NDE of composite structures using ultrasonic guided waves

Composite honeycomb sandwich structures are being widely used in the aerospace industry as a light weight structural design. However, the residual strength (damage tolerance) analysis required to address the vulnerability of such structures after the impact damage has proven difficult to perform with confidence. An integrated progressive failure analysis method is developed herein that simulates the progressive intra/inter laminar failure mechanisms observed in composite honeycomb sandwich structures. This method adopts a continuum damage mechanics approach and modifies the stiffness degradation scheme to enable prediction of the intralaminar matrix cracking and fiber fracturing. It also applies integrated VCCT/Cohesive method to allow the explicit modeling of interlaminar failure – the debond/delamination growth from the structural flaw, as well as the formation of the secondary debond/delamination that eventually merges with the primary damage to precipitate the ultimate failure of certain composite honeycomb sandwich structures. The formulation of the method are derived rigorously and summarized in this paper. The analysis predictions are correlated against existing test data for various honeycomb sandwich structures with an embedded flaw that fails in different failure modes.

Composite honeycomb sandwich structure is widely used in aircraft wing leading edge, rudder surface, engine fairing, etc. It is susceptible to strong aerodynamic noise and impact loads from birds, hails, and stones. Therefore, it is essential to evaluate the life of structure with sound and collision load. In this paper, an acoustic fatigue life evaluation method based on vibro-acoustic coupling with impact damage is proposed. A representative composite honeycomb sandwich structure is built up to implement the proposed method. Firstly, the low-velocity impact process is simulated by the finite element (FE) method. This analysis case is used to get the structure with damage and material degradation. Secondly, the FE model is linked with the acoustic indirect boundary element (BE) model. A white Gaussian noise load is applied to the coupled FE/indirect BE model, and the power spectral density (PSD) curve of the structural dangerous point is obtained by modal-based vibro-acoustic coupling response case and random post-processing analysis case. At last, the fatigue life of the honeycomb structure is computed by the PSD theory. The result shows that with the increase of the impact energy, the structural life under the same acoustic load decreases. This method reveals high computational efficiency and excellent feasibility. The analytical result has reference value for the acoustic and mechanical properties design of composite honeycomb sandwich structure.

This study experimentally and numerically investigated the lightning strike (LS) damage in carbon fibre reinforced polymer (CFRP) composite honeycomb sandwich structures (CHSSs), aiming to address the limited understanding and lack of predictive capability for LS damage in such widely used yet structurally complex aerospace materials. Firstly, a series of simulated LS experiments were conducted on CHSSs, and the resulting LS damages were characterised using visual inspection, ultrasonic scanning and micro-computed tomography to identify typical material and structural failure modes. Then, a novel coupled electrical-thermal finite element (FE) model of CHSSs under LS was developed to predict the lightning-induced damage in such sandwich structures for the first time, addressing a knowledge gap in previous numerical studies which primarily focused on conventional CFRP laminates rather than structurally complex CHSSs. It was found that the FE-predicted LS damage area and depth showed good agreement with the experimental results with deviations of less than 16%, demonstrating the effectiveness of the FE model. Finally, the LS damage mechanism of CHSSs was explored and the effect of variations in honeycomb core thickness on the LS damage was examined by the proposed FE model, providing new perspectives on tailoring CHSS designs for enhanced LS resistance.

View Video Presentation: https://doi.org/10.2514/6.2022-0161.vid Studies dealing with the guided wave (GW) propagation through composite sandwich structures comprising of a honeycomb core bonded to two face sheets have been of continued interest. Numerous researches have dealt predominantly with the GW propagation through laminated composite plates and honeycomb composite sandwich structures (HCSSs) with constant core thickness. However, several components in aerospace and aeronautics industries incorporate HCSS with their core thickness varying smoothly. In this paper, a finite element simulation of the fundamental anti-symmetric (A0) mode of guided waves has been accomplished through HCSS possessing a smooth variation of core thickness. A three-dimensional transient analysis of the aforementioned structure has been performed using a finite element (FE) package, ABAQUS. A cumulative change in the wavefield pattern concerning the core thickness variation is observed. Also, a progressive change in the wave properties in response to the structural thickness variation is noticed thereby vindicating the adiabatic (A0 mode) behaviour of the propagating guided waves.

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

Currently, pultruded glass fibre-reinforced polymer (pGFRP) composites have been extensively applied as cross-arm structures in latticed transmission towers. These materials were chosen for their high strength-to-weight ratio and lightweight characteristics. Nevertheless, several researchers have discovered that several existing composite cross arms can decline in performance, which leads to composite failure due to creep, torsional movement, buckling, moisture, significant temperature change, and other environmental factors. This leads to the composite structure experiencing a reduced service life. To resolve this problem, several researchers have proposed to implement composite cross arms with sleeve installation, an addition of bracing systems, and the inclusion of pGFRP composite beams with the core structure in order to have a sustainable composite structure. The aforementioned improvements in these composite structures provide superior performance under mechanical duress by having better stiffness, superiority in flexural behaviour, enhanced energy absorption, and improved load-carrying capacity. Even though there is a deficiency in the previous literature on this matter, several established works on the enhancement of composite cross-arm structures and beams have been applied. Thus, this review articles delivers on a state-of-the-art review on the design improvement and mechanical properties of composite cross-arm structures in experimental and computational simulation approaches.

In this paper, an experimental study of pull-off load-bearing capacity and damage mode was conducted for tenon-and-mortise glued T-joints of composite honeycomb sandwich structures. Five test pieces of T-joints with different tenon widths and different numbers of tenons with the same total tenon width were investigated. The results of the pull-off tests showed that the tensile failure of the gluing surface occurs first in the tenon shoulder region during loading of the T-joint, after which the tenon region takes most of the load until the shear failure of the gluing surface of the bottom plate and the web in the tenon region or the panel pulls off. For the composite honeycomb sandwich structure tenon-and-mortise glued T-joints, increasing the width of the tenon makes an important contribution to improving the pull-off load capacity of the T-joints; when the total width of the tenon is the same, the pull-off load capacity of the T-joints decreases when the number of tenons increases.

