Journal of Sandwich Structures and Materials recent issues

Special Issues of the Journal of Sandwich Structures and Materials dedicated to full manuscripts of selected papers presented at the 8th International Conference on Sandwich Structures (ICSS-8), 6--8 May 2008 Porto, Portugal

Mendes Ferreira, A. J. — Thu, 15 Oct 2009 09:17:02 PDT

Evaluation of New Crack Suppression Method for Foam Core Sandwich Panel Via Fracture Toughness Tests and Analyses Under Mode-I Type Loading

Hirose, Y., Matsuda, H., Matsubara, G., Inamura, F., Hojo, M. — Thu, 15 Oct 2009 09:17:02 PDT

A new crack arrester was proposed, in which a different material with higher stiffness is installed on the crack propagation path. The effect of this crack arrester was experimentally evaluated for interfacial crack propagation between a carbon fiber reinforced plastic surface skin and a foam core. The experimental results indicated that the crack arrester increased the critical load of the crack growth, and approximately five times larger apparent fracture toughness was obtained near the leading edge of the arrester by considering the energy release rate. It was also confirmed that the fabrication of the crack arrester had no detrimental effect on the intrinsic properties of the sandwich panel structures.

Development of a Mechanical Technology for Joining Sandwich Elements

Feldhusen, J., Warkotsch, C., Kempf, A. — Thu, 15 Oct 2009 09:17:02 PDT

Wide spread use of sandwich structures is still inhibited by the lack of a consistent joining technology one which is easy to work with, quantifiable and not dependent on special knowledge about manufacturing processes. A mechanical technology for joining sandwiches as developed in this article shall achieve these goals. The developing process follows the design methodology according to Pahl et al. [1]. The resulting systematic approach to the problem facilitated the generation of a complete solution field. This approach led to 18 promising concepts of 783 possible original solutions. Prototypes of those concepts showed that a reliable mechanical technology for joining sandwich elements is feasible.

Mechanical Behavior of Sandwich Structures using Natural Cork Agglomerates as Core Materials

Reis, L., Silva, A. — Thu, 15 Oct 2009 09:17:02 PDT

Cork is a material of great value to the Portuguese economy. Unfortunately, its use is still restricted to traditional areas, with the agglomerate form in particular not being used to its full potential. The objective of this article is to analyze the viability of using cork-based material as core materials in sandwich structures in aeronautical and aerospace applications. The use of cork-based material is proposed because of its isolation properties (both thermal and acoustic) and there is no significant performance loss, when compared with the currently used materials. It presents other advantages, as well as, less wastage of energy in manufacturing and a better environmental integration, both in the transformation stage and in the end of life recycling stage. The objective of this work is to study the mechanical behavior of different sandwich specimens, with carbon/epoxy faces, and cores of different cork agglomerates and their comparison with the results obtained with similar specimens using current material cores. Experimental shear tests and three-point bending tests were carried out and the evolutions of the load— displacement curves of the different cork agglomerates/sandwiches were analyzed and discussed. The obtained results show that significant room for improvement still exists in use of cork-based core materials.

Strength Variability of Inserts in Sandwich Panels

Raghu, N., Battley, M., Southward, T. — Thu, 15 Oct 2009 09:17:02 PDT

Bonded inserts are commonly used to introduce localized loads into sandwich panels. This article investigates sources of variability in the pull-out strength of metallic inserts in aramid honeycomb sandwich panels. Finite element modeling is used to investigate the sensitivity of core stresses to potting depth in partially potted inserts, which is found to be significant. Experimental specimens showed large variations in potting radii and cell wall dimensions, both critical parameters since the failure mode is predominantly core shear, adjacent to the potting. Foam core specimens were also tested and showed significantly lower variability in pull-out strength compared with honeycomb specimens.

