The yield point Re (yield strength) is a material characteristic value and is determined from the results obtained in tensile testing (e.g. ISO 6892 (metals) or ISO 527 (plastics)). The yield point is always measured in N/mm².
Often an upper yield point ReH and a lower yield point ReL are set.
The upper yield point designates the stress up to which no permanent plastic deformation occurs in a material. The material does undergo deformation, however after withdrawal of the stress it returns to its original form. If the upper yield point is exceeded deformation remains; in tensile testing the specimen is elongated.
The yield point ratio can be calculated from the yield point Re and the tensile strength Rm: Re / Rm
Oftentimes the yield point cannot be clearly determined in a tensile test. These materials do not have a distinctive yield point. In this case offset yields of 0.2% are used (Rp 0,2).
The highest stress value before its significant first drop is designated as the upper yield point ReH. At this point the material undergoes plastic deformation for the first time. Instead of constricting the specimen (necking), the stress oscillates briefly in favor of greater elongation. The lowest measured stress value here corresponds to the lower yield point. This effect occurs exclusively in steel with little or no alloy.
For the computer-aided test, the highest stress value was defined as the upper yield point, followed by a stress reduction of at least 0.5% and in the subsequent strain range of at least 0.05% this stress is not exceeded again.
The upper yield point ReH is calculated from the results of the tensile test: Upper yield point ReH = force at the upper yield point FeH / stress cross section S0
The lower yield point ReL is the lowest stress value in the range of the plastic deformation of the material following the upper yield point ReH, whereby transient oscillation occurrences (e.g. due to a change in test speed or control mode) are not taken into account.
In a case where the upper yield point is not recognized (the reduction in force is less than 0.5%) or yielding occurs at a constant force over a larger range, this stress value is generally referred to as yield point.
The lower yield point ReL is calculated from the results of the tensile test: Lower yield point ReL = force at the lower yield point FeL / stress cross section S0
Materials with continuous onset of yielding do not have a pronounced yield point. Generally for these materials an offset yield of 0.2 % (Rp0,2) is specified. This 0.2 % offset yield can always be clearly determined from the stress-strain diagram (which is not always the case for the yield point).
The 0.2 % offset yield is the stress whereby the specimen exhibits a residual elongation of 0.2 % (relative to the initial length of the specimen).