DESIGN OF LAMINATES 



6-43 



Table 6-1 gives the values for — ratios based on E below the proportional limit, E a 

 above the proportional limit and E r the reduced modulus above the proportional limit. A 

 comparison of these values indicate that for materials that have continuous sloping stress- 

 strain curves with no abrupt point of inflection, the — values based on E a and E r are al- 

 most identical. Therefore, the more complex method of solving for Per based on the reduced 

 modulus need not be used. For practical purposes the value of F cr based on E a can be used 

 without appreciable error. The — values above the proportional limit in Tables 6-2 and 

 6-3 have been based on E a . 



DESIGN EXAMPLE 6-15. CRITICAL COMPRESSIVE LOAD OF MAT LAMINATE 



Compute the critical compressive load, P cr ,for a mat-polyester laminate, 6 in. wide 

 x 1/2 in. thick and 15 in. long, simply supported at the ends, Fig. 6-28. 



1/2" 



Fig. 6-28. Mat Laminate in Compression 



Area = 6 x 0.50 = 3.0 sq.in. 



bd3 



Moment of Inertia, I = 



12 



(6.23) 



xx 



L yy 



6 x (o.5o) 3 , H I, 

 Jr — = 0.0625 in^ 



(0.50) 6 3 



12 



9.0 in'- 



