Face Sheet Material Research Articles

Experimental and numerical characterizations of flexural behavior of VARTM-infused composite sandwich structures

Experimental and numerical characterizations of sandwich materials are required to fully exploit the benefits offered by these materials and efficiently design them. In this study, the flexural behavior of resin-infused composite sandwich structures was investigated. Panels with two different polyvinylchloride foam densities and thicknesses were studied. The S-2 glass fabric face sheets were resin-infused and bonded to the core in a single-step process using epoxy resin. Sandwich beams machined from the panels were subjected to three- and four-point bending tests. Finite element-based simulations predicting the flexural response of the sandwich panels were performed and compared with experiments. Excellent agreement in finite element-predicted failure loads and experiments were observed.

Designs for construction of quiet composite sandwich structures.

Sandwich structures are comprised of a low-density core sandwiched between two thin, stiff facesheets, often composite laminates. The construction of the sandwich structure is designed to optimize the high-bending stiffness-to-weight ratio necessary for aerospace applications. Although the high stiffness-to-weight ratio of sandwich structures is ideal for bearing mechanical loads, a trade-off exists between mechanical and acoustic performance, in that increases in one generally come at the expense of the other. Improvement of TL of sandwich panels can be divided into three general categories: altering the panel construction by changing the core/facesheet materials, use of active or semi-active vibration suppression, and finally, added treatments such as foams, fibers, or other structures to either the exterior of the panel, or inside the core. TL of sandwich structures using these strategies is presented, in addition to explanation of the physical principals responsible for the decrease in transmitted sound. Featured approaches include design of core and facesheet materials, attachment of layered gasses to the panel facesheet, and insertion of midplane membrane-type metamaterial to the core. Advantages and disadvantages of each strategy are also considered and discussed.

Response and Damage Tolerance of Composite Sandwich Structures under Low Velocity Impact

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.

Edge Reinforcement of Honeycomb Sandwich Panels

Protection of the fragile honeycomb core material in hollow-core panels has long been a subject of interest for the manufacturers because it is necessary to seal the panel edges to prevent damage. Traditionally this has been accomplished by using edge banding, which has the added benefit of improving panel bending strength and stiffness. This study focuses on evaluating the effects of edge banding on the bending strength and stiffness properties of honeycomb core panels. The honeycomb panels were made with a combination of different face sheet materials (3-mm hardboard or 6-mm medium-density fiberboard [MDF]), rail types (particleboard or yellow poplar [Liriodendron tulipifera]), and rail widths (10 or 38 mm), and had edge-band materials fixed to their long edges using either direct coating, stabilizer edge, or surface folding techniques. Panels made with the 6-mm MDF face sheet and 38-mm poplar rails had the highest strength properties. To safely apply edge banding to honeycomb core panels, a solid edge reinforcement material is required to prevent the core material from being crushed during the process. The surface folding technique was the best method of edge banding and significantly enhanced the maximum bending moment of honeycomb core panels—more than twice that of panels without edge banding.

The Response of Honeycomb Core Sandwich Panels, with Aluminum and Composite Face Sheets, to Blast Loading

The results of blast tests on sandwich panels with honeycomb cores are reported. Two core heights (13 mm and 25 mm) and two face sheet materials (glass fiber epoxy composite and aluminum alloy) were investigated. Increasing the core thickness reduced the permanent displacements exhibited by the sandwich panels. The panels with composite face sheets also exhibited smaller residual displacements than the aluminum face sheet counterparts. Damage took the form of core crushing, core shearing, debonding of the face sheet from the core, permanent displacement, cracking of the composite face sheets, and tearing. Higher damage levels were observed at elevated impulse levels. Load localization was found to concentrate the damage to the central portion of the panel, preventing the whole panel being employed in resisting the blast.

Performance assessment of aerospace sandwich secondary structure panels using thermoelastic stress analysis

Full scale tests carried out on polymer composite sandwich panels, which model the loads experienced by aircraft secondary wing structure, are described. The results from the tests form the basis of a performance assessment of the sandwich construction. The sandwich panel face sheets were manufactured from five different carbon fibre material architectures and were consolidated with epoxy resin using a variety of approaches. In previous work, it was shown that it was possible to significantly reduce the manufacturing costs of producing the sandwich structure by replacing the standard certificated process of unidirectional prepreg cured in an autoclave with a method that used non-crimp fabric infused in a conventional oven. In the present paper, thermoelastic stress analysis (TSA) is used during the full scale tests to obtain stress data from the five different panel types. The TSA data and measured deflection data are used to validate finite element (FE) models of each panel. The experimental validation highlighted some interesting features in the modelling approach. The face sheet materials were treated as homogeneous orthotropic blocks, which resulted in a conservative prediction of deflections. The models did not consider the woven and stitched material face sheet material configurations and therefore, omitted some of the features apparent in the experimental work. However, the validation showed that this did not affect the performance evaluation and most importantly, the validated FE showed that using different face sheet materials had little effect on the stresses in the panels.

