Including Crack Propagation Research Articles

Modeling of the Effect of Excavation Mode on the Extent of the Anisotropic Zone Surrounding Deep Tunnels

A series of numerical model tests were performed to investigate the behaviour of the anisotropic rock surrounding circular excavations under high confining pressures. The aim was to provide information on the formation of fractures and failure around deep level rock tunnels under controlled conditions. Solid cubes containing a circular hole were confined to a vertical pressure with same as the confinement in the horizontal directions. In this modeling, the inhomogeneous rock is generated by using Weibull parameters which are related to the microstructural properties determined by crack size distribution and grain size. The fracture angle is assumed to be 45o. The observed failure zone around the excavation was simulated using both the maximum tensile strain criterion and Mohr-Coulomb criterion respectively (as the damage threshold). And RFPA (Realistic Failure Process Analysis) code was used as the calculating tool in this modelling, three opening modes are simulated and compared. Computational model predictions that include crack propagation and failure modes of rock show a good agreement with those of the observation in site. It is pointed out that the damage evolution of EDZ strongly depends on the inhomogeneous, the excavation mode, anisotropic property, and the various loading conditions. Concerning the existence of a weak plane, the amount of displacement at the side wall of the tunnel was quite large, since the shear deformation occurred in EDZ. The model is implemented in RFPA code and is able to represent the change in fracture patterns between the solid and jointed parts. This provides confidence for the application of the numerical model to the design of rock tunnels at great depth.

Self-similar evolution of the surface morphology of a stressed amorphous alloy foil

A time-ordered sequence of topographic images of a stressed amorphous Fe70Cr15B15 ribbon is presented. It is shown that the surface of this material (unlike polycrystalline metal foil) has a fractal structure due to the nonequilibrium conditions of its formation. As a tensile stress of about 500 MPa is applied to the surface, the fractal dimension of the surface increases from 1.21±0.02 to 1.34±0.03, then drops to 1.12±0.03, and finally increases to 1.22±0.02. In about 1.5 hours, a complex surface morphology characterized by a roughness amplitude of several tens of nanometers evolves into a regular pattern of shear bands with amplitude of about 300 nm. Self-affine changes in surface morphology are explained by competition between several processes, including crack propagation, surface smoothing, and self-diffusion.

Kunststoffe stoßartig belastet

This paper deals with a computer-controlled and fully instrumented dropping sledge test equipment for measuring the endurance strength of polymers. The measuring program enables an automatically flow of the test and a complete registration ofspecimen response including crack propagation. First tests of specimens of different thermoplastic materials demonstrated the capability of the equipment to registrate the specimen response during a single impact as well as in endurance loading up to some 10 5 impacts with good reproducibility. On the basis of the obtained results so far it can be concluded that the endurance impact test enables to distinguish the influence of different parameters on the mechanical endurance strength of polymers.

A COMPARISON OF SMALL FATIGUE CRACK GROWTH, LOW CYCLE FATIGUE AND LONG FATIGUE CRACK GROWTH IN A1‐Li ALLOYS

AbstractThe behaviour of small fatigue cracks in the A1‐Li‐Cu‐Mg‐Zr alloy 8090 has been studied. Growth rates of small fatigue cracks in these alloys were found to be proportional to ΔK2. This relationship is compared to that suggested by Murakami where crack growth rate was proportional to ε2pa. The difference between the equations is attributed to the relative importance of plastic strain for the two sets of data. It is suggested that the two equations are equivalent if crack growth rate is related to total strain. The common observation of a Paris law exponent of two in different alloys suggests that there is a single common mechanism for small fatigue crack growth. This contrasts with long fatigue crack growth, where a variety of Paris law exponents are observed. Other characteristics of this fatigue mechanism include crack propagation on {111} slip bands, insensitivity of crack propagation rate to precipitate distribution and a plastic zone comparable in size to the crack itself.

Probabilistic fatigue — theory and experimentation

The possibility of testing to destruction a significant number of large and/or expensive engineering components in order to assess their fatigue reliability statistics is, in most cases, simply not feasible. Motivated by this and the ever increasing demands for reliability estimates, this paper reports on the present status of an ongoing research endeavour whose aim is to make meaningful judgements as to the likelihood of fatigue failure of polycrystalline metals based on a combined theoretical and experimental investigation of the stochastic nature of actual material degradation processes. It first reviews the modelling, experimental and analytic steps that have recently been completed by the author and his associates to relate probabilistic and stochastic descriptions of microstructural damage processes, including crack propagation, in polycrystalline metals under constant amplitude cyclic strain loading, with the fatigue reliability of the metal involved. The results of these theoretical and experimental investigations are then used to give an indication as to the number of cycles that are involved in initiating a fatigue crack and to distinguish this number from those involved in propagating a crack to failure in ductile materials. Using this approach in the case of O.F.H.C. copper tested in the intermediary range of cycles to failure, i.e., 20,000 – 200,000, the preliminary indications, fully discussed in the paper, are that approximately 90% of the total number of cycles to failure are involved in initiating a crack, the remainder being associated with propagation. With this delineation in mind, the results of an initial acoustic emission test program designed to detect differences in the characteristics of stress energy waves originating in the neighbourhood of an initiating as opposed to a propagating crack are also described. After a discussion of these results, the paper draws some important conclusions that already influence the direction of current and future research efforts.

Dugdale crack closure analysis of fatigue cracks under constant amplitude loading

Fatigue fracture mechanics which means structural analysis including crack propagation and crack closure in conjunction with a local failure criterion provides detailed insight into mechanisms of cyclic loaded crack sheets. Using the Dugdale method and rigid-perfectly plastic strip material law the infinite sheet with colinear cracks was parametrically investigated in respect of: SSY limits, crack closure occurrence, formation of contact stresses and displacements, influences of material parameters, R- ratio and maximum load on effective stress ratio. Rationales for the load ratio, mean load and maximum load dependence and the form of da/dN- curves are given based on crack closure analysis.