// Fatigue Testing — Strain-Controlled
L.C.F. Testing
Low Cycle Fatigue — Strain-Controlled Axial Fatigue per ASTM E606 and ISO 12106
SMT srl performs Low Cycle Fatigue (LCF) testing on servo-hydraulic machines under strain control, characterising the cyclic elasto-plastic response of metallic materials and generating the fatigue life data required for damage-tolerant and life prediction calculations.
// Understanding the Test
What is Low Cycle Fatigue?
Low Cycle Fatigue (LCF) describes the fatigue failure regime characterised by relatively few loading cycles to failure — typically fewer than 104 to 105 cycles — under strain amplitudes large enough to induce significant plastic deformation in every cycle. Unlike High Cycle Fatigue (HCF), where stresses remain predominantly elastic and cycles to failure are in the millions, LCF involves repeated yielding that accumulates damage rapidly.
Because the applied strain amplitudes exceed the elastic limit, LCF testing is conducted under strain control (displacement- or extensometer-controlled), not force control. This ensures that the strain amplitude remains constant and well-defined throughout the test, even as the material's cyclic stress response evolves due to hardening or softening.
The primary output of an LCF test is the fatigue life curve in strain (Δɛ vs. Nf) and the cyclic stress–strain curve, from which the material constants of the Coffin–Manson relationship are derived. These parameters are essential inputs for fatigue life prediction in finite element analysis and damage-tolerant design.
SMT performs both fully-reversed strain-controlled LCF tests (Rɛ = −1) and controlled-amplitude cyclic tests, including strain-aging (strain-controlled cycles with defined amplitude, such as ±2% or ±4%), on round machined specimens using servo-hydraulic test machines equipped with clip-on or laser extensometers.
Coffin–Manson Relationship
- ɛf' — fatigue ductility coefficient
- c — fatigue ductility exponent (−0.5 to −0.7 typical)
- σf' — fatigue strength coefficient
- b — fatigue strength exponent (−0.05 to −0.12 typical)
- Nf — number of cycles to failure
// Fatigue Regimes
LCF vs HCF — Key Differences
Understanding which fatigue regime applies to a given application determines the appropriate test method, specimen geometry, control mode, and data analysis approach.
| Parameter | LCF — Low Cycle Fatigue | HCF — High Cycle Fatigue |
|---|---|---|
| Cycles to failure (Nf) | Typically < 104 – 105 cycles | Typically > 105 – 107+ cycles |
| Strain per cycle | Significant plastic strain in every cycle — repeated yielding | Predominantly elastic; plastic strain per cycle negligible |
| Test control mode | Strain-controlled (extensometer required) | Force-controlled (constant force amplitude) |
| Primary life law | Coffin–Manson (Δɛ vs. Nf) | Basquin / Wöhler (S–N curve, σ vs. Nf) |
| Key outputs | Hysteresis loops, cyclic stress–strain curve, Coffin–Manson constants (ɛf', c, σf', b) | S–N curve, fatigue limit (σD), stress ratio effects |
| Typical applications | Thermal cycling, pressure cycles, turbine blades, reactor components, riser pipes | Rotating machinery, springs, fasteners, structural members at low stress |
| SMT capability | ASTM E606, ISO 12106 — servo-hydraulic, strain control Available | ASTM E466, ISO 1099 — servo-hydraulic & resonance ACCREDIA |
// How the Test Works
Test Procedure
An LCF test follows a defined sequence from specimen preparation through cyclic loading to data analysis. The procedure is governed by ASTM E606/E606M and ISO 12106.
Specimen Machining
Round, smooth specimens with a cylindrical gauge section are precision-machined to ASTM E606 or ISO 12106 geometry. The gauge section surface is ground to a fine finish to eliminate machining marks that would act as stress concentrators. Dimensional verification is performed before testing. SMT's in-house workshop machines LCF specimens directly from raw material supplied by the client.
Extensometer Attachment & Setup
A clip-on extensometer is attached directly to the specimen gauge section to measure and control strain throughout the test. The servo-hydraulic machine is programmed to impose a constant, fully reversed strain amplitude (Rɛ = −1) at the required rate. The extensometer signal serves as the control variable — the machine continuously adjusts force output to maintain the target strain amplitude.
Cyclic Loading — Hysteresis Recording
The specimen is subjected to fully reversed strain cycling at the prescribed amplitude. Load and strain are continuously recorded, generating hysteresis loops (stress–strain cycles). At selected cycle counts, complete loops are saved to document the evolution of the material's cyclic response — hardening, softening, or saturation. Testing continues until specimen fracture or a defined load drop criterion (typically 50% of peak load) is reached.
