Sandwich composite laminates

The work presented in this article is directed towards the application of CFRP foam core sandwich structures as primary structures in commercial aviation. With closed cell rigid foams, it is possible to produce comparatively low priced high-integral sandwich components having a complex geometry in terms of a curved and a variable lateral cut. Sandwich structures are offering a good bending stiffness and strength to weight ratio. Thus, they are suited for using in structures which are at risk to fail by buckling. The investigations are focused on a CFRP sandwich structure with polymethacrylimide (PMI) foam core, named ROHACELLRIST. Besides good structural stability at thermal conditions, the foam is characterized by a good strength and stiffness to weight ratio. Primary structures in aircraft applications are exposed to a superposition of in-service loads and environmental conditions. The typical working loads in combination with environmental conditions were investigated. The structure needs a sizing with respect to large temperature changes and influences of humidity. Thus, the time, temperature, and moisture dependency of the mechanical behavior were studied for the single components of the structure and for the composite itself. Therefore, Finite Element Models on macroscopic level were built with reference to the experiments. For each in-service case, the residual stresses arising during manufacturing have to be regarded and quantified. During manufacturing, the sandwich structure is cured at 180C. Due to the different stiffnesses and coefficients of thermal expansion of the foam and the CFRP face sheets, residual stresses are induced by cooling down to service temperature. Among others, some tests were made at laterally closed CFRP sandwich structures with a storage time up to half a year at certain climate conditions. The aging process is not only controlled by external conditions, but also by a rearrangement of molecules, for example, the relaxation behavior of the PMI foam. Another question of the long-term behavior of the CFRP foam core sandwich structure is the characteristic of delamination of the face sheets from the inner core after an impact has occurred. To describe the crack growth behavior of the sandwich structure fracture mechanical principles can be used estimating the damage tolerance. Sandwich specimens with initial damage were loaded up to 3 million mechanical load cycles.

A composite material is one of the most vital attributes in the manufacturing of aerospace and wind turbine structures. In order to protect composite surfaces against environmental influences (e.g., ultraviolet [UV] light, moisture, oxygen, and pollutants), protective coatings are typically employed; however, these coatings tend to degrade during service and fail prior to the life expectancy as the result of debonding, buckling, cracking, and blistering of coating materials. In the present study, unidirectional and woven pre-impregnated (pre-preg) carbon fiber-reinforced composites (CFRCs) were prepared in a vacuum oven, and then a number of different surface treatment processes-oxygen plasma (0, 4, 8, and 12 min), UV light (0, 2, 4, and 8 days) and fine sandpapering (1500 mesh)-were applied on the CFRC panels. Two coats of primer and a topcoat were subsequently introduced to the surfaces of treated composite panels. The coatings were analyzed via Fourier transform infrared spectrometry (FTIR), optical microscopy, water contact angle (WCA), and cross-cut tests before and after water soaking/immersion and extensive UV light exposure tests. It was determined that plasma treatment, UV exposure, and sandpapering drastically reduced the WCA values (e.g., from 106°to 33°) of the composite panels prior to the coating process. The study showed that the surfaces are highly functionalized during these treatments, which in turn create strong bonds between the composite and the primed surfaces. The cross-cut and optical microscopy studies showed that the coating surfaces were severely damaged through flaking, deboning, and delaminating during the water immersion and UV exposure tests; however, the surface-treated composite panels were substantially less damaged. This study confirms that the surface treatment of composite panels is critically important for a longer service time of the composites in aerospace, wind turbine, and other industrial applications.

The liquid foam laydown technique and its optimal parameterization is an important final product quality factor in the rigid PU foam insulating core sandwich panels production. Inappropriate setup can lead to issues such as:

- Range and distribution of liquid PU foam: element underpacking in the joints area, widespread surface flatness failure (internal as well as external facing) and significant density variations across the width.

- Liquid mixture residence time within poker: regional mixing quality issues leading to irregular shrinkage during the element cooling phase and consequent dimensional instability.

- Liquid foam discharge intensity: irregular interface between foam and facings with foam rise impression on upper metal facing (external to the building) which can be visible at certain angles (especially with 0.4mm thick metal facings).

- Deterioration of mechanical properties such as adhesion and compression.

These metal pipe Z-pokers can be easily (and cheaply) manufactured in any local machine shop and can be reused several times after a thorough cleanup using drills sets and appropriate solvents. This routine cleanup however results in extensive wear (pipe inner roughness) as well as significant discharge port diameter distortions, which eventually affect liquid PU foam flow patterns with the entailed issues described previously.

Since around 2010, all major chemical system suppliers in the PU foam industry (BASF, Covestro, Dow & Huntsman) introduced fixed liquid foam laydown methodologies and supplied customers with expendable one use plastic static rakes such as the below illustrated from BASF.

