What is Strain-Controlled Fatigue Testing?
ASTM E606 defines strain-controlled fatigue testing. It is a specialized mechanical evaluation method designed to determine the fatigue properties of nominally homogeneous materials under conditions where cyclic plastic strain is the dominant damage mechanism. Unlike stress-controlled (load-controlled) fatigue testing (ASTM E466), which is typical for high-cycle fatigue (>10^5 cycles), strain-controlled testing is critical for low-cycle fatigue (LCF) applications (<10^5 cycles) where components experience significant plastic deformation in each cycle. This standard is essential for evaluating materials used in pressure vessels, turbine disks, nuclear reactors, and aerospace structures subjected to thermal cycling or severe mechanical loading, where life is governed by ductility and cyclic stress-strain response rather than just endurance limits.
What is a typical Strain-Controlled Fatigue Test Result?
The results provide quantitative data describing the material’s resistance to crack initiation and propagation under cyclic plastic strain:
① Cyclic Stress-Strain Curve: A plot of stable stress amplitude versus strain amplitude, distinct from the monotonic stress-strain curve. It reveals whether the material cyclically hardens (stress increases with cycles) or cyclically softens (stress decreases).
② Strain-Life Curve ( ϵ−N ): The primary output, plotting total strain amplitude ( Δϵ/2 ) against the number of reversals to failure ( 2Nf ). This curve is often described by the Coffin-Manson relationship, separating elastic and plastic strain components.
③ Hysteresis Loops: Closed loops of stress vs. strain recorded at various intervals (e.g., 1st cycle, half-life, near failure), showing energy dissipation per cycle and changes in mean stress (relaxation) or mean strain (creep/ratcheting).
④ Fatigue Life ( Nf ): The number of cycles to reach a defined failure criterion, such as complete separation, a 50% drop in load capacity, or the initiation of a specific crack size.
⑤ Cyclic Hardening/Softening Coefficients: Parameters ( K′ and n’) derived from the cyclic stress-strain curve that quantify the material’s resistance to cyclic plastic flow.
⑥ Relaxation and Creep Data: For tests with hold periods, data on stress relaxation (drop in stress at constant strain) or creep (increase in strain at constant load) during the hold time.
Discovery and Evolution of Strain-Controlled Testing
In the mid-20th century, engineers observed that many failures in high-temperature applications (jet engines, power plants) occurred after relatively few cycles but involved significant plastic deformation. Traditional S-N (Stress-Life) curves, based on elastic behavior, failed to predict these lives accurately. Researchers like Coffin and Manson independently developed empirical relationships linking plastic strain amplitude to fatigue life. ASTM E606 was established to standardize the rigorous experimental procedures required to generate this data, ensuring that the complex interaction between servo-hydraulic control, specimen alignment, and extensometry yielded reproducible results across different laboratories. The standard has evolved to include precise definitions for failure criteria and data recording intervals to support modern damage tolerance analysis.
The standard defines specific mechanical properties for material acceptance:
Plastic Deformation Focus: Specifically designed for regimes where plastic strain is a significant portion of the total strain.
Feedback Mechanism: Requires a closed-loop system using an extensometer signal for control, not just load cells.
Data Density: Mandates recording of full hysteresis loops at specific intervals to track cyclic hardening/softening behavior.
Failure Definition: Provides specific guidelines for defining failure based on load drop, acknowledging that complete fracture may occur long after a crack has initiated.
The key contents covered by the standard include:
Scope and definitions specific to strain-controlled fatigue (e.g., inelastic vs. plastic strain, hold periods).
Apparatus requirements: Servohydraulic testing machines, high-stiffness fixtures, Class B-2 extensometers, and load transducers.
Specimen design: Detailed dimensions for uniform and hourglass specimens, including sheet metal alternatives.
Specimen preparation: Machining, surface finishing, and storage procedures to minimize variability.
Test procedure: Alignment verification, control mode selection (continuous vs. limit), waveform definition (triangular/trapezoidal), and strain rate settings.
Failure definition: Options for defining end-of-life (separation, load drop, modulus change).
Data reporting: Comprehensive lists of required metadata, including material properties, environmental conditions, and cyclic parameters.
Referenced Standards
ASTM E8/E8M: Tension Testing of Metallic Materials (For monotonic properties).
ASTM E83: Verification and Classification of Extensometers (Critical for strain measurement accuracy).
ASTM E466: Force-Controlled Constant-Amplitude Axial Fatigue Tests (The counterpart for high-cycle fatigue).
ASTM E647: Measurement of Fatigue Crack Growth Rates (Complementary for crack propagation studies).
ISO 12106: Metallic materials — Fatigue testing — Axial strain-controlled method.
ASTM E2714: Standard Test Method for Creep-Fatigue Testing (Combines hold times with strain cycling).
ASTM E2368: Standard Practice for Strain Controlled Thermomechanical Fatigue Testing.
ASME Boiler and Pressure Vessel Code, Section III: Rules for Construction of Nuclear Facility Components (Uses strain-life data for design).
SAE J1099: Technical Report on Fatigue Properties (Often utilizes ASTM E606 data).
Environment:
Temperature: Standard practice is at room temperature (20°C to 25°C). Stability is required to prevent thermal drift in the extensometer.
Atmosphere: Ambient air, unless environmental effects are being specifically studied (which would require a chamber).
Frequency: Determined by the desired strain rate. Typical frequencies range from 0.1 Hz to 5 Hz depending on the strain amplitude.
Test Procedure:
- Specimen Preparation: Machine specimens to specified geometry (uniform gage preferred). Polish gage section to minimize surface defects (max roughness 0.2 µm). Measure dimensions accurately.
- Alignment Check: Install a trial specimen with strain gages. Verify bending strains are <5% of axial strain. Adjust fixtures as needed.
- Instrumentation: Attach axial extensometer (for uniform specimens) or diametral extensometer (for hourglass). Connect load cell.
- Setup: Install specimen in machine. Ensure grips are tight and aligned. Set up environmental chamber if needed.
- Control Setup: Configure controller for strain control. Select waveform (triangular for continuous, trapezoidal for holds). Set strain rate/frequency.
- Initialization: Start test in tensile or compressive direction (consistent for all tests). Ramp to target strain amplitude over first few cycles if necessary.
- Data Acquisition: Record hysteresis loops (stress vs. strain) at initial, intermediate (logarithmic intervals), and half-life cycles. Monitor peak loads and mean stress.
- Termination: Stop test upon reaching failure criterion (e.g., 50% load drop or fracture).
- Post-Mortem: Examine fracture surface (SEM/optical) to confirm failure mode and location. Check for buckling or grip failures.
- Reporting: Compile data into strain-life curves, cyclic stress-strain plots, and detailed test reports per Section 9.
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