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In this study, the critical buckling temperature of a lattice sandwich panel was investigated by using FEM. In particular, the effects of geometric properties, for example a core height h, a core length L, and the number of cells Nc on the buckling temperature, were discussed. It is confirmed that the lattice sandwich panel also has the global and local buckling modes that are common in a latticed members, and a correction factor which includes the effect of the number of cells Nc is presented for estimating the global buckling temperature of the panel. Multiplying the analytical equation of the buckling temperature which was derived from the conventional homogenization method by the correction factor, the buckling temperature can be estimated with a good accuracy. On the other hand, the local buckling temperature can be estimated by considering the buckling behaviour of a small plate surrounded by the cells and supported by four corners.

Hybridization of sandwich panels and their different components have drawn huge attention due to the significant improvement in their attributes. Hybrid core of 'Polyester Pin-reinforced Foam filled Honeycomb Sandwich panels' (PFHS) were fabricated and compared with unreinforced 'Foam filled Honeycomb Sandwich panels' (FHS) in terms of low velocity impact and Compression After Impact (CAI) performance. The impact damage area was calculated by employing MATLAB image processing technique. Incorporating through thickness pins for connecting faces and core is an effectual way to improve interfacial bonding, specific bending stiffness and also imparts out of plane properties for sandwich panels. The low velocity impact tests performed on the sandwich panels revealed that the polyester pin reinforcement in foam filled honeycomb sandwich panel improved the load bearing capacity, total absorbed energy and reduced the impact damage area significantly. In CAI test, debond, wrinkling of face sheet, and buckling of face sheet and core are the major modes of failure. The addition of the pins enhanced the compressive strength for all the impact energy levels. [ABSTRACT FROM AUTHOR]

A major obstacle to obtaining cost-effective experimental data on the fatigue life of sandwich panels is the prohibitive amount of time and cost required to carry out millions of cycles. On the other hand, vibration techniques applied to sandwich geometries fail to match the stress patterns that are obtained from standard flexural fatigue tests. To overcome such limitations, a vibration-based fatigue technique is proposed, which entails the use of sandwich specimens whose geometries are optimized to reproduce the stress distribution observed during three point bend loading while vibrating at the first resonant frequency. The proposed vibration technique was experimentally validated. The results, compared with the average number of cycles to failure at different stress ratios obtained via the Three-Point Bending test, showed high levels of accuracy. The proposed method is robust and time effective and indicates the possibility of attaining fatigue lifetime prediction of a wide class of composite elements, such as sandwich panels. [ABSTRACT FROM AUTHOR]

Present article has experimentally determined the flexural strength of carbon fiber reinforced polymer sandwich panel reinforced by nano‐SiO2 with novel M‐shaped lattice core. For this purpose, a polymer composite sandwich panel with M‐shaped core made of carbon‐epoxy fiber has been fabricated in this experiment. In order to fabricate the sandwich panels, the vacuum assisted resin transfer molding has been used to achieve a laminate without any fault. Afterward, polyurethane foam (PU) has been injected into the core of the sandwich panel. In order to determine the flexural strength of the sandwich panels, they have been subjected to three‐point bending loads. From the results of the study, it was figured out that: a) Adding 1 to 3 wt% of nano‐SiO2 into the carbon fiber had the most desirable effects on the enhancement of flexural strength of the sandwich panels. b) PU has improved the flexural strength of sandwich panels. c) The new M‐shaped core can well postpone the buckling and failure of the composite sandwich structures. [ABSTRACT FROM AUTHOR]

Present article has experimentally determined the flexural strength of carbon fiber reinforced polymer sandwich panel reinforced by nano‐SiO2 with novel M‐shaped lattice core. For this purpose, a polymer composite sandwich panel with M‐shaped core made of carbon‐epoxy fiber has been fabricated in this experiment. In order to fabricate the sandwich panels, the vacuum assisted resin transfer molding has been used to achieve a laminate without any fault. Afterward, polyurethane foam (PU) has been injected into the core of the sandwich panel. In order to determine the flexural strength of the sandwich panels, they have been subjected to three‐point bending loads. From the results of the study, it was figured out that: a) Adding 1 to 3 wt% of nano‐SiO2 into the carbon fiber had the most desirable effects on the enhancement of flexural strength of the sandwich panels. b) PU has improved the flexural strength of sandwich panels. c) The new M‐shaped core can well postpone the buckling and failure of the composite sandwich structures. [ABSTRACT FROM AUTHOR]

Validation of wind loads on structures with sandwich elements In 2009, the harmonized standard DIN EN 14509 was introduced with the title "Selbsttragende Sandwichelemente mit beidseitigen Metalldeckschichten". This is still valid today in the updated form of 2013 and regulates, among other things, the design procedures for sandwich elements. Since its introduction, the wind load modeling for the large‐surface facade elements has been discussed, since the new regulation in connection with the wind load standard DIN EN 1991‐1‐4:2012 led to higher wind loads, especially in the corner area. Therefore, in this study, the impact effects on the structural level were determined and statistically evaluated in order to obtain a realistic wind load model for the considered construction method. High‐resolution wind tunnel measurements were performed as well as structural tests to identify real load transfer effects on full‐scale sandwich systems. The superposition of both and the statistical validation led to the conclusion that the existing models do not show any general load reduction and that the verification of the elements is essentially correct also with respect to the load value in the corner of the building. However, it was shown that the correlation effect that is decisive for the large‐area elements can also be applied to the connecting elements, contrary to current design practice, so that the modeling of local suction peaks is not necessary and the cpe,10 coefficients from DIN EN 1991‐1‐4:2012 can also be used here. [ABSTRACT FROM AUTHOR]

Roof with metal sheets and PV‐systems – practical knowledge Recently it was legally stipulated in some German states that a solar power system has to be installed on new buildings. A legislative initiative on a federal level is also prepared. A solar power system (named PV‐system in the following) can consist of photovoltaic elements for producing electricity or solar elements for heating primarily water. Light weight metal sheets are often used for the roofing of industrial buildings, on which the PV‐system is installed. Prefabricated metal sheets consist of e. g. trapezoidal profiles, standing seam profiles and sandwich panels. Special fixing systems are used that fit for the individual type of metal sheet. The experience has shown that even though PV‐systems are put on metal sheets since many years, the special topics for the application are not known or not taken into account. The following article goes into detail to the before mentioned metal sheets regarding the formal basis, loadings, static design, occasional or repeated access of persons and requirements due to corrosion. [ABSTRACT FROM AUTHOR]

