DESIGN OF LAMINATES 



6-181 



different core materials which vary with density have not been completely established. For 

 these reasons, alternate conservative methods of analysis which sacrifice accuracy for speed 

 and ease in design are generally used and are recommended for most marine applications. 



Sandwich construction is generally used for flat or curved panels subject to flexure and 

 edgewise compression. 



Flexure 



The flexural analysis of sandwich construction includes the determination of the stresses 

 in the facings and core materials and the deflection of the panel. When considering a one- 

 way panel the analysis is similar to the analysis for a one-way composite plate or beam as 

 previously discussed. The flexural properties of some of the core materials, such as foamed 

 plastics or honeycombs are quite low and may be ignored in the analysis. Some core ma- 

 terials such as balsa wood have appreciable flexural moduli of elasticity, particularly when 

 the grain of the wood is parallel to the plane of the panel and may be considered effective. 



As an example, consider a 1 in. wide strip of a sandwich panel having a 1 in. thick core 

 of 4 lb. density balsa wood and 1/8 in. thick facings of 10 ounce cloth-polyester laminate. 

 Ignoring the effect of the core, the flexural rigidity of the sandwich from equation 6. 16 is, 

 EI = 0. 1270 x 10° in. -lbs. Including the effect of the balsa core, the flexural rigidity be- 

 comes, EI = 0. 1397 x 10°\ which is an increase of approximately 10 per cent. When the 

 core is considered effective, the allowable tensile or compressive stress should not be ex- 

 ceeded at the outermost fiber of the core. 



Simple One- Way Panels: In the determination of the deflection of one-way sandwich 

 panels, the effect of the shear deformation of the low density core may be appreciable and 

 should be considered. Shear deformation effect has been previously discussed for lami- 

 nates and beam sections. 



As an example of this effect, the approximate deflection of a simple cantilever sandwich 

 beam section with a concentrated load at the unsupported end, Fig. 6-52, has been investi- 

 gated (12,21) and the resulting expression in a simplified form is: 



2e = 





3 I g 



(6. 86) 



(6. 87) 



whe re 



z£ 



^xc 



(6. 88) 



P> EI and G are as previously defined. 

 Other symbols as indicated in Fig. 6-52. 



A comparison of the deflection obtained by this expression and that obtained by various 

 other approximate methods is presented in Design Example 6-27. 



