Glass fiber reinforced polymer (GFRP) materials are recently widely applied in marine industry, especially for hulls structures. The high popularity of composites and safety requirements result in development of Structural Health Monitoring (SHM) systems. One of the sensors types that can be permanently integrated with a composite structure and used as a part of SHM system are fiber Bragg grating (FBG) sensors. FBG sensors advantages like small size, high corrosion resistance, and absence of electric current are especially important in marine applications. The goal of the paper is to present FBG sensors application to SHM of a composite fast patrol boat. An array of the sensors was embedded into a boat hull structure. Moreover, accelerometer was utilized to determined maximum accelerations during the power boat operations. The analyses were performed during normal exploitation of the boat performed on the Gulf of Gdansk (southeastern bay of the Baltic Sea) under different sea states and wind speeds.
Typically, material properties originate from destructive tensile tests and are used in computational models in the design and analysis process of structures. This approach is well-established in relation to isotropic homogeneous materials. However, if this approach is used for composite laminates, inaccuracies can arise that lead to vastly di erent stress distributions, strain rates, natural frequencies, and velocities of propagating elastic waves. In order to account for this problem, the alternative method is proposed, which utilizes Lamb wave propagation phenomenon and optimization techniques. Propagating Lamb waves are highly sensitive to changes in material parameters and are often used for structural health monitoring. In the proposed approach, the elastic constants, which are utilized to determine dispersion curves of Lamb waves, are optimized to achieve a good correlation between model predictions and experimental observations. In the first step of this concept, parametric studies have been carried out in which the influence of mass density, Young's modulus, Poisson's ratio of reinforcing fibers as well as a matrix of composite laminate and volume fraction of reinforcing fibers on dispersion curves of Lamb waves was investigated. The dispersion curves of Lamb waves were calculated by using the semi- analytical spectral element method considering the variability of properties of composite constituents. The resulting dispersion curves were also compared with experimental measurements of full wavefield data conducted by scanning laser Doppler vibrometer and processed by 3D Fourier transform. It allowed formulating fitness function which will be next used in a genetic algorithm for optimization and identification of elastic constants.
In this paper results of numerical and experimental analysis of S0/A0 elastic wave mode conversion phenomenon at various discontinuities in glass fibre reinforced polymer (GFRP) panels are presented. Analysis of S0/A0 mode conversion effect at Teflon inserts simulating delamination and impact damage with energies 5 J, 10 J and 15 J is conducted. In the case of Teflon inserts circular inserts with the same diameters located at different depth and inserts with various shapes located at the same depth were investigated. Numerical analysis is based on the time domain spectral element method (SEM). SEM computational algorithm is adapted for parallel calculation using graphic processor units (GPUs). Numerical model takes also into account the influence of material damping. Numerical results are compared with experimental results based on full wavefield measurements using scanning laser Doppler vibrometry (SLDV). It is shown that based on the effects of S0/A0 mode conversion it is possible to detect the damage and determine its shape and size. In the research, auxiliary Non-Destructive Testing (NDT) method is also utilised. The aim of its application is to indicate the depth of discontinuity and to prove that delamination was created in the case of impact damage. The auxiliary method is based on terahertz spectroscopy (THz) where the analysis of propagation of electromagnetic waves in the terahertz band is conducted. The THz spectroscopy is a useful technique for damage assessment in the dielectric materials.
In this paper idea of load and structural health monitoring (LSHM) system for assessment of composite powerboat has been presented. One of the structural health assessment method utilised in this system is electromechanical impedance (EMI). Authors have presented results related to influence of load and temperature on EMI method. EMI method is based on electromechanical coupling of piezoelectric transducer (PZT) with host structure. Due to this coupling, mechanical resonances of structure can be seen in electrical impedance characteristic of PZT. According to the literature EMI method is sensitive to change of loads in the structure and to changing temperature. Influence of changing operation and environmental conditions should be eliminated by compensation algorithms in order to reduce risk of false alarms in damage assessment system. Results of research related to simple specimen experienced the influence of changing temperature and transverse load and EMI method have been presented. Strains at bending condition for beam have been measured using fibre Bragg grating strain sensors. Some preliminary results of measurements related to powerboat have been also presented and discussed. Measurements have been related to self-diagnosis of piezoelectric transducers using EMI method and to measurements of accelerations using MEMS accelerometer.
