64 research outputs found
Damage identification in various types of composite plates using guided waves excited by a piezoelectric transducer and measured by a laser vibrometer
Composite materials are widely used in the industry, and the interest of this material is growing rapidly, due to its light weight, strength and various other desired mechanical properties. However, composite materials are prone to production defects and other defects originated during exploitation, which may jeopardize the safety of such a structure. Thus, non-destructive evaluation methods that are material-independent and suitable for a wide range of defects identification are needed. In this paper, a technique for damage characterization in composite plates is proposed. In the presented non-destructive testing method, guided waves are excited by a piezoelectric transducer, attached to tested specimens, and measured by a scanning laser Doppler vibrometer in a dense grid of points. By means of signal processing, irregularities in wavefield images caused by any material defects are extracted and used for damage characterization. The effectiveness of the proposed technique is validated on four different composite panels: Carbon fiber-reinforced polymer, glass fiber-reinforced polymer, composite reinforced by randomly-oriented short glass fibers and aluminum-honeycomb core sandwich composite. Obtained results confirm its versatility and efficacy in damage characterization in various types of composite plates
Damage identification in composite laminates and sandwich structures using ultrasonic guided waves and a 3D laser vibrometer
This thesis addresses the feasibility of using ultrasonic guided waves (UGWs) to detect and characterise the barely visible impact damages (BVID) that can develop in thin composite laminates and composite sandwich structures (CSS) by carrying out a fundamental investigation into wave-damage interaction. The interaction of UGWs with BVID in a structure is analysed using full wavefield data obtained by a Laser Doppler Vibrometer (LDV) and by numerical simulations. Multiple signal and image processing techniques are proposed to enhance the features relating to damage. The findings from this analysis are then incorporated into an in-service structural health monitoring (SHM) methodology using a sparse network of piezoelectric transducers.
For the laminated composite panels, isolated subsurface delaminations between plies and
complex BVID caused by a low velocity impactor are investigated. Both cases show that the
first symmetric mode, S0, causes mode conversions when interacting with the defects whilst
the first anti-symmetric mode, A0, mainly causes a change in phase and amplitude across the defects. Both cases also show that as the damaged area got more severe, the effects of the damage became more pronounced. The findings are then integrated and validated by a delay & sum algorithm to show the UGWs potential as an in-service SHM methodology.
The focus of research then turns to the theoretical fundamentals of UGW propagation
through CSS. The underlying mechanics of UGWs in CSS, including the relation between panel thickness and the UGW wavelength as well as the energy transfer through the core are presented. It is noted that three main types of propagation can exist in CSS which are global Lamb waves, leaky Lamb waves and Rayleigh waves. Dispersion curves are obtained for the CSS and polar plots of group velocities show their anisotropic nature. The final part of the thesis focuses on damage detection and localisation in CSS using full wavefield analysis and a sparse network of transducers. Both fundamental modes can localise the BVID in the structure, even with the anisotropic behaviour of the core.
Based on these results, this thesis concludes that UGW based SHM shows great promise as an in-service damage detection technique that can detect, localise and, in some cases, characterise impact induced defects in thin composite laminates and CSS.Open Acces
Numerical simulation of the Lamb wave propagation in honeycomb sandwich panels: A parametric study
a b s t r a c t The paper aims to describe the guided Lamb wave propagation in honeycomb sandwich panels. The application of Lamb waves is a well-known method in modern online structural health monitoring techniques. To analyze the wave propagation in such a complicated geometry with an analytical solution is hardly possible; therefore a dimensional finite element simulation is used. A piezoelectric actuator is used to excite the waves on the sandwich panel surface. The extended model of the honeycomb sandwich panel is consisting of two plate layers and a mid-core layer. In addition, a simplified model is used to reduce the computing costs, where the mid-core of the sandwich panel is replaced by a homogeneous layer. The results from the extended model and the simplified model are in most cases in a good agreement; however the limitations of using the simplified model are discussed. A parametric study is used to show the influence of the geometrical properties of honeycomb plates, the material properties of the skin plates and the loading frequency on the group velocity, the wave length and the energy transmission. Each of these properties provides valuable information to design an efficient health monitoring system. Finally, an experimental test is presented
Impact Induced Delamination Detection and Quantification With Guided Wavefield Analysis
This paper studies impact induced delamination detection and quantification by using guided wavefield data and spatial wavenumber imaging. The complex geometry impact-like delamination is created through a quasi-static indentation on a CFRP plate. To detect and quantify the impact delamination in the CFRP plate, PZT-SLDV sensing and spatial wavenumber imaging are performed. In the PZT-SLDV sensing, the guided waves are generated from the PZT, and the high spatial resolution guided wavefields are measured by the SLDV. The guided wavefield data acquired from the PZT-SLDV sensing represent guided wave propagation in the composite laminate and include guided wave interaction with the delamination damage. The measured guided wavefields are analyzed through the spatial wavenumber imaging method, which generates an image containing the dominant local wavenumber at each spatial location. The spatial wavenumber imaging result for the simple single layer Teflon insert delamination provided quantitative information on delamination damage size and location. The location of delamination damage is indicated by the area with larger wavenumbers in the spatial wavenumber image. The impact-like delamination results only partially agreed with the damage size and shape. The results also demonstrated the dependence on excitation frequency. Future work will further investigate the accuracy of the wavenumber imaging method for real composite damage and the dependence on frequency of excitation
Recent Advances in Piezoelectric Wafer Active Sensors for Structural Health Monitoring Applications
In this paper, some recent piezoelectric wafer active sensors (PWAS) progress achieved in our laboratory for active materials and smart structures (LAMSS) at the University of South Carolina: http: //www.me.sc.edu/research/lamss/ group is presented. First, the characterization of the PWAS materials shows that no significant change in the microstructure after exposure to high temperature and nuclear radiation, and the PWAS transducer can be used in harsh environments for structural health monitoring (SHM) applications. Next, PWAS active sensing of various damage types in aluminum and composite structures are explored. PWAS transducers can successfully detect the simulated crack and corrosion damage in aluminum plates through the wavefield analysis, and the simulated delamination damage in composite plates through the damage imaging method. Finally, the novel use of PWAS transducers as acoustic emission (AE) sensors for in situ AE detection during fatigue crack growth is presented. The time of arrival of AE signals at multiple PWAS transducers confirms that the AE signals are originating from the crack, and that the amplitude decay due to geometric spreading is observed
Damage identification in FRP-retrofitted concrete structures using linear and nonlinear guided waves
Structural health monitoring (SHM) involves the implementation of damage identification
methods in engineering structures to ensure structural safety and integrity. The paramount
importance of SHM has been recognised in the literature. Among different damage
identification methods, guided wave approach has emerged as a revolutionary technique.