Multiple Strain Amplitudes
To characterise a material fully, LCF tests are performed at a minimum of four to six different strain amplitude levels, covering a range from low-cycle (short life) to the transition regime with high cycle fatigue. Each amplitude level requires at least one specimen; statistical scatter is assessed by running replicate tests at selected amplitudes. The full programme delivers the complete Δɛ–Nf dataset.
Post-Test Metallography
Where specified, the fracture surface of tested specimens is examined to verify the failure initiation mode and confirm specimen validity per the standard (no multiple crack initiations outside the gauge section, no material defects invalidating the test). Metallographic cross-sections can also be prepared to characterise cyclic deformation structures or crack propagation features.
Data Analysis & Report
Raw data is processed to determine Nf at each strain amplitude, plot the complete Δɛ–Nf curve, construct the cyclic stress–strain curve from half-life hysteresis loops, and perform a regression fit of the Coffin–Manson and Basquin equations to extract the four material fatigue constants. The final report includes all raw data, hysteresis loop plots, the Δɛ–Nf curve, and the fitted material parameters.
// Test Deliverables
Test Results & Outputs
An LCF characterisation programme delivers a complete set of material fatigue data for use in FEA, life prediction codes, and fitness-for-service assessments.
Δɛ–Nf Fatigue Life Curve
The primary result: total strain amplitude plotted against cycles to failure on a log–log scale, with separate elastic and plastic components shown. Provides a direct visualisation of the fatigue life at each tested strain level and confirms the transition point between plastic-dominated and elastic-dominated regimes.
Hysteresis Loops
Recorded at specific cycle counts throughout the test, hysteresis loops document the cyclic stress–strain response of the material over time. The area enclosed by the loop is proportional to the energy dissipated per cycle. Evolution of loop shape reveals cyclic hardening, cyclic softening, or stable behaviour — critical data for constitutive model calibration.
Cyclic Stress–Strain Curve
Constructed from the half-life stress amplitudes across all tested strain amplitudes. The cyclic stress–strain curve characterises the stabilised material response and is compared to the monotonic (tensile) stress–strain curve to reveal whether the material cyclically hardens or softens. Essential for non-linear FEA with cyclic loading.
Coffin–Manson Material Constants
Regression fit of the Coffin–Manson and Basquin equations to the experimental data yields the four material fatigue constants: ɛf' (fatigue ductility coefficient), c (fatigue ductility exponent), σf' (fatigue strength coefficient), and b (fatigue strength exponent). These are the standard input parameters for fatigue life prediction in ANSYS, Abaqus, fe-safe, nCode, and similar tools.
// Normative Framework
Applicable Standards
// Laboratory Equipment
Test Equipment
Servo-Hydraulic Test Machine
Clip-On Extensometer
High-accuracy clip-on extensometer attached directly to the specimen gauge section provides continuous strain measurement and feedback signal throughout the entire test. Extensometer calibration traceable to national standards is maintained under SMT's quality system.
Temperature-Controlled Testing
An environmental chamber mounted around the specimen allows LCF testing at controlled temperatures from −80°C to +180°C, covering cryogenic service conditions and elevated temperature applications such as steam turbines and hot section components. Temperature uniformity within the gauge section is verified before testing.
In-House Specimen Workshop
LCF specimens require a ground, low-roughness gauge section to eliminate surface stress concentrations. SMT's in-house mechanical workshop machines and grinds specimens directly from client-supplied material — pipe, plate, bar, or forging — with orientation and dimensional records maintained throughout.
// Industrial Relevance
Where LCF Data Matters
LCF characterisation is required wherever components undergo repeated plastic cycling in service — thermal cycles, pressure cycles, or mechanical overloads that repeatedly exceed the elastic range.
Power Generation & Turbines
Turbine blades, rotors, discs, and casings in gas and steam turbines experience severe thermal and mechanical cycling at start-up and shutdown. LCF data per ASTM E606 is mandatory for life prediction and component qualification in power generation designs.
Nuclear Industry
Reactor pressure vessels, piping, nozzles, and primary circuit components are subject to thermal transients that drive low-cycle fatigue. LCF characterisation per ISO 12106 and ASME III design fatigue curves is a standard part of nuclear material qualification and fitness-for-service assessments.
Oil & Gas — Risers & Pipelines
Subsea risers, flexible pipes, and pressure vessel nozzles accumulate LCF damage from pressure cycling, wave loading, and installation bending. Coffin–Manson data is used directly in finite element fatigue assessments to predict inspection intervals and replacement life.
Aerospace Structures
Aircraft fuselage panels, landing gear, and engine structural components are designed against LCF damage arising from pressurisation cycles and high-g manoeuvres. LCF testing per ASTM E606 generates the material data underpinning safe-life and damage-tolerant fatigue analysis.
