Sandwich Specimens Research Articles

Acoustic emission wave propagation in honeycomb sandwich panel structures

This paper studies acoustic emission (AE) wave propagation in a glass fibre aluminium honeycomb sandwich panel (HSP). Of particular novelty is the quantification of the through-thickness propagation of AE from one surface of the HSP to the other, which is a real-world monitoring consideration for applications where AE instrumentation is only be permissible on one surface, i.e. aerofoils. Complexity was introduced to the specimens in stages to enable a thorough understanding; first investigating propagation in a large glass fibre laminated plate (GFLP) alone; then in bespoke sandwich specimens with a limited number of honeycomb cells; and, finally, in a large HSP. The results of this paper demonstrate that, whilst some energy is transmitted through the honeycomb core, AE propagating in an HSP becomes bound in the outer plates. Despite this, propagation in these outer plates differs to that in an equivalent plate of the same material in isolation due to the complex interactions with the structural elements of the HSP. Further, propagation of AE transmitted from one surface to the other was quantified for the first time, giving insight into expected attenuation rates and characteristics for practical applications.

Experimental and numerical analysis of dynamic response of graded PVC foam sandwich panel under impact load

Polyvinyl chloride (PVC) foam sandwich panels have a wide application prospect in the field of warship impact protection due to the low moisture absorption, low density, excellent energy consumption and chemical resistance of PVC foam. The dynamic response of layered density-graded PVC foam sandwich panel under impact load was experimentally and numerically investigated here. A three-layer PVC foam sandwich specimen was designed and fabricated. The graded strategy of foam core was achieved by setting various foam layers with different densities. A large-diameter split-Hopkinson pressure bar device was used to perform the experimental study. The effect of density-graded strategy of foam core on the transmitted force, transmitted impulse and energy absorption were discussed. The software ANSYS/LS-DYNA was used to simulate the dynamic impact test of the foam sandwich structure. The simulation results are in good agreement with the experimental results. The results reveal that, the order of the core layer of density-graded sandwich panel has little effect on the impact resistance of the structure, while the impact resistance can be improved by increasing the density difference between foam layers. The uniform configuration UD has the largest impact force and the most specific energy absorption, while the large gradient configuration large gradient configuration has the smallest transmitted impulse under various impact velocities of the projectile. Thus, large gradient configuration has better protection effect.

Fire resistance behaviour of GFRP-polyurethane composite sandwich panels for building floors

This paper presents an experimental study about the fire resistance behaviour of composite sandwich floor panels made of glass-fibre reinforced polymer (GFRP) faces and polyurethane foam core subjected to a standard fire scenario and a mechanical load. Three full-scale sandwich specimens were tested: an unprotected specimen and two specimens protected with calcium silicate (CS) boards, applied either adherent to the underside of the panel or suspended from it and forming an air cavity. The study addressed the thermal and mechanical responses of the panels, including the evolution of (i) temperature distributions, (ii) deflections and deformations, (iii) failure modes, and (iv) time to failure, as well as the effects of the fire protection systems in those responses. The results obtained confirm the susceptibility of composite sandwich panels to fire: the unprotected panel failed after only ~25 min, due to a failure mode that involved different damage mechanisms, such as rupture of the glass fibres of the lower face, total degradation of the core, and flexural failure of the top face. On the other hand, with CS board protections, due to the thermal insulation conferred to the sandwich panels, the fire resistance was considerably increased to ~80 min (adherent) and ~100 min (air cavity). These tests also provide new insights about the thermo-mechanical behaviour in fire of composite sandwich panels, indicating a progressive change of their structural system, from a beam system to an arch system and finally to a simple laminate (the top face sheet) in bending.

