colorized debond locations caused by laser shock testing in PNNL plates are shown. Note: The images are not to scale.</p> Full article ">Figure 12
<p>Ultrasonic C-scan images of the FF fuel plates that have been laser shock tested. The dark regions indicate debonds in interfaces that reflect sound and keeps sound from traveling through the fuel plate to the back side. While the HIP process appears to have healed the coating blisters on foil 21E-2B1, the bond strength is low and the debond areas are large. Note that the exceptionally large debond areas for foil 21E-2B1 corresponds to the blistered side of the foil.</p> Full article ">Figure 13
<p>Location of the debond used for laser shockwave technique (LST) and section from an FF electroplated plasma fuel plate.</p> Full article ">Figure 14
<p>The separation of the DU fuel foil from the electroplated Zr is shown in in the right center portion of the figure. The white line is to emphasize the surface indentation which is most likely due to happenstance.</p> Full article ">Figure 15
<p>Laser UT inspection of the 102-D1 fuel plate is shown. The A-scan image shows ringing between the two yellow lines indicative of sound bouncing between a debonded ligament on the back interface (see <a href="#applsci-09-00249-f006" class="html-fig">Figure 6</a>).</p> Full article ">
15 pages, 4180 KiB  
Article
An Efficient Time Reversal Method for Lamb Wave-Based Baseline-Free Damage Detection in Composite Laminates
by Liping Huang, Junmin Du, Feiyu Chen and Liang Zeng
Appl. Sci. 2019, 9(1), 11; https://doi.org/10.3390/app9010011 - 20 Dec 2018
Cited by 12 | Viewed by 3388
Abstract
Time reversal (TR) concept is widely used for Lamb wave-based damage detection. However, the time reversal process (TRP) faces the challenge that it requires two actuating-sensing steps and requires the extraction of re-emitted and reconstructed waveforms. In this study, the effects of the [...] Read more.
Time reversal (TR) concept is widely used for Lamb wave-based damage detection. However, the time reversal process (TRP) faces the challenge that it requires two actuating-sensing steps and requires the extraction of re-emitted and reconstructed waveforms. In this study, the effects of the two extracted components on the performance of TRP are studied experimentally. The results show that the two time intervals, in which the waveforms are extracted, have great influence on the accuracy of damage detection of the time reversal method (TRM). What is more, it requires a large number of experiments to determine these two time intervals. Therefore, this paper proposed an efficient time reversal method (ETRM). Firstly, a broadband excitation is applied to obtain response at a wide range of frequencies, and ridge reconstruction based on inverse short-time Fourier transform is applied to extract desired mode components from the broadband response. Subsequently, deconvolution is used to extract narrow-band reconstructed signal. In this method, the reconstructed signal can be easily obtained without determining the two time intervals. Besides, the reconstructed signals related to a series of different excitations could be obtained through only one actuating-sensing step. Finally, the effectiveness of the ETRM for damage detection in composite laminates is verified through experiments. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>Schematic diagram of the time reversal process (TRP). PZT: Piezoelectric.</p> Full article ">Figure 2
<p>Schematic diagram of the proposed efficient time reversal method (ETRM).</p> Full article ">Figure 3
<p>Illustration of ridge and its neighbor area in time-frequency domain.</p> Full article ">Figure 4
<p>Schematic diagram of the composite plate with damage and transducers.</p> Full article ">Figure 5
<p>(<b>a</b>) The time interval for extracting re-emitted components varies with <span class="html-italic">weight1</span>; (<b>b</b>) the extract re-emitted components when <span class="html-italic">weight1</span> = 0.3; (<b>c</b>) the time interval for extracting reconstructed components varies with <span class="html-italic">weight2</span>; and (<b>d</b>) the comparison between the reconstructed components and the original input signal when <span class="html-italic">weight2</span> = 0.3.</p> Full article ">Figure 6
