ASTM E328 Stress Relaxation Test for materials and structures

Information on the most widely used ASTM standards within the materials testing industry

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ASTM E328 Standard Test Methods for Stress Relaxation for Materials and Structures

ASTM E328 focuses on stress relaxation: the time-dependent decrease in stressin a material or structural element that is held under approximately constant constraint (constant strain) at a constant test environment, with negligible vibration.

Stress relaxation of materials is an important characteristic when designing mechanical connectors, which must maintain permanent fastening. long-term failure of the connector may occur if stress relaxation is not considered.

Test Principle

A specimen is loaded to a preset initial strain (or deflection/twist) and kept under approximately constant constraint throughout the test. As time passes, the internal stress of the material gradually decays due to stress relaxation. Testers continuously or periodically adjust the external applied force, moment or torque to maintain the fixed constraint, and record the variation of force/stress versus time to characterize the relaxation performance.

Initial stress (σ₀): The stress imposed on the specimen when the target constraint is fully achieved (defined as zero time, t₀).

Remaining stress: The residual stress in the specimen at any time during the test.

Relaxed stress: The difference between initial stress and remaining stress.

Stress-relaxation rate: The absolute slope of the stress-time curve at a given moment.

ASTM E328 Stress Relaxation Test for materials and structures

Four Specific Test Methods

Part	Name	Typical Equipment required	Where It's Used Conceptually
Part A	Tension Stress-Relaxation (uniaxial constant tensile strain)	Servo tensile frame + extensometer (or locked-displacement rig), furnace/oven	Bolting, tendons, springs, any part whose function depends on held stretch
Part B	Compression Stress-Relaxation (uniaxial constant compressive strain)	Compression platens/blocks, alignment device, anti-buckling considerations	Gaskets/seals, crush-loaded shims, bearing housings, interference fits (load-path parts)
Part C	Bending Stress-Relaxation (constant bending curvature)	4-point flexure fixtures orstatic fixtures (mandrels/clamps/cantilevers)	Leaf/flat springs, strips, thin sheet formed parts, clips
Part D	Torsion Stress-Relaxation (constant twist/angle of twist)	Torsion testing machine, coaxial grips, angular displacement transducer	Helical springs, torsion bars, threaded fastener under-torquestudies (less common than tension/compression)

ASTM E328 allows more than one practical way to get the relaxation data:

1, Continuous / closed-loop: While the apparatus actively adjusts forceto keep constraint within specified bounds, the force indicator is read continuously.

2, Periodic lift-off / check loads: Periodically measure the force required to just freethe specimen from part of its constraint.

3, Elastic springback (post-test): At the end of the hold, removethe constraint and measure the recovered elastic strain (springback) → back-calculate remaining stress.

Test Specimen Info

Uniform cross-section in gauge; tight dimensional tolerances (diameter/width tolerance bands by size).

Wire can be tested as-received; plate/sheet/strip/bar/rod maybe machined to shape.

Machining must avoid altering residual stress state.

Tension: round specimens per ASTM E8/E8M geometry philosophy; gauge lengths longerthan typical tensile coupons to improve strain control; successful examples given: 0.375 in dia × 7.0 in gauge, etc.; threaded/shouldered ends; concentricity control.

Compression: solid cylinders, L/D = 8–10; bearing-block hardness matters; E9-style prep.

Bending: thickness & surface condition should represent service stock; width/taper geometry chosen by fixture type (four-point vs cantilever vs mandrel); rounded edge-radius ≤ 1/10 thickness; no twist.

Torsion: cylinders or tubes; L/D ≥20; straight axis before torque; avoid nicks in gauge.

ASTM E328 Stress Relaxation Test Equipment Requirements

Tensile stress relaxation testing machine	The force accuracy across the full working range shall be within 1%. It must be calibrated under both loading and unloading conditions, and equipped with automatic force adjustment functions to maintain constant constraint.
Tension:	Extensometer for strain monitoring; axial misalignment shall cause elastic strain deviation no more than 15%.
Compression:	High-rigidity parallel bearing blocks, anti-buckling fixtures and alignment devices; platens must remain parallel without lateral movement.
Bending:	Four-point bending fixtures, spherometers (for curvature measurement) or fixed mandrels; static clamps for batch testing are allowed.
Torsion:	Angular displacement transducers and coaxial collet grips; bending and axial strain shall not exceed 1% of the target torsional strain.
Heating & Temperature Chamber	Furnace, cold box or temperature-controlled chamber is allowed. The overall temperature variation of the specimen shall not exceed ±3 °C (±5 °F) or ±0.5% (whichever is larger) during the entire test. Thermal expansion-induced additional strain shall not exceed 0.000025 mm/mm. Thermocouples complying with ASTM E139. Continuous temperature recording is recommended.

