Fracture Types: 2 Main Types of Fracture in Metals | Metallurgy

The following points highlight the two main types of fracture. The types are: 1. Ductile Fracture and Rupture 2. Brittle Fracture.

Type # 1. Ductile Fracture and Rupture:

A ductile fracture is characterized by extensive plastic deformation in the vicinity of an advancing crack. Ductile- fracture-surfaces have their own distinctive features on both macroscopic and microscopic levels. Fig. 15.3 illustrates several forms of ductile fractures. Fractures illustrated in Fig. 15.3 (a) to 15.3 (c) may be called as ruptures. A rupture is a slow separation process. It differs from the normal concept of fracture, which usually involves the propagation of the crack. These figures also illustrate the influence of slip on the rupture.

The configuration illustrated in Fig. 15.3 (a) is for a properly oriented single-crystal of hexagonal metal (Zn, Cd and Mg) that deforms by easy glide (mono-slip system) on the basal plane under low rates of straining. The process of slip occurs on that slip-system which has the highest resolved shear stress. When it becomes critical (Schmid’s law), the dislocation-sources get activated.

Slip continues on slip planes with very little, if any, strain-hardening. The stress-concentration occurs at the notches thus produced where the slip lines cut the crystal-surface. The stress-concentration confines the process of slip on these slip-bands, until finally the crystal separates by shear in two parts along a slip band, i.e., separation occurs by sliding off on a slip plane. Such a fracture is called shear-fracture as extensive slip occurs on active slip plane before separation, and is promoted by shear stresses.

Fig. 15.3 (b), illustrates the process of rupture characteristic of single crystal of ductile cubic metals to result in a chisel-edge type of fracture-profile. The single crystal has slip occurring simultaneously on two slip-systems with slip-planes perpendicular to the plane of the paper and with slip-directions in the plane of the paper.

Such metals show small rate of strain-hardening with increasing strain. Initially the central section of the crystal deforms at a slightly faster rate to decrease the cross-sectional area, and form a neck there, with a corresponding increase of shear-stress. Slip on double slip- system with continued-growth of neck leads to chisel-edge type rupture.

Single crystal of NaCI at higher temperatures having two slip-systems operating, shows such chisel-edge type of fracture-profile. Fig. 15.3 (c) illustrates rupture of very soft metals such as gold and lead at room temperature. Here the single crystal deforms with more than two slip-systems operative, and when initially a neck forms.

Fully-ductile polycrystalline metals too, such as gold and lead, finally rupture, when the cross-section necks down to a point (such as a sharpened pencil-tip), and it cannot bear the load any longer. Such metals show virtually 100 percent reduction in area. Rupture is favoured when the void-nucleation and/or growth is inhibited. The original orientation of a single crystal is an important factor in deciding the character of fracture or rupture.

Polycrystalline materials necessarily need five independent slip- systems for plastic deformation to occur, or to have ductility. Randomly-oriented polycrystalline materials having less than five deformation slip systems are inherently brittle. In suitable single-phase polycrystalline materials, the extent of necking depends on their purity. In highly pure materials, necking may proceed until almost 100 percent reduction in area has been attained with the specimen drawing down to a point, i.e., rupture in a fully-ductile manner.

The most common type of tensile fracture-profile (and the stages in the process) for moderately ductile metals is illustrated in Fig. 15.3 (d), when the fracture is preceded by only a moderate amount of necking. The crack nucleates at brittle particles; either the natural kind present in the multi-phase materials, e.g., cementite in steel, or foreign inclusions such as carbides, sulphides or silicates in steels.

When a metal is undergoing plastic deformation and if a brittle particle is present, it is difficult to maintain compatibility in the necked region between the continuously deforming matrix and the non- deforming brittle particle. This leads to the formation of very tiny micro-voids by decohesion at matrix-particle interface as illustrated in Fig. 15.4 (a) at MnS inclusions in steel during hot-rolling.

Microvoids can also form by the cracking of brittle particles as illustrated in Fig. 15.4 (b) when MnS particle cracks during drawing. In a tensile-test-piece, when a neck has formed, voids are more likely to be formed within the neck because of the triaxial stresses in that region. Once the voids have formed, they continue to grow with increasing deformation, eventually coalesce to form a crack. Such a crack has its long-axis perpendicular to the stress-direction.

