Foam Core Sandwich Composites Research Articles

Damage Assessment Due to Repeated Slamming of Foam Core Sandwich Composites

Foam core sandwich composite panels repeatedly slammed onto the body of calm water indicate greater accumulation and progression of damage accompanied by lower lifetimes as a function of increasing slamming energy or lower deadrise angle. E—N (slamming energy vs. lifetime) curves showed an exponentially decreasing trend with extensive scatter in the data. E—N curves also differed dramatically when compared with the conventional S—N fatigue life curve. A significant reduction observed in fatigue life of slammed specimens as compared with the non-slammed specimens was used as a basis for the development of a life assessment methodology. Catastrophic failure under slamming resulted from a major crack formation near the chine or the keel depending on the deadrise angle, however, during post-slamming fatigue, failure always occurred near the keel. Core tearing along the interface and core shear were observed to be the dominant modes of failure, while facesheet damage activity was largely absent prior to catastrophic failure.

Study on impact properties of through-thickness stitched foam sandwich composites

Abstract Through-thickness stitched foam core sandwich composites were fabricated by using RTM process; and impact performance and damage extent were studied at 1–70 J impact energy levels. The results show that two sharp peak loads and a low-loading plateau appear on the load-time plots at 1–30 J impact energy levels; both sharp peak loads can be considered as the course of penetrating top and bottom facings, a low-loading plateau has the characteristics of penetrating foam cores. Compare to the unstitched samples, the average damage angle of stitched samples increase by 48%, the maximal cracking width and penetration depth of the stitched samples decrease by 67% and 4% at 25 J impact energy levels.

Work on Impact Properties of Foam Sandwich Composites with Different Structure

Foam core sandwich composites with various facings composed of glass, carbon woven, carbon/Kevlar hybrid, and Kevlar fabrics were fabricated by using RTM process, as well as the through-thickness stitched sandwich samples with glass fabrics; and impact performance was studied at three energy levels. This article proposes that the damage extent of foam core sandwich construction may be characterized by the average damage angle, penetration depth, and maximal cracking width. The results show that the foam core samples with Kevlar facing are optimal for the peak load at load—time plots and the lowest impact damage extent at the same energy level. Compared to the unstitched samples, the average damage angle of stitched samples increases by 48%, the maximal cracking width and penetration depth of stitched samples decrease by 67% and 4% at 25J impact energy level.

Damage Assessment Due to Single Slamming of Foam Core Sandwich Composites

Foam core sandwich composite panels were slammed onto the body of calm water as a function of slamming energy (161—779 J) and deadrise angle (0°—45°). Higher slamming energy and lower deadrise angle resulted in greater damage to the material. Discrete pressure data though critical in ship design, failed to yield any relevant damage information. Catastrophic failure was observed to occur beyond a threshold strain of 0.0035 mm/mm. With the introduction of a hydroelasticity function, quasi-static analysis accurately predicted strain behavior under slamming. Post-failure analysis of noncatastrophically damaged specimens indicated very little reduction in flexural capacity in spite of a measurable change in the acoustic emission activity. Core shear along the interface with the facesheets and local buckling of the facesheet and resin fragmentation were observed to be the dominant modes of failure under slamming.

Analytical Modeling of Composite Sandwich Panels under Blast Loads

Analytical solutions were derived for the transient response and damage initiation of a foam-core composite sandwich panel subjected to blast loading. The panel response was modeled in two consecutive phases: (1) a through-thickness wave propagation phase leading to permanent core crushing deformations and (2) a transverse shear wave propagation phase resulting in global panel deflections. The predicted transient deformation of a sandwich panel consisting of E-glass vinyl ester facesheets and H100 PVC foam core compared well with ABAQUS predictions. Analytical predictions of the critical impulse for damage initiation in several foam sandwich panels also compared well with ABAQUS predictions.

