I. Surface materials
Most honeycomb structures used in aircraft construction have aluminum, fiberglass, kevlar® or carbon fiber surface materials. Carbon fiber surface panels cannot be used with aluminum honeycomb core materials because it causes corrosion of the aluminum. Titanium and steel are used for specialty applications in high temperature structures. The facing materials for many components, such as spoilers and flight controls, are very thin, sometimes only 3 to 4 thicknesses (meaning mm). Parametric reports have shown that these facing plates do not have good impact resistance.
Ⅱ. Core materials
2.1 Honeycomb core
Each honeycomb core material can have certain good properties. As shown in Figure 19, the most common core material used for aircraft honeycomb structures is aramid paper (Nomex®or Korex®). Glass fibers are used for higher strength applications.
Figure 19: Honeycomb core materials
-Kraft paper - Its strength is relatively low, used in large quantities for its good insulating properties and low cost.
-Thermoplastic - Its thermal mass insulates well, absorbs well or can be reset for orientation, moisture and chemical resistance. It is environmentally compatible, aesthetically pleasing and relatively low cost.
Aluminum - optimal strength, high weight-to-weight ratio and energy absorption, good heat transfer properties, electromagnetic shielding properties, easy processing, relatively low cost.
-Steel-Good heat transfer properties, electromagnetic shielding properties and heat resistance.
-Specialty metals (titanium)-with relatively high strength, weight ratio, good heat transfer performance, chemical resistance, and heat resistance to high temperatures.
-Aramid paper- With fire resistance, flame retardant, good insulation properties, low dielectric properties, and easy molding.
-Glass Fiber- Its has easy shear, low dielectricity, good insulation, and easy molding.
-Carbon fiber-its maintains the stability of carbon, high temperature, high stiffness, and very low coefficient of thermal expansion, easy to control the thermal conductivity, shear modulus is relatively high, but expensive.
-Ceramics - Its high temperature heat resistance is good, good insulation, and has a very small cell structure, but expensive.
Honeycomb cores for aerospace applications are usually hexagonal. These cores are made of specially positioned bonded stacked thin sheets. The stacked sheets are stretched into a hexagonal shape. Those that extend in the horizontal direction are referred to as strip direction.
A dichotomous hexagonal core has another layer of material cross-cut into each hexagon. A dichroic honeycomb is harder and stronger than a hexagonal core. An overstretched core is made by expanding the paper to create hexagons. Overstretched cores have a rectangular core. Overstretched cores are flexible perpendicular to the direction of the band, using simple curves. A bell-shaped core, or curved core, has a curved core material that makes it flexible in all directions. Bell-shaped kernel cores are used in the direction of panels with complex curves.
Honeycomb cores are available in different core sizes. Smaller sizes provide better strength for sandwich panel strength. Honeycomb cores also come in different densities. Higher density honeycomb cores are stiffer and stronger than lower density cores. As shown in Figure 20.
Figure 20: Honeycomb core
2.2 Foam
Foam core is used in residential construction and light aircraft to provide support and shape for wing tips, flight controls, fuselage sections, wings and wing ribs. Foam core is not commonly used in commercial airplanes. Foam is usually heavier and less robust than honeycomb cores. Various foams that can be used as core material include:
-Polystyrene (more commonly known as polystyrene foam)-Aerospace grade polystyrene foam with a tightly closed cellular cellular core structure with no voids between the cells; high compressive strength and good resistance to water penetration; can be cut with a hot wire and made into wing shapes.
-Phenolic - Good fire resistance, can have very low density, but its mechanical properties are relatively low.
-Polyurethane - Used in the production of fuselages, wingtips, and other curved parts for small aircraft; relatively inexpensive, flame-resistant, and compatible with most adhesives; polyurethane foams cannot be cut with a hot wire; easy to contour with large knives and sanding equipment.
-Polypropylene - Used to create winged shapes; can be cut with hot wire; compatible with most adhesives and epoxy resins; not for use with polyester resins, soluble in fuels and solvents.
