Strain Gauge Creep Performance Testing: Methods, Standards, and Applications

Strain gauge creep performance testing evaluates the time-dependent stability of strain gauges and strain gauge-based transducers under sustained mechanical load, a critical quality parameter for load cells, force sensors, and precision measurement systems. This article explains the underlying mechanisms of strain gauge creep, the applicable international standards such as ASTM E251, GB/T 13992, and OIML R60, typical test procedures, and how accredited third-party laboratories assess creep and creep recovery. Understanding these testing practices helps manufacturers select, validate, and certify strain gauges for demanding metrological applications.

Technical Overview

Strain gauge creep refers to the time-dependent change in the output signal of a bonded resistance strain gauge while the applied strain is held constant. Although the mechanical input remains fixed, the indicated strain gradually drifts, typically within the first minutes to hours after loading. This behavior originates primarily from the viscoelastic relaxation of the adhesive layer and the polymeric backing film that bond the metallic grid to the measured structure, as well as from minor stress redistribution within the grid itself.

In precision instrumentation, creep directly affects measurement uncertainty. For load cells and force transducers used in legal metrology, weighing systems, and process control, creep is one of the dominant error sources and is strictly limited by accuracy class requirements. Consequently, strain gauge creep performance testing is an essential part of product qualification, incoming inspection, and type evaluation for both discrete strain gauges and finished transducers. Testing is commonly performed by accredited third-party laboratories in accordance with recognized standards to ensure impartial, traceable results.

Testing Samples

Foil Strain Gauges

The most widely tested sample type, consisting of a photo-etched metallic grid (constantan, karma, or modified alloys) on a flexible backing. Foil gauges are evaluated for intrinsic creep characteristics after bonding to a standardized test beam.

Semiconductor Strain Gauges

Piezoresistive silicon elements that offer high gauge factors but exhibit more pronounced creep and temperature sensitivity. Creep testing is especially important to characterize their long-term stability under constant load.

Weldable and High-Ttemperature Strain Gauges

Gauges designed for harsh environments, attached by spot welding or high-temperature cements. Their creep behavior must be verified at both room temperature and operating temperature.

Strain Gauge-Based Transducers

Complete sensor assemblies, including load cells, torque sensors, pressure transducers, and force sensors, in which the combined creep of the elastomer, gauges, adhesive, and compensation circuitry is measured as a system-level parameter.

Calibration Beams and Test Specimens

Constant-stress cantilever beams or proving frames instrumented with candidate gauges, used to compare creep performance across gauge batches, adhesives, and installation processes.

Testing Items

Creep performance evaluation typically covers the following measurable characteristics:

• Creep under constant load: the maximum deviation of output from the reference reading taken shortly after load application, expressed in µm/m (microstrain) or as a percentage of full-scale output.
• Creep return (creep recovery): the residual zero shift observed over time after the load is removed, indicating how completely the gauge system returns to its unloaded state.
• Zero drift: slow variation of the unloaded output over extended durations, distinguishing pure zero instability from load-dependent creep.
• Creep at elevated or low temperatures: verification of creep behavior at the rated operating temperature range of the gauge or transducer.
• Humidity and environmental influence: assessment of moisture ingress effects on the adhesive and backing, which can amplify creep in unprotected installations.
• Direction-dependent creep: separate evaluation under tensile and compressive strain, since grid and adhesive behavior is often asymmetric between the two loading directions.

Testing Methods

Reference Standards

Testing is generally conducted in accordance with established national and international standards, including ASTM E251 for performance characteristics of metallic bonded resistance strain gauges, GB/T 13992 for resistive strain gauges, OIML R60 for load cell metrological evaluation, and ISO 376 for the calibration of force proving instruments. Where applicable, IEC and EN documentation requirements are followed for reporting.