Indentation of Foam-Based Polymer Composite Sandwich Beams and Panels Under Static Loading

Foam core composite sandwich structures are highly susceptible to damage when subjected to localized loading. Therefore, thorough study of the role of factors such as face sheet thickness, indentor diameter value, and crosshead displacement rate in indentation events is important. The objective of the present work is to investigate experimentally and numerically the influence of these factors on the nonlinear static indentation behavior of sandwich beams and panels consisting of glass fiber/resin face sheets and PVC (polyvinylchloride) foam core. Static indentation tests are carried out on sandwich composite beam and panel specimens using steel cylindrical and spherical indentors, respectively. Numerical models are developed for simulating the mechanical response of sandwich structures subjected to localized indentation beyond the limit of elastic deformation in the foam core. In this relation, the *CRUSHABLE FOAM and the *CRUSHABLE FOAM HARDENING options in the ABAQUS finite element program system are used. The numerical analysis results demonstrate good agreement with experimental data. It is found that increasing the face sheet thickness and indentor diameter value leads to increase in the load (for a given displacement). It is shown also that the indentation behavior does not exhibit sensitivity to crosshead displacement rate over the conditions considered in the present work.

Work on Low-velocity Impact Properties of Foam Sandwich Composites with Various Face Sheets

Foam core sandwich composites in conjunction with various facings composed of glass, stitching glass, carbon, carbon/Kevlar hybrid, and Kevlar fabric were fabricated by using RTM process; and low-velocity impact performance was studied at three energy levels. This article proposes that the damage extent of foam core sandwich construction may be characterized by the average damage angle, penetration depth, and maximal cracking width. The results show that the foam core samples with Kevlar facing are optimal for the peak load at load versus time plot, the shortest history course and the lowest impact damage extent at 25 J; the range of average damage angle of Kevlar facing samples is biggest at 90—67°, and the range of maximal cracking width and penetration depth of Kevlar facing samples are smallest: one is at 4.5—15.5 mm, the other is at 16.8— 18.0 mm at 25, 45, and 70 J.

Moisture absorption in foam‐cored composite sandwich structures

AbstractThe hygroscopic behavior of sandwich structures composed of E‐glass/polyester face sheets bonded to a PVC foam core exposed to 95% relative humidity and immersed in sea‐water is examined herein. Moisture uptake was monitored for 11 months yielding absorption curves for samples of polyester resin, laminated composites, PVC foam core, and a sandwich structure. The coefficients of diffusion and moisture saturation values extracted from the curves are significantly greater for the water immersed condition than for the exposed to elevated moisture one, and point to the foam core as the most absorbing material in the sandwich structure. The measured absorption curves are compared to a diffusion model which employs the calculated coefficient of diffusion, showing good agreement. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Impact performance of nanophased foam core sandwich composites

In this study, sandwich panels were fabricated with neat and nanophased foam core and three-layered plain weave carbon fabric/Sc-15 epoxy composite face sheets. Neat and nanophased foam cores with Nanocor ® I-28E nanoclay at a loading of 0.5% and 1% by weight were prepared. Sandwich panels were then fabricated using co-injection resin transfer molding process. Samples of size 100 mm × 100 mm were then cut from the panels and subjected to low-velocity impact loading using an instrumented impact test setup. Impact response of the panels was recorded and analyzed in terms of peak load, absorbed energy, time and deflection at peak load. The tested samples were then sectioned into two halves and scanned using a scanner, optical and scanning electron microscopes to understand the failure patterns. Samples with nanophased foam sustained higher loads and had lower damage areas as compared with neat counterparts. Nanophased foam cores exhibited relatively more brittle fracture.

Failure Analysis of Bolted Joints in Foam-core Sandwich Composites

This study represents an effort to predict the bearing strength, failure modes, and failure load of bolted joints in foam-core sandwich composites. The studied joints have been used in a light full composite airplane. By using solid laminates, a new design for the joint zone is developed. These solid laminates include a number of glass plies with total thickness equal to core thickness. The effect of solid laminate size and interface angle of foam—solid laminate in the bonding zone on the bearing strength, failure loads and type of modes are investigated. The numerical study is performed using 3D FEM in ANSYS commercial code. Tsai—Wu failure criterion is used in the failure analysis. The results indicate that the most important parameter in the proposed joint zone design is the foam—solid laminate interface angle which plays an important role on the value of failure criterion (damage) in the bonding zone. Also, the use of squared shaped solid laminate as compared with a circular laminate will decrease the criterion value significantly. Finally, the influence of solid laminate size and interface angle on the buckling strength was discussed. As obtained through eigenvalue buckling analysis, the increase of solid laminate size or interface angle could result in considerable higher buckling strength.

