90 MECHANICAL TESTING OF RUBBER 



Only a brief notice of this important subject is possible 

 here, and for further details reference should be made to 

 the literatm-e on the subject.* 



Tensile Tests. — A number of machines have been 

 devised for determining the tensile properties of rubber, 

 one of the best known of which is the Schopper machine. 

 Another useful machine is that of Schwartz, which is 

 specially designed for making hysteresis tests on strip 

 rubber. 



The tensile tests include the determination of the 

 breakmg- strain and the elongation at the breaking- point ; 

 the elongation with intermediate loads ; the elongations 

 with fixed load ; the loads requned for fixed elongation ; 

 the amount of subpermanent set either after elongation 

 to a constant limit or under a fixed load ; the behaviour 

 on repeated elongations, etc. 



A most important point in connection with these tests 

 is the form of the test piece. Experiments made by 

 Memmler have shown that the rod form of test piece 

 with thickened ends is not satisfactory in many cases, 

 especially with rubbers exhibiting good elasticity ; if 

 gripped sufficiently tightly to prevent slipping it fre- 

 quently breaks near one of the jaws. The ring form 

 is much better; but stationary rings do not give con- 

 stant results, and the best method appears to be that 

 adopted in the Schopper machine — viz. a ring of standard 

 dimensions which is evenly rotated during the application 

 of tension. 



Compression Tests. — These tests can be made by an 

 ordinary testing machine, or by means of a special machine 

 designed by Professor Martens in which cylindrical pieces 

 of rubber, from the same or different mixings, can be 

 subjected to varying pressures and the extent of the com- 

 pressions read off on a scale. The effect of repeated 

 compressions is usually also determined. 



Abrasion Tests. — These are usually made by holding 

 the specimens against a revolving emery or carborundum 

 wheel by means of a constant weight or pressure, or by 

 making balls or rings of the rubber to revolve between 

 metallic discs or rollers which are pressed together by a 

 constant weight. The loss of weight after a fixed number 



* See Dcr Kautschuk imd acinc Prufung by Hinrichscn and Memmler 

 (S, Hirzol, Leipzig). 