Test Apparatus and Setup

A typical creep test configuration includes:

• A constant-strain device, most commonly a dead-weight loaded cantilever beam producing a stable reference strain of approximately 1000 µm/m at the gauge location.
• A temperature-controlled laboratory environment, typically maintained at 23 °C ± 1 °C with controlled relative humidity.
• A high-resolution strain indicator or data acquisition system with resolution and stability significantly better than the creep magnitude to be measured, often 0.1 µm/m or finer.
• A stable, regulated excitation voltage to eliminate bridge supply drift as an error contribution.
• Timing instrumentation capable of capturing output readings at defined intervals from the instant of load application.

Test Procedure

The specimen is first prepared by bonding the gauge to the test beam or transducer elastomer using the manufacturer-specified adhesive and curing cycle, followed by complete thermal conditioning and moisture protection as appropriate. After environmental stabilization, the test proceeds as follows:

• Record the unloaded (zero) output after the specimen has stabilized in the test environment.
• Apply the constant strain or load as rapidly and smoothly as possible, ideally within one second, to establish a defined loading time reference.
• Acquire output readings at prescribed intervals, for example at 5 s, 10 s, 20 s, 30 s, 1 min, 5 min, 10 min, 20 min, and 30 min, extending to 60 minutes or longer where required by the standard or specification.
• Remove the load and record the recovery output over an equivalent period to determine creep return.
• Repeat the cycle as required to confirm repeatability of the creep signature.

Data Reduction and Evaluation

Creep is calculated as the difference between the output at each recorded time and the output at the defined reference time, normalized to the full-scale output or expressed directly in microstrain. The reported creep value is usually the maximum absolute deviation observed within the specified test duration. For transducers evaluated under OIML R60, the creep error is compared against the maximum permissible error of the assigned accuracy class over a 30-minute loading interval. Results are documented with test conditions, loading history, and instrument traceability in a formal test report.

Applications

Creep performance testing supports quality assurance and certification across many industries:

• Weighing technology: type evaluation and production verification of load cells for platform scales, batch weighing, and checkweighers, where sustained loads are common.
• Industrial automation: qualification of force and torque sensors used in robotics, assembly verification, and press-fit monitoring.
• Aerospace and defense: long-duration structural load monitoring, ground test instrumentation, and fatigue test programs requiring stable static readings.
• Automotive engineering: validation of wheel force transducers, crash test instrumentation, and durability rigs.
• Civil and structural engineering: long-term health monitoring of bridges, dams, and buildings where multi-hour and multi-day stability is essential.
• Medical and laboratory equipment: precision force measurement in infusion systems, material testing machines, and analytical instruments.

FAQ

What causes creep in strain gauges?

Creep is caused mainly by the viscoelastic flow and stress relaxation of the adhesive layer and the gauge backing, which gradually transfer strain differently to the metallic grid. Incomplete adhesive curing, excessive adhesive thickness, elevated temperature, and humidity absorption all increase creep. The sensing grid and the transducer elastomer also contribute smaller relaxation components.

How is creep differentiated from zero drift?

Creep is observed only while a constant strain is applied and is measured relative to the loaded reference reading, whereas zero drift is the slow change of the unloaded output over time. A complete evaluation measures both, together with creep return after unloading.

What are typical acceptance criteria?

For high-accuracy load cells, creep over 30 minutes is commonly limited to a small fraction of rated output, for example 0.02 % to 0.03 % FS or tighter, consistent with the applicable accuracy class. For discrete strain gauges, manufacturers specify creep in µm/m, and high-quality foil gauges typically remain within a few µm/m over one hour at room temperature.

Can creep be reduced or compensated?

Yes. Proper surface preparation, correct adhesive selection with a fully executed curing and post-curing cycle, controlled bondline thickness, and adequate moisture protection all reduce creep. Many manufacturers also supply creep-compensated gauge patterns whose grid geometry deliberately counteracts adhesive relaxation, and transducer designers may add compensation resistors to balance positive and negative creep contributions.

Which laboratories can perform creep testing?

Accredited third-party laboratories with force and strain measurement capabilities, traceable reference standards, and environmental control can perform creep testing of strain gauges and transducers in accordance with ASTM E251, GB/T 13992, OIML R60, and related standards, providing objective evidence for product certification and customer acceptance.