Experimental and Numerical Simulation Studies of Low-Velocity Impact Responses on Sandwich Panels for a BIMODAL Tram

This paper describes the results of experiments and numerical simulation studies on the impact and indentation damage created by low-velocity impact subjected onto honeycomb sandwich panels for application to the BIMODAL tram. The test panels were subjected to low-velocity impact loading using an instrumented testing machine at six energy levels. Contact force histories as a function of time were evaluated and compared. The extent of the damage and depth of the permanent indentation was measured quantitatively using a 3-dimensional scanner. An explicit finite element analysis based on LS-DYNA3D was focused on the introduction of a material damage model and numerical simulation of low-velocity impact responses on honeycomb sandwich panels. Extensive material testing was conducted to determine the input parameters for the metallic and composite face-sheet materials and the effective equivalent damage model for the orthotropic honeycomb core material. Good agreement was obtained between numerical and experimental results; in particular, the numerical simulation was able to predict impact damage area and the depth of indentation of honeycomb sandwich composite panels created by the impact loading.

Influence of Core and Face Sheet Materials on Quasi‐Static Mechanical Properties and Failure in Aluminium Foam Sandwich

Aluminium foam sandwich (AFS) with metallurgical bonding between face sheets and core has high potential for lightweight design. The present paper studies failure mechanisms and mechanical characteristics under quasi-static bending load to support safe dimensioning of parts. For this purpose, failure charts are provided based on a weakest-link approach which incorporates local material properties and reflects scatter of sandwich properties.

Hypervelocity impact on CFRP: Testing, material modelling, and numerical simulation

Abstract This paper describes the derivation and validation of a numerical material model that predicts the highly dynamic behaviour of CFRP (carbon fibre reinforced plastic) under hypervelocity impact. CFRP is widely used in satellites as face sheet material in CFRP-Al/HC sandwich structures (HC = honeycomb) that can be exposed to space debris. A review of CFRP-Al/HC structures typically used in space was performed. Based on this review, a representative structure in terms of materials and geometry was selected for study in the work described here. An experimental procedure for the characterisation of composite materials is documented by Riedel et al. [ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003.]. The test results from the CFRP of the current study allow for the derivation of an experimentally based orthotropic continuum material model data set that is capable of predicting the mechanical behaviour of CFRP under hypervelocity impact. Such a data set was not previously available. In the work by Riedel et al. [Hypervelocity impact damage prediction in composites: part II – experimental investigations and simulations. International Journal of Impact Engineering, 2006;33:670–80.] an orthotropic material data set was used for modelling HVI on AFRP (aramid fibre reinforced plastic), which shows relatively high deformability before failure. The enhancements of the modelling approaches in previous studies [Riedel W, Harwick W, White DM, Clegg RA. ADAMMO – advanced material damage models for numerical simulation codes. ESA CR(P) 4397, EMI report I 75/03, Freiburg; October 31, 2003. Hiermaier S, Riedel W, Hayhurst C, Clegg RA, Wentzel C. AMMHIS – advanced material models for hypervelocity impact simulations. Final report, EMI report E 43/98, ESA CR(P) 4305, Freiburg; July 30, 1999.] necessary to model brittle CFRP are specified. An experimental hypervelocity impact campaign was performed at two different two-stage light gas guns which encompassed both normal and oblique impacts for a range of impact velocities and projectile diameters. Validation of the numerical model is provided through comparison with the experimental results. For that purpose measurements of the visible damage of the face sheets and of the HC core are conducted. In addition, the numerically predicted damage within the CFRP is compared to the delamination areas found in ultrasonic scans.

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time. In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell ® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.

The collapse response of sandwich beams with a Y-frame core subjected to distributed and local loading