Creep of axially loaded sandwich panels In the design of sandwich panels subjected to compressive axial loads (apart from uniformly distributed loads vertical to the plane of the panel and temperature difference) effects according to 2nd order theory must be taken into account. Deflections are determined considering shear deformation of the core material which is highly influenced by creep effects. For pure bending the wide variation of creep coefficients depending on the composition of the core material is considered by sandwich panel producers by declaring creep coefficients for different load time periods. The question is how these coefficients may also be used for the design of axially loaded panels taking into account deformation due to creeping. For this purpose axial creep tests were performed at iS‐mainz. Based on the results of both this research and the FP7‐project EASIE coordinated by iS‐mainz a design procedure for axial loaded sandwich panels is proposed which will also be contained in the draft of prEN 1993‐7 Design of steel structures – sandwich panels. [ABSTRACT FROM AUTHOR]

Fire resistance of walls made of sandwich panels with mineral wool core – experimental verification of the extrapolation rules in DIN EN 15254‐5 for the fire resistance of walls made of sandwich panels Sandwich panels fulfil different requirements under fire load depending on their composition. For the selection and application, a classification of the fire behaviour and the fire resistance class is required. Since the classification cannot be investigated for all applications, verified extrapolation rules are required to provide a classification based on the building application. As part of a research program which was conducted by the Versuchsanstalt für Stahl, Holz und Steine of the KIT, walls made of sandwich panels with a mineral wool core were tested and the test results were compared with the extrapolation rules of DIN EN 15254‐5 and suggestions for extrapolation based on the test results were developed. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations carried out on various composite floor deck panel systems. The effects of parameters such as concrete shear length, steel thickness, and concrete thickness on the shear and flexural behavior of panels were examined. Shear tests were carried out on identical specimens with four different shear span lengths. Four-point flexural tests were carried out on single-span and double-span specimens. Shear stress-slip, loaddeformation, and moment-curvature behaviors were compared. From the test results, the effect of shear strength between the corrugated sheets and concrete governs the failure mechanisms of the flexural test, if there is no shear connector. In all flexural tests the failure is became due to the slippage between the steel panel and concrete. [ABSTRACT FROM AUTHOR]

The impact response of an ultrahigh molecular weight polyethylene (UHMWPE) fiber reinforced polymer matrix composite laminate has been investigated. The laminate encapsulated an aluminum alloy sandwich panel whose corrugated core was filled with prismatic alumina inserts. The laminate encased hybrid core target could sustain ceramic prism base impacts by a spherical, 12.7 mm diameter steel projectile with velocities in excess of 2.7 km s-1. This was 150% higher than the ballistic limit of an equal areal density, similarly encapsulated aluminum plate target. By contrast, when the projectile impacted a hybrid core target at the apex of a ceramic prism insert, failure of the UHMWPE laminate on the rear face occurred at a lower impact velocity. High-speed imaging, three-dimensional digital image correlation and x-ray tomography measurements are used to show that upon impact the projectile and the ceramic insert fragment. These fragments then load the UHMWPE laminate on the rear face with a significantly reduced pressure compared to the impact pressure of the projectile on the front surface of the target. The loading area on the inner surface of the rear laminate was highest for a prism base impact and lowest for a prism apex impact. The inability to penetrate the rear laminate of the base impacted samples is consistent with the recent identification of an impact pressure controlled mechanism of progressive penetration in this class of laminate.

Sandwich panels’ exceptional mechanical properties and low density, owing to their multifunctional characteristics and innovative design, made them a popular choice in numerous industries. Sandwich panels with cellular cores are known for their exceptional energy absorption properties, which make them effective energy absorbers for high-impact scenarios such as accidents or explosions. For advancing research on sandwich panels, it is vital to develop innovative designs that can enhance their energy absorption and flexural stiffness. This review outlines the most essential topological parameters that influence the mechanical properties of cellular core structures. This paper gives insight into recent advancements related to optimizing sandwich panel structures for various engineering applications. The topological parameters investigated by researchers include core structure, thickness, number of layers, and material. The choice of core material governs the overall mechanical behavior of the panel. In this paper, various structures, including foam, honeycomb, lattice, corrugated, bioinspired, and various materials, are compared. Functionally graded structures were also explored in the literature as they can significantly optimize the response of sandwich panels in high and low-velocity impact applications. Similarly, a multi-layered core structure can enhance the total stiffness and specific energy absorption of the panel.

Sandwich panels that can be used as an explosion shield are important structures for absorbing explosion energy. Crushing and plastic deformation of the core with the plastic bending of the faces are the main factors in absorbing the explosion energy in this sandwich panel. Structural components undergo permanent deformation after explosion and energy absorption. In this paper, the energy absorption of the structure and the deformation of circular metal sandwich panels with tubular core under explosion load have been investigated analytically, numerically, and experimentally. The tubes are arranged radially and symmetrically in the core constructions. The experiment has been performed by making sandwich panels under free blast load to evaluate and validate analytical and numerical results. The analytical solution is performed using the energy method by balancing the kinetic energy and the plastic work which is done by the different components of the sandwich panels. A numerical solution is performed in finite element software, ABAQUS, and the pressure function is generated by CONWEP method. The amount of energy absorbed by the structure and different parts of it is obtained. There is good agreement between the results in different ways. [ABSTRACT FROM AUTHOR]

