Stress suction surfaces

The craze front velocity v can be governed by one of two distinct mechanisms of craze matter production. As Argon and Salama have discussed in detail, under the usual levels of service stresses or stresses under which most experiments are carried out, craze matter in single phase homopolymer is produced by the convolution of the free surface of the sohd polymer at the craze tip. This occurs by a fundamental interface instability present in the flow or deformation of all inelastic media when a concave, meniscus-like surface of the medium is being advanced locally by a suction gradient. This is the preferred mechanism of craze advance in homopolymers. In block copolymers with uniform distributions of compliant phases of a very small size, and often weaker interfaces than either of the two phases in bulk, craze advance can also occur by cavitation at such interfaces to produce craze matter as has been discussed by Argon et al. Both of these mechanisms of craze advance lead to very similar dependences of the craze front velocity on apphed stress and temperature that is of the basic form... 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

For the analysis of the FEB EX in situ test, a soil mechanics state-surface model was implemented. This state-surface approach provides a better representation of bentonite behavior under partially saturated conditions than a single effective stress approach. The logarithmic state surface model proposed by Lloret and Alonso (1985) was adopted in this analysis. In their model, void ratio (e) is a function of both net mean stress, (Ora" = Om - Pg. where = total mean stress and Pg is gas pressure) and suction (s = Pg -Pi, where Pg and pi is gas and liquid pressures, repectively). 

As the microstructural suction decreases to low values the stress path crosses the yield surface, SD, (Figure 7.b) and some plastic re-arrangements take place. In the model this phenomenon is described by the interaction function fp- A net volumetric compression was observed (and computed) at this stage. 

Effective stress of unsaturated soil will change with the variation of saturation. Ning (2006 and 2008) proposed that it is unreasonable to use matric suction as stress variable. He defined suction stress as a inter particle physico-chemical stresses contributed by cementation, van der Waals attraction, double layer repulsion, capillary stress arising from surface tension, and negative pore water pressure. Shear strength for unsaturated soil based on suction stress conception can be expressed as equation 4 ... 

To understand the mechanism further, a microscopic investigation on the moisture content in clay is required. The osmotic suction potential was introduced as the driving force of moisture movement, described in Section 31.3, and was successively applied to the prediction of moisture movement in wet clay [15,16]. The theoretical analysis on the two-dimensional moisture transfer of cylindrical clay was performed taking into account the effects of both osmotic suction and strain-stress caused by the shrinkage [17,18]. However, only the transient mass-transfer equation was analyzed, assuming a constant drying rate on the external surface of the... 

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