The effect of facesheet/core delamination in sandwich structures under transverse loading

The present work is concerned with the problem of a delamination crack along the facesheet/core interface of a sandwich structure which is submitted to transverse loading. In contrast to a loading by compressive inplane forces or a bending loading the presumed transverse loading does not lead to buckling of the delaminated facesheet but it may provoke further delamination crack growth. As a kind of crack driving force the energy release rate is studied for a virtual crack growth by means of Irwin's crack closure integral within a finite element modelling. The resultant energy release rate is dependent on various geometrical and material parameters which is investigated in some detail.     

The influence of core height and face plate thickness on the response of honeycomb sandwich panels subjected to blast loading

This paper reports on an investigation into the behaviour of circular sandwich panels with aluminium honeycomb cores subjected to air blast loading. Explosive tests were performed on sandwich panels consisting of mild steel face plates and aluminium honeycomb cores. The loading was generated by detonating plastic explosives at a pre-determined stand-off distance. Core height and face plate thickness were varied and the results are compared with previous experiments. It was observed that the panels exhibited permanent face plate deflection and tearing, and the honeycomb core exhibited crushing and densification. It was found that increasing the core thickness delayed the onset of core densification and decreased back plate deflection. Increasing the plate thickness was also found to decrease back plate deflection, although the panels then had a substantially higher overall mass.     

Experimental investigation of compression failure of sandwich specimens with face/core debond

An experimental study of the in-plane compressive failure mechanism of foam cored sandwich specimens with an implanted through-width face/core debond is presented. Tests were conducted on sandwich specimens with glass/vinylester and carbon/epoxy face sheets over various PVC foam cores. Observation of the response of the specimens during testing showed that failure occurred by buckling of the debonded face sheet, followed by rapid debond growth towards the ends of the specimen. The compression strength of the sandwich specimens containing a debond decreased quite substantially with increasing debond size. A high-density core resulted in less strength decrease at any given debond size. Examination of the failure surfaces after separation of the face sheet and core revealed traces of core material deposited on the face sheet evidencing cohesive core failure. The amount of core material adhered to the face sheet decreased with increasing foam density indicating increasing tendency for core/resin interfacial failure.     

Analysis of the sandwich DCB specimen for debond characterization

Analysis of the compliance and energy release rate of the sandwich double cantilever beam (DCB) specimen is presented. It is assumed that there is a starter crack at the upper face/core interface and that the crack remains at or near this interface during crack propagation. Beam, elastic foundation, and finite element analyses are presented and compared to experimentally measured compliance data, and compliance calibrated energy release rate over a range of crack lengths for foam cored sandwich DCB specimens. It is found that the beam analysis provides a conservative estimate on the compliance and energy release rate. The elastic foundation model is in agreement with finite element analysis and experimental compliance data. Recommendations for specimen design and an expression for an upper limiting crack length are provided.     

Torsion of carbon fiber composite pyramidal core sandwich plates

A combined theoretical, experimental and numerical investigation of carbon fiber composite pyramidal core sandwich plates subjected to torsion loading is conducted. Pyramidal core sandwich plates are made from carbon fiber composite material by a hot compression molding method. Based on the Classical Laminate Plate Theory and Shear Deformation Theory, the equivalent mechanical properties of laminated face-sheet are obtained; based on a homogenization concept combined with a mechanical of materials approach, the equivalent in-plane and out-of-plane shear moduli of pyramidal core are obtained. A torsion solution is derived with Prandtl stress function and can be used in the sandwich plate with orthotropic face-sheets and orthotropic core. The influences of material properties and geometrical parameters on the equivalent torsional stiffness are explored. In order to verify the accuracy of the analytical torsion solution, experimental tests of sandwich plate samples with different face-sheet thicknesses are conducted and two types of finite element models are developed. Good agreements among analytical predictions, finite element simulations and experimental evaluations are achieved, which prove the validity of the present derivation and simulation. The proposed method could also be applied in design applications and optimization of the pyramidal core sandwich structures.     

