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MSE_2610_HW_9_solutions

# MSE_2610_HW_9_solutions - MSE 2610 Introduction to the...

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Introduction to the Mechanical Properties of Materials Fall 2008 Homework Solutions #9 1.) Cracks were observed in 25% of the steel Liberty ships during World War II. Some even fractured in half on cold nights while docked in calm waters. These failures were due to a ductile to brittle transition that occurs with decreasing temperature in steels. a. Use a sketch of a Davidenkov diagram to explain why the Liberty ships failed in brittle fracture. Stress, σ σ fracture σ yield σ UTS Temperature Nil ductility temperature The steel used to construct the Liberty ships has a Davidenkov diagram that looks similar to the schematic above. Due to the instability of the cracks and the relatively low loads required to propagate them in cold conditions, it is fair to assume an almost exclusively brittle fracture. Therefore, the ambient temperature must have been close to the nil ductility temperature for the steel. Below the nil ductility temperature, the fracture stress is lower than the yield or the ultimate tensile stress. There is no plasticity at failure and the steel fails in brittle fracture. Above the nil ductility temperature the steel yields but eventually fails in fracture. At high temperature, the ultimate tensile strength becomes less than the fracture strength, and the steel fails completely in the ductile rupture. b. Should the Navy have raised or lowered the nil ductility temperature (NDT) of the steel to obtain greatest toughness during low temperature service? Why? The Navy should have specified a steel with a lower nil ductility temperature because the steel would still deform plastically when the ambient temperature was low. The plastic deformation not only absorbs energy that would have gone into propagating the crack, but it also would give inspectors more warning if a component was about to fail. c.

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MSE_2610_HW_9_solutions - MSE 2610 Introduction to the...

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