Automotive & Drivetrain
Highly loaded engine components — connecting rods, crankshafts, cylinder heads, and suspension knuckles — experience LCF loading at peak torque and temperature. LCF material data feeds durability simulation in fatigue-life prediction software used in automotive component development.
Pressure Vessels & Process Equipment
Pressure equipment subject to repeated pressurisation cycles — autoclave vessels, heat exchangers, reactors — must be evaluated for fatigue life per design codes (ASME VIII Div. 2, EN 13445). LCF data, particularly the cyclic stress–strain curve, is directly required for elastic-plastic fatigue analysis under these codes.
// Why Choose Us
A Partner You Can Count On
Servo-Hydraulic Machines — Full Strain-Control Capability
SMT operates servo-hydraulic test machines configured for strain-controlled LCF testing, equipped with clip-on extensometers and data acquisition capable of recording complete hysteresis loops at programmable cycle intervals. Both ASTM E606 and ISO 12106 programmes are supported, across a wide range of strain amplitudes and strain rates.
LCF and HCF in One Laboratory
SMT offers both LCF (strain-controlled, ASTM E606 / ISO 12106) and HCF (force-controlled, ASTM E466 / ISO 1099, ACCREDIA accredited) testing from a single laboratory. Complete fatigue characterisation programmes spanning both regimes — including S–N curves at high cycles and Coffin–Manson parameters at low cycles — can be managed from a single order with coordinated data delivery.
Job Manager — Single Point of Contact
Each LCF programme is assigned a dedicated Job Manager who coordinates specimen machining, test execution at the required strain amplitudes and temperature, and report delivery. For multi-amplitude characterisation programmes, the Job Manager ensures consistent test conditions across all specimens and manages scheduling to optimise turnaround.
40+ Years of Fatigue Testing Experience
Founded in 1979, SMT has built deep expertise in axial fatigue testing across all regimes. The laboratory holds ACCREDIA accreditation No. 00588 for force-controlled axial fatigue and applies the same rigorous quality standards to strain-controlled LCF programmes. SMT is a recognised leader in fatigue testing of rebar and other challenging geometries.
// All Testing Services
Discover All Services
Fracture & Fatigue
LCF, HCF, CTOD, K₂C, J₂C, crack propagation — full fracture and fatigue programme.
Mechanical Testing
Tensile, hardness, impact, bend, creep, and DWT testing on metallic materials.
Metallographic Examinations
Macro and micrographic examinations, fractographic analysis, and surface measurements.
Failure Analysis
Root cause investigation of component failures through integrated testing and analysis.
Chemical Analysis
Compositional analysis to verify conformance with material specifications and standards.
Corrosion Testing
HIC, SSC, pitting, crevice, intergranular, CSCC — full corrosion testing programme.
Non-Destructive Testing
UT, MT, PT and other NDT methods for integrity assessment of components.
Mechanical Workshop
In-house machining of specimens to international standards with full dimensional control.
// Company Overview
About SMT srl
Founded in 1979, SMT srl is one of Italy's leading independent materials testing laboratories, located in Pozzo d'Adda near Milan, Lombardia. The laboratory holds ACCREDIA accreditation No. 00588 under UNI CEI EN ISO/IEC 17025:2018 and issues over 20,000 test reports per year.
The fatigue laboratory is equipped with servo-hydraulic machines capable of both force-controlled (HCF) and strain-controlled (LCF) axial fatigue testing, as well as resonance test systems (vibrofori) for high-frequency HCF programmes. Temperature-controlled testing between −80°C and +180°C is available on all servo-hydraulic machines.
Each project is managed by a dedicated Job Manager — a direct technical contact who accompanies the client from test programme definition through specimen preparation, test execution, data analysis, and report delivery.
Discover SMT srl →
SMT srl — Company Presentation Video
// Frequently Asked Questions
LCF Testing FAQ
Low Cycle Fatigue (LCF) is the fatigue failure regime that occurs when a material is subjected to cyclic strain amplitudes large enough to cause significant plastic deformation in every cycle. Failure occurs at relatively few cycles — typically fewer than 104–105 — because the repeated plastic flow rapidly accumulates damage. LCF is the relevant regime for components subject to thermal cycling, pressure cycles, or repeated mechanical overload, as opposed to High Cycle Fatigue (HCF) where stresses remain predominantly elastic and life is measured in millions of cycles.
In the LCF regime, the material undergoes cyclic plastic deformation in every cycle. If the test were run in force control at a fixed force amplitude, cyclic softening would cause the strain amplitude to grow uncontrollably, eventually leading to runaway deformation and a non-interpretable result. Conversely, if the material cyclically hardens, strain amplitude would decrease below the intended test level. Strain control — achieved by feeding back the extensometer signal to the servo-hydraulic controller — fixes the strain amplitude regardless of how the material's stiffness evolves, yielding well-defined, reproducible test conditions throughout the test life.