This paper describes the electromechanical impedance (EMI) method based structural health monitoring using sensor data fusion approach. The data fusion of different attributes is more effective over a single data in achieving reasonable accuracy and precision. The paper investigates an electromechanical impedance (EMI) method applied to the structural health monitoring of sensor network of thin composite plates using distributed sensor data fusion techniques and a single sensor for different levels of delamination. The information from multiple sensors and single sensor is studied in the frequency domain and a new optimized fused criteria of variable admittance (Y) and conductance (G) is explained by damage metric root mean square deviation (RMSD). The experiments are performed on a thin composite plate with attached piezoelectric transducers at different locations and a plate with single transducer with different delamination levels.
Electro-mechanical impedance based damage detection has been shown to detect small changes in structure. Due to the large frequency band which may be assessed with the EMI technique, it has been shown to detect very local damage irrespective of the change in the boundary conditions. Studies have also been carried out to compensate for ambient temperature effects. Unfortunately, the drawback of the local nature of the EMI approach is the relatively low range of sensing. As a result most of the studies using EMI approach are limited to applications where the sensitive region is apriori known. This is not always the case, and thus the study of the technique at the array level is necessary. Thus, the present study tries to establish the approach for optimization of the array of PZTs for EMI based approach. The primary task is to establish the range of the sensors to damage in an anisotropic GFRP plate. The range of the sensors and their directionality then may be used to optimize the sensor placement to ensure the maximum coverage of the plate for damage detection. Once the cost function of the optimization has been established, genetic algorithm (GA) is employed for optimization. GA offers several advantages over brute force based methods as well as other optimization approaches. GA is ideally suited for multi-objective optimization which then paves the way for incorporating other optimization objectives in the search.
In this paper results of numerical and experimental analysis of guided wave mode conversion phenomenon are presented. Research presented in this paper is focused on S0/A0’ mode conversion caused by discontinuities in the form of notches and delaminations. In the numerical research Spectral Element Method is utilized for modelling of elastic wave propagation. Two kinds of structures are investigated: aluminium beam with notch and composite panel with delamination. In both cases influence of symmetrical and non-symmetrical (in respect to the thickness of structure) location of discontinuities on S0/A0’ mode conversion is investigated. Numerical results lead to the conclusion that necessary condition for mode conversion is the non-symmetric location of a discontinuity in respect to the thickness of the structure. Experimental research is based on scanning laser Doppler vibrometry and full wave-field measurements. In this approach, guided wave generation is conducted based on piezoelectric transducer while sensing process is performed for a dense mesh of points that span over an investigated area of the composite part. Only composite panel with teflon inserts with different shapes is investigated in experimental research. Moreover, only symmetric locations of teflon inserts are investigated. Mode conversion S0/A0’ was noticed clearly for the symmetrical location of teflon inserts (in respect to the panel thickness). Experimental results lead to the conclusion that in real condition always a small distortion of symmetry exists. In reality, perfect symmetry is extremely rare, so it is expected that in real-world damage scenarios mode conversion will always occur.