Wet-sand impulse loading of metallic plates and corrugated core sandwich panels

We have utilized a combination of experimental and modeling methods to investigate the mechanical response of edge-clamped sandwich panels subject to the impact of explosively driven wet sand. A porthole extrusion process followed by friction stir welding was utilized to fabricate 6061-T6 aluminum sandwich panels with corrugated cores. The panels were edge clamped and subjected to localized high intensity dynamic loading by the detonation of spherical explosive charges encased by a concentric shell of wet sand placed at different standoff distances. Monolithic plates of the same alloy and mass per unit area were also tested in an identical manner and found to suffer 15-20% larger permanent deflections. A decoupled wet sand loading model was developed and incorporated into a parallel finite-element simulation capability. The loading model was calibrated to one of the experiments. The model predictions for the remaining tests were found to be in close agreement with experimental observations for both sandwich panels and monolithic plates. The simulation tool was then utilized to explore sandwich panel designs with improved performance. It was found that the performance of the sandwich panel to wet sand blast loading can be varied by redistributing the mass among the core webs and the face sheets. Sandwich panel designs that suffer 30% smaller deflections than equivalent solid plates have been identified.     

Fundamental frequency and design of the CFCF composite sandwich plate

The design efficiency of sandwich panels is often associated with the value of fundamental frequency. This paper investigates the free vibrations of rectangular sandwich plates having two adjacent edges fully clamped and the remaining two edges free (CFCF). The vibration analysis is performed by applying Hamilton’s principle in conjunction with the first-order shear deformation theory. The analytical solution determining the fundamental frequency of the plate is obtained using the generalised Galerkin method and verified by comparison with the results of finite element modal analysis. The approach developed in the paper and equations obtained are applied to the design of sandwich plates having composite facings and orthotropic core. Design charts representing the effects of the thickness of the facings and core on the mass of composite sandwich panel for a given value of the fundamental frequency are obtained.     

In-plane effective elastic properties of a novel cellular core for sandwich structures

Within this paper an analytical model is presented for the calculation of the in-plane effective elastic properties E_x and E_y of a novel cellular structure which is proposed to be used as a core in sandwich structures. The proposed cellular core may represent a less expensive and easily to produce alternative to the already known cellular structures used for the construction of sandwich structures. The developed analytical model is validated through experimental tests. The results obtained by analyzing the theoretical model show a good agreement with the tests. The structure topology is studied using a parameterized unit cell and it is shown the way in which the in-plane stiffness depends on the geometric parameters and relative density of the core.     

Modelling of composite sandwich structures with honeycomb core subjected to high-velocity impact

In this study the perforation of composite sandwich structures subjected to high-velocity impact was analysed. Sandwich panels with carbon/epoxy skins and an aluminium honeycomb core were modelled by a three-dimensional finite element model implemented in ABAQUS/Explicit. The model was validated with experimental tests by comparing numerical and experimental residual velocity, ballistic limit, and contact time. By this model the influence of the components on the behaviour of the sandwich panel under impact load was evaluated; also, the contribution of the failure mechanisms to the energy-absorption of the projectile kinetic energy was determined.     

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

Effects of core machining configuration on the debonding toughness of foam core sandwich panels

Core machining is often applied to improve the formativeness of foam core and the manufacturing effectiveness of sandwich panels. This paper investigates the effects of core machining configuration on the interfacial debonding toughness of foam core sandwich panels fabricated by vacuum-assisted resin transfer molding process. Several machining configurations are conducted to foam core, and skin–core debonding toughness of fabricated sandwich panels is evaluated using double-cantilever-beam tests. The sandwich panels with core cuts exhibited higher apparent fracture toughness than the panels without core cut, specifically in the case of perforated core. The relationship between core machining configuration and measured fracture toughness is discussed based on the experimental observations and the numerical analyses of energy release rates.     