The Coffin–Manson relationship describes fatigue life in the LCF regime as a function of plastic strain amplitude: Δɛp/2 = ɛf' (2Nf)c. The four material constants — ɛf' (fatigue ductility coefficient), c (fatigue ductility exponent), σf' (fatigue strength coefficient), and b (fatigue strength exponent) — are determined by regression fit to the experimental Δɛ–Nf data. Once derived, these constants are used as direct inputs to fatigue life prediction routines in FEA post-processors (fe-safe, nCode DesignLife, ANSYS Fatigue, Abaqus) to calculate expected component life under complex cyclic loading histories.
A minimum of four to six strain amplitude levels is required to define the Δɛ–Nf curve with sufficient resolution across the LCF regime and into the LCF/HCF transition zone. ASTM E606 recommends at least one specimen per strain level for a screening programme, with replicates at two or more levels for statistical characterisation. A typical basic LCF programme therefore requires 6–10 specimens; a full statistical programme with replicates and cyclic stress–strain curve construction may require 12–20 specimens. SMT will advise on the appropriate programme scope based on the intended use of the data.
A hysteresis loop is the closed stress–strain curve traced by the material in a single loading cycle during an LCF test. Its width in the strain direction represents the plastic strain range; the area enclosed is proportional to energy dissipated per cycle. By recording loops at regular cycle intervals throughout the test life, it is possible to track cyclic hardening (loop progressively narrows and peak stress rises), cyclic softening (loop widens and peak stress drops), or cyclic stability. The stabilised (half-life) loop at each strain amplitude is used to construct the cyclic stress–strain curve. Hysteresis data is also used directly in kinematic hardening model calibration for non-linear FEA.
Yes. SMT's servo-hydraulic machines are fitted with environmental chambers enabling LCF testing across a temperature range of −80°C to +180°C. Testing at elevated temperature is particularly relevant for power generation and aerospace applications where LCF damage accumulates during thermal cycling of components operating at service temperature. At cryogenic temperatures, LCF characterisation is relevant for liquefied gas storage and cryogenic process equipment. Temperature is controlled and monitored throughout the test, and stability within the gauge section is verified before loading commences.
LCF specimens are solid, round bars with a cylindrical (or slightly hourglass-profiled) reduced gauge section, machined and ground to a specified surface finish. ASTM E606 and ISO 12106 define the acceptable specimen geometries, gauge length-to-diameter ratios, and surface roughness requirements. The ground surface in the gauge section is critical — machining marks act as stress concentrators and can cause anomalously early initiation, invalidating the test. SMT's workshop specialises in precision specimen machining and grinding to these exacting requirements, including specimens from tube wall, plate, or forging stock.
Force-controlled axial fatigue testing per ASTM E466 and ISO 1099 (High Cycle Fatigue) is included within SMT's ACCREDIA accreditation No. 00588. Strain-controlled LCF testing per ASTM E606 and ISO 12106 is performed on the same servo-hydraulic machines under SMT's ISO/IEC 17025 quality system, but as a non-accredited service. Both services benefit from the same rigorous equipment calibration, traceability chain, and technical competence. For applications where accredited fatigue data is specifically required, SMT can discuss the specific need and advise accordingly.
Both tests use strain-controlled cycling on a servo-hydraulic machine, but they serve different purposes. A full LCF test (ASTM E606 / ISO 12106) runs until specimen fracture at multiple strain amplitude levels, with the primary objective of determining fatigue life (Nf) and Coffin–Manson parameters. A strain-aging or hysteresis test applies a defined number of cycles (or a fixed strain history) and characterises the cyclic mechanical response — typically the shape and evolution of the hysteresis loop — without necessarily running to failure. Hysteresis tests at ±2% or ±4% are used, for example, to assess dynamic strain ageing in linepipe or rebar steels, or to calibrate cyclic plasticity models for FEA.
Yes. SMT's fatigue laboratory offers both strain-controlled LCF (ASTM E606 / ISO 12106) and force-controlled HCF (ASTM E466 / ISO 1099, ACCREDIA accredited) testing, as well as resonance testing (vibrofori, 250 kN and 700 kN) for high-frequency HCF programmes. A combined programme allows a complete total strain amplitude–life curve to be constructed from the LCF regime through to the high-cycle fatigue limit, covering the full range required for modern fatigue design codes. All testing is managed from a single order by a dedicated Job Manager.
// Request a Quote
Request an LCF Test Quote
Describe your material, required strain amplitudes, test temperature, specimen orientation, and reference standard. We will provide a detailed quotation and programme timeline.