Contents 1 Fracture strength 2 Types 2.1 Brittle fracture 2.2 Ductile fracture 3 Fracture modes and characteristics 4 Notable fracture failures 5 See also 6 Notes 7 References 8 Further reading 9 External links


Fracture strength[edit] Stress vs. strain curve typical of aluminum 1. Ultimate tensile strength 2. Yield strength 3. Proportional limit stress 4. Fracture 5. Offset strain (typically 0.2%) Fracture strength, also known as breaking strength, is the stress at which a specimen fails via fracture.[2] This is usually determined for a given specimen by a tensile test, which charts the stress–strain curve (see image). The final recorded point is the fracture strength. Ductile materials have a fracture strength lower than the ultimate tensile strength (UTS), whereas in brittle materials the fracture strength is equivalent to the UTS.[2] If a ductile material reaches its ultimate tensile strength in a load-controlled situation,[Note 1] it will continue to deform, with no additional load application, until it ruptures. However, if the loading is displacement-controlled,[Note 2] the deformation of the material may relieve the load, preventing rupture.


Types[edit] Brittle fracture[edit] Brittle fracture in glass Fracture of an aluminum crank arm of a bicycle, where Bright= brittle fracture, Dark= fatigue fracture. In brittle fracture, no apparent plastic deformation takes place before fracture. Brittle fracture typically involves little energy absorption and occurs at high speeds (up to 7000 ft/sec in steel).[3] In most cases brittle fracture will continue even when loading is discontinued.[4] 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) σ t h e o r e t i c a l = E γ r o {\displaystyle \sigma _{\mathrm {theoretical} }={\sqrt {\frac {E\gamma }{r_{o}}}}} where: – E {\displaystyle E} is the Young's modulus of the material, γ {\displaystyle \gamma } is the surface energy, and r o {\displaystyle r_{o}} is the equilibrium distance between atomic centers. On the other hand, a crack introduces a stress concentration modeled by σ e l l i p t i c a l   c r a c k = σ a p p l i e d ( 1 + 2 a ρ ) = 2 σ a p p l i e d a ρ {\displaystyle \sigma _{\mathrm {elliptical\ crack} }=\sigma _{\mathrm {applied} }\left(1+2{\sqrt {\frac {a}{\rho }}}\right)=2\sigma _{\mathrm {applied} }{\sqrt {\frac {a}{\rho }}}} (For sharp cracks) where: – σ a p p l i e d {\displaystyle \sigma _{\mathrm {applied} }} is the loading stress, a {\displaystyle a} is half the length of the crack, and ρ {\displaystyle \rho } is the radius of curvature at the crack tip. Putting these two equations together, we get σ f r a c t u r e = E γ ρ 4 a r o . {\displaystyle \sigma _{\mathrm {fracture} }={\sqrt {\frac {E\gamma \rho }{4ar_{o}}}}.} Looking closely, we can see that sharp cracks (small ρ {\displaystyle \rho } ) and large defects (large a {\displaystyle a} ) both lower the fracture strength of the material. Recently, scientists have discovered supersonic fracture, the phenomenon of crack propagation faster than the speed of sound in a material.[5] This phenomenon was recently also verified by experiment of fracture in rubber-like materials. The basic sequence in a typical brittle fracture is: introduction of a flaw either before or after the material is put in service, slow and stable crack propagation under recurring loading, and sudden rapid failure when the crack reaches critical crack length based on the conditions defined by fracture mechanics.[4] Brittle fracture may be avoided by controlling three primary factors: material fracture toughness (Kc), nominal stress level (σ), and introduced flaw size (a).[3] Residual stresses, temperature, loading rate, and stress concentrations also contribute to brittle fracture by influencing the three primary factors.[3] Under certain conditions, ductile materials can exhibit brittle behavior. Rapid loading, low temperature, and triaxial stress constraint conditions may cause ductile materials to fail without prior deformation.[3] Ductile fracture[edit] Ductile failure of a specimen strained axially In ductile fracture, extensive plastic deformation (necking) takes place before fracture. The terms rupture or ductile rupture describe the ultimate failure of ductile materials loaded in tension. Rather than cracking, the material "pulls apart," generally leaving a rough surface. In this case there is slow propagation and an absorption of a large amount energy before fracture.[6] Schematic representation of the steps in ductile fracture (in pure tension) 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.[citation needed] Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modelled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation ahead of the crack as it propagates. The basic steps in ductile fracture are: void formation, void coalescence (also known as crack formation), crack propagation, and failure, often resulting in a cup-and-cone shaped failure surface.


