ASTM E 328-13 Standard test method for Stress Relaxation for Materials and Structures
ASTM E 328-12 covers a broad range of testing activities. These test methods cover a broad range of testing activities. To help decide the subject matter for particular testing, the standard is divided into a general four-section, which applies to all materials and structures for stress relaxation test.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM E328 provides a standard test method for measuring stress relaxation in metallic materials and structures — the decrease in stress over time under constant strain (fixed deformation). Stress relaxation is a time-dependent viscoelastic or creep mechanism that reduces the residual clamping force in bolted joints, spring preloads, and gasket seals, leading to potential leakage, loosening, or loss of functional performance.
This test method is essential for evaluating spring materials, fasteners, gaskets, and structural materials intended for long-term applications that require sustained load-bearing without deformation under constant displacement.
Scope, Applications, and Benefits
Scope
ASTM E328 evaluates:
- Stress relaxation behavior under defined initial strain and temperature
- Percent stress remaining after defined time periods
- Relaxation rate and long-term extrapolation
- Effect of temperature on stress relaxation kinetics
Applications
- Spring material qualification for constant-force applications
- Fastener bolt load retention in flange assemblies
- Gasket and sealing material preload retention
- Electrical contact spring reliability evaluation
- High-temperature structural component design data
Benefits
- Quantifies load retention critical for bolted and spring assemblies
- Enables life prediction for stress-relaxation-sensitive applications
- Supports material selection for long-term load retention
- Provides data for creep-relaxation constitutive model development
- Essential for nuclear, aerospace, and pressure vessel design
Test Process
Specimen Preparation
Tensile or spring specimens are machined to ASTM E328 geometry; initial dimensions are measured precisely.
1Initial Loading
cThe specimen is loaded to the specified initial stress level at a controlled strain rate; initial strain and stress are recorded and the crosshead/fixture is locked at constant displacement.
2Relaxation Monitoring
Stress (or load) is monitored as a function of time at the test temperature for the defined test duration (hours to thousands of hours); data are recorded at defined intervals.
3Reporting
Percent stress remaining (PSR) at defined time intervals and test temperature is calculated and plotted; long-term behavior may be extrapolated using a logarithmic or power-law model.
4Technical Specifications
| Parameter | Details |
|---|---|
| Test Mode | Constant strain (tensile, torsional, or bending) |
| Temperature Range | Ambient to 800 °C (material dependent) |
| Test Duration | 1 hour to 10,000+ hours |
| Measured Output | Percent stress remaining (PSR) vs. time |
| Applicable Materials | Metals, alloys, spring steels, superalloys |
Instrumentation Used for Testing
- Servo-hydraulic or deadweight testing machine with displacement locking
- High-temperature furnace with accurate thermal control (±2 °C)
- Load cell or stress monitoring system
- Extensometer or displacement transducer
- Data acquisition system (long-duration logging)
Results and Deliverables
- Stress relaxation curves (PSR vs. time at defined temperatures)
- Percent stress remaining at 100, 1000, and 10,000 hours
- Activation energy for relaxation (Arrhenius analysis)
- Long-term PSR extrapolation
- Full stress relaxation test report per ASTM E328
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Frequently Asked Questions
Creep is the time-dependent increase in strain under constant stress. Stress relaxation is the time-dependent decrease in stress under constant strain. In bolted joints, both occur simultaneously — the bolt relaxes (stress decreases) while the gasket creeps (deformation increases under the bolt load).
Requirements vary by application. Precision spring applications may require >90% stress remaining after 1000 hours at service temperature. Automotive and industrial springs may tolerate 80–90% PSR. Critical sealing or structural applications may require >95%.
Short-term relaxation data (up to 1000 hours) can be extrapolated to longer durations using logarithmic or power-law relationships derived from ASTM E328 test results. This provides estimated bolt load retention over the design service life, which is then used to set initial preload requirements.
Austenitic stainless steels and nickel-based superalloys have superior stress relaxation resistance. Carbon steels, aluminum alloys, and some copper alloys show significant relaxation above 100–200 °C. Spring steels (Si-Cr, Si-Mn grades) are specifically designed for high stress retention.
Yes. For materials used in liquefied gas or cryogenic applications, stress relaxation testing at liquid nitrogen temperatures (-196 °C) or liquid helium temperatures is feasible with appropriately instrumented cryogenic test fixtures. However, most materials show very low stress relaxation rates at cryogenic temperatures.
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