Fracture and the Titanic Flashcards
What is the purpose of impact testing?
To measure response of a body to sudden applied loads
How is impact testing carried out?
Specimen has notch put into it; hammer is released from a fixed height; hammer strikes specimen; difference between fixed height released from and how high hammer rises on other side indicates energy expended in fracture; if hammer rises to a very high height, little energy was absorbed by the specimen, and so the specimen is brittle
In what circumstances might ductile materials fracture in a brittle fashion?
If temperature is lowered, strain rate is increased, or notch is introduced
What are the characteristics of a ductile failure?
Substantial plastic deformation forms around the crack tip; crack is stable at a constant stress/ increasing stress is required to propagate crack; high energy absorption; crack propagation is slow
What are the characteristics of brittle failure?
Little or no plastic deformation formed; applied stress causes an unstable crack; crack propagation is rapid
What is the DBTT?
The temperature at which a ductile material starts behaving in a brittle manner
What is stress amplification?
Stress is amplified around a defect tip in stress concentrations, such that the stress is no longer uniform across the structure; can occur in both micro- and macro-scopic flaws
What happened to the de Havilland Comet?
The first commercial passenger jet in 1952, had square window; corners of the windows acted as stress-raisers, developing a crack; one plane broke up due to structural damage, two due to fatigue
What happened to the Liberty Ship?
Was welded, but had lots of locked-up stresses; a drop in temperature made the material act in a brittle fashion
What were the metallurgical features of the Titanic?
Steel plates fastened with wrought iron rivets, which were heated until austenitic and hydraulically squeezed to deform; high P and S content, as well as relatively large Mn-S inclusions
Why was the P and S content bad for the Titanic?
Are embrittling
What happened to the Titanic?
Waters were below the DBTT point, and so when the iceberg struck, the steel plates fractured catastrophically
How does the steel of the Titanic compare to modern day steel?
50% shear fracture would occur in modern steel at ~3 Celsius, whilst Titanic steel would occur @49-59 Celsius