Key Test Parameters & Mandatory Stipulations

Parameter	E328 Stipulation / Guideline
Constraint definition	Specified in terms of strain (not just "load"), because strain is the true boundary condition being held constant
Zero time t₀	Defined as instant the desired constraint condition is first achieved— not "when you walk away"— and loading history before t₀ is acknowledged as important
Loading rate	"Reasonably rapid, without impact or vibration" so relaxation during loading stays small; compression part caps at ≤100 ksi/min (690 MPa/min) for stress application rate
Strain-hold control band	Tight: ±0.000025 in./in. class effectively demanded by the control philosophy & torsion ±1% strain note — the point is: drift = poisonfor relaxation data
Temperature stability	±3 °C (±5 °F) or ±0.5% class; excursions can be reject-level unless provably insignificant
Alignment / nonaxiality	Tension: ≤15% side-to-side strain difference at RT; verified per E1012
Environment	Draft-free lab (±3 °C), dwell for dimensional stability before loading at temperature; strain-gage/expansivity considerations at non-ambient
Duration	Hours–thousands of hours; E328 freely admits these are long-time tests, often unsuited for routine QC, but essential for design data

ASTM E328 Stress Relaxation Standard Test Procedures

Specimen Mounting & Inspection: Fix the specimen on fixtures, check axial alignment and concentricity, and install extensometers, thermocouples or displacement sensors.

Thermal Stabilization (for non-ambient tests): Heat/cool to the specified temperature and hold until the specimen’s size and temperature are stable.

Initial Loading: Apply force, moment or torque quickly without impact until the preset initial strain/deflection/twist is reached; mark this moment as zero time (t₀).

Formal Relaxation Test: Activate the automatic control system to maintain constant constraint. Continuously or periodically record force, stress, temperature and time data.

Post-Test Operation: Stop loading after the scheduled test duration. For the elastic springback method, unload and measure springback deformation.

Data Processing & Report Generation: Calculate remaining stress, relaxed stress and relaxation rate, draw relaxation curves, and compile a complete test report.

Industry / Application Fields

Bolted / riveted / fastened joints — ensures permanent tightness: preload loss via relaxation can cause leaks, joint slip, or fatigue failures.

Press/shrink/interference fits — held strain is the whole point of the design; relaxation = fit loses grip.

Springs — constraining force decay (leaf springs, flat springs, torsion bars).

Gaskets & seals — relaxation reduces contact pressure that keeps fluid/gas contained.

Electrical/mechanical contacts — e.g. solderless wrapped connections (hoop stress relaxation).

Prestressed concrete — wire tendon constraining force stability.

Residual-stress thermal relief — judging heat-treatment effectiveness by how stresses relax at temperature.

Related Standard:

ASTM E328	Standard Test Methods for Stress Relaxation for Materials and Structures
ASTM E139	Creep / creep-rupture methodology & temperature-measurement discipline (referenced for temp practice)
EN 10319-1	Metallic materials. Tensile stress relaxation testing. Procedure for testing machines
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.

Torsion Testing Machine is mainly used to execute torsion test on various materials, measure torsion angles, angular stiffness and other content; record the testing data. Replace only with the appropriate clamps, it can complete the other component, bolts torsion test.

Q1: What does ASTM E328 cover that a standard tensile test (E8) does not?

A: A tensile test (E8) measures how much load a material can hold before it breaks. ASTM E328 measures how much load a material loses over time while it is held at a fixed length (constant strain).

In an E328 test, you stretch a specimen to a specific length and then lock that length in. Over days, weeks, or months, the stress inside the metal "relaxes" due to micro-creep. This is the only standardized way to predict if a bolt will stay tight or a spring will stay strong after years of service.

Q2: My part is a spring. Should I use Tension (A), Bending (C), or Torsion (D)?

A: ASTM E328 provides a decision matrix in the Introduction:

Match the service condition: If you are testing a bolt, use Tension (Part A). If you are testing a leaf spring, use Bending (Part C). If you are testing a torsion bar, use Torsion (Part D).

