Uniform Elongation (UEL)
For all tensile tests (MPQT & Production tests), in addition to recording / reporting the
YS, TS, and YS/TS, the UEL value shall also be measured and recorded, in accordance
with the following procedure.
Uniform elongation shall be understood as “percentage plastic extension at maximum
force”, as denoted by Ag in ISO 6892-1.
The extension shall be determined with an extensometer or by another method with
equivalent or better accuracy (i.e. the extensometer or other equipment shall
measure up till and slightly beyond maximum force).
For materials, which exhibit a plateau at maximum force, the percentage total
extension at maximum force is the extension at the mid-point of the plateau as
defined in ISO 6892-1, Clause 17.
If the recorded uniform elongation value is < 6% (as per the evaluation method stated
above), the following measurement method shall be applied;
The stress – strain graph shall be exported into format in which the graph can be
plotted and analysed
The plateau is defined as the area within 0.5% of the maximum load
The UEL value shall be defined as the strain at the mid-point of the area selected
in accordance with the stated applied range.
Note that the JIP 2017-0173 Guideline for installation of HFW pipe states that the tensile strain should be kept below minimum of 5% and UEL/1.25,
where, UEL is min(min. avg, min. ind + 2%).
In consideration of the FEA predicted strain and to allow some additional margin with increased conservatism, it may be worth requesting that supplier target the following
Un el: min. ind > 3.5% and min. avg > 5.5%
This will permit allowable FEA tensile strain up to 4.4% (= 5.5%/1.25) per the JIP 2017-0173 .
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