Assessment of debonding in sandwich CF/EP composite beams using A0 Lamb wave at low frequency

The aim of this study is to quantitatively assess debonding in sandwich CF/EP composite structures with a honeycomb core using acoustic waves activated and captured by surface-mounted PZT elements. For experimental investigation, debonding was introduced at different locations in sandwich CF/EP composite beams. The fundamental anti-symmetric A₀ Lamb mode was excited at a low frequency. The transmitted and reflected wave signals in both surface panels were captured by PZT elements after interacting with the debonding damage and specimen boundaries. Aided by finite element analysis (FEA), the differences in wave propagation characteristics in sandwich composite beams and composite laminate (e.g. skin panel only) were investigated. The debonding location was assessed using the time-of-flight (ToF) of damage-reflected waves, and the severity of debonding was evaluated using both the magnitude of the reflected wave signal and the delay in the ToF of Lamb wave signals. Good correlation between the experimental and FEA simulation results was observed. The results demonstrate the effectiveness of Lamb waves activated and captured by surface-mounted PZT elements on either surface of sandwich composite structures in assessing debonding.     

Interfacial debonding of a spherical inclusion embedded in an infinite medium under remote stress

In the study of strength of particle reinforced composites, it is important to understand the energy release rate due to interfacial debonding between the particle and the matrix which is induced by manufacturing imperfection. This paper is aimed at the investigation of the critical condition for growth of the interfacial debonding and the corresponding volume increase due to void formation. The model used in the study is an isotropic elastic spherical inclusion embedded in an infinite isotropic elastic matrix under remote stress. Initial axisymmetrical interfacial debondings are assumed to exist in the vicinity of poles of the spherical inclusion. Axisymmetrical deformations of the matrix and the inclusion are analyzed based on the theory of three-dimensional elasticity in spherical coordinates. In order to avoid oscillatory stress singularity at the interfacial debonding front between two dissimilar materials, a condition of free slipping without friction at the interface is imposed. A Fredholm integral equation of the first kind is formulated based on the continuity conditions in the normal components of stress and displacement at the contact interface. The kernel function of the integral equation is expressed in terms of an infinite series of Legendre functions. Two types of remote stresses are considered in this study. The first type is the remote tension in the axial direction of the spherical inclusion and the second type is the remote compression in the transverse direction with respect to the axis of the spherical inclusion. Energy release rate is determined according to the rate of change of work done by remote stresses. In this paper, energy release rate and volume of the deformed void due to debonding are computed for any given size of initial interfacial debonding.     

A layerwise finite element formulation for vibration and damping analysis of sandwich plate with moderately thick viscoelastic core

Abstract

Most previous studies of viscoelastic sandwich plates were based on the assumption that damping was only resulting from shear deformation in the viscoelastic core. However, extensive and compressive deformations in the viscoelastic core should also be considered especially for sandwich plates with moderately thick viscoelastic core. This paper presents a finite element formulation for vibration and damping analysis of sandwich plates with moderately thick viscoelastic core based on a mixed layerwise theory. The face layers satisfy the Kirchhoff theory while the viscoelastic core meets a general high-order deformation theory. The viscoelastic core is modeled as a quasi-three-dimensional solid where other types of deformation such as longitudinal extension and transverse compression are also considered. To better describe the distribution of the displacement fields, auxiliary points located across the depth of the sandwich plate are introduced. And based on the auxiliary points, the longitudinal and transverse displacements of the viscoelastic core are interpolated independently by Lagrange interpolation functions. Quadrilateral finite elements are developed and dynamic equations are derived by Hamilton’s principle in the variation form. Allowing for the frequency-dependent characteristics of the viscoelastic material, an iterative procedure is introduced to solve the nonlinear eigenvalue problem. The comparison with results in the open literature validates the remarkable accuracy of the present model for sandwich plates with moderately thick viscoelastic core, and demonstrates the importance of the higher-order variation of the transverse displacement along the thickness of the viscoelastic core for the improvement of the analysis accuracy. Moreover, the influence of the thickness and stiffness ratios of the viscoelastic core to the face layers on the damping characteristics of the viscoelastic sandwich plate is discussed.     

