Frictional behavior of planar and rough granite fractures subjected to normal load oscillations of different amplitudes

The dynamic frictional behaviors of natural discontinuities(joints,fractures,faults)play an important role in geohazards assessment;however,the mechanisms of the dynamic fault weakening/strengthening are still unclear.In this paper,a dynamic shear box was used to perform direct shear tests on saw-cut(planar)and natural(rough)granite fractures,with different normal load oscillation amplitudes.Based on the recorded shear forces and normal displacements,the shear forces,apparent friction coefficients and normal displacements are found to change periodically with oscillated normal loads and are characterized by a series of time shifts.The observed changing patterns are similar for the rough and planar fractures.Compared with the test data under constant normal load(CNL),small/large normal load oscillation amplitude enhances/reduces the peak shear strength,with a critical point.The magnitude of critical normal load oscillation for the rough fractures is smaller than the planer fractures.The results imply that dynamic fault weakening/strengthening can be achieved by both normal load oscillation amplitudes and slip surface topography.The rough fractures with larger normal oscillation amplitude can easily cause frictional weakening under stress disturbance.

Yoctonewton force detection based on optically levitated oscillator

Optically levitated oscillators in high vacuum have excellent environmental isolation and low mass compared with conventional solid-state sensors,which makes them suitable for ultrasensitive force detection.The force resolution usually scales with the measurement bandwidth,which represents the ultimate detection capability of the system under ideal conditions if sufficient time is provided for measurement.However,considering the stability of a real system,a method based on the Allan variance is more reliable to evaluate the actual force detection performance.In this study,a levitated optomechanical system with a force detection sensitivity of 6.33±1.62 zN/Hz^(1/2)was demonstrated.And for the first time,the Allan variance was introduced to evaluate the system stability due to the force sensitivity fluctuations.The force detection resolution of 166.40±55.48 yN was reached at the optimal measurement time of 2751 s.The system demonstrated in this work has the best force detection performance in both sensitivity and resolution that have been reported so far for optically levitated particles.The reported high-sensitivity force detection system is an excellent candidate for the exploration of new physics such as fifth force searching,high-frequency gravitational waves detection,dark matter research and so on.