Transcript Liberty Ships pres.pptx

Liberty Ships
BRITTLE FRACTURE
CRACKING
History
•WW2 BROKE OUT IN 1939. THE MERCHANT FLEET
CAUGHT UNAWARE.
•SUDDEN LARGE DEMAND FOR CARGO SHIPS TO SUPPLY
SUPPLIES FOR WAR
•WELDS REPLACE RIVETS IN DESIGN, REDUCE COST AND
TIME
•END OF WORLD WAR TWO, 2710 SHIPS BUILT
•BY 1953, 25% AFFECTED BY SOME FORM OF SERIOUS OR
POTENTIALLY SERIOUS BRITTLE FRACTURE CRACKING
Analysis
• The cause of these fractures was a mystery and
suspucion fell on the new welding techniques.
• In 1957, a British woman working in Cambridge named
Constance Tipper discovered the cause for the large
amount of ship casulties.
• She demonstrated that there is a critical temperature
below which the fracture mode changes from ductile
to brittle. Because ships in the North Atlantic were
subjected to low temperatures, they were susceptible
to brittle failure.
•Brittle fracture is a fast
fracture and absorbs
very little energy.
•Welded joints allowed
the brittle fractures to
spread for large
distances unimpeded.
•This caused severe
damage to the ships
with some ship such as
the SS John P. Gaines
breaking completely in
two.
Observing the BCC line (body close contact), this
demostrates the drop in energy required to break
the steel bonds at reduced temperatures
Improvements
• Fractures occured because:
 Welds preformed by inexperienced operators
 Local stress concentration at hatch corners
 Steel had poor toughness
Improvements
• 3 types of solution:
 Improvements to shipyard practice
 Retrofits of the completed ships
 Changes to design
Questions?