A machine learning algorithm was applied to predict the onset of flashover in archival experiments in a 1/5 scale ISO 13784‐1 enclosure constructed with sandwich panels. The experiments were performed to assess whether a small‐scale model could provide a better full‐scale correlation than the single burning item test. To predict the binary output, a regularized logistic regression model was chosen as ML environment, for which lasso‐regression significantly reduced the amount of variance at a negligible increase in bias. With the regularized model, it was possible to discern the predictive variables and determine the decision boundary. In addition, a methodology was put forward on how to use the to update the learning algorithm iteratively. As a result, it was shown how a learning algorithm can be used to facilitate ongoing experimentation. At first as a crude guideline, and in later stages, as an accurate prediction algorithm. It is foreseen that, by iteratively updating the algorithm, by compiling existing and new experiments in databases, and by applying fire safety knowledge, the final learned algorithm will be able to make accurate predictions for unseen samples and test conditions. [ABSTRACT FROM AUTHOR]

The mechanical properties of sandwich panels with a polyurethane core and fiberglass face layers produced by a continuous lamination process have been studied. Tensile, compression and shear tests were performed on isolated face layer materials. Using a nonlinear finite-element model, the mechanical behavior of the sandwich panels in flatwise compression, edgewise compression, and bending was simulated, and a good agreement with experiments was found to exist. [ABSTRACT FROM AUTHOR]

This paper experimentally and numerically investigates the free vibration, quasi-static compressive and split Hopkinson pressure bar impact responses of hybrid composite pyramidal truss sandwich panels. Such sandwich panels made of carbon fibre composite face sheets and aluminium alloy pyramidal truss cores are fabricated using an interlocking and adhesive bonding approach. Modal tests and quasi-static compression tests are conducted. A good consistency for natural frequencies, modal shapes and static stress–strain curves of the specimen with the same specification is obtained, which ensures the good repeatability of the present specimens. Considering the effect of strain rate, a series of split Hopkinson pressure bar tests combined with numerical simulations is carried out to investigate their dynamic compression responses. A good agreement between simulation results and experimental data is observed, which shows that the adopted split Hopkinson pressure bar testing device and the modified Johnson-Cook model are reasonable and reliable. Results show that the dynamic compression modulus and strength of specimen are strongly influenced by the relative density of the truss cores and much higher than the corresponding static compression modulus and strength. Furthermore, it is also revealed that all the specimens have excellent energy absorption performance, which may have greatly advantage in shock isolation application. [ABSTRACT FROM AUTHOR]

Balsa wood is increasingly considered as a core material in sandwich panels for applications in several industries. However, there is still very limited information about its mechanical behaviour at elevated temperature. This article presents an experimental and analytical study about the effects of elevated temperature on the shear behaviour of end-grain balsa with nominal density of 109 kg/m3. The Iosipescu/V-notch test method was used to investigate the shear behaviour of balsa specimens extracted along the two material directions that are relevant for core materials subjected to shear: (i) transverse to the wood grain and (ii) parallel to the wood grain. The shear tests were conducted under steady-state conditions for temperatures between room temperature and 240°C. For the various temperatures, the stress vs. strain behaviour in shear was non-linear, with such non-linearity becoming more accentuated at higher temperatures. The shear modulus and strength presented a linear and comparable reduction with increasing temperature, and the residual properties at 240°C dropped to less than 20% of the room temperature properties. In the final part of the article, four alternative analytical models available in the literature were assessed regarding their ability to describe the reduction with temperature of the shear properties of balsa. All models were able to successfully fit the test data. [ABSTRACT FROM AUTHOR]

By the wider spread of sandwich panels as a cladding system on industrial steel buildings, the research on various design aspects is evolving as well. The principal aim of the research introduced in this paper is to study the rotational restraint provided by sandwich panels at both ambient and elevated temperatures, and derive the temperature dependent rotational spring stiffnesses. In the frame of an ongoing international RFCS research program named STABFI, several experimental test series were carried out. This paper has a main focus on the part of the small scale tests, in which the rotational behavior of the screwed connection between sandwich panels with mineral wool core and H‐beam was investigated. The panel‐substructure connections were tested to define rotational stiffness at different inner panel sheet temperatures (20, 200, 250, 300, 450 and 600 C°). During testing the temperature of the inner sheet, the outer sheet, and the core of the sandwich panel were recorded. The applied torsional load at the axis of the connection and the rotation was measured and recorded, from which the moment‐rotation diagrams of the joints were derived. Based on these curves the temperature dependent rotational spring stiffnesses were also determined. [ABSTRACT FROM AUTHOR]

Sandwich panel constructions are getting popular from the second half of the 20th century however, the restraint provided by sandwich panels in regards to the instability of structural members was not investigated at elevated temperature, which became the scope of the research introduced in this paper. An experimental test series was conducted at the Department of Structural Engineering at the Budapest University of Technology and Economics in order to study the stability behavior of structural members restrained by sandwich panels at ambient and elevated temperature. In this paper 18 tests are introduced, in which centrally located HEA120 sections of a 3.00 m x 3.00 m sized supporting sandwich panel diaphragm were tested for axial compression. The results indicate, that the decrease in load bearing capacity of PIR panels is higher, that in the case of mineral wool panels. The data derived from experimental tests were used for developing a complex numerical model. The primary objective of this paper is the comparison of numerical results with the results derived from experimental testing. This research has been financially supported by the European Union, in the frame of the Research Fund of Coal and Steel (RFCS) program. The authors gratefully acknowledge the support. [ABSTRACT FROM AUTHOR]

The nonlinear stochastic behavior of soft-core sandwich panels and the evolution of uncertainty along the nonlinear path are investigated. The paper focuses on the way the geometrically nonlinear and unstable behavior is affected by uncertainty and considers, for the first time, the evolution of uncertainty along the unstable structural response. The structural modeling relies on the EHSAPT and its integration into the geometrically nonlinear stochastic regime. A tailored nonlinear FE model that is based on a variational formulation and an arc-length solution scheme are developed. The stochastic analysis adopts the Perturbation-based SFEM and its generalization for the evolution of uncertainty along the process axis. A numerical study demonstrates the stochastic methodology and explores the impact of uncertainty on the nonlinear structural behavior. The evolution of the stochastic stress resultants at the displaced end is studied along the two axes of evolution. The differences between the two representations and their physical and engineering significance are highlighted revealing significant changes to the uncertainty evolution pattern with the shift from global buckling to localized wrinkling instabilities. The representation of the stochastic results along the process axis also quantifies the level of uncertainty near critical and singular points where conventional analyses diverge. [ABSTRACT FROM AUTHOR]