Vibration analysis of sandwich plates with lattice truss core

An effective methodology is developed to investigate the vibration of the sandwich plate with pyramidal lattice core. Equation of motion of lattice sandwich plate is established by Hamilton's principle. Displacement fields are expressed with a simple method, and the natural frequencies of the lattice sandwich plate are conveniently calculated. The correctness of the analytical method is verified by comparing the present results with published literatures. The effects of structural and material parameters on the vibration characteristics of lattice sandwich plate are analyzed. The present method will be useful for vibration analysis and design of lattice sandwich plates.     

Assessment of the delamination hazard of the core face sheet bond in structural sandwich panels

Delamination of the adhesive bond between face sheets and cellular core of structural sandwich panels is a major problem in sandwich construction. Due to incompatibilities in the modes of deformation associated with the face sheets and the cellular core, stress concentrations and singularities can occur even in absence of cracks. These stress concentrations are assumed to govern the onset of delamination. In the present study, a mesoscale concept for a first-order assessment of the delamination hazard induced by the incompatibility in the modes of deformation at the interface between core and face sheets is presented. The approach is based on a fourth order tensor which can easily be derived from the effective elasticity tensor for the cellular core. Due to the general formulation, the concept is applicable to all types of two dimensional cellular sandwich cores irrespectively of cell geometry and loading conditions. The approach is illustrated by an analysis of three examples concerning commercial sandwich core geometries as well as a more general non-orthotropic cellular structure.     

Fundamental frequency of the CCCF composite sandwich plate

The analytical closed-form solution providing the value of fundamental frequency of the CCCF composite sandwich plate is proposed in this article. The variational equations of motion are derived using Hamilton’s principle in conjunction with the first-order shear deformation theory. The solution approach involves reduction of the governing equations of motion to a system of ordinary equations using Kantorovich procedure and subsequent application of the generalised Galerkin method. The analytical solution is verified using the finite element modal analysis. The comparisons of computational results have shown that the fundamental frequency of the CCCF sandwich plates can be calculated with sufficient accuracy using the analytical technique developed in this work.     

The analysis of skin-to-stiffener debonding in composite aerospace structures

A comprehensive finite element (FE) analytical tool to predict the effect of defects and damage in composite structures was developed for rapid and accurate damage assessment. The structures under consideration were curved, T-stiffened, multi-rib, composite panels representative of those widely used in aerospace primary structures. The damage assessment focussed on skin-to-stiffener debonding, a common defect that can critically reduce the performance of composite structures with integral or secondary bonded stiffeners. The analytical tool was validated using experimental data obtained from the structural test of a large stiffened panel that contained an artificial skin-to-stiffener debond. Excellent agreement between FE analysis and test results was obtained. The onset of crack growth predictions also compared well with the test observation. Since the general damage tolerance philosophy in composite structures follows the “no-growth” principle, the critical parameters were established based the onset of crack growth determined using fracture mechanics calculations. Parametric studies were conducted using the analytical tool in order to understand the structural behaviour in the postbuckling range and to determine the critical parameters. Parameters considered included debond size, debond location, debond type, multiple debonds and laminate lay-up.     

High-order free vibrations of doubly-curved sandwich panels with flexible core based on a refined three-layered theory

There are few reports on the free vibration of soft core doubly-curved sandwich shells. Previous studies are largely based on the equivalent single layer theories in which the natural frequencies are grossly overestimated. This study deals with the analytical free vibrations of doubly curved sandwich shell with flexible core based on a refined general-purpose sandwich panel theory. In this theory, equations of motion are formulated based on displacements and transverse stresses at the interfaces of the core. The first order shear deformation theory and assumptions of linear distribution of transverse normal stress and uniform shear stresses over the thickness of core (based on 3D-elasticity solution of weak core) are used in the present theory. In this model, the in-plane displacements take cubic polynomial distributions and the transverse displacement has a quadratic one thorough the core thickness. Hamilton’s principle is used to obtain the equations of motion. The obtained results are validated by the analytical and numerical results published in the literatures. Parametric study is also included to investigate the effects of radius of curvature, thickness and flexibility of core.