ASTM D7337 Tensile Creep-Rupture Testing for FRP Composite Bars
Tensile creep rupture of a Fibre-Reinforced Polymer (FRP) matrix composite bars can be tested using ASTM D7337 standard test method. Creep is the time-dependent deformation (strain) under sustained force (stress). Creep rupture is a material failure caused by sustained force (stress) over time. Creep rupture strength is the stress-causing failure after a specified period from the initiation of a sustained force.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
ASTM D7337 Tensile Creep-Rupture Testing-Overview
ASTM D7337 covers tensile creep-rupture testing of fibre-reinforced polymer (FRP) composite bars — primarily GFRP and CFRP rebar used as internal reinforcement in concrete structures. The test determines how long an FRP bar can sustain a specified fraction of its short-term tensile strength before stress-rupture failure occurs. Unlike conventional creep, which measures slow deformation under load, creep-rupture is catastrophic failure after sustained loading below the short-term tensile strength — a safety-critical failure mode for FRP rebar in bridges, parking structures, and marine infrastructure.
The test generates a creep-rupture envelope — a log-log plot of sustained load fraction versus time to failure — which design engineers use to establish the maximum allowable sustained stress for FRP reinforcement over a 50 to 100-year service life. This data is a direct input to ACI 440.1R and CSA S807 design guidelines, which typically limit sustained stress in GFRP bars to 20–25% of short-term tensile strength for a 50-year service life.
Scope, Applications, and Benefits
Scope
ASTM D7337 covers the determination of tensile creep-rupture strength of FRP composite bars subjected to sustained axial tensile loads over extended periods. Specimens are loaded to specified fractions of their short-term tensile strength and held until rupture or until the prescribed test duration is reached. The resulting time-to-failure data at multiple load levels is plotted on a log-log scale to construct the creep-rupture envelope, which is extrapolated to predict performance at design service life durations. The standard applies specifically to FRP bars intended for use as internal reinforcement in concrete, including GFRP and CFRP rebar and FRP prestressing tendons.
- Determines tensile creep-rupture strength of GFRP and CFRP composite bars under sustained axial loading
- Applies to FRP bars used as internal reinforcement in concrete structures and FRP prestressing tendons
- Sustained load levels typically range from 20–70% of short-term tensile strength
- Minimum test duration per load level: data points required out to at least 1,000 hours for valid extrapolation
- Minimum 3 specimens per load level; multiple load levels required to construct the full creep-rupture envelope
- Time-to-failure data plotted on a log-log scale; linear regression used for extrapolation to design service life
- Test conducted at ambient or elevated temperature as specified
- Data output feeds directly into ACI 440.1R and CSA S807 sustained stress limit calculations
Applications
Applications
- Bridge deck FRP rebar qualification for GFRP and CFRP bars under sustained dead load and traffic loading over 75–100 year design life.
- Parking structure internal reinforcement where chloride-induced corrosion of steel is a concern and long-term load capacity must be demonstrated.
- Marine piling and substructure FRP bar design in permanently loaded structures exposed to sustained stress in aggressive environments.
- Prestressed FRP tendon qualification to establish long-term stress limits where creep-rupture is the governing failure mode.
- Tunnel lining and underground structure reinforcement in permanently loaded applications requiring verified long-term strength.
- Seismic zone infrastructure where non-magnetic, non-corrosive FRP reinforcement with confirmed sustained load capacity is specified.
Benefits
- Provides the creep-rupture envelope required by ACI 440.1R and CSA S807 to set the sustained design stress limit for FRP reinforcement.
- Distinguishes safe long-term stress levels from short-term tensile strength, preventing unconservative design in sustained-load applications.
- Supports FRP rebar product qualification for infrastructure projects with 50–100 year design life requirements.
- Enables direct comparison of GFRP and CFRP bar performance under sustained load for material selection decisions.
- Log-log extrapolation method allows long-term predictions from test durations of 1,000–3,000 hours, avoiding impractically long test programs.
- Applicable to both new product development and third-party qualification testing for project specification compliance.
ASTM D7337 Tensile Creep-Rupture Testing-Test Process
Specimen Preparation & Setup
FRP composite bars are aligned axially and secured in appropriate gripping or anchoring fixtures.
1Load Application
A constant tensile force is applied smoothly within the specified loading time to avoid impact effects.
2Creep Rupture Monitoring
The specimen is maintained under sustained load and monitored until rupture occurs.
3Data Recording & Verification
Time-to-failure and applied force levels are recorded and reviewed in accordance with test requirements.
4ASTM D7337 Tensile Creep-Rupture Testing-Technical Specifications
| Parameter | Details |
|---|---|
| Applicable Materials | Fibre-reinforced polymer matrix composite bars |
| Loading Mode | Tensile |
| Load Application Time | Approximately 20 seconds to 5 minutes |
| Test Duration | Until creep rupture occurs |
| Test Environment | Ambient laboratory conditions |
| Measured Outputs | Force ratio and creep rupture time |
| Sustained load levels | 20–70% of short-term tensile strength (Fu) |
| Minimum test duration | Data points required to at least 1,000 hours per load level |
Instrumentation Used for Testing
- Universal testing machine or creep rupture testing frame
- Tensile gripping and anchoring fixtures
- Load the application and control system
- Load measurement devices
- Data acquisition and recording software
- Calibration and verification equipment
Results and Deliverables
- Time-to-creep rupture data
- Force ratio versus rupture time curves
- Semi-logarithmic creep rupture plots
- Stress level versus time-to-failure relationship
- compliance report
Frequently Asked Questions
ASTM D7337 determines the creep-rupture life of FRP reinforcing bars by subjecting them to a sustained constant load and determining the time to failure. This helps to establish the long-term load limit.
FRP rebar is prone to static fatigue when subjected to sustained loads. The ASTM D7337 standard offers reduction factors that enable the transformation of short-term strength into safe long-term strength.
ASTM D7337 is used in transport infrastructure, coastal engineering, healthcare facilities, and tunnel construction where high corrosion resistance, non-magnetic properties, and long-term structural integrity of FRP reinforcement are required.
ASTM D7337 specifies that full sections of FRP rebar with anchored ends are required. Steel pipe anchors filled with high-strength resin are used to transfer loads without crushing the bar during long-duration tests.
This standard measures the bearing strength and bearing stress–strain response of composite materials when loaded through a fastener hole, helping evaluate joint performance, damage progression, and failure behavior.
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