When waves of seismic or other origin are propagated through fractured rock mass not only will the reflection and transmission coefficients depend on the acoustic impedance mismatch and the nature of the interface layer, but also the physico-mechanical behaviour of the interface will change as a function of time. Degradation and aging processes of the interface material, which may quantitatively be formulated by means of continuum damage mechanics principles, may be made responsible in part. Elastic wave behaviour across imperfectly bonded linear slip interfaces separating two elastic media is investigated in Schoenberg (1980), where the displacement discontinuity, or slip, is taken to be linearly related to the stress traction which is continuous across the interface for both the normal and tangential direction. Applying the theoretical results of Schoenberg (1980) to seismic wave propagation model experiments in a jointed columnar basaltic rock formation, acceptable agreement between observed and predicted results has been found by Myer et al (1990). A proper constitutive model for rock joints must be able to predict the deformation behaviour involving aging effects on a long time scale as well as deterioration effects due to repeated loading and unloading. In percussion drilling, the interface is dynamically loaded at high intensity and the interface characteristics will change with each consecutive percussive blow ofthe tool. Improved knowledge about the impact-induced time-variation of the mechanical behaviour of the rock joints appears to be instrumental and vital for the improvement of drill efficiency and the development of smart mining machinery.