Stress Relaxation Testing of Fiber Fabrics for Structural Maintenance and Strengthening

Stress relaxation testing of fiber fabrics for structural maintenance quantifies the time-dependent loss of tension in carbon, glass, aramid, and basalt fiber sheets impregnated with structural resin and held under constant strain. This behavior is a decisive quality indicator for prestressed FRP strengthening systems and for the long-term reliability of repaired concrete, masonry, and steel structures. This article explains the purpose, specimens, test items, procedures, and applications of the stress relaxation test in accordance with recognized standards such as GB 50728, GB 50608, and ASTM D2990.

Technical Overview

Fiber fabrics used for structural maintenance—unidirectional or woven sheets of carbon, glass, aramid, or basalt fibers saturated with epoxy or vinyl ester resin—are among the most widely used materials for strengthening and rehabilitating concrete bridges, buildings, tunnels, chimneys, masonry walls, and steel members. When these fabrics are installed under tension, as in prestressed carbon fiber strengthening systems, or when they sustain load for decades, the internal stress gradually decays even though the imposed strain remains constant. This phenomenon is called stress relaxation, and it directly controls the long-term effectiveness of the strengthening system, the magnitude of prestress loss, and the durability margin of the strengthened member.

Stress relaxation testing provides a quantitative measure of this decay. A resin-impregnated fabric specimen is loaded in tension to a specified initial stress, the strain is then held constant, and the force (and therefore the stress) is recorded continuously over time. The resulting relaxation curve supports material qualification, design allowances for prestress loss, and factory production control. In China, the fitness-for-purpose requirements for strengthening-grade fiber composites are defined in GB 50728, while design provisions for FRP-strengthened structures are given in GB 50608; internationally, the general relaxation test methodology follows ASTM D2990, with tensile reference values determined per ASTM D3039, ASTM D7565, or ISO 527, and design context provided by ACI 440.2R.

Testing Samples

Unidirectional Carbon Fiber Fabric (CFRP Sheet)

Carbon fiber fabric is the dominant material for prestressed maintenance applications because of its high elastic modulus, high strength, and superior relaxation resistance. Specimens are straight strips cut along the fiber direction, typically 15–25 mm wide with a gauge length of at least 200 mm. Because field performance is governed by the cured laminate rather than the dry fabric, the sheet is fully impregnated with the specified structural resin using the wet lay-up process and allowed to cure before testing. Bonded aluminum or composite tabs are used at the ends to prevent premature grip failure.

Glass Fiber Fabric (GFRP Sheet)

E-glass and S-glass fabrics are commonly used for shear strengthening, seismic retrofit, and confinement wrapping. Their lower fiber modulus makes the cured laminate comparatively more resin-sensitive, so relaxation behavior tends to be less favorable than that of carbon systems and must be verified separately for each resin system. Specimen geometry and preparation follow the same principles as for carbon fiber sheets.

Aramid and Basalt Fiber Fabrics

Aramid fabrics, used where impact and blast resistance are required, and basalt fabrics, used as an economical intermediate option between glass and carbon, are tested with identical procedures. The relaxation characteristics of these fibers differ significantly from carbon and glass, so system-specific data are essential before use in prestressed maintenance applications.

Resin-Impregnated Laminate Coupons

All relaxation specimens are cured fabric-resin laminates with a typical fiber volume fraction of 30–60% for wet lay-up systems. Laminates are cured for at least seven days at 23 ± 2 °C and conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours before testing, consistent with ISO 291 or ASTM D618 conditioning practice. Thickness and width are measured at several locations, and at least five valid specimens are tested for each condition.

Testing Items

The stress relaxation test program for maintenance-grade fiber fabric typically reports the following results:

• Initial stress at the start of the holding period
• Residual stress at defined time intervals, commonly 1 h, 10 h, 100 h, 500 h, and 1000 h
• Stress relaxation rate at time t, calculated as R(t) = (σ0 − σt) / σ0 × 100%
• Complete stress-versus-time relaxation curve on linear and logarithmic axes
• Time-dependent relaxation modulus E(t) = σ(t) / ε0
• Relaxation loss percentages at 100 hours and 1000 hours
• Extrapolated long-term relaxation for design service lives using power-law fitting
• Influence of elevated temperature and humidity on relaxation loss
• Batch-to-batch comparison of relaxation behavior for quality control purposes

Testing Methods

Specimen Preparation and Conditioning

Fabric is cut with the fibers aligned to the load axis, impregnated with the project resin by the wet lay-up method, and cured according to the manufacturer's schedule. After conditioning, specimens are inspected for voids, fiber misalignment, or resin-starved areas, and any defective coupon is discarded. Reference tensile tests per ASTM D3039, ASTM D7565, or ISO 527 are performed beforehand to establish the short-term tensile strength and modulus, which define the target initial stress for the relaxation test.

