100 th. Anniversary of the TITANIC DISASTER 14 th. April 1912
Unsinkable: 16 watertight compartments.
Sank in less than three hours. Lifeboat space had been provided for only about half of the passengers and crew Californian, close to the scene, had not come to the rescue because its radio operator was off duty and asleep. Reforms: lifeboat space for every person on a ship lifeboat drills full-time radio watch while at sea an international ice patrol.
Why did the Titanic sink so easily?
"All kids dream a marvelous image of what they want to do. But then society tells them they can't do it. I didn't listen. I wanted to live my dream." Robert D. Ballard Professor of Oceanography Director, Institute for Archaeological Oceanography University of Rhode Island Discovered the wreck of the Titanic in 1985
Liberty Ships : the first all-welded pre-fabricated cargo ships Mass produced in the United States. 2,751 Liberty Ships were built between 1941 and 1945 Only two now remain afloat Many of the remainder were destroyed by cracking of the type shown.
What was the cause of the failure of the Liberty ships?
Two ways for the tire/tube to fail Silent puncture Noisy burst ?
Balloon Experiment
Surprise !!! A balloon bursts when all we are trying to do is to make a small hole ???
Phenomena of rupture and flow in solids, Philosophical Transactions of the Royal Society of London, A221, (1921) A.A. Griffith ( )
Experiment on paper tearing Compressive stress Tensile stress Cracks propagate when there is tensile stress perpendicular to the crack surface. They do not propagate when there is compressive stress perpendicular to the surface.
A critical tensile stress is required to propagate the crack. What is the driving force for crack propagation? Reduction in strain energy But what prevents the propagation of crack even in the presence of a tensile stress less than a critical value? Surface energy
Griffith’s relation continued: ΔEΔE c c* c*= critical crack size Crack is stable Crack will grow
15 Safe Stable cracks Unsafe Unstable cracks c
16 f = fracture stress E = Young’s modulus = surface energy c = crack size Griffith’s equation 2c Competition between strain energy and surface energy
17 c = half crack size for internal crack Griffith’s equation c c =full crack size for surface crack Surface crack of depth c as effective as an internal crack of length 2c 2c
Distance from the crack tip For c = 1 m ; =1 Å; max =201 app =crack tip radius
Two types of fracture: Ductile fracture Brittle fracture Brittle ductile stress strain Q: Which of the two absorb more energy?
Charpy Impact Test ductile brittle
Ductile-to-brittle transition y (bcc) T T DBTT y (fcc) T ff ff y < f ductilebrittle y > f
Ductile-to-brittle transition 1.FCC materials do not show DBT Good for cryogenic applications Stainless steel (austenite: fcc)containers for Liq O 2 rocket fuel mild steel not good ( : bcc) 2.Fine grain size give lower transition temperature 3.High strain rate increase the transition temperature 4. Notches increase the transition temperature
Protection against fracture Introduce compressive stresses in the surface Tempering of glass Ion- exchange of glass Carburization of steel (chapter 8) Fine-grains in ceramic: glass-ceramics discussed in chapter 9 Reduction of sulphur in steel Reduction in cross-section: glass fiber
24 2 May 1952 A new era in civil aviation BOAC- the envy of world airlines Smooth and Silent Travel time reduced by half Pressurised cabins COMET First Passanger jet aircraft
25 Comet Tragedy 2 nd May 1952First flight 2 nd May 1953First crash near Kolkata 10 th Jan 1954Second crash near Elba Islands 8 th April1954Third crash between Rome to cairo WITHDRAWN FROM SERVICE
26 What were the causes of these failures ? ? ? ?
27 Cyclic Loading Fatigue
28 Can a metal get tired? Ans: Yes, Indeed they do!! max Fracture yy