<p>Damage index (DI) values change with (<b>a</b>) <span class="html-italic">weight1</span> and (<b>b</b>) <span class="html-italic">weight2</span> in the health and damage condition.</p> Full article ">Figure 7
<p>The broadband response signal and the extracted direct wave of A0-mode Lamb wave.</p> Full article ">Figure 8
<p>The short-time Fourier transform (STFT) spectrogram of (<b>a</b>) the response signal and (<b>b</b>) the filtered direct wave of A0 mode.</p> Full article ">Figure 9
<p>Reconstructed signals to (<b>a</b>) five-cycle, Hanning windowed tone bursts at various central frequencies 40 kHz, 50 kHz, 60 kHz and 70 kHz and (<b>b</b>) 40 kHz Hanning windowed tone bursts with various pulse numbers 3, 5, 7, 9 as generated by the ETRM.</p> Full article ">Figure 10
<p>The DI value changes with <span class="html-italic">weight</span>.</p> Full article ">Figure 11
<p>Photo of the composite laminates and sensing path of the sensor array.</p> Full article ">Figure 12
<p>(<b>a</b>) The comparison between the reconstructed signals and the original input signal of Path 1 and Path 2 and (<b>b</b>) the DI values of the 16 paths.</p> Full article ">Figure 13
<p>ETRM-based damage imaging results at different <span class="html-italic">weight</span> values (<b>a</b>) <span class="html-italic">weight</span> = 0.1, (<b>b</b>) <span class="html-italic">weight</span> = 0.2 and (<b>c</b>) <span class="html-italic">weight</span> = 0.3.</p> Full article ">Figure 14
<p>The traditional TRM based damage imaging results at different <span class="html-italic">weight1</span> values (<b>a</b>) <span class="html-italic">weight1</span> = 0.1, (<b>b</b>) <span class="html-italic">weight1</span> = 0.2 and (<b>c</b>) <span class="html-italic">weight1</span> = 0.3.</p> Full article ">
19 pages, 9360 KiB  
Article
A Full-Process Numerical Analyzing Method of Low-Velocity Impact Damage and Residual Strength for Stitched Composites
by Hongjian Zhang, Mingming Wang, Weidong Wen, Ying Xu, Haitao Cui and Jinbo Chen
Appl. Sci. 2018, 8(12), 2698; https://doi.org/10.3390/app8122698 - 19 Dec 2018
Cited by 1 | Viewed by 3410
Abstract
The failure and residual strength after low-velocity impact of stitched composites are very important in their service and maintenance phases. In order to capture the failure and residual strength more accurately, a full-process numerical analyzing method was developed in this paper. The full-process [...] Read more.
The failure and residual strength after low-velocity impact of stitched composites are very important in their service and maintenance phases. In order to capture the failure and residual strength more accurately, a full-process numerical analyzing method was developed in this paper. The full-process numerical analyzing method includes two parts: (1) Part 1 is the progressive low-velocity impact damage prediction method for stitched composites; (2) Part 2 is the progressive residual strength prediction method by introducing all types of damage that are caused by the low-velocity impact as the analysis presuppositions. Subsequently, the failure and residual strength of G0827/QY9512 stitched composites were simulated by the full-process numerical analyzing method. When compared with experiments, it is found that: (1) the maximum error of low-velocity impact damage areas was 17.8%, and their damage modes were similar; (2) the maximum error of residual strength was 8.9%. At last, the influence rules of stitched density and stitching thread thickness were analyzed. The simulation results showed that, if there is no suture breakage failure, stitched density affects the mechanical properties of the stitched composites, while stitching thread thickness has little effect on it; otherwise, both factors have a significant effect on the mechanical properties. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>Transient analysis model of laminate and punch.</p> Full article ">Figure 2
<p>Relationship between two coordinate systems.</p> Full article ">Figure 3
