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Unformatted text preview: Brittle transcrystalline steel fracture
http://www.tescan.com/gallery‐gallery.php?obr 40&menu 2 • Defects are the key to brittle fracture Fig. 9.3(b) Callister
9 •
–
• specimen still a single piece
• extensive deformation –
• many pieces
• little deformation 10 Figures from V.J. Colangelo and F.A.
Heiser, Analysis of Metallurgical Failures
(2nd ed.), Fig. 4.1(a) and (b), p. 66 John
Wiley and Sons, Inc., 1987. Used with
permission. •
•
•
– Crack orientation
– Crack geometry • t max
a
2
Kt 0
t • When max stress exceeds the yield strength in a ductile material, plastic deformation occurs
• Consider an elliptical hole in a plate: 13 Magnitude of stress diminishes with distance from the crack tip • What shape are the window panels in airplanes? • Answer: –
–
–
14 • According to what we have so far:
As rt 0 for any 0
max • That’s NOT POSSIBLE. • In 1921, Griffith developed an energy criterion.
– The crack will only propagate if:
elastic energy released > work needed to create fracture surfaces U Gc Acritical strain energy el release rate, “toughness” 15 • We find that a crack propagates if: EGc
o c 2
aY Toughness,
a material parameter Compliance function, depends on geometry G c =2γ s
16 •
1/ 2 2 E s c a E – Modulus of Elasticity
s – specific surface energy
a – one half the length of an internal crack • Fracture toughness Kc : a property that is a measure of a material’s resistance to brittle fracture when a crack is present
• when specimen thickness crack dimensions, Kc
becomes independent of thickness plane strain exists K c Y c a
Units: A unit‐less parameter dependent on :
•Crack geometry
•Specimen size
•Manner of load application MPa m KIc plane strain fracture toughness decreases with increasing strain rate and decreasing temperature •
KI 2r where K I Y o a
K IC Y c a K IC EGIC
KIC and GIC are both material parameters related to toughness.
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 Spring '11
 Ghosh
 Stress

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