Crack Initiation And Propagation Ppt Slides
In this study, the rock crack propagation induced by blasting under in situ stress is firstly analyzed theoretically, and then a numerical model with a decoupled charge in the LS-DYNA code is developed to investigate the crack evolution of rock mass subjected to coupled static stress and blast loading. Through simulation, the mechanisms of blast-induced crack evolution under various hydrostatic and nonhydrostatic pressures are investigated, and the differences in rock fracturing under high in situ stress with different decoupling coefficients are also compared and analyzed. This study provides not only an analysis of the crack evolution under high in situ stress but also a reference for resolving excavation difficulties in deep mining.
crack initiation and propagation ppt slides
It is well known that, in rock blasting, two basic forms of energy, i.e., the shock wave energy and the gas energy, are released . During the blasting, explosion shock waves are generated immediately and they travel outwards from the coupling medium or borehole followed by a longer duration gas pressure. Furthermore, it has been revealed that the explosion shock waves are responsible for the formation of the crushed zone and the initiation of surrounding radial cracks, while the gas pressure further extends these cracks [3, 7, 19, 39, 40, 45]. Kutter and Fairhurst  divided the blast-induced rock fragmentation process into four consecutive stages. This division has been extensively accepted, and it is briefly introduced here for offering a comparison with the latter numerical results, which will be presented in Section 4.1.
Figure 1 shows the sketch of the consecutive stages in the cracking process of rock blasting. When the strong shock waves propagate away from the coupling medium or the wall of borehole, the rock in the vicinity of the borehole is immediately fully crushed with relatively uniform and smallest particle size, and the first zone; i.e., the strong-shock (hydrodynamic) zone  or the crushed zone [3, 4, 38, 40, 41] (Stage 1) is formed (in some literatures, although the divisions of the consecutive stages of blast-induced rock fragmentation process are same, they are different in the nomenclature of the damage zones). Then, the shock waves pass through the second zone, i.e., the transitional and nonlinear zones  or the cracked zone  where the rock is severely fractured, but the particle size is rapidly increasing as the increase of radial distance (Stage 2). Thereafter, because of the tensile stress component of the propagating stress wave, radial cracks extend from the edge of the transitional and nonlinear zones and propagate in the third zone, i.e., the elastic zone  or the fragment formation zone  (Stage 3). Finally, the high-pressure gas promotes the propagation of the cracks; i.e., the crushed zone and the transitional and nonlinear zones are expanded, and the radial cracks are further developed due to the penetration of the high-pressure gas into the cracks (Stage 4).
For a continuous cylindrical charge hole which is subjected to in situ stress, a simplified 2D theoretical model was proposed by Nilson et al.  to predict the propagation of blast-induced cracks in a cross section of the cylindrical borehole and rock mass. In this model, it is assumed that the crack propagation is stably driven by quasistatic gas pressure, after the cracked zone is generated by explosion stress waves. The opening displacement of crack is calculated using linear elastic theory, and the inelastic response areas are assumed in small regions including the crack tips and in the vicinity of the borehole. Figure 2 shows the schematic diagram of the planer wedge-shaped crack propagation model, which is subjected to in situ stress. σ is the in situ stress; R is the radius of the borehole; is the initial length of the crack which is induced by explosion stress waves, and it is set to be 3R according to the study of Persson et al. ; and is the total length of crack induced by explosive stress waves and detonation gas.
Through comparison the results obtained from simulation and theoretical calculation, as shown in Figure 9(a), it can be seen that a similar tendency that the crack length decreases and the reduction of crack length in per MPa decrease with the increase in in situ stress, and it indicates that both the simulation and theoretical analysis provide good insights into the mechanism of blast-induced crack propagation when the rock is subjected to in situ stress. Nevertheless, there are still some differences between the simulation and theoretical results. The crack length obtained through theoretical calculation is slightly shorter than that of simulation under low in situ stress, while it is a little longer than the simulated value under high in situ stress. Meanwhile, as shown in Figure 9(b), the simulated velocity of crack propagation is higher than the theoretical value. This is mainly caused by the error accumulation during theoretical calculation on account of the simplified description of rock mass. Moreover, some influencing factors about rock blasting are not considered in this theoretical analysis such as strain hardening and damage softening. Therefore, it can be concluded that the theoretical analysis can be applied to predict the crack evolution, but due to the complexity of rock blasting under the influence of in situ stress, numerical simulation is a better way to precisely investigate the process of crack propagation.