Effects of Core Height, Cell Angle and Face Thickness on Vibration Behavior of Aircraft Sandwich Structure with Honeycomb Core: An Experimental and Numerical Investigations

The aim of the present paper is to study the vibration behavior of a sandwich structure with honeycomb core experimentally and numerically with different design parameters. The natural frequency and damping ratio were obtained. Core height, cell angle and face thickness were considered as design parameters. Finite element models for the honeycomb sandwich were developed and analyzed via ANSYS finite element analysis (FEA) software. Response Surface Method (RSM) is used to establish numerical methodology to simulate the effect of the design parameters on natural frequency and damping ration. The employment of (RSM) provides a study of the effect of design parameters on natural frequency and damping ratio, numerical modeling of them in term of design parameters and specifying optimization condition. The experimental tests were conducted on sandwich specimens for the validity goal of the previous models created via the finite element analysis. The obtained results show that the natural frequency is directly proportional to the core height and face thickness, while it is inversely proportional to cell angle, Vice versa for damping ratio. Moreover, the optimum value of natural frequency (209.031 Hz) as minimum and damping ratio (0.0320) as maximum were found at 4.8855 mm of core height, 26.770 cell angle and 0.0614 mm face thickness.

A novel test fixture for mode III fracture characterization of monolithic laminates and composite sandwich specimens

This work presents a novel test fixture for mode III fracture characterization of delaminations in monolithic laminates and face-core debonds in foam core composite sandwich specimens. The test fixture is configured as an extension of the already existing shear-torsion-bending (STB) test designed for monolithic laminates. The specimen sizing, lay-up configuration and the manufacturing process are presented. Accordingly, an overview of the test fixture is provided along with the data reduction method employed to compute the energy release rate. The results from representable fracture characterization tests are presented as force vs. displacement curves, where different definitions of the critical load for crack propagation can be defined. Thus, the critical value of the energy release rate is computed using analytically based equations for the different definitions given for the critical loads. The results show a stable crack growth for monolithic laminate specimens. However, a highly non-linear response of the sandwich specimens, before the onset of crack propagation, is observed. A non-linear numerical analysis and destructive specimen inspections are carried out in order to identify the source of the non-linear behaviour observed in the experimental results.

Microtensile Bond Strength Between Resin-Matrix CAD/CAM Ceramics and Resin Cement after Various Surface Modifications and Artificial Aging.

To evaluate the effect of different surface modification methods on the microtensile bond strength (μTBS) of four resin-matrix CAD/CAM ceramics after artificial aging. Specimens of four CAD/CAM materials (Shofu Block HC, Lava Ultimate, Brilliant Crios, and Vita Enamic) were prepared and divided into four groups. Each group received one of the following treatments: group 1 (INT): no surface modification; group 2: sandblasting with 29-μm Al2O3 particles (SB); group 3: hydrofluoric acid etching (9%) + silane (HF+Si); group 4: sandblasting with 30-μm particles of the CoJet system (CJ). The specimens of each group were luted together in pairs using resin cement (RelyX Ultimate). After one week of water storage (37°C), the sandwich specimens were sectioned into rectangular microspecimens and half of them were immediately subjected to μTBS testing, while the other half was tested after six months. Data were statistically analyzed using FFANOVA including the factors of material, treatment, and storage time, with α = 0.05. After one week, the lowest μTBS was observed for INT, while the highest was found for either mechanical (SB and CJ) or chemical (HF+Si) treatments (p < 0.05). After six months, a significant decrease in μTBS was observed depending on treatment (p < 0.05), while artificial aging significantly influenced the μTBS of all experimental groups (p < 0.05). During the two storage periods, the failure type was mainly interfacial and was associated with the type of surface modification. After artificial aging, the μTBS appeared to depend on srface modification, while the parameter "material" did not influence the results. Consequently, adhesive strategies should be oriented towards surface modification techniques.