• Stressed-skin full scale test on steel monopitch portal frame was conducted. • Comparison of test data provided clear influence of the cladding on the horizontal structure stiffness. • FE numerical model was created, validated based on the test data. • The approach for taking into account the cladding horizontal shear stiffness was derived and compared. This paper presents a study on the influence of sandwich panel roof cladding on the horizontal stiffness of load-bearing steel structures (stressed skin action). A full-scale test was carried out on a steel monopitch portal frame structure made in two stages – without (bare steel structure) and with roof sandwich panel cladding. Data obtained from both stages of the test are presented and are used to demonstrate the significant increase in horizontal stiffness caused by the cladding system. The test data were then used to validate an FE numerical model. A roof cladding made of composite panels is relatively stiff in its own plane. However, the critical detail is the connection of the panels to the sub-structure (e.g. purlins or rafters), i.e. the screw bearing capacity. Even within the serviceability limits, the internal sheet of the composite panel suffers a certain level of permanent bearing damage. Based on the test data, the results of the FE numerical model and the analytical approach for calculating the shear stiffness of the sandwich panels, the approach for taking into account the influence of the cladding on the horizontal stiffness was derived and compared. This approach is believed to be safe and simple for practical use, and it leads to load-bearing structure members with more slender cross-sections. In addition, the data and results that are obtained expand the limited amount of available data related to stressed skin action. [ABSTRACT FROM AUTHOR]

• The novel honeycomb core sandwich structures were fabricated by combinations of solid-state joining and adhesive bonding processes. • The combined friction stir spot welded, with disc inserts and adhesive bonded, demonstrated exceptional peak load-to-weight ratio and fracture energy. • The peak load of the hybrid bonding specimen with disc insert improved by 705 % and 854 % in the peel and shear test compared to the conventional adhesive bonded specimen. • The stiffness and peak load results of FEA of three-point bending were in good agreement with the experimental results for all types of adhesive-bonded specimens. In the present work, the novel sandwich structures with honeycomb core are fabricated by a solid-state joining process — friction stir spot welding (FSSW), without and with disc inserts (abbreviated as FSSW and FSSW_D) and hybrid joints without and with disc inserts (abbreviated as FSSW_AB and FSSW_D_AB). Their mechanical performance was compared with adhesive-bonded (AB) joints through the lap-shear, peel, bend, and scaled-up forming tests. Further, numerical simulations of a few FSSW strategies and AB joints are performed in Abaqus to predict the load-displacement response and Mises stress during bending. Proposing the fabrication stretegies, implementation and testing, along with numerical prediction of behavior of such joints, are the novel contributions. The peel tests showed substantial improvement in load capacity from 232 % to 705 % for FSSW, FSSW_AB, FSSW_D, and FSSW_D_AB joints, compared to AB joints. In the lap-shear tests, all joint types displayed significantly enhanced peak load over AB joints, ranging from 568 % to 1015 %. The hybrid joints, such as FSSW_AB and FSSW_D_AB, consistently outperformed the corresponding spot-welded joints, FSSW and FSSW_D, due to the sharing of load between the AB and metallurgical bonding. The FSSW_D_AB joints demonstrated exceptional peak load-to-weight ratio and fracture energy. The failure modes were consistent, with disc-inserted structures showing failure at the front-side welds for lap-shear and peel tests. During bending, the numerical modelling results were in good agreement up to the peak load. Bending panels made by FSSW_D_AB demonstrated improvement in specific stiffness and peak load by 46 % and 6.12 %, respectively, compared to the AB panel. Lastly, the scaled-up hybrid sandwich panel (FSSW_D_AB) showed significant improvement in deliverables when compared with the AB panel. [ABSTRACT FROM AUTHOR]

Impact resistance and energy absorbing capability are of great interest in the design of composite sandwich structures. This paper experimentally studies damage, failure and energy absorption properties of foam core sandwich panels with aluminum and glass fiber reinforced plastic (GFRP) facesheets subjected to low velocity impact. Tests are performed using a drop weight impact tower at different impact velocities. The energy absorbing capabilities of aluminum and composite facesheet sandwich panels with PUR and PS foam core are evaluated by means of absorbed energy-time histories and by specific parameters as normalized absorbed energy, specific energy absorption, and crush force efficiency. Stiffer panels behave better at lower impact velocities, while more ductile ones do better if impact energy is increased.

Abstract This work investigates the mechanical behaviour of a sustainable sandwich panel, consisting of bamboo rings core, treated aluminium skins and epoxy adhesive. A Design of Experiment (DoE) is used to identify the effects of bamboo diameters (30 and 45 mm) and aluminium skin treatments (alkaline degreasing and application of primer) on the mechanical and physical properties of sandwich panels. The aluminium skins treated with the wash primer significantly increase adhesion to the polymer, resulting in greater maximum load, flexural strength, maximum skin stress and maximum core shear stress; while the skins treated with NaOH resulted in a greater flexural and core shear modulus. Relatively more rigid and resistant structures are obtained with Ø30 mm rings, due to the increased surface contact area and the number of constraints on the core. The samples fail due to the skin fracture, implying an efficient face-core bond that is attributed to the proper absorption of the polymer by bamboo and the treatment of the aluminium surface. The proposed panels present good mechanical performance, proving to be a feasible and promising alternative for secondary structural applications.

