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Concept
Toughening
Résumé
In materials science, toughening refers to the process of making a material more resistant to the propagation of cracks. When a crack propagates, the associated irreversible work in different materials classes is different. Thus, the most effective toughening mechanisms differ among different materials classes. The crack tip plasticity is important in toughening of metals and long-chain polymers. Ceramics have limited crack tip plasticity and primarily rely on different toughening mechanisms.
For the case of a ductile material such as a metal, this toughness is typically proportional to the fracture stress and strain as well as the gauge length of the crack. The plane strain toughness in a metal is given by:
where is the plane strain toughness, is a constant that incorporates the stress state, is the tensile flow stress at fracture, is the tensile fracture strain, and is the radius of crack tip.
In a low yield strength material, the crack tip can be blunted easily and larger crack tip radius is formed. Thus, in a given metallic alloy, toughness in a low-strength condition is usually higher than for higher strength conditions because less plasticity is available for toughening. Therefore, some safety-critical structural part such as pressure vessels and pipelines to aluminum alloy air frames are manufactured in relatively low strength version. Nonetheless, toughness should be improved without sacrificing its strength in metal. Designing a new alloy or improving its processing can achieve this goal.
Designing a new alloy can be explained by different toughness in several ferrous alloy.18%Ni-maraging steel has a higher toughness than the martensitic steel AISI 4340. In an AISI 4340 alloy, interstitial carbon exist in a bcc (body centered cubic) matrix and show an adverse effect on toughness. In 18%Ni-maraging steel, the carbon content is lower and martensite is strengthened by substitutional Ni atoms. In addition, transformation induced plasticity (TRIP) effects in steel can provide additional toughness.
Source officielle
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