Question about physical materials whose yield stresses are highly strain-rate dependent

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Why are materials whose yield stresses are highly strain-rate dependent
more susceptible to brittle fracture than those materials whose yield stresses
do not exhibit marked strain-rate dependence
 

Bystander

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I did some search but with not much luck. Thanks for the link provided which is helpful but not addressing the question. I am preparing for an exam for the course of Fracture and Failure Analysis and this is one of the past questions that I have seen.
 

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Not a trick question? "Answer: By definition?" Paging @Chestermiller .
 
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Sorry @Bystander. No clue. This is a little too far removed from by background.
 
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Isn't this like asking why it is that a dropped wineglass will probably shatter, while a dropped plastic ball does not?
 

Astronuc

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Why are materials whose yield stresses are highly strain-rate dependent
more susceptible to brittle fracture than those materials whose yield stresses do not exhibit marked strain-rate dependence
Do you have some specific examples of materials?

The relationships between strain (hardening) and strain rate sensitivity has to do with the behavior of dislocations in the grains/crystals of the metal, which depends on the microstructure and crystal morphology.

Thinking about metals (since they are ductile), the yield strength depends on composition, grain size, and cold work (dislocation density). Think about the role of dislocations in the strengthening of a metal/alloy. Strain rate sensitivity is influenced by the same mechanisms that influence creep and flow of a metal/alloy.

Fracture and Failure Analysis and this is one of the past questions that I have seen.
See Chapter 2 of this book, section 2.4.2 Speed of Loading
http://www.springer.com/us/book/9789814560375
With a very high rate of application of stress there may be insufficient time for plastic deformation of a material to occur under normal conditions, a ductile material will behave in a brittle manner.
But it is important to know why that is.

Strain rates can vary from about 1 E-6 /s (essentially static) to ~1 E4 /s, or about 10 orders of magnitude. At the upper end, one has to consider shock waves in the material.
 
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