<p>Simulation procedures of full-process analyzing method.</p> Full article ">Figure 4
<p>Finite element model of the stitched laminate.</p> Full article ">Figure 5
<p>Progressive damage process of the laminate and the damage projection diagrams obtained by experiment [<a href="#B18-applsci-08-02698" class="html-bibr">18</a>] and simulation after the impact of 16.8 J. Note: (1) DF: delamination failure; (2) FTF: fiber tensile failure; (3) FCF: fiber compressive failure; (4) MTF: matrix tensile failure; (5) MCF: matrix compressive failure; (6) FMSF: fiber-matrix shear failure; and, (7) SBF: suture breakage failure.</p> Full article ">Figure 6
<p>Final failure of each layer after the impact of 16.8 J.</p> Full article ">Figure 7
<p>Final failure of each interface after the impact of 16.8 J.</p> Full article ">Figure 8
<p>Finite element model of the stitched laminate.</p> Full article ">Figure 9
<p>Progressive damage process of the laminate after the impact of 16.8 J.</p> Full article ">Figure 10
<p>Full-process of low-velocity impact damage and residual strength for laminates with different stitched densities.</p> Full article ">Figure 11
<p>Distributions of different damage modes with different stitched densities after the low-velocity impact damage: (<b>a</b>) MTF; (<b>b</b>) FCF; (<b>c</b>) FTF; and, (<b>d</b>) DF.</p> Full article ">Figure 12
<p>The predicted residual strengths with different stitched densities.</p> Full article ">Figure 13
<p>SBF of the laminates after the impact of 10 J.</p> Full article ">Figure 14
<p>Progressive damage processes of the laminates after the impact of 10 J.</p> Full article ">Figure 15
<p>Distribution of different damage modes with different stitching thread thicknesses after the low-velocity impact damage: (<b>a</b>) MTF; (<b>b</b>) FCF; (<b>c</b>) FTF; and, (<b>d</b>) DF.</p> Full article ">Figure 16
<p>The effect of stitching thread thickness on residual strength under the impact of 10 J.</p> Full article ">
14 pages, 5248 KiB  
Article
Acoustic Emission Based on Cluster and Sentry Function to Monitor Tensile Progressive Damage of Carbon Fiber Woven Composites
by Wei Zhou, Peng-fei Zhang and Yan-nan Zhang
Appl. Sci. 2018, 8(11), 2265; https://doi.org/10.3390/app8112265 - 16 Nov 2018
Cited by 30 | Viewed by 4211
Abstract
Understanding the tensile failure mechanisms in carbon fiber woven composites based on the acoustic emission (AE) technique is a challenging task. In this study, the mechanical behaviors of composites were studied under uniaxial tensile loading. Meanwhile, the internal damage evolution process in composites [...] Read more.
Understanding the tensile failure mechanisms in carbon fiber woven composites based on the acoustic emission (AE) technique is a challenging task. In this study, the mechanical behaviors of composites were studied under uniaxial tensile loading. Meanwhile, the internal damage evolution process in composites was monitored by AE and the recorded AE signals were analyzed. To achieve the dominant damage mechanisms in composites, five AE parameters such as rise time, duration, energy, peak amplitude, and frequency were selected for cluster analysis by a k-means algorithm. The results show that AE signals can be divided into three clusters based on microscopic observations and frequency range. The three clusters correspond to three kinds of damage modes such as matrix cracking, fiber/matrix debonding, and fiber breakage. In addition, the sentry function (SF) was adopted to investigate AE signals originated from the internal damage evolution in composites. It was found that the drop in the SF curve corresponds to the serious damage of the composites. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>(<b>a</b>) The schematic illustration of weave composite. (<b>b</b>) Images of composite specimens.</p> Full article ">Figure 2
<p>Experimental system of tensile tests and acoustic emission (AE) monitoring.</p> Full article ">Figure 3
<p>A typical waveform for a burst-type AE signal.</p> Full article ">Figure 4
<p>Five different behaviors of sentry function (SF) vs. displacement.</p> Full article ">Figure 5
