Debond Size Research Articles

Dynamic Analysis of Honeycomb Sandwich Beam with Multiple Debonds

Analytical formulation for the evaluation of frequency of CFRP sandwich beam with debond, following the split beam theory, generally underestimates the stiffness, as the contact between the honeycomb core and the skin during vibration is not considered in the region of debond. The validation of the present analytical solution for multiple-debond size is established through 3D finite element analysis, wherein geometry of honeycomb core is modeled as it is, with contact element introduced in the debond region. Nonlinear transient analysis is followed by fast Fourier transform analysis to obtain the frequency response functions. Frequencies are obtained for two types of model having single debond and double debond, at different spacing between them, with debond size up to 40% of beam length. The analytical solution is validated for a debond length of 15% of the beam length, and with the presence of two debonds of same size, the reduction in frequency with respect to that of an intact beam is the same as that of a single-debond case, when the debonds are well separated by three times the size of debond. It is also observed that a single long debond can result in significant reduction in the frequencies of the beam than multiple debond of comparable length.

Applications of Nondestructive Evaluation Techniques in Concrete Inspection

Ground penetrating radar (GPR) and infrared thermography (IRT), as two nondestructive evaluation-civil engineering (NDE-CE) technologies, are introduced in this paper to facilitate building inspection under the Mandatory Building Inspection Scheme (MBIS), as well as public housing blocks and government buildings in Hong Kong. Recognition of anomaly signal by GPR can be used for compliance check with design drawings by mapping embedded objects, and to distinguish variation of material properties such as wet or dry concrete. Post-image processing of IRT images by gradient computation provides an estimate of boundary and size of debond over external wall envelop of high-rise buildings. Both techniques are promising in MBIS because these techniques see through the ‘unseen’, and are completely nondestructive, rapid, relatively low-cost and offer data in very high resolution.

Analytical approach for modal characteristics of honeycomb sandwich beams with multiple debond

An analytical formulation for the evaluation of vibration characteristics of sandwich beams with multiple debond at the interface between the carbon fiber reinforced plastic (CFRP) face sheets and honeycomb core is obtained and verified through test. The well-known split theory is modified considering the core stiffness of sandwich beam model and accordingly the equations of motion are set up for both the undebonded and the debond regions to solve the Eigenvalue problem. Honeycomb sandwich beams with aluminium as well as quasi-isotropic carbon epoxy laminate face sheets of same thickness and core height are fabricated and tested. In the case of cantilever beam with single debond, the region of validity of the analytical solution for natural frequencies and mode shapes are found to be within a debond size of 12% of the beam length for sandwich beams with either CFRP or aluminium skin. Studies on multiple debond reveal that a single long debond can result in significant reduction in the frequencies of the beam than multiple debond of comparable length located along the same axial direction compared to intact beam.

An incremental damage theory for micropolar composites taking account of progressive debonding and particle size effect

The paper presents an incremental damage model for determining the overall mechanical behaviors of particulate-reinforced micropolar composites. The particle size effect and progressive-debonding damage are fully accounted for within the present framework, which makes use of the micropolar Eshelby’s tensor for characterizing the size-dependent constraint of the matrix microstructure on the particles, a critical stress criterion for the interfacial debonding, and Qiu-Weng’s energy approach, together with Hu’s variation method for the equivalent stress of the matrix. Finally, the present model was applied for the predictions of the overall stress–strain relations of the composites that reinforced by either particles or voids, and are subjected to the uniaxial tension. It is shown that all the predictions are in good agreement with the experiments and finite element computations.

Debond Propagation in Honeycomb Structure under Pure Bending Load

This paper used Finite Element Method to investigate the influence of the circular face-core debonding dimension on the further debond propagation under the pure bending load in honeycomb structure. We also summarized the debonding rules and looked further into debonding area effect on the critical load for debond propagation. The conclusions are as follows: For a honeycomb panel, there exists a certain debonded radius . When the initial debonded radius is greater than , it will experience local buckling which takes the form of snap-through buckling. The debonding propagation is only discovered on the wide direction of the plate. The critical load for debonding propagation decreases with increased debond size.

PDMS-Glass Interface Adhesion Energy Determined Via Comprehensive Solutions for Thin Film Bulge/Blister Tests

This paper provides comprehensive relationships for pressure, deflection, energy release rate, and phase angle for bulge testing, which are valid for all combinations of the testing length-scales (film thickness, debond size, and bulge height) and materials. These solutions can be used to design experiments that vary relative contributions of opening and sliding displacements at the crack tip by modulating the film thickness, debond size, and bulge volume. Their closed-form nature greatly facilitates property extraction via regression, e.g., modulus from experimental pressure/deflection data or interface toughness from debond size/injected volume data. This is illustrated using experiments to quantify the interfacial adhesion energy between an initially dry polydimethylsiloxane-glass interface via bulge testing under controlled volume injection. The results indicate that the mode-mixity has no effect on the energy required for debonding, which suggests that wetting of the crack faces behind the debonding front eliminates friction.