A New 2D Local Vibrational Model for Unidirectional Clamped--Clamped Sandwich Structure with Edge Stiffeners

Karczmarzyk, S. — Thu, 15 Oct 2009 09:17:02 PDT

Flexural vibration of a clamped—clamped, three-layer, one-span sandwich, unidirectional structure composed of isotropic layers is considered in the article. A new single series model of the 2D problem, obtained within the local theory of linear elastodynamics, is presented. Very realistic boundary conditions at edges of the structure, which correspond to the engineering solution with the edge stiffeners, are introduced and satisfied. Eight eigenfrequencies of the structure for two sets of input data are calculated, according to the new model, and compared with numerical and experimental results existing in the literature. A detail comparative analysis of the results is done and some conclusions are presented.

An Approach for the Evaluation of Effective Elastic Properties of Honeycomb Cores by Finite Element Analysis of Sandwich Panels

Aydincak, I., Kayran, A. — Wed, 12 Aug 2009 08:03:09 PDT

Different detailed finite element model alternatives are developed to come up with the most reliable finite element model of the sandwich panel, with the actual honeycomb core geometry, to evaluate the existing equivalent continuum models of aluminum honeycomb cores. Finite element models of sandwich panels with effective elastic constants of the honeycomb core are generated based on the existing continuum models of the honeycomb core. The evaluation of the effective elastic constants of honeycomb cores is based on the comparison of the total reaction forces, calculated by both finite element models, on the supported faces of sandwich panels due to different in-plane and out-of-plane uniform input displacements applied to the faces of the panels. The results show that the reliability of the individual in-plane and out-of-plane effective elastic constants of the existing continuum models of the honeycomb cores can be successfully evaluated based on the comparative study.

Repeated Slamming of Sandwich Composite Panels on Water

Charca, S., Shafiq, B., Just, F. — Wed, 12 Aug 2009 08:03:09 PDT

Wave slamming was simulated by repeatedly slamming rectangular sandwich composite specimens mounted on a rigid wedge with constant deadrise angle onto the body of calm water at various energy levels. Under single slamming, peak pressures and strains on the specimens were consistently found near the keel, whereas the maximum damage was localized near the chine. Significant reduction in strength was observed resulting from a single slam even at a moderate slamming energy level that left no apparent/visible damage to the test panel. Similarly, a substantial reduction in strength was observed under repeated slamming at various energy levels. The results were corroborated with acoustic emission observations that indicated a substantial reduction in AE activity in slammed specimens. A methodology was developed for the quantitative assessment of remaining strength and damage accumulation in slammed specimens using AE technique. Face yielding and core crushing were found to be the dominant modes of failure.

Flexural Properties of Nanoclay Syntactic Foam Sandwich Structures

Woldensenbet, E., Sankella, N. — Wed, 12 Aug 2009 08:03:09 PDT

Flexural properties of syntactic foam sandwich structures under three-point and four-point bending, and short beam shear tests are studied. Four different types of plain and nanoclay syntactic foam sandwich structures are fabricated and tested for flexural properties. The volume fraction of microballoons in the foam core is maintained at 60%, while 1% volume fraction of nanoclay is included in the nanoclay syntactic foam. The flexural properties of the nanoclay syntactic foam sandwich structures are compared with those of plain syntactic foam sandwich structures. Nanoclay incorporation hasimproved the flexural load bearing capacity of syntactic foam sandwich structures in three-point and four-point bending tests. However, the short beam shear test shows that nanoclay incorporation degrades the property of the sandwich structure. It is also found that the microballoons radius ratio effect on the strength of the sandwich structures is affected by the nanoclay.

Special Issues of the Journal of Sandwich Structures and Materials Dedicated to Full Manuscripts of Selected Papers Presented at the 8th International Conference on Sandwich Structures (ICSS-8), 6--8 May 2008 Porto, Portugal

Mendes Ferreira, A. J. — Wed, 24 Jun 2009 07:31:28 PDT

Nondestructive Evaluation of Sandwich Structures: A Review of Some Inspection Techniques

Hsu, D. K. — Wed, 24 Jun 2009 07:31:28 PDT

This article is a summary of research and development conducted by the author's group at Iowa State University for the nondestructive evaluation (NDE) of sandwich structures in aviation, space, and marine applications. The targets of inspection include manufacture defects, in-service damage due to human and natural causes, and the inspection of repairs on sandwich structures. The emphases adopted in the development of NDE techniques and instruments for sandwich structures are imaging capability, quantitative inspection, and the ease of use in the field. This article describes the various NDE methods for sandwich structures, especially those techniques and instruments developed at Iowa State University that had gone through the research, development, and field test stages.