Composite materials are commonly used in many branches of industry. One of the effective methods to join CFRP parts is to use adhesives. There is a search of effective methods for quality assurance of bonded parts. There is a need for pre- and post-bond inspection to ensure proper bonding and verify its quality. Research reported here focuses on post-bond inspection of bonded CFRP parts. In this paper we report investigations of samples that were modified in order to simulate the conditions that can be encountered during the bonding repair processes. The modifications were made before adhesive bonding, and their effect on the quality of the bond is studied. The first case was the thermal treatment. It was made by exposure of samples to elevated temperatures. This case accounts for parts that may be exposed to external heat source or lightening impact. Second of the investigated cases was deicing fluid contamination prepared by dip-coating of clean CFRP plates. When cleaning the aircraft for a repair this fluid can be transported to bonding areas and weaken the joint. The third type of modification was faulty curing of the adhesive. It was prepared by local pre-curing of the adhesive. Pre-curing causes irregularities in the curing of the adhesive joint. Apart from single modifications, also mixed cases were studied, as well as scarf bonding. The samples were studied in non-destructive approach. It was shown that for some of the cases the detection is possible.
Composite materials are commonly used in many branches of industry. One of the effective methods to join CFRP parts is the adhesive bonding. There is a search of effective methods for quality assurance of bonded parts. There is a need for pre- and post-bond inspection to ensure proper bonding and verify its quality. Research reported here focuses on post-bond inspection of bonded CFRP parts. In this paper we report investigations of samples that were modified with contamination that can be encountered during the manufacturing process of the CFRP parts. The contaminations were introduced before adhesive bonding, and the effect of the contamination on the quality bond is studied. First of the investigated cases was release agent contamination prepared by dip-coating of clean CFRP plates. The release agent is used during the production of composite elements and can contaminate the surface to be bonded. The second case was the moisture contamination. It was obtained by conditioning of the samples in humid conditions. Moisture contamination can be gained from water-coupled ultrasonics or during transportation of unprotected parts. The third type of contamination had more local character. It simulated fingerprints. Artificial sweat was used. The fingerprint contamination can be caused by improper handling of the parts. Apart from single contamination, also mixed contamination cases were studied, as well as curved samples. The samples were studied in non-destructive approach. It was shown that for some of the cases the detection is possible.
Composite materials are commonly used in many branches of industry. One method to join or repair CFRP parts is by the use adhesive bonding. There is a search of effective methods for pre-bond assessment of bonded parts and post-bond inspection. Research reported here focuses on post-bond inspection of bonded CFRP plates. In this paper we reported results of two methods. We used noncontact ultrasonic testing (UT) technique as reference method. Ultrasonic testing was made in an immersion tank using phased-array probes. The second method was the electromechanical impedance (EMI). A piezoelectric sensors were surface mounted on each of the samples. Due to piezoelectric effect the electrical response of the sensor is related to mechanical response of the structure to which the sensors is bonded to. Measurements were conducted using HIOKI Impedance Analyzer IM3570. In order to perform a detailed study three samples of each kind were tested. There were three reference samples. The samples with modified adhesive bonds had three levels of severity, so there were three samples with each level of modification. The ultrasonic testing was focused on C-scan analysis taking into consideration the amplitude and time of flight (TOF). Two probes were used, one with 5 MHz frequency, second with 10 MHz. The EMI spectra were gathered up to 5 MHz and they were processed with signal processing algorithms in order to extract differences between reference samples and samples with modified bonds. The UT results provided relevant information about the investigated samples, while the EMI showed sensitivity to the level of adhesive bond modification.
Parallel implementation of the time domain spectral element method on GPU (Graphics Processing Unit) is presented. The proposed spectral element method implementation is based on sparse matrix storage of local shape function derivatives calculated at Gauss–Lobatto–Legendre points. The algorithm utilizes two basic operations: multiplication of sparse matrix by vector and element-by-element vectors multiplication. Parallel processing is performed on the degree of freedom level. The assembly of resultant force is done by the aid of a mesh coloring algorithm. The implementation enables considerable computation speedup as well as a simulation of complex structural health monitoring systems based on anomalies of propagating Lamb waves. Hence, the complexity of various models can be tested and compared in order to be as close to reality as possible by using modern computers. A comparative example of a composite laminate modeling by using homogenization of material properties in one layer of 3D brick spectral elements with composite in which each ply is simulated by separate layer of 3D brick spectral elements is described. Consequences of application of each technique are explained. Further analysis is performed for composite laminate with delamination. In each case piezoelectric transducer as well as glue layer between actuator and host structure is modeled.
