EN 10319-1 Metal Tensile stress relaxation testing

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EN 10319-1 Metallic materials - Tensile stress relaxation testing - Part 1: Procedure for testing machines

EN 10319-1 specifies the test method for determining stress relaxation of metallic test pieces under nominally constant tensile strain and constant temperature. Paired with EN 10319-2 (Procedure for model bolts) for targeted bolt relaxation tests.

In stress relaxation, lock in a total strain (mostly elastic at the start); as time passes at temperature, the stress decays because the material creeps internally while the external strain is held constant.

Test Principle

The fundamental principle is to hold a metallic test piece at a constant temperature and apply a fixed total tensile strain throughout the test. As time elapses, the internal stress of the specimen gradually decreases (stress relaxation). The test continuously or periodically records the residual stress to evaluate the stress relaxation performance of metals. Total strain consists of elastic strain plus plastic strain; stress is calculated as applied force divided by the original cross-sectional area of the specimen.

Test Specimen

Shape & Dimensions:

Standard specimen: Machined proportional cylindrical specimen with circular cross-section, following the rule L₀=k√S₀. The proportional coefficient k ≥ 11.28; reference length ≥ 100 mm.

For limited material supply, k can be reduced to a minimum of 3, and this change shall be noted in the report.

Parallel length L_c shall not exceed the original gauge length L₀ by more than 20% for circular specimens.

Transition radius between parallel section and grip ends: 0.25d ~ 1d (d = specimen diameter).

Preparation & tolerances:

Machined to avoid residual deformation/surface defects.

Diameter shape tolerance per Table 3 (max deviation along entire parallel length), e.g.:

3<d≤6 mm → 0.02 mm

6<d≤10 mm → 0.03 mm, etc.

Original cross-sectional area Sₒ:

Calculated from measured dimensions.

Dimensions measured to ±0.1% or ±0.01 mm, whichever is greater.

Test equipment required for EN 10319-1 Metal Tensile stress relaxation testing

Applies axial tensile force without shock and minimizes bending/torsion. Recommend UnitedTest Steel strand tensile stress relaxation testing machine.

Steel strand tensile stress relaxation testing machine

Computer controlled steel strand tensile stress relaxation testing machine mainly used to check the prestressing steel material relaxation performance.

This machine is use mechanical mode loading, computer control to complete the test.

Machine verification required: Class 1 / 0.5 per ISO 7500‑1.

EN 10319-1 Metal Tensile stress relaxation testing

Extensometer (strain measuring device)

Recommended minimum gauge length: 100 mm.

Dual-sided measurement of elongation is preferred; single-side contact measurement is permitted but must be recorded.

Heating & Temperature Chamber (optional)

Temperature indicator accuracy: At least 0.5 °C; overall temperature system accuracy: ±1 °C.

2 thermocouples for parallel length ≤ 50 mm; 3 thermocouples (two at ends + one in the middle) for parallel length ＞ 50 mm.

Key Test Parameters & Stipulations

1, Strain Control

The total strain shall be maintained within ±1% of the initial strain value during the entire test. Manual force adjustment can only decrease stress; servo-controlled systems allow both increase and decrease to stabilize strain.

2, Data Recording Precision

Temperature: Recorded to 1 °C

Specimen diameter: Recorded to 0.01 mm

Ratio of gauge length to diameter L₀/d: 1 decimal place

Initial stress & residual stress: 3 significant figures

Time: 3 significant figures

Time recording error: Within ±1%

3, Test Termination

At the end of the test, cool the specimen under residual force. Unloading modulus can be measured at room temperature for potential reloading in follow-up tests.

Complete Test Procedures of EN 10319-1 Metal Tensile stress relaxation testing

Pre-test Preparation: Measure the original cross-sectional area, diameter and gauge length of the specimen; inspect and calibrate all testing equipment (extensometer, thermocouples, testing machine).

Determine Ambient Elastic Modulus: Conduct tensile loading at room temperature to obtain the elastic modulus of the specimen.

EN 10319-1 Metal Tensile stress relaxation testing

Heating: Install the specimen on grips, turn on the heating device, and heat to the specified temperature. Hold the temperature until fully stabilized and verify temperature uniformity via thermocouples.

Apply Total Strain: Gradually apply tensile load to reach the preset total strain without impact, and confirm the initial stress value.

Formal Relaxation Test: Activate strain control mode to keep total strain constant. Continuously or periodically record temperature, time and residual stress throughout the test.

Post-test Treatment: Stop loading after the required test duration. Cool the specimen under residual force, then unload and sort out all test data.

Data & Report Compilation: Draw the stress relaxation curve (stress vs. time) and compile a formal test report covering specimen info, equipment type, test conditions and results.

Application Industries

EN 10319-1 is widely applied across industries where metallic components work under long-term constant strain or pre-tension:

Steel & Metallurgical Industry: Routine performance inspection for carbon steel, alloy steel, high-temperature alloys, non-ferrous metals and their products.

Construction & Prestressed Engineering: Quality testing for prestressed steel wires, steel strands and reinforcing steel bars to ensure long-term safety of bridges, buildings and infrastructure.

