steel structures 3-03

Question 1

what is cleavage fracture?

Answer 1

Failure mechanism in which in consequence of local loading a micro crack arises because
the cleavage fracture stress is exceeded due to a pile-up of dislocations at barriers such
as grain boundaries or non-metallic inclusions.

• material separation ┴ to the largest principal
  normal stress along the crystal planes with low
  packing density
• the separation takes place transgranular at the
  atomic level, or intergranular along the grain boundaries
• low deformation before fracture (no necking)
• planar, macroscopically shiny

Question 1

fractures due to mechanical load:

Answer 1

overload failure: single overloading
-sliding fracture
-cleavage fracture

fatigue failure: repeated loading

Question 2

what is sliding fracture?

Answer 2

Fracture mechanism in which pores are formed as a result of local
capabilities for plastic deformation. The pores grow under further loading
and finally unify at failure.

• large deformations in the micrometer range before breaking
• dimple-like fracture surface; macroscopically matt
• microscopically ductile; (macroscopically does not need to be a ductile failure)

Question 3

name the 3 phases or microscopic phenomena of ductile fracture:

Answer 3

1-. blistering / pore formation at the point of the
greatest necking
2-. void expansion: concentration of individual pores
in the center of specimen; formation of
microcracks
3-. merging (coalescence) of the microcracks / voids
and crack propagation to the surface of the
specimen in the direction of principal stresses

Question 4

conditions supporting brittle fracture:

Answer 4

-structural design: restraint and stress triaxiality due to notches, abrupt transitions (change in stiffness), large wall thicknesses.
-production: surface defects, inhomogeneous structure and cracks due to welding, grinding, hardening by thermal cutting; cold-forming; insufficient steel toughness.
-loading conditions: high loading rate (e.g. impact forces); multiaxial state of stress (tension-tension); cyclic loading
-surrounding conditions: low temperature, corrosive media → stress corrosion cracking, ionising radiation → neutron embrittlement
-microstructure: coarse grained structure, grain boundary precipitates, non-metallic inclusions; high nitrogen content (aging), micro-structural changes.

Question 5

thick components - more brittle fracture

Answer 5

• increase in residual stresses from the welding process due to greater
  stiffness of the component
• plane state of stress (thin components) plane state of strain (thick components: triaxial stress state)
• increase in cooling rate during welding (formation of martensite)
• increase in metallurgical difficulties (inhomogeneity of the material)
• uneven distribution of the toughness throughout the product thickness:
  
  • relatively uniform: normalized and heat-rolled
    plates
  • non-uniform: incompletely hardened water-
    quenched and tempered plates (fy > 620 MPa) and
    plates in as-rolled condition with t > 100 mm

Question 6

what test is used for evaluation of brittle fracture behavior?

Answer 6

notched-bar impact-bending test/ chary impact test

Question 7

name two types of cracks in welded joints:

Answer 7

-lamellar fracture
-hydrogen cracking