Influence of Process Pressure on Local Facesheet Instability for Ultra-light Sandwich Structures

Rion, J., Stutz, S., Leterrier, Y., Manson, J.-A. E. — Wed, 24 Jun 2009 07:31:28 PDT

Skin wrinkling phenomenon is investigated in the case of ultra-light sandwich structures with a honeycomb core manufactured by one-shot vacuum bag processing. The interplay between process pressure and compressive strength of the skin is established. It is observed that the size of the adhesive menisci between honeycomb cell walls and skin, and the waviness of the skin increases with process pressure. As these two effects exert opposing influences on the compressive strength of the skin, an optimal process pressure equal to 0.7 bar is identified experimentally and confirmed by an analytical model.

Vibration and Buckling of Sandwich Laminates having Interfacial Imperfections

Chakrabarti, A., Sheikh, A. H. — Wed, 24 Jun 2009 07:31:28 PDT

Vibration and buckling of sandwich laminates are studied for different degrees of imperfections at the layer interfaces using a refined plate theory. In this plate theory, the through-thickness variation of transverse shear stresses is represented by piecewise parabolic functions, where the continuity of these stresses is satisfied at the layer interfaces. The transverse shear stresses free condition at the top and bottom surfaces of the plate is also satisfied. The interlaminar imperfections are represented by in-plane displacement jumps at the layer interfaces by a linear spring layer model. Many new results are generated that should be useful for future research as there is no such published result on imperfect sandwich plates.

Impact Damage and Energy-absorbing Characteristics and Residual In-plane Compressive Strength of Honeycomb Sandwich Panels

Zhou, G., Hill, M.D. — Wed, 24 Jun 2009 07:31:28 PDT

An experimental study of the in-plane compressive behavior of both aluminium and nomex composite sandwich panels with 8 ply carbon/epoxy skins was conducted. All sandwich panels were impact-damaged with a range of impact energies from 1 to 55 J. Dominant damage mechanisms were found to be core crushing, skin delamination, and fracture with the former two absorbing most of the impact energy. While the intact panels failed in region close to one loaded end, all the impact-damaged nomex panels failed around the mid-section region. Two thirds of the aluminium panels also failed in the mid-section region and one third failed in the loaded end region. The presence of the core played a unique role in in-plane compression with a substantial stabilizing support to the skins, which counteracted the deleterious effect of impact damage. The in-plane compressive behavior has shown the combined effects of impact damage and the core in a complex manner.

Analytical Modeling of Composite Sandwich Panels under Blast Loads

Hoo Fatt, M. S., Palla, L. — Wed, 24 Jun 2009 07:31:28 PDT

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.

Predicting Failure Loads of Undamaged Sandwich Honeycomb Panels Subject to Bending

Staal, R.A., Mallinson, G.D., Jayaraman, K., Horrigan, D.P.W. — Thu, 02 Apr 2009 07:59:08 PDT

This paper describes a method to predict failure loads accurately in undamaged sandwich panels subject to bending. The current method of predicting failure loads using linear wrinkling models is limited. A general lack of correlation exists between experimental and analytical/numerical wrinkling models. In some cases, the linear wrinkling models over-estimate wrinkling failure loads in undamaged panels by 100%. Various authors have tried to account for this discrepancy by modifying their linear models to take into account imperfections or irregularities in the structure or in some cases by using correction factors to get them to agree.

This research uses these Finite Element models to find failure stresses in undamaged panels and to track the failure mode and determine the failure mechanism. Based on the failure mechanism found from the Finite Element analyses, a revised analytical expression is developed. These numerical and analytical models are verified against four point bend tests for a range of panel configurations. The results show that the panels collapse due to localised core crushing as a consequence of face sheet wrinkling. By using correct core failure criteria an almost perfect correlation is found between experimental, numerical and analytical results.

