Lamb wave-based BVID imaging for a curved composite sandwich panel

The aim of this study is to investigate the capability of the zero-lag crosscorrelation (ZLCC) imaging condition of an A0 Lamb wave mode in imaging a barely visible impact damage (BVID) in a carbon fiber reinforced plastic (CFRP) using a fully non-contact Lamb wave based non-destructive inspection (NDI) system. A 16- ply [45/0/-45/90]2S CFRP laminate was impacted at three different locations with different impact energies using a drop ball at three drop heights causing three BVIDs with different dimensions, ~7 mm, ~12 mm and ~17.5 mm in diameter. The A0 Lamb wave mode is generated inside the laminate using a circular air-coupled transducer (ACT) and detected along the damaged region using a laser Doppler vibrometer (LDV). The measured wavefield is then decomposed into a forward and backward propagating wavefields by applying a frequency-wavenumber filtering postprocessing technique. The decomposed wavefields are then cross-correlated in the frequency domain using ZLCC method producing a detailed cumulative damage image. The experimental images highlight the three damaged areas with higher ZLCC values compared to other parts of the inspected areas. However, it was found that depending on the damage size and if delaminations are happening all around the impacted spot, the delamination boundary on the right side of the impact is difficult to detect if the generated wavefield is traveling from left to right which interacts with the delaminations of the left side and vice versa. In this case, the region of interest around one of the damages (12 mm) was scanned twice with the excitation being to the left and then to the right of it and overlapping the images have given a good approximation of the actual damaged area. The experimental investigation has shown a good correlation between the ZLCC imaging condition and C-scan images, which demonstrate a strong capability of guided wave ZLCC imaging condition technique in approximating the location and size of relatively small BVIDs in thin composite structures. The presence of three separate delaminations in a 2.4 mm thick CFRP laminate impacted by 13.56, 19, and 27 Joule drop ball energies were detected in a fully non-contact ultrasonic technique. doi: 10.12783/SHM2015/319

Nondestructive evaluation (NDE) for detection and quantification of damage in composite materials is fundamental in the assessment of the overall structural integrity of modern aerospace systems. Conventional NDE systems have been extensively used to detect the location and size of damages by propagating ultrasonic waves normal to the surface. However they usually require physical contact with the structure and are time consuming and labor intensive. An automated, contactless laser ultrasonic imaging system for barely visible impact damage (BVID) detection in advanced composite structures has been developed to overcome these limitations. Lamb waves are generated by a Q-switched Nd:YAG laser, raster scanned by a set of galvano-mirrors over the damaged area. The out-of-plane vibrations are measured through a laser Doppler Vibrometer (LDV) that is stationary at a point on the corner of the grid. The ultrasonic wave field of the scanned area is reconstructed in polar coordinates and analyzed for high resolution characterization of impact damage in the composite honeycomb panel. Two methodologies are used for ultrasonic wave-field analysis: scattered wave field analysis (SWA) and standing wave energy analysis (SWEA) in the frequency domain. The SWA is employed for processing the wave field and estimate spatially dependent wavenumber values, related to discontinuities in the structural domain. The SWEA algorithm extracts standing waves trapped within damaged areas and, by studying the spectrum of the standing wave field, returns high fidelity damage imaging. While the SWA can be used to locate the impact damage in the honeycomb panel, the SWEA produces damage images in good agreement with X-ray computed tomographic (X-ray CT) scans. The results obtained prove that the laser-based nondestructive system is an effective alternative to overcome limitations of conventional NDI technologies.

Barely visible impact damage assessment in laminated composites using acoustic emission

Uncertainty quantification for acoustic nonlinearity parameter in Lamb wave-based prediction of barely visible impact damage in composites

This paper exploits the implementation of a delay-and-sum imaging method using Lamb wave signals to localise barely visible impact damage (BVID) in quasi-isotropic composite panels. The structural discontinuities, such as opening and stiffener, has also been tested to reflect the common structural features of an aircraft and to examine the feasibility of the proposed detection technique. The prediction results are compared with ultrasonic C-scan images, which indicate location error for three different panels –flat panel, flat panel with an opening and stiffened panel. The accuracy is believed to be improved by increasing the number of transducers. Overall the proposed damage detection technique, with the use of only four transducers, demonstrated sufficient accuracy and efficiency in impact damage detection and can be applied alongside the traditional NDT inspections for providing a priori information of the impact damage location.

Nondestructive testing (NDT) of composite materials is of paramount importance to the aerospace industry. Several NDT methods have been adopted for the inspection of components during production and all through the aircraft service life, with infrared thermography (IRT) techniques, such as line scan thermography (LST) and pulsed thermography (PT), gaining popularity thanks to their rapidity and versatility. On one hand, LST is an attractive solution for the fast inspection of large and complex geometry composite parts during production. On the other hand, PT can be employed for the characterization of composite materials, e.g., the determination of thermal diffusivity and defect depth estimation. In this study, the use of LST with an uncooled microbolometer camera is explored for the identification of artificially produced porosity and barely visible impact damage (BVID) on academic samples. The performance of LST is quantitatively assessed with respect to PT (considered the gold standard in this case) using a high-definition cooled camera through the contrast-to-noise ratio (CNR) criterium. It is concluded that, although in most cases the measured CNR values were higher for PT than for LST (as expected since a high-definition camera and longer acquisition times were used), the majority of the defects were clearly detected (CNR ≥ 2.5) by LST without the need of advanced signal processing, proving the suitability of LST for the inspection of aerospace composite components. Furthermore, the deepest defect investigated herein (z ≈ 3 mm) was detected solely by LST combined with signal processing and spatial filtering (CNR = 3.6) and not by PT (since pulse heating was not long enough for this depth). In addition, PT was used for the determination of the thermal diffusivity of all samples and the subsequent depth estimation of porosity and damaged areas by pulsed phase thermography (PPT).