-Polyvinyl Chloride (PVC) (Divinycell, Klegecell, and Airex)-It is a closed-cell medium to high density foam with high compression strength, durability, and excellent fire resistance; can be vacuum formed into composite shapes and molded using thermoforming; compatible with polyester, vinyl ester, and epoxy resins.
-Poly(methacrylimide) (Rohacell) - closed-cell foam for lightweight sandwich structures; excellent mechanical properties, stable at high temperatures, good solvent resistance, outstanding creep compression resistance; more expensive than other types of foams, but with superb mechanical properties.
III. Damage in manufacture and use
3.1 Manufacturing defects
Manufacturing defects include:
-Delamination (delamination)
-Areas of resin deficiency
-Areas of resin excess
-Blisters, bubbles
-Wrinkles
-Hollows
-Thermal decomposition
Manufacturing damage encompasses anomalies such as porosity, micro-cracking, and delamination caused by machining differences. It also includes such matters as unintentional edge cuts, surface gouges and scratches, damaged fastener holes, and impact damage. Examples of defects occurring during the manufacturing process include contaminated bonding surfaces or inclusions, such as prepreg liners or release films, which are inadvertently left between layers during the layup process. Unintentional (non-processing) damage to detail parts or components may occur during assembly, transportation or handling.
If too much resin is used in a part, it may be resin overloaded, which is not necessarily bad for non-structural applications, but it adds weight. If too much resin runs out during the curing process, or if not enough resin is applied during the wet layup process, the part is said to be resin starved. Resin-deprived areas are revealed by the fiber surface. A fiber to resin ratio of 60:40 is considered optimal.
Sources of manufacturing defects include:
-Improper curing or processing
-Improper processing
-Improper handling
-Improper drilling
-Tool drippings
-Contamination
-Improper grinding
-Unqualified materials
-Improper tools
-Bore shaft or detailing problems
In structural configurations of composites, damage can occur in several plies. This ranges from damage to the matrix and fibers to failure of broken elements and bonded or bolted attachments. The degree of damage controls the repetitive load life and residual strength and is critical to damage tolerance.
3.2 Fiber breakage
Fiber breakage can be critical because structures are typically designed to be fiber-dominated (i.e., fibers carry most of the load). Fortunately, fiber breakage is usually confined to the area near the point of impact and is limited by the size and energy of the impact object. Only a few service-related elements of the previous unit may result in extensive fiber damage.
3.3 Substandard matrix (inhomogeneous cell)
Matrix defects usually occur at the matrix-fiber interface or at the matrix parallel to the fibers. These defects slightly degrade some of the material's properties, but rarely have a critical effect on the structure unless matrix degradation is widespread.
Accumulation of cracks in the matrix can lead to degradation of matrix-dominated properties. For laminates designed to transmit loads with fibers (fiber-dominated), only minor degradation of properties is also observed when the matrix is severely damaged. Matrix cracking or microcracking can significantly degrade properties dependent on the resin or fiber-resin interface, such as interlaminar shear and compression strength. Microcracking can have a very detrimental effect on the performance of high temperature resins. Matrix defects may develop into delaminations, a more severe type of damage.
3.4 Delamination and De-sticking
Delamination forms at the interface between layers in a laminate. Delaminations may be formed by matrix cracks or low-energy impacts that extend from the base to the interlayer. Bonds can also be formed by the manufacturing process along the bondline between two elements and begin to delaminate (delaminate) in neighboring laminates. Under certain conditions, delamination or bonding can grow during repeated loading and can lead to catastrophic damage when the laminate is loaded. The criticality of delamination or bonding depends on:
-Dimensions.
-Number of delaminations at a given location.
-Location - in the thickness of the laminate, in the structure, near free edges, areas of stress concentration, geometric discontinuities, etc.
-Loadings - The behavior of delamination and bonding depends on the type of loading. They have little effect on the response of a tensile laminate. However, under compressive or shear loading, sub-layers adjacent to delaminated or peeled units may buckle and lead to load redistribution mechanisms that can lead to structural damage.