A finite element model based on continuum shell elements available in ABAQUS software has been developed for the free vibration analysis of FGM sandwich panels. Applications to sandwich plates with different combinations of FGM core and/or FGM face sheets satisfying through-the-thickness material gradations in the form of either power (P-FGM) or sigmoid (S-FGM) or exponential (E-FGM) distributions are considered. A user-defined material subroutine UMAT was used to implement functionally graded properties of the constitutive FGM layer. Numerical studies are given for free vibrations of sandwich plates subjected to different boundary conditions and with different structural parameters such as span to thickness, face sheet–core–face sheet thickness and aspect ratios, and volume fraction exponent. The studies showed very good agreement between the present results and those existing in the literature that confirmed the accuracy of the developed model. A series of numerical solutions obtained in the research extends the results of testing examples and may serve as additional benchmark data for other researchers. [ABSTRACT FROM AUTHOR]

This paper is focused on the assembly of lightweight sandwich panels built upon the patented 'Opencell' structure concept. The objective is to investigate the possibility of joining the connection members of the core to the adjoining skin sheet by plastic deformation at ambient temperature, instead of welding or adhesive bonding. The methodology draws from earlier developments of the authors in joining by forming using the mortise-and-tenon concept to experimentation and finite element modelling of the assembly process in unit cells that are representative of the sandwich panels. It is shown that replacing welding by joining by forming allows fabricating sandwich panels from sheet materials that are difficult or impossible to weld while preventing thermal cycles that are responsible for causing metallurgical changes, distortions, and residual stresses. Replacing adhesive bonding by joining by forming circumvents the need of surface preparation, time for the adhesive to cure and environmental compliance, among other requirements. [ABSTRACT FROM AUTHOR]

The present work proposes a baseline‐free method to assess debonding on composite panels. The method is based on regressive Gaussian processes (GP) used to obtain vibration modes free of noise. A Bayesian scheme is implemented to allow the automatic determination of the model hyperparameters. The estimation of the undamaged vibration modes is carried out by modifying the optimal hyperparameters determined employing the damaged plate. The curvatures associated with the vibration modes are calculated as the second analytical derivative of the kernel function of the GP; thus, the use of numerical methods is avoided. A damage index is obtained comparing the differences between the curvatures of the damaged and undamaged modes. Finally, using k‐means clustering, the damage size and location could be identified. The effectiveness of the proposed method is verified by means of numerical simulations and experimental tests. The predicted damage is compared with the actual damage in terms of size, location, and intersection. Both numerical simulations and experimental tests offer favorable results. [ABSTRACT FROM AUTHOR]

The aerospace, automotive, and marine industries are heavily reliant on sandwich panels with cellular material cores. Although honeycombs with hexagonal cells are the most commonly used geometries as cores, recently there have been new alternatives in the design of lightweight structures. The present work aims to evaluate the mechanical properties of metallic and polymeric honeycomb structures, with configurations recently proposed and different in-plane orientations, produced by additive and subtractive manufacturing processes. Structures with configurations such as regular hexagonal honeycomb (Hr), lotus (Lt), and hexagonal honeycomb with Plateau borders (Pt), with 0°, 45°, and 90° orientations were analyzed. To evaluate its properties, three-point bending tests were performed, both experimentally and by numerical modeling, by means of the finite element method. Honeycombs of two aluminum alloys and polylactic acid were fabricated. The structures produced in aluminum were obtained either by selective laser melting technology or by machining, while polylactic acid structures were obtained by material extrusion using fused filament fabrication. From the stress distribution analysis and the load–displacement curves, it was possible to evaluate the strength, stiffness, and absorbed energy of the structures. Failure modes were also analyzed for polylactic acid honeycombs. In general, a strong correlation was observed between numerical and experimental results. The results show that the stiffness and absorbed energy increase in the order, Hr, Pt, Lt, and with the orientation through the sequence, 45°, 90°, 0°. Thus, Lt structures with 0° orientation seem to be good alternatives to the traditional honeycombs used in sandwich composite panels for those industrial applications where low weight, high stiffness, and large energy-absorbing capacity are required. [ABSTRACT FROM AUTHOR]

Sandwich structures are frequently used in automotive, aerospace and marine industries, as they provide adequate functional properties. The two-dimensional regular hexagonal cell shape, i.e. honeycomb is the most used core structure in sandwich panels. Recently, a new type of cellular structures composed of lattice struts has been proposed, as they combine high stiffness, strength and energy absorption with low weight. The main purpose of this research is to investigate the effect of the lattice topology on the flexural behaviour of sandwich panels. Five lattice geometries inspired in crystalline structures were designed, namely, body-centred parallelepiped, body-centred parallelepiped with struts in z -axis, body- and face-centred parallelepiped with struts in z -axis, face-centred parallelepiped with struts in z -axis and parallelepiped simple. The relative density of all the lattices was kept constant as 0.3. Both numerical and experimental approaches were used to evaluate the flexural properties and failure behaviour of the sandwich structures under three-point bending tests. The numerical analysis was undertaken with the finite element software NX Nastran. Taking advantage of additive manufacturing technologies, material extrusion was used to produce polylactic acid samples with the configurations aforementioned. The sandwich panels are composed by a single layer formed by the lattice core and two thin plates, at the bottom and top. The three parts of the panel were manufactured all together. The simulation results indicate that, among the lattices studied, topologies body-centred parallelepiped with struts in z -axis and body- and face-centred parallelepiped with struts in z -axis exhibit higher strength, while body- and face-centred parallelepiped with struts in z -axis shows higher stiffness and higher energy absorption, attaining values that do not differ much from the ones obtained with a two-dimensional hexagonal cellular structure, with the same relative density. As a consequence, some of the geometries studied may have the potential to be considered as alternatives to conventional structures in the design of sandwich structures. [ABSTRACT FROM AUTHOR]

abstract: Insulated pre-cast concrete wall panels, also called sandwich panels, consist in two external concrete layers, in which an internal layer of thermal insulation material is inserted between, aiming better acoustic and thermal performance. One of the main concerns regarding the performance of these panels refers to the elimination of thermal bridges caused by metallic connectors, which jeopardize the panels’ thermal efficiency. One of the proposals to solve this problem consists in the use of PERFOFRP connectors, which are plane plates with perforated holes through its thickness, which are embedded into the concrete plates, creating anchorage pins that enhance the shear strength and the layers’ debonding resistance. This research had the objective of evaluating the production quality of this type of connector, produced with a low-cost and easy-to-use vacuum assisted resin infusion system; considering the effects of: (a) resin plate homogeneity, by taking samples from various locations on the plate; (b) fabrication repeatability; and (c) raw production materials’ origin; on the results of: (I) ultimate tensile stress, (II) modulus of elasticity, and (III) volume fraction of fibre. Also, 18 specimens in the form of representative models of the shear connector in insulated pre-cast concrete wall panels, with six different hole configurations achieved by varying the holes’ diameter and spacing, were subjected to push-out tests, to assess the holes’ diameter and spacing effects on the mechanical performance of the connection in terms of ultimate load capacity and stiffness. The results indicated a production quality with a satisfactory level of characteristics variation, considering: the variability in different parts of a single composite plate, the variability between composite plates from different infusion process, and the variability between different production batches. Furthermore, the push-out tests demonstrated that the perforated connectors presented, when compared to non-perforated connectors: a gain in shear strength from 8% to 25%, lower relative displacements, and higher levels of stiffness. It was also observed that connectors with 25.40 mm diameter holes presented better performance than connectors with 31.75 mm diameter holes; and that the reducing the hole spacing from 2.00 to 1.75, for the 25.40 mm diameter specimens, caused a decrease in the load capacity of the connector. Thus, it was verified that variations on the hole’s diameter and spacing influenced the load carrying capacity of the connection.

