Straincontrolled

Strain-controlled refers to a method used in mechanical testing and material characterization in which strain, rather than load or force, is the primary control variable. In a strain-controlled test, the specimen is deformed according to a predefined strain program, and the testing machine adjusts the applied load to track the target strain within specified tolerances. The strain is typically measured with an extensometer, a traveling displacement sensor, or through digital image correlation, and is fed back into a control loop to maintain the desired deformation trajectory.

This approach is commonly contrasted with load-controlled testing, where the specimen is subjected to a prescribed load or force and the resulting deformation is recorded. Strain-controlled tests are particularly useful for exploring constitutive behavior and strain-rate effects, and they are widely used for polymers, metals, composites, and emerging materials. In fatigue testing, strain control can be applied as strain-controlled fatigue, where fixed strain amplitudes drive cyclic deformation, causing corresponding stress responses.

Equipment for strain-controlled testing typically includes a servo-hydraulic or servo-electric universal testing machine, accurate strain measurement devices, and closed-loop control systems to regulate strain with time. The main outputs are stress–strain curves, modulus estimates, yield and hardening behavior, and information on rate sensitivity. While provides smooth and repeatable deformation histories, strain-controlled testing can be more sensitive to measurement accuracy and may be challenging near instability or rapid, localized failure modes.