Structure-Property Correlation [9] Failure of Materials In Services

Transcription

1 MME 297: Lecture 12 Structure-Property Correlation [9] Failure of Materials In Services Dr. A. K. M. Bazlur Rashid Professor, Department of MME BUET, Dhaka Topics to discuss today... Failure: An Introduction Common Failure Modes: Fracture Fatigue Creep References: 1. W D Callister, Jr. and D G Rethwisch. Materials Science and Engineering An Introduction 9th Ed, Wiley, pp /30

2 Failure of engineering materials is almost always an undesirable event human lives are in jeopardy economic losses interference with the availability of products and services Usual causes of failure improper materials selection and processing inadequate design of the component misuse It is the responsibility of the engineer to anticipate and plan for possible future failure in the event that failure does occur, to assess its cause and then take appropriate preventive measures against future incidents 3/30 Breaking of material i.e. total destruction of material By plastic deformation or yielding e.g., by bending a paper clip By (instantaneous) impact fracture e.g., by breaking a pencil or a toothpick by impact By fatigue (delayed fracture) e.g., by bending that paper clip back and forth several times By creep (temperature-assisted delayed fracture) e.g., by sagging of gold archway in ancient churches By wear (surface damage) e.g., by simply wearing something out Corrosion and Oxidation i.e. gradual loss of material By corrosion (i.e., dissolution of material) By oxidation (i.e., the formation of nonmetallic scale or film) 4/30

3 Failure by Plastic Deformation Plastic (permanent) deformation of a bridge Deformation led to eventual collapse Suspension bridge failed after only having been open for traffic a few months (1940) 5/30 Instantaneous Impact Fracture 500 T2 tankers and 2700 Liberty ships were built during WW2 prefabricated all-welded construction with brittle steel one vessel was built in 5 days!! SS John P. Gaines split in two (1943) initially, some 30% of Liberty ships suffered catastrophic failure cracks started at stress concentrations (e.g., hatchways) and propagated rapidly through the steel hull as the metal became too brittle at low temperature Brittle fracture of SS Schenectady (1943) 6/30

4 Air France charter flight from Paris to New York (25 July 2000). The Concorde crushed into a hotel shortly after take-off, 5 miles from airport, with 109 fatalities. Attributed to a piece of metal on the runway causing the bursting of a tire. The impact of the tire debris on the fuel tank punctured it, leading to loss of engine power, and the subsequent crack. This is an example of foreign-object damage (FOD). 7/30 Fatigue and Delayed Fracture De Havilland Comet, first commercial jet aircraft, had five major crashes in period Caused by fatigue cracks initiated at square windows, driven by cabin pressurisation and depressurisation Aloha Airline Boeing 737, in route from Hilo to Honolulu (April 1998) undergoes explosive decompression 1 fatality Caused by a weakening of the fuselage due to corrosion and small cracks 8/30

5 Failure due to Wear A major wear problem is with railroad tracks, where surface wear from metal-to-metal rolling contact can damage the rails leading to derailment Derailment of 100 T tank wagon and the rest of the train in UK (1982) Rail collapse lead to derailment of a locomotive in UK (1981) 9/30 Fracture is the separation of a body into two or more pieces in response to an applied static stress at a temperature that is low relative to its melting point. Any fracture process involves two steps: Crack formation or initiation Crack propagation The mode of fracture is highly dependent on the mechanism of crack propagation. 10/30

6 Fracture strengths for most materials are significantly lower than those predicted by theoretical calculations based on atomic bonding energies Presence of microscopic flaws (cracks, voids, notches, etc.) acts as stress concentrator. For a long crack oriented perpendicular to the applied stress, the maximum stress near the crack tip is: σ m 2σ 0 a ρ t 1/2 elliptical hole in a plate s 0 = applied external stress a = half length of crack (internal flaw) (full length for surface flaw) r t = radius of curvature of crack tip s 0 stress distribution in front of hole The stress concentration factor K t = σ m σ 0 2 a ρ t 1/2 11/30 The critical stress s c required for crack propagation in a brittle material is σ c = 2 E γ s π a 1/2 E = modulus of elasticity g s = the specific surface energy a = one-half the length of an internal crack The relation between the critical stress for crack propagation (s c ) and crack length (a) K c = Y σ c πa 1/2 Thus, the condition for crack propagation: K c = the fracture toughness, a property that is a measure of a material s resistance to brittle fracture when a crack is present (Unit: MPa m 1/2 ) Y = a dimensionless parameter or function that depends on both crack and specimen sizes and geometries as well as on the manner of load application. Stress intensity factor Depends on applied stress, crack length, and component geometry K t K c Fracture toughness Depends on material, temperature, environment, and rate of loading 12/30

7 13/30 Depending on the ability of material to undergo plastic deformation before fracture, two fracture modes can be defined - ductile or brittle. Ductile fracture very extensive plastic deformation ahead of crack tip Brittle fracture very little or no plastic deformation ahead of crack tip A. Very ductile fracture soft metals (e.g. Pb, Au) at room temperature; other metals, polymers, glasses at high T. B. Moderately ductile fracture typical for ductile metals C. Brittle fracture cold metals, ceramics. 14/30

