Cores & inserts - A202

Cores & inserts
Document Type	Article
Document Identifier	202
Relevant Class	Material
Tags	Sandwich Panels Inserts

Introduction

Composite structures frequently utilize lightweight core materials within components for a variety of reasons. The most popular use is to construct sandwich panels. The core material increases the moment of inertia by spacing the high strength material away from the neutral axis where the tensile or compressive loads are high in bending. Core materials can also provide a layer with high permeability to assist in resin flow during a liquid moulding process. Common core materials are wood, metallic and non-metallic honeycomb, and open and closed cell foams.

Scope

This section will discuss the manufacturing processes and common use for honeycomb, wood, and foam core materials. The core material properties and the selection of the core material when designing a composite product are also discussed. A list of core materials and insert suppliers are included.

Significance

Sandwich panels are widely used in the aerospace sector and increasingly in transportation, energy, marine sectors, etc. It is crucial to understand the properties of the core materials in order to select the right material and design and manufacturing sandwich panels that meet performance requirements.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Thermoset polymers - A105
• Thermoplastic polymers - A161
• A144
• A145
• A146

Core material types

Wood

Common wood core materials include balsa, mahogany, spruce and poplar. While any type of wood can be laminated between two face sheets and form a sandwich structure in principle, balsa (Ochroma pyramidale) wood is the most common wood core material in composites applications due to its low cost, low density, relatively high mechanical properties and other properties. Balsa wood is a cellular material consisting of fibers (66-76%), rays (20-25%) and vessels (3-9%) . To be used as a core with composite materials, balsa wood is cut transversely to its grain direction, and re-joined (by adhesive) side by side such that the grain direction is perpendicular to the core surface; therefore called end-grain balsa. [pic] Density of end-grain balsa core can range from 100 to 300 kg/m^3

Eng-grain balsa

Aside from the advantages mentioned above, end-grain balsa cores have high compression and shear strength, good fatigue performance and are easy to machine. Depending on the face sheet material and manufacturing process, faces sheets can also be directly laminated onto the core without using adhesives. Disadvantages of end-grain balsa cores include low conformability, high variability in properties (since the core is made from smaller blocks bonded together) and high moisture sensitivity. Instead of bonding the small blocks side by side using adhesive, balsa can be produced in flexible sheet form where the small blocks are held by a fabric scrim backing, allowing the core to conform to curvatures to some extend. Moisture sensitivity can be mitigated if the core is properly sealed.

End-grain mahogany or spruce have also been used as high strength inserts to close out the sandwich panels. However, aluminum extrusions are more recent replacements. In general, the thermal conductivity of wood along the grain direction is higher (about 2.5 times) than that in the direction parallel to the grain.

End-grain cores are widely used in marine, sporting goods applications. Major manufacturers of balsa core include:

• Alcan Baltek Corp
• DIAB
• Nida-Core

Honeycomb

Please visit Honeycomb

Foam

Please visit Foam

Core material properties

General properties

The density of core are typically measured in pounds per cubic feet (lbs/ft³) of grams per cubic centimeter (g/cm³).

Mechanical properties

Mechanical properties of the core heavily depend on the core material and density. For honeycomb cores, cell size, configuration and manufacturing process can also affect the core mechanical properties. Mechanical properties for honeycomb cores are usually directional.

Sandwich panels under bending can fail by face sheet yield, face sheet buckling (wrinkling), core shear failure, or face sheet indentation. Depending on the failure mode, Young's modulus, shear modulus, yield strength and shear yield strength of the core or a combination of these mechanical properties can play roles in the failure of sandwich panels. Values for the compressive modulus and strength are typically provided by suppliers. In general, honeycomb has better shear and compressive strength and modulus than foam, see table

Thermal properties

Depending on the functions of the sandwich panel or the stand alone core, the specific heat C_p and thermal diffusivity of the core should be taken into consideration when selecting a core material. The core, face sheets and adhesive must also have compatible coefficient of thermal expansion to withstand thermal cycles during service and provide dimensional stability.

