Everything about Fracture totally explained
A
fracture is the (local) separation of an object or material into two, or more, pieces under the action of
stress.
The word
fracture is often applied to
bones of living creatures, or to
crystals or crystalline materials, such as
gemstones or
metal. Sometimes, in crystalline materials, individual crystals fracture without the body actually separating into two or more pieces. Depending on the substance which is fractured, a fracture reduces
strength (most substances) or inhibits
transmission of
light (
optical crystals).
A detailed understanding of how fracture occurs in materials may be assisted by the study of
fracture mechanics.
Types of fracture
Brittle fracture
In
brittle fracture, no apparent
plastic deformation takes place before fracture. In brittle crystalline materials, fracture can occur by
cleavage as the result of
tensile stress acting normal to crystallographic planes with low bonding (cleavage planes). In
amorphous solids, by contrast, the lack of a crystalline structure results in a
conchoidal fracture, with cracks proceeding normal to the applied tension.
The theoretical strength of a crystalline material is (roughly)
»
Looking closely, we can see that sharp cracks (small
) and large defects (large
) 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. This phenomenon was recently also verified by experiment of fracture in
rubber-like materials.
Ductile fracture
In
ductile fracture, extensive plastic deformation takes place before fracture. Many ductile metals, especially materials with high purity, can sustain very large deformation of 50–100% or more
strain before fracture under favorable loading condition and environmental condition. The strain at which the fracture happens is controlled by the purity of the materials. At room temperature, pure
iron can undergo deformation up to 100% strain before breaking, while
cast iron or
high-carbon steels can barely sustain 3% of strain..
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.
Further Information
Get more info on 'Fracture'.
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