These aimed to eliminate issues associated with the gradual performance deterioration of metal Z-pokers from repeated use as well as improving final product mechanical properties and of course significantly simplifying process set up, online corrective actions and maintenance work load for the operators (http://www.polyurethanes.basf.de/pu/solutions/de/function/conversions:/publish/content/group/News_und_Medien/Polyurethan/Flyer_PU_en.pdf).   

The benefits of static rake utilization are undoubtedly significant and according to the author’s opinion usually come at a reasonable price. However, due to the additional external dependency and cost, entailed logistics, less than expected performance in some production lines as well as the fact that these are not available from all chemical system suppliers, many PU panels manufacturers prefer their existing in-house Z-poker designs & methodology. 

This paper aims to provide a guide on important parameters correlation - interaction for optimizing liquid PU foam laydown in high pressure continuous insulating panels production lines using Z-pokers.

The performance of laminated glass, which consists of two or more glass plies bonded together by polymeric
interlayers, depends upon shear coupling between the plies through the polymer. This is commonly
considered by defining the effective thickness, i.e., the thickness of a monolithic beam with
equivalent bending properties in terms of stress and deflection. General expressions have been proposed
on the basis of simplified models by Newmark and Wölfel–Bennison, but they are either difficult to apply
or inaccurate. Here, a variational approach to the problem is presented. By choosing appropriate shape
functions for the laminated-beam deformation, minimization of the strain energy functional gives new
expressions for the effective thickness under any constraint- and load-conditions, embracing the classical
formulations as particular cases. Comparisons with numerical experiments confirm the better accuracy of
the proposed approach with respect to the previous ones.

In this study, a new core design is introduced for sandwich composite structures. Its strength and failure behavior are investigated via three-point bending tests. E-glass-fiber-reinforced epoxy resin is selected as the material for both the core and the face sheets. The core has an egg-crate shape. Acoustic emission (AE) method is used to detect the progression of damage. Signals due to elastic waves caused by activated damage mechanisms are investigated in order to identify the corresponding failure modes. A finite element model of the sandwich structure is developed to predict the failure behavior of the specimens under the loading conditions in the tests. A promising agreement between the results of the finite element model and the experiments is observed. The force-deflection-relation, the failure load as well as the region where damage initiates are accurately predicted.

The results of a major research programme into the cost effective use of fibre reinforced composites offshore, undertaken as a Joint Industry Project by a consortium of universities and contractors are presented in this report.
The report includes a detailed (54 page) summary of current offshore composite applications, followed by a review of the 43 projects conducted during the programme which spanned 13 years and four phases of research and considered the following performance issues – fire, blast/impact, environmental durability, elevated temperatures, jointing, long term behaviour, non-destructive inspection and structural design. These are related to the use of pipes, panels, secondary structures
e.g. gratings, walkways etc. and top-side structures.
The report is intended for use as an information source for engineers involved in the use of composites offshore. The most significant advances have been in the areas of pipework and fluid handling, driven by their light weight and corrosion resistance compared to metals. Until recently, interest focussed on the use of glass fibre-based composites and on the topsides of offshore platforms. However, the prospect of deepwater production is generating a new impetus for high performance composites for demanding subsea applications. The probable scale of some new subsea developments, such as rigid risers and tethers is likely to result in greatly increased demand for carbon based composites as well as for the manufacturing facilities to process them. Composite down-hole tubing is also beginning to find applications subsea and in well intervention.
The original barriers to the use of composites offshore were regulatory requirements, lack of design information and the fragmented structure of the composites industry. The project has made significant progress in their removal, as well as generating increased awareness of the major economic, safety and environmental benefits that a wider use of composite materials can bring. The Composites Offshore programme provided valuable underpinning information for a number of applications, including glass reinforced plastic (GRP) piping, offshore structures, repair systems and composite fire protection of the oil and gas industry.
This report and the work it describes were funded in part by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

CP07 Fire resistant sandwich panels for offshore structures

The objective of the work was to develop fire resistant panels of lighter construction than existing products.
A low cost computer-controlled furnace was developed, which enabled laminate specimens to be subjected to a prescribed temperature-time curve. Tests were carried out to evaluate the performance of commercial non-structural core materials including polystyrene, polyurethane and mineral wool. Similar tests were carried out to assess the surface protection offered by a number of commercial materials, including vermiculite, phenolic foam, mineral fibre and calcium silicate. A number of formulations based on sodium silicate, vermiculite, perliote, ball clay and latex were also investiged