Detection of edge delamination in surface adhered active fiber composites

A simple method has been developed to detect the bonding condition of active fiber composites (AFC) adhered to the surface of a host structure. Large deformation actuating capability is one of important features of AFC. Edge delamination in adhesive layer due to large interfacial shear stress at the free edge is typically resulted from axial strain mismatch between bonded materials. AFC patch possesses very good flexibility and toughness. When an AFC patch is partially delaminated from host structure, there remains sensing capability in the debonded part. The debonding size can be determined through axial resonance measured by the interdigitated electrodes symmetrically aligned on opposite surfaces of the patch. The electrical impedance and modal response of the AFC patch in part adhered to an aluminum plate were investigated in a broad frequency range. Debonding ratio of the AFC patch is in inverse proportion to the resonant frequency of the fundamental mode. Feasibility of in-situ detecting the progressive delamination between AFC patch and host plate is demonstrated.

Wave propagation in 2D elastic composites with partially debonded fibres by the null field approach

Time-harmonic plane wave propagation in a two-dimensional (2D) elastic matrix with partially debonded elastic fibres of nonclassical cross-section is investigated. The modified null field approach, taking into account the asymptotic behaviour of the solution at the interface crack-tips, is exploited to obtain the numerical results for a single scatterer. The effective medium approach based on Foldy's approximation is applied to estimate the average dynamic parameters of the composites containing randomly distributed partially debonded fibres of dilute concentration. Numerical results concern the longitudinal wave dispersion and attenuation owing to scattering by both randomly oriented and aligned fibres. The effects of the fibre shape, debonding (interface crack) size and direction of wave incidence on the effective P-wave velocity and attenuation coefficient are analysed.

Influence of skin/core debonding on free vibration behavior of foam and honeycomb cored sandwich plates

The dynamic behavior of partially delaminated at the skin/core interface sandwich plates with flexible cores is studied. The commercial finite element code ABAQUS is used to calculate natural frequencies and mode shapes of the sandwich plates containing a debonding zone. The influence of the debonding size, debonding location and types of debonding on the modal parameters of damaged sandwich plates with various boundary conditions is investigated. The results of dynamic analysis illustrated that they can be useful for analyzing practical problems related to the non-destructive damage detection of partially debonded sandwich plates.

The Research of the Effective Moduli of Particle Reinforced Polymer Composites Based on Interface Debonding

This paper is devoted to studying influences of matrix/particle interface debonding and particulate size in micromechanical predictions of the effective moduli of particulate reinforced polymer composites (PRPC). The PRPC is regarded as a three-phase composite that includes the matrix, particle and interphase. The formulation for the effective moduli of the interphase is derived by the cohesive zone model, and combined with the Mori-Tanaka method, the micromechanical model for the effective moduli of the PRPC is formulated with emphasis on the effects of the matrix/particle interface, particulate size and volume fraction. The numerical example shows that the interface debonding, the particulate size and volume fraction have significant influences on the effective moduli of PRPC. The effective moduli of the PRPC can be used to characterize its damage degree.

Post-buckling and debond propagation in sandwich panels subject to in-plane compression

Nonlinear finite element analysis is conducted to predict initiation of debond propagation in compression loaded foam cored sandwich panels containing a circular face/core debond embedded at the panel center. A three-dimensional geometrically nonlinear finite element model of the debonded sandwich panel combined with linear elastic fracture mechanics is used to determine the stress intensity factors K I and K II and energy release rate at the debond (crack) front parallel and perpendicular to the applied load. A range of core densities and debond sizes are analyzed. The opening mode (mode I) was found to dominate the fracture process. The critical load for crack propagation predicted using fracture mechanics concepts was found to agree with measured collapse loads for smaller debonds, but fell below measured debond propagation loads for larger debonds. In all cases the predicted direction of crack propagation was perpendicular to the loading direction, in agreement with experimental observations.

Three-dimensional Finite Element Buckling Analysis of Debonded Sandwich Panels

A three-dimensional (3D) finite element analysis has been conducted to examine the local buckling behavior of foam-cored composite sandwich panels containing a face-core debond. The influences of core stiffness and debond size and shape are examined. The core stiffness was found to have a profound influence on the local buckling load, especially for low modulus cores and smaller debonds. Panels with square debonds buckled at lower loads than those with circular debonds of the same dimension, and buckling loads for both circular and square debonds decreased with increased debond size. The results of the analysis were compared to local buckling loads determined experimentally. Reasonable agreement was found between the predicted and experimentally measured local buckling loads for both circular and square debonds.

