ASTM D1415 Rubber Hardness Test

Scope:

ASTM D1415 test provides a method for measuring the hardness of thermoplastics or vulcanized rubber samples. This test is also used to determine the penetration depth of a rigid ball into the surface of a rubber sample that is converted to a hardness scale value known as International Rubber Hardness Degrees (IRHD). On the IRHD scale, a material having an elastic modulus of zero is denoted by 0, and a material with an incalculable elastic modulus is denoted by 100. 

Test Procedure:

For ASTM D1415, two different methods are provided to accommodate samples of different dimensions. In both methods, IRHD is obtained from the differences in penetrations. The test is usually carried out at 232°C (73.43.6°F). 

Firstly, the upper and lower surfaces of the test sample are lightly dusted with talc. The sample is then supported on a horizontal rigid surface, and the foot is lowered to rest on the sample’s surface. The plunger is pressed with a slight force on the indenting ball and continued vertically onto the sample for 5 s. If the gauge is graduated straight to IRHD, the bezel of the gauge is turned so that the indicator indicates 100. 

Then, the major force to the plunger is added and maintained for 30 sec on the ball. The reading on the gauge is noted as the hardness value in IRHD. In case if the measuring device is graduated in metric or inch units, the movement of the plunger is recorded by the application of the major load for 30 s. On the other hand, if the micro tester is used, the movement is multiplied by the scale factor of 6. The value obtained is converted into IRHD by using the table as specified by ASTM D1415. Then, one measurement at each of the three or five different points distributed over the sample is made. The median of these measurements is taken which is rounded to the nearest IRHD to get the hardness value.

Sample size:

The standard sample for ASTM D1415 should be between 8 and 10 mm in thickness. The non-standard samples can be measured but should be no thinner than 2 mm. The sample measure for the micro-test should be between 1.5 mm and 2.5 mm in thickness. 

Data:

For ASTM D1415, Hardness is expressed in International Rubber Hardness Degrees (IRHD). It is based on the following:

The relation between penetration and Young’s modulus for a perfectly elastic isotropic material is:

D=61.5 R^-0.48([F/E]^0.74-[f/E]^0.74)

where:

D = known relationship for a perfectly elastic isotropic material between indentation,

R = radius of the ball, mm,

F = total indenting force,

E = Young’s modulus expresses in megapascals, and

f  =  contact force

Conclusion:

ASTM D1415 is used for evaluating the hardness of thermoplastics or vulcanized rubber samples (Rubber Property) and the penetration depth of a rigid ball into the surface of a rubber sample that is converted to a hardness scale value (International Hardness).