A full mechanical characterisation of three types of 3-D printed lattice cores was performed to evaluate the feasibility of using additive manufacturing (AM) of lightweight polymer-based sandwich panels for structural applications. Effects of the shape of three selected lattice structures on the compression, shear and bending strength has been experimentally investigated. The specimens tested were manufactured with an open source fused filament fabrication-based 3-D printer. These sandwich structures considered had skins made of polypropylene (PP)-flax bonded to the polylactic acid (PLA) lattice structure core using bi-component epoxy adhesive. The PP-flax and the PLA core structures were tested separately as well as bonded together to evaluate the structural performance as sandwich panels. The compression tests were carried out to assess the in-plane and out of plane stiffness and strength by selecting a representative number of cells. Shear band and plastic hinges were observed during the in-plane tests. The shear and three-point bending tests were performed according to the standard to ensure repeatability. The work has provided an insight into the failure modes of the different shapes, and the force-displacement history curves were linked to the progressive failure mechanisms experienced by the structures. Overall, the results of the three truss-like lattice biopolymer non-stochastic structures investigated have indicated that they are well suited to be used for potential impact applications because of their high-shear and out of the plane compression strength. These results demonstrate the feasibility of AM technology in manufacturing of lightweight polymer-based sandwich panels for potential structural uses.

Debonding of face-core interface is the most important damage mechanisms which make loss of structural integrity in sandwich structures. In this paper, mode-I and mode-II fracture of face-core interface in sandwich structures have been investigated under both static and fatigue loadings. The considered sandwich structures contain of different face sheet fiber-metal laminates and the core material is polyvinyl chloride foam. Several specimens are fabricated and the experiments are carried out to find the effects of initial debonding location and various fiber-metal laminate face sheets on the fracture toughness under static and fatigue loadings. Double cantilever beam specimens are used for mode-I and end notch flexure specimens for mode-II loading conditions. The resistance strength curves are plotted for mode-I and mode-II under static loading to find the instability point which is the border of stable and unstable crack growth and determine the critical crack length too. The strain energy release rates of mode-I and mode-II are also obtained for fatigue loading to investigate the resistance against damage evolution. Also, the global damage parameter is defined for both static and fatigue loading which is the combination of all damage mechanisms occurred in sandwich structures. Finally, the more efficient layup configurations under static and fatigue loadings among the investigated layups are introduced in mode-I and mode-II fracture conditions separately.

The stochastic buckling behaviour of sandwich plates is presented considering uncertain system parameters (material and geometric uncertainty). The higher-order-zigzag theory (HOZT) coupled with stochastic finite element model is employed to evaluate the random first three buckling loads. A cubic in-plane displacement variation is considered for both face sheets and core while quadratic transverse displacement is considered within the core and assumed constant in the faces beyond the core. The global stiffness matrix is stored in a single array by using skyline technique and stochastic buckling equation is solved by simultaneous iteration technique. The individual as well as compound stochastic effect of ply-orientation angle, core thickness, face sheets thickness and material properties (both core and laminate) of sandwich plates are considered in this study. A significant level of computational efficiency is achieved by using artificial neural network (ANN) based surrogate model coupled with the finite element approach. Statistical analyses are carried out to illustrate the results of stochastic buckling behaviour. Normally in case of various engineering applications, the critical buckling load with the least Eigen value is deemed to be useful. However, the results presented in this paper demonstrate the importance of considering higher order buckling modes in case of a realistic stochastic analysis. Besides that, the probabilistic results for global stability behaviour of sandwich structures show that a significant level of variation with respect to the deterministic values could occur due to the presence of inevitable source-uncertainty in the input parameters demonstrating the requirement of an inclusive design paradigm considering stochastic effects.

Sandwich panels are increasingly used in aircraft structures due to their high specific bending stiffness and hence excellent buckling stability. The objective of this work is to find rough sizing process of a composite sandwich panel incorporating multi-scale modelling techniques to substantiate structural capability of each level of test pyramid, and find a minimum number of tests required to validate the approaches. The numerical analysis of such hybrid structures by means of the finite element method (FEM) requires specific strategies regarding the degree of homogeneity of each component. The common modelling approach for solid laminate of carbon fibre reinforced plastics (CFRP) structures using an extended layered shell element formulation can also be applied in modelling of sandwich panels in the global FE-model. The sandwich core, which is thicker compared to the solid laminate CFRP skins, can be formulated as an additional layer between the two face skin layers. For more detailed modelling of the sandwich structure, e.g. a cut out of the global FE model (GFEM), a solid shell approach can be applied. Both of the monolithic skins are idealised using layered shell, while the sandwich core is represented by solid elements. For a hard foam core the solid elements are assigned to the core system according to their type, isotropic material for unreinforced foam or homogenised, anisotropic properties for a reinforced foam core system. The homogenised mechanical properties of the reinforced foam core can be determined using analytical or numerical approaches, in which for a numerical approach the textile profile or needles shaped foam reinforcements are modelled with shell and beam elements respectively. With some modifications the meso-mechanical FE-model can further be used e.g. through the application of explicit FEM for the determination of an impact damage or using the virtual crack closure technique (VCCT) method and a higher level of FE discretisation to assess the growth behaviour of damages. The non-linear FE analyses show a good agreement with the recorded panel deformations.

