Cracking behaviour of clayey geomaterials

  • Ian William Murray

Student thesis: Doctoral Thesis


Cracking is a significant problem in the ceramics industry. It results in a substantial loss of production due to the discarded cracked wares associated. There is an understanding that cracking relates from restrained shrinkage and/or uneven drying. One of the solutions to this is to slow the rate of evaporation so that the wares dry more evenly. Such remedial measures are impractical due to the increase in drying space required. What is therefore required for the ceramics industry is a greater understanding of cracking and methods, which work within their manufacturing limitations, to reduce cracking susceptibility.Despite a number of studies, there is a lack of understanding surrounding the mechanisms of crack initiation. It is clear from experimental evidence that the vast majority of cracking occurs while the soil is still saturated or at the transition from saturated to unsaturated. Many researchers assume that cracking is governed by total stress, i.e. a crack initiates when the tensile stress generated in a soil during drying exceeds the tensile strength of the material.This research investigates the mechanism of crack initiation using an effective stress based approach. A new tensile testing device utilising high capacity tensiometers to allow for the measurement of (negative) pore-water pressure during testing and, hence, the characterisation of the effective stress state has been developed. The device has been designed to test clays in both saturated and unsaturated states.A series of tensile tests were performed on saturated clay samples prepared using non-de-aired and de-aired water. The results show that for non-de-aired tests, when suction approached the air-entry value, failure occurred at a deviatoric stress lower than the one corresponding to the critical state line derived from triaxial and uniaxial compression tests. For specimens with the slurry de-aired before slip casting the de-airing process realigned the deviator stress at failure recorded in the tensile test with the critical state line derived from uniaxial and triaxial compression tests. It could therefore be speculated that water cavitation is one of the mechanisms that can control rupture of clay when subjected to a (total) tensile stress state. Finally, failure data from tests on unsaturated specimens could be fairly modelled by the Mohr-Coulomb criterion extended to unsaturated states. These tests confirm that tensile failure is associated with failure in shear for both saturated and unsaturated states.Further validation of the shear failure mechanism is attempted via a numerical study using a simple coupled hydro-mechanical Finite Element Method (FEM) model to simulate the desiccation of a clayey soil to the point of crack initiation. The results of four laboratory desiccation tests of specimens with non-uniform geometry and different hydraulic boundary conditions are presented. These tests were simulated via FEM, and the time and location of cracking compared to test the validity of the model and the failure criterion.A greater understanding of the mechanism of cracking has then be used to test more practical remedial measures that can be used in the manufacture of ceramics to reduce cracking. These measures are based around altering the material mixes used in production process, as to this point the adjustment of the slip material relating to cracking has been done by a process of trial and error without guidance. The results of Finite Element Method simulations suggest creating a material with a more graded grain and pore-size distribution can reduce deviatoric stress development during uneven drying.
Date of Award1 Apr 2015
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorAlessandro Tarantino (Supervisor) & Rebecca Lunn (Supervisor)

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