The energy efficiency of buildings drives the replacement of traditional construction materials with lightweight insulating materials. However, energy-efficient but combustible insulation might contribute to the building’s fire load. Therefore, it is necessary to analyse the reaction-to-fire properties of various insulating materials to provide a better understanding of designing a fire-safe structure. In this study, reaction-to-fire tests were carried out to assess the fire behaviour of lightweight polystyrene insulating panels commonly employed in high-rise buildings. The flammability characteristics of expanded polystyrene (EPS) and extruded polystyrene (XPS) were determined using a cone calorimeter under two distinct external irradiance regimes, 35 kW/m2 and 50 kW/m2, to approximate small to medium fire exposure situations. To investigate the effect of a fire-rated (FR) foil layer on a sandwich panel, three distinct test configurations were used: (i) sample without FR layer (standard sample), (ii) sample with FR layer (FR foil), and (iii) damaged layer (foil and vent) for EPS. Except for the smoke toxicity index (STI), the overall fire performance of EPS is superior to that of XPS. The findings of this study are useful in analysing fire performance and fire safety design for lightweight insulation panels.

This paper presents an analysis for the dynamic stability of sandwich beams/wide plates subjected to axial impulsive loads. The formulation and solution of the problem is done by use of the extended high-order sandwich panel theory (EHSAPT). With the initial geometric imperfection included, the equations of motion in terms of seven generalized displacements are derived. The dynamic response of sandwich panels subjected to three different types of impulsive loads, namely, step, linear decay, and triangular impulse, is studied. Furthermore, the effects of the oscillation mode number, face/core materials, and geometries are investigated. It is observed that all measurements of the dynamic response, such as the maximum displacements, strains, and stresses, change at the same rate as the change of the impulse load magnitude and duration, for a specific impulse load profile. When the impulse load is lower than the static buckling load, the dynamic response is bounded no matter how long the load is applied. A step impulsive axial load with magnitude lower than the static buckling load can lead a sandwich panel to have a dynamic response as high as twice the static response. When the impulse load is higher than the static critical load, the dynamic response is unbounded with increasing load duration. However, it is possible that the dynamic response can be controlled at a low level if the duration of the impulse load is short enough, and thus, in this case, the load can safely exceed the static critical load. [ABSTRACT FROM AUTHOR]

Precast sandwich panels, characterized by external textile reinforced concrete (TRC) layers and an inner insulation core, represent a convenient system for energy retrofitting of existing façades. These elements fulfil all the requirements for façade systems and constitute a valid alternative to both external thermal insulation composite systems (ETICS) and ventilated façades. The main feature of this kind of panel is that it could be applied on an existing façade through four punctual connectors by means of a crane, without any scaffolding. The paper provides a solution that has been firstly designed within the European project 'EASEE' and it is now being developed in the 'Smart P.I.QU.E.R.' project, supported by Lombardy Region (Italy). Basing on the experience gained in the European project, the partners are trying to overcome some criticisms previously encountered, related to the use of expanded polystyrene as insulation material, the cost of the anchoring system and aesthetical issues related to TRC cracking. The main innovations concern: the development of a new eco-friendly insulation material based on inorganic diatomite; the optimization of TRC layers; the development of a new anchoring system. This paper focuses on the research developed at the material level referring to both external layers and insulating core. [ABSTRACT FROM AUTHOR]

Composite Sandwich panels find wide applications in areas like aerospace, automobile, marine, and civil infrastructures due to its exceptional properties like high specific strength, flexural stiffness, resistance to corrosion, specific energy absorption, acoustic and damping features. However, the de-bonding of face and core is the most critical failure in sandwich panels. In this study, the interfacial bonding between glass-fiber face sheets and aluminum-honeycomb sandwich composites and their mechanical behavior was investigated under flexural load. The influence of loading rates on the flexural properties of sandwich panels was also assessed. The viability and influence of resin-fillet reinforcement and interfacial toughening by the inclusion of nano clay in the glass-fiber face sheets and aluminum-honeycomb core were studied. The presence of resin and nano clay at the interface and fillet of honeycomb core effectually act as a composite. The concentration of nano-clay in the face sheet and honeycomb core sandwich laminates had increased the interfacial toughness, thereby increased the flexural properties of the panels. Furthermore, increasing the content of nano-clay had a higher effect, whereas, the loading rate had a moderate effect on the failure load of sandwich panels. [ABSTRACT FROM AUTHOR]

The trend for utilizing sustainable products is becoming increasingly important in the construction industry. Precast concrete sandwich systems containing expanded polystyrene (EPS) is gradually replacing the conventional blockwork systems due to its lightweight, enhanced insulation properties and rapid installation. Accordingly, there is a need to optimize the design and production of this system to provide superior insulation, durability and rapid installation while ensuring adequate bonding, strength and mechanical properties to sufficiently fit its purpose. Research has shown that the system’s technical properties are highly dependent on the mix design and the production and installation methods. This study involved the design and production of a sustainable lightweight precast concrete sandwich panel for non-load bearing partition wall systems. An experimental program was developed to identify the optimum mixture proportions for the EPS concrete core. Practical procedures for system production and installation were proposed. Physical and mechanical properties for the proposed EPS concrete system were determined based on experimental testing and calculations. A relationship between the plastic density of fresh EPS concrete and its compressive strength was established to ensure quality control before concrete is cast. Results revealed a high degree of correlation of 0.97 between the core density and compressive strength. Technical comparison revealed that the proposed system is superior in comparison to the conventional lightweight blockwork system. Furthermore, the installation time for the proposed partition wall system was 3 times faster in comparison with the conventional block work system, which would lead to significant reduction in the total cost.