In this study, the free vibration analysis of functionally graded materials (FGMs) sandwich beams having different core metals and thicknesses is considered. The variation of material through the thickness of functionally graded beams follows the power-law distribution. The displacement field is based on the classical beam theory. The wide applications of functionally graded materials (FGMs) sandwich structures in automotive, marine construction, transportation, and aerospace industries have attracted much attention, because of its excellent bending rigidity, low specific weight, and distinguished vibration characteristics. Design/methodology/approach: A mathematical formulation for a sandwich beam comprised of FG core with two layers of ceramic and metal, while the face sheets are made of homogenous material has been derived based on the Euler-Bernoulli beam theory. Findings: The main objective of this work is to obtain the natural frequencies of the FG sandwich beam considering different parameters. Research limitations/implications: The important parameters are the gradient index, slenderness ratio, core metal type, and end support conditions. The finite element analysis (FEA), combined with commercial Ansys software 2021 R1, is used to verify the accuracy of the obtained analytical solution results. Practical implications: It was found that the natural frequency parameters, the mode shapes, and the dynamic response are considerably affected by the index of volume fraction, the ratio as well as face FGM core constituents. Finally, the beam thickness was dividing into frequent numbers of layers to examine the impact of many layers' effect on the obtained results. Originality/value: It is concluded, that the increase in the number of layers prompts an increment within the frequency parameter results' accuracy for the selected models. Numerical results are compared to those obtained from the analytical solution. It is found that the dimensionless fundamental frequency decreases as the material gradient index increases, and there is a good agreement between two solutions with a maximum error percentage of no more than 5%.

This research aims to inspect through the infrared thermography technique damages by
drop weight impact and by almost static indentation in aeronautical composite sandwich
materials type. The motivation arose from the study of aircraft with structures and elements
made of composite materials, where it was found that the load capacity as well as the reliability
of the aircraft to perform flights decreases dramatically when they suffer low speed impacts.
The methodology used made use of a thermal insulation chamber previously designed for
thermographic tests. Drop weight impacts and almost static indentation tests were used to
include different energy levels of damage, followed by the thermographic tests of the plates in
composite materials with and without damages, where we defined the best heating time to
obtain the thermograms. As results, we analyzed the damages in the upper and posterior faces
of the thermographic test, where the damages imposed on the composites with drop weight tests
were more severe than the damages by almost static indentation. For the thermographic test,
thermograms of composites with and without damage were obtained and it was possible to
compare them with the damaged composites, where it was observed that damage type of fiber
breakage was easily identified. Therefore, it was conclude that it is possible to inspect through
thermography in composites sandwiches materials in laboratory environment, but this method
did not show viable to detect lower damages impacts of 15.6 J or lower

The NACA 4digit series aerofoil shapes are universally accepted standard designs generally used for wind turbine blades, helicopter rotor blades and car spoilers. The manufacturing of these complex shapes is a challenging task for the manufacturing engineers. These components need to be made of materials having high specific strength and fatigue properties. The composites with sandwich construction fulfill the above requirements. The main aim of the present Investigation is to select the best fiber orientation for the fabrication of automotive rear spoiler. The Design FOIL software provides different shapes of aerofoil from which NACA 2412 has been selected. The spoiler is modeled using CATIA software and is analyzed for the static deflection using ANSYS for various orientations of the fiber. The spoiler is then fabricated using woven E-glass fiber (with the best orientation) and epoxy resin. Static test is performed on the fabricated model to validate the results obtained from the software. This confirms the design feasibility and software adoptability for the design of sandwich aerofoil shape

Sandwich panels with uni-directional stiffeners in the core are lightweight structures. When the voids between the stiffeners are empty, the global bending of the panel is accompanied by the secondary bending of the faces between the stiffeners. This study investigates the effect of the secondary bending on local buckling of the sandwich panel. Web-core steel sandwich panel is analyzed with finite element method using shell elements. The panel is studied under uni-axial loading in the direction of the stiffeners. The results show that the local buckling strength is higher at the unloaded edges where the secondary bending deformation is larger. This has a positive effect on the strength of the panel since it postpones the reduction of the load-carrying capacity. Local buckling occurs first in the center of the panel. The buckles have atypical shape with the wave-length significantly smaller than the width of the face plate between the stiffeners.

Sandwich structures with face sheets made of carbon fiber reinforced plastics (CFRP) demonstrate a high potential to reduce both production costs and the weight of primary aircraft structures. This potential has however yet not been realized mainly due to the complex structural and material behavior of sandwich structures under different kinds of loading and adverse effects like moisture take up of honeycomb core material [1]. Using closed cell hard foams such as PMI foam, better known as Rohacell®, the issue of moisture take-up has been solved. One more critical issue however is the often criticized lack of damage tolerance of sandwich structures [2].
Impact damage resistance is part of the larger topic of damage tolerance. It can be treated separately as impacts may cause a damage that is used a starting point for a damage tolerance analysis while not vice versa. In consequence the resulting damage size of a predefined impact scenario is of great importance.