Guided wave-based damage identification has been the subject of intensive research in the past
two decades. Meanwhile, applications of fibre reinforced polymer (FRP) composites for
strengthening and retrofitting concrete structures have been growing dramatically. FRP
composites offer high specific stiffness and high specific strength, good resistance to corrosion
and tailorable mechanical properties. On the other hand, there are grave concerns about longterm
performance and durability of FRP applications in concrete structures. Therefore, reliable
damage identification techniques need to be implemented to inspect and monitor FRPretrofitted
concrete structures.
This thesis aims to explore applications of Rayleigh wave for SHM in FRP-retrofitted
concrete structures. A three-dimensional (3D) finite element (FE) model has been developed
to simulate Rayleigh wave propagation and scattering. Numerical simulation results of
Rayleigh wave propagation in the intact model (without debonding at FRP/concrete interface)
are verified with analytical solutions. Propagation of Rayleigh wave in the FRP-retrofitted
concrete structures and scattering of Rayleigh waves at debonding between FRP and concrete
are validated with experimental measurements. Very good agreement is observed between the
FE results and experimental measurements. The experimentally and analytically validated FE
model is then used in numerical case studies to investigate the scattering characteristic. The scattering directivity pattern (SDP) of Rayleigh wave is studied for different debonding size
to wavelength ratios and in both backward and forward scattering directions. The suitability
of using bonded mass to simulate debonding in the FRP-retrofitted concrete structures is also
investigated. Besides, a damage localisation method is introduced based on the time-of-flight
(ToF) of the scattered Rayleigh wave. Numerical case studies, involving different locations
and sizes of debonding, are presented to validate the proposed debonding localisation method.
Nonlinear ultrasonics is a novel and attractive concept with the potential of baseline-free
damage detection. In this thesis, nonlinear Rayleigh wave induced at debondings in FRPretrofitted
concrete structures, is studied in detail. Numerical results of nonlinear Rayleigh
wave are validated with experimental measurements. The study considers both second and
third harmonics of Rayleigh wave. A very good agreement is observed between numerical and
experimental results of nonlinear Rayleigh wave. Directivity patterns of second and third
harmonics for different debonding size to the wavelength ratios, and in both backward and
forward scattering directions, are presented. Moreover, a damage image reconstruction
algorithm is developed based on the second harmonic of Rayleigh wave. This method provides
a graphical representation for debonding detection and localisation in FRP-retrofitted concrete
structures. Experimental case studies are used to demonstrate the performance of the proposed
technique. It is shown that the proposed imaging method is capable of detecting the debonding
in the FRP-retrofitted concrete structures.
Overall, this PhD study proves that Rayleigh wave is a powerful and reliable means of
damage detection and localisation in FRP-retrofitted concrete structures.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 201
Multi step structural health monitoring approaches in debonding assessment in a sandwich honeycomb composite structure using ultrasonic guided waves
This paper aims to investigate the use of ultrasonic guided wave (GW) propagation mechanism and the assessment of debonding in a sandwich composite structure (SCS) using a multi-step approach. Towards this, a series of GW propagation-based laboratory experiments and numerical simulations have been carried out on the SCS sample. The debonding regions of variable size and locations were assessed using a pre-defined network of piezoelectric lead zirconate transducers (PZT). Besides, several artificial masses were also placed in the SCS to validate the multi-step structural health monitoring (SHM) strategy. The SHM approach uses a proposed quick damage identification matrix maps and an improved elliptical wave processing (EWP) strategy of the registered GW signals to detect the locations of debonding and other damages in the SCS. The benefit of the proposed damage identification map is to locate the damaged area (sectors) quickly. This identification step is followed by applying the damage localization step using the improved EWP only on the previously identified damage sector region. The proposed EWP has shown the potential to effectively locate the hidden multiple debonding regions and damages in the SCS with a reduced number of calculations using a step-wise approach that uses only a selected number of grid points. The paper shows the effectiveness of the proposed approach based on data gathered from numerical simulations and experimental studies. Thus, using the above-mentioned SHM strategy debondings and damages present within and outside the sensor network are localized. The results were cross verified with nondestructive testing (NDT) methods such as infrared thermography and laser Doppler vibrometry
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