6-190 



DESIGN OF LAMINATES 



Method of Analysis 



Summary of Panel Bending Moments 

 Bending Moment 



Balsa Core 



Foamed Plastic Core 



M, 



M 



y 



Exact 



Normal Panel Formulas 



Simple Beam 



5.24 in. -lbs. 2.31 in. -lbs. 

 2. 76 in. -lbs. 2. 62 in. -lbs. 

 7. 20 in. -lbs. 7. 20 in. -lbs. 



M x 



4. 27 in. -lbs. 

 2. 76 in. -lbs. 

 7.20 in. -lbs. 



M y 



4. 06 in. -lbs. 

 2. 62 in. -lbs. 

 7. 20 in. -lbs. 



A study of the summary indicates that the use of normal plate panel formulas for sand- 

 wich panels results in non-conservative values for both deflection and bending moment. This 

 is due to the effect of the very low shear moduli of the core materials. It is interesting to 

 note the effect of core orthotropicity on the bending moment as evidenced by the balsa wood. 

 The use of the simplified method of analysis gives conservative results and can be used in 

 lieu of the exact method which is cumbersome and tedious to evaluate. 



Edgewise Compression 



Sandwich panels loaded in edgewise compression may fail in one of two ways; by in- 

 stability of the facings and by column instability of the entire sandwich as a unit. The critical 

 stress for a particular sandwich section is the lower of the two stresses. 



The critical facing buckling stress, below which rippling of the facing will not occur, may 

 be conservatively predicted from the formula (13, 25). 



1 



cr 



= -V 



2 V E 



E f E 



ZC G XZC 



(6.95) 



where J 



E|. is the flexural modulus of the facing material 

 in the direction of load. 



E zc is the tensile or compressive modulus of the core 

 material in the direction perpendicular to the 

 plane of the sandwich. 



G is the shear rigidity of the core material associated 

 with the direction of the load and the perpendicular 

 to the plane of the sandwicho 



The critical facing buckling stress for buckling of the panel as a unit is given by (24, 26): 



•yfcr 



J yf 



K AA 



a 2 Hv 



K 



(6. 96) 



Where the x axis is taken parallel to side a, the y axis parallel 

 to side b, and the load is uniformly distributed along side a. 



K = K f ♦ ^ 



