Precursor Damage Quantification in Composite Structures Using Coda Wave Interferometry and Nonlinear Ultrasonics

Guided wavefield curvature imaging of invisible damage in composite structures

Fatigue induced cracks in welds may result in the failure of welded structures and therefore it is important to detect the fatigue crack in the early stage. In this paper, we proposed a coda wave interferometry (CWI)-enabled fatigue cracks detection method using piezoceramic transducers. Because of its high sensitivity to the tiny change in the medium, the CWI method employs the multiple scattered coda waves to detect the slight change of welding fatigue cracks. Firstly, a theoretical model is developed by combining the CWI theory with the acoustoelastic effect. The derivation shows that there is a linear relationship between the crack width change and the coda wave relative velocity variation. To verify the correctness of the derived result, the CWI technique is applied to the detection of micro fatigue cracks of three butt welded specimens under fatigue loading. Experimental results show that the relative velocity variation of coda wave increases linearly with the increase of the width of welding fatigue cracks. In addition, energy-based active sensing (EAS) approach was also used as a comparison. Compared with the EAS method, the CWI technique is more sensitive to slight changes of welding fatigue cracks and therefore holds a great potential for the monitoring of micro fatigue crack in the early stage.

A hybrid structural health monitoring (SHM) system, consisting of a piezoelectric transducer and fiber optic sensors (FOS) for generating and monitoring Lamb waves, was investigated to determine their potential for damage detection and localization in composite aerospace structures. As part of this study, the proposed hybrid SHM system, together with an in-house developed algorithm, was evaluated to detect and localize two types of damage: a through thickness damage (hole of 2 mm in diameter) and a surface damage (2 mm diameter bore hole with a depth of 0.65 mm) located on the backside of the plate. The experiments were performed using an aircraft representative composite plate skin, manufactured from carbon fiber reinforced polymer (CFRP).

Carbon fiber composite laminates, consisting of highly anisotropic ply layers, are widely used in aerospace structures due to their good strength to weight ratio. However, due to poor interlaminar strength, composite components are prone to barely visible impact damage during aircraft operation. Sparse array guided wave imaging, using a network of distributed sensors, is an important Structural Health Monitoring (SHM) tool for the detection and localization of in-service damage in composite structures. However, the anisotropy of composite laminates influences guided wave scattering, impacting imaging performance. Defect characterization can be improved by considering the scattering characteristics of various damage types for the sparse array signal processing. Guided wave scattering (A0 Lamb wave mode) was investigated around an artificial insert delamination in a quasi-isotropic carbon fiber reinforced polymer (CFRP) panel. Permanent magnets, mounted on an undamaged region of the plate, were also used as scattering targets and compared to the delamination case. Full 3D Finite Element (FE) simulations were performed for both the delamination and magnet cases and compared to wavefield data obtained from non-contact laser measurements. Good agreement was found between the experimental measurements and simulations. Scattered guided wave amplitudes around each damage type show strong directional dependency with energy focusing along the fiber directions of the outer ply layers of the laminate. Distinct scattering behavior was observed for each damage type. A forward scattered wave was observed for the delamination, whereas the magnet blocked forward wave transmission. Implications of anisotropy and angular scattering on sparse array SHM of different defect types are discussed.

It is well known that the amount of damage caused by lightning strikes to protected composite airframe structures depends on the paint characteristics, often applied on the surface of composite structures to protect from environmental effects and to personalise a product. In this work, physically based models of the mechanical loads induced by lightning strikes are employed in the generation of the mechanical overpressure fields due to a simulated lightning strike, while accounting for the paint thickness. These fields are then implemented into a three-dimensional finite element framework and combined with a damage model to predict the effect of paint thickness on the mechanical damage in composite structures subjected to this type of events. These models accurately predict the increase of damage extent with the increase of paint thickness, which is corroborated by experimental observations from industry and by the experimentally observed trends reported in literature.

: In this project, computational tools were developed that help in designing and analyzing multi-functional composite structures that have sensing and actuation capabilities. Magnetic actuation of composite structures using built-in or embedded electromagnetic devices was studied. The main application of interest is micro air vehicle wings that are designed to flap to generate thrust and lift. Computational tools were developed to design and analyze such structures actuated by magnetic forces. Coupled magnetostatic-elastostatic analysis capability was implemented into a pre-existing software (named IBFEM) developed at the University of Florida that can perform finite element analysis without the need for generating mesh. After a systematic comparison of several actuator designs, it was concluded that a coil actuator built into composite structures is an ideal means for actuation of composite structures. A conceptual design of a flapping wing vehicle was developed. A method for sensing load application and damage in composite structures by determining change in resistivity/conductivity was also studied. An algorithm for solving inverse problem to determine average resistivity values in composite structures was demonstrated.