Facesheet Wrinkling of Damaged Honeycomb Sandwich Structures

Wadsworth, D.J., Horrigan, D.P.W., Moltschaniwskyj, G., Collins, I. — Thu, 02 Apr 2009 07:59:08 PDT

Critical facesheet wrinkling stresses are determined for thin gauge sandwich structure constructed from thick, highly orthotropic, honeycomb-cored materials containing sub-surface damage due to soft-body impact. A closed-form, analytical expression is extended to account for regions of crushed core extending a uniform depth into the core material. The results show that sub-surface core damage can significantly decrease facesheet support, further reducing the critical wrinkling stress computed from classical formulae. The model accounts for shear traction at the core/facesheet interface and provides a means of assessing the fundamental parameters governing the initiation of facesheet wrinkling in initially flat panels constructed from thick orthotropic cores containing sub-surface damage. The results are discussed in terms of the damage tolerance of panels used in aerospace applications.

Delamination Growth and Residual Strength of Compressively Loaded Sandwich Panels with Stiffness Tailored Face Sheets

De Xie, , Biggers, S. B. — Thu, 02 Apr 2009 07:59:08 PDT

This study shows that a simple approach to stiffness tailoring of composite face sheets can improve the performance of sandwich panels under compressive loading when delaminations are present. The simple stiffness tailoring concept used here is to reposition of all 08 material (aligned with the loading direction) into regions of certain width near the edges of the sandwich panel. This concept has been shown to improve the buckling and postbuckling performance of solid plates and to offer control of delamination growth. To evaluate this tailoring design concept in sandwich panels, numerical simulations of onset and propagation of the delamination growth were conducted with uniform face sheets and tailored face sheets. The delamination front tracing method previously developed by the authors was used to perform the delamination growth analysis. The interfacial elements, which allow the growth of the delamination front to be traced, are placed at the interface between the top face sheet and the core in the undelaminated region. They enable calculation of the strain energy release rates and application of a fracture mechanics delamination growth criterion. Gap elements were used to avoid interfacial overlap. Based on the numerical study, the improvement in the ultimate load provided by the simple stiffness tailoring concept ranges from 80% to 100%, depending on the extent of tailoring. Substantial improvements in the residual strength and stiffness, after significant delamination growth, can also be observed. Therefore, this article provides a potential design concept that can improve the damage tolerance of sandwich panels without adding weight.

Investigation of Continuously Produced Thermoplastic Honeycomb Processing -- Part I: Thermoforming

Fan, X., Verpoest, I., Pflug, J., Vandepitte, D., Bratfisch, P. — Thu, 02 Apr 2009 07:59:08 PDT

A continuous process of thermoplastic honeycomb core, ThermHex, has been patented and is under development at K. U. Leuven. This new concept of thermoplastic honeycomb provides more affordable choices for structural applications than the previous expensive honeycombs thanks to the automated and continuous processing technology. The ThermHex process starts from one continuous thermoplastic sheet and consists of three main processing steps: thermoforming of half-hexagonal webs, folding of webs to honeycomb geometry, and internal fusion bonding. In this article, the polymeric material behavior during the thermoforming step is investigated in detail. First the material is characterized by a series of polymer tests to obtain basic data for the next numerical calculation. Then a 1D viscoelastic model and a 2D finite element (FE) model are used to analyze the polymer behavior during the thermoforming step, to acquire the correct understanding and hence to minimize the effect of the anelastic strain recovery (ASR) during the production. It is found that the material stress relaxation is one of the key features of the final process quality.

Investigation of Continuously Produced Thermoplastic Honeycomb Processing -- Part II: Fusion Bonding

Fan, X., Verpoest, I., Pflug, J., Vandepitte, D., Bratfisch, P. — Thu, 02 Apr 2009 07:59:08 PDT

Following paper Part I, this article investigates the last step of ThermHex process namely fusion bonding. The critical point of the fusion bonding step is to prevent catastrophic deformation (collapsing) of cell walls under heat and pressure. This demands a rapid heating rate and short heating time to keep sufficient stiffness for the inner part of cell walls against the pressure applied on the honeycomb. Hence, in this work the transient heat transfer behavior during the fusion bonding is investigated by using a finite element model for the detailed hexagonal unit cell. Furthermore, a mathematical model is developed to analyze the deformation of molten cell walls and their gradual contact to the skin strips. The experiments based on the talc-filled polypropylene material have been performed to validate the analytical model. Consequently, the optimal fusion bonding conditions can be determined to ensure good quality for the final products.

