Large tensile stresses

FFs that are parameterized for high-pressure conditions can still lead to behavior that differs from that observed in experiments. For instance, it is common practice to treat the interatomic interactions with Lennard-Jones (LJ) potentials. Although this method is convenient from a computational standpoint, it is known that LJ potentials do not reproduce experimentally observed behavior such as necking, where a material attempts to minimize surface area and will break under large tensile stresses. Many other examples exist where particular types of FFs cannot reproduce properties of materials, and once again, we emphasize that one should ensure that the FF used in the simulation is sufficiently accurate. 

Consequently the upper part of the crack zone (Fig. 8.11) is free to relax (contract) in response to the compression applied by the liquid, but the gel network ahead of the crack (zone II, Fig. 8.11) is constrained. Large (tensile) stresses occur especially in the zone around the crack tip (zone W). Fracture occurs as soon as the value of surpasses the strength of the gel and this occurs sooner the larger the (effective) crack length is and the larger are the drying stresses (and thus drying rates). 

On January 15, 2001, in the upstream surface at both the elevation of 38 m and 103m, there appear large tensile stresses, of which the components in the vertical and dam axial directions are 1.25 MPa and 1.10 MPa at the elevation of 38 m, and 2.12 MPa and 1.51 MPa at the elevation of 103 m, respectively. And, the zones of tensile and compressive stress alternatively appear along the vertical direction. In addition, after the reservoir water filling, the tensile stresses are very small or disappear. 

Initially (at tQ), the aluminum plate is cooled and contracts, compressing the warmer foam, as shown in Fig. 3b. As time progresses (tj, t2, and t3), the insulation cools and the foam tends to contract in accordance with the local temperature and its coefficient of thermal expansion. However, near the tank surface, the insulation is constrained from contracting by the much stiffer aluminum plate, which, because of a lower coefficient of expansion, experiences less contraction than the foam. Ultimately, at t3 (as shown in Fig. 3c), the combination of the thermal contraction mismatch between the insulation and the aluminum tank and temperature distribution through the insulation lead to an in-plane stress pattern with large tensile stresses in the insulation near the tank and smaller compressive stresses at the free surface. 

Polymers show a similar effect in the presence of solvents. Solvents pre-ferredly enter the material near the crack tip because the distance between the molecules is increased there by the large tensile stresses. If, for instance, a rod made of polymethylmethacrylate (Plexiglas) is bent and the tensile side is wetted with acetone or alcohol, brittle fracture can occur after a short exposure time. In this case, the cleavage strength is reduced because the dipole bonds between the molecules are replaced by bonds formed with the solvent (see also section 8.8). 

FIG. 13-23. Tensile creep and creep recovery of polyisobutylene of high molecular weight, under large tensile stress X = 1 +... 

Efforts to alleviate these large tensile stresses, induced by the bending deformation, have led researchers to develop a double-lap specimen. The standardized... 

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