MME 131: Lecture 18 Fracture of Metals. Today s Topics

Transcription

1 MME 131: Lecture 18 Fracture of Metals Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics How do things break? Fracture fundamentals Ductile vs. brittle fracture Characterstics of ductile failure Characterstics of brittle failure Impact fracture testing Ductile-to-brittle transition Reference: W. D. Callister, Jr. Materials Science and Engineering An Introduction, 5 th Edition, John Wiley & Sons, 2001, Ch. 7, pp Lec 18, Page 1/12

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 By plastic deformation yielding e.g., by bending a paper clip By (instantaneous) impact fracture e.g., by breaking a pencil or a toothpick by impact fracture 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 Lec 18, Page 2/12

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) 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) Lec 18, Page 3/12

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). 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 Lec 18, Page 4/12

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) 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. Lec 18, Page 5/12

6 Fracture Mechanics Presence of microscopic flaws (cracks, voids, notches, etc.) acts as stress concentrator. s 0 Elliptical hole in a plate Stress distribution in front of hole For a long crack oriented perpendicular to the applied stress, the maximum stress near the crack tip is: s m 2 s 0 a ½ r t 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 The stress concentration factor K = 2 s m s 0 a ½ r t Lec 18, Page 6/12

7 Fracture Toughness, K C Crack propagates when the stress concentration factor exceeds to a certain critical value K C, known as fracture toughness. K C = s y (pa) ½ Condition for crack propagation: Stress intensity factor Depends on applied stress, crack length, and component geometry K K C Fracture toughness Depends on material, temperature, environment, and rate of loading Lec 18, Page 7/12

8 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. Dislocation mediated crack grows perpendicularly to applied stress Steps in ductile fracture 45 maximum shear stress (a) (b) (c) (d) (e) Necking Cavity formation Cavity coalescence to form elliptical crack Crack propagation at 45 deg. Fracture Lec 18, Page 8/12

9 Limited dislocation mobility No appreciable plastic deformation Crack propagation is very fast Crack propagates nearly perpendicular to the direction of applied stress Crack often propagates by cleavage breaking of atomic bonds along specific crystallographic planes (cleavage planes) Comparison at a glance Ductile Failure Extensive plastic deformation ahead of advancing crack High energy absorption before failure (high toughness) Process proceeds relatively slowly as the crack length extended Such crack is stable (i.e., it resists any further deformation unless an increased stress is applied) Brittle Failure Very little plastic deformation at the crack front Little energy absorption before failure (low toughness) Crack advances extremely rapidly Such crack is unstable and the crack propagation, once started, continues spontaneously Lec 18, Page 9/12

10 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 intergranular fracture transgranular fracture fracture crack propagated along grain boundaries fracture cracks pass through grains Lec 18, Page 10/12

11 Stress Testing of fracture characteristics under high strain rate Two standard tests, the Charpy and Izod, measure the impact energy (that required to fracture a test piece under an impact load), which is also called the notch toughness. 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 Lec 18, Page 11/12

12 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!! Next Class MME 131: Lecture 19 Fatigue and Creep of Metals Lec 18, Page 12/12