In the new century, electrospun nanofibrous webs are widely employed in various applications due to their specific surface area and porous structure with narrow pore size. The mechanical properties have a major influence on the applications of nanofiber webs. Lamination technology is an important method for improving the mechanical strength of nanofiber webs. In this study, the influence of laminating pressure on the properties of polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) nanofibers/laminate was investigated. Heat-press lamination was carried out at three different pressures, and the surface morphologies of the multilayer nanofibrous membranes were observed under an optical microscope. In addition, air permeability, water filtration, and contact angle experiments were performed to examine the effect of laminating pressure on the breathability, water permeability and surface wettability of multilayer nanofibrous membranes. A bursting strength test was developed and applied to measure the maximum bursting pressure of the nanofibers from the laminated surface. A water filtration test was performed using a cross-flow unit. Based on the results of the tests, the optimum laminating pressure was determined for both PAN and PVDF multilayer nanofibrous membranes to prepare suitable microfilters for liquid filtration.

Sandwich construction is increasingly used as wall and roof cladding for building structures. Typically, a cladding panel may consist of two plane or profiled metal faces with a foamed plastic core. The core may be polyurethane, polyisocyanurate, polystyrene or phenolic resin. When such a panel is subject to static loading due, for instance, to wind, snow or temperature gradient, one face is compressed and becomes liable to local buckling. If this face has a trapezoidal or similar profile the failure mode is similar to that for profiled steel sheeting, though the failure stress is enhanced by the presence of the core. The compressed face element first forms a series of buckling waves which increase in amplitude in the postbuckling phase. Failure takes place when one buckle in the region of maximum bending moment cripples.
In light gauge steel applications, the conventional design treatment for this phenomenon utilises the concept of effective width. In order to investigate the extension of the effective width concept to plate elements supported by plastic foam material, a series of tests were undertaken on foam-filled steel beams. This paper describes these tests and their interpretation in terms of an enhanced effective width formula.

An aircraft structural assembly includes an area element which has a core and an outer layer. A first surface of the area element is convexly curved, at least in certain sections, relative to an imaginary central area of the area element extending through the core of the area element.

This paper presents analytical and case studies of sandwich structures. In this study, the Euler−Bernoulli beam equation is solved analytically for a four-point bending problem. Appropriate initial and boundary conditions are specified to enclose the problem. In addition, the balance coefficient is calculated and the Rule of Mixtures is applied. The focus of this study is to determine the effective material properties and geometric features such as the moment of inertia. The effective parameters help in the development of a generic analytical correlation for complex sandwich structures from the perspective of four-point bending calculations. The case study is built for a sandwich structure made of two materials; Aluminium and Steel. This case is solved using MATLAB R. The main outcomes of the Al-Steel sandwich structure are the maximum lateral displacements and longitudinal stresses varying with number of sandwich layers.

► SUMMARY:

♦ The Resilient Composite Systems (R.C.S.) could be counted as "The Methodically Reinforced Nonlinear Porous Materials", also having the high specific modulus of resilience in flexure. They are the compound materials, with some particular structural properties, in which, contrary to the basic geometrical assumption of the flexure theory in Solid Mechanics, the strain changes in the beam height during bending is typically "Nonlinear". The Resilient Composite Systems are made by expediently creating the disseminated suitable hollow pores and/or by distributing the appropriate lightweight aggregates throughout the methodically reinforced, fibered conjoined matrix so that; "the strain changes in the beam height during bending" is typically "nonlinear". Thereby, by applying the mentioned method to make the said composite systems, "considerably increasing the modulus of resilience and the bearing capacity in bending" together with "the significant decrease of the weight" and "the removal of the beam fracture possibility of primary compressive type" have been feasible. Through making these particular consistently functioning systems, the above-stated paradoxical virtues have been concomitantly fulfilled in "one functioning unit" altogether.

♦ The RCS (with the mentioned general structural properties and specific functional criteria) whose cement materials include the "C-S-H (Calcium Silicate hydrate) crystals" have been termed "Elastic Composite, Reinforced Lightweight Concrete (E.C.R.L.C.)".

♦ The RCS and ECRLC as a kind of RCS could be used in many cases. For instance, "Constructing Lightweightly and Consistently", also with practically utilizing the RCS and ECRLC in particular, can be counted as the pivotal and practical tactic to effectively increase the resistance & safety of the constructions against "earthquake" and lateral forces, in the large extent. For example, using some lightweight and insulating, "non-brittle", reinforced sandwich panels or 3D-panels, "with the high modulus of resilience and appropriate behavior against the bending loads and impacts", for construction could be taken into consideration according to the case... [It is worth remarking that; contrary to the ECRLC, the usual reinforced lightweight concretes, especially in the very low densities, are dramatically brittle; they do not have the appropriate behavior and resistance against the high bending loads and impacts.]
.
● "Resilient Composite Systems (RCS)":

♦ "Resilient Composite Systems" (RCS) are the compound materials, having some particular structural properties, in which, contrary to the basic geometrical assumption of the flexure theory in Solid Mechanics, the strain changes in the beam height during bending is typically "Nonlinear". The RCS could be counted as "The Methodically Reinforced Nonlinear Porous Materials", also having the high specific modulus of resilience in flexure. (Elastic Composite, Reinforced Lightweight Concrete; ECRLC is a type of RCS with the mentioned specifics.)