Fracture modes and characteristics[edit] Main article: Fracture mechanics There are three standard conventions for defining relative displacements in elastic materials in order to analyze crack propagation[3] as proposed by Irwin.[7] In addition fracture can involve uniform strain or a combination of these modes.[4] Fracture crack separation modes Mode I crack – Opening mode (a tensile stress normal to the plane of the crack) Mode II crack – Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front) Mode III crack – Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front) The manner in which a crack propagates through a material gives insight into the mode of fracture. With ductile fracture a crack moves slowly and is accompanied by a large amount of plastic deformation around the crack tip. A ductile crack will usually not propagate unless an increased stress is applied and generally cease propagating when loading is removed.[4] In a ductile material, a crack may progress to a section of the material where stresses are slightly lower and stop due to the blunting effect of plastic deformations at the crack tip. On the other hand, with brittle fracture, cracks spread very rapidly with little or no plastic deformation. The cracks that propagate in a brittle material will continue to grow once initiated. Crack propagation is also categorized by the crack characteristics at the microscopic level. A crack that passes through the grains within the material is undergoing transgranular fracture. A crack that propagates along the grain boundaries is termed an intergranular fracture. Typically, the bonds between material grains are stronger at room temperature than the material itself, so transgranular fracture is more likely to occur. When temperatures increase enough to weaken the grain bonds, intergranular fracture is the more common fracture mode.[4]


Notable fracture failures[edit] Failures caused by brittle fracture have not been limited to any particular category of engineered structure.[3] Though brittle fracture is less common than other types of failure, the impacts to life and property can be more severe.[3] The following notable historic failures were attributed to brittle fracture: Pressure vessels: Great Molasses Flood in 1919,[3] New Jersey molasses tank failure in 1973[4] Bridges: King Street Bridge span collapse in 1962, Silver Bridge collapse in 1967,[3] partial failure of the Hoan Bridge in 2000 Ships: Titanic in 1912,[4] Liberty ships during World War II,[3] SS Schenectady in 1943[4]


See also[edit] Environmental stress fracture Fracture (mineralogy) Fracture (geology) Fractography Forensic engineering Forensic materials engineering Gilbert tessellation Microvoid coalescence


Notes[edit] ^ A simple load-controlled tensile situation would be to support a specimen from above, and hang a weight from the bottom end. The load on the specimen is then independent of its deformation. ^ A simple displacement-controlled tensile situation would be to attach a very stiff jack to the ends of a specimen. As the jack extends, it controls the displacement of the specimen; the load on the specimen is dependent on the deformation.


References[edit] ^ Cherepanov, G.P., Mechanics of Brittle Fracture  ^ a b Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, p. 32, ISBN 0-471-65653-4.  ^ a b c d e f g h i j Rolfe, John M. Barsom, Stanley T. (1999). Fracture and fatigue control in structures : applications of fracture mechanics (3. ed.). West Conshohocken, Pa.: ASTM. ISBN 0803120826.  ^ a b c d e f g h Campbell, edited by F.C. (2012). Fatigue and fracture : understanding the basics. Materials Park, Ohio: ASM International. ISBN 1615039767. CS1 maint: Extra text: authors list (link) ^ C. H. Chen; H. P. Zhang; J. Niemczura; K. Ravi-Chandar; M. Marder (November 2011). "Scaling of crack propagation in rubber sheets". Europhysics Letters. 96 (3): 36009. Bibcode:2011EL.....9636009C. doi:10.1209/0295-5075/96/36009.  ^ Perez, Nestor (2016). Fracture Mechanics (2nd ed.). Springer. ISBN 3319249975.  ^ Jin, C.T. Sun, Z.-H. (2012). Fracture mechanics. Waltham, MA: Academic Press. ISBN 9780123850010. 


Further reading[edit] Alireza Bagher Shemirani, Haeri, H., Sarfarazi,V., Hedayat, A., A review paper about experimental investigations on failure behaviour of non-persistent joint.Geomechanics and Engineering, Vol. 13, No. 4, (2017), 535-570, [1] 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.


External links[edit] Virtual museum of failed products at http://materials.open.ac.uk/mem/index.html Fracture and Reconstruction of a Clay Bowl Ductile fracture Authority control LCCN: sh85051154 BNF: cb119469536 (data) NDL: 00562833 v t e Patterns in nature Patterns Crack Dune Foam Meander Phyllotaxis Soap bubble Symmetry in crystals Quasicrystals in flowers in biology Tessellation Vortex street Wave Widmanstätten pattern Causes Pattern formation Biology Natural selection Camouflage Mimicry Sexual selection Mathematics Chaos theory Fractal Logarithmic spiral Physics Crystal Fluid dynamics Plateau's laws Self-organization People Plato Pythagoras Empedocles Leonardo Fibonacci Liber Abaci Adolf Zeising Ernst Haeckel Joseph Plateau Wilson Bentley D'Arcy Wentworth Thompson On Growth and Form Alan Turing The Chemical Basis of Morphogenesis Aristid Lindenmayer Benoît Mandelbrot How Long Is the Coast of Britain? Statistical Self-Similarity and Fractional Dimension Related Pattern recognition Emergence Mathematics and art Retrieved from "https://en.wikipedia.org/w/index.php?title=Fracture&oldid=816156139" Categories: Materials scienceBuilding defectsElasticity (physics)Plasticity (physics)Solid mechanicsMechanicsGlass physicsHidden categories: CS1 maint: Extra text: authors listUse dmy dates from November 2017Articles needing additional references from September 2010All articles needing additional referencesAll articles with unsourced statementsArticles with unsourced statements from February 2007Wikipedia articles with LCCN identifiersWikipedia articles with BNF identifiers


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