6-1! 



DESIGN OF LAMINATES 



C.Ii990 x O.U998 



1 0.2l[98 O.2I498 



11800 2 J t l800 2 37800 2 



(6. 11a) 



5960 psi 



Stress at 30°, F ? 



sin 2 30 



0.25 

 os 2 30° = 0.750 



1 0.563 0.0625 



o ~ 7 + : ~ + 



F 2 2 141800^ 



37800 2 



sinU 300 = 0.0625 



cos^ 30° = 0.563 



0.25 x 0.750 

 5960 2 



= 13,300 psi 



FACTOR OF SAFETY 



Before actual design problems can be considered it is important to discuss factor of 

 safety. Factor of safety is defined as the ratio of the ultimate strength of the material to 

 the allowable working stress. 



F.S. 



Ultimate Strength 



Allowable Working Stress 



In many fields the allowable working stresses or design stresses are specified by codes or 

 recognized authorities. When allowable stresses are not specified, the factor of safety 

 must be carefully chosen based upon the following considerations: 



1. Accuracy of the estimated load on the structure. A precise load value allows 

 for a lower factor of safety. 



2. Precision of analysis and stress determination. Where analysis for stresses 

 are accurate and precise a lower factor of safety can be used. Inexact or 

 approximate analysis requires a higher factor of safety. 



3. The probable homogeneity and consistency of behavior of the material. Since 

 stress patterns are a function of the homogeneous character of the material 

 this variable must be taken into account in establishing the factor of safety. 

 Metals are considered homogeneous and consistent in behavior while fiberglass 

 laminates, even those made under controlled experimental conditions, show 

 some inconsistency in the physical properties. 



4. Deterioration due to environmental conditions. 



Nature of Loading. Certain types of loads have greater effect than others. 

 The effect on the ultimate strength of the material produced by different types 