8 comparison at a glance Ductile Failure Extensive plastic deformation ahead of advancing crack Brittle Failure Very little plastic deformation at the crack front High energy absorption before failure (high toughness) Little energy absorption before failure (low toughness) Process proceeds relatively slowly as the crack length extended Crack advances extremely rapidly Such crack is stable (i.e., it resists any further deformation unless an increased stress is applied) Such crack is unstable and the crack propagation, once started, continues spontaneously 15/30 Tensile loading Shear loading Dimples Typical Cup-and-Cone fracture in ductile aluminium Fractographic studies at high resolution using SEM Brittle fracture in a mild steel Scanning electron fractograph of brittle failure 16/30

9 Stress Ductile-to-Brittle Transition Temperature dependency of absorbed impact energy of material ductile failure brittle failure Temperature impact energy drops suddenly over a narrow temperatures range BCC and HCP metals Show DBTT Depends on composition and microstructure (grain size DBTT) (DBTT) FCC metals Remains ductile even at extremely low temperatures DBTT -100 to +100 C 17/30 Case Study Pre WW2: The Titanic WW2: Liberty Ship Problem: Used steel with a DBTT about equal to atmospheric temperature!! Safe design strategy: Stay above the DBTT!! 18/30

10 Failure occurs at prolonged application of dynamic and fluctuation stress, the value of which is much lower than tensile or yield stress of material (for a static load) bridges, aircrafts, machine components Single largest cause of material failure ( 90% of all material failure) It is catastrophic and insidious, occurring very suddenly and without warning brittle-like failure, even in ductile materials 19/30 Failure process occurs by the initiation and propagation of surface-initiated crack, and the fractured surface is usually perpendicular to the direction of the applied stress. The crack propagates during the tensile stage of loading and the effective cross-sectional area of sample is reduced. Finally the remaining section can no longer support the applied stress and the material fails catastrophically. crack origin smooth, circular beachmark dull, fibrous brittle failure Practical and schematic representation of a fatigue fracture surface in a steel shaft, showing the initiation region, the propagation of fatigue crack (with beach markings), and catastrophic rupture when the crack length exceeds a critical value at the applied stress 20/30

11 Transmission electron fractograph showing fatigue striations. 21/30 Laboratory fatigue test rotating bend test periodic and symmetrical about zero axis LOAD periodic and asymmetrical about zero axis Result is commonly plotted as: S (stress) vs. N (# of cycles to failure) graph Low cycle fatigue high loads, plastic and elastic deformation High cycle fatigue low loads, elastic deformation (N > 10 5 ) random stress fluctuation 22/30

12 The S-N Curve Fatigue limit, or endurance limit, S fat stress below which fatigue failure would not occur for steels, S fat 35-60% of TS Example: Steel Most nonferrous materials do not show any fatigue limit (i.e., S fat = 0!!) Example: Aluminium Fatigue strength, S f stress to cause fracture after specific # of cycles S f Fatigue life, N f number of cycles to cause failure at a specific stress N f Factors Improving Fatigue Life Reducing working stress (magnitude, amplitude) Imposing compressive surface stress (by shot peening, case hardening, etc.) (to suppress crack growing) Improving quality of surface (removing defects e.g., sharp edge, notch, groove, etc.; applying surface treatments) Removing environmental effects (thermal fluctuations, corrosive environment) 24/30

13 Creep Failure Creep is time-dependent permanent deformation of material when subjected to prolonged constant load at high temperature (T > 0.4 T m ) Objects commonly failed under creep: turbine blades in jet engines, steam generators, etc. fractured surface of creep failured material showing oxide films fractograph of fractured surface 25/30 Obtaining Creep (e-t) Curve in laboratory experiment 1. Instantaneous deformation mainly elastic. Constant load Steady-state creep rate, De/Dt Time of rupture, t r 2. Primary (or, transient) creep decreasing creep strain with time due to work-hardening 3. Secondary (steady-state) creep rate of straining is constant: balance of hardening and recovery (longest stage) 4. Tertiary creep rapidly accelerating strain rate up to failure due to microstructural changes (formation of internal cracks, voids, cavities, grain boundary separation, necking, etc.) 26/30

14 Effect of Temperature and Applied Stress on Creep Failure Dependency of steady-state creep rate on s and T:. e s = K 1 s n. e s = K 2 s n - Q c RT K 1, K 2 and n = materials constant Q c = activation energy for creep With increasing stress or temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases 27/30 Factors Reducing Creep Rate/Failure High-melting point of material Increased Young s modulus Coarse-grained structure (reduces grain boundary sliding) (Opposite effect to strength!!) Stainless steels Contain Cr and/or Ni. Materials Resilient to Creep high temperature alloys Refractory metals High melting point elements, like Nb, Mo, W, Ta. Superalloys Co, Ni based alloys: solid solution hardening and secondary phases. Use of advanced processing technique Directional solidification producing highly elongated grains or single crystals. 28/30

15 Failure of Materials: A Summary Failure Type Description Characteristic Property General Dislocation motion at: s y, TS Yielding s s y Fracture Crack growth to rupture at: K c s < TS (ductile) s < s y (brittle) Fatigue Cyclic crack growth at: S N f curve s < s fracture K max < K c Creep High temperature (T > 0.4 T m ) Q c, n deformation by diffusion at: s < s y 29/30 Next Class MME297: Lecture 13 Classification of Biomaterials [1] The diversity and versatility of biomaterials