Proc. SPIE. 10170, Health Monitoring of Structural and Biological Systems 2017
KEYWORDS: Actuators, Carbon, Data modeling, Waveguides, Signal attenuation, Composites, Numerical simulations, 3D modeling, Scanning electron microscopy, Wave propagation, Transducers, Vibrometry, 3D metrology, Spectral models
In this paper problem of guided wave damping in composite materials is investigated. Material damping is estimated from experimental measurements based on energy of propagating guided waves. Simply Rayleigh damping is introduced into the model in the form of damping matrix proportional to the mass matrix. The numerical model is based on Spectral Element Method (SEM). Numerical model includes the piezoelectric transducer and bond layer between actuator and the host structure. In this paper each ply of composite laminate is simulated by separate layer consisting of 3D brick spectral elements. Numerical results are experimentally validated using Scanning Laser Doppler Vibrometry (SLDV). Guided waves are excited using piezoelectric transducer and registered using non-contact device – the laser vibrometer. Validation is based on signals gathered in dispersed points as well as on full wavefield measurements. The full wavefield measurements are conducted on dense grid of points. In this paper results for simple carbon fiber reinforced polymer are presented. Paper presents result for composite structure for damaged case. Investigated damage is in the form of delamination.
Carbon Fibre Reinforced Polymers (CFRP) are more and more used in many branches of industry. Researchers are developing numerous techniques of non-destructive assessment of the structures made out of CFRP such as guided waves, ultrasonics, laser induced fluorescence and others. In this research we focus on electromechanical impedance (EMI) technique. In this technique a piezoelectric sensor is either surface mounted or embedded into investigated host structure. The electrical quantities of the sensor are measured for wide frequency range. Due to piezoelectric effect the electrical response of the sensor is related to mechanical response of the structure to which the sensors is bonded to. In the reported research impedance spectra in the vicinity of the transducer thickness mode were investigated as well as the lower frequency range. The spectra that were analysed were gathered from samples with surface treatment such as thermal degradation and samples adhesively bonded with film adhesive with symmetric and unsymmetric bond. Moreover, the samples with modified adhesive bonds were investigated. These spectra for different cases were compared with reference measurement results gathered from pristine samples. Numerical indexes for comparison of the EMI characteristics were proposed. The comparison of the indexes was also conducted. In the experimental part of the research the piezoelectric transducer was mounted at the sample surface. Measurements were conducted using HIOKI Impedance Analyzer IM3570.
The paper presents methods and techniques oriented towards structural health monitoring (SHM) and (ENDT) for both metallic and composite structures. Particularly the paper is dedicated to elastic waves propagation phenomenon, scanning laser vibrometry, electromechanical impedance and terahertz spectroscopy. Selected numerical modelling aspects of the phenomena related to the mentioned methods are addressed. Moreover it covers the main disciplines which are related to above mentioned techniques as piezoelectric sensors and transducers, and signal processing. The signal processing approach is crucial allowing extracting damage related features from the gathered signals.
Investigated damage is in the form of mechanical failures as cracks, delaminations, debonding, voids. Also methods dedicated to thermal degradation, moisture and chemical contamination are shown. Presented methods are also suitable for performance of bonded joints assessment. Problem of external factor (temperature, load) on investigated methods is also discussed in this paper. The characteristic of each method is summarized by a critical look. Promising combination of selected techniques should lead to an innovative approach to ensure safety operation of structures. All problems have been dealt with a hybrid experimental – numerical approach.
