UEL for HFW pipes

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 .