Sandwich Structures in Aircraft | Sandwich Structure materials & Uses

Sandwich-Structures in Aircraft

Sandwich-structured composites, an exclusive subcategory of composite materials, are made by bonding two stiff but thin skins to a light but thick core. Due to the increased thickness of the core material, which is frequently a low strength material, the sandwich composite has a high bending stiffness and a low density in general.

Both open- and closed-cell structural foams, such as those made of polyethersulfone polyvinyl chloride, polyurethane, polyethylene, or polystyrene, balsa wood, syntactic foams, and honeycombs, are frequently used as core materials. The honeycomb structure occasionally has different foams put in it for added strength. Both open- and closed-cell metal foam are suitable as core materials.

Description of Sandwich Structures

Theory A sandwich construction is a type of structural panel that, in its most basic form, consists of two parallel face sheets that are very thin and connected by a core that is relatively thick and light. The core prevents out-of-plane shear loads from acting on the face sheets and supports them against buckling. High shear strength and compression stiffness are required for the core. The most popular fabrication methods for composite sandwich construction are vacuum bag cure, press cure, and autoclave cure. Skin laminates may be co-cured to the core in one operation, precured and then bonded to the core, or a mix of the two techniques. Wing spoilers, fairings, ailerons, flaps, nacelles, floor boards, and rudders are a few examples of honeycomb structures.

Properties of Sandwich Structures

Comparing sandwich structure to aluminium and composite laminate construction, the composite structure has higher bending stiffness at a lower weight. The majority of honeycombs have directional features, or they are anisotropic. Using a honeycomb architecture has advantages. The stiffness of the honeycomb architecture is considerably increased by increasing the core thickness, although the weight increase is barely noticeable. The great stiffness of a honeycomb design eliminates the need for external stiffeners like frames and stringers.

Facing Materials

The majority of honeycomb structures used in the manufacture of aircraft contain face sheets made of carbon fibre, Kevlar®, fibreglass, or aluminium. Aluminum honeycomb core material cannot be utilised with carbon fibre face sheets because the aluminium will corrode. Specialty applications in high-temperature constructions utilise steel and titanium. Many components, including spoilers and flight controls, have very thin face sheets—occasionally only 3 or 4 plies. These face sheets do not have a good impact resistance, according to field reports.

Core Materials

The following types of core materials are used for the construction of sandwich structures:

Honeycomb

Each type of honeycomb material has particular qualities and advantages. Aramid paper (Nomex® or Korex®) is the most popular core material used for aircraft honeycomb structures. For applications requiring more strength, fibreglass is employed.

Kraft paper

Kraft paper Kraft paper offers strong insulation qualities, a low cost, a large supply, and a relatively low strength.

Thermoplastics

Thermoplastics are advantageous in terms of insulation, energy absorption and/or redirection, smooth cell walls, chemical and moisture resistance, compatibility with the environment, aesthetic appeal, and cost.

Aluminum

Aluminum has the best strength-to-weight ratio and energy absorption, is machinable, has smooth, thin cell walls, is electromagnetically shielding, and has superior heat transmission qualities.

Steel

Steel is heat resistant, has good electromagnetic shielding qualities, and good heat transmission properties.

Titanium

Titanium is a specialty metal with a relatively high strength-to-weight ratio, excellent heat transfer characteristics, chemical resistance, and heat resistance to extremely high temperatures.

Aramid paper

Aramid paper has excellent formability, low dielectric constant, good insulating and fire resistance.

Fiberglass

Fiberglass is excellent formability, low dielectric constant, tailorable shear properties by layup, and strong insulating qualities.

Carbon fiber

Carbon is very expensive and has several desirable properties, including good dimensional stability and retention, high stiffness, a very low coefficient of thermal expansion, tailorable thermal conductivity, and a reasonably high shear modulus.

Ceramics

Ceramics are highly pricey, have excellent insulating qualities, can withstand very high temperatures, and come in very small cell sizes.

Hexagonal honeycomb core cells are typically used in aerospace applications. By joining stacked sheets at specific spots, the cells are created. Hexagons are created by expanding the stacked sheets. Ribbon direction is the plane perpendicular to the sheets.

A second sheet of material is cut diagonally through each hexagon of the bisected hexagonal core. More rigid and durable than a hexagonal core is a bisected hexagonal honeycomb. The sheets are inflated beyond what is required to form hexagons, creating an overexpanded core. Overexpanded core cells are rectangular in shape. In panels with straightforward curves, overexpanded core, which is flexible perpendicular to the ribbon direction, is used. Curved cell walls in the bell-shaped core, also known as the Flexicore, provide it flexibility in all directions. Panels with intricate curves employ a bell-shaped core.

There are several cell sizes for honeycomb cores. Sandwich face sheets are better supported by smaller sizes. Also available in various densities is honeycomb. Stronger and more rigid than a lower density core is a higher density core.

Foam

On homebuilt and lighter aircraft, foam cores are utilised to offer wing tips, flight controls, fuselage sections, wings, and wing ribs strength and shape. The usage of foam cores in commercial aeroplanes is uncommon. Foams are often heavier and weaker than honeycomb. As a core material, a number of foams can be employed, including:

Polystyrene

Polystyrene, sometimes known as Styrofoam, is a type of foam that is used in aeroplanes. It has a tightly packed cell structure with no spaces between the cells, a high compressive strength, and good water resistance. It can also be cut with a hot wire to create airfoil forms.

Phenolic

Phenolic has excellent fire resistance and the potential for very low density, but it also has rather poor mechanical qualities.

Polyurethane foam

Using a big knife and sanding tools to readily contour polyurethane foam, which is used to make the fuselage, wing tips, and other curved elements of tiny aircraft. Polyurethane is also reasonably affordable, fuel-resistant, and compatible with most adhesives.

Polypropylene

Airfoil shapes are made of polypropylene, which can be cut with a hot wire and is compatible with most adhesives and epoxy resins. Polyester resins should not be used with polypropylene, which also dissolves in fuels and solvents.

PVC (Divinycell, Klegecell, and Airex)

PVC (Divinycell, Klegecell, and Airex) is a closed-cell medium- to high-density foam that can be bent with heat or vacuum moulded into complex shapes. It has excellent fire resistance and is compatible with resins made of polyester, vinyl ester, and epoxy.

Closed-cell foam made of polymethacrylimide (Rohacell

Closed-cell foam made of polymethacrylimide (Rohacell), which is more expensive than other types of foam but has better mechanical properties, is used to make lightweight sandwich constructions. It has high dimensional stability under heat, good solvent resistance, and outstanding creep compression resistance.

Balsa Wood

Elongated closed cell natural wood product known as balsa is offered in a number of grades that correspond to its structural, aesthetic, and physical features. Balsa has a density that is less than half that of typical wood products. However, compared to other kinds of structural cores, balsa has a significantly higher density.

Application of sandwich structures

Sandwich panels, such as FRP sandwich panels and aluminium composite panels, among others, can make extensive use of sandwich structures. FRP polyester reinforced composite honeycomb panel (sandwich panel) is created using multi-axial high-strength glass fibre, PP honeycomb panel, and polyester reinforced plastic in an unique antiskid tread pattern mould under constant temperature vacuum.