Analyzing Ductile and Brittle Failure Vander Voort

From macroscopic standpoint, a ductile failure exhibits the following characteristics

A large amount of plastic deformation precedes the fracture

Shear lips may be present

The fracture may appear to be fibrous, or have a matte or silky texture

The cross section at the fracture may be reduced by necking

Crack growth will be slow

From macroscopic standpoint, brittle fractures are characterized following

Little or no plastic deformation precedes the fracture

The fracture is generally flat and perpendicular to the surface of the component

The fracture may appear granular or crystalline, and is often highly reflective to light

Herringbone, or chevron, patterns may be present

Crack lengthen rapidly, often with a loud noise

The apexes of the chevron point back toward the origin of the fracture

From microscopic standpoint, the brittle fracture is characterized by transgranular cleave, intergranular fracture path, and river marks or tongues

Factors affecting the toughness and ductility of a material are: the stress system, notch sensitivity, size, design, workmanship, and enviornment effects.

Also of concern are material variables, including chemical composition, grain size, cleanliness, anisotrophy, processing problem, and embrittlement phenomena

Influence of a stress raiser, or concentrator, is much greater for a brittle material than for a ductile material

Aluminium and Silicon when used as deoxidizer to promote fine grain size also improve toughness.

Silicon in excess of that required for fine grain size lower the toughness

Careful control of the oxygen, nitrogen, sulfur and phosphorous contents is also important to prevent loss of toughness

Toughness is raised by refining the grain size of steel ferrite or prior austenite grain size

Steel cleanliness is important in obtaining good toughness. Cleanliness depends on choice of melting procedure; control of residual impurity elements through scrap control, slag control, degassing, or special melting techniques such as electron slag or vacuum arc re-melting (ESR or VAR); and choice of deoxidation process and level of addition. It also important to maintain clean moulds and ladles, minimize erosion of refractories, and avoid or minimize entrapment of slag, scum, or foreign material

In general, steel exhibits the highest when the test sample is oriented parallel to the hot-working direction.

Toughness is lowest when specimen is oriented in the short transverse (through-thickness)

Due to unfavorable carbide morphology, upper bainite generally has poor toughness. Lower bainite exhibits better toughness as the isothermal transformation temperature decreases.