6-6 



DESIGN OF LAMINATES 



accordingly. Fig. 6-4 illustrates the difference in the distribution of ultimate tensile 

 strengths for isotropic and orthotropic materials. Values given in Fig. 6-4b are assumed 

 and will vary with different materials. 



100$ -» 



1 00^ 



8 4$ 



I SOTROP I C - EQUAL 



STRENGTHS IN ALL DIRECTIONS 



ORTHOTROPIC - DIFFERENT 

 STRENGTHS AT VARIOUS ANGLES 



Fig. 6-4. Distribution of Tensile Strength for 

 Isotropic and Orthotropic Materials 



For an example of the orthotropic characteristics of fiberglass laminates, Fig. 6-5 

 illustrates the directional properties of a 10 ounce cloth laminate with the warp fibers placed 

 in the vertical or degree direction. 



If a load P in tension is applied in the direction of the longitudinal L or degree axis, 

 the laminate exhibits an ultimate tensile strength or tensile strength at rupture that is equal 

 to an average value of 24, 100 psi, Table 5-6. This is the strongest value that the 10 ounce 



cloth laminate will exhibit since it is in the 

 warp direction or the direction with the 

 maximum number of fibers. A tensile load 

 applied in the 45 degree direction will cause 

 the laminate to rupture when the tensile stress 

 reaches a value of approximately 14, 000 psi. 

 This will be the weakest value for this lami- 

 nate. At 90 degrees, or the transverse direc- 

 tion, the laminate will fail when a tensile 

 (90°) load, placed in this direction causes a tensile 



PSI stress of 20,200 psi. 



It is apparent from observation of Fig. 

 6-5 that in order to properly design an ortho- 

 tropic laminate the strength of the laminate 

 must be known in all directions and its be- 

 havior under load must be thoroughly under- 

 stood. 



Before a thorough stress analysis can be 

 performed it becomes necessary to have the 

 values of the physical constants for all angles 

 (8-12). The general procedure for the deter- 



Fig. 6-5. Orthotropic Characteristics 

 of a 10 Ounce Fiberglass Cloth Polyester 

 Laminate 



