518 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



Organic materials are in ever-increasing use for mechanical and electrical 

 devices where satisfactory performance is required over long periods.of time and 

 under a wide assortment of atmospheric influences. Although the dimensional 

 stabiUty of plastics and electrical insulating materials under no-load conditions 

 has been estabUshed reasonably well, there is an important gap in existing 

 knowledge with regard to a material's abihty to maintain adequate counter- 

 stresses under compressive loading. 



A.S.T.M. Method of Test D 621 - 5P uses a constant load system and meas- 

 ures the material's resistance to gross deformation. However, this fails to simu- 

 late the usual appHcation where the material is subjected to constant deflection 

 (such as fastening devices, inserts, etc.) and is required to maintain adequate 

 counter-stresses for the foreseeable life of the part. Therefore, it has been neces- 

 sary to integrate data from Method D 621 with long practical experience in 

 order to extrapolate between two dissimilar systems. 



This paper describes a constant-deflection procedure for direct measurement 

 of stress relaxation or change thereof, thereby permitting evaluation in terms of 

 a material's abihty to maintain a tight assembly under conditions simulating 

 actual use. Apparatus for carrying out the test is described and typical data 

 illustrating its usefulness are included. 



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