Abbott Aerospace – Analysis Method

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Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Sandwich Structure Design Guidelines

Abbott Aerospace – Analysis Method

XL-VIKINGDisplay Your Math in Excel

Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Sandwich Structure Design Guidelines

• The difference between FAR part 23 and part 25 aircraft core. In general foamcore is not used in part 25 primary structures.

a. Foam Core: Foam core is commonly used in part 23 aircraft. Foam core is more readilyprocessed, it is easier to machine and can often be used without any additional filmadhesive product. Foam core is closed-cell and is generally less prone to moistureabsorption than honeycomb core. Foam core also gives more reliable results whenused ‘out of autoclave’. Some brittle foam cores have poor damage tolerancecharacteristics and it is recommended that if possible the softer foam cores are used.Foam cores can also be used with wet laminate composites and also with infusedlaminates (with some significant weight penalty though). Most foam cores can bethermally pre-formed to gentle compound contours. For large sections of foam core,different foam core panels can be butt joined together with adhesive.

b. Honeycomb core creates sandwich panels with superior weight and durabilitycharacteristics but is more expensive and more difficult to process. Honeycomb core ismore difficult to machine and often requires autoclave processing to give an adequatequality laminate.

Abbott Aerospace – Analysis Method

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Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Sandwich Structure Design Guidelines

• Face sheets should be designed to minimize people-induced damage duringhandling or maintenance of the component.

• Where possible avoid laminate buildup on the core side of the laminate.

• The minimum possible core density should be used to minimize weight. Theparameters to consider in selecting core density are as follows:

a. Out-of-plane shear strength required for service loadsb. Crushing Strength required for service loadsc. Crushing strength required to withstand process (autoclave cure)d. A working temperature that can withstand the cure temperaturee. Thermal formability (for foam cores)f. Chemical compatibility – Most commercial core materials are formulated for

use with epoxy resins, regardless, this should be checked for all materials

Abbott Aerospace – Analysis Method

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Taken from: Analysis and Design of Composite and Metallic Flight Vehicle Structures

Sandwich Structure Design Guidelines

• If foam cores have been removed from the packaging or thermally formed sometime prior to lamination, foam cores should be ‘baked’ in order to drive anymoisture out of the core that may have been absorbed.

• The (MIL-HNDBK-17F Vol 3, 2002) source material states that core chamfershould be no greater than 20 degrees from the horizontal plane. In practice, 30degree chamfer angles have been widely used with little or no issues.

• Fasteners through primary sandwich structure must always be installed using apotted insert. For secondary structure other means such as filler alone can beused.