FRACTURE
- DEFINITION
separation of a body into 2 / more pieces
due to stress (static)
tensile
compressive
shear
torsional
- FRACTURE PROCESS
imposed stress
at temp relatively low than material's melting temp
- RELATED TERMS
FRACTURE MECHANICS
- crack geometry, material strength & toughness & stress system
- FRACTURE TYPE
- depends on type & condition of material
FACTORS
type of stress applied
rate of stress applied
temp (creep)
temp & environment conditions (corrosion)
component geometry
(stress concentrators)
size & nature of internal flaws
(vacancies, precipitates, cracks)
external flaws
(crack, oxides)
material bonding & structure
- MODE OF FRACTURE
- see table
FAST FRACTURE
- caused by growth of cracks, flaws / defects
- suddenly become unstable & propagate at speed of sound
- TOUGHNESS
- energy needed to break a unit volume material
Material toughness
- slow absorption of energy by the material
Good combination: high strength + high ductility
Unit: J/m2 (energy/volume)
Area under stress-strain curve (measure from tensile test)
VARIABLES INFLUENCING TOUGHNESS OF MATERIAL
STRAIN RATE (RATE OF LOADING)
- metal: satisfactory toughness under static loads, but fail under dynamic load/ impact
- rate of loading increase - toughness decrease
- PP: at low SR - ductile; at high SR - brittle
TEMPERATURE
- temp decrease - ductility & toughness decrease
- Glass: at 800K - ductile; at 273K - brittle
NOTCH EFFECT
- material may have good toughness when stress is uniaxial
- when multiaxial tress state due to presence of notch, might not withstand simultaneous elastic & plastic deformation in various direction
- FRACTURE TOUGHNESS (FT)
- ability of material containing a crack to resist fracture
METAL
- highest FT (cracks cannot propagate easily in tough materials)
- highly resistant to cracking under stress - large zone of plastic flow
ENGINEERING CERAMICS
- lower FT
- easy to crack
- exceptional improvement in stress fracture due to 1.5 orders of magnitude strength increase - relative to metal
Measured by Stress-Intensity Factor, SIF (K)
- function of loading, crack size & structural geometry
Material Thickness
Ductile-to-Brittle Transition
*static: constant/ slowly changing with time
2) CRACK PROPAGATION
- Mode of fracture highly dependent on mechanism of crack propagation
1) CRACK FORMATION
usually in metals with BCC and HCP structure
motion of dislocation assisted by thermal agitation of atoms
at low temp, less thermal agitation - dislocation cannot move easily - intrinsic lattice resistance increase
yield strength rises - plastic zone shrinks - fracture mechanism change from ductile tearing to cleavage
similar to glass-rubber transition
below transition temp, polymer much more brittle
other polymer (e.g. epoxy) have low Gc at all temp becoz heavily cross-linked by covalent bond - does not flow at crack tip - no blunting
CONDITION FOR CRACK PROPAGATION
- fracture in material occurs when stress intensity factor (K) exceeds FT of material (Kc): K > Kc
different thickness value - different SIF value
when thickness exceed critical dimension, SIF becomes constant = value of FT