Web-core Sandwich Panels Research Articles

The Effect of the Secondary Bending Moment on the Fracture Strength Evaluation of the Laser Welded Joints from a Web Core Sandwich Structure

During the last period the interest on the sandwich structures has became more favorable due to the strength to weight ratio. In the same manner, in ship building field the lightweight structures became more and more attractive. With increasing the usage fields has increased the need to study the behavior of these structures. In general all the sandwich structures loaded in bending shows an effect of the secondary bending moment. In the case of web core sandwich panels used in ship structures has been observed a pronounced effect of the secondary bending moment on laser welded joints. Considering this, the paper presents an analysis of the fracture strength of laser welded joints of a web core sandwich structure, due to the effect of secondary bending moment. In the first part, the paper analytical formulation of the secondary bending moments and their effect on welded joints. This effect is explained on the basis of angle α defined in the paper and which depends on the thickness of the face plate, the thickness of the web plate and respectively the height. The paper continues with a numerical analysis of the stress and strain state from a web core sandwich beam and where also it is analyzed the effect of the secondary bending moment on the fracture strength of laser welded joints. Based on the carried out study it was observed that for high thickness of the web plate the effect of secondary bending moments is the overloading of the welded joints, instead for small thickness of the web plate the effect of the secondary bending moments can be of the unloading the welded joints. However, a small thickness of the web plate can affect the rigidity of the structure. Therefore, based on this study was proposed a solution to reduce the secondary bending moment without reducing the stiffness of the sandwich panel. The analysis conducted in this paper can be a design criterion for the web core sandwich structures.

Deflection of a Web-Core Sandwich Beam in Weak Direction

The method for calculating the deflection of a web-core sandwich panel in its weak direction is based on com- plex shell and plate theory. A simple method was presented by assuming inflection points are located at the mid-span of the facing plate segment between two adjacent web cores. It also assumes that an inflection point also exists at the mid- height of the web core. Based on these assumptions, a very simple equation was proposed. However, these assumptions are necessary to be evaluated for its suitability since the geometry of a web-core sandwich can be much different in theo- retical design. This study then carries out a parametric study to find the validity range of the presented equation for calcu- lating the deflection of a web-core sandwich panel in its weak direction. In the parametric study, finite element method is used to analyze such deflection. The parametric study shows that the accuracy of the presented equation is acceptable when the flexural stiffness per length of the web core is not less than that of the facing plates. This conclusion can be con- sidered in practical design stage.

Experimental validation of the modified couple stress Timoshenko beam theory for web-core sandwich panels

The paper presents experimental validation of the modified couple stress Timoshenko beam theory for web-core sandwich panels. The face and web-plates are assumed to be isotropic and to behave according to the kinematics of the Euler–Bernoulli beam theory. First, a modified couple stress theory for Timoshenko beams is reviewed. Then shear, bending and couple stress responses of homogenized web-core sandwich beams are examined. The developed theory is validated with experiments from open literature for beams in 3- and 4-point bending. The beams have 4, 9 and 15 unit cells along their length. It is seen that the developed theory is in excellent agreement with experiments. It is also seen that the theory converges to physically correct solutions in the cases of infinite and zero shear stiffness; while the first corresponds the case of Euler–Bernoulli sandwich beam, the second corresponds the case where the sandwich effect is lost and the bending is carried out purely by the bending of the faces. The paper also gives explicit expression for the couple stress stiffness in terms of unit cell dimensions and materials.

