6-26 



DESIGN OF LAMINATES 



Part (a) above states that if the stress in the mat laminate is assumed as the controlling 

 stress, then for the ultimate values selected, the maximum stress in the cloth laminate will 

 never exceed 13,200 psi. Since this value of 13,200 psi is less than the ultimate for the 10 

 ounce cloth laminate of 17,900 psi, the composite laminate is not overstressed in any lamina. 



Part (b) states that if the ultimate stress for cloth, 17,900 psi, is assumed as the con- 

 trolling stress then the stress indicated in the mat lamina will reach a value of 8, 950 psi. 

 Since this value exceeds the ultimate value of 6, 600 psi given for the mat lamina this clearly 

 indicates that the mat lamina will be overstressed if the cloth lamina is assumed to govern. 

 Consequently, this condition of overstress cannot be allowed for a safe design. 



The stresses of part (a) are then chosen as the ultimate values for the composite 

 laminate, namely: 



f m = 6,600 psi 



f cl = 13,200 psi 



The ultimate load that the laminate can carry can now be computed by using the basic 

 equation, P = f t A where f+A is the summation for both mat and cloth laminae as follows: 



f tcl A cl + f tfm A m 



(6.15) 



= 6600 x r + 13,200 x i 



= 3300 lbs. per in, of width 



Example 6-9 indicates the need for establishing the correct values of the stress f in the 

 basic formula P = fA. The value of the stress f to be used is dependent upon the compatibility 



condition that all layers of a composite laminate are equally strained and follow the equa- 



f 

 tion e = —. 



E 



DESIGN EXAMPLE 6-10. ULTIMATE TENSILE LOAD OF A COMPOSITE LAMINATE 



The composite laminate of Fig. 6-21 is assumed to be made of several different 

 materials as follows: 



One lamina of 1/8 in. thick woven roving 



One lamina of 1/4 in. thick mat 



One lamina of 1/16 in. thick cloth 



Dl RECTI ON OF WARP_ 

 FOR ALL PL I ES 



T , 1 , 1 , > ■ 



Ill ' 



) , T T T T t 



llllllilllll 



1/8 



CLOTH 



MAT 



WOVEN ROVING 



-1/1 en 



Fig. 6-21. Composite Laminate Under Tensile Load 



