Stress concentration

About: Stress concentration is a research topic. Over the lifetime, 23250 publications have been published within this topic receiving 422911 citations.

Corrosion fatigue crack growth of titanium alloys in aqueous environments

Abstract: The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency in distilled water, while addition of Na2SO4 produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence takes place at ΔKscc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the crack tip. Above ΔKscc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching KIsco only for very slow cycling frequencies.

Mechanics of fault junctions

Abstract: A sharp bend in a fault must be part of a triple junction, and slip on the three fault segments at the junction must have a vector sum of zero, in order to avoid an unphysical 1/r stress singularity. Accompanying the slip, a volume change occurs at the junction; either a void opens or intense local deformation is required to avoid material overlap. The energy absorbed due to the volume change is proportional to the slip increment times the total past slip accumulated at the junction. At a new junction the energy absorbed is a small fraction of the energy released by slip on the fault system, but after a number of earthquakes (proportional to shear modulus over confining stress) the junction becomes a strong barrier to further slip. Although slip occurs more easily on old rupture surfaces than on fresh fractures, the growing barrier strength of junctions requires that there be some fresh fracture in earthquakes. Perhaps a small fraction of the surface that slips in any earthquake is fresh fracture, which could provide the instability needed to explain earthquakes. A numerical model in two-dimensional static plane strain shows, even without accounting for energy absorbed at the junction, that a bend in a fault acts as a barrier if slip is impeded on the associated fault spur. The stress concentration at the bend will tend to induce slip on the spur. Slip occurring on the spur unstably (with a drop in the coefficient of friction) can induce increased slip on the main fault segments with no change in the coefficient of friction. A fault junction provides a natural realization of barrier and asperity models without appealing to arbitrary variations of fault strength. The location of the fresh fracture that occurs after a junction becomes a strong barrier remains an unanswered question. It is likely to be initiated near the stress concentration at the old junction. A model simulating the effect of fresh fracture near old junctions might explain earthquakes without appealing to an unstable friction law anywhere.

Fracture Evolution Around a Cavity in Brittle Rock Under Uniaxial Compression and Coupled Static–Dynamic Loads

Abstract: To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks.