Brittle

Brittle

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A material
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...

 is brittle if, when subjected to stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

, it breaks without significant deformation (strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...

). Brittle materials absorb relatively little energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 prior to fracture, even those of high strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

es (which do not deform plastically) and some polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s, such as PMMA and polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

. Many steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

s become brittle at low temperatures (see ductile-brittle transition temperature), depending on their composition and processing.

When used in materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...

, it is generally applied to materials that fail when there is little or no evidence of plastic deformation
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...

 before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred.

When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing grain
Crystallite
Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...

 size, precipitation hardening, work hardening
Work hardening
Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material. Any material with a reasonably high melting point such as metals and...

, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...

 is therefore a balancing act.

Toughening


This principle generalizes to other classes of material. Naturally brittle materials, such as glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, are not difficult to toughen effectively. Most such techniques involve one of two mechanisms
Fracture toughening mechanisms
In materials science, fracture toughening mechanisms are processes that increase energy absorption during fracture, resulting in higher fracture toughness.-Intrinsic toughening mechanisms:...

: to deflect or absorb the tip of a propagating crack, or to create carefully controlled residual stresses so that cracks from certain predictable sources will be forced closed. The first principle is used in laminated glass
Laminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...

 where two sheets of glass are separated by an interlayer of polyvinyl butyral
Polyvinyl butyral
Polyvinyl butyral is a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility. It is prepared from polyvinyl alcohol by reaction with butyraldehyde. The major application is laminated safety glass for automobile...

, which as a viscoelastic polymer absorbs the growing crack. The second method is used in toughened glass
Toughened glass
Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of...

 and pre-stressed concrete. A demonstration of glass toughening is provided by Prince Rupert's Drop
Prince Rupert's Drop
Prince Rupert's Drops are a glass curiosity created by dripping hot molten glass into cold water. The glass cools into a tadpole-shaped droplet with a long, thin tail. The water rapidly cools the molten glass on the outside of the drop, while the inner portion of the drop remains significantly...

. Brittle polymers can be toughened by using rubber particles to initiate crazes when a sample is stressed, a good example being high impact polystyrene or HIPS. The least brittle structural ceramics are silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

 (mainly by virtue of its high strength) and transformation-toughened zirconia.

A different philosophy is used in composite materials, where brittle glass fibres, for example, are embedded in a ductile matrix such as polyester resin
Polyester resin
Polyester resins are unsaturated resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Polyester resins are used in sheet moulding compound, bulk moulding compound and the toner of laser printers...

. When strained, cracks are formed at the glass-matrix interface, but so many are formed that much energy is abosrbed and the material is thereby toughened. The same principle is used in creating metal matrix composite
Metal matrix composite
A metal matrix composite is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite...

s.

Effect of pressure


Generally, the brittle strength of a material can be increased by pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

. This happens as an example in the brittle-ductile transition zone
Brittle-ductile transition zone
The brittle-ductile transition zone is the strongest part of the Earth's crust. For quartz and feldspar rich rocks in continental crust this occurs at an approximate depth of 13–18 km . At this depth rock becomes less likely to fracture, and more likely to deform ductilely by creep...

 at an approximate depth of 10 kilometres (6.2 mi) in the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

, at which rock becomes less likely to fracture, and more likely to deform ductilely
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...

.

Crack growth


Supersonic fracture
Supersonic fracture
Supersonic fractures are fractures where the fracture velocity moves faster than the speed of sound in the material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart and IBM Almaden Research Center in San Jose, California Supersonic...

 is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research
Max Planck Institute for Metals Research
The Max Planck Institute for Metals Research is a research institute of the Max Planck Society located in Stuttgart. The institute was founded 1921 as Kaiser Wilhelm Institute for Metal Research in Berlin and closed 1932. 1934 it was reopened in Stuttgart and incorporated into the Max Planck...

 in Stuttgart
Stuttgart
Stuttgart is the capital of the state of Baden-Württemberg in southern Germany. The sixth-largest city in Germany, Stuttgart has a population of 600,038 while the metropolitan area has a population of 5.3 million ....

 (Markus J. Buehler
Markus J. Buehler
Markus J. Buehler is an American materials scientist and engineer at the Massachusetts Institute of Technology . He joined MIT in 2005 and is currently a faculty member in the Department of Civil and Environmental Engineering, where he directs the Laboratory for Atomistic and Molecular Mechanics...

 and Huajian Gao
Huajian Gao
Huajian Gao is an American materials scientist and engineer. He joined the Max Planck Society in 2001 and is currently Director of the Max Planck Institute for Metals Research, Stuttgart.-Education and career:...

) and IBM Almaden Research Center in San Jose
San Jose, California
San Jose is the third-largest city in California, the tenth-largest in the U.S., and the county seat of Santa Clara County which is located at the southern end of San Francisco Bay...

, California (Farid F. Abraham
Farid F. Abraham
Farid F. Abraham is an American scientist.Abraham earned his Bachelor of Science and Ph.D from the University of Arizona in 1959 and 1962, respectively. By pioneering new methods of using computer modelling in research, he has made seminal contributions to science in the fields of fracture...

).

See also

  • Izod impact strength test
    Izod impact strength test
    Izod impact strength testing is an ASTM standard method of determining impact strength. A notched sample is generally used to determine impact strength....

  • Charpy impact test
    Charpy impact test
    The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent...

  • Fractography
    Fractography
    Fractography is the study of fracture surfaces of materials. Fractographic methods are routinely used to determine the cause of failure in engineering structures, especially in product failure and the practice of forensic engineering or failure analysis...

  • Forensic engineering
    Forensic engineering
    Forensic engineering is the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability. The field also deals with...

  • Ductility
    Ductility
    In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...

  • Strengthening mechanisms of materials
    Strengthening mechanisms of materials
    Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the...