and compression with tensile strength being lower than the compressive. A 

 conservative approach was used to circumvent this problem. The tensile 

 strength was applied to combined states of stress at any time when at least 

 one principal stress was significantly in a tension zone, and the higher com- 

 pressive strength was used strictly in the all-compression failure octant. 



Because acrylic is a typical thermoplastic, two additional variables 

 significantly affect its material properties: temperature and time. 



In general, when temperatures are increased, acrylic material 

 properties decrease in value. This feature of acrylic makes it an ideal material 

 for its proposed use as a viewport for an undersea vehicle because the ocean 

 provides a low-temperature environment which enhances properties over 

 those measured at room temperature. 



The design recommendations set forth in this report are based upon 

 material response at room temperature, thus allowing the additional strength 

 of the acrylic due to lower temperatures at operational depths to increase the 

 margin of safety. 



The second variable, time, is not as easily dealt with as temperature, 

 but is certainly important due to the creep properties of acrylic. For this 

 reason every effort has been made to rationally treat the effect of time on 

 the stress distributions of the viewport and the yield strength of the acrylic. 

 The details of this approach are given in Appendix C as a separate topic 

 because this treatment is not limited to viewports, the approach may be used 

 for any structure utilizing acrylic. 



Briefly, the approach is to choose the "worst" stress distribution 

 and utilize this for any value of the parameter time, while the development 

 of a yield-stress versus load-duration curve provides the factor of time in the 

 design parameters. Appendix C deals with the concepts in a straightforward 

 and rational manner. 



Method of Analysis. As discussed in Appendix C, the highest stress 

 concentrations result from the viscoelastic solution when time is equal to 

 zero. This is identical to elastic solutions; consequently, elastic solutions 

 based on all the classical assumptions of linear, infinitesimal-strain, elastic 

 theory were assumed and the solutions were obtained by the finite element 

 program for an axisymmetric solid written by Wilson.^ References 4 and 5 

 verify the capability of using a finite element analysis on conical acrylic 

 viewports. 



The sequence of analysis was as follows: for each viewport 

 configuration, the finite element program computed the local state of stress 

 at every element and the corresponding effective stress defined as 



