Chapter_09.pdf

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Chapter 9

MECHANICAL FAILURE

¤ How do flaws in a material initiate failure?

¤ How is fracture resistance quantified; how do different

material classes compare?

¤ How do we estimate the stress to fracture?

¤ How does temperature affect the failure stress?

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FAILURE

¤ Ductile failure:

--Plastic deformation around cracks

--Slow crack propagation

¤ Brittle failure:

--Little plastic deformation

--fast crack propagation

Moderately

ductile

ductile

brittle

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MODERATELY DUCTILE FAILURE

¤Evolution to failure:

void growth

and linkage

void

nucleation

shearing

at surface

necking

fracture

s

45°

100 µ m

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BRITTLE FRACTURE SURFACES

Transgranular

Intergranular

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IDEAL VS REAL MATERIALS

¤Stress-strain behavior (Room T):

- Real fracture strength is 10 – 1000 times

below theoretical value ( E/10 ).

Cohesive force

¤DaVinci (500 yrs ago!) observed...

--the longer the wire, the smaller the load to fail it.

¼ Reasons:

--The larger the sample is, the more chance to have flaws

--Flaws cause premature failure.

--Larger samples are more flawed!

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Chapter_01.pdf (3787 KB)
Chapter_02.pdf (1367 KB)
Chapter_03.pdf (9244 KB)
Chapter_04.pdf (2097 KB)
Chapter_06.pdf (553 KB)

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