What Is Strain Control?

Strain control is a testing and process-control methodology in which the deformation of a material or structure, rather than the applied force, is used as the regulated variable in a feedback loop. Instead of commanding a fixed load and measuring the resulting deformation, a strain-controlled system measures the actual strain continuously and adjusts the actuator output to hold it at a prescribed value or waveform. This approach is essential whenever the material response is strongly nonlinear or when plastic deformation accumulates during cyclic loading, conditions under which force-controlled tests can produce unpredictable and irreproducible results.

Strain control is closely related to displacement control but is more direct: displacement measures a physical position, while strain normalizes that displacement to the gauge length of the specimen, making results independent of specimen geometry. The method is standard in low-cycle fatigue characterization, constitutive model development, and structural health monitoring of components that cycle through plastic strains in service.

Closed-Loop Control Architecture

A strain-controlled test system consists of a servo actuator, a feedback sensor, a controller, and a command signal generator. The sensor, typically a clip-on extensometer or bonded strain gauge mounted directly on the specimen, reports instantaneous strain to the controller, which computes the error between the measured and commanded values and drives the servo valve or motor to correct it. The controller uses a PID (proportional-integral-derivative) algorithm or its digital equivalent, with gains tuned to the stiffness of the specimen and the bandwidth of the hydraulic or electromechanical drive. Servohydraulic testing machines are the dominant hardware platform for high-force, high-cycle applications because the fast response of the servo valve allows precise waveform tracking at frequencies from fractions of a hertz up to several hertz even under cyclic plasticity.

Strain-controlled systems can switch between control modes, running in load control during the elastic portion of a test and transitioning to strain control when the material yields. This switching capability is implemented in most modern digital test controllers and is specified as a requirement in several ASTM and ISO fatigue standards.

Low-Cycle Fatigue Testing

The primary application driving the development of strain control is ASTM E606/E606M strain-controlled fatigue testing, the standard test method for measuring cyclic stress-strain behavior and fatigue life at low and high cycles. In low-cycle fatigue, materials undergo significant plastic straining each cycle, typically at total strain amplitudes above 0.5 percent, and fail within fewer than 100,000 cycles. Controlling strain rather than load ensures that the cyclic plastic strain range is reproducible from specimen to specimen, which is necessary for constructing fatigue life curves (strain-life or epsilon-N curves) and calibrating constitutive models used in finite element analysis.

The test outputs include cyclic stress-strain curves, fatigue life data as a function of strain amplitude, and the cyclic hardening or softening behavior of the material. These data feed directly into design codes for power generation turbine blades, nuclear pressure vessel components, and aircraft structural parts that experience repeated thermomechanical loading.

Applications

Strain control has applications in a wide range of disciplines, including:

• Low-cycle fatigue characterization of metals and alloys for aerospace and power generation components
• Constitutive model calibration for finite element simulations of plastic deformation
• NIST intermediate strain rate research for automotive and defense materials at elevated loading rates
• Geotechnical laboratory testing to measure soil deformation under cyclic earthquake loads
• Biomechanical testing of soft tissues and orthopedic implants under physiological strain cycles