<p>(<b>a</b>) Typical AE waveforms. (<b>b</b>) The relationship between amplitude and duration of the AE signals.</p> Full article ">Figure 6
<p>The tensile loading curve versus displacement for the composite specimen.</p> Full article ">Figure 7
<p>(<b>a</b>) The image of the whole fractured sample. (<b>b</b>,<b>c</b>) SEM micro-photographs of the fractured composite: delamination and matrix crack; fiber breakage and fiber pull out.</p> Full article ">Figure 8
<p>Tensile load, accumulative energy, counts, energy release rate, and AE event count rate vs. time for the composite specimen.</p> Full article ">Figure 9
<p>Progressive amplitude and peak frequency distribution at 30%, 60%, and 90% of the failure load. (<b>a</b>) amplitude distribution; (<b>b</b>) peak frequency distribution.</p> Full article ">Figure 10
<p>Silhouette value as the function of the number of clustering class, <span class="html-italic">k</span>.</p> Full article ">Figure 11
<p>Average properties of three clusters with regard to the five AE parameters. (<b>a</b>) Class CL-1, (<b>b</b>) CL-2, and (<b>c</b>) CL-3.</p> Full article ">Figure 12
<p>The mechanism-based analysis of AE signals. (<b>a</b>) The distribution of frequency regarding the three clusters and (<b>b</b>) the SF diagram associated with the applied tensile load.</p> Full article ">Figure 13
<p>Summary of frequency range for damage mechanisms.</p> Full article ">
20 pages, 4348 KiB  
Article
Multiaxial Damage Characterization of Carbon/Epoxy Angle-Ply Laminates under Static Tension by Combining In Situ Microscopy with Acoustic Emission
by Kalliopi-Artemi Kalteremidou, Brendan R. Murray, Eleni Tsangouri, Dimitrios G. Aggelis, Danny Van Hemelrijck and Lincy Pyl
Appl. Sci. 2018, 8(11), 2021; https://doi.org/10.3390/app8112021 - 23 Oct 2018
Cited by 17 | Viewed by 2893
Abstract
Investigating the damage progression in carbon/epoxy composites is still a challenging task, even after years of analysis and study. Especially when multiaxial stress states occur, the development of damage is a stochastic phenomenon. In the current work, a combined nondestructive methodology is proposed [...] Read more.
Investigating the damage progression in carbon/epoxy composites is still a challenging task, even after years of analysis and study. Especially when multiaxial stress states occur, the development of damage is a stochastic phenomenon. In the current work, a combined nondestructive methodology is proposed in order to investigate the damage from the static tensile loading of carbon fiber reinforced epoxy composites. Flat angle-ply laminates are used to examine the influence of multiaxial stress states on the mechanical performance. In situ microscopy is combined with acoustic emission in order to qualitatively and quantitatively estimate the damage sequence in the laminates. At the same time, digital image correlation is used as a supporting tool for strain measurements and damage indications. Significant conclusions are drawn, highlighting the dominant influence of shear loading, leading to the deduction that the development of accurate damage criteria is of paramount importance. The data presented in the current manuscript is used during ongoing research as input for the damage characterization of the same material under fatigue loads. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>Bi-axiality ratios, <span class="html-italic">λ</span>, for different angles, <span class="html-italic">θ</span>, of [0°/<span class="html-italic">θ</span>]<sub>2s</sub> laminates.</p> Full article ">Figure 2
<p>Experimental set-up. DIC—digital image correlation; AE—acoustic emission.</p> Full article ">Figure 3
<p>Stress-longitudinal strain curves of the angle-ply laminates.</p> Full article ">Figure 4
<p>Damage state of [0°/30°]<sub>2s</sub> laminates at (<b>a</b>) 70%, (<b>b</b>) 80%, (<b>c</b>) 85%, and (<b>d</b>) 87% of σ<sub>ult</sub>.</p> Full article ">Figure 5
<p>Damage state of [0°/60°]<sub>2s</sub> laminates at (<b>a</b>) 40%, (<b>b</b>) 65%, (<b>c</b>) 80%, and (<b>d</b>) 90% of σ<sub>ult</sub>.</p> Full article ">Figure 6
<p>Damage process during the quasi-static tests for the [0°/30°]<sub>2s</sub> laminates.</p> Full article ">Figure 7
