Compression rupture:In this case (see figure below),note that the weak com- pression resistance of Kevlar fibers'leads to a compression strength about two times less than for sandwich panels made using glass fibers. Fo rupture Fo rupture 2 glass Kevlar 4.4 FABRICATION AND DESIGN PROBLEMS 4.4.1 Honeycomb:An Example of Core Material These well-known materials are made of hexagonal cells that are regularly spaced. Such geometry can be obtained using a technique that is relatively simple.Many thin sheets are partially bonded.Starting from stacked bonded sheets,they are expanded as shown in Figure 4.11. The honeycomb material can be metal (light alloy,steel)or nonmetal (carton impregnated with phenolic resin,polyamide sheets,or impregnated glass fabrics) Metallic honeycombs are less expensive and more resistant.Nonmetallic hon- eycombs are not sensitive to corrosion and are good thermal insulators.The following table shows the mechanical and geometric characteristics of a few current honeycombs,using the notations of Figure 4.11. Table 4.1 Properties of Some Honeycomb Bonded Sheets of Light Alloy Light Alloy Polyamide:Nomex" AG3 2024 Dia.(D):inscribed 6;8;12 4 6 circle (mm) Thickness e (mm) 0.05 0.04 Specific mass(kg/m) 64 80 46 Shear strength 1.7 3.2 1.5 T rup (MPa) Shear modulus: 58 520 280 G (MPa)# 1.5 Gmat(e/D) Shear strength trup 0.85 2 0.9 (MPa) Shear modulus:G= 24 250 140 (MPa) Compression strength: 2.8 4.4 2 C=rup (MPa) Nomex is a product of Du Pont de Nemours. See Section 3.3.3. 2003 by CRC Press LLCCompression rupture: In this case (see figure below), note that the weak com￾pression resistance of Kevlar fibers7 leads to a compression strength about two times less than for sandwich panels made using glass fibers. 4.4 FABRICATION AND DESIGN PROBLEMS 4.4.1 Honeycomb: An Example of Core Material These well-known materials are made of hexagonal cells that are regularly spaced. Such geometry can be obtained using a technique that is relatively simple. Many thin sheets are partially bonded. Starting from stacked bonded sheets, they are expanded as shown in Figure 4.11. The honeycomb material can be metal (light alloy, steel) or nonmetal (carton impregnated with phenolic resin, polyamide sheets, or impregnated glass fabrics). Metallic honeycombs are less expensive and more resistant. Nonmetallic hon￾eycombs are not sensitive to corrosion and are good thermal insulators. The following table shows the mechanical and geometric characteristics of a few current honeycombs, using the notations of Figure 4.11. 7 See Section 3.3.3. Table 4.1 Properties of Some Honeycomb Bonded Sheets of Polyamide: Nomexa Light Alloy AG3 Light Alloy 2024 Dia. (D): inscribed circle (mm) 6; 8; 12 4 6 Thickness e (mm) 0.05 0.04 Specific mass (kg/m3 ) 64 80 46 Shear strength txz rup (MPa) 1.7 3.2 1.5 Shear modulus: Gxz (MPa) # 1.5 Gmat(e/D) 58 520 280 Shear strength tyz rup (MPa) 0.85 2 0.9 Shear modulus: Gyz (MPa) 24 250 140 Compression strength: sz rup (MPa) 2.8 4.4 2 a Nomex® is a product of Du Pont de Nemours. TX846_Frame_C04 Page 60 Monday, November 18, 2002 12:07 PM © 2003 by CRC Press LLC