Sandwich materials consisting of a low-density core and stiff face sheets offer significant potential for weight savings in panel applications, Where the main loads are based on bending. While Sandwich materials of interest for car and van body panels, seat shells, etc. include steel/plastic laminates, integrally foamed plastic shells, and fiberglass-reinforced polyester shells with foamed plastic cores.
We will discuss basic design formulas for the flexural stiffness and strength of such Sandwich materials and gives a method for designing optimum Sandwich structures at the lowest weight or cost.
Data are presented on the mechanical properties of several Sandwich materials that may be of interest for automotive trim applications.
It is then shown how the application of the weight-optimized design method enables the determination of core and skin thicknesses and provides a means of improving the flexural properties of existing Sandwich structures.
Fabrication And Properties Of Natural Fiber-reinforced Polyester Composites.
Attempts have been made to find new uses for natural fibers, a renewable resource that is otherwise underutilized. The structure and properties of the fibers, as well as the manufacture and physical and mechanical properties of their polyester-based composites, are described. The performance of these composites is evaluated after indoor and outdoor weathering by destructive and non-destructive testing methods.
The fabrication of various consumable items such as a voltage stabilizer cover, a mirror housing, a projector cover, and a canopy is also reported. This study highlights the potential of natural fibers for non-conventional applications and points out some of their limitations.
This review outlines the development of Sandwich panels based on recent work and older sources, focusing on trends in Sandwich panel achievements and applications:
- Core materials
- Core designs
- Types of failure mechanisms
- Factors contributing to Sandwich panel failure.
The review begins with a detailed discussion highlighting the accomplishments and trends related
to Sandwich panels over the past 50 years, including recently published work. The purpose of this paper is to reevaluate the current core design of metal-based Sandwich panels and to further explain the core design, core materials, and types of failure mechanisms that occur in Sandwich panels under certain conditions.
The main factors contributing to the failure phenomena of Sandwich panels, such as the geometries of the core design. A different configuration of the core design, the adhesive interaction effect between the interconnecting layers of Sandwich panels, and Sandwich panels’ effect under high-speed impact and blast loading are considered.
Future issues related to metal-based Sandwich panels, including the use of new materials for the core,
new concepts in the core design, and the possibility of extending Sandwich panels for heavier applications, such as in the defense industry, are highlighted and discussed. At the end of this review, the authors draw attention to other researchers by suggesting a list of topics that need to be addressed by researchers soon.
Joining Metal And Polymer Sheets In Sandwich Panels For Highly Improved Interface Strength
Wire mesh interlayers are used to fabricate metal-polymer-metal Sandwich panels with high interfacial strength. While the metal sheets are joined to the wire mesh by resistance welding at predetermined locations, the polymer is introduced into the wire mesh by vacuum hot pressing. The Sandwich panels’ peel strength is about 300% higher than that of the bonded joints described in the literature. Since the interfacial strength is achieved by welding and mechanical interlocking, it does not deteriorate in the presence of moisture.
Uniaxial tensile tests are used to characterize the strength and strain of the individual layers and the fabricated Sandwich panels. Interfacial strength is characterized by peel and double lap shear tests. V-bend and stretch-forming tests are used to demonstrate the deformability of the developed Sandwich panels. The presented method is an attractive alternative to the use of adhesives for joining metals with polymers.
Composites are preferred in various applications due to their advantages over conventional materials:
- Enhanced mechanical
- Electrical properties
Sandwich panels are typically composites of two thin face sheets and core material with comparatively higher thickness and lower density. Sandwich panels provide higher bending stiffness with lower areal density. The lower areal density increases fuel efficiency and the insulating core helps reduce heat conduction through Sandwich panels. The core also acts as a damper and provides better acoustic properties.
These properties have increased their demand in roof structures, automobiles, refrigerated vehicles, boats, aerospace applications, etc. Sandwich panels, which have satisfactory formability and flexural properties, are also used in railway carriages, automobile parts, partition walls, compartment doors, etc.
Outer skin sheets of Sandwich panels are usually designed to resist normal and bending loads, while the core material is designed to resist shear loads.
Steel and aluminum alloys are usually used for the face sheets, while low-density wood, metal foams, and polymers are used for the cores.
Large differences in the skin and core’s mechanical properties lead to unequal deformations in these layers, which can cause delamination. Therefore, interfacial bonding between skin and core plays an important role in the manufacture of Sandwich panels. Usually, skin and core are joined by rolled or glued joints.
The bond strength of the metal-polymer interface bonded with epoxy adhesives deteriorates over time, usually due to moisture in the environment.
Once a crack develops in the adhesive, delamination occurs within a short period of time.
In most metal-polymer-metal Sandwich panel manufacturing processes, adhesives are used to bond metal and polymer layers.
The roughness of the metal sheets is increased by sandblasting to increase
the adhesive contact area and provide some mechanical interlock. Adhesive strength is also increased by chemical treatment, e.g. plasma treatment, primer coating, etc.
Mechanical inserts are sometimes provided to prevent delamination, but these affect the Sandwich panels’ deformability and lead to stress concentrations. Sandwich panels with wood or metal foam cores cannot be deformed due to these materials’ brittleness. Polymer core is preferred over others because of its ductility, corrosion resistance, lightness, low cost, durability, and damping properties.
Multiple layers of metal and fiber-reinforced plastic sheets with small thickness joined with adhesives to
achieve high strength are called fiber-metal laminates (FMLs). Among FMLs, aramid-reinforced aluminum laminate (ARALL), which is a composite material
made of aluminum and metal fibers, was developed in 1982.
Aluminum sheets bonded with glass fibers using epoxy resin (GLARE) and with carbon fibers (CARALL) are used in aircraft body construction. Hylite aluminum polymer laminates and Lite cor steel polymer laminates bonded with epoxy resins can be deep drawn and bent. However, FMLs take a long time to manufacture due to curing and are quite expensive.
Besides, their interfacial strength degrades with time.
Here, a new method for fabricating metal-polymer-metal Sandwich plates with higher interfacial strength is proposed. In this fabrication method, a wire mesh is used at the metal-polymer interfaces.
The wire mesh is bonded to the steel skin by resistance welding, while mechanical interlocking (filling the gaps in the wire mesh with polymer to obtain a void-free mesh-polymer core), achieved by vacuum hot pressing, bonds the wire mesh to the polymer layer. Surface treatments necessary for bonding, such as surface roughening, chemical etching, primer coating, etc., are not required here.
Unlike adhesive bonding, there is no weakening of the bond strength due to moisture or water. The formability of the developed Sandwich panels is proven by stretch forming and V-bending tests.