The crack continues to grow (to ultimately reach size of an order of a mm) in a direction parallel to its major-axis by this microvoid-coalescence process. Ultimately, fracture occurs by the rapid propagation of a crack around the outer perimeter of the neck by shear deformation at an angle of about 45° with the tensile axis.

This is the angle at which shear stress is maximum (Schmid’s law). A fracture having such a characteristic surface-profile is called a cup-and-cone fracture, because one of the broken surfaces is in the form of a cup, and the other like a cone as illustrated in Fig. 15.5 and schematically in Fig. 15.3 (d). The central interior region of the surface has an irregular and fibrous appearance, which is indicative of plastic deformation there.

When this fibrous surface is examined with electron microscope at a high magnification, Fig. 15.6 (b), it is found to consist of numerous spherical ‘dimples’, each such depression (hollow) being associated with a hard particle, either a carbide, or non-metallic inclusion. Each dimple is one-half of a microvoid that formed, and then separated during fracture process. Such a structure is characteristic of ductile fracture under uniaxial tensile load.

Normally these dimples are round, or equiaxed in the fibrous central region of cup-and-cone fracture, but are oval-shaped or elongated on the shear-lip with the ovals pointing towards the centre of tensile test piece.

Such a parabolic shape of dimples is indicative of shear failure. In the fracture of a thinner plate, lesser necking may be seen, and the entire fracture-surface may be a shear fracture as elongated dimples are observed on fractured surface indicating larger proportion of shear part. Ductile fractures are usually due to simple overloads, or too high a stress on the material.

Type # 2. Brittle Fracture:

Fig. 15.7 illustrates a brittle fracture. It is characterized by a flat surface where separation has occurred perpendicular to the tensile stress. Table 15.1 distinguishes brittle and ductile fractures based on appearance. A brittle fracture occurs with no macro deformation but failure occurs abruptly without localised reduction in area. X-ray diffraction analysis is able to detect a thin film of deformed metal at the fracture surfaces.

If the fractured pieces of a brittle fracture are fitted together, the original shape and dimensions of the part are restored. Initiation of the crack normally occurs at small flaws, which cause a stress-concentration. The crack may move at a rate approaching the velocity of sound in the metal.

The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress, and yields a relatively flat fractured surface, Fig. 15.7. In an ideal material, as illustrated in Fig. 15.1 (a), fracture can be visualised as the pulling apart and breaking of the interatomic bonds across two neighbouring atomic planes.

For most brittle crystalline materials, crack propagates by the successive and repeated breaking of atomic bonds along certain crystallographic planes. This is called cleavage. Such a fracture is called transgranular as it passes through the grains, Fig. 15.2. Such a fracture has grainy, or faceted, or rock-candy appearance macroscopically. Intergranular brittle fracture can occur in alloys in which some processes have weakened the grain boundary regions.

Any metal that exhibits strong temperature and strain-rate dependence of the yield strength is susceptible to brittle fracture. BCC and HCP-metals show brittle fracture but FCC-metals do not show brittle fracture unless there are other reasons for the grain- boundary embrittlement.

Brittle fracture is identified by observing the typical features on the fractured surface. In an inorganic glass, the origin of fracture is usually a surface defect. In its immediate vicinity is the area called ‘mirror zone’, Fig. 15.8 (a). The crack spreads gradually through the mirror zone, and then more rapidly.

A useful characteristic feature of such a fracture is that markings beyond the mirror zone, called tear lines, point backward toward the origin of the fracture. Such a shell-like appearance of the tear lines is also called ‘conchoidal fracture’.

Another common feature of fracture as observed in electron microscope is that tear lines often form ‘river’ pattern in which the lines follow the direction of crack propagation. As a crack-front crosses a grain boundary, numerous tear lines initiate because of the change in the orientation, Fig. 15.8 (b).

Chevron pattern is also a common feature visible with naked eye, or a magnifying glass such as seen in steel plates fractured in brittle manner, Fig. 15.8 (c). Chevron pattern is produced by separate crack-fronts propagating at different levels in the material. The specific feature is that the apex of the pattern points towards the origin of the fracture.