What is fatigue cracking?
Fatigue cracking is one of the primary damage mechanisms of structural components. Fatigue cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress of the material. This leaves tiny steps in the surface that serve as stress risers where tiny cracks can initiate.
What is a Griffith crack?
Definition. The Griffith theory states that a crack will propagate when the reduction in potential energy that occurs due to crack growth is greater than or equal to the increase in surface energy due to the creation of new free surfaces. This theory is applicable to elastic materials that fracture in a brittle fashion …
What is polymer crazing?
Crazing is a phenomenon that frequently precedes fracture in many glassy polymers and that occurs in regions of high hydrostatic tensions. It leads to the formation of interpenetrating micro-voids and small fibrils, as schematically presented in Fig. 13.79.
What is stress corrosion cracking testing?
The crack plane is indicated by the direction normal. Tests for Stress-Corrosion. Stress-corrosion cracking is a time-dependent process in which a metallurgically susceptible material fractures prematurely due to the synergistic interaction of a corrosive environment and sustained tensile stress at the metal surface.
How do you know if your fatigue is cracking?
A quick analysis of the fracture surface of a fatigue failure will often show features casually referred to as “beach marks”. These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a critical level, it will propagate very rapidly until the fracture is complete.
How do you fix fatigue cracks?
Fatigue crack repair generally falls into one of two categories: Small, localized fatigue cracking indicative of a loss of subgrade support. Remove the cracked pavement area then dig out and replace the area of poor subgrade and improve the drainage of that area if necessary. Patch over the repaired subgrade.
What is an R curve?
A tearing resistance curve, or R-curve, represents a material’s resistance to progressive crack extension (this implies that a material’s fracture toughness can change with crack extension). Stable tearing in metals occurs predominantly by the process of microvoid coalescence.
How do cracks initiate?
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. Crack initiation may also be known as crack formation.
Why does crazing happen?
Crazing is caused by the glaze being under too much tension. This tension occurs when the glaze contracts more than the clay body during cooling. Because glazes are a very thin coating, most will pull apart or craze under very little tension. Crazing can make a food safe glaze unsafe and ruin the look of the piece.
How does stress corrosion cracking occur?
Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. SCC usually occurs in certain specific alloy-environment-stress combinations.
What is chloride stress corrosion cracking?
Chloride stress corrosion cracking (CSCC) is a type of intergranular corrosion. Chloride stress corrosion involves selective attack of a metal along its grain boundaries. It occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions and high temperature.
Where do most fatigue cracks start?
Fatigue cracks propagate as a result of alternating mechanical stresses. In technical components they usually start (on surfaces) at points of stress concentration caused by abrupt changes of the component cross-section and all kinds of surface defect.
What are the stages of fatigue failure?
There are three stages of fatigue fracture: initiation, propagation, and final rupture. Indeed, this is the way that most authors refer to fatigue fracture, for it helps to simplify a subject that can become exceedingly complex. Stage 1- Initiation.
What causes block cracking?
Block cracking is mainly caused by shrinkage of the asphalt concrete and daily temperature cycling, and it is not load associated. The occurrence of block cracking usually indicates that the asphalt has hardened significantly. Block cracking normally occurs over a large portion of pavement area.
What causes crocodile cracking?
It is caused by lack of support of the road edge, sometimes due to poorly drained or weak shoulders. If left untreated, additional cracks will form until it resembles fatigue cracking.
How do you calculate energy released?
Mathematically, the energy release rate is expressed as the decrease in total potential energy per increase in fracture surface area, and is thus expressed in terms of energy per unit area.
When does a crack propagation?
From the energy criterion theory, the crack will propagate when the energy needed for a crack to propagate is sufficient to overcome the resistance of the material. The energy release rate, G, is the change in potential energy with crack surface area for a linear elastic material, (3.13)
What is crack coalescence?
3.2. Crack Propagation Process of Pre-Cracked Specimens with Crack Systems. In the specimens containing pre-existing crack systems, because of the propagation of wing cracks and secondary cracks, crack coalescence phenomenon occurred. Wing cracks propagated toward the direction of the maximum compressive stress.