This paper deals with analysis of guided waves mode conversion phenomenon in fiber reinforced composite materials. Mode conversion phenomenon may take place when propagating elastic guided waves interact with discontinuities in the composite waveguide. The examples of such discontinuities are sudden thickness change or delamination between layers in composite material. In this paper, analysis of mode conversion phenomenon is based on full wave-field signals. In the full wave-field approach signals representing propagation of elastic waves are gathered from dense mesh of points that span over investigated area of composite part. This allow to animate the guided wave propagation. The reported analysis is based on signals resulting from numerical calculations and experimental measurements. In both cases defect in the form of delamination is considered. In the case of numerical research, Spectral Element Method (SEM) is utilized, in which a mesh is composed of 3D elements. Numerical model includes also piezoelectric transducer. Full wave-field experimental measurements are conducted by using piezoelectric transducer for guided wave excitation and Scanning Laser Doppler Vibrometer (SLDV) for sensing.
The scope of the paper includes non–destructive assessment of the structure's material condition, for the aerospace
structures during its useful lifetime. The paper presents multidisciplinary technologies devoted to development and
implementation of methods and systems that realize inspection and damage detection by non–destructive methods.
The paper covers several disciplines which are based on topics such as piezoelectric transducers, elastic waves
propagation phenomenon, structural vibrations analysis, electro–mechanical impedance method, terahertz technique,
laser induced fluorescence and 3D laser vibrometry applications.
Among various techniques available the paper presents selected numerical simulations and experimental validations of
considered structures. Authors address also the problem of adhesive bonding in the case of carbon fiber reinforced
polymers (CFRP). Techniques for detection of weak bonds are presented together with signal processing approaches.
The reported investigations concern weak adhesive bonds caused by both manufacturing (e.g. release agent, poor curing)
and in–service contaminations (e.g. moisture). Also the paper provides helpful information about dispersion, mode
conversion and wave scattering from stiffeners and boundaries. It addresses the problem of optimisation of excitation
signal parameters and sensor placement, as well as analysis of signals reflected from damage. It also includes a variety of
techniques being related to diagnostics (damage size estimation and damage type recognition) and prognostics.
Many studies have been published in recent years on Lamb wave propagation in isotropic and (multi-layered) anisotropic structures. In this paper, adiabatic wave propagation phenomenon in a tapered composite panel made out of glass fiber reinforced polymers (GFRP) will be considered. Such structural elements are often used e.g. in wind turbine blades and aerospace structures. Here, the wave velocity of each wave mode does not only change with frequency and the direction of wave propagation. It further changes locally due to the varying cross-section of the GFRP panel.
Elastic waves were excited using a piezoelectric transducer. Full wave-field measurements using scanning Laser Doppler vibrometry have been performed. This approach allows the detailed analysis of elastic wave propagation in composite specimen with linearly changing thickness. It will be demonstrated here experimentally, that the wave velocity changes significantly due to the tapered geometry of the structure. Hence, this work motivates the theoretical and experimental analysis of adiabatic mode propagation for the purpose of Non-Destructive Testing and Structural Health Monitoring.
In aircraft industry the Carbon Fiber Reinforced Polymer (CFRP) elements are joint using rivets and adhesive bonding.
The reliability of the bonding limits the use of adhesive bonding for primary aircraft structures, therefore it is important
to assess the bond quality. The performance of adhesive bonds depends on the physico-chemical properties of the
adhered surfaces. The contamination leading to weak bonds may have various origin and be caused by moisture, release
agent, hydraulic fluid, fuel, poor curing of adhesive and so on. In this research three different causes of possible weak
bonds were selected for the investigation: 1. Weak bond due to release agent contamination, 2. Weak bond due to
moisture contamination, 3. Weak bond due to poor curing of the adhesive. In order to assess the bond quality
electromechanical impedance (EMI) technique was selected and investigation was focused on the influence of bond
quality on electrical impedance of piezoelectric transducer. The piezoelectric transducer was mounted at the middle of
each sample surface. Measurements were conducted using HIOKI Impedance Analyzer IM3570. Using the impedance
analyzer the electrical parameters were measured for wide frequency band. Due to piezoelectric effect the electrical
response of a piezoelectric transducer is related to mechanical response of the sample to which the transducers is
attached. The impedance spectra were investigated in order to find indication of the weak bonds. These spectra were
compared with measurements for reference sample using indexes proposed in order to assess the bond quality.