Residual In-plane Mechanical Properties of Transversely Crushed Structural Foams

Koissin, V., Shipsha, A. — Thu, 02 Apr 2009 07:59:08 PDT

The mechanical properties of structural polymer foams are investigated after crushing in the rise direction (out-of-plane axis of a foam material block). The crushed foams are loaded in uniaxial compression, tension, or shear. All tests are performed in the plane of the foam block, i.e., perpendicular to the crushing direction. For comparison, virgin foams are also characterized. The results are discussed featuring the properties of crushed foams, which can be important for the damage tolerance analysis of a foam core sandwich structure.

Predicting Failure Loads of Impact Damaged Honeycomb Sandwich Panels

Staal, R.A., Mallinson, G.D., Jayaraman, K., Horrigan, D.P.W. — Thu, 02 Apr 2009 07:59:08 PDT

This article examines the effect of impact damage on minimum gauge sandwich structures. This type of damage can cause panels to fail at significantly lower loads than an equivalent undamaged panel. For this reason, impact damage in sandwich structures has been a focus for numerous studies, including those that examine damage tolerance of structures and also their post-impact residual strength. This study specifically examines thin-gauge Nomex honeycomb panels and is concerned with barely visible impact damage (BVID), the type of damage that can be found on aircraft components. An overview of BVID, including an in-depth examination of the failure modes and failure loads of damaged panels is presented in this article. The study uses nonlinear finite element models to accurately predict failure loads, and understand how this type of damage causes panels to fail at reduced loads. A simplified, closed-form analytical model is developed to predict failure stresses through a complete range of damage sizes and depths. This study finishes by comparing numerical and analytical models against experimental values. Good agreement is seen in all cases for a wide variety of damage sizes.

Fillets Formed by Adhesive Bonding of Axially Oriented Webs to Flat grain Wood Pieces and their Effects on Bond Strength

Follrich, J., Frybort, S., Teischinger, A., Muller, U. — Thu, 02 Apr 2009 07:59:08 PDT

In order to study the influence of adhesive fillet radius on bond strength as well as notch-stress t-shaped specimens of Norway spruce wood (Picea abies Karst.) bonded with a rapidly curing epoxy adhesive were tested in tension. In general, increasing tensile strength was observed with increasing fillet radii and web thickness. The effect of small fillet radius was more pronounced for thin webs. Highest notch-stress reduction was found for samples of small fillet radii and thin webs.

Improved Response of Unsymmetrically Laminated Sandwich Plates by Using Zig-zag Functions

Brischetto, S., Carrera, E., Demasi, L. — Thu, 02 Apr 2009 07:59:08 PDT

This article shows the advantages of using the zig-zag function (-1)^k_k (ZZF) in the bending analysis of unsymmetrically laminated sandwich flat panels with a soft core. Higher order theories are developed by adding ZZF to displacement fields of known theories. From linear to seventh order cases in displacement are considered. The main advantage of ZZF lies in the fact that it introduces a discontinuity in the first derivative (so called zig-zag effect) of the displacement distribution corresponding to the core-face interfaces. Results including and discarding ZZF are compared in the bending response of sandwich plates loaded by an harmonic distribution of transverse pressure at the top surface. Different values of face-to-core stiffness ratio (FCSR) as well as length-to-thickness ratio (LTR) have been analyzed. It is concluded that: (1) ZZF is highly recommended in the bending analysis of unsymmetrically laminated sandwich plates; (2) the use of ZZF makes the error almost independent of the FCSR parameter; (3) ZZF is easy to implement and its use should be considered with respect to other theories.