_ 10 



L s 



DESIGN OF LAMINATES 6-185 



Shear stress at the neutral axis in the balsa core 



x L 1.60 x 10 6 x 1 x .125 x .5625 + 0.52 x 10 6 x 1 x .5 x .25 J 



1 x 0.1397 x 10 



= 12.71 psi 



As an alternative method, consider all the shear 

 stress carried by the core only. Then the maximum 

 shear stress at the center of the core is: 



f s=lw < 6 ' 33) 



c c 



This approximate method gives a conservative estimate of the shear stress and is con- 

 sidered applicable. 



Fig. 6-53 gives the section moduli and moments of inertia about the neutral axis for a 

 1 in. wide strip for two types of sandwich constructions. In both cases the core is regarded 

 as ineffective in determining the above properties. The section moduli values are corrected 

 to give stress values directly, similar to the values given for the single laminates. The 

 moment of inertia, I, for the Type A sandwich is based on cloth equivalence and may be 

 used with a modulus of elasticity of 1.95 x 10 6 psi. The moment of inertia, I, for the Type 

 B sandwich is based on woven roving equivalence, and may be used with a modulus of 

 elasticity of 2. 06 x 10 6 psi. 



Flat Rectangular Panels: Methods of analysis of laterally loaded sandwich panels have 

 been developed (12, 24), but all conditions of loading and edge supports have not been fully 

 investigated for orthotropic sandwich panels. The behavior of a simply supported uniformly 

 loaded orthotropic sandwich panel (24) is presented since it is applicable to the sandwich 

 panels with cloth and woven roving fiberglass laminate facings presented in Fig. 6-53. 



Referring to Fig. 6-47 for flat plates, the lengths of the edges of the panel are denoted 

 as a and b and are parallel to the directions of the x and y axes which are the warp and fill 

 directions of the laminates and core. The formulas given are applicable only to panels 

 with small deflections that do not introduce direct stresses to the panel. 



For a uniformly loaded simply supported panel, the deflection at the center of the panel: 



16 a 2 b 2 r 



71 ' 



w=^x — = ^-£ - (6.92) 



/D^Dy +[K f ^J 



