Fracture

The fracture in mechanics is a phenomenon that occurs due to the stresses acting on the material and consists, at macroscopic level, in the disintegration of the material itself (or object) in smaller fragments.

This phenomenon is characterized by the disintegration of chemical bonds that hold together and cohesive the atoms constituting the material; these bonds are broken due to an external energy provided for example by an external stress too intense.

In each material, there are different types of fracture: brittle, ductile, fatigue, creep and so on, all phenomena resulting from the more or less immediate breaking of the interatomic bonds of the material.

The fracture can occur even with low stresses both as brittle fracture and as fatigue collapse. The fracture can propagate along the edge of the grains (metal crystals) that make up the fractured material (intergranular fracture) or through a fracture within the grains themselves (transgranular fracture).

Fracture mechanisms

Generally we distinguish two types of fracture:

  • Ductile: the material subjected to stress before reaching fracture deforms significantly in the plastic field, thus absorbing part of the energy as plasticization energy;
  • Brittle: the material subjected to stress does not undergo (at least macroscopically) plastic deformation and fractures in the elastic field.

Ductile fracture

Ductile fracture is the one that develops as a result of a significant plastic deformation, even localized, and is initiated by a nucleation of pores that can be: heterogeneous due to decohesion of the matrix by particles of second phases foreign to it, homogeneous due to coalescence of microporosities existing in the matrix itself.

The propagation of the fracture occurs by further formation of pores, caused by the increasing plastic deformation and, by connection between them of these pores, with breaking surfaces determined by the action of tangential tension. By nullifying the causes that produce plastic deformation, fracture propagation stops.

Because of the low rate of fracture propagation, there is no definite time at which this occurs. The fracture is not accompanied by a dry crash, but its progress causes a prolonged crackling, which is easily detected with appropriate acoustic detectors. A characteristic of ductile fracture in pieces subjected to traction is the phenomenon of section reduction (striation), in correspondence of the fracture zone. This characteristic, in very ductile metals, may be so marked as to reduce the contact area between the two parts that will result from fracture to a line or even a point, before their detachment actually takes place.

In cylindrical pieces subjected to normal traction, the ductile fracture surface assumes the shape of a truncated cone funnel, while its macroscopic characteristic is a remarkable fibrosity.

Brittle fracture

Brittle fracture is that which occurs under external action, by extension of a pre-existing crack, at nominal stresses that may be even considerably below the elastic limit of the material. At a critical combination of the crack severity conditions and the intensity of the stress field in the vicinity of the crack, the rate of crack extension increases to a limit near 40% of the speed of sound. The high speed with which the phenomenon proceeds in the last stage allows us to consider the fracture, under such conditions, as instantaneous. The fracture propagates entirely at the expense of the elastic energy stored in the piece and therefore cannot be stopped by eliminating the external stress.

The excess elastic energy is released at the time of fracture, resulting in a dry crash (noise). The external appearance of a piece broken by brittle fracture shows no evidence of macroscopic plastic deformation phenomena, while microdeformations are minimized.

The brittle fracture can take place by cleavage (transcrystalline or infracrystalline fracture), recognizable at high magnifications, for a considerable roughness of the fracture surface, or be intercrystalline (with smoother surface), usually due to the fracture proceeding through a film formed by a brittle constituent of the alloy, segregated at the grain boundaries. The danger of brittle fracture is determined by the fact that it occurs, in workpieces, suddenly, without any plastic deformation being previously noticed. Cases of this type of fracture occur in all those situations in which there are factors capable of causing brittleness in metals.

Metallurgy

In metallurgy, the fracture is a sudden break of metal materials due to stresses, caused by stress, applied at a point far from that in which the fracture begins. There are two major types of fractures: fragile and ductile (or tenacious).

Fracture toughness

Fracture toughness is a property which describes the ability of a material to resist fracture and is one of the essential properties of any material for many design applications.

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