Universal testing machine creep performance testing is a critical method for evaluating the time-dependent deformation behavior of materials under sustained loads. By maintaining a constant force over extended periods while precisely measuring strain, engineers can determine creep strength, steady-state creep rates, and stress rupture life for metals, polymers, and composites. This article provides a comprehensive technical guide to creep testing equipment, sample requirements, test procedures, applicable standards, and typical industrial applications.
Technical Overview
Creep is the tendency of a solid material to deform progressively under the influence of sustained mechanical stress, particularly at elevated temperatures relative to the material melting point. Universal testing machine creep performance testing applies a constant tensile, compressive, or flexural load to a specimen and records strain as a function of time, generating the characteristic creep curve that defines the material long-term load-bearing capability.
A typical creep curve consists of three stages. Primary creep exhibits a high but decelerating strain rate as the material work-hardens. Secondary creep, also called steady-state creep, shows a nearly constant minimum strain rate that governs service life predictions. Tertiary creep displays accelerating strain caused by necking, internal void formation, or microstructural damage, terminating in rupture. Modern servo-controlled universal testing machines equipped with high-resolution load cells, precision extensometers, environmental chambers, and long-duration data acquisition software can reproduce these curves with laboratory-grade accuracy while maintaining load stability over test durations ranging from minutes to several thousand hours.
- Load frame stiffness and closed-loop force control ensure constant stress throughout the test.
- High-accuracy extensometry, including contact and non-contact video or laser systems, captures minute strain changes over time.
- Temperature chambers and furnaces with multi-zone control enable isothermal testing from ambient conditions up to 1200 °C or higher.
- Continuous data logging records load, strain, temperature, and elapsed time for full traceability.
Testing Samples
Metallic Specimens
Metallic creep testing typically uses cylindrical threaded-end or shoulder-loaded specimens machined per ASTM E8 and ASTM E139, with gauge lengths of 25 mm to 100 mm and gauge diameters selected to match the available load capacity. Flat rectangular specimens may be used for sheet products when geometry permits. Materials commonly evaluated include carbon and low-alloy steels, stainless steels, aluminum alloys, titanium alloys, copper alloys, and nickel-based superalloys intended for high-temperature service.
Polymer and Plastic Specimens
Polymeric materials are tested using dumbbell-shaped tensile bars per ISO 527 or ASTM D638 geometry, since polymers exhibit significant creep even at room temperature. Creep behavior of engineering plastics, thermoplastic composites, films, and pipes is evaluated under constant tensile or flexural loading in accordance with ASTM D2990 and ISO 899.
Elastomer and Composite Specimens
Rubber specimens are prepared as dumbbell or cylindrical compression buttons to assess long-term set and load retention. Fiber-reinforced polymer composites, adhesives, and welded or bolted joints may also be tested to characterize viscoelastic creep and joint relaxation behavior under sustained load.
Testing Items
Universal testing machine creep performance testing covers a full portfolio of long-term mechanical property measurements:
- Total creep strain and creep strain versus time curves at specified stress and temperature.
- Minimum (steady-state) creep rate derived from the secondary creep stage.
- Creep rupture strength and stress rupture life, defining time to failure at a given stress level.
- Creep limit or creep strength, the stress producing a specified strain, commonly 0.1% to 1%, after a defined duration such as 10,000 or 100,000 hours.
- Rupture elongation and reduction of area as ductility indicators.
- Creep modulus and isochronous stress-strain curves for polymers.
- Stress relaxation behavior, where decay in stress is monitored under constant strain.
- Notched specimen creep rupture data to assess notch sensitivity and multi-axial stress effects.
Testing Methods
Creep testing on a universal testing machine follows a standardized sequence to ensure repeatable, comparable results:
- Specimen preparation and measurement: verify gauge dimensions, surface finish, and alignment to eliminate bending stresses.
- Instrumentation: attach the extensometer to the gauge length and install the specimen in the grips with care to avoid preloading errors.
- Thermal conditioning: ramp the environmental chamber or furnace to the target temperature and hold until the specimen reaches thermal equilibrium, typically at least one hour.
- Load application: apply the test force smoothly at a controlled rate, then hold constant for the entire test duration using closed-loop control or a lever-arm dead-weight system.
- Data acquisition: record strain at logarithmic or scheduled intervals, with increased sampling frequency during primary creep and near anticipated rupture.
- Termination and analysis: conclude the test at rupture, at a specified strain limit, or at the planned duration, then calculate creep rates, rupture parameters, and extrapolated long-term strength.
Governing standards include ASTM E139 for tension creep and stress rupture of metallic materials, ISO 204 for metallic uniaxial creep testing, ASTM D2990 and ISO 899 for tensile and compressive creep of plastics, and ASTM E328 for stress relaxation tests. For long-term prediction, data are frequently extrapolated using the Larson-Miller parameter or Norton power-law creep models to estimate 100,000-hour strength from shorter-duration tests.
Applications
Creep performance testing supports design, qualification, and life assessment across many industries where components sustain static loads over long service periods:
- Power generation: life evaluation of boiler tubes, steam piping, turbine rotors, blades, and bolts operating under sustained stress at high temperature.
- Aerospace: qualification of nickel-based superalloys and titanium structures for jet engine hot-section components.
- Petrochemical and chemical processing: integrity assessment of reactor vessels, pressure vessels, furnace tubes, and high-temperature piping.
- Automotive: validation of exhaust system alloys, engine components, and high-temperature polymer under-hood parts.
- Polymer and packaging industries: long-term load performance of plastic pipes, fittings, geomembranes, films, and load-bearing plastic assemblies.
- Infrastructure: creep characterization of prestressing steel, bridge cables, cable stays, and structural adhesives.
- Research and development: ranking of new alloys, polymers, and composites, and generation of input data for finite element creep analysis.
FAQ
What is creep performance testing? Creep performance testing measures how a material slowly deforms under a constant applied load over time. It produces strain-time data used to determine steady-state creep rates, creep strength, and time to rupture, which are essential for components subjected to sustained loading.
Can a standard universal testing machine perform creep tests? Yes, when equipped with appropriate accessories. An electromechanical UTM with closed-loop load control, a stable load cell, long-travel or high-resolution extensometry, a temperature chamber for elevated-temperature work, and software capable of multi-day or multi-week test scheduling can conduct reliable creep and stress rupture testing.
How long does a creep test take? Test duration depends on the objective. Screening tests may run several hours, while stress rupture and creep strength evaluations often extend to 1,000, 10,000, or more hours. Extrapolation methods such as the Larson-Miller parameter allow estimation of 100,000-hour properties from shorter tests.
What is the difference between creep and stress relaxation testing? In creep testing, the load is held constant and strain increases with time. In stress relaxation testing, the strain is held constant and the stress decays over time. Both reveal the same underlying time-dependent deformation mechanisms but require different control modes.
Which standards govern creep testing? ASTM E139 and ISO 204 cover metallic creep and stress rupture testing, ASTM D2990 and ISO 899 govern creep of plastics, and ASTM E328 addresses stress relaxation. Selecting the correct standard depends on material class, temperature range, and end-use requirements.
Why is temperature control so important in creep testing? Creep rates are extremely sensitive to temperature, often changing by an order of magnitude with a temperature shift of only 10 to 20 °C. Precise, stable temperature control and uniform heating of the specimen gauge section are therefore mandatory for valid, repeatable results.