♦ Generally, the Resilient Composite Systems comprise these components as the main, necessary elements:
• 1) Mesh (Lattice);
• 2) Fibers or strands;
• 3) Conjoined matrix, having the disseminated suitable pores and/or the disseminated appropriate lightweight aggregates beads or particles. [Here, the general term of "lightweight aggregate" has a broad meaning, also including the polymeric and non-polymeric beads or particles.]

♦ The Resilient Composite Systems are made by creating the disseminated suitable hollow pores and/or by distributing the appropriate lightweight aggregates throughout the reinforced, fibered conjoined matrix so that; "the strain changes in the beam height during bending" is typically "nonlinear". Thereby, by applying the mentioned method to make the said particular composite systems, "considerably increasing the modulus of resilience and the bearing capacity in bending" together with "the significant decrease of the weight" and "the removal of the beam fracture possibility of primary compressive type" have been feasible. Through making these particular consistently (unitedly, integratedly) functioning systems, the stated paradoxical virtues have been concomitantly fulfilled in one functioning unit altogether.

♦ Generally, in these consistently functioning units, the amount and the use manner of the mentioned components in the organized system are so that; the reciprocal interactions among the components finally lead to the "typically nonlinear strain changes in the beam height during bending" (as the "basic functional character" of these systems with the specific testable criteria and indices) and the functional specifications fulfillment of the system.

♦ In the RCS in general, the main strategy to raise the modulus of resilience in bending is "increasing the strain capability of the system in bending" within the elastic limit.

♦ Here, the main tactic to realize the stated strategy is: "creating the suitable hollow pores and/or using the appropriate lightweight aggregates, all disseminated throughout the methodically reinforced conjoined matrix", to provide the possibility for occurring of the expedient internal deformities in the matrix during the bending course, which could lead to the more appropriate distribution of the stresses and the strains throughout the system and the more strain capability of the beam in flexure. On the other hand, only creating the hollow pores and/or using the lightweight aggregates in the matrix, by itself, not only cannot lead to the mentioned goals but also brings about weakening of the matrix and its fragility. Hence, concomitantly, the matrix should be well supported and strengthened. Here, this essentially ameliorating and strengthening the matrix are performed by giving attention to "the internal consistency of the matrix" and also via "employing the expedient reinforcements in two complementary levels": 1) Using the fibers to the better distribution of the tensile stresses and strains in the matrix, and to increase the matrix endurance and the modulus of resilience in tension and bending; 2) Using the mesh or lattice to better distribution of the tensile stresses and strains in the system, and to increase the system endurance and the modulus of resilience in tension and bending.

♦ In these systems, the presence of the mentioned hollow pores and/or lightweight aggregates disseminated throughout the conjoined matrix (which has been ameliorated through making "an integrated, reticular structure") provides the possibility for occurring of the expedient internal deformities in the matrix during the bending course. By the way, this can lead to the less accumulation of the internal stresses in the certain points of the matrix during bending, the better absorption and control of the stresses, and providing the more strain capability of the beam, especially within the elastic limit.

♦ The occurrence of the remarked internal deformities in the said methodically reinforced matrix during the bending course also means; the occurrence of the deformities in the said hollow pores and/or lightweight aggregates well disseminated throughout the conjoined matrix, in two different forms. Indeed, we have the internal deformities in the fibered lightweight matrix of the system throughout the bending course, in two main different forms, leading to: A) The comparative increase of the thickness (height) of the in-compressing layers (particularly in the upper parts of the beam) and the conversion of some internal compressive stresses to the internal tensile stresses (on the axis perpendicular to the mentioned internal compressive tensions) in the in-compressing layers; B) The comparative decrease of the thickness (height) of the in-tension layers (particularly in the lower parts of the beam) and the conversion of some internal tensile stresses to the internal compressive stresses (on the axis perpendicular to the mentioned internal tensile tensions) in the in-tension layers.

♦ In the under-bending sections of the "Resilient Composite Systems", the deformities occurring in the "conjoined layers perpendicular to the applied load direction" during the bending course are so that; "the initially plane sections perpendicular to the beam axis" typically shift from "the plane status" to "the curve status" during the bending course. Thereby, the basic geometrical assumption of the flexure theory in Solid Mechanics ("linearly" being of the strain changes in the beam height during bending) and the respective trigonometric equations & equalities are mainly overshadowed in these systems.