Investigation into the arc profiles and penetration ability of axial magnetic field-enhanced K-TIG welding by means of a specially designed sandwich

To investigate the influence of an axial magnetic field on K-TIG welding, experiments are carried out with an specially designed axial magnetic field device and specimen. The real-time profiles of the arc-keyhole are captured through a steel/glass sandwich specimen, and the arc voltage is measured synchronously. The arc shape appears as a symmetrical bell under typical welding conditions without an AMF. When an AMF is exerted, the arc shape appears like a gong, with the top of the arc constricted to the torch while the bottom spreads in the radial direction. The rear endpoint of the arc can represent the penetration status of the weldment. A single particle description model is established to analyze the arc shape under the influence of the AMF. The mechanism of the improvement effect of the AMF on the arc penetration ability is clarified. The arc emitting area decreases with the exertion of the AMF. This decrease leads to an increase in the arc force and arc pressure, which ultimately results in improvement in the penetration ability. This work thus lays a solid foundation for better understanding the influence of an AMF on K-TIG welding.

A novel and robust method to quantify fatigue damage in fibre composite materials using thermal imagine analysis

This work proposes a novel and robust thermal imaging-based method suitable to quantify damage evolution stages in fibre-reinforced composite materials under fatigue loading. The method uses adiabatic hysteretic heating of the material to calculate the change of enthalpy by thermal imaging when the material undergoes fatigue damage. This work analytically corroborates the validity of using the calculated change of enthalpy to identify the three different fundamental fatigue damage stages of the material. The method is applied to two different types of notched sandwich specimens subject to constant amplitude fatigue loading. It is practically demonstrated that the characteristic curve progressions of the mechanically measured hysteretic dissipation curves are in good agreement with those of the enthalpy change computed by the proposed method.

Infill shape effects on bending stiffness of additively manufactured short fibre reinforced polymer sandwich specimens

Additively manufactured polymer parts are often designed like composite sandwich structures. In this work, sandwich beam-like specimens with hexagonal, triangular and rectangular infills were manufactured with different infill densities. An influence of the shape of the infill on the overall stiffness was observed. The bending stiffness of the hexagonal specimens was between 13% and 25% lower than that of the other two cases that instead showed similar performances. Numerical simulations were performed using both shell and solid elements for the infill, to check if it was possible to model the differences observed experimentally. All simulations lead to accurate bending stiffness predictions, except for the rectangular infills with higher infill densities, for which overprediction between 20% and 25% was obtained. Based on these results, strategies for the finite element analysis of additively manufactured composite structures are discussed.

Experimental estimation of the damping ratio of metallic foam sandwich panels with sand intrusions

The interest to use steel foam sandwich structures is expanding in various industrial applications, and more attention is paid to improve the properties of these materials. However, liquid and solid intrusions can severely hamper their useful characteristics. This study aims to provide an experimental investigation on the potential of steel foam (hollow sphere) sandwich specimens to operate as passive dampers in flexural vibration and to preserve their capabilities when affected by the intrusion of external particles. The authors utilized two different experimental examinations, random noise (white noise) and impact (hammer) tests. The specimens consist of a hollow sphere foam core sandwiched between two mild steel sheets, bonded with a thermosetting epoxy resin. To simulate the intrusions of granular materials in operating conditions, the metallic foam cores of the samples were partially filled with different percentages of quartz sand particles. The two-phase specimens were then compared to the pristine (single-phase) ones. The resulting estimates of the vibrational damping ratio for single and double-phase metallic foam specimens were used to calibrate the respective Finite Element models, which proved to be suitable for replicating the damping characteristics of the specimens.

Analysis of sandwich single cantilever beam test specimen with graded core

In previous research, an analysis model named the ‘beam on Vlasov foundation model’ was proposed to theoretically analyze the compliance and energy release rate of sandwich single cantilever beam (SCB) test specimens. The validity of the analytical model was confirmed when it was applied to sandwich specimens with homogeneous cores. In this study, the analysis model is applied to the analysis of a sandwich SCB test specimen where the Young’s modulus of the core varies along the thickness direction. The procedures to obtain the solution for two specific cases are described. One is the case that the Young’s modulus of the core varies in the thickness direction as an exponential function. The other is the case that the Young’s modulus of the core varies as a combination of exponential functions which is symmetric with respect to the midplane of the core. By comparing with finite element analysis, it is confirmed that the beam analysis model can provide reliable results even when the core is inhomogeneous in the thickness direction.