Lightweight and heavy-duty fiber-reinforced polymer (FRP) composite honeycomb sandwich structures have been increasingly used in civil infrastructure. Unique cellular core configurations, such as sinusoidal core, have been applied in sandwich construction. Due to specific core geometry, the solutions for core effective stiffness properties are not readily available. This paper presents a mechanics of materials approach to evaluate the effective stiffness properties of sinusoidal cores. In particular, the internal forces of a curved wall in a unit cell are expressed in terms of resultant forces, and based on the energy method and principle of equivalence analysis, the in-plane stiffness properties of sinusoidal cores are derived. Both finite-element modeling and experimental testing are carried out to verify the accuracy of the proposed analytical formulation. To illustrate the present analytical approach as an efficient tool in optimal analysis and size selection of sinusoidal cores, several design plots are provided and discussed. The simplified analysis and formulation presented for sinusoidal cores can be used in design application of FRP honeycomb sandwich and optimization of efficient cellular core structures.

Principles for determining material allowable and design allowable values of composite aircraft structures

Adhesive-bonded components and structures have become an important part of manufacturing in the aerospace industry. These components often rely on honeycomb composite structures for strong yet lightweight design. However, the quality of the bonds is very important to the overall integrity of the composite structures. Due to their wide range of laminate and core configurations, these materials pose inspection challenges, especially during inspection for damage in the core, for example, disbonds and crushed core. For improved probability of detection (POD) on honeycomb composite structures, a multiple frequency C-scan-based approach exploiting both amplitude and phase C-scans is proposed.

In aeronautical industry, composite structures under testing often have complex and variable geometries. In this case, an optimal use of ultrasonic transducer arrays requires specific algorithms in electronic systems in order to achieve rapid and reliable inspections. To fulfil such requirements, a new real time and adaptive technique is presented. This technique is based on an iterative algorithm that does not require a prior knowledge of the geometrical and acoustical properties of the component undergoing inspection. All different parts of one given component (with flat, concave, convex parts...) can be controlled using a same transducer array, such as a conventional linear array with a flat shape. In this paper, the adaptive processing is demonstrated and illustrated through acquisitions performed with different typical aircraft composite structures.

Composite structures are generally more susceptible to impact damage than non-composite structures, and early identification of damage is the primary goal of structural health monitoring (SHM). If such damage remains undetected or reaches a critical size, it can lead to sudden collapse and catastrophic failure. Modern SHM methods aim to preserve the integrity of composite structures through continuous inspection, monitoring, and damage assessment, including detection, localization, quantification, classification, and prognosis. These methods use sensor-based technologies to assess vibration, extension, and acoustic and thermal emission. This paper provides a review of various computational methods including physics-based methods (signal processing techniques, modal analysis, and finite element model updating) and optimization methods (inverse problems, particle swarm optimization, topology optimization, genetic algorithms, time series analysis, and hybrid techniques), alongside machine learning methodologies employing neural networks as well as deep learning for damage identification in composite structures. These computational and learning-based techniques are widely applied in the development of algorithms, optimization strategies, and hybrid frameworks for SHM. The review further summarizes the applications, advantages, and limitations of each method according to structure type and damage characteristics. The key emphasis of this review is on integrating computational approaches, as well as machine learning, to enhance the efficiency of damage identification. The conclusion is drawn based on an overview of the literature, focusing on the contributions of different computational methods and machine learning for damage identification in composites.

In composite structures, damages are often invisible from the surface and can grow to reach a critical size, potentially causing catastrophic failure of the entire structure. Thus safe operation of these structures requires careful monitoring of the initiation and growth of such defects. Ultrasonic methods using guided waves offer a reliable and cost-effective method for structural health monitoring in advanced structures. Guided waves allow for long monitoring ranges and are very sensitive to defects within their propagation path. In this work, the relevant properties of guided Lamb waves for damage detection in composite structures are investigated. An efficient numerical approach is used to determine their dispersion characteristics, and these results are compared to those from laboratory experiments. The experiments are based on a pitch-catch method, in which a pair of movable transducers is placed on one surface of the structure to induce and detect guided Lamb waves. The specific cases considered include an aluminum plate and an aluminum honeycomb sandwich panel with woven composite face sheets. In addition, a disbond of the interface between one of the face sheets and the honeycomb core of the sandwich panel is also considered, and the dispersion characteristics of the two resultant waveguides are determined. Good agreement between numerical and experimental dispersion results is found, and suggestions on the applicability of the pitch-catch system for structural health monitoring are made.

An overview of the durability design requirements and substantiation approaches, with an emphasis on aerospace applications, is provided in this chapter. The service history of both commercial and military composite aircraft structures has provided numerous examples of both good and bad designs from which current design specification and guidance are derived. Issues to be considered in the design of composites used in primary and secondary composite structures including corrosion prevention measures associated with joining composite and metallic components are described. Design details, material selection, and demonstration that designs meet performance criteria are dependent on a structure’s thermal and mechanical loading environments, service and economic life requirements, manufacturing constraints, and inspectability requirements. Based on an assessment of the durability of composite materials at the coupon level through modeling and testing, a description of the building block approach used to validate durability predictions of elements, subcomponents, components, and full-scale structural testing is given. The unique requirements for developing load spectra for accelerated full-scale durability testing for both composite and combined composite/metallic structures are discussed.

Concentric honeycomb composite sandwich structures with tunable connection and gradient configurations