Tensile Loading to the Target Stress Level

The specimen is mounted in a universal testing machine or a dedicated relaxation rig with mechanical wedge grips and carefully aligned to eliminate bending. Loading proceeds at a controlled rate, typically 1–2 mm/min, until the specified initial stress is reached—commonly 30% to 60% of the short-term tensile strength, or a level representative of the intended field prestress. The initial stress σ0 is recorded after the load stabilizes, normally within one to two minutes of reaching the target level.

Constant-Strain Holding Phase

Once the target strain is achieved, the deformation is locked by arresting the crosshead or by closed-loop control on the extensometer signal. The decaying force is recorded continuously or at logarithmically spaced intervals throughout the holding period. Standard test durations are 100 hours for screening and quality control and 1000 hours for qualification testing. The test environment is maintained at 23 ± 2 °C and 50 ± 10% relative humidity, and temperature is logged continuously because even small thermal fluctuations measurably affect the relaxation response.

Data Processing and Evaluation

The relaxation rate at each interval is computed as R(t) = (σ0 − σt) / σ0 × 100%, where σ0 is the initial stress and σt is the stress at time t. Plotted on log-log axes, relaxation data for well-consolidated laminates generally follow a straight line, which permits extrapolation from a 1000-hour test to design service lives of 50 years with appropriate safety factors. Project specifications and product standards commonly limit the 1000-hour relaxation loss of prestressing-grade fabric systems to no more than a few percent; higher losses must be compensated by additional prestress or by reduced design stress. The test report includes specimen details, σ0, a table of σt values, the relaxation curve, the calculated relaxation rates, and the complete environmental record.

Applications

Stress relaxation data for maintenance fiber fabrics are used throughout the engineering lifecycle of strengthening projects:

• Qualification of prestressed carbon fiber fabric systems for bridge girder, slab, and cable-stay strengthening
• Verification of prestress loss assumptions in design calculations per GB 50608 and ACI 440.2R
• Factory production control and batch release testing of fabric and resin systems
• Comparative screening of resin formulations, curing agents, and fiber surface finishes for time-dependent performance
• Long-term durability assessment when combined with accelerated aging involving temperature, moisture, and alkaline exposure
• Input parameters for finite-element and service-life models of strengthened structures
• Technical documentation for maintenance contracts, tender submissions, and acceptance of repair works

FAQ

What is the difference between stress relaxation and creep?

Creep is the progressive increase in strain of a material held under constant stress, whereas stress relaxation is the progressive decrease in stress of a material held under constant strain. Both originate from the viscoelastic behavior of the fiber-resin composite; relaxation is the governing mechanism in prestressed fabric applications because the fabric length is fixed once installed.

Why is stress relaxation critical for prestressed fabric strengthening?

In prestressed maintenance systems, the fabric is tensioned and then anchored or bonded to the member. Any relaxation of the fabric permanently reduces the effective prestress, lowering the crack-closing force and the load-carrying benefit. Designers must quantify this loss through testing and compensate for it during installation.

How long does a stress relaxation test take?

Screening tests of approximately 100 hours are used for routine quality control, while qualification tests of 1000 hours are standard for system approval. Longer durations or validated extrapolation methods are used when service-life predictions of 50 years or more are required.

What initial stress level is applied in the test?

The initial stress is set at a level representative of the intended field prestress, typically between 30% and 60% of the short-term tensile strength of the cured laminate. Comparative testing must always use the same initial stress level, because relaxation loss increases with the applied stress.

Which standards govern stress relaxation testing of maintenance fiber fabrics?

GB 50728 defines the fitness-for-purpose requirements for strengthening-grade fiber composites in China, and GB 50608 provides the corresponding design provisions. ASTM D2990 supplies the general framework for relaxation testing of polymeric materials, ASTM D3039, ASTM D7565, and ISO 527 establish the tensile reference properties, and ACI 440.2R offers design guidance for FRP-strengthened structures.

How are short-term results extrapolated to a design life of decades?

Relaxation data plotted on logarithmic stress-loss versus logarithmic time axes usually follow a straight line, allowing a fitted power law to extrapolate from a 1000-hour test to a 50-year design life. Conservative safety factors are applied to account for temperature, humidity, and statistical scatter.

Can the test be performed at elevated temperature or humidity?

Yes. Tests at elevated temperatures such as 40 °C or 60 °C, and at high relative humidity, accelerate the relaxation process and support time-temperature superposition analysis. These conditions are specified for structures in hot climates, industrial environments, or locations near heat sources.

Are dry fabrics tested, or only impregnated laminates?

Relaxation testing is performed on fully impregnated, cured laminates, because the interaction between fibers and resin governs the time-dependent behavior seen in the field. Dry fabric tensile tests remain part of incoming inspection for verifying fiber areal weight and tensile strength, but they do not substitute for laminate relaxation data.