Design and characterization of a multilayered hybrid cored-sandwich panel stiffened by thin-walled lattice structure

Low-density polyurethane (PU) foams have been commonly used for some structural purposes in marine applications. However, they have not been exploited as a structural core because of damage tolerance and durability concerns. The major drawbacks of sandwich panels having PU foam core are their poor mechanical properties and vulnerability to marine environmental conditions. Therefore, the present study aimed to contribute to enhancing both the mechanical properties and environmental durability of the traditional PU foam cored-sandwich panels through introducing a potentially suitable alternative. For this purpose, an innovative panel with a multilayered hybrid core consisting of agglomerate cork and PU foam was proposed which was orthogonally stiffened by a thin-walled lattice structure and sandwiched between two composite skins. To characterize the environmental durability and mechanical properties of this novel panel, a wide range of experimental tests including water immersion conditioning, three-point bending, edgewise and flatwise compression, quasi-static indentation, and high-velocity impact perforation were conducted. The sandwich specimens made of composite skins and PU foam core were also manufactured and tested for comparative purposes. Additionally, a nonlinear finite element (FE) analysis was also carried out and the numerical results were compared with the experimental data. The principal findings were that the proposed structural modifications caused a significant enhancement in all the studied mechanical and durability properties of PU foam-cored sandwich panels. It was also found that the FE model could predict load-displacement behavior of the studied sandwich panels under a variety of experimental conditions.

Three-point bending properties of carbon fiber/honeycomb sandwich panels with short-fiber tissue and carbon-fiber belt interfacial toughening at different loading rate

The bending performances at different loading rate are studied for carbon fiber/honeycomb sandwich panels toughened by short aramid fiber tissues and carbon fiber belts. The carbon fiber belts are stitched cross the pores of honeycomb core for interfacial improvements. Three-point bending tests at different loading rates are carried out to investigate the effect of short aramid fiber tissue and carbon fiber belt on mechanical properties of carbon fiber/honeycomb sandwich specimens. Experimental results firstly indicate that the short-aramid-fiber interfacial toughening and carbon fiber belts toughening could both enhance the energy absorption and peak load of sandwich specimens at loading rates ranging from 2 mm/min to 500 mm/min. The failure modes and microstructures of toughened specimens are observed to analyse and explain the underlying mechanism of enhancing effect. It is illustrated that crack isolation phenomenon is found to be the main mechanism for avoiding interfacial damage of the sandwich specimen.

Experimental and numerical study of square sandwich panels with layered-gradient foam cores to air-blast loading

The blast response of clamped sandwich panels with layered-gradient aluminium foam cores was studied experimentally and numerically. The effective blast impulse acting on specimens in ballistic pendulum tests was first calibrated, and the influences of blast impulse and core-layer arrangement on the deformation modes and shock resistance were revealed. Then, the corresponding numerical simulations were conducted. The finite element model was validated via the displacement–time curves of the ballistic pendulum system and the deformation profile and deflection response of sandwich specimens. The blast pressure, deformation process, energy absorption, strain evolution, and distribution of blast-loaded sandwich panels are discussed. The experimental and numerical results show that all the layered-gradient core sandwich panels have a weaker blast resistance capability than the ungraded sandwich panels because of the reduction in structural integrity of the specimens. For a given effective impulse, the specific energy absorption value of the positive gradient sandwich panels is the largest, followed by that of the ungraded sandwich panels, whereas that of the negative gradient sandwich panels is the lowest.

