Brittleness

Brittle redirects here. For other uses, see Brittle (dis-ambiguation).

A material is brittle if, when subjected to stress,

Brittle fracture in glass

Brittle fracture in cast iron tensile testpieces

it breaks without signicant deformation (strain). Brit-tle materials absorb relatively little energy prior to frac-

Graph comparing stressstrain curves for brittle and ductile ma-terials

ture, even those of high strength. Breaking is often ac-companied by a snapping sound. Brittle materials in-clude most ceramics and glasses (which do not deformplastically) and some polymers, such as PMMA andpolystyrene. Many steels become brittle at low tempera-tures (see ductile-brittle transition temperature), depend-ing on their composition and processing.When used in materials science, it is generally applied tomaterials that fail when there is little or no evidence ofplastic deformation before failure. One proof is to matchthe broken halves, which should t exactly since no plasticdeformation has occurred.When a material has reached the limit of its strength, itusually has the option of either deformation or fracture.A naturally malleable metal can be made stronger by im-peding the mechanisms of plastic deformation (reducinggrain size, precipitation hardening, work hardening, etc.),but if this is taken to an extreme, fracture becomes themore likely outcome, and the material can become brit-tle. Improving material toughness is therefore a balancingact.

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2 5 REFERENCES

1 TougheningThis principle generalizes to other classes of material.Naturally brittle materials, such as glass, are not di-cult to toughen eectively. Most such techniques in-volve one of two mechanisms: to deect or absorb thetip of a propagating crack, or to create carefully con-trolled residual stresses so that cracks from certain pre-dictable sources will be forced closed. The rst principleis used in laminated glass where two sheets of glass areseparated by an interlayer of polyvinyl butyral, which as aviscoelastic polymer absorbs the growing crack. The sec-ond method is used in toughened glass and pre-stressedconcrete. A demonstration of glass toughening is pro-vided by Prince Ruperts Drop. Brittle polymers can betoughened by using metal particles to initiate crazes whena sample is stressed, a good example being high impactpolystyrene or HIPS. The least brittle structural ceramicsare silicon carbide (mainly by virtue of its high strength)and transformation-toughened zirconia.A dierent philosophy is used in composite materials,where brittle glass bres, for example, are embedded ina ductile matrix such as polyester resin. When strained,cracks are formed at the glassmatrix interface, but somany are formed that much energy is absorbed and thematerial is thereby toughened. The same principle is usedin creating metal matrix composites.

2 Eect of pressureGenerally, the brittle strength of a material can be in-creased by pressure. This happens as an example in thebrittle-ductile transition zone at an approximate depth of10 kilometres (6.2 mi) in the Earths crust, at which rockbecomes less likely to fracture, and more likely to deformductilely (see rheid).

3 Crack growthSupersonic fracture is crack motion faster than the speedof sound in a brittle material. This phenomenon was rstdiscovered by scientists from the Max Planck Institutefor Metals Research in Stuttgart (Markus J. Buehler andHuajian Gao) and IBM Almaden Research Center in SanJose, California (Farid F. Abraham).

4 See also Izod impact strength test

Charpy impact test

Fractography

Forensic engineering Ductility Strengthening mechanisms of materials Stressstrain curve

5 References Lewis, Peter Rhys; Reynolds, K; Gagg, C (2004).

Forensic Materials Engineering: Case studies. CRCPress. ISBN 978-0-8493-1182-6.

Rsler, Joachim; Harders, Harald; Bker, Martin(2007). Mechanical behaviour of engineering ma-terials: metals, ceramics, polymers, and composites.Springer. ISBN 978-3-642-09252-7.

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TougheningEffect of pressureCrack growthSee alsoReferencesText and image sources, contributors, and licensesTextImagesContent license