What Is Creep Testing? Definition, Methods & Material Applications
TEM image of precipitation strengthening microstructure in heat-treated aluminum alloyWhat Is Creep?
Creep is the time-dependent, progressive plastic deformation of a material under sustained constant stress, occurring at temperatures above approximately 30–40% of the material’s absolute melting temperature (0.3–0.4 Tm). Unlike instantaneous elastic or plastic deformation, creep accumulates continuously over time — even at stresses below the material’s yield strength — causing components to change dimensions, develop cracks, and eventually rupture if the design does not account for this time-dependent behaviour.
Creep is a critical design consideration for high-temperature structures — gas turbine blades, power plant steam pipes, nuclear reactor components, and glass furnace structures — where operating temperatures approach Tm and service lives extend for decades.
The Three Stages of Creep
A typical creep test at constant temperature and stress produces a characteristic strain-time curve with three distinct stages:
Stage I — Primary Creep (Transient Creep)
The creep rate (dε/dt) is initially high and decreases with time as the material work-hardens and establishes a balance between dislocation multiplication and recovery. Primary creep dominates at the start of loading — the strain rate decreases rapidly as obstacles to dislocation motion accumulate.
Stage II — Secondary Creep (Steady-State Creep)
A constant minimum creep rate (ε̇_min) is achieved when work hardening and dynamic recovery are in balance. Secondary creep dominates for the majority of the creep life — most design calculations and creep data use the secondary creep rate (minimum creep rate) as the primary parameter.
The Norton power law describes steady-state creep rate: ε̇_min = A × σⁿ × exp(−Q/RT)
where σ is stress, n is the creep stress exponent, Q is activation energy, R is the gas constant, and T is absolute temperature.
Stage III — Tertiary Creep (Accelerating Creep)
Creep rate increases rapidly as damage accumulates — void formation and coalescence at grain boundaries, necking, oxidation damage, and microstructural degradation accelerate deformation. Tertiary creep ends in rupture.
Creep Testing Methods
ASTM E139 — Conducting Creep Tests of Metallic Materials
ASTM E139 defines the procedure for constant-load or constant-stress creep testing of metals. Specimens are loaded with calibrated dead weights or servo-controlled load frames while heated in a furnace at defined temperature (±1°C precision). Axial strain is measured by high-resolution extensometers or dilatometers. Tests may run for 100 to 10,000+ hours depending on the stress/temperature combination and data required.
Data outputs: Creep strain vs. time curve, minimum creep rate, time to specified strain, and rupture time (for creep rupture testing).
ASTM D2990 — Tensile, Compressive, and Flexural Creep of Plastics
For polymeric materials, ASTM D2990 measures creep at temperatures typically 23–80°C — low relative to polymer Tm but above the glass transition temperature where polymer chain mobility enables significant creep. Isochronous creep data (stress vs. strain at defined times) and isostress creep curves (strain vs. time at defined stress) provide the data for polymer component design under sustained load.
ISO 899 — Plastics Creep Behaviour Under Multiaxial Stress
ISO 899 characterises creep under uniaxial tensile and flexural loading for engineering plastics — complementing ASTM D2990 for international applications.
Extrapolation of Creep Data for Design
Practical component design requires creep data at temperatures and stresses relevant to service — but running 100,000-hour creep tests is impractical. Standard extrapolation methods including the Larson-Miller parameter (LMP) enable extrapolation of creep rupture data from short-term elevated-temperature tests to predict long-term service life at lower temperatures. LMP = T × (C + log t), where T is absolute temperature, t is rupture time, and C is a material constant.
Industrial Applications
In the power generation industry, steam turbine rotors and boiler superheater tubes operating at 550–620°C must demonstrate adequate creep rupture life (minimum 100,000 hours) per ASME Boiler and Pressure Vessel Code creep allowable stress data. In aerospace, nickel superalloy turbine blades in gas turbine engines operate near their melting point and their creep life governs engine inspection intervals and blade replacement schedules.
Why Choose Infinita Lab for Creep Testing Services?
Infinita Lab provides ASTM E139 and ASTM D2990 creep testing for metals and polymers through our nationwide accredited high-temperature mechanical testing laboratory network, with extended test durations and elevated temperature capabilities.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
At what temperature does creep become significant for engineering metals? A useful rule of thumb: significant creep occurs above approximately 0.4 Tm (absolute melting temperature). For steel (Tm ≈ 1500°C = 1773 K), significant creep begins above ~430°C. For aluminium alloys (Tm ≈ 660°C = 933 K), creep becomes significant above ~100°C. For engineering polymers near their glass transition, creep occurs at room temperature.
What is the difference between creep testing and stress relaxation testing? Creep testing maintains constant applied stress and measures strain increase with time. Stress relaxation testing maintains constant applied strain and measures stress decrease with time. Both involve the same underlying deformation mechanisms but address different design problems — creep is relevant for displacement-controlled designs (bolt elongation, pipe bending); stress relaxation for load-controlled designs (bolted joint pre-load retention, spring force).
What is creep rupture and how is it different from regular creep? Creep rupture (stress rupture) testing is creep testing continued until specimen fracture — providing time to rupture in addition to creep strain data. Creep testing per ASTM E139 may be terminated before rupture (to collect creep rate data only) or continued to rupture (for Larson-Miller extrapolation and design allowable development). Rupture time is used for high-temperature component life prediction.
Why do nickel superalloys maintain strength at temperatures approaching 1000°C? Nickel superalloys contain ordered precipitate phases (γ' — Ni₃Al and γ'' — Ni₃Nb) coherently embedded in the FCC nickel matrix. These precipitates resist dislocation motion — including thermally activated climb — at temperatures up to 80% of Tm. The precipitate coarsening rate (Ostwald ripening) and dissolution temperature govern the upper service temperature limit, which is increased by single-crystal casting that eliminates grain boundary sliding creep.
Can polymer creep data predict the long-term performance of plastic structural components? Yes — with appropriate modelling. Isochronous stress-strain curves from ASTM D2990 enable creep-corrected design stress selection for the required service life. Power law or time-hardening models fit to short-term creep data (100–1000 hours) can extrapolate to 10–50 year design lives, subject to the assumption that the creep mechanism does not change at longer times. This is the basis for design of plastic pipe systems (ISO 9080 regression analysis), plastic gears, and structural polymer brackets.