Hypervelocity Impacts on Honeycomb Core Sandwich Panels Filled with Shear Thickening Fluid

The use of honeycomb core sandwich panels filled with a shear thickening fluid (STF) as a component of spacecraft micrometeoroid/orbital debris (MMOD) shielding was investigated using hypervelocity impact (HVI) testing. Incorporating a STF into shielding has the potential to reduce damage to the core and the likelihood of back-side facesheet perforation in the event of a HVI. The sandwich panels tested consisted of 1.27 cm thick hexagonal aluminum honeycomb core bonded between 0.064 cm thick aluminum facesheets. The STF displayed a marked rise in viscosity with increasing shear rate above a critical shear rate. It was based on low molecular weight polyethylene glycol (PEG) and hydrophilic fumed silica. Sandwich panel target specimens filled with the STF were subjected to HVIs by 1 mm diameter stainless steel spheres at nominal temperatures of -80°C or 21°C with nominal impact velocities of 4.8 km/s or 6.8 km/s. Additional specimens filled with PEG only were also impacted for comparison.Visual inspections and X-ray computerized tomography were used to assess impact damage. All of the panels experienced perforation of the impacted facesheet, facesheet bulging, localized delamination, and the formation of a cavity in the damaged core. STF-filled panels sustained significantly less damage than PEG-filled panels. None of the STF-filled panels were completely perforated during impact. In contrast, one of the PEG-filled panels impacted at the peak velocity was perforated. The remaining PEG-filled panel sustained substantially more honeycomb core damage and facesheet-core delamination compared to an analogous STF-filled panel. Sandwich panels filled with the STF provide superior HVI mitigation in comparison to panels filled with a Newtonian fluid (i.e., PEG). These experiments show that incorporation of STFs into MMOD shielding components has the potential to dramatically improve the HVI penetration resistance over a broad range of impact velocities and temperatures.

Modeling of interfacial crack propagation and kinking in sandwich panels

The paper presents an analytical model for the nucleation, propagation, and kinking of cracks in sandwich panels. The analytical approach adopts the variational principle of minimum of the total potential energy and it is based on cohesive interface modeling. The mathematical modeling considers two face sheets, a core layer, and an interfacial core-face sheet crack that can kink into the core and form a diagonal shear crack. This phenomenon, which has been observed in experiments, divides the core into two bodies. The core is therefore modeled using such two body approach. Three interfaces are introduced between the four components and appropriate interfacial laws are used to describe the interfacial phenomenon. The First-Order Shear Deformation Theory with geometrically nonlinear kinematic relations is adopted for the face sheets. The two core layers use linear geometrical relations and assume the high order polynomial displacement fields of the Extended High-Order Sandwich Panel Theory. Linear elastic laws are considered for the two face sheets and the core. The model is validated through a comparison with experimental observations of pre-cracked Double Cantilever Beam sandwich specimens that show a diversity of crack paths. The comparison and the quantitative investigation explore the capabilities and the limitations of the approach and shed light on the nucleation, debonding, and kinking phenomena in soft-core sandwich panels.

Upper and Lower Bounds for the Parameters of One-Dimensional Theories for Sandwich Fracture Specimens

AbstractUpper and lower bounds for the parameters of one-dimensional theories used in the analysis of sandwich fracture specimens are derived by matching the energy release rate with two-dimensional elasticity solutions. The theory of a beam on an elastic foundation and modified beam theory are considered. Bounds are derived analytically for foundation modulus and crack length correction in single cantilever beam (SCB) sandwich specimens and verified using accurate finite element results and experimental data from the literature. Foundation modulus and crack length correction depend on the elastic mismatch between face sheets and core and are independent of the core thickness if this is above a limit value, which also depends on the elastic mismatch. The results in this paper clarify conflicting results in the literature, explain the approximate solutions, and highlight their limitations. The bounds of the model parameters can be applied directly to specimens satisfying specific geometrical/material ratios, which are given in the paper, or used to support and validate numerical calculations and define asymptotic limits.

