These curves are only for room temperature, but a temperature of 

 35°F will increase the yield strength by about 20% according to References 

 4 and 10. The strength is, of course, much lower with a higher temperature 

 and, therefore, temperatures considerably above room temperature should 

 be avoided. 



Figure 18 is used concomitantly with Figure 19 for calculating design 

 strength. 



Structure. Figure 19 represents the structural facet of the design. 

 The curves include the parameters: t/d ratio, included angle, safety factor, 

 yield stress, and operating pressure. The t/d ratios range from 0.25 to 1 .75 

 while the included angles range from 60° to 120°. 



The curves are based on a finite element stress analysis. With four 

 t/d ratios, three included angles, and two boundary conditions (fixed and 

 free), there were a total of 24 computer runs. 



Figure 19a represents the maximum effective stress in the 

 compression-compression-compression octant and the worst of the two 

 boundary conditions, the free case. This point always occurred at the low- 

 pressure face corner for the complete range of t/d ratios and angles. This is 

 attributed to the plugging action creating a higher effective stress at this 

 point. 



o 6,000 





\ 





Plexigl 

 Temp 



asG 

 = 700F 







''■^^^^ compress 



ive 







■--.^ensile 





^ 















___________ 



















0.1 



10 100 1,000 10,000 



Time of Load Duration, T (hr) 



Figure 18. Yield stress-time curves for acrylic plastic. 



28 



