Direct shear behavior of a plane joint under dynamic normal load (DNL) conditions

• A significant phase shift between peak normal force and peak shear force is observed.
• The relative time shift between peak normal force and peak shear force decreases with increasing normal load.
• The dynamic normal stiffness is more than one order of magnitude higher than the static one.
• A new shear strength criterion is proposed for a joint under constant shear velocity and sinusoidal normal loading.

The effect of dynamic normal loading (DNL) on the direct shear behavior of a plane joint has been first time investigated using a dynamic direct shear box device. Multi-stage normal loading tests with superimposed normal cyclic excitation at different frequencies were conducted at constant horizontal shear velocity of 3000 μm/min. Experimental results show, that peak shear force increases with increase of normal stress, and amplitude ratio between shear and normal stress decreases with the increase of normal stress. A significant phase shift between peak normal stress and peak shear stress is observed, which decreases with increasing normal load, while the relative time shift between peak normal stress and peak friction coefficient is nearly constant. Impact frequency has little effect on relative time shift. The dynamic normal stiffness is significantly higher than the static one. Finally, a new shear strength criterion is proposed for a joint under constant shear velocity and sinusoidal normal loading.