♦ This way, through occurring of the remarked internal deformities in the strengthened matrix during the flexure course, the stresses are more distributed and absorbed, and the rise rate of the internal stresses in the matrix ... reduces.  ...
●●●
► [sites.google.com/site/newstructure1]

PAPERS
AND 
POST GRADUATE 
PAPERS 
FROM
 THE
 SPECIAL
 TRACK HELD
 AT 
THE
 CIB WORLD 
BUILDING CONGRESS
 2010,
10‐13
 MAY
 2010
THE 
LOWRY,
 SALFORD
 QUAYS, 
UNITED 
KINGDOM
Selected 
papers
 from 
the 
Proceeding s
of
the
18th
 CIB 
World
Building
 Congress.
Proceedings 
edited
 by: Professor
 Peter
 Barrett, 
Professor
 Dilanthi
Amaratunga,
Dr.
Richard
 Haigh,
Dr.
Kaushal
 Keraminiyage
and
Dr.
Chaminda
 Pathirage
W056
S pecial
 Track
 Papers 
(excluding
 Postgraduate 
Papers)
reviewed
by:
Professor 
Jose
 Amorim 
Faria,
Professor 
Jukka 
Aalto,
Professor 
Jörg 
Lange,
Dr.
Lars 
Heselius
 and 
Professor
J. Michael
 Davies
CIB 
Publication
342

Polyurea elastomer is known to exhibit advantageous impact-mitigation characteristics and thus can improve the dynamic performance of various components and structures. This study identifies the mechanisms of dynamic response of thin metallic plates, covered by a frontal polyurea layer, using a physically verified, custom material model for two-part polyurea implemented within a finite-element-method framework. A linear increase in the ballistic performance of a target with polymer coating is consistent with experimental work captured for the first time in a numerical study. A reported ballistic-limit improvement of 7.4 m s-1 per millimetre increase of polyurea thickness for frontal-layer thicknesses higher than 4 mm on the thin monolithic plate was established. In contrast, the application of polyurea coating thinner than 4 mm resulted in a diminished ballistic performance of the target. These outcomes are attributed to significant alterations in the energy-absorbing capacity of thin plates with the introduction of the polyurea layer that strongly depend on the impact velocity, polymer thickness, and interfacial interactions.

In this paper, the behavior of free vibrations and buckling of the sandwich panel with a flexible core was investigated using a new improved ‎ high-order sandwich panel theory. In this theory, equations of motion were formulated based on shear stresses in the core. First-order shear deformation theory was ‎ applied for the procedures. In this theory, for the first time, incompatibility problem of velocity and acceleration field existing in Frostig's ‎ first theory was solved using a simple analytical method. The main advantage of this theory is its simplicity and less number of equations than the ‎ second method of Frostig's high-order theory. To extract dynamic equations of the core, three-dimensional elasticity theory was utilized. ‎ Also, to extract the dynamic equations governing the whole system, Hamilton's principle was used. In the analysis of free vibrations, the ‎ panel underwent primary pressure plate forces. Results demonstrated that, as plate pre-loads got closer to the critical buckling loads, the natural frequency of the panel tended zero. The results obtained from the present theory were in good correspondence with the ‎ results of the most recent papers. ‎

Davies and Hakmi (1990) [5] studied the local buckling behaviour of a compressed plate element supported by relatively weak isotropic core medium. When the sandwich panel is subjected to uniform compression the authors represented the panel as a simply supported plate
resting on half space linear elastic foundation. The critical buckling stress was determined using the principle of minimum strain energy method following Timoshenko and Gere‟s method (1961). For simply supported plates without foundation they added the strain energy contribution for the core assuming it to be the elastic foundation with exponentially decaying displacement. Parameter representing the decay in the displacement of the core was found by minimizing the
strain energy. This leads to a formula for the buckling coefficient of sandwich pane which is used to achieve the ultimate strength of plate using the effective width concept (i.e.Winter formula). The authors found that the results did not quite agree with the experimental results. Improved correlation was obtained by arbitrarily reducing the value of an intermediate parameter.
Davies and Hakmi (1991) [6] reviewed the analytical methods for the evaluation of
buckling stress of sandwich panels for plane and profiled faces. They developed analytical formula for wrinkling stress based on considering the core as an elastic half space, by treating the
width of the plate as wider flat face (i.e. width of the plate increases to infinity), this formula is modified for lightly profiled faces by introducing the effect of flexural rigidity of profiled face. Also the authors introduced practical design reduction factors for both cases due to imperfection and material non-linearity. For the profiled sandwich panels authors adopted design formula similar to Winter formula based on experimental mean values and numerical post-buckling analysis.The sandwich panels, one of the attractive engineering structures mixed between two different materials to achieve high ultimate strength associated with light weight structures. Usually, the sandwich panels comprise of foam core and thinner high strength steel faces. This report discusses currently design formulae of local buckling behaviour of sandwich panels with profiled faces using finite element method. Multiple wave finite element models adopted to investigate and examine the adequacy of currently approach for design. This report presents the details of examining the FEA model including geometry, dimensions, load pattern and boundary conditions. The FEA model gives well agreement using experimental programme of Pokharel and Mahendran (2003). However, it appears the currently design formulae are conservative for the plate elements with low b/t ratios while over conservative for high b/t ratios (slenderness plate). A unified design formula of local buckling behaviour is developed.

