Transcript LIBERTY SHIPS presentation.ppt

LIBERTY SHIPS
Brittle Fracture
Background
• Lightly armed cargo vessels built during
WWII – 2700 built from 1942 to end of war
• Prefabricated, all-welded construction
• Took only 5 days to complete one ship
Construction of Ships
Brittle Failure
• 30% of ships
suffered
catastrophic failure,
initially the cause
was unknown
Causes of failure
• Failure thought to be due :
– All-welded construction: eliminated crackarresting plate boundaries which are present
in riveted joints
– Inexperienced operators pressed in to service:
crack-like flaws in the welds
– Material: Low toughness, ie. resistance to
crack advance, further decreased by low
temperatures
Constance Tipper
• Established the main reasons
for brittle failure:
– A triaxial state of stress
– A low temperature
– High strain rate or rapid rate of
loading
• Not all 3 have to be present,
the majority of failures
probably due to temperature
and strain rate (ie waves)
Low temperature
• All metals will begin to become brittle at transition
temperature
• As temperature decreases, the likelihood of a metal
behaving in a brittle manner increases
• Reduction in temperature inhibits the
movement of particles
– Steel becomes stronger but less ductile –
cracks propagate rapidly
Improvements to design
• Higher grade steels
used
• Rivet holes added:
– Plate boundaries to
prevent crack
propagation
• Better hull flexibility
• Stronger hulls