A material isbrittle if, when subjected tostress, itfractures with littleelastic deformation and without significantplastic deformation. Brittle materials absorb relatively littleenergy prior to fracture, even those of highstrength. Breaking is often accompanied by a sharp snapping sound.

When used inmaterials science, it is generally applied to materials that fail when there is little or noplastic deformation before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred.

Mechanical characteristics ofpolymers can be sensitive to temperature changes near room temperatures. For example,poly(methyl methacrylate) is extremely brittle at temperature 4˚C,^[1] but experiences increased ductility with increased temperature.

Amorphous polymers are polymers that can behave differently at different temperatures. They may behave like a glass at low temperatures (the glassy region), a rubbery solid at intermediate temperatures (the leathery or glass transition region), and a viscous liquid at higher temperatures (the rubbery flow and viscous flow region). This behavior is known asviscoelastic behavior. In the glassy region, the amorphous polymer will be rigid and brittle. With increasing temperature, the polymer will become less brittle.

Somemetals show brittle characteristics due to theirslip systems. The more slip systems a metal has, the less brittle it is, because plastic deformation can occur along many of these slip systems. Conversely, with fewer slip systems, less plastic deformation can occur, and the metal will be more brittle. For example, HCP (hexagonalclose packed) metals have few active slip systems, and are typically brittle.

Ceramics are generally brittle due to the difficulty of dislocation motion, or slip. There are few slip systems in crystalline ceramics that a dislocation is able to move along, which makes deformation difficult and makes the ceramic more brittle.

Ceramic materials generally exhibitionic bonding. Because of the ions’ electric charge and their repulsion of like-charged ions, slip is further restricted.

Materials can be changed to become more brittle or less brittle.

When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturallymalleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducinggrain size,precipitation hardening,work hardening, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving materialtoughness is, therefore, a balancing act.

Naturally brittle materials, such asglass, are not difficult to toughen effectively. Most such techniques involve one of twomechanisms: to deflect or absorb the tip of a propagating crack or to create carefully controlledresidual stresses so that cracks from certain predictable sources will be forced closed. The first principle is used inlaminated glass where two sheets of glass are separated by an interlayer ofpolyvinyl butyral. The polyvinyl butyral, as aviscoelastic polymer, absorbs the growing crack. The second method is used intoughened glass andpre-stressed concrete. A demonstration of glass toughening is provided byPrince Rupert's Drop. Brittlepolymers can be toughened by using metal particles to initiate crazes when a sample is stressed, a good example beinghigh-impact polystyrene or HIPS. The least brittle structural ceramics aresilicon carbide (mainly by virtue of its high strength) and transformation-toughenedzirconia.

A different philosophy is used incomposite materials, where brittleglass fibers, for example, are embedded in a ductile matrix such aspolyester resin. When strained, cracks are formed at the glass–matrix interface, but so many are formed that much energy is absorbed and the material is thereby toughened. The same principle is used in creatingmetal matrix composites.

Generally, thebrittle strength of a material can be increased bypressure. This happens as an example in thebrittle–ductile transition zone at an approximate depth of 10 kilometres (6.2 mi) in theEarth's crust, at which rock becomes less likely to fracture, and more likely to deformductilely (seerheid).

Supersonic fracture is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered^{[citation needed]} by scientists from theMax Planck Institute for Metals Research inStuttgart (Markus J. Buehler andHuajian Gao) andIBM Almaden Research Center inSan Jose, California (Farid F. Abraham).

• Charpy impact test
• Ductility
• Forensic engineering
• Fractography
• Izod impact strength test
• Strengthening mechanisms of materials
• Toughness

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• Rösler, Joachim; Harders, Harald; Bäker, Martin (2007).Mechanical behaviour of engineering materials: metals, ceramics, polymers, and composites. Springer.ISBN 978-3-642-09252-7.
• Callister, William D.; Rethwisch, David G. (2015).Fundamentals of Materials Science and Engineering. Wiley.ISBN 978-1-119-17548-3.