6 1 = ~ 2 ~ 



E cl E m 



Equation 6. 14 can be rearranged to give the following: 



DESIGN OF LAMINATES 6-25 



f cl _ f m 



f -f.xJL < 6 - 14b > 



m cl E cl 



Examination of equations 6 .14a and 6 .14b reveals that the actual stress values for 

 either material will be dependent upon the ratio of the moduli or the values of the right hand 

 portion of the equations. The only restriction that must be placed on equations 6 .14a and 

 6 .14b is that the actual stress in any lamina cannot exceed its ultimate stress value. The 

 ultimate stress is used since no actual proportional limit has been established for these 

 materials. The stress strain curves are almost straight lines indicating that the ultimate 

 value can be used without appreciable error. Example 6-9 will serve to illustrate the 

 recommended procedure. 



DESIGN EXAMPLE 6-9. ULTIMATE TENSILE LOAD OF A COMPOSITE LAMINATE 



Determine the ultimate tensile load P, per inch of width, that the composite laminate 

 of Fig. 6-19 can support. 



From Table 5-7, the following lower limit values for E are chosen for the individual 

 laminae: 



6 

 S = 0.70 x 10 psi 



E = l.i|0 x 10 6 psi 

 cl 



From Table 5-6, the following lower limit values for ultimate tensile strength are chosen: 



F m = 6600 psi 

 F cl = 17900 psi 



Values for f c j and f m can now be calculated using equations 6.14a and 6.14b: 



(a) f = 6600 x ^'Q * 10 l - 13200 psi 



C1 0.70 x 10° 



(b) f = 17900 x 0<7 ° x 10 , = 89^0 Dsi 



l.hO x 10° 



