Fracture



Mechanical failure modes
Buckling
Corrosion
Creep
Fatigue
Fracture
Melting
Rupture
Thermal shock
Wear
Yielding
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A fracture is the (local) separation of a body into two, or more, pieces under the action of stress.

The word fracture is often applied to bones of living creatures, or to light (optical crystals).

A detailed understanding of how fracture occurs in materials may be assisted by the study of fracture mechanics.

Contents

Types of fracture

Brittle fracture

 

In conchoidal fracture, with cracks proceeding normal to the applied tension.

The theoretical strength of a crystalline material is (roughly)

\sigma_\mathrm{theoretical} = \sqrt{ \frac{E \gamma}{r_o} }

where: -

On the other hand, a crack introduces a stress concentration modeled by

\sigma_\mathrm{elliptical\ crack} = \sigma_\mathrm{applied}(1 + 2 \sqrt{ \frac{a}{\rho}}) = 2 \sigma_\mathrm{applied} \sqrt{\frac{a}{\rho}} (For sharp cracks)

where: -

  • σapplied is the loading stress,
  • a is half the length of the crack, and
  • ρ is the radius of curvature at the crack tip.

Putting these two equations together, we get

\sigma_\mathrm{fracture} = \sqrt{ \frac{E \gamma \rho}{4 a r_o}}

Looking closely, we can see that sharp cracks (small ρ) and large defects (large a) both lower the fracture strength of the material.

Recently, scientists have discovered supersonic fracture , the phenomenon of crack motion faster than the speed of sound in a material.[citation needed] This phenomenon was recently also verified by experiment of fracture in rubber-like materials.  

Ductile fracture

In cast iron or high-carbon steels can barely sustain 3% of strain.[citation needed].

 

Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modeled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates.

The basic steps of ductile fracture are necking (which results in stress localization at the point on the sample of smallest cross-sectional area), void formation, void coalescence (also known as crack formation), crack propagation, and failure, often resulting in a cup-and-cone shaped failure surface.

 

Crack Separation Modes

There are three modes of fracture. Mode I, or the opening mode, is characterized by a stress normal to the crack faces. Mode II, the sliding mode or forward shear mode, has a shear stress normal to the crack front. Finally Mode III is the tearing mode, with a shear stress parallel to the crack front.

For more information, see fracture mechanics.


See also

Bibliography

  • Dieter, G. E. (1988) Mechanical Metallurgy ISBN 0-07-100406-8
  • A. Garcimartin, A. Guarino, L. Bellon and S. Cilberto (1997) " Statistical Properties of Fracture Precursors ". Physical Review Letters, 79, 3202 (1997)
  • Callister, Jr., William D. (2002) Materials Science and Engineering: An Introduction. ISBN 0-471-13576-3
  • Peter Rhys Lewis, Colin Gagg, Ken Reynolds, CRC Press (2004), Forensic Materials Engineering: Case Studies.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fracture". A list of authors is available in Wikipedia.