01 Core Principles of the Two Control Modes
Displacement Control (Speed Control)
--Most stable for general tests Core principle: The crosshead moves at a fixed speed to stretch or compress the specimen at a constant rate. The system collects force and deformation data synchronously.
Advantages: Stable operation throughout the test, no jitter, no runaway. Highly repeatable curves, fully compliant with mainstream national and ASTM standards. Disadvantages: Cannot maintain a constant load precisely. Not suitable for tests requiring pressure stabilization or load holding.
Force Control (Load Control)
--Dedicated for constant force loading Core principle: The equipment dynamically adjusts motor speed in real time to achieve a steady rise or stable constant load.
Advantages: Uniform load application; supports constant load holding and uniform pressure loading. Critical drawback: Prone to instability when the material yields, necks or fractures. Once the load drops, the motor will rapidly accelerate to compensate, resulting in crosshead runaway and stroke overshoot, which may easily damage load cells and fixtures.
02 Industry Guidelines: How to Select for Different Tests
① Use Displacement Control for all conventional tests measuring strength, curves and elongation
Metal tensile test (yield strength, tensile strength, percentage elongation after fracture)
Tensile test of plastics, rubber and films (large-deformation materials)
3-point / 4-point bending test
Shear, peel and tear test
Mechanical testing of pipes, profiles and plates
Reason: After yielding, most materials undergo stress reduction and plastic deformation. Force control will lose stability, leading to invalid tests and equipment failure.
Supplementary knowledge: What does the material yield curve look like? In metal tensile tests, the stress-displacement / stress-strain curve presents distinctive yield features, the basis for judging material yield, falling into two main types:
Curve with obvious yield (mild steel, ordinary carbon steel and other soft metals) The curve rises steadily in the elastic stage. After reaching a certain value, the force stops increasing and forms a horizontal plateau or small zigzag fluctuations. Key feature: displacement keeps increasing (specimen continues to elongate) while tensile force remains nearly constant or slightly decreases. This plateau is the yield stage, and the corresponding value is yield strength.
Curve without obvious yield (alloy steel, stainless steel, aluminum, plastics) The curve rises smoothly with no horizontal plateau or zigzag fluctuation, and no force drop. Such materials have no distinct yield point. The specified non-proportional extension strength Rp0.2 is universally adopted in the industry as the yield criterion.
Key point: Once the material shows yield features (plateau, stagnant or dropping force), force control interprets it as "insufficient load" and commands the motor to speed up sharply for compensation. This is the root cause of crosshead runaway, distorted curves and invalid data, and why tensile tests must adopt displacement control.
② Use Force Control for all tests requiring uniform pressure loading or pressure stabilization
Compressive test of concrete, bricks and foams (uniform force loading mandated by national standards)
Constant force compression test for bolts and fasteners
Creep test (long-term constant load holding)
Stress relaxation and cyclic fatigue loading
Process press fitting, seal compression and constant pressure verification
03 Why Are Many Test Results Inaccurate?
Two common mistakes:
❌ Performing metal tensile tests under force control Once the specimen yields, the machine runs away instantly. Yield point and tensile strength readings are completely distorted.
❌ Performing compressive tests under displacement control Pressure rises unevenly. Compressive strength data shows large dispersion and fails audit.
04 Summary
Do not select control modes based on intuition. Most test deviations, chaotic curves and equipment faults stem from mismatched control modes.
✅ Displacement control: material property measurement, breaking test, bending test, curve acquisition
✅ Force control: uniform pressure loading, constant load holding, fatigue & creep tests
Choose the correct control mode to greatly improve test success rate and ensure repeatable data.
