What is crack initiation and propagation?
initiation of micro-cracks due to local accumulation of dislocations, high stresses at local points, plastic deformation around inhomogeneous inclusions or other imperfections in or under the contact surface; crack propagation, which causes permanent damage to a mechanical element.
How does the initiation of crack takes place in fatigue?
Fatigue is one of the primary reasons for the failure of structural components. Dislocations play a major role in the fatigue crack initiation phase. It has been observed in laboratory testing that after a large number of loading cycles dislocations pile up and form structures called persistent slip bands (PSB).
What is crack initiation stress?
In laboratory tests on intact rocks, the crack initiation stress or threshold is defined by the onset of stable crack growth. It is sometimes defined as the point where the crack volumetric strain deviates from zero. AE starts to appear systematically when the stress level is above σci.
What are the factors influencing fatigue crack growth?
Factors found to influence ΔKThEAc and subsequent plateau crack growth rates were fatigue cycle frequency, water chemistry and flow rate, temperature and the size and distribution of MnS inclusions in the steel.
Why do cracks propagate?
The theory predicts that a crack will propagate in order to lower the total energy of the system, by dissipating the elastic strain energy due to loading into the creation of a new surface.
What is fatigue propagation?
The propagation stage of fatigue causes the microcrack to change direction and grow perpendicular to the tensile stress. The second, or propagation, stage of fatigue is usually the most readily identifiable area of a fatigue fracture.
How many modes are there for crack propagation?
Modes of fracture refers to the decomposition of crack tip stresses into three loadings, or “modes.” The modes are Mode-I (stress orthogonal to the local plane of the crack surface), Mode-II (stress parallel to the crack surface but orthogonal to the crack front), and Mode-III (stress parallel to the crack surface and …
How can you improve the fatigue life of a component?
Fatigue life is improved by reducing the stress concentration on the weld joint, softening the weld toe shape, improving the metal structure of the weld zone, and adequately changing the pin shape and treatment conditions of friction stir welding as shown in Figure 2.
What is stress intensity factor in fracture mechanics?
The stress intensity factor (K) is used in the field of fracture mechanics. It predicts stress intensity near the tip of a crack caused by a remote load or residual stresses.
What is propagation of cracks?
Crack growth is defined as the widening, lengthening or increase in the number of cracks on a particular surface. Crack growth is also known as crack propagation.
Why is similitude important for fatigue crack growth?
The concept of similitude is important for fatigue crack growth, as it provides the basis for applying fracture mechanics to fatigue crack growth. Similitude implies that the crack tip conditions are uniquely defined by a single loading parameter such as the stress intensity factor.
What is the mechanism of fatigue crack propagation in ductile and brittle solids?
Mechanisms of fatigue-crack propagation in ductile and brittle solids 59 In general, ductile materials are toughened intrinsically, e.g., through mobile dislocation activity to induce a significant plastic-zone size, although under cyclic loading extrinsic mech- anisms play an important role in the form of crack closure.
Is fatigue crack growth rate dependent on the applied stress ratio?
When the crack growth rates observed under different applied stress ratio R are compared, it is noted that fatigue crack growth rate exhibits a dependence on the R ratio, particularly at both extremes of the crack growth curve.
What is the process of fatigue failure?
The process of fatigue failure itself consists of several distinct processes involving initial cyclic damage (cyclic hardening or softening), formation of an initial ‘fatal’ flaw (crack initiation), macroscopic propagation of this flaw (crack growth), and final catastrophic failure or instability.