Experimental Study of Debonded Sandwich Panels under Compressive Loading

An experimental study of the in-plane compression failure of sandwich panels consisting of glass/epoxy face sheets over a range of PVC foam cores (H45, H100, and H200) and a balsa wood core containing one or two circular or square interfacial debonds is conducted. For the great majority of the specimens, failure occurred by local buckling of the debonded face sheet followed by rapid debond growth towards the panel edges, perpendicular to the applied load. Panels with the largest debond (10 cm diameter or 9 cm length) displayed some post-buckling strength. Examination of the face and core failure surfaces after total separation showed that the tendency for interface (core/resin) failure increases with increasing core density. It was found that the compression strength strongly decreases with decreasing core stiffness and increasing debond size. The compression strength of panels with H45 core decreased with reduced core thickness. Failure loads for panels with symmetric debonds at both face/core interfaces were practically the same as those for panels containing a single debond. Panels with square debonds failed at lower loads than those with circular debonds of the same area. Circular debonds of 50 mm diameter and square debonds of 45 mm length set the threshold for debond failure of the panels considered here.

Face Sheet Buckling of Debonded Sandwich Panels Using a Two-Dimensional Elastic Foundation Approach

A two-dimensional elastic foundation model for analysis of the face sheet buckling behavior of sandwich panels with an embedded rectangular debond has been developed. The model is based on an energy method which assumes that the core behaves as an elastic foundation supporting the face sheets everywhere except over the debond. The face sheet buckling load is presented in a versatile form expressed into two buckling coefficients. Parametric studies are conducted to evaluate the influences of the foundation modulus, face sheet modulus, face sheet anisotropy, debond and panel dimensions, and aspect ratios. It is shown that the critical buckling load is very sensitive to the core modulus and flexural stiffness of the face and less sensitive to the debond size and face anisotropy. Model predictions are compared to experimental results obtained for sandwich panels consisting of glass/epoxy face sheets over a H100 PVC foam core with a range of square face/core debond sizes. Reasonable agreement with experimentally measured local buckling loads is observed for relatively small debonds. For large debonds the model tends to overpredict the buckling load.

ICS-11: Identification of Debond Size in Adhesively Bonded Lap Joint by Inverse Method(ICS-II: INTERFACES AND CONTACT SURFACE MECHANICS)

ICS-11: Identification of Debond Size in Adhesively Bonded Lap Joint by Inverse Method(ICS-II: INTERFACES AND CONTACT SURFACE MECHANICS)

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.

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.

Experimental verification of the effects of friction and residual stress on the analysis of interfacial debonding and toughness in single fiber composites

Single-fiber fragmentation tests were done on AS4 carbon fiber/epoxy and E-glass/epoxy specimens. Using a new interpretation of the photoelasticity fringes around fiber breaks we measured debonds that occurred instantaneously after each fiber break. The new techniques led to measured debond lengths that were longer than in prior studies. An energy balance analysis of the debond size when the breaks are far apart was used to investigate the interfacial fracture toughness. The best analysis was one that accounted for both residual stress effects and interfacial friction. It was not possible to determine all effects by debonding experiments alone and thus the most accurate results for interfacial toughness require supplemental experiments such as Raman spectroscopy or additional fragmentation observations. The best estimate for interfacial toughness was 220 J/m2 for carbon-fiber/epoxy and 120 J/m2 for glass-fiber/epoxy.

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.

Stresses, deformation, and void nucleation in locally debonded metal interconnects

Three-dimensional analyses of stresses and deformation in metal interconnects were carried out using the finite element method within the continuum framework. Particular attention is devoted to the preexisting local debond between the metal line and dielectric, which serves as the void nucleation site. Thermal mismatch induced stresses in the aluminum line near the interface defect are considered. The local reduction of stresses as well as the stress gradient along the line are quantified for various debond sizes. It is found that, for aluminum lines with an aspect ratio of unity, the influence of local debond on the stress profile along the line direction becomes negligible in regions greater than about one line height away from the debond edge. A unique stress pattern due to the preexisting debond is identified, which forms the basis of constructing a void nucleation model in terms of crystallographic slip. This three-dimensional modeling provides quantitative information on the initial stress field useful for modeling stress and electromigration induced voiding; it also confirms the qualitative features of stress evolution obtained from a previous two-dimensional analysis.