A hybrid method for determination of fatigue limit and non-destructive evaluation of composite structures after low-velocity impact loading

Correlative intelligent approach to predicting damage level in composite structures is presented, tested and proved to be effective. Using energy flow analysis under MATLAB environment and in connection with a neural networks intelligent algorithm (Weight Elimination Algorithm-WEA) proved to be very successful in predicting level of impact damage from correlated features obtained from static images taken for samples under test.

Damage localization for composite structure using guided wave signals with Gramian angular field image coding and convolutional neural networks

Structural health monitoring is the process of acquisition and analyzing technical data obtained from structures to determine the present condition of the structure and residual life. Composites have been widely in use because of their low weight and better mechanical properties compared to conventional metals. They are more prone to damage during cyclic loading and the impact of foreign objects. So, usage of the nondestructive techniques is important to detect such damage in composites at the beginning stage itself, which further helps to avoid catastrophic failure. Many review articles are discussing a single nondestructive technique to monitor the health of the structure, but a single technique is not sufficient in most of the cases. This review is focused on the most commonly used nondestructive health monitoring techniques such as acoustic emission, vibration testing, ultrasonic testing, infrared thermography, and shearography to detect and characterize the damage in composite structures used in aerospace, automotive, and marine applications. The comparison among the techniques also has been presented in this review.

Acoustic method of damage sensing in composite materials : Workman, G.L.; Walker, J.; Lansing, M. Alabama Univ., Huntsville (United States), N95-18135/0/GAR, 39pp. (20 Oct. 1994)

Delamination is a type of representative damage in composite structures, severely degrading structural integrity and reliability. The identification of delamination is commonly treated as an issue of nondestructive testing. Differing from existing studies, a hybrid optimization algorithm (HOA), combining particle swarm optimization (PSO) with simplex method (SM), is proposed to identify delamination in laminated beams. The objective function of the optimization problem is created using delamination variables (optimization parameters) together with actually measured modal frequencies. The HOA adopts a hierarchical and cooperative regime of global search and local search to optimize the objective function. The PSO performs global search for objective function space to achieve a preliminary solution specifying a local potential space. Initialized by this preliminary solution, the SM executes local search for the local potential space to explore the optimal solution. The HOA is validated by a series of simulated delamination scenarios, and the results show that it can identify delamination in laminated beams with decent accuracy, reliability and efficiency. The method proposed holds promise for establishing online damage detection system beneficial for health monitoring of laminated composite structures.

In aeronautics delamination damage in composite structures are a crucial factor for the timing of maintenance cycles. Structural health monitoring systems based on ultrasonic guided waves have the potential to find them quickly and consequently save airlines money. For the development of such a system removable pseudo damage are a cost-efficient alternative to enable the necessary quantity of testing. However, to develop a removable pseudo damage, a strategy is needed to classify the wave interactions of delaminations in a way which can also be used to classify those of removable pseudo damage. Therefore, this paper will showcase a methodology to classify the Guided waves reflection, transmission and S0 to A0 mode conversions induced by delaminations and discuss potential pseudo damage types. The analysis is based on a b-scan taken by an air coupled ultrasonic scanner starting at the actuator and ending behind the damage. The key findings are that the suggested methodology was able to capture the wave interactions of the delaminations as well as the experimental pseudo damage. Thus, it could be used as a tool to test the interactions of pseudo damage and compare it to the ones of delamination. Hence, being the first step towards enabling high-volume damage localisation tests of structural health monitoring systems.

Modification of basalt fibre using pyrolytic carbon coating for sensing applications

In this study, we investigate the interactions of Lamb wave A0 mode with different sizes of delaminations in composites using finite element code Abaqus®. According to Lamb wave dispersion curves, the group velocity of A0 mode increases rapidly as the frequency-thickness increases in the relatively low frequency region. In the delamination region, the frequency-thickness product decreases compared to the healthy laminate since the damage causes ply separation at the lamina interface. In the current study this observation is investigated in detail using finite element simulations. The resulting phase delay is analyzed by Empirical Mode Decomposition (EMD) and instantaneous phase approach. Finite element simulations are performed using Abaqus® and signal processing is performed in joint time-frequency domain using Hilbert-Huang Transform (HHT) method. The unwrapped instantaneous phase difference is correlated with the extent of delamination (quantitative level of damage).