This paper gives a brief overview of sandwih application history in general and present composite sandwich structures at Airbus. Current R&D developments for sandwich in primary structure s are being outlined followed by a discussion of potentials and challenges for composite sandwich structures.

This study aimed to investigate the forming limit diagrams(FLDs), mechanical properties, and bending behavior of the three kinds of sandwich composites. The core layer in all sandwich sheets consisted of polyethylene, and just the skin layers were chosen to be AA5754 or AA3105. The results showed that the sandwich sheets are an appropriate choice to be used instead of the monometallic
sheets. According to the 3-point bending test results, the flexural strength of the AL 3105/Polyethylene/AL 3105 sandwich composite is about 30 percent less than the AA3105 monometallic layer. Also, the formability of the metallic sandwich composites with non-identical skin layers was studied. The results demonstrated that using one layer AA 3105 and one layer AA5754 instead of AA5754 for both skin layers, although formability decreased about 11% in the plane strain region, the
cost of fabricating the composite was decreased sheet reduces almost 75%. As a result, using skin layers with the different grades of aluminum is an appropriate way to reduce the cost of fabricating the aluminum-polymer sandwich composites, in which the formability does not decrease significantly.

Sandwich structures with  face sheets made of carbon fibre reinforced plastics  (CFRP) have a high potential in  reducing production costs and  weight of primary  aircraft structures. Their  advantage is based on their superior bending stiffness compared to solid laminates. This allows a less complex and highly integrated structural design which typically reduces the number of parts and joints significantly. The highlighted potential has however yet not been realized within primary aircraft structures mainly due to the complex structural behaviour. Another adverse effect often mentioned is moisture take up of the core material. Using closed cell hard foams such as PMI foam, better known as Rohacell®, for the core, the problem of moisture take-up has been solved. Damage tolerance however has been one of  the most critical  issues regarding the  use of sandwich structures [1]  [2]. Extensive research has been conducted on CFRP foam core sandwich structures with mostly PVC foam for application in marine vessels, which provides a large benefit for today’s level of understanding. Past and current research  concentrates on describing material behaviour with damage, damage growth under cyclic loading  and consequently  maximum  damage  sizes [3]-[5]. Less attention  has however  been given to damage initiation by localized impact events. Low Velocity impact may lead to face sheet
and/or core damages [6]-[8].

Sandwich structures are nowadays widely used due its excellent characteristics of matching light weight with high bend stiffness. However the complex nature of its composition turns the estimation of its mechanical elastic properties challenging. The objective of the present article is to propose an analytical/experimental model to estimate the orthotropic elastic properties of this composite material. To accomplish this objective a theory of anisotropic plates is used together to an extensometric experimental approach. A sandwich laminate specimen, instrumented with strain gages, was submitted through pure bending loading by a mechanical testing machine on a 4-point bending apparatus. The result of this research was the determination of 4 orthotropic elastic properties of the sandwich laminate specimen, namely: the longitudinal and transversal Young moduli, the shear modulus, and the Poisson's ratio.

Sandwich panels are extensively analyzed and used in the transportation field due to their specific strength and stiffness values, higher than the metal homogeneous. Often, it is highlighted that one of the main drawbacks is related to their increased acoustic emissions, which especially in the past has limited their use in the application where a high level of comfort is required. In the last decade, new types of core for sandwich panels have been designed to obtain good structural and acoustical performances. In order to create a reference for future analysis on new configurations of truss-core, in the present study the experimental modal analysis on sandwich panels with honeycomb and foam cores is carried out in the low-mid frequency range. The results obtained from modal analysis are used to estimate the acoustic power radiated by both panels. The panels’ faces are in carbon/epoxy so that all the attention is concentrated on the cores which represent the variables to be investigated. This activity has been developed inside the framework offered by the
SUPERPANELS project (www.superpanels.unina.it).

In the new century, electrospun nanofibrous webs are widely employed in various applications due to their specific surface area and porous structure with narrow pore size. The mechanical properties have a major influence on the applications of nanofiber webs. Lamination technology is an important method for improving the mechanical strength of nanofiber webs. In this study, the influence of laminating pressure on the properties of polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) nanofibers/laminate was investigated. Heat-press lamination was carried out at three different pressures, and the surface morphologies of the multilayer nanofibrous membranes were observed under an optical microscope. In addition, air permeability, water filtration, and contact angle experiments were performed to examine the effect of laminating pressure on the breathability, water permeability and surface wettability of multilayer nanofibrous membranes. A bursting strength test was developed and a...