COMPOSITE MATERIALS HANDBOOK VOLUME 6. Structural Composite Sandwich
2.3 CORE MATERIALS
Cores are usually one of 3 types: honeycomb, foam, or balsa wood. Each has their own pros and some obvious cons
Testing methods for properties
In selecting a core, the maximum service temperature, flammability, moisture pickup, corrosion resistance, impact resistance, and heat transfer all can be important depending on the application. The shear properties for honeycomb cores are different in the L (ribbon) and W (transverse) directions depending on the cell geometry. For hexagonal honeycomb core, the L shear properties are about two times the W shear properties. The honeycomb core shear strengths also vary with core thickness
3.2 FACE SHEETS
The flatwise tensile test consists of cutting a small specimen from the panel, usually 2" by 2" (50 mm
by 50 mm), and bonding it to metal blocks. The specimen is then fixtured into a test machine and pulled apart with the maximum load and mode of failure recorded. The failure mode is extremely important as it can tell if the panel was made properly. The modes of failure are the following: core tearing, cohesion failure of the core-to-face sheet adhesive, and adhesion failure (either as adhesion failure between the core and interfacial adhesive, adhesion failure between the face sheet and interfacial adhesive, or adhesion failure between the core and face sheet for self-adhesive face sheets). If the bond between the block and the panel fails, this is not considered a valid failure and the test should be repeated. If the failures are adhesion to core or face sheet, this suggests that the core or face sheet is contaminated and the cleaning process should be reviewed. In some cases a core failure may simply not be achievable due to extremely high-strength core or in cases where the test temperature has exceeded the practical use temperature for the adhesive.
The climbing drum peel test consists of peeling off one face sheet from the panel. This test only
works well on relatively thin face sheets, and the modes of failure are the same as above. For thicker
face sheets, the cleavage test may give better results, as the face sheet does not have to be bent around drum.
The core will also be affected by the same environmental conditions, as
will the core-to-face sheet bond. In general, high temperature, moisture, and fluids all degrade the sandwich properties. Common problems with sandwich structures subjected to temperature and moisture/fluids are those associated with poor core sealing and porous or easily damaged face sheets. Face sheets that have been damaged can provide moisture paths to the core, which then may become degraded.
The following tables are general properties of Nomex, Aluminum Honeycomb, and foam core
If properties at a certain density are known, a linear extrapolation can be used to obtain estimates of properties at a different density. The following equation can be used to estimate the density of a metallic honeycomb core:
where wc is the core density, w0 is the material density, t0 is the thickness and s is the cell size (Diameter of inscribed circle).
The modulus of elasticity of the core can be approximated by the ratio of core density to material density times the modulus of elasticity of the material.
3.2 FACE SHEETS
the face sheet is incorporated into the sandwich structure resulting in three basic categories:
Adhesively bonded pre-fabricated face sheets
Pre-cured sheets that are bonded using some kind of adhesive, common with metallic face sheets. The adhesive used is generally a film adhesive but may also be a paste adhesive.
Co-cured or co-bonded face sheets with adhesive
Sandwich co-cure is defined as an approach where the adhesive and both face sheets are assembled to the core in the uncured state and the adhesive and face sheets are cured in a single cure cycle.
Sandwich co-bond refers to an approach where one of the face sheets is pre-cured and the other face sheet is cured simultaneously with the adhesive that bonds the core to that face sheet (and even perhaps the adhesive that bonds the core to the pre-cured face sheet)
Self-adhesive face sheets (prepreg with no separate adhesive or liquid molding construction)
Self-adhesive face sheets refers to a category of composite face sheet materials that are assembled in their uncured state with the core during layup operations, relying exclusively on the face sheet resin material to bond the face sheet to the core. As implied by the category title, no separate adhesive is used to establish face sheet-to-core bonding.
3.3 ADHESIVES
Film Adhesives
semi-solid adhesive with fibrous reinforcement. This form is easy to handle
and control during lay-up, and it provides a controlled thickness or amount of adhesive during lay-up. In terms of structural performance, film adhesives with knitted carriers can provide higher peel strength in a sandwich structure, while film adhesives with a mat carrier limit the co-mingling of the prepreg resin with the film adhesive, and may result in lower peel strength.
Paste Adhesives
Paste adhesives are a family of adhesive materials that are provided in a semi-solid state and typically procured as either a single-part or two-part compound. generally not used for bonding face sheets to core
Liquid Resins
Liquid resins may be used to bond face sheets to core materials during liquid resin molding processes, provided that the core is relatively solid and non-porous such as balsa and foam materials.
Foaming Adhesives
Foaming adhesives are used to join core sections when the size of the part exceeds that which is available in standard core sheet stock sizes. Foaming adhesives are also used for bonding replacement sections of damaged core for repairs
3.3.4 Adhesive Chemistry
Epoxy
Epoxies offer relatively high strength and modulus, low levels of volatiles, low cure shrinkage, good chemical resistance, ease of processing, and excellent adhesion to a range of substrate materials. Among the disadvantages of epoxy adhesives are the mixing requirements (two-part systems), limited pot life (both two-part paste and one-part films), relative brittleness, and a reduction of properties with continued exposure to moisture. The processing or curing of epoxies is often slower than polyester resins and the cost of the resin is also higher than the polyesters
Bismaleimide
Bismaleimide (BMI) resins, so called because of the two maleimide chemical moieties responsible for cure conversion, are a class of addition-type polyimide thermosetting resins. They are used most often in high temperature applications due to their excellent physical property retention at elevated temperatures.
Phenols
a.k.a. phenolics
excellent fire resistance, high-temperature performance, long-term durability, and resistance to hydrocarbon and chlorinated solvents
broadly used in the walls, ceilings and floors of aircraft interiors, so that passengers may have increased evacuation time during an airplane fire
among the disadvantages of phenolics are their relative brittleness, volatiles generated during cure for some types, and modest health and safety issues for some formulations
Polyester
Class of thermosetting resins that are relatively inexpensive and fast
processing compared to epoxies. Polyester resins have good fatigue resistance, UV stability, and retain good performance in the presence of moisture. Polyester resins have excellent adhesion to glass fibers, but lack equivalent compatibility with carbon fibersDue to the free-radical cure of the polyesters, the presence of oxygen can inhibit cure conversion, especially at the surface, resulting in a tacky surface if exposed to air during cure. In general, polyester resins are used for lower-cost applications and are preferred when quick processing is needed.
Polyimide
Polyimide matrix composites are used in high temperature applications where their thermal resistance, oxidative stability, low coefficient of thermal expansion, and solvent resistance justify their higher cost and processing difficulties
Polyimide resins typically require cure temperatures in excess of 550°F (290°C),
consequently demanding special, higher-temperature bagging films, bleeder and breather cloths, and steel (or other) tooling that can accommodate the higher processing temperatures; standard lower cost nylon bagging films and polytetrafluoroethylene (PTFE) release films used with epoxies will not survive the processing temperatures required for polyimide resins.
4.4 SANDWICH PANEL FAILURE MODES
Some of the more important issues specific to sandwich design are:
• Core shear
• Core crushing
• Core buckling
• Face sheet dimpling
• Face sheet wrinkling
• Face sheet buckling
• Strength of core-to-face sheet attachment
• Hardpoints (inserts and attachment points)
• Ramps (areas of transition from sandwich to solid laminate)
Face Sheet Failure
Occurs when one or both face sheets exceed their allowable stress or strain, leading to yielding or fracture.
Core Shear Failure
Happens when the core fails in shear, often forming 45° cracks or cell wall buckling under transverse loads.
Core Crushing
Compression failure where face sheets move toward each other because the core lacks sufficient compressive strength.
Core Tensile Failure
Occurs when the core fails in flatwise tension due to low tensile strength through the thickness.
Face Sheet-to-Core Debonding
Separation between face sheet and core caused by insufficient shear, peel, or tensile bond strength.
Local Indentation
Failure under concentrated loads when local stresses exceed the core’s compressive strength; avoided by spreading loads.
Face Sheet Wrinkling
Local buckling of thin face sheets, often with core crushing or debonding, common in low-density cores.
Face Sheet Dimpling (Intracell Buckling)
Buckling of a thin face sheet within individual core cells; occurs with large cell size and thin skins.
General Buckling
Overall panel buckling resembling plate or column instability, with face sheets and core still intact.
Shear Crimping
Local instability where core shear failure and lateral face sheet displacement occur, with buckle wavelengths similar to cell size.