ASTM E8/E8M describes the uniaxial tensile test on metals at ambient temperature and the determination of characteristic values such as offset yield, yield strength, yield point elongation, tensile strength, strain at break and reduction of area.
The values can be used to make predictions about the strength and toughness of the material.
Strictly speaking, this ASTM standard includes two standards, and therefore ASTM E8 must be differentiated from ASTM E8M.While ASTM E8 refers to units of measurement “inch” and “pound”, ASTM E8M uses SI units.This leads to the fact that characteristic values determined with one system of units are not exactly equal to those determined with the other system of units.In practice however, this usually does not present a problem, because there is no change between the units when determining and comparing characteristic values.
Within this context, it is however important to note that the initial gauge length for the determination of strain in ASTM E8 refers to 4D, or four times the diameter of the round specimens, while in ASTM E8M it refers to 5D, or five times the diameter of the round specimens.A mix-up or failure to observe this difference can lead to characteristic values that are no longer comparable.
ASTM E8/ASTM E8M goes into detail about the testing machine and the type of specimen grips.It provides information on the correct use of specimen grips.
Important information is provided for specimen preparation, which is intended to ensure that the sampling process and the subsequent specimen preparation do not influence the material, since this could in turn affect the results of the tensile test.
There can be a wide range of shapes for tensile specimens.ASTM E8/ASTM E8M lists standard flat specimens for sheet metals and thin sheet metals, for tubular products, for special specimen grips, and standard round specimens for other metal products, and specifies the corresponding initial gauge lengths to which all strain values refer.With a few exceptions, all dimensions that are needed for specimen preparation are specified, or else minimum dimensions are indicated.
Particular focus is placed on the test speed.ASTM E8/ASTM E8M allows five different ways of specifying test speeds.They are designated as
- (a) Rate of straining the specimen,
- (b) Rate of stressing the specimen,
- (c) Rate of separation of the two crossheads of the testing machine during a test,
- (d) The elapsed time for completing part or all of the test, or
- (e) Free-running crosshead speed (rate of movement of the crosshead of the testing machine when not under load).
For determination of the so called yield properties, that is the yield strength, yield point elongation, and offset yield, in general all characteristic values related to the change of material behavior from elastic to plastic, it is important to define a suitable control of the test speed.Because in the case of metallic materials, these characteristic values can be significantly dependent on the actual test speed, and therefore the test speeds must be maintained within specified tolerances.ASTM E8/ASTM E8M takes this into account with three different control methods.They are designated as Method A, B and C.
Method A is based on the increase in tensile stress during load application.In the linear elastic part of the tensile test, that is at the very beginning of the test, the rate of stress application must be between 1.15 and 11.5 MPa/s (this corresponds to 10000 and 100000 psi/min).However, ASTM E8/ASTM E8M clearly indicates that these specifications and this method do not mean that the increase in stress application must be kept constant up to plastic behavior or that the closed loop control of the force increase may be applied beyond the linear elastic range.
Method B is based on the increase in strain during load application.For this method, the testing machine must keep closed loop strain rates constant, using the strain measurement of an extensometer.The strain rate shall be set and maintained at tolerances of 0.015 ± 0.006 in./in./min (or mm/mm/min*)).ASTM E8/ASTM E8M provides information on what factors should be taken into consideration.
Method C is based on constant speed of the crosshead.The crosshead speed shall be set and kept constant so that the initial parallel length of the specimen undergoes an elongation of 0.015 ± 0.003 in./in./min (or mm/mm/min*)).This method is recommended if the material does not deform continuously.
All figures for the three methods are to be used unless product standards or standards for special applications specify other values.
If determination of the yield strength and offset yield have been completed (or do not have to be determined) and a specimen elongation of more than 5% is expected, the test speed can be increased to 0.05 up to 0.5 in./in./min (oder mm/mm/min*)).This specification refers to either the initial gauge length of the measuring extensometer or the initial parallel length of the specimen.The test speed is therefore indicated as a strain rate.
At this test speed, all other characteristic values for the tensile test are then determined according to ASTM E8/ASTM E8M.
*) These cases refer to relative values for which the different units of length do not matter (they are minimized)