BUCKLING OF A PLATE ON A PASTERNAK FOUNDATION UNDER UNIFORM IN-PLANE BENDING LOADS

In-plane bending loads occur in many thin-walled structures, including web core sandwich panels (foam-filled panels with interior webs) under transverse loading. The design of such structures is limited in part by local buckling of the thin webs and the subsequent impact on stiffness and strength. However, the core material can have a significant impact on web buckling strength and thus must be considered in design. This paper presents solutions for the buckling strength of simply supported plates under in-plane bending loads. The location of the neutral bending axis is allowed to vary and is characterized by a load parameter. A Pasternak model is used to account for the resistance of the foundation to compression and shear. Using the principle of minimum potential energy, buckling solutions are developed for infinitely long plates and representative foundation materials. The solutions match known results for two special cases: Uniform loading with variable foundation, and bending loads with no foundation. An order of magnitude increase in buckling strength is possible, depending on loading and foundation stiffness. The results suggest an important avenue for future development of lightweight structures, including sandwich panels and structures such as plate girders that are not typically associated with the use of foam filling.

Local shear buckling and bearing strength in web core sandwich panels: Model and experimental validation

A model and experimental validation of shear buckling and local bearing failure of web core sandwich panels are presented. Of particular interest are steel-faced panels with stiffening metal webs and a polymer core. The metal webs provide the required panel stiffness and the foam core serves the dual purposes of preventing local buckling and providing thermal insulation. In applications, such as the building sector, in which thermal performance is crucial, the webs are thin and widely spaced to reduce conduction between the face sheets. The models of shear buckling and bearing failure account for the influence of the core material on web strength and provide closed-form solutions. The models are validated by symmetric four-point bending tests to evaluate shear buckling and asymmetric three-point bending for bearing failure. The shear buckling model predicted buckling strength to within 4% of the test results. The bearing failure model overpredicted the observed strength by 11% on average, similar to test results reported in the literature for the bearing strength of webs with no foam support.

Measuring thickness changes of edgewise compression loaded corrugated board panels using digital image correlation

This study examines thickness changes in web-core sandwich panels under edgewise compressive loading. Both undamaged and damaged panels were examined. Three-dimensional full-field digital image correlation systems were used to determine deflections on both sides of loaded panels. The change in thickness at any given point in the panel was obtained as the difference between the two displacement fields. It was observed that the thickness was reduced in the post-buckling regime. Damage introduced into the corrugated core by lateral compression proved to significantly reduce the load-carrying capability panels and elevate the thickness reduction of the panels.

Design Procedure for Web Core Sandwich Panels for Residential Roofs

Panelized construction of residential buildings is gaining popularity due to the architectural and energy efficiency benefits that can be achieved. An important challenge to the design of panel structures for buildings is the balance between long-term structural performance and the thermal insulating requirement. In this study, foam core and web core panels are designed for residential roofs. Both panels are comprised of two face sheets and an insulating foam core. In the web core panel, thin metal webs that connect the face sheets are added to improve panel shear stiffness and enable longer spans. A design procedure is developed that considers R-value, panel deflection, core shear failure, bearing failure, and buckling of the face sheets and webs. The buckling model includes the ability of the foam core stiffness to restrain the buckling deformation. Panel designs are presented that provide R-5.3 m2K/W for roof loads of 1500, 2000, and 3000 N/m2, corresponding to climate zones in the US. It is demonstrated that the web core panel can be designed for these structural and thermal requirements with unsupported span lengths as long as 7 m, while span lengths for foam core panels are limited to 4 m. Web shear buckling and R-value are the two performance criteria that limit panel span length and depth.

Energy Absorption Characteristics of Web-Core Sandwich Composite Panels Subjected to Drop-Weight Impact

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.

Minimum weight web-core sandwich panels subjected to uniaxial compression.

Analytical methods are developed by which web-core sandwich panels can be designed with minimum weight while retaining their structural integrity (i.e., structurally optimum) for a given load index, panel length and width, and specified face and web materials. These methods also provide a means for rational material selection through the comparison of weights of the minimum weight construction for various material system as a function of load index, and an assessment of weight penalties associated with the use of commercially available thicknesses. The methods account for both isotropic and orthotropic face and core material as well as various boundary conditions. Numerical results clearly show the weight savings associated with the use of second generation composite materialssuch as boron-epoxy and graphite-epoxy.