Simplicity vs. Accuracy:

Tension is the most "unequivocal" (cleanest math), but you must worry about buckling and alignment.

Bending is great for thin strips and uses simpler, cheaper fixtures (like mandrels or clamps), allowing you to put many specimens in one oven. However, the stress state is complex (higher on the outside, zero in the middle).

Compression (Part B) is specifically for seals and gaskets where a crush load is held over time.

Q3: I already do Creep testing per ASTM E139. Why do I need E328?

A: They are two sides of the same coin, but they answer different questions:

ASTM E139 (Creep): You hang a weight (constant load/stress) and measure how much the part stretches (strain) over time. Result: "How much will my pipe sag?"

ASTM E328 (Relaxation): You lock a part to a fixed length (constant strain) and measure how much the load drops over time. Result: "Will my bolt still be tight next year?"

In many high-temperature joints (like a flange), both happen simultaneously: the bolt relaxes (E328) while the gasket creeps (E139).

Q4: I already do Creep testing per ASTM E139. Why do I need E328?

A: They are two sides of the same coin, but they answer different questions:

ASTM E139 (Creep): You hang a weight (constant load/stress) and measure how much the part stretches (strain) over time. Result: "How much will my pipe sag?"

ASTM E328 (Relaxation): You lock a part to a fixed length (constant strain) and measure how much the load drops over time. Result: "Will my bolt still be tight next year?"

In many high-temperature joints (like a flange), both happen simultaneously: the bolt relaxes (E328) while the gasket creeps (E139).

Q5: What is the key difference between stress relaxation and creep?

A: They are two typical time-dependent mechanical behaviors of materials:

Stress relaxation (tested by ASTM E328): Constant strain/constraint, stress gradually decreases over time.

Creep: Constant stress, strain gradually increases over time.

In practical components like bolted joints, these two phenomena often occur simultaneously.

Q6: Why is the ASTM E328 stress relaxation test so important for materials and engineering?

A:Guarantee structural safety: Most components like bolts, springs and prestressed parts work under constant strain for decades. Excessive stress relaxation causes preload loss, joint loosening or structural failure. This test predicts long-term service reliability.

Guide material selection and design: Engineers select low-relaxation materials for critical components according to test data and optimize structural design based on different loading modes.

Optimize manufacturing processes: Test results help evaluate residual stress in forgings, weldments and machined parts, and optimize heat treatment and machining techniques.

Unify test data: As a widely accepted American standard, it ensures data comparability across laboratories and supports product certification and cross-regional trade.

Support failure analysis: It provides reference data to trace the root cause of component failure due to prestress or elastic force attenuation.

Q7: What is the requirement for the initial loading speed?

A: Apply the initial force, moment or torque rapidly but without impact or vibration. This minimizes extra stress relaxation caused during the loading process. The moment the target strain is reached is defined as zero time t0.

Q8: How to present and analyze test data?

A:Common plotting forms: Remaining stress/relaxed stress vs. time, log stress vs. log time.

For material comparison: Calculate and plot the fraction of initial relaxed stress ( (Initial stress − Remaining stress) / Initial stress ).

For bending and torsion tests: Use specified elastic mechanics formulas to calculate stress, and clearly list all formulas in the test report.

Q9: Why is a test judged invalid if the specimen has obvious axial misalignment?

A: Axial misalignment will introduce extra bending or shear stress, resulting in uneven stress distribution on the specimen. This completely distorts the real relaxation data under uniaxial strain and makes the test results non-repeatable.

Q10: What are the typical acceptable remaining stress ratios for spring products?

A: It depends on service scenarios:

Critical sealing and structural springs: Remaining stress > 95% after 1000 hours.

Precision springs: Remaining stress > 90% after 1000 hours.

Ordinary automotive and industrial springs: 80% ~ 90% remaining stress is acceptable.

Q11: Are there unified precision and bias data for ASTM E328 test results?

A: No official unified precision and bias data has been established via sufficient inter-laboratory tests. Existing round-robin tests show that the result deviation among laboratories is generally within 10%.

Q12: Can multiple specimens be tested simultaneously in one furnace for bending relaxation tests?

A: Yes. Bending relaxation uses relatively simple static fixtures and requires lower load. Multiple specimens can be placed in one temperature-controlled chamber for batch testing, which improves test efficiency.

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