Sandwich beams comprising a Y-frame core have been manufactured by assembling and brazing together pre-folded sheets made from AISI type 304 stainless steel. The collapse responses of the Y-frame core have been measured in out-of-plane compression, longitudinal shear and transverse shear; and the measurements have been compared with finite element predictions. Experiments and calculations both indicate that the compressive response is governed by bending of the constituent struts of the Y-frame and is sensitive to the choice of lateral boundary conditions: the energy absorption for a no-sliding boundary condition exceeds that for free-sliding. Under longitudinal shear, the leg of the Y-frame undergoes uniform shear prior to the onset of plastic buckling. Consequently, the longitudinal shear strength of the Y-frame much exceeds its compressive strength and transverse shear strength. Sandwich beams were also indented by a flat bottomed punch, and a relatively high indentation strength was observed. It is argued that this is due to the high longitudinal shear strength of the Y-frame. While finite element calculations capture the measurements to reasonable accuracy, a simple analytical model over-predicts the indentation strength. Finally, the finite element method was used to investigate the energy absorption capacity of the sandwich beams under indentation loading. The calculations reveal that for a given tensile failure strain of the face-sheet material, a sandwich beam with Y-frame core has a comparable performance to that of a sandwich beam with a metal foam core. The relative performance is, however, sensitive to the choice of design parameter: when the indentation depth is taken as the design constraint, the sandwich beam with a Y-frame core outperforms the sandwich beam with the metal foam core.

Effect of face sheet material on the indentation response of metallic foams

Sandwich panels comprising metallic foam cores fail by localized indentation when subjected to impact and blast loads. In this paper the indentation response of aluminum foams with face sheets, whose behavior represent elastic, elastic–ideally plastic and elastic–plastic strain hardening, were investigated experimentally under quasi-static loading conditions. The tests were carried out using flat and hemispherical indenters made of stiff tool steel on the blocks of aluminum foam with and without face sheets. Thickness of the face sheets was varied from 0.5 mm to 1.0 mm to evaluate the thickness effect on the indentation behavior. Competing failure modes for the initiation of failure are discussed. Results show that the indentation behavior is strongly dependent on the type and thickness of the face sheets used.

Response surface characterization of the mechanical behavior of impact-damaged sandwich composites

In this research, Response Surface Methodology (RSM) is employed to characterize the influence of material configuration on the damage tolerance and residual strength characteristics of sandwich composites. Test specimens used were comprised of carbon-epoxy woven fabric facesheets, and Nomex honeycomb cores. The ranges of the material configuration used are typical of those employed in aircraft applications. A series of carefully selected tests were used to isolate the coupled influence of various combinations of the number of facesheet plies, core density, and core thickness on the damage formation and residual strength degradation due to normal impact. Response surface estimates suggest that impact damage development and residual strength degradation are highly material and lay-up configuration dependent. Increasing the core thickness for a specific number of facesheet plies resulted in decreasing the impact damage, whereas increasing the number of facesheet plies for a given core thickness resulted in enhancing the residual strength. The derived damage tolerance and residual strength models can lead to a better understanding of the mechanical behavior of the impact-damaged sandwich composites, and hence improve their design and expand their applications.

The Effect of a Honeycomb Core on the Mechanical Properties of Composite Sandwich Plates

When comparing composite sandwich analysis with an exact solution, the results of finite element modeling with an ANSYS shell 91 element agreed well with the exact solution. The practical applications of the shell 91 element are demonstrated with a four-point bend test conducted on sandwich beam specimens. The specimens comprised carbon/epoxy fabric face sheets and a honeycomb core. Two kinds of honeycomb cores were used to fabricate the composite sandwich specimens: an aluminum one and a glass/phenolic one. The predictions with the shell 91 element were also agreed well with the experimental results. A variety of tests was conducted; namely, a long beam flexural test, a short beam shear test, a flatwise tensile test, a flatwise compression test and an edge compression test. The sandwich plate with the aluminum honeycomb core had a specific bending stiffness that was 1.7 to 2.0 times higher than that of the sandwich plate with the glass/ phenolic honeycomb core.

Analysis of Sandwich Panels for an Energy Efficient and Self-Supporting Residential Roof

The structural and thermal feasibility of a self-supporting sandwich panel for energy efficient residential roof applications is assessed. The assessment is limited to symmetric sandwich panels comprising two face sheets and an insulating core. Feasible panel designs are presented for loading conditions, corresponding to southern and northern climates in the United States. The base case panel is 5.5m long for a nominal 4.6m horizontal span and an 8∕12 roof pitch. Face sheet materials considered are oriented strand board, steel, and fiber reinforced plastic. Core materials considered are expanded polystyrene, extruded polystyrene, polyurethane, and poly(vinyl-chloride) foams. A wide range of material options meet building code limits on deflection and weight and prevent face sheet fracture and buckling, and core shear failure. Panels are identified that have structural depths similar to conventional wood rafter construction. Shortening the overall panel length provides greater choice in the use of materials and decreases the required panel thickness. Suggestions for improved panel designs address uncertainty in the ability of the plastic core to withstand long term loading over the expected life of residential buildings.

