Deformation Characteristics of Kinking in Rocks

Kinking in rocks is a well-known phenomenon particularly among structural geologists. It is commonly observed in thinly layered sedimentary rocks subjected to folding and metamorphic rocks with well pronounced schistosity, cleavage such as schist, phyllite, slate, etc. The authors got involved with this phenomenon when they tested wooden specimens to obtain their mechanical characteristics. The authors report their experimental results and observations in this article and discuss their implications in rock mechanics. Our conclusions are; (1) Kinking always initiates from either a material or imposed mechanical singularity and grows into a band in which the strain is localised. (2) The visible kinking develops after the peak strength is exceeded. The orientation of fibers within the band decreases as the straining continues. Furthermore, the length of kining fibers in the band of the wooden specimens grows as the strain increases, while those of the rock specimens are constant as the strain increases. (3) However, the global orientation of the kink band with respect to the loading direction remains almost constant throughout the deformation process, provided that the confining pressure is kept constant. The inclination of kink bands is different from that of shear fractures which is predicted from Mohr-Coulomb yield criterion under compression.