Experimental and Analytical Study on the Mechanical Behavior of Stitched Sandwich Composite Panel with a Foam Core

To quantify the effect of structural through-thickness reinforcement in foam core sandwich composite panels, an experimental study was carried out which included three-point bending tests, core shear tests, flatwise tensile and compression tests, as well as edgewise compression tests. Standard test procedures based on ASTM guidelines are followed to test the behavior of the stitched panels with reinforcement at 90 degree orientation with respect to the sandwich faces. The test specimens were manufactured by using polyurethane foam Rohacell 71 IG and carbon fiber reinforced composite facesheets. The dry perform facesheets and foam core were then assembled in a dry lay-up already stitched. Kevlar 29 yarn was used to stitch both sets of panels. The results showed a significant effect of the stitching on the in-plane Young’s modulus which was attributed to local displacements of the in-plane fibers and changes in the fiber volume fraction. Stitching of sandwich panels significantly increases the maximum failure loads under flexure, core shear, flatwise tensile, flatwise compression, and edgewise compression loading. A finite element based unit-cell model was developed to estimate the elastic constants of structurally stitched foam core sandwich composite panels taking into consideration the yarn diameter, the stitching pattern and direction as well as the load direction. Depending on these parameters, local changes of the fiber volume fraction as well as regions with undisturbed and disturbed fiber orientations within the laminate plies are taken into account. A good match between the finite element modeling and the experimental data was obtained. The present work should be considered as a step towards developing a more sophisticated numerical model capable of describing mechanical behavior of sandwich structures.

Numerical Study of Fractured Sandwich Composites under Flexural Loading

Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated numerically. Extensive fatigue data from foam core sandwich composites under flexural loading were analyzed. A static finite element analysis is adopted for the uncracked sandwich beam in order to predict crack initiation positions. A face/core debond parallel to the beam axis is considered in accordance with experimental data. A static two-dimensional finite element analysis of the cracked sandwich beam is accounted for the evaluation of stress intensity factors at the crack tips investigating the crack growth behavior. In this way it is succeeded in making a significant contribution to understand the initiation and propagation of a fatigue crack in the foam core of sandwich beams loaded by flexural loading.

Mechanisms of Fatigue Damage in Foam Core Sandwich Composites with Unsymmetrical Carbon/Glass Face Sheets

This paper attempts to describe the fatigue response of a foam the core sandwich composite with unsymmetrical carbon/glass face sheets under tension-tension cyclic loading. The laminate is a critical element of the structure of a light full composite airplane which was determined through the stress analyses for numerous load cases. The face sheets, over a PVC foam the core, consist of two similar woven E-glass fibers following unidirectional carbon fibers with epoxy resin. Extensive fatigue data were generated for the S-N diagram of this laminate. Based on the static FEM analyses, two designs for the configuration of foam to solid laminate bonding zone have been investigated. Hence, the effects of using each of these different proposed designs have been determined. The damage events causing final failure were traced during fatigue tests. Four distinct damage events were found. Two of them were initiated and propagated in the foam the core, which could result in lower dispersion in comparison with other composite materials, because of foam the core homogeneity. The first damage event in the core is crack initiation and propagation along thickness, followed by the second damage event, which is crack propagation parallel to the length of the gauge. The parallel crack length is shortened by increasing the load ratio.

Reliable low-cost NDE of composite marine sandwich structures

The research summarized in this paper centered on the development of reliable low-cost NDE methods for marine composites, especially composite sandwich structures. It has involved the investigation of vibration-based mechanical identification of thick composite structures, low velocity impact and damage studies of foam-cored composite sandwich structures, as well as the development- and application-oriented investigation of various other NDE methods. Composite materials are being increasingly deployed into ship structures, and generally into both the civilian and military sectors of the US economy. It thus becomes essential to have rapid, low-cost and reliable methods for obtaining mechanical property data for these materials, and assessing damage in them. The research has sought to apply low-frequency vibration, ultrasonic, acoustic absorption, thermosonic and especially, acoustic emission methods to the characterization and integrity assessment as well as strain rate and nano-phasing behaviors of composite marine structures.

