Tension Spring Stress Relaxation Testing: Methods, Standards, and Applications

Tension spring stress relaxation testing measures the time-dependent loss of load that occurs when an extension spring is held under constant deflection, providing essential data for predicting long-term performance in service. Performed by accredited third-party laboratories in accordance with standards such as ASTM E328, GB/T 10120, and EN 10319, this test helps manufacturers qualify materials, validate spring designs, and prevent premature load loss in critical assemblies. This article explains the test principle, sample requirements, measured parameters, test methods, and typical industrial applications of stress relaxation testing for tension springs.

Technical Overview

A tension spring, also known as an extension spring, stores mechanical energy by resisting a tensile load through closely coiled helical windings that separate as the spring is stretched. When a tension spring is held at a fixed extension for a prolonged period, the force it exerts gradually decreases even though the deflection remains constant. This phenomenon is known as stress relaxation, and it is one of the most important degradation mechanisms limiting the service life of springs.

Stress relaxation differs fundamentally from creep. In a creep test, the specimen elongates under a constant applied stress, whereas in a relaxation test the total strain is held constant and the resisting stress (spring load) decays with time. The underlying mechanism is micro-plastic flow within the wire material: dislocation movement, grain-boundary sliding, and metallurgical changes allow the material to redistribute elastic strain into small permanent deformation. The rate of relaxation accelerates strongly with increasing torsional stress in the wire, elevated temperature, and time under load.

For tension springs, uncontrolled relaxation can lead to insufficient pulling force, loss of preload in latches and linkages, degraded electrical contact pressure, and eventual functional failure of the assembly. Stress relaxation testing therefore quantifies the load retention capability of a spring design under simulated service conditions, enabling engineers to establish safe working deflections, select appropriate wire materials, and demonstrate compliance with customer or regulatory durability requirements.

Testing Samples

Cold-Coiled Extension Springs

The most common test articles are cold-coiled helical tension springs manufactured from cold-drawn spring wire, including music wire (ASTM A228), oil-tempered carbon steel wire (ASTM A229), chromium-silicon wire (ASTM A401), chromium-vanadium wire (ASTM A231), and stainless steel spring wire (ASTM A313). These springs cover the broad range of automotive, industrial, and electronics applications and are typically tested in the as-delivered, stress-relieved condition.

Hot-Coiled Heavy-Duty Extension Springs

Large-diameter tension springs coiled from hot-rolled bar stock are used in railway, marine, and energy-sector equipment. Because of their high spring loads, these springs require high-capacity load frames or dedicated relaxation rigs with fixtures designed to grip the prominent end hooks without slipping or stress concentration artifacts.

Special Alloy Springs

Springs intended for high-temperature or corrosive environments, such as those made from nickel-based superalloys or copper alloys like phosphor bronze and beryllium copper, are tested at their rated service temperature. Special alloy testing is essential for valve actuators, exhaust-system components, and electrical contact hardware where sustained load retention at elevated temperature is a safety requirement.

Raw Wire Specimens

In addition to finished springs, straight lengths of spring wire can be tested in tensile relaxation fixtures to characterize the intrinsic relaxation behavior of a wire lot. This baseline material data supports material qualification, incoming inspection, and finite-element or analytical predictions of relaxation in finished spring geometries.

Testing Items

A comprehensive tension spring stress relaxation program evaluates the following parameters:

• Initial spring load at the specified test deflection (P0), verified before the holding phase begins
• Residual spring load at defined time intervals (Pt), for example at 1, 10, 100, and 1,000 hours
• Load loss percentage and relaxation rate, calculated as (P0 − Pt) / P0 × 100%
• Spring rate (stiffness) measured before and after the relaxation exposure to detect stiffness degradation
• Permanent set, evaluated as the change in free length after the spring is released at the end of the test
• Calculated initial torsional stress in the wire at test deflection, including Wahl stress correction
• Relaxation curves fitted to logarithmic or exponential decay models for extrapolation to design life
• Temperature-dependent relaxation behavior, when testing is performed at elevated temperature

Testing Methods

The predominant method for springs is the constant-deflection (fixed-strain) relaxation test. The spring is extended to a specified length, typically corresponding to the maximum working deflection, and rigidly held in a fixture. The decay of the tensile load is then recorded either continuously through a calibrated load cell or periodically by removing the spring and measuring its load at the reference deflection on a calibrated spring rate tester. Periodic measurement is common for long-duration tests because it allows several specimens to share one chamber while each load measurement is traceable to a calibrated reference machine.

