Dilatancy (solid body)
Dilatancy in solids describes the effect of the increase in volume of solids under shear stress , especially in rocks .
In contrast to the dilatance in granular materials , cohesion and ductility cannot be neglected in the dilatance in solids . The increase in volume is therefore initially preceded by a reduction in volume due to ductile compression. Only damage to the material then enables the increase in volume and thus dilatant behavior.
Dilatancy is a process that takes place over a long period of time and as such must be clearly distinguished from the short-term structural failure of the material when the shear stress is even higher.
Dilatance limit
The value of the shear stress at which the minimum of the sample volume is measured in the shear test at a given normal stress is usually referred to as the dilatance limit . In a diagram with mean stress and shear stress as coordinates, it separates the area of ductile compaction from that of dilatance.
Alternative methods for determining the dilatance limit are based on
- the increase in permeability
- the sudden increase in acoustic emissions during deformation or
- the decrease in the speed of propagation of ultrasonic waves .
In most cases, the dilatance limit can only be specified within a certain range, since with increasing mechanical tension the minimum volume during the shear test can be determined less and less sharply. It decreases with increasing loading speed and with increasing pore pressure (the pressure of a fluid in the pores of the rock).
Practical meaning
Dilatancy in solids is the expression of mechanical damage. This can result in the loss of the load-bearing capacity of load-bearing parts such as pillars in mines . The increase in porosity associated with dilatation can increase the permeability for fluids, which is a danger for underground storage facilities or in the final storage of waste. In particular in salt rock , penetrating fluids can increase ductility due to dissolution processes.
Knowledge of the rock's dilatancy limit is important for the long-term safety of an underground structure . If this limit is exceeded, increasing porosity and, with prolonged loading, creep rupture are inevitable.
Individual evidence
- ^ A b N. Cristescu, U. Hunsche: Time effects in rock mechanics . Wiley, Chichester / New York 1998, pp. 342 .
- ↑ O. Schulze, T. Popp. H. Kern: Development of damage and permeability in deforming rock salt . In: Engineering Geology . tape 61 , 2001, p. 163-180 .
- ↑ H. Alkan, Y. Cinar, G. Pusch: Rock salt dilatancy boundary from combined acoustic emission and triaxial compression tests . In: International Journal of Rock Mechanics and Mining Sciences . tape 44 , 2007, pp. 108-119 .
- ↑ U. Hunsche, O. Schulze: Effect of humidity and confining pressure on creep of rock salt . In: M. Ghoreychi, P. Berest, H. Hardy Jr., M. Langer (Eds.): The Mechanical Behavior of Salt III; Proceedings of the Third Conference, Palaiseau, 1993 . 1996, p. 237-248 .