Fatigue of Foam and Honeycomb Core Composite Sandwich Structures: A Tutorial

This article is intended to be a tutorial on the subject of fatigue of foam and honeycomb core composite sandwich structures. First, several different analytical models for predicting the fatigue life of sandwich composites are presented. Then representative publications which have reported on the major failure modes in sandwich beams under dynamic fatigue loading are summarized, along with several related publications dealing with static and impact loading. Papers dealing with the effects of loading frequency, environmental factors, and block loading on the fatigue life of sandwich composites are discussed. Finally, recent research on different types of non-destruction evaluation (NDE) techniques employed for failure investigations during fatigue testing of sandwich structures is reviewed. Conclusions and generalizations that can be drawn from the literature are presented along with discussions of areas in which further research is needed.

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.

On the dynamic response of sandwich composites and their core materials

Sandwich composites are being aggressively pursued as structural materials by various defense and commercial industries. These include navy, air force, army, automotive and sporting industries to name a few. In the context of structural load bearing members and absorbing dynamic loads, foam core sandwich composites offer unique advantages over traditional composites. The cellular construction of the foam materials not only provides lightweight capability but also a deformation mechanism that allows efficient absorption of energy. Of particular interest in this study is to investigate the behavior of the foam materials and their sandwiches under high strain rate (HSR) loading which are very much prevalent in their actual applications. A systematic approach has been taken to study the response of PVC foam materials at strain rates ranging from quasi-static to around 2000 S −1. First, room temperature response has been extracted for various density foams, and determined their strain rate sensitivities and failure characteristics. Next, two microstructures in the construction of the core materials, namely linear and cross-linked foams, were considered in the investigation. This study revealed that both the categories of foam had direct dependence on temperatures well below the T g of the base polymer. Consequently, the next phase of the investigation included HSR tests at elevated and sub-ambient temperatures. The study has established that at room temperature (RT), the delamination at the sub-interface zone underneath the actual core–skin interface is the critical mode of failure. This sub-interface is characterized by the separation of the dry and resin-soaked cells near the core–skin interface. Failure modes however changed as the test temperature changed from RT to sub-ambient.

Characterization of Flexural Properties of Syntactic Foam Core Sandwich Composites and Effect of Density Variation

Microballoons (hollow particles) of the same outer radius but with five different inner radius values are used to fabricate five types of syntactic foam slabs. These five types of slabs are used as the core material to fabricate sandwich composites. Three- and four-point bending and short beam shear strength tests are carried out to characterize the flexural behavior of syntactic foam core sandwich composites. The effect of change in microballoon radius ratio (ratio of the inner to the outer radius) on the flexural properties of the sandwich composites is also studied. The results show that in three- and four-point bending tests, the failure is governed by tensile properties of the foam core and the strength is not affected by the microballoon radius ratio. Shear failure takes place in short beam shear tests, which makes the microballoon radius ratio an important factor in determining the strength of the sandwich composite.

Fatigue characteristics of foam core sandwich composites

In situ AE and post test microscopic analysis of quasi-static and flexural fatigue testing of sandwich composites indicated core damage to be the predominant failure activity while fiber rupture served as a precursor to catastrophic failure. Multiple crack initiation sites were observed under fatigue loading conditions in the vicinity of the notch tip. Both modes I and II cracking was observed in the core and along the interface between the core and the facesheets. Flexural fatigue life was found to be beyond a 500,000 cycle arbitrarily established cutoff below 75% of the ultimate static load. A statistical model based on experimental lifetime and corresponding AE data was developed to quantify the extent of damage; a reasonably good correlation with the experimental results has been obtained.

Buckling of Sandwich Composites; Effects of Core?Skin Debonding and Core Density

Foam–core sandwich composites have been fabricated using innovative co-injection resin infusion technique and tested under in-plane compression. The sandwich construction consisted of Klegcell foam as core materials and S2-glass/vinyl ester composites as face sheets. Tests were conducted with various foam densities and also with implanted delamination between the core and the face sheet. The intent was to investigate the effect of core density, and the effect of core–skin debonds on the overall buckling behavior of the sandwich. Analytical and finite element calculations were also performed to augment the experimental observations. It has been observed that core density has direct influence on the global buckling of the sandwich panel, while embedded delamination seem to have minimal effect on both global as well as local buckling. Detailed description of the experimental work, finite element modeling and analytical calculations are presented in this paper.