The energy absorption and structural isolation performance of axially-compressed sandwich structures constructed with stiff face plates separated with an auxetic lattice core metamaterial is studied. Advances in additive manufacturing increasingly allow bespoke, carefully designed, structures to be included within the core lattice to enhance mechanical performance. Currently, the internal structure of the lattice core is deliberately disrupted geometrically to engineer suitable post-buckling behavior under quasi-static loading. The desirable properties of a high fundamental stiffness and a practically zero underlying stiffness in the post-buckling range ensure that energy may be absorbed within a limited displacement and that any transfer of strain to an attached structure is minimized as far as is feasible. It is demonstrated that such disruptions can be arranged to enhance the panel performance. The concept may be extended to promote cellular buckling where the internal lattice buckles with densification occurring at defined locations and in sequence to absorb energy while maintaining a low underlying mechanical stiffness.

A newly developed innovative steel–geopolymer concrete composite floor slab for use in modular construction is investigated in this study. We present experimental results on the flexural behaviour of eight modular sandwich composite floor slabs with different configurations containing self-compacting geopolymer concrete (SCGC) as infill and Basalt FRP (BFRP) bars as reinforcement. The use of sustainable infill material such as SCGC and non-corrosive BFRP in the proposed composite floor slabs is beneficial from the perspective of environmental sustainability. This study also compares the performance of these composite floor slabs against their hollow counterparts. The overlap between the cells in multi-cell panels acts as additional partitioning walls. The infill material offers the sandwich composite floor slabs significant advantages by improving their load-carrying capacity. A critical analysis of the composite floor slabs for load displacement, failure modes, and strain behaviour is also conducted. The study concludes that the sandwich panels with multiple smaller cells and infill materials exhibit a sound structural performance, reporting a 6–8 times higher load-carrying capacity than their hollow counterparts. A comparison of hollow and infilled panels shows that the infill sandwich panels are suitable as structural slabs. At the same time, the former is more suitable for temporary formworks, shelter, and pedestrian platform applications.

Bu çalışmada, otomotiv sektöründe taşıyıcı elemanlardan olan şasi malzemesinin mukavemet değerlerini yükseltirken ağırlığını düşürmek amacıyla şasi malzemesi olarak sandviç panel teknolojisinin kullanılabilirliği araştırmak amaçlanmıştır. Yüzey malzemeleri olarak; karbon elyaf ve cam elyaf takviyeli polyester kompozit ile alüminyum plaka kullanılırken, çekirdek malzemeleri olarak; alüminyum bal peteği, polipropilen (PP) bal peteği ve polietilen terafitalat (PET) sert köpük kullanılmıştır. Yüzey kompozit malzemeleri, cam ve karbon elyaf kumaşlarının katmanlı olarak polyester reçinesi ile ıslatılarak el yatırma ve basınçlı kalıplama metoduyla üretilmiştir. Sandviç plakaların soğuk preste 50 bar basınç altında 720 dk. boyunca kürünü tamamlaması sağlanmıştır. Sandviç kompozitler, yüzey ve çekirdek malzemelerinin epoksi bant ile birleştirilerek ve sıcak preste 90 °C’de 30 dk. ve 120 °C’de 60 dk. 3 bar basınç altında bekletilerek üretilmiştir. Üretilen sandviç panel kompozitlerden standartlara uygun olarak; eğilme, yüzey ve kenar doğrultusunda basma numuneleri alınıp eğilme ve basma deneylerine tabi tutulmuştur. Deney sonuçlarına göre, sandviç panellerde çekirdek malzemelerinin yüzey malzemelerine kıyasla panel mekanik özelliklerinde daha belirgin değişimler oluşturduğu belirlenmiştir. Çekirdek malzeme olarak eğilme dayanımı ve kenara basma dayanımı açısından PET çekirdek malzemesi, eğilmede elastikiyet ve yüzeye basma mukavemeti açısından alüminyum bal peteği en iyi sonuçları göstermiştir. Bununla birlikte, PP köpük malzemeleri ile üretilen sandviç paneller ise diğer çekirdek malzemelerden üretilenlere göre önemli oranda yüksek uzama oranı göstermiştir.

Bu çalışmada, otomotiv sektöründe taşıyıcı elemanlardan olan şasi malzemesinin mukavemet değerlerini yükseltirken ağırlığını düşürmek amacıyla şasi malzemesi olarak sandviç panel teknolojisinin kullanılabilirliği araştırmak amaçlanmıştır. Yüzey malzemeleri olarak; karbon elyaf ve cam elyaf takviyeli polyester kompozit ile alüminyum plaka kullanılırken, çekirdek malzemeleri olarak; alüminyum bal peteği, polipropilen (PP) bal peteği ve polietilen terafitalat (PET) sert köpük kullanılmıştır. Yüzey kompozit malzemeleri, cam ve karbon elyaf kumaşlarının katmanlı olarak polyester reçinesi ile ıslatılarak el yatırma ve basınçlı kalıplama metoduyla üretilmiştir. Sandviç plakaların soğuk preste 50 bar basınç altında 720 dk. boyunca kürünü tamamlaması sağlanmıştır. Sandviç kompozitler, yüzey ve çekirdek malzemelerinin epoksi bant ile birleştirilerek ve sıcak preste 90 °C’de 30 dk. ve 120 °C’de 60 dk. 3 bar basınç altında bekletilerek üretilmiştir. Üretilen sandviç panel kompozitlerden standartlara uygun olarak; eğilme, yüzey ve kenar doğrultusunda basma numuneleri alınıp eğilme ve basma deneylerine tabi tutulmuştur. Deney sonuçlarına göre, sandviç panellerde çekirdek malzemelerinin yüzey malzemelerine kıyasla panel mekanik özelliklerinde daha belirgin değişimler oluşturduğu belirlenmiştir. Çekirdek malzeme olarak eğilme dayanımı ve kenara basma dayanımı açısından PET çekirdek malzemesi, eğilmede elastikiyet ve yüzeye basma mukavemeti açısından alüminyum bal peteği en iyi sonuçları göstermiştir. Bununla birlikte, PP köpük malzemeleri ile üretilen sandviç paneller ise diğer çekirdek malzemelerden üretilenlere göre önemli oranda yüksek uzama oranı göstermiştir.