Mechanical & Fastener Industry: Relaxation performance evaluation for bolts, studs, springs and pre-tightened mechanical parts.

Aerospace & Power Equipment: High-temperature relaxation tests for turbine parts, engine components and high-temperature pipeline metals operating under sustained load.

Automotive Industry: Testing for chassis parts, suspension springs and preloaded structural metal components.

Related Standard:

ASTM E328	Standard Test Methods for Stress Relaxation for Materials and Structures
JIS Z 2276	Method of tensile stress relaxation test for metallic materials
KS D 0301	Method of tensile stress relaxation test for metallic materials
CNS 14312	Method of tensile stress relaxation test for metallic materials
GB/T 10120	Metallic materials - Tensile stress relaxation - Method of test
ISO 6934-4	Steel for the prestressing of concrete - Part 4: Strand
GB/T 5224	Steel strand for prestressed concrete
JIS G 3536	Steel wires and strands for prestressed concrete
BS 5896	High tensile steel wire and strand for the prestressing of concrete. Specification
ISO 6934-3	Steel for the prestressing of concrete; part 3: quenched and tempered wire
ISO 15630-3	Steel for the reinforcement and prestressing of concrete. Test methods - Part 3: Prestressing steel
AS/NZS 4672.2	Steel prestressing materials - Testing requirements
EN 10138-3	Prestressing steels - Part 3. Bars

Computer controlled steel strand tensile stress relaxation testing machine mainly used to check the prestressing steel material relaxation performance. This machine is use mechanical mode loading, computer control to complete the test. PC with control card to realize real time data collection.

High temperature furnace chamber for tensile testing machine, furnace have a columnar split structure, three stage control function. Mainly consists of furnace, temperature controller, high temperature pull rod, high temperature clamp, high temperature extensometer and supporting device.

Steel Strand Extensometer is a high-precision electronic instrument specifically designed for measuring the deformation (strain) of steel strands, prestressed tendons, and similar elongated specimens during tensile tests.

The Hydraulic universal testing machine can perform tension, compression, flexure/bending and shearing test for kinds of specimens. Suitable for metal and nonmetal material, such as iron, steel, steel bar, rebar, cement, concrete, rod and so on. Specially design for production, research, student training and industrial laboratory. Available in wide range of loadframe capacities (300kN to 2000kN)/(10Ton to 200Ton) and with combination of control panels (Digital, Computerized) makes United Test product range suitable for every customer requirement.

UnitedTest Fatigue Testing Machine mainly used at a variety of material and components, parts for both dynamic and static mechanical property testing, include tension, compression, low cycling test, match with high-low temperature chamber can execute the temperature test.

Q1: What is the main purpose of the EN 10319-1 test?

A: It determines the tensile stress relaxation behaviour of metallic materials. The test maintains a constant total tensile strain and constant temperature on a specimen, then records how internal stress decreases over time. It provides standardised data for material performance assessment.

Q2: What is the difference between EN 10319-1 and EN 10319-2?

A: EN 10319-1 specifies general test procedures and requirements for common metallic specimens. EN 10319-2 is the supplementary part specially for model bolts. They together form the complete EN 10319 series for tensile stress relaxation testing.

Q3: Why do we even need a stress relaxation test? Isn't a tensile test enough?

A: A tensile test tells you peak strength— what happens until the material breaks. Stress relaxation tells you something a tensile test cannot: when a metal part is held stretched at a fixed length (constant total strain) while hot, the internal stress quietly decays over time via micro-creep. That decay is what kills bolted-joint preload, spring force, and seal integrity — even though nothing looks "broken."

Real-world consequences:

Bolted flanges loosen → hot gas / steam leaks (turbines, pressure vessels, exhaust manifolds)

Gasket contact pressure drops → seal failure

Springs lose set-point force in high-temp environments

Pre-stressed steel elements lose tension over decades.

Q4: What exactly is "stress relaxation" vs. "creep"?

A: They're two faces of the same time-dependent behavior:

Creep	Stress Relaxation
What's held constant	Stress (load)	Strain (deformation / displacement)
What changes	Strain increases over time	*Stress decreasesover time*
Physical picture	Hang a weight → wire slowly stretches	Pull a bolt to length, lock the nut → tension slowly drops

Q5: What temperature range does it cover?

A: Formally up to 1000 °C with defined tolerances:

Temperature range	Permitted deviation (Tᵢ vs. T)	Max temp gradient along gauge
T ≤ 600 °C	±3 °C	2 °C
600 < T ≤ 800 °C	±4 °C	3 °C
800 < T ≤ 1000 °C	±5 °C	3 °C
>1000 °C	by agreement	by agreement

Q6: Why is tensile stress relaxation testing so important for metals?

A: Many metal components (bolts, prestressed steel, springs, high-temperature parts) work under permanent strain in service. Stress relaxation causes gradual loss of preload or bearing stress, which may lead to loosening, deformation or structural failure. This test evaluates long-term stability, guides material selection, product design and factory quality control.

Q7: Why must we isolate the test machine from external vibration?

A: External shock and vibration will interfere with force and strain control, cause unstable readings of extensometer and load cell, and finally lead to inaccurate relaxation test results.

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