Control of a mechanism for the application of variable axial loads in a multiaxial fatigue testing machine

Abstract

The loads acting on mechanical components are usually complex, and the more traditional fatigue analysis criteria may be non-conservative, especially in out-of-phase and non-proportional loadings. Many multiaxial fatigue models have been developed and need to be experimentally validated. This work describes the load control mechanism of a low-cost electromechanical multiaxial fatigue testing machine, which applies variable axial and torsional loads. A load cell interconnected to the motor control system was manufactured and instrumented with extensometers. For the axial load, a four-bar mechanism coupled to a power screw was designed to apply alternating axial loads, thus ensuring a higher load frequency and preservation of the motor, which will not need to alternate its direction of rotation. The machine’s control system, focused on out-phase loading, is based on the data sent by the load cell and pulse encoders per degree for each motor, ensuring good accuracy in applied loads. Tests were carried out to check the system’s response, in which it was possible to apply load curves of torque and axial force in different phase shift angle. Therefore, several existing multiaxial fatigue models can be tested with good reliability.