<p>Damage process during the quasi-static tests for the [0°/60°]<sub>2s</sub> laminates.</p> Full article ">Figure 8
<p>Stress–shear strain curves of the angle-ply laminates.</p> Full article ">Figure 9
<p>Evolution of delamination length in the [0°/30°]<sub>2s</sub> laminates.</p> Full article ">Figure 10
<p>Evolution of delamination length in the [0°/60°]<sub>2s</sub> laminates.</p> Full article ">Figure 11
<p>Increase of matrix crack density in the [0°/30°]<sub>2s</sub> laminates.</p> Full article ">Figure 12
<p>Increase of matrix crack density in the [0°/60°]<sub>2s</sub> laminates.</p> Full article ">Figure 13
<p>DIC σ pattern for a [0°/30°]<sub>2s</sub> laminate (<b>a</b>) before and (<b>b</b>) after delamination.</p> Full article ">Figure 14
<p>DIC σ pattern for a [0°/60°]<sub>2s</sub> laminate (<b>a</b>) before and (<b>b</b>) after delamination.</p> Full article ">Figure 15
<p>Evolution of the Poisson’s ratio of the angle-ply laminates.</p> Full article ">Figure 16
<p>Total AE events recorded for the two laminates during the static tests.</p> Full article ">Figure 17
<p>AE events rate recorded for the two laminates during the static tests.</p> Full article ">Figure 18
<p>Typical schematic of an AE waveform.</p> Full article ">Figure 19
<p>Evolution of average rise time values for the angle-ply laminates.</p> Full article ">Figure 20
<p>Evolution of average rise time versus the delamination length in the [0°/30°]<sub>2s</sub> laminates.</p> Full article ">
17 pages, 5089 KiB  
Article
Challenges and Limitations in the Identification of Acoustic Emission Signature of Damage Mechanisms in Composites Materials
by Nathalie Godin, Pascal Reynaud and Gilbert Fantozzi
Appl. Sci. 2018, 8(8), 1267; https://doi.org/10.3390/app8081267 - 31 Jul 2018
Cited by 51 | Viewed by 4998
Abstract
Acoustic emission is a part of structural health monitoring (SHM) and prognostic health management (PHM). This approach is mainly based on the activity rate and acoustic emission (AE) features, which are sensitive to the severity of the damage mechanism. A major issue in [...] Read more.
Acoustic emission is a part of structural health monitoring (SHM) and prognostic health management (PHM). This approach is mainly based on the activity rate and acoustic emission (AE) features, which are sensitive to the severity of the damage mechanism. A major issue in the use of AE technique is to associate each AE signal with a specific damage mechanism. This approach often uses classification algorithms to gather signals into classes as a function of parameters values measured on the signals. Each class is then linked to a specific damage mechanism. Nevertheless, each recorded signal depends on the source mechanism features but the stress waves resulting from the microstructural changes depend on the propagation and acquisition (attenuation, damping, surface interactions, sensor characteristics and coupling). There is no universal classification between several damage mechanisms. The aim of this study is the assessment of the influence of the type of sensors and of the propagation distance on the waveforms parameters and on signals clustering. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>Schematic diagram of an instrumented specimen with four sensors, (<b>a</b>) on ceramic matrix composites (CMC) samples without a waveguide (<b>b</b>) on CMC samples with a waveguide.</p> Full article ">Figure 2
<p>Calibration curve with the reciprocity method. Sensibility in reception for a sensor μ80 on steel material for the Rayleigh waves.</p> Full article ">Figure 3
<p>(<b>a</b>) Amplitude and frequency recorded by a μ80 sensor for signals of different frequencies and same energy generated by an acousto-ultrasonic card. (× Weighted Frequency, * Peak Frequency, + Central Frequency and ◊ Average Frequency) (<b>b</b>) PPi versus input frequency. (The input signal is generated with a specific frequency equal to 150 kHz up to 950 kHz, amplitude 5 volts and rise time 20 μs. (Propagation distance of 100 mm, composite material propagation medium with undamaged SiC<sub>f</sub>/SiC, actuator μ80 sensor).</p> Full article ">Figure 3 Cont.
