Lecture 14 Fracture and Impact ME 330 Engineering Materials Fracture Process Ductile Fracture Brittle Fracture Impact Testing Stress Concentration Factor, K t Case Study - Silver Bridge Disaster Read Chapter 8 Fracture mechanics: Study of cracks and crack-like defects with a view of understanding and predicting the cracks growth tendencies. Such growth may be either stable (slow and safe) or unstable (instantaneous and catastrophic). FRACTURE MECHANICS: An Introduction ISSUES TO ADDRESS... How do flaws in a material initiate failure? How is fracture resistance quantified; how do different material classes compare? How do we estimate the stress to fracture? How do loading rate, loading history, and temperature affect the failure stress? Ship-cyclic loading from waves. Computer chip-cyclic thermal loading. Hip implant-cyclic loading from walking. Adapted from Fig (b), Callister 7e. (Fig (b) is courtesy of National Semiconductor Corporation.) Adapted from Fig (b), Callister 7e. Chapter 8: Mechanical Failure Adapted from chapter-opening photograph, Chapter 8, Callister 7e. (by Neil Boenzi, The New York Times.) Collapse of Molasses tank in Boston in 1919 Cargo ships failures (World War II) (Liberty ships) -Due to presence of stress concentrations in the ship structure -ability of cracks to transverse welds joining adjacent steel plates -faulty welds -inferior steel quality These ships were first to have an all welded hull, could be fabricated much faster than earlier riveted designs. Today all ships are welded but sufficient knowledge was gained to avoid similar failures. FRACTURE FAILURES: Examples Several bridges fractured and collapsed in Belgium, Canada, Australia, US (last 50 years), e.g. Duplessis Bridge in Quebec, Canada due to preexistent crack in steel microstructure Minnesota Bridge in Minneapolis, due to design flaw and extra weight Sinking of Titanic in due to brittle fracture of steel with high sulfur content Challenger Space Shuttle - O-ring seal failed in one of the main boosters due to cold weather Columbia Shuttle Accident (2003) - hole was punctured in one of the wings made of carbon- carbon composite (a piece of insulating foam from the external fuel tank peeled off, puncturing the wing) FRACTURE FAILURES: Examples (cont) FRACTURE MECHANICS : Historical Perspective 1.Experiments of Leonardo da Vinci (500 years ago) He measured strength of iron wires and found that the strength varied inversely with wire length. These results implied that flaws in the material controlled strength. 2.Inglis (1913) Stress field in the presence of elliptical hole 3. Griffith (1920) Quantitative connection between fracture stress and flaw size He applied a stress analysis of an elliptical hole to the unstable propagation of a crack. Theory applied to brittle materials only. 4.Westergaard (1938) semi-inverse technique for analyzing stresses and displacements near a sharp crack tip. 5.Irwin (1956) energy release rate concept 6.Irwin (1957) introduced crack tip characterizing parameter called stress intensity factor All above: Linear elastic fracture mechanics (LEFM) 7.Irwin, Dugdale, Barenblatt, Wells (1960s) analysis of yielding at crack tip 8. Rice (1968) J- integral (parameter to characterize nonlinear behavior of crack tip) 9. Begley and Landes (1971) used J integral to characterize fracture toughness of steels 10. Shih and Hutchinson (1976) mathematical relationship between fracture toughness, stress, and flaw size. FRACTURE MECHANICS : Historical Perspective Fracture Process Crack formation (initiation) Crack propagation Dictates mode of fracture Ductile Extensive plastic deformation - often visible deformation Proceeds relatively slowly Stable - crack resists further extension unless there is an increased stress Brittle Small plastic deformation Spread rapidly Unstable - crack propagation will occur without increased stress Fracture mechanisms Ductile fracture Occurs with plastic deformation Brittle fracture Little or no plastic deformation Catastrophic Ductile failure: --one piece --large deformation Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., Used with permission. Example: Failure of a Pipe Brittle failure: --many pieces --small deformation Typical Tensile Fracture Surfaces Soft metals (Ag, Au) Metals, polymers, glasses at high temperatures Ductile Metals Brittle Metals (Cast Iron) Ceramics Brittle Plastics From: Callister Ductile vs Brittle Failure Very Ductile Moderately Ductile Brittle Fracture behavior: LargeModerate%AR or %EL Small Ductile fracture is usually desirable! Adapted from Fig. 8.1, Callister 7e. Classification: Ductile: warning before fracture Brittle: No warning Evolution to failure: Resulting fracture surfaces (steel) 50 mm particles serve as void nucleation sites. 50 mm From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig , p. 294, John Wiley and Sons, Inc., (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp ) 100 mm Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission. Moderately Ductile Failure necking void nucleation void growth and linkage shearing at surface fracture (MPa) (%) Necking begins at UTS * Microvoids form in neck 3-D stress state * Microvoid enlargement, coalescence, & growth * Further microvoid growth Rapid crack propagation around outer neck * Fibrous region Shear region ~45 Ductile Tensile Fracture- Cup-and-cone formation From: Callister Ductile vs. Brittle Failure Adapted from Fig. 8.3, Callister 7e. cup-and-cone fracturebrittle fracture Brittle Tensile Fracture No appreciable deformation Rapid crack propagation Perpendicular to applied load Flat fracture surface Fracture surfaces show point of crack formation Often visible with naked eye Chevron marks point to center Fan-like patterns Transgranular - bond breaking along crystallographic planes Cracks pass through grains, change orientation Also called cleavage Intergranular - cracking along grain boundaries From: Hertzberg, Callister Intergranular (between grains) Intragranular (within grains) Al Oxide (ceramic) Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78. Copyright 1990, The American Ceramic Society, Westerville, OH. (Micrograph by R.M. Gruver and H. Kirchner.) 316 S. Steel (metal) Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.) 304 S. Steel (metal) Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.) Polypropylene (polymer) Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., mm 4 mm 160 mm 1 mm (Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p ) Brittle Fracture Surfaces Intergranular (between grains) Intragranular (within grains) Stress-strain behavior (Room T): Ideal vs Real Materials TS E/150) polymers More Ductile Brittle Ductile-to-brittle transition temperature FCC metals (e.g., Cu, Ni) Adapted from Fig. 8.15, Callister 7e. Ductile-to-Brittle Transition Higher temperatures show higher energy absorption Can see a distinct temperature (or range) where absorption characteristics change drastically Appearance of fracture surface also changes Ductile - fibrous or dull (shear), 45 surfaces Brittle - granular or shiny (cleavage) Usually see some combination Most materials show some form of transition BCC and HCP metals (not FCC - yield stress always lower than stress to cause brittle failure, regardless of temperature) Most polymers - closely related to glass transition temperature Ceramics at very high temperatures Pre-WWII: The Titanic WWII: Liberty ships Problem: Used a type of steel with a DBTT ~ Room temp. Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and Sons, Inc., (Orig. source: Dr. Robert D. Ballard, The Discovery of the Titanic.) Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and Sons, Inc., (Orig. source: Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad. Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.) Design Strategy: Stay Above The DBTT!