Core Material For Sandwich Structures Research Articles

Simple stiffness tailoring of balsa sandwich core material

A concept for improving the shear stiffness properties of balsa core material for sandwich structures is presented. The concept is based on utilization of the strongly orthotropic properties of the balsa wood, applying an appropriate transverse layup sequence. The effective core material shear modulus is modeled using basic laminate theory. This is subsequently validated through sandwich beam bending and lap shear experiments. Compared to the standard balsa core systems, a substantial increase in the shear stiffness is demonstrated, whereas the transverse stiffness is reduced. The concept is suitable for mass production, using standard plywood fabrication technology.

Characterization of Integrated Functionally Gradient Syntactic Foams

Light weight high strength composites can be obtained by reinforcing resin with fillers such as hollow or solid particles and fibers. Composites were fabricated using microballoons (hollow particles) called syntactic foams. These foams can be used in various low density applications such as buoyancy aid materials for deep sea exploration and aerospace vehicles. These foams are usually utilized as light weight core materials for sandwich structures. The present study explores the procedure to fabricate functionally gradient syntactic foams (FGSFs) and further analyze their mechanical properties. The FGSFs produced are gradient structures consisting of four layers with four different types of microballoons, namely, S22, S32, S38, and K46, each having different wall thickness. The volume fraction of all microballoons is maintained constant at 60% to maintain light weight structures. Several FGSF specimens having similar density are fabricated with different layer arrangements. The different layers are integrated before major solidification takes place. Quasistatic compression testing is then performed on the cured FGSF samples using MTS-810 servohydraulic machine. Compressive strength and energy absorption values for each arrangement are compared. The stress plateau in integrated FGSF composites extends from 10% to 60% strain compared with plain syntactic foams. The integrated FGSF shows increment in yield strength and energy absorption compared with adhesively bonded FGSF. It is found that the compressive strength and energy absorption of integrated FGSF composites can be varied based on arrangement of the layers.

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

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.

Water immersion effect on swelling and compression properties of Eco-Core, PVC foam and balsa wood

Syntactic foam, balsa wood and PVC foams are commonly used as core materials in sandwich structures for weight critical applications such as aircraft and ship structures. Water absorption is highly undesirable in these applications. The present study evaluates the effect of water immersion on three types of core materials, namely, Eco-Core, balsa wood and PVC foam. Eco-Core is a new fire resistant core material under development that utilizes about 83% by weight of fly ash. Designers of naval ships and aircraft commonly specify balsa wood and PVC foam as core materials for sandwich structures. These three core materials were subjected to water immersion to determine the relative resistance to property change. Both tap water as well as seawater was used. Core samples were studied for dimensional change, weight gain and compression properties after water immersion and the results were compared with the test results of dry core samples. Time periods included in the study ranged from 4 h to 500 days. The results showed that Eco-Core is as good as PVC foam in resisting swelling, water absorption and changes in compression properties due to water immersion. Where as balsa wood showed a significant swelling, water absorption and deterioration of compression properties.

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.

Criteria for Yielding or Failure of Cellular Materials

Sandwich structures are used extensively in marine structures and other applications. Cellular materials such as foams, honeycomb, and balsa wood used as core materials in sandwich structures are subjected to multiaxial loadings. Concentrated loads, impacts, and explosions can result in large deformations and damage, and yield or failure criteria are needed to predict the end of the elastic phase. Yielding or failure of isotropic foams is generally dependent on the hydrostatic stress, a feature that these materials share with many polymers, concrete, and soils. Appropriate criteria which are different from those traditionally used for metals have been classified in four major categories. Similarities between seemingly different criteria and between foams and other materials are brought out. Foams can also be anisotropic as honeycomb and balsa wood, and the anisotropy has to be accounted for in addition to the differences in strength in tension and compression in various directions. Suitable anisotropic criteria are shown to be the extensions of criteria previously used for isotropic materials and methods to present these criteria in a unified manner or to systematically extend isotropic criteria are discussed. Finally the strain rate effects and size effects are discussed.

Experimental evaluation of a balsa sandwich core with improved shear properties

End grain balsa is widely used as a core material in sandwich structures, especially in marine applications where the shear properties are of major importance. In a recent study, a new balsa core structure that better utilizes the anisotropy of balsa was designed and numerically analyzed. An improved shear stiffness was predicted with this new core. In the present study, manufacturing methods for this core were developed and more extensive experiments were performed to verify the improvement in shear stiffness as well as to evaluate the shear strength.

Analyses and preliminary tests of a balsa sandwich core with improved shear properties

End grain balsa is a common core material in sandwich structures. The main properties of interest of a sandwich core, in particular for marine applications, are the shear stiffness and shear strength. The end grain arrangement does not fully utilize the anisotropy of balsa wood for the benefit of shear properties. A new arrangement of balsa blocks in a sandwich core was developed and numerically analyzed. This new core, which consisted of an assembly of oddly shaped balsa blocks, allowed the grains in various balsa blocks to be oriented for example at 45° to the normal of the core. The effective out-of-plane shear stiffness was isotropic. Numerical analyses predicted a superior shear stiffness, which was confirmed by preliminary tests.

Fatigue in sandwich structures loaded in transverse shear

The main purpose has been to evaluate the fatigue properties for crosslinked PVC foams with closed cells which are used extensively as core material in sandwich structures for marine applications. Different fatigue crack propagation criteria were investigated from four point bending fatigue tests on sandwich beams. This was done by placing a crack in the midplane of the beam in a Finite Element model, and determining the stress intensity factors for mode I and mode II, as the crack propagates. The criteria were: the maximum hoop stress criteria, the minimum energy criteria and the maximum energy release rate criteria. As a result, the maximum hoop stress seems to be suitable for cross-linked PVC foams.