<p>(<b>a</b>) Amplitude and frequency recorded by a μ80 sensor for signals of different frequencies and same energy generated by an acousto-ultrasonic card. (× Weighted Frequency, * Peak Frequency, + Central Frequency and ◊ Average Frequency) (<b>b</b>) PPi versus input frequency. (The input signal is generated with a specific frequency equal to 150 kHz up to 950 kHz, amplitude 5 volts and rise time 20 μs. (Propagation distance of 100 mm, composite material propagation medium with undamaged SiC<sub>f</sub>/SiC, actuator μ80 sensor).</p> Full article ">Figure 4
<p>Evolution of the recorded acoustic energy, the frequency centroid and the relative modulus versus strain for a tensile test on CMC at room temperature. (The input signal is generated with a frequency in the range of 150 kHz and 950 kHz, amplitude 5 volts and rise time 20 μs, actuator μ80 sensor).</p> Full article ">Figure 5
<p>Stress-strain curve and the cumulated recorded energy during a tensile test on glass fibres/vinylester matrix monitored with two types of sensors (μ80 and pico HF) located at the same place on the gauge length on each face.</p> Full article ">Figure 6
<p>Frequency centroid versus amplitude for the signals recorded during a tensile test with two types of sensors μ80 and pico HF located on the gauge length at the same place (<b>a</b>) Glass fibres/polyamide 6.6 matrix and (<b>b</b>) glass fibres/vinylester matrix.</p> Full article ">Figure 7
<p>Results of the classification in the plane Frequency Centroid/Amplitude for the data recorded during tensile tests of glass fibre/Vinylester composites (<b>a</b>) data recorded with the pico HF sensors and (<b>b</b>) with the μ80 sensors. (For the classification, the selected descriptors are Rise time, Duration, amplitude, energy, FP (frequency peak) and FC (frequency centroid)).</p> Full article ">Figure 8
<p>Radar chart for the four classes obtained with the two kinds of sensors for the data recorded during tensile tests of glass fibre/Vinylester composites (<b>a</b>) μ80 sensor and (<b>b</b>) picoHF sensor (class blue: highest rise time, Black class: Highest energy, green class: second class in energy term, red class: the last one class) (E energy, RT rise time, D duration, A amplitude, RA rise angle, AF average frequency and FC frequency centroid).</p> Full article ">Figure 9
<p>(<b>a</b>) Frequency centroid versus amplitude for the data collected during a tensile test on CMC composite with four similar sensors applied on the surface of the specimen (<b>b</b>) Peak Frequency versus amplitude for the signals located along the gauge length during a tensile test on CMC composite, data recorded with and without waveguides.</p> Full article ">Figure 9 Cont.
<p>(<b>a</b>) Frequency centroid versus amplitude for the data collected during a tensile test on CMC composite with four similar sensors applied on the surface of the specimen (<b>b</b>) Peak Frequency versus amplitude for the signals located along the gauge length during a tensile test on CMC composite, data recorded with and without waveguides.</p> Full article ">Figure 10
<p>(<b>a</b>) Initial dataset with four artificial classes (<b>b</b>) results of the segmentation, accordingly to the DB (Davies and Bouldin) and SI (Silhouette) indices, with the 18 descriptors selected.</p> Full article ">Figure 11
<p>Energy cumulative distribution functions for a fatigue tests at 450 °C on CMC composite (±5 mm interval around the middle of the gauge length for strain lower than 0.1%).</p> Full article ">
16 pages, 5017 KiB  
Article
The Influence of Sensor Size on Acoustic Emission Waveforms—A Numerical Study
by Eleni Tsangouri and Dimitrios G. Aggelis
Appl. Sci. 2018, 8(2), 168; https://doi.org/10.3390/app8020168 - 25 Jan 2018
Cited by 14 | Viewed by 4425
Abstract
The performance of Acoustic Emission technique is governed by the measuring efficiency of the piezoelectric sensors usually mounted on the structure surface. In the case of damage of bulk materials or plates, the sensors receive the acoustic waveforms of which the frequency and [...] Read more.
