Biaxial Strain Control Fatigue Testing Strategies for Composite Materials

Background Efficient biaxial strain control to study fatigue damage growth in composite materials at the coupon scale level under realistic multi-axial stress states observed in structures is needed. Objective Two biaxial cyclic strain control algorithms, referred to here as the active and the passive control method, are presented as a fatigue testing strategy for composite materials. Method The strain is measured at a sampling frequency of 116 Hz through a real-time digital image point tracking system. The active control method utilizes a cascade control algorithm to accommodate the low sampling rate of the digital image point tracking system relative to the servo-hydraulic PID controller. The passive control method is a conditional control algorithm where the force command is adjusted when the measured peak-valley strain violates the predefined strain tolerance. The performance of these control techniques are initially evaluated on uniaxial coupon specimens to investigate the effect of testing frequency, strain magnitude, accuracy of the stiffness parameter and the tolerance band. Subsequently, the two control methods are verified on cruciform specimens under different biaxial strain states. Results Under biaxial loading, the peak feedback strains had a maximum error of 6.4% and 9.0% for the active and the passive control method respectively, depending on the required biaxial strain state. Conclusions The active control method was found to offer a better accuracy relative to the passive control method. However, it suffered from a lower fidelity as the test stopped whenever the real-time digital image point tracking system lost track of the point markers.