Honeycomb sandwich panels entice continuously enhanced attention due to its excellent mechanical properties and multi-functional applications. However, the principal problem of sandwich panels is failure by face/core debond. Novel lightweight sandwich panels with hybrid core made of honeycomb, foam and through-thickness pin was developed. Reinforcing polyester pins between faces and core is an effectual way to strengthen the core and enhance the interfacial strength between the face/core to improve the structural performance of sandwich panels. To provide feasibility for pin reinforcement, honeycomb core was pre-filled with foam. Mechanical properties enhancement due to polyester pinning were investigated experimentally under flatwise compression, edgewise compression and flexural test. The experimental investigations were carried out for both “foam filled honeycomb sandwich panels” (FHS) and “polyester pin-reinforced foam filled honeycomb sandwich panels” (PFHS). The results show that polyester pin reinforcement in foam filled honeycomb sandwich panel enhanced the flatwise, edgewise compression and flexural properties considerably. Moreover, increasing the pin diameter has a larger effect on the flexural rigidity of PFHS panels. PFHS panels have inconsequential increase in weight but appreciably improved their structural performance.

Recent studies on innovative solutions in structural engineering fields highlight the effectiveness of closed-cell aluminum foams as traditional material/architecture core replacement in sandwich structures. This study aims to investigate the mechanical behavior and the collapse modes of innovative sandwich panels with aluminum foam as the core and stainless steel wire mesh-grid as the skins; the manufacturing process was based on the powder compact melting technique. The results of quasi-static three-point bending tests point out the beneficial effects, in terms of mechanical performance, of the reinforcement skin on the sandwiches, compared to the plain foams; this joins by their effectiveness in withstanding thermal and mechanical loads. Moreover, the investigation of the collapse mechanisms of the sandwiches with different thicknesses allows for testing the goodness-of-fit of the experimentally observed collapses with the ones predicted by the failure map from analytical models available in the literature. [ABSTRACT FROM AUTHOR]

Sandwich structures comprising glass-fibre reinforced polymer faces and low-density core constitute an efficient and versatile constructive system for civil and ocean engineering structures. However, being multilayered with relatively soft core materials, they are particularly susceptible to damage under concentrated loads. Whilst numerous studies exist on the indentation and impact behaviour of sandwich composites, the great majority considers the thin-skinned laminates used by the aeronautical industry. To mitigate the lack of studies on the significantly thicker and more robust civil and ocean engineering sandwich laminates, the quasi- static indentation and low-velocity impact behaviour of such panels is experimentally studied. Three types of core materials (polyurethane and polyethylene terephthalate foams and end-grain balsa) and five different indenters, of varying shape (hemispherical versus flat) and diameter (10, 20 and 30 mm), are considered. Flat and larger indenters required higher loads and energies for first damage and perforation. The first damage and peak resistance values of the polyethylene terephthalate panels were, respectively, 15 and 8% higher than in the polyurethane panels; for the balsa panels, such figures were 20 and 10%. The polyurethane panels showed the highest energy absorption capacity. Predictions of first damage resistance given by two analytical models (for flat and hemispherical indenters) were assessed against the gathered experimental data. The obtained predictions were reasonably accurate, but indicate a need for further calibration, mainly concerning the effects of core material. [ABSTRACT FROM AUTHOR]

The trend for utilizing sustainable products is becoming increasingly important in the construction industry. Precast concrete sandwich systems containing expanded polystyrene (EPS) is gradually replacing the conventional blockwork systems due to its lightweight, enhanced insulation properties and rapid installation. Accordingly, there is a need to optimize the design and production of this system to provide superior insulation, durability and rapid installation while ensuring adequate bonding, strength and mechanical properties to sufficiently fit its purpose. Research has shown that the system's technical properties are highly dependent on the mix design and the production and installation methods. This study involved the design and production of a sustainable lightweight precast concrete sandwich panel for non-load bearing partition wall systems. An experimental program was developed to identify the optimum mixture proportions for the EPS concrete core. Practical procedures for system production and installation were proposed. Physical and mechanical properties for the proposed EPS concrete system were determined based on experimental testing and calculations. A relationship between the plastic density of fresh EPS concrete and its compressive strength was established to ensure quality control before concrete is cast. Results revealed a high degree of correlation of 0.97 between the core density and compressive strength. Technical comparison revealed that the proposed system is superior in comparison to the conventional lightweight blockwork system. Furthermore, the installation time for the proposed partition wall system was 3 times faster in comparison with the conventional block work system, which would lead to significant reduction in the total cost. [ABSTRACT FROM AUTHOR]

Sandwich panels have proven to be excellent energy absorbents. Such panels may be used as a protective structure in, for example, façades subjected to explosions. In this study, the dynamic response of sandwich structures subjected to blast loading has been investigated both experimentally and numerically, utilizing a shock tube facility. Sandwich panels made of aluminium skins and a core of extruded polystyrene (XPS) with different densities were subjected to various blast load intensities. Low-velocity impact tests on XPS samples were also conducted for validation and calibration of a viscoplastic extension of the Deshpande-Fleck crushable foam model. The experimental results revealed a significant increase in blast load mitigation for sandwich panels compared to skins without a foam core, and that the back-skin deformation and the core compression correlated with the foam density. Numerical models of the shock tube tests were created using LS-DYNA, incorporating the new viscoplastic formulation of the foam material. The numerical models were able to capture the trends observed in the experimental tests, and good quantitative agreement between the experimental and predicted responses was in general obtained. One aim of this study is to provide high-precision experimental data, combined with a validated numerical modelling strategy, that can be used in simulation-based optimisation of sandwich panels exposed to blast loading. [ABSTRACT FROM AUTHOR]