Material used for Sandwich Construction

Material used for Sandwich Construction:

FACING MATERIALS

The facings of a sandwich part serve many purposes, depending upon the application, but in all cases they carry the major applied loads. The stiffness, stability, configuration, and, to a large extent, the strength of the part are determined by the characteristics of the facings as stabilized by the core. To perform these functions the facings must be adequately bonded to a core of acceptable quality. Facings sometimes have additional functions, such as providing a profile of proper aerodynamic smoothness, a rough non-skid surface, or a tough wear-resistant floor covering. To better fulfil these special functions, one facing of a sandwich is sometimes made thicker or of slightly different construction than the other.

Any thin, sheet material can serve as a sandwich facing. A few of the materials usually used are discussed briefly in the following:

Metals (ref. 2-35)

Aluminum Alloys. –The stronger alloys of aluminum, such as 7075-T6, 2024-T3, or 2014-T6, are commonly used as facings for structural as well as for non-structural sandwich applications.

Steel Alloys. –Stainless steel sheets are finding increasing use as a facing material in airframe sandwich construction. The chief advantage of stainless steel sheet is its high strength at elevated temperatures. Alloys such as 18-8, 17-7PH, and PH15-7Mo are currently finding use because high stresses can be realized. The 18-8 alloys can be rolled to various degrees of hardness to produce high strength but it should be understood that a sheet rolled full hard has a longitudinal compressive yield stress about one-half of the compressive yield stress in the transverse direction. This discrepancy can be closed by subsequent stress relief. Alloys of the 17-7PH and PH15-7Mo are precipitation hardenable and can be strengthened by heat treatment–usually to condition TH1050.

Titanium Alloys. –Alloys of titanium are currently of interest as facing materials because of their high strength-weight ratios and because they can be utilized for moderately high temperature applications.

Magnesium Alloys. –Magnesium alloy sheets have been used only experimentally as facing materials, but may find increasing application because of their low density.

Nickel Base Alloys. –Nickel base alloys such as René 41 can be utilized for heat-resistant sandwich at temperatures of 1200°-1500° F. René 41 is a precipitation-hardening alloy that needs protection from the atmosphere during heat treating. The alloy can be welded.

Cobalt Base Alloys. –Alloys of cobalt with chromium, nickel molybdenum, and tungsten are available for use in moderately stressed applications at temperatures of 1000°-1800° F. Alloys such as L605 can be brazed, or fusion or resistance welded.

Columbium Alloys. –Columbium alloys D-36, D-43, and Cb-752 are suitable for use at temperatures up to 2500° F if they are protected from oxidation by thin suicide coatings. These alloys can be brazed in an inert atmosphere or can be welded; however, degradation can be minimized by joining parts by diffusion bonding.

Molybdenum Alloys. –Alloy TZM of molybdenum can resist temperatures up to 2800° F. Need for protection and means of joining parts are the same as for the columbium alloys.

Beryllium. –The low weight and high elastic modulus of beryllium make it most attractive for use in sandwich composites. The metal is heat resistant in the range 1000°-1200° F. Parts can be joined by brazing or welding. Precautions must be taken to prevent individuals from inhaling toxic beryllium particles during fabrication of parts.

Reinforced Plastic Materials (ref. 2-34)

Glass-Fabric Reinforced. –Resin-impregnated glass-fabric facings possess acceptable properties for structural sandwiches when properly fabricated. Because of its excellent dielectric characteristics when fabricated with the proper resin, this type of facing is used almost universally for radomes of sandwich construction. A variety of weaves are available commercially, which makes it practicable, by orienting the fiber directions in the facing, to achieve a wide range of directional strength properties.

In many airframe applications, facings are exposed to moisture, either in the form of high humidity or free water. Even though the amounts of moisture absorbed by glass-reinforced plastic are quite small (on the order of 0.5 to 1.5 percent), the strength properties are decreased, with the amount of decrease depending upon the type of finish applied to the glass fabric. Current specification requirements permit only small losses of strength, after exposure to moisture, that are consistent with results of tests on fabrics made with more recent and effective finishes (such as Volan A, A-1100, Garan RS-49, T-31, NOL-24, and A-172). The most suitable finish for a given application is selected by the glass fabricator. For chemical resin types, such as phenolic, epoxy, and triallyl cyanurate-polyester resins, optimum properties are obtained by use of specific finishes with each resin formulation. The acceptable finishes for each approved resin are given in the qualified products lists that accompany the military specification for each chemical type of resin.

Glass Mats Reinforced. –Glass fibers are also commercially available in the form of mats, but owing to the relative non uniformity in thickness and resin content and because of the low strength when compared to glass fabric, mats have found little use in aircraft sandwich construction.