Concentric honeycomb composite sandwich structures with tunable connection and gradient configurations

Composite structures require a rigorous program of nondestructive inspection and maintenance to detect and characterize hidden defects at an early stage of their occurrence so that preventive measures can be taken before the structure loses its load carrying capacity or suffers from catastrophic failure. Current methods for defects detection in large aircraft and aerospace structures are slow, labor intensive and costly. This is especially true for composite structures where conventional techniques are often ineffective. Ultrasonic guided waves offer an attractive complementary tool for improving inspection techniques in relatively large plate-like structural components due to their large propagation range and sensitivity to defects in their propagation path. Since the waves are affected by the geometrical structural features (e.g. stringers) as well as harmful defects (e.g. delaminations), the application of guided waves in the NDE or SHM of real structures requires a good understanding of these interaction effects. This will help identify the defects from their distinguishing features in the signal in structural components with complex geometry. In this paper a detailed study of the interaction of guided waves with defects in an aluminum plate and a honeycomb composite sandwich structure is carried out using numerical simulations and laboratory experiments. The simpler aluminum plate is used for model validation and understanding the basic characteristics of the interaction phenomena. The agreement between the simulated waveforms and those measured from the experiments are found to be excellent in both cases indicating the possibility of applying guided wave based techniques to more realistic structures.

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.

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

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.

Sandwich honeycomb structures (aluminum core bonded to Carbon Fiber Reinforced Polymer (CFRP) sheets on either side) are widely employed in the aerospace industry for their high strength to mass ratio. However, they might be subjected to damages such as delaminations of the composite sheets or debondings between the face sheets and the core due to impacts or thermo-mechanical aging. In order to reduce maintaining costs and extend the service time, Guided Waves (GW) based Structural Health Monitoring (SHM) systems are considered an adequate solution. Indeed, GW propagate over long distances and exhibit low attenuation, thus allowing to monitor wide areas with a limited number of sensors. Defect imaging on CFRP composites and honeycomb composite structures using both Delay-And-Sum [1] and correlation-based algorithm Excitelet [2] is presented in this communication. A machine learning algorithm is finally implemented in order to automatically identify and isolate defects on a given cartography map. The machine learning algorithm is trained on an experimental database of false and positive results obtained on representative specimens.

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.

The smart sandwich structures have been widely used in the aerospace, automobile, marine, and civil engineering applications. A typical smart sandwich structure is usually comprised of two stiff face skins separated by a thick core with variety of embedded sensors to monitor the performance of the structures. In this study, the smart composite sandwich structure (CSS) samples are fabricated with glass microballoons syntactic foam core and resin infused glass-fiber face skins (with piezoelectric fiber composite sensors (PFCS) embedded inside the resin infused glass-fiber face skins). One of the main concerns associated with embedding sensors inside composite structures is the structural continuity, compatibility, and interface stress concentrations caused by the significant differences in material property between sensor and host structures. PFCS are highly flexible, easily embeddable, highly compatible with composite structures and their manufacturing processes, which makes them ideal for composite health monitoring applications. In this study, in-plane tensile, tension–tension fatigue, short beam shear, and flexural tests are performed to evaluate the effect on strengths/behavior of the CSS samples due to embedded PFCS. Then carefully planned experiments are conducted to investigate the ability of the embedded PFCS to monitor the stress/strain levels and detect damages in CSS using modal analysis technique. The tensile tests show that both the average ultimate strength and the modulus of elasticity of the tested laminate with or without embedded PFCS are within 7% of each other. The stress–life (S-N) curves obtained from fatigue tests indicates that the fatigue lives and strengths with and without the PFCS are close to each other as well. From short beam and flexural test results, it is observed that embedded PFCS leads to a reduction of 5.4% in the short beam strength and 3.6% in flexural strength. Embedded PFCS’s voltage output response under tension–tension fatigue loading conditions has been recorded simultaneously to study their ability to detect the changes in input loading conditions. A linear relationship has been observed between the changes in the output voltage response of the sensor and changes in the input stress amplitude. This means that by constantly monitoring the output response of the embedded PFCS, one could effectively monitor the magnitude of stress/strain acting on the structure. Experiments are also performed to explore the ability of the embedded PFCS to detect the damages in the structures using modal analysis technique. Results from these experiments show that the PFCS are effective in detecting the initiations of damages like delamination inside these composite sandwich structures through changes in natural frequency modes. Hence embedded PFCS could be an effective method to monitor the health of the composite sandwich structures’ in-service conditions.

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.

Compression, bending, energy absorption properties, and failure modes of composite Kagome honeycomb sandwich structure reinforced by PMI foams

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

The demand for biodegradable, recyclable, natural composites with lightweight structures is driven by the fact that advanced structures can withstand quasi-static and dynamic loadings. This study examined the energy-absorbing characteristics of interlocking periodic honeycomb sandwich structures made from short sugar palm, kenaf, and pineapple leaf fibres (PALFs) reinforced with a polylactic acid (PLA) composite. The biodegradable sugar palm, kenaf, and PALF/PLA composite sheets were subjected to hot compression and cut into single- and double-slot square plates. The interlocking technique was used to assemble periodic two-dimensional square-honeycomb sandwich structures. Moreover, new and recyclable PLA-based composites with three fibres were tested for tensile properties. The biodegradable PLA-based composite honeycomb sandwich structure underwent a quasi-static compression test. Finite element modelling was used to simulate the load–displacement curve, energy-absorption characteristics, and failure behaviour, incorporating tensile properties and geometric imperfections. The results revealed that the double-slot design of the pineapple/PLA sandwich structure significantly increased by 1.33 times compared to the sugar palm/PLA sandwich structure. Notably, it reduced the compressive strength of recyclable pineapple/PLA (66.4%) and recyclable sugar palm/PLA (31.5%) composite sandwich structures compared to the new pineapple/sugar palm PLA-based composite. In addition, finite element analysis (FEA) showed reasonable agreement with experimental data, with a 7.11% error in energy absorption (EA). It was highlighted that biodegradable, recyclable, interlocking sandwich-structured composites have potential for advanced, sustainable energy-absorbing structures.

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