Crack initiation is a process that forms cracks on the surface of a material. The primary reason for the formation of cracks on any surface is fatigue. Fatigue leads to progressive and localized structural damage when any material experiences cyclic loading. Due to cyclic loading, the material experiences continuous and repeated loads or forces at various points on the material. When such loads are high enough, they lead to crack initiation, growth of cracks and ultimately a fracture.
Abstract:In this work, an optimization proposal for a model based on the definition of regions for crack propagation by means of the micromechanical comparison by SEM images and its application to failure assessment diagrams (FADs) is presented. It consists in three approaches. (1) The definition of the crack propagation initiation in the elastic-plastic range. (2) A slight modification of the zones in which the FAD is divided for hydrogen induced cracking (HIC) conditions. (3) The introduction of a simple correction for the definition of the Kr coordinate of the FAD to take into account the fracture toughness reduction caused by an aggressive environment, instead of using a fracture parameter obtained from a test in air. For the experimental work, four medium and high strength steels exposed to a cathodic charge and cathodic protection environments were employed, studying two different loading rates in each case, and testing C(T) samples under slow rates in the environment. The study was completed with a subsequent fractographic analysis by SEM. A good degree of fulfilment was appreciated in both materials and environmental conditions, showing the validity of the predictions supplied by the FAD optimization model proposal, which constitutes an advance in the accuracy of the FAD predictive model.Keywords: failure assessment diagram (FAD); hydrogen induced cracking (HIC); high strength low alloy steels (HSLA); cathodic protection (CP); cathodic polarization or cathodic charge (CC); subcritical propagation; micromechanisms; crack initiation
The aim of the present investigation is to follow the evolution of IML thickness and crack initiation/propagation during thermal shock tests of THT SAC solder joints produced by through hole reflow soldering and multiwave soldering. The microstructural characterization of SAC 305 is presented in our previous publication .
A methodology is proposed to experimentally and numerically evaluate the initiation and propagation of skin/stiffener separation in postbuckled panels subjected to cyclic loads. The response mechanisms that characterize skin/stiffener separation were investigated using a three point bend (3PB) test specimen, which is a simple specimen consisting of a laminate representing the skin and a doubler representing the stiffener flange. The 3PB tests were performed under quasi-static and cyclic loads. A fatigue cohesive model was used to analyze the initiation and propagation of skin/stiffener separation in the 3PB specimen. The results indicate that the fatigue cohesive model with model parameters obtained from quasi-static fracture tests can provide accurate predictions of initiation from a pristine state, and the rate of fatigue crack propagation for a variety of mode mixities, load levels, and stress ratios.
The purpose of this investigation was to apply the techniques of fracture mechanics to a study of fatigue crack propagation in compact bone. Small cracks parallel to the long axis of the bone were initiated in standardized specimens of bovine bone. Crack growth was achieved by cyclically loading these specimens. The rate of crack growth was determined from measurements of crack length versus cycles of loading. The stress intensity factor at the tip of the crack was calculated from knowledge of the applied load, the crack length, and the specimen geometry. A strong correlation was found between the experimentally determined crack growth rate and the applied stress intensity. The relationship takes the form of a power law similar to that for other materials. Visual observation and scanning electron microscopy revealed that crack propagation occurred by initiation of subcritical cracks ahead of the main crack.
The interaction between microscopic particles within a material is the essence of cohesion . The cohesion region represents the area along the front of the crack tip where the crack initiation is imminent. The crack in the region is not a real crack either. It is a virtual numerical crack, which is used to describe the stress field between two virtual crack interfaces.