Other properties

Other properties to consider when selecting a core material include:

• Chemical and corrosion resistance
• Moisture resistance
• Flammability
• Acoustic damping
• EMI/RFI shielding

Core material processing

Cleaning and drying

Core materials should be clean and free of contamination or solvent residual prior to be bonding or assembled into sandwich panels. Contamination that occurs during transportation, receiving, storage, forming or machining can heavily impact the bond strength. Dust from machining, for example, can be wiped with clean cheese cloth or blown by compressed air. Grease or wax on metallic honeycomb can be cleaned by solvent vapor or bath. Depending on the core material and contamination type, different cleaning methods may be employed.

Moisture within the core prior to bonding can have detrimental effects on the core-face sheet bond quality. During thermal transformation, the steam pressure can result in blisters between the core and face sheet or in the case of open cell cores (such as honeycomb), damage or destroy the core. To remove moisture, cores can typically be dried in an oven. Depending on the core material, the drying temperature can range from 60 °C to 121 °C (140 °F to 250 °F). Drying time is usually 120 minutes at minimum. Storing conditions should also be specified to avoid moisture absorption. During material deposition, the core laid into a tool should be covered with adhesive film or prepreg within 24 hours from being taken out of the storage. Exposed core should be cover with vacuum bags sealed to the tool.

Forming

Curvature and geometrical complexity can be formed into cores prior to the material deposition step. In general, the denser and thicker the core, the harder it is to form. Some common core materials and their forming methods are:

• Metallic honeycombs can be mechanically formed with or without heat
• Non-metallic honeycombs are typically thermal formed (heat)
• Foam core are typically thermal formed (heat)
• Wood core can be formed with heat and moisture. Wood becomes pliable after soaking in hot water. Wood can then be formed into shape and dried

Thermal forming involves heating the core to the forming temperature then forcing the core into the desired shape (with vacuum, mechanical weight, dies, etc). Thermal forming can alter mechanical properties and may have a tendency to spring back.

As an extra processing step, forming the core may involve extra tooling and equipment. The increase in time and cost to manufacture a product and should be carefully considered during design.

Splicing

Smaller core sections can be spliced together to construct a larger core with more complicated geometries. Different core materials can also be spliced together to take advantage of the different properties. It is not uncommon to splice/insert a core into another core such that it is completely surrounded to alter the local properties. When performing such insertion, the core being inserted is typically cut slightly larger than the nominal such that the fit would be snug. When splicing cores together, it is also a good practice to match the cores' directions. For example, matching the ribbon directions of two honeycomb cores.

When splicing two or more different core materials together, it is important to make sure the final sandwich panel will be flat without steps at the splice. If one core material is more compliant, it can be slightly thicker to "match" the compressive modulus of the others. The spliced cores can also be machined smooth before being deposited into a sandwich panel.

Splicing adhesives should be carefully selected while considering the entire thermal history of the sandwich panels. If the sandwich panel is being co-cured, the adhesive needs to be compatible with the face sheet and adhesive curing requirements. Certain foaming characteristics might be required when splicing honeycomb cores.

Machining

Cores can also be machined to achieve complex geometries and contours. During core machining, it is very important to avoid contamination so that any subsequent core bonding is not affected. No oil or other types of lubricants should be used unless specified and have a plan for removing them before bonding. It is also common to have equipment and work cells dedicated to core machining to eliminate contamination.

The machined surface finish also can have an impact on the subsequent bonding. For example, the burr/fuzz on honeycomb cell edges after machining can either facilitate the bonding by "biting" into the adhesive or prevent proper fillet formation. Evaluation should be conducted on a per case basis.

Very low density or light weight honeycomb cores may need to be stabilized by taping or potting before machining. Similar to core forming, machining involves extra tools and equipment, adding cost and complexity to product manufacturing.

Explore this area further

• Cores & inserts - A202
  • Foam - A328
  • Honeycomb - A326

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material	Cores & inserts - A202 Foam - A328 Honeycomb - A326
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Sandwich Panels - A325
Case Studies
Perspectives Articles	Introduction to Sandwich Structures - Materials and Processing - A327 Sandwich Panels in Aerospace - A341

References

About	Help

Permeability refers to the resistance to fluid flow through a porous material.

• Resin flow through fibre
• Gas flow through prepreg

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.