As the yield strength increases, the amount of stress a metal can support without deforming increases. Alternatively, as yield strength increases, a smaller cross-section of metal is required to support a given load without deforming.
As tensile strength increases, the amount of stress a metal can support without cracking and fracturing increases. And as fracture toughness increases, the energy required to cause a crack to grow to fracture increases. Low fracture toughness corresponds to low ductility. For example, glass has very low toughness and is very brittle.
Conversely, for a certain load, as fracture toughness increases, a component can tolerate a longer crack before fracturing. Need help figuring out a component failure or quality problem?
We can help. However, this can inadvertently lead to using a material with insufficient fracture toughness to withstand fracturing if a small crack forms in the material during component manufacturing or during use. Fatigue stress is one possible cause of cracks. The formation of cracks in components exposed to fatigue conditions is often expected.
In these situations, knowledge of the fracture toughness is required to determine how long the component can remain in service before a crack grows so long that the intact cross-section of the component cannot support the load, and the component fractures. This applies to aerospace components and pressure vessels such as boilers. The strength must be large enough that the material can withstand the applied loads without deforming.
The toughness must be sufficient for the metal to withstand the formation of fatigue cracks without failing catastrophically. More information about the relationship between strength, toughness and fracture behavior is in Deformation and Fracture Mechanics of Engineering Materials by R. Reprinted with permission. Michael Pfeifer, Ph. He provides metallurgy training and metallurgical engineering consulting to companies involved with product development and manufacturing. He has over 20 years of experience working on failure analysis, root cause analysis, product design, cost reduction, and quality improvement for a wide variety of products and materials.
Michael has a Ph. Stress-corrosion cracking results from the combined action of an applied tensile stress and a corrosive environment , both influences are necessary.
SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks that are subject to corrosion, which are then corroded further, weakening the metal by further cracking. The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching.
Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile. SCC can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. Stress-corrosion cracking may cause, for example, a failure of nuclear fuel rod after inappropriate power changes, rod movement and plant startup.
Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides and mild steel cracks in the presence of alkali boiler cracking. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions. Nickel-based alloys, however, are not effected by chloride or hydroxide ions. An example of a nickel-based alloy that is resistant to stress-corrosion cracking is inconel. Special Reference: U. Department of Energy, Material Science.
January Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. There are various fuel failure root causes , that have been identified in past. One of possible causes is also the pellet-cladding interaction PCI , which may be caused by stress-corrosion cracking. In this case, a difference in thermal expansions between fuel cladding and fuel pellets causes an increase in stress in the fuel cladding.
PCI fuel failure is caused by stress-corrosion cracking on the inside surface of the cladding, which results from the combined effects of fuel pellet expansion especially at pellet radial cracks and the presence of an aggressive fission product environment especially gaseous iodine. Hydrogen embrittlement is one of many forms of stress-corrosion cracking. Hydrogen embrittlement results from the combined action of an applied tensile stress and a corrosive hydrogen environment, both influences are necessary.
In this case the corrosive agent is hydrogen in its atomic form H as opposed to the molecular form, H 2 , which diffuses interstitially through the crystal lattice , and concentrations as low as several parts per million can lead to cracking.
Although embrittlement of materials takes many forms, hydrogen embrittlement in high strength steels has the most devastating effect because of the catastrophic nature of the fractures when they occur.
Hydrogen embrittlement is the process by which steel loses its ductility and strength due to tiny cracks that result from the internal pressure of hydrogen H 2 , which forms at the grain boundaries.
In case of steels, hydrogen than diffuses along the grain boundaries and combines with the carbon to form methane gas. The methane gas collects in small voids along the grain boundaries, where it builds up enormous pressures that initiate cracks and decrease the ductility of the steel. If the metal is under a high tensile stress, brittle fracture can occur.
It is a complex process that is not completely understood because of the variety and complexity of mechanisms that can lead to embrittlement.
A number of mechanisms have been proposed to explain hydrogen embrittlement. Mechanisms that have been proposed to explain embrittlement include the formation of brittle hydrides, the creation of voids that can lead to bubbles and pressure build-up within a material. Hydrogen is introduced to the surface of a metal and individual hydrogen atoms diffuse through the metal structure.
Because the solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen. In zirconium alloys , hydrogen embrittlement is caused by zirconium hydriding. One of possible causes is also:.
Mode I. Also known as the opening mode, which refers to the applied tensile loading. The most common fracture mode and used in the fracture toughness testing.
And a critical value of stress intensity determined for this mode would be designated as KIC. A Knowledge and Innovation Community KIC , is a highly autonomous partnership of leading higher education institutions, research organisations, companies and other stakeholders in the innovation process that tackles societal challenges through the development of products, services and processes and by nurturing.
The ability of a metal to deform plastically and to absorb energy in the process before fracture is termed toughness. The key to toughness is a good combination of strength and ductility.
A material with high strength and high ductility will have more toughness than a material with low strength and high ductility. The general factors that influence toughness are alloying elements, fabrication techniques, microstructure, temper condition and service application e. Fracture toughness is an indication of the amount of stress required to propagate a preexisting flaw.
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