FRACTURE

  1. DEFINITION

separation of a body into 2 / more pieces

due to stress (static)

tensile

compressive

shear

torsional

  1. FRACTURE PROCESS

imposed stress

at temp relatively low than material's melting temp

  1. RELATED TERMS

FRACTURE MECHANICS

  • crack geometry, material strength & toughness & stress system
  1. 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

  1. MODE OF FRACTURE
  • see table

FAST FRACTURE

  • caused by growth of cracks, flaws / defects
  • suddenly become unstable & propagate at speed of sound
  1. 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
  1. 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