For elevated-temperature testing, the entire fixture assembly is placed inside a temperature-controlled environmental chamber or laboratory oven. The specimen is pre-loaded at room temperature, thermal soak is applied until the spring reaches uniform temperature, and the load decay is monitored at temperature. Thermocouples attached near the spring verify temperature stability throughout the exposure. Accelerated evaluation is possible by testing at temperatures above the service temperature and applying time-temperature extrapolation techniques, such as the Larson-Miller parameter, to estimate long-term load loss at operating conditions.

Testing is generally conducted within the framework of the following material and spring standards, adapted to the geometry of helical tension springs:

• ASTM E328 — Standard Test Methods for Stress Relaxation Tests for Materials and Structures
• GB/T 10120 — Metallic materials, stress relaxation test method
• EN 10319-1 — Metallic materials, tensile stress relaxation testing, testing machines and procedures
• JIS Z 2276 — Method of stress relaxation test for metallic materials
• ASTM A228, A229, A401, A231, and A313 — material specifications for common spring wire grades
• GB/T 1239 series — specifications governing dimensions, tolerances, and load verification of cold-coiled helical tension springs

Typical acceptance criteria are agreed between the customer and the laboratory; a common requirement in automotive and industrial specifications is a maximum load loss of 5% to 10% after 100 hours at the maximum service temperature, with extrapolated loss limits over the design life.

Applications

Stress relaxation testing of tension springs supports product reliability and regulatory compliance across many industries:

• Automotive: throttle return springs, clutch and brake return springs, hood and door latch springs, seat and belt retractor mechanisms, and transmission control linkages
• Aerospace: actuator return springs, access-panel latches, valve mechanisms, and safety-critical locking hardware requiring demonstrated load retention
• Electronics and electrical equipment: relay and switch contact springs, connector retention springs, and circuit-breaker trip mechanisms
• Medical devices: surgical instrument return springs, drug-delivery actuators, and disposable device mechanisms with tight force tolerances
• Industrial equipment: safety relief valves, counterbalance systems, door closers, and agricultural machinery components
• Railway and heavy industry: heavy-duty tension springs in coupling, suspension, and braking systems
• Oil and gas: downhole tool springs exposed to elevated temperature and corrosive media over extended service periods

FAQ

What is the difference between stress relaxation and creep?

Creep is time-dependent elongation under constant stress, while stress relaxation is the time-dependent decay of stress under constant strain. A tension spring in service, held at a fixed deflection by its mounting, experiences relaxation: its length stays the same while the pulling force it delivers gradually decreases.

How long does a stress relaxation test take?

Common test durations range from 24 hours for screening tests to 1,000 hours or longer for qualification programs. Long-term behavior is often estimated by fitting a relaxation curve to the measured data and extrapolating, or by performing accelerated tests at elevated temperature with time-temperature extrapolation methods.

What load-loss criterion is typically considered acceptable?

Acceptance limits depend on the application, but specifications frequently require less than 5% to 10% load loss after 100 hours at maximum working deflection and temperature, with a defined maximum extrapolated loss over the design life. Safety-critical springs may have stricter limits defined by the end customer or product standard.

Can stress relaxation be tested at room temperature?

Yes. Although relaxation proceeds faster at elevated temperature, cold-drawn spring steels exhibit measurable load loss even at ambient temperature, particularly at high working stresses. Room-temperature tests over several hundred hours are routinely used for quality control and design margin verification.

What information is needed to start a test?

The laboratory typically requires the spring drawing with dimensions and material grade, the specified test deflection or initial load, the test temperature and duration, the required measurement intervals, and the applicable acceptance criterion. Sample quantity should also be confirmed, since replicate specimens improve the statistical confidence of the load-loss determination.