What is brittle fracture & types of brittle fracture

What is Brittle Fracture?

Brittle Fracture is the sudden, very fast cracking of material or equipment under stress where the material showed little or no sign of ductility or plastic degradation before the fracture happens. Unlike other tensile failures conditions, where the material plastically strains under loaded conditions and becomes thinner (necking) until the point of rupture, when a material go through a brittle fracture, there is no thinning or necking in the material (little or no plastic deformation). Rather, this damage mechanism often causes cracking without warning, sometimes fracturing equipment into many pieces. In a Charpy Impact test, brittle metal will absorb a small amount of energy when impact tested, a tough ductile metal a large amount of energy.

Mechanism of Brittle Fracture

Brittle fracture is often caused by low temperatures in ferritic & martensitic materials whereas austenitic materials exhibits ductile behavior even at low temperatures. If the material temperature or let’s assume Steel is at or below its brittle-to-ductile transition temperature, then it will be highly susceptible to brittle fracture. Combine this condition with a serious sized flaw and high stress on that flaw (either applied or residual), and its likely to face a brittle fracture.

Factors affecting Brittle Fracture in a material

Below are the factors that can increase the susceptibility to brittle fracture:

Metallurgical Degradation

Metallurgical degradation can occur in some steels at higher temperatures and can include things like temper embrittlement, graphitization, sigma phase embrittlement, and 885°F or 475°C embrittlement.

Material Cleanliness and Grain Structure

In case of steel, large grain sizes and steel contaminants can reduce steel toughness, thus making material more brittle. Addition of alloying elements which can affect its microstructural properties are can make material brittle.

High Material Thickness

When it comes to material thickness, thicker components have a higher degree of susceptibility to brittle fracture because they have higher tri-axial stresses. Also, thicker materials produce a state of higher constraint, and are therefore less likely to deform under stress as opposed to crack initiation and propagation.

High residual stresses

Brittle fracture is more likely in the presence of high residual stresses or if the structure is highly loaded, particularly under high strain rate (impact loading). Stress-concentrations (from weld toes, change of section, notches) and weld defects (such as cracks or lack-of-fusion) can have a major effect on the likelihood of brittle fracture.

Low temperature

The ductile to brittle transition in steels at low temperature influences whether a failure will be ductile or brittle. At low temperatures the material has lower fracture toughness and is more prone to brittle fracture. Low toughness is more likely in materials with a crystalline structure which is body centered cubic (bcc) ferritic steels because they show the toughness transition, compared with those with face centered cubic (fcc) crystal structures, such as austenitic stainless steel or aluminum which do not show a marked transition between ductile and brittle behavior. Low toughness can result from the steel’s microstructure as a fine grain size has high toughness whereas martensite or coarse grain HAZ have low toughness. Material thickness also has an effect on the fracture toughness with thick material having lower effective toughness than thinner plate made from the same material.

Appearance of Brittle Fracture

characteristic of brittle fracture is there is little or no plastic deformation before failure. The fracture surface may show chevron marks or river lines pointing back to the fracture initiation point. With a brittle impact fracture the surface is rough but not torn and will sometimes have a crystalline appearance (particularly under high strain rate loading, for example in a Charpy specimen).

Types of Brittle fracture

There are two major types of brittle fractures:

  1. transgranular and
  2. intergranular.

With transgranular fractures, the fracture travels through the grain of the material. It changes direction from grain to grain due to the different lattice orientation of atoms in each grain, following the path of least resistance.

Intergranular brittle fracture, on the other hand, occurs when a crack travels along the grain boundaries, as opposed to through the grains themselves. Intergranular fracture usually occurs when the phase in the grain boundary is weak and brittle.

In order to reduce the risk of brittle fracture, one must be sure to keep materials operating at or above their brittle-to-ductile transition temperature during both service and testing. Likewise, while conducting repairs, taking steps to establish and find flaws that might weaken the material while in-service or during pressure testing will reduce the chances of brittle fracture.

brittle fracture vs ductile fracture

Brittle fracture means fracture of material without plastic deformation or with very small plastic deformation before fracture. Rock, concrete, glass, and cast iron all have such property, so they are called brittle materials. Ductile fracture means fracture of material with large plastic deformation before fracture. Fracture of soft steel and other soft metals, rubber, and plastics is ductile fracture.

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