TABLE 5.5 PROPERTIES OF MATERIALS [FROM REF. (22)] 



ence (22) there are three major disadvan- 

 tages with glass as viewport material: 



1) Poor reproducibility in mechanical prop- 

 erties from one manufactured specimen 

 to another stemming from inadequate 

 quality control. 



2) Occurrence of tensile stresses in designs 

 for a material that has low tensile 

 strength and no tolerance for localized 



yielding. The tensile stresses usually 

 stem from glass/metal interfaces. 

 3) Glasses fail catastrophically without 

 any prior yielding or permanent defor- 

 mation that might serve as warning. 

 On the other hand, plastic offers the follow- 

 ing advantages: 



1) Reproducibility in physical properties 

 from one viewport to another is excel- 

 lent. 



259 



