Total fatigue lives of notched structural components are considered as a summation of crack initiation and propagation phases and a two-phase approach to fatigue life prediction is presented. Furthermore, plasticity effects during crack branching might cause premature closure of fatigue cracks. Hence, the barrier strength of a phase or a grain boundary is directly related to the crystallographic twist and tilt misorientation between the slip systems of the neighboring grains. A similar approach is used in this work but considering multiple micromechanical and microstructural variables to first predict the crack path and then the associated speed of crack propagation. Simulation of short fatigue crack propagation in a 3D experimental microstructure.
Accordingly, the stress distribution in the model microstructure is rearranged by a finite-element calculation. A stack of grains, separated by twin boundaries as shown in Fig. By the use of fracture mechanics principles it is possible to predict the number of cycles spent growing a crack to some specified length or to final failure. In a macrozone oriented favourably for basal or prismatic slip systems, the short crack growth is non-deviated with minimal resistance on the grain boundaries. Immediately after initiation, microcracks grow at a very high rate that decreases when approaching the first microstructural barrier. For this purpose, a polymer resin is vacuum-infiltrated into the crack and hardened. Schematic representation of the monotonic tensile and compression behavior of cellular metals.
A comparison with results from full-scale brake rig experiments shows that predictions by any of the models that have been calibrated using data from a well-designed thermomechanical test are in reasonable agreement with the estimated crack initiation phase for actual brake disc lives. Finally, the models are used to assess the life of a brake disc for a simulated brake dynamometer experiment. Modern techniques of material characterization, like the automated electron backscatted diffusion technique for crystallographic orientation mapping in the scanning electron microscope or in situ observation of fatigue damage using electron microscopy or synchrotron computer tomography, in combination with sophisticated computer modeling concepts, define the road map for efficient materials design and optimization in the 21st century. Similar data are derived with the assumption of identical initial crack sizes and weld toe radii, which can be explained by a more severe notch effect in the thicker plate, accelerating crack propagation in the range of small crack size. When plasticity-induced crack closure is determined by dislocation motion along activated slip systems ahead of the crack tip, there must be a relationship with the crystallographic orientation of the respective grains, at least in the case of short cracks.
Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. When discussing grain size effects, one should take a possible superimposition by texture effects into consideration. For instance, one can correlate the striation spacing with the crack-propagation increment per cycle: Measuring striation spacing at different locations of the crack path yields the crack propagation vs. For slip on the alternate slip plane the dislocation may dissociate into partial dislocations again. Furthermore, Fatemi and Socie suggested that the ratio between the shear and opening stress terms is approximately 0.
It has been widely concluded that short cracks grow faster rate than those of long cracks subjected to the same nominal stress intensity factor range, even grow at the stress intensity range below the threshold values of long cracks. This paper reviews fatigue life prediction techniques for metallic materials. This can be attributed to a smaller extent of plasticity-induced crack closure less plastic deformation at the crack tip of the stronger material , leading to a decrease in the contribution of frictional corrosion. Major simplifications and assumptions have been required in the development of life prediction methodologies. For this purpose, Vickers microhardness indentations were placed above and below a propagating microcrack and illuminated by laser light. This equation gives the approximate ideal radius of the plastic zone deformation beyond the crack tip, which is useful to many structural scientists because it gives a good estimate of how the material behaves when subjected to stress. A very effective and, if carefully calibrated, very sensitive method to measure crack propagation during monotonic or cyclic deformation resolution ~ 1 μm is provided by means of the potential-drop technique.
Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth—microstructure relationship of nanocrystalline alloys. Such a calibration can be 3. Once the critical resolved shear stress on a slip plane in a favorably oriented grain is exceeded cf. Contrary to the prediction of the Paris law, for microstructurally short cracks, e. The crack propagation direction and the associated propagation rate were reconstructed by utilizing a slip direction-based procedure. Fatigue-crack propagation can be identified in many cases by the presence of fatigue striations within the fracture surface, particularly in the case of ductile alloys.
At this point it should be mentioned that the present overview on methods of service-strength analysis and fatigue-life prediction is a coarse and incomplete one. The so-called initiation life is the number of cycles required from the time of initiating a surface imperfection or a grain size crack to the time of the formation of a not inconsiderable length of the crack. Cracks are defined as being short i when their length is small compared to relevant microstructural dimensions a continuum mechanics limitation , ii when their length is small compared to the scale of local plasticity a linear elastic fracture mechanics limitation , or iii when they are simply physically small e. A failure locus is defined for the material using basic mechanical properties. Mimicking microstructure at the macroscale makes polycrystal-like structures comparable to macroscale cracks. The inspection intervals correspond to the time to grow the crack to a critical length, which is calculated for the assumed initial crack length a0 inspection intervals 1 and 2 or the actual real crack length a inspection intervals 3 and 4. This may be due to the wedge action of metal oxide in cracks.
Small crack propagation follows crystallographic directions and planes, and thus is said to be a slip-mediated process. Architected materials that consist of periodic arrangements of nodes and struts are lightweight and can exhibit combinations of properties such as negative Poisson ratios that do not occur in conventional solids. The probability of detection of an existing crack depends on inspection technique and crack size. There are inconsistencies in the crack propagation approach with regard to the material parameters controlling crack propagation. The acceleration in the attainment of scientific knowledge in fatigue research is, last but not least, a consequence of improved methods to measure microstrains and crack propagation as well as of the introduction of servohydraulic mechanical testing systems in the late 1950s. Mechanics of Fatigue Crack Closure, J. Fatigue phenomena occur when a material is subjected to fluctuating stresses and strains, which lead to failure due to damage accumulation.