The performance of Acoustic Emission technique is governed by the measuring efficiency of the piezoelectric sensors usually mounted on the structure surface. In the case of damage of bulk materials or plates, the sensors receive the acoustic waveforms of which the frequency and shape are correlated to the damage mode. This numerical study measures the waveforms received by point, medium and large size sensors and evaluates the effect of sensor size on the acoustic emission signals. Simulations are the only way to quantify the effect of sensor size ensuring that the frequency response of the different sensors is uniform. The cases of horizontal (on the same surface), vertical and diagonal excitation are numerically simulated, and the corresponding elastic wave displacement is measured for different sizes of sensors. It is shown that large size sensors significantly affect the wave magnitude and content in both time and frequency domains and especially in the case of surface wave excitation. The coherence between the original and received waveform is quantified and the numerical findings are experimentally supported. It is concluded that sensors with a size larger than half the size of the excitation wavelength start to seriously influence the accuracy of the AE waveform. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Simulation model geometry.</p> Full article ">Figure 2
<p>Waveforms received by different size sensors for the case of surface source and excitation frequency of 1 MHz. The excitation wave (yellow starting at time 0) was reduced to the graph axes for clarity (nominal amplitude of 1).</p> Full article ">Figure 3
<p>(<b>a</b>) Snapshot of displacement field corresponding to 22 μs after excitation (frequency at 1 MHz). The source is set on the surface; (<b>b</b>) Fast Fourier Transforms (FFT) functions of the waveforms received by different size sensors.</p> Full article ">Figure 4
<p>(<b>a</b>) Waveforms received by different size sensors for the case of surface source and excitation frequency of 50 kHz; (<b>b</b>) FFT functions of waveforms received by different size sensors (exc. frequency at 50 kHz and surface source). The corresponding waveforms are depicted in <a href="#applsci-08-00168-f004" class="html-fig">Figure 4</a>a.</p> Full article ">Figure 5
<p>(<b>a</b>) Waveforms received by different size sensors for the case of vertical source and excitation frequency of 1 MHz; (<b>b</b>) Waveforms received by different size sensors for the case of vertical source and excitation frequency of 50 kHz.</p> Full article ">Figure 6
<p>(<b>a</b>) Waveforms received by different size sensors for the case of diagonal source and frequency excitation of 1 MHz; (<b>b</b>) Snapshot of displacement field corresponding to 10 μs after excitation (exc. frequency at 1 MHz).</p> Full article ">Figure 7
<p>(<b>a</b>) FFT functions of waveforms received by different size sensors (exc. frequency at 1 MHz and diagonal source); (<b>b</b>) FFT functions of waveforms received by different size sensors (exc. frequency at 50 kHz and diagonal source).</p> Full article ">Figure 8
<p>Relative amplitude of the received signal for point (1 mm, in grey), medium (5 mm, in blue), long (20 mm, in red) along the frequency range, from 50 kHz to 1 MHz, and concerning the three source positions: (<b>a</b>) surface; (<b>b</b>) diagonal; (<b>c</b>) vertical.</p> Full article ">Figure 8 Cont.
<p>Relative amplitude of the received signal for point (1 mm, in grey), medium (5 mm, in blue), long (20 mm, in red) along the frequency range, from 50 kHz to 1 MHz, and concerning the three source positions: (<b>a</b>) surface; (<b>b</b>) diagonal; (<b>c</b>) vertical.</p> Full article ">Figure 9
<p>Relative amplitude vs. sensor size over wavelength parameter (<span class="html-italic">D</span>/<span class="html-italic">λ</span>).</p> Full article ">Figure 10
<p>(<b>a</b>) Waveforms received by the 5 mm size sensor emitted from source set on the surface (in black) and vertically beneath the sensor (in blue) with excitation frequency equal to 1 MHz. Original waveform is added in red color and in reduced scale to fit the graph; (<b>b</b>) Respective coherence functions between received waves and original.</p> Full article ">Figure 11
<p>Average coherence up to 2 MHz for different sensor size and angles of propagation relatively to the sensor surface and excitation frequencies (<b>a</b>) 1 MHz; (<b>b</b>) 500 kHz.</p> Full article ">Figure 12
<p>Experimental setup showing the receiver at the top concrete surface and pencil lead breakage applied at the horizontal and vertical direction.</p> Full article ">Figure 13
<p>Individual waveforms received by Pico (5 mm) sensor in the case of (<b>a</b>) horizontal excitation; (<b>b</b>) vertical (<b>c</b>) the average FFT curves of previous waveforms.</p> Full article ">Figure 14
<p>Waveforms received by long size (R15, 20 mm) sensor in the case of (<b>a</b>) horizontal surface; (<b>b</b>) vertical excitation; (<b>c</b>) The average FFT curves of previous waveforms.</p> Full article ">Figure 15
<p>(<b>a</b>) Simulated waveforms received by 20 mm size sensor after surface source excitation with frequency of 500 kHz and different number of cycles; (<b>b</b>) Relative amplitude vs. waveform length over sensor size.</p> Full article ">Figure 16
<p>Simulated waveforms received by different size sensors from surface source with excitation frequency equal to 500 kHz (curves overlap).</p> Full article ">

Review

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21 pages, 5243 KiB  
Review
Digital Shearography for NDT: Phase Measurement Technique and Recent Developments
by Qihan Zhao, Xizuo Dan, Fangyuan Sun, Yonghong Wang, Sijin Wu and Lianxiang Yang
Appl. Sci. 2018, 8(12), 2662; https://doi.org/10.3390/app8122662 - 18 Dec 2018
Cited by 71 | Viewed by 10704
Abstract
Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques [...] Read more.
Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques for nondestructive testing (NDT) of composite structures. Of these techniques, digital shearography is one the most sensitive and accurate methods for NDT. Digital shearography can directly measure strain with high sensitivity when combined with different optical setups, phase-shift techniques, and algorithms. Its simple setup and less sensitivity to environmental disturbances make it particularly well suited for practical NDT applications. This paper provides a review of the phase measurement technique and recent developments in digital shearographic NDT. The introduction of new techniques has expanded the range of digital shearography applications and made it possible to measure larger fields and detect more directional or deeper defects. At the same time, shearography for different materials is also under research, including specular surface materials, metallic materials, etc. Through the discussion of recent developments, the future development trend of digital shearography is analyzed, and the potentials and limitations are demonstrated. Full article
(This article belongs to the Special Issue Damage Inspection of Composite Structures)
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<p>Fundamentals of digital shearography.</p> Full article ">Figure 2
<p>Comparison between the NDT results of holography and shearography.</p> Full article ">Figure 3
<p>(<b>a</b>) Holographic NDT result, (<b>b</b>) shearographic NDT result.</p> Full article ">Figure 4
<p>Comparison between (<b>a</b>) real-time subtraction version and (<b>b</b>) phase-shift shearography.</p> Full article ">Figure 5
<p>Schematic of single-detector spatial phase-shift technique.</p> Full article ">Figure 6
<p>Spatial phase-shift digital shearography (SPS-DS) based on Mach–Zehnder interferometer.</p> Full article ">Figure 7
<p>Schematic of the Fourier spectrum of the image.</p> Full article ">Figure 8
<p>(<b>a</b>) The modified Michelson-based SPS-DS system with a 4f system; (<b>b</b>) measurement result by traditional shearography system; (<b>c</b>) measurement result by modified 4f system.</p> Full article ">Figure 9
<p>Results of an application of Michelson-based SPS-DS system during dynamic loading.</p> Full article ">Figure 10
<p>(<b>a</b>) The SPS-DS setup for out-of-plane deformation and its first derivative measurement; (<b>b</b>) phase map of a hologram; (<b>c</b>) phase map of a shearogram.</p> Full article ">Figure 11
<p>(<b>a</b>) The dual-shearing direction SPS-DS system; (<b>b</b>) measurement result of circular flaw and bar flaw in x direction; (<b>c</b>) measurement result of circular flaw and bar flaw in y direction.</p> Full article ">Figure 12
<p>(<b>a</b>) The modified shearography system for specular surfaces; (<b>b</b>) experimental result of the metal plate with a cone-shape deformation; (<b>c</b>) experimental result of the metal plate with an irregular deformation.</p> Full article ">Figure 13
<p>Experimental setup of directed acoustic shearography.</p> Full article ">Figure 14
<p>(<b>a</b>) The new shearography system with a spatial light modulator; (<b>b</b>) experimental result of a thimble-loaded aluminum plate; (<b>c</b>) experimental result of a composite plate with three flaws.</p> Full article ">Figure 14 Cont.
<p>(<b>a</b>) The new shearography system with a spatial light modulator; (<b>b</b>) experimental result of a thimble-loaded aluminum plate; (<b>c</b>) experimental result of a composite plate with three flaws.</p> Full article ">
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