The application of Carbon Fibre Reinforced Polymers (CFRP) in aeronautics has been increasing. The CFRP elements
are joint using rivets and adhesive bonding. The reliability of the bonding limits the use of adhesive bonding for primary
aircraft structures, therefore it is important to assess the bond quality. The performance of adhesive bonds depends on the
physico-chemical properties of the adhered surfaces. This research is focused on characterization of surfaces before
bonding. In-situ examination of large surface materials, determine the group of methods that are preferred. The analytical
methods should be non-destructive, enabling large surface analysis in relatively short time. In this work a spectroscopic
method was tested that can be potentially applied for surface analysis. Four cases of surface condition were investigated
that can be encountered either in the manufacturing process or during aircraft service. The first case is related to
contamination of CFRP surface with hydraulic fluid. This fluid reacts with water forming a phosphoric acid that can etch
the CFRP. Second considered case was related to silicone-based release agent contamination. These agents are used
during the moulding process of composite panels. Third case involved moisture content in CFRP. Moisture content
lowers the adhesion quality and leads to reduced performance of CFRP resulting in reduced performance of the adhesive
bond. The last case concentrated on heat damage of CFRP. It was shown that laser induced fluorescence method can be
useful for non-destructive evaluation of CFRP surface and some of the investigated contaminants can be easily detected.
In this paper the investigation of a structural health monitoring method for thin-walled aircraft part is presented.
The concept is based on the guided elastic wave propagation phenomena. This type of waves can be used in order
to obtain information about structure condition and possibly damaged areas. In reported investigation piezoelectric
transducer was used to excite guided waves in chosen structural element. Specimen was a riveted panel from an
aircraft structure. Dispersive nature of guided waves results in changes of velocity with the wave frequency,
therefore a narrowband signal was used to minimize the dispersion phenomenon. The generated signal was
amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided wave.
Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses
at a points belonging to a defined mesh. This non-contact tool allowed to investigate phenomena related to wave
propagation in considered aircraft element. Due to high complexity of the element baseline measurements were
taken before measurements for component with the introduced discontinuity. Signal processing procedures were
developed in order to visualize the interaction of elastic waves with specimens components (rivets, etc.). In the
second stage of research the signals gather by laser vibrometry method were input to the damage detection
algorithms. Signal processing methods for features extraction from signals were proposed. These features were
applied in order to detect and localize the presence of damage. In the first step damage detection was based on full
wavefield measurements. In this way it was possible to obtain amplitude contrast between region with
discontinuities and without them. In the second step a point-wise damage detection was conducted. It was based on
several laser measurement points treated as sensors. The signal processing was conducted in MATLAB with the
procedures developed by authors. The results of damage detection were compared with each other and conclusions
In this paper algorithm for discontinuities localisation in thin panels made of aluminium alloy is presented.
Mentioned algorithm uses Lamb wave propagation methods for discontinuities localisation. Elastic waves were
generated and received using piezoelectric transducers. They were arranged in concentrated arrays distributed on
the specimen surface. In this way almost whole specimen could be monitored using this combined distributed-concentrated transducer network. Excited elastic waves propagate and reflect from panel boundaries and
discontinuities existing in the panel. Wave reflection were registered through the piezoelectric transducers and used
in signal processing algorithm. Proposed processing algorithm consists of two parts: signal filtering and extraction
of obstacles location. The first part was used in order to enhance signals by removing noise from them. Second part
allowed to extract features connected with wave reflections from discontinuities. Extracted features damage influence maps were a basis to create damage influence maps. Damage maps indicated intensity of elastic wave reflections which corresponds to obstacles coordinates. Described signal processing algorithms were implemented in the MATLAB environment. It should be underlined that in this work results based only on experimental signals were presented.