3.5 Damage combinations
Generally, impact events can cause a variety of damage. High-energy impacts from large objects (e.g., turbine blades) may result in component fragmentation or attachment failure. The resulting damage may include significant fiber failure, matrix cracking, delamination, fastener breakage, and stripped components. Damage from low-energy impacts is more easily controlled, but may also include a combination of fiber breakage, matrix cracking, and multiple delaminations.
3.6 Fastener hole defects
Improperly drilled holes, poorly installed fasteners, and missing fasteners may occur during the manufacturing process. During service, piece hole elongation may occur due to repeated loading cycles.
3.7 Defects in service
Service defects include:
- Environmental damage
- Impact damage
- Fatigue
- Cracks caused by localized overloads
- Debonding (gluing)
- Delamination
- Fiber rupture
- Corrosion
Most honeycomb core structures, such as wing spoilers, fairings, flight controls and landing gear doors, have very thin surface panels. Experiencing durability problems, they can be broadly categorized into three groups: low impact resistance, liquid ingress, and erosion (corrosion). These structures have adequate stiffness and strength, but are less resistant to service environments where parts are crawled over, tools are dropped, and service personnel usually do not realize the vulnerability of thin-skinned sandwich components. Damage to these components, such as core crush, impact damage, and dislodgement, is usually easily detected by visual inspection because of their thin surfaces. However, they are sometimes overlooked or damaged by service personnel who do not want to delay aircraft departures or draw attention to accidents that could affect their performance record. As a result, damage is sometimes left unchecked, often leading to increased damage due to liquid entering the core. Undurable design details (e.g., improperly cut honeycomb core edges) can also lead to the entry of liquid.
Restoration due to fluids getting into the part may vary from fluid to fluid, most commonly water or hydraulic fluid. Water tends to cause additional damage in repaired parts unless all moisture is removed from the part. Most restoration material systems cure at temperatures above the boiling point of water, which can lead to debonding at the skin-core interface, resulting in water pooling everywhere. For this reason, core cycle drying is usually performed prior to any restoration. Some operators take the extra step of drying damaged but not repaired parts in a high-pressure tank to prevent any additional damage from occurring during the repair. Hydraulic fluid is a different issue. Once the core of the sandwich panel is saturated, it is nearly impossible to completely remove the hydraulic fluid. Even during the curing process, the section will continue to leak fluid until the leaking contamination is completely removed. Removal of the contaminated honeycomb core and adhesive is highly recommended as part of the restoration. As shown in Figure 21
Figure 21: Damage to radome honeycomb sandwich structure
Composites are known to have a lower erosive capacity than aluminum, so they are often avoided for application to tip surfaces. However, composites have been used in highly complex geometries, but usually in conjunction with corrosion coating applications. Some corrosion coatings are not ideal for abrasion resistance or maintenance. Another problem, not as obvious as the first, is the erosion of the edges of doors or panels if they are exposed to air currents. This erosion may be due to design or installation (improper installation). On the other hand, metal structures in contact with or in the vicinity of these composite components may show corrosion damage due to improper selection of aluminum alloys, corrosive sealant damage to the metal components during assembly or splicing, insufficient sealant, or lack of fiberglass barrier at the interface of beams, ribs, and fittings. As shown in Figure 22
Figure 22: Corrosion damage to wingtip (tip)
3.8 Corrosion
Most fiberglass and kevlar® parts have an excellent aluminum mesh for lightning protection. This aluminum mesh often corrodes around bolt or screw holes. Corrosion affects the electrical bonding of the panel and requires removal of the aluminum mesh and installation of a new mesh to restore the electrical bonding of the panel. As shown in Figure 23
Figure 23: Corrosion of Aluminum Lightning Protection Grid
UV rays affect the strength of composites. Composite structures need to be protected from the effects of UV light with a top coat. Specialized UV primers and coatings have been developed to protect composites.