A shape memory-based multifunctional structural actuator panel

A multifunctional panel concept is presented in which a lightweight, structurally efficient sandwich panel is able to undergo a reversible change in shape upon application of a localized thermal stimulus while supporting significant load. The shape change is effected by shape memory alloy face sheet elements, which exploit a “one-way” shape memory effect only. Unlike other related designs, no external or bias forces are required to complete the full cycle of shape change. The reversible shape change is accomplished by a core design which forces an inactive face sheet to martensitically deform in tension when the opposing face sheet undergoes a length change. By alternately heating one face sheet and then the other, the sandwich panel is able to perform fully reversible cyclic shape changes. The performance of the sandwich panel, in terms of required thermal power, actuation frequency, peak load bearing capacity, stiffness, and weight, can be optimized by proper selection of face sheet material and its thickness, the overall core thickness, core member thickness and length, and the design of the joint connecting core members and face sheet.

Enhancement of low velocity impact damage resistance of sandwich plates

Conventionally designed sandwich plates used in marine and ship structures consist of thin, laminated composite face sheets bonded to a relatively thick, compressible structural foam core layer. In service, the external face sheet often makes low velocity contact with floating objects or moorings that may cause reversible local deflection of the face sheet and permanent compression or crushing of the underlying foam, resulting in interface delaminations. Extension of such interfacial cracks under service loads may impair structural integrity. Modified designs that protect the foam core by inserting a ductile interlayer under the external face sheet, first proposed in our recent paper [Dvorak, G.J., Suvorov, A.P., in press. Protection of sandwich plates from low velocity impact. Journal of Composite Materials 38], are analyzed here in much greater detail and compared with their conventional counterparts. Two different interlayer materials are used, a stiff and incompressible polyurethane (PUR), and a compliant and compressible elastomeric foam (EF), together with the same face sheet and core materials. Response of the three different sandwich plate configurations of constant overall thickness is analyzed under uniformly applied load to a span of a continuous plate, and under forces imparted by two shapes of rigid indenters in a low velocity impact. The results show that the PUR interlayer reduces both overall and local deflections of the face sheet, and also the local compression of the foam core and the residual stresses left after unloading. The EF interlayer offers a much better protection against compression of the foam core, while magnifying both overall and local deflections. Both interlayers substantially reduce the strain energy release rates of interfacial cracks driven by residual stresses generated by foam core compression.

Low-Velocity Impact Response of Polyurethane Foam Composite Sandwich Structures

Low-velocity instrumented impact tests were carried out on sandwich panels made of glass fiber-reinforced plastic facesheets and polyurethane foam core. The tests were carried out using a drop weight instrumented impact tester, connected to a data acquisition system. Four different types of sandwich samples using polyester/e-glass and epoxy/e-glass facesheet materials and polyurethane foam were considered for investigation. Two different face sheet materials were chosen to experimentally examine the effect of their elastic modulus on the impact response of the sandwich structures. The data acquisition system records the impact data such as impact force, penetration time and depth of penetration, and plots impact force versus depth of penetration and penetration time versus depth of penetration curves. From the recorded data the impact parameters such as maximum impact force, penetration time and depth of penetration versus impact energy were plotted to study the impact behavior. The results show that higher impact energy is required to break the epoxy/e-glass facesheet and backsheet sandwich specimens than other types of specimens examined. The impact damage caused to the facesheet, the core and the backsheet were thoroughly studied experimentally and the extent of damage caused to the facesheets and the core were also compared by finite element analyses.

Temperature Effects on the Fatigue Behavior of Foam Core Sandwich Structures

The effects of temperature on the fatigue behavior of foam core sandwich structures have been investigated. Two types of PVC cores of the same nominal density were used; one was linear HD foam and the other a slightly cross-linked H foam. In both cases the face sheet material was S2-glass/vinylester. Fatigue tests were conducted on these sandwich structures at room temperature (RT), 40° and 80 °C. The tests were conducted on an MTS machine equipped with an environmental chamber. The stress ratio was set at R=0.1 and the frequency, f=3Hz. S-N diagrams were generated and revealed that fatigue life increased with decreased stress and decreased with increased temperatures. At RT, HD structures revealed much longer fatigue life compared to H structures however at elevated temperatures, a complete reversal of this trend was seen. Further, significant differences in the failure between H and HD structures were observed. Details of the experimentation, the analysis of the fatigue data, and damage mechanisms are presented.

Measurement and Simulation of the Performance of a Lightweight Metallic Sandwich Structure With a Tetrahedral Truss Core

Metallic sandwich panels with tetrahedral truss cores have been fabricated and their structural performance evaluated. A fabrication technique involving deformation-shaping and brazing has been used. The responses of the structure in core shear and panel bending have been measured. The results demonstrate robust behavior beyond the limit load. A finite element simulation of the core shear response duplicates the features found experimentally. When combined with the constitutive properties of the face sheet material, these shear characteristics have been shown to predict, with good fidelity, the limit load for panels in bending.