Innovative sandwich panels made of wood bio-concrete and sisal fiber reinforced cement composites

This paper presents the results of the development of innovative sandwich panels made by combining wood bio-concretes and long sisal fibers reinforced composites. An experimental study of the wood bio-concretes (WBC) was carried out by producing several cement-to-wood ratios (0.5, 1, 2, 2.5 and 3) in order to allow the development of panels of different density. Before producing the WBC, the influence of hot water washings of the wood shavings on the cement hydration was investigated. The effect of the cement-to-wood ratios on the density, thermal conductivity, compressive strength and elastic modulus of the WBC were also investigated. Lightweight bio-concretes were obtained with densities ranging between 700 and 1250 kg/m3 and compressive strengths within 0.44–16 MPa, after 28 days cure. The lighter and denser bio-concretes were used as cores in sandwich system with faces made of cement composites reinforced with 6% of long sisal fibers distributed in three layers. The results of compressive and flexural tests performed on the sandwich specimens revealed that the WBC2.5 were more effective in terms of promoting compressive strength, shear transfer capacity and allowing a pure bending failure, while WBC0.5 promoted core shear cracks and the inferior face crush.

Research on the compatibility of waterproof layer materials and asphalt mixture for steel bridge deck

The current structure of the steel bridge deck often made of steel plate, waterproof adhesion layer (WAL) and paving layer (wearing course and protecting course). The WAL acts as an interlayer between the bridge deck and the paving layer, which enhances the interface adhesion and ensures the pavement layer can follow the deformation of the steel plate well. In addition, it constitutes the barrier against the environmental agent, especially the water. Therefore, it is crucial to understand the interfacial adhesive performance of the WAL, namely its adhesion to the coatings of steel and asphalt mixture. To improve the level of the design for the steel bridge deck, the compatibility of WAL and asphalt mixture (such as epoxy asphalt concrete (EA) and guss asphalt concrete (GA)) is the focus of discussion. In this paper, the orthogonal experimental is used to study the adhesive behavior of the epoxy binder and methacrylate-based adhesive (MMA) and steel plate under the influence of various factors such as the anticorrosive layer, temperatures, spraying quantity. Following the determination of optimal scheme, a WAL-asphalt mixture sandwich specimen was subjected to tensile and shear tests, to evaluate the compatibility of the WAL and protecting course. The results show that the anticorrosive layer is necessary and feasible to improve the interfacial adhesive property. Shear strength and bond strength are easily affected by temperature and decrease with the increase of test temperature. Sometimes, an increase in the spraying quantity of the adhesive layer may decrease the connection efficiency. The MMA material shows poor compatibility with EA, and the epoxy binder has better compatibility with EA. Finally, it is recommended to use the “GA + 1.5 kg/m2 MMA + anticorrosive layer + steel plate” scheme for perennial high-temperature zone; the “EA + 0.6 kg/m2 epoxy binder + anticorrosive layer + steel plate” scheme for the zone with small temperature difference; and the “EA + 1.0 kg/m2 epoxy binder + anticorrosive layer + steel plate” scheme for low-temperature zone.

Design analysis of end notch flexure sandwich specimen with honeycomb core

Laminate beam theory based design analysis of the sandwich End Notch Flexure (ENF) test specimen is presented. The analysis is specifically considering specimens with honeycomb core, prone to in-plane compression failure of the core, other undesirable failure modes such as face indentation and core shear failure are analyzed. In addition, crack stability and energy dissipation due to frictional sliding between the crack surfaces are analyzed. Parametric analysis of ENF specimens with a range of face and core thickness, is presented to guide the design. In-plane compression failure of the core, but shorten the range of crack lengths were stable growth occurs. A thicker face will also increase the critical load for crack propagation and by so elevate the risk for indentation and core shear failures. Increased core density is beneficial for preventing indentation and core shear failures, but may increase the risk for in-plane compression failure of the core. The thickness of the core does not strongly influence the critical load for crack propagation. A thicker core increases frictional energy loss, but reduces the risk for core shear failure. A thicker core, however, makes the ENF specimen more prone to in-plane compression failure. ENF sandwich specimens with foam and honeycomb core were tested. The test results support the design analysis.