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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
• 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
– Crack orientation
– Crack geometry • t max
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
a material parameter Compliance function, depends on geometry G c =2γ s
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 :
•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