Composite materials are susceptible to barely visible impact damages (BVID) due to low-velocity impact. Therefore, an automated damage detection and quantification technique is highly desirable for quick inspection of large number of composite structures. Among various different non-destructive techniques (NDTs), active thermography NDT can be used for detecting damage on aircraft structures, using an infrared camera to capture the temperature distribution on the structure after it is exposed to heat using a flash lamp. In this paper, an image analysis algorithm that analyzes the infrared image, acquired using NDTherm NT, by determining the changes in the colormap values to automate the detection and quantification of the damage size was proposed. An area of the second derivative pre-processed grayscale image acquired using NDTherm NT is scanned in the x-direction and y-direction, and for each scan region the histogram of colormap values is extracted and stored. Irregularities in the structure result in non-uniform temperature distributions, which cause the infrared image to have a wide-range of grayscale colormap values in the damaged area. Therefore, the damage region is identified by monitoring the changes in the number of detected grayscale colormap values. The proposed image analysis technique was implemented for automated damage detection on Boeing 787 skin’s curved CFRP panel, with dimensions of 1.3 × 1.3 m. The proposed method detected the damage and determined the maximum damage length in the x-direction and y-direction to be 70.1 mm and 57.8 mm, respectively. Moreover, the proposed technique is suitable for feature identification applications.

Strength, stiffness, and panel peeling strength of carbon fiber-reinforced composite sandwich structures with aluminum honeycomb cores for vehicle body

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.

Composite sandwich structures have been widely used in aerospace structures, ship building, infrastructure, etc. due to their light weight and high strength to weight ratio. Traditionally, light-weight core materials such as foam core, truss core, honeycomb core have been used in fabricating sandwich structures with limited success. In this study, a new composite sandwich structure with a hybrid core was proposed, fabricated, tested, and modeled. The hybrid core consists of a fiber reinforced polymer grid skeleton that is filled in by extremely light-weight syntactic foam in the bay areas. The new sandwich structure was manufactured using a new manufacturing process. The behavior of these structures under impact and compression was investigated. Experimental results show that the grid structure with smaller bay area has higher impact resistance compared to grids with larger bay areas. For the hybrid core with a smaller bay area, the damage was localized to a much smaller area as evidenced by ultrasonic inspection. The nodal region was observed to have the highest impact resistance while the bay area was the weakest and had the least impact resistance. Compression testing was done after subjecting the specimens to impact testing. The results obtained show that the residual load carrying capacity decreases as the bay area increases. However, compared to the control group, which was the traditional laminated composite, the sandwich structure with grid stiffened core shows better impact tolerance and higher residual strength, although the fiber volume fraction used was the same. SEM images of impact region were taken and the mechanisms involved in the failure of foam, ribs and nodes were observed. From impact analysis it was found that grid stiffened sandwich structures with a smaller bay area have a higher impact loading capacity. The damage was localized to a smaller area. The residual load carrying capacity of grid structures was found to be higher than that of the laminates. Finite element analysis was conducted on the hybrid structure using ANSYS. A 3-D finite element model was developed and appropriate material properties were given to each component. Boundary conditions similar to those used in compression testing were utilized. The model was firstly validated by the experimental results. Parametric analysis was then performed on the validated model by varying important design parameters like skin thickness, skin modulus, rib width, rib modulus and bay area. Results obtained by changing these parameters were analyzed. The experimental and finite element results were

Although much effort has been made to explore the characteristics of composite lattice truss core sandwich structures, little work has been undertaken into non-destructive evaluation (NDE) and structure health monitoring (SHM). This paper presents a baseline-free NDE method to determine the debonding of truss bar in composite lattice truss core sandwich structures. The method is proposed based on the uniform load surface (ULS) curvature and developed according to the combination of gapped smoothing method (GSM) and Teager energy operator (TEO), which is denoted as GSM-TEO method. The effects of truss bar on the vibration and wave motion characteristics of composite lattice truss core sandwich structures are firstly investigated, and then, experiment and numerical simulation are conducted to assess the performance of the proposed method. Results verify the method is reliable and applicable for damage localization in composite lattice truss core sandwich structures. doi: 10.12783/SHM2015/74

Mechanical and oxidation properties of C/SiC corrugated lattice core composite sandwich panels

Post-fire mechanical properties of sandwich composite structures

Applications of sandwich composites are increasing as they continue to develop. The reason for the growth in development and the rising demand for sandwich composites in numerous industries is due to its ability to achieve high stiffness and strength, while simultaneously being lightweight in comparison to traditional materials. 3D printed plastic hollow core sandwich composites were focused on in this study where a bioplastic, Polylactic Acid (PLA), was compared to a conventional plastic, Polypropylene (PP). The study investigated the effects of varying core and face sheet parameters on the mechanical properties of 3D printed plastic core sandwich composites to obtain the optimum sandwich composite structure. A finite element model was developed in ANSYS Workbench to conduct the study. Numerical results were compared with experimental results, and both results revealed that the mechanical properties of sandwich composite structures are certainly influenced by the core and face sheet parameters. An optimum sandwich composite structure cannot be defined for a general case, as it depends on the application for which it is being designed. Hence, different optimum structures were determined for different desired mechanical properties.

Determination of optimal combination of pumping and probing inputs in vibro-acoustic modulation for sensitive BVID detection of CFRP