Why brittle fracture occurs

The brittle fracture of normally ductile steels occurs primarily in large, continuous, box-like structures such as:. Brittle fractures that occur in service are invariably initiated by defects that are initially present in the manufactured product or fabricated structure, or by defects that develop during service.

A brittle fracture is a breakage or cracking of a material into discernible parts, from which no deformation can be identified a clean break. It is characterized by rapid crack propagation with low energy release and without significant plastic deformation. The fracture may have a bright granular appearance. The fractures are generally of the flat type and chevron patterns may be present.

In brittle crystalline materials, fractures can occur by cleavage as the result of a tensile stress acting normal to crystallographic planes with low bonding cleavage planes. In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.

In a brittle fracture, cracks run close to perpendicular to the applied stress. This perpendicular fracture leaves a relatively flat surface at the break. Besides having a nearly flat fracture surface, brittle materials usually contain a pattern on their fracture surfaces. Some brittle materials have lines and ridges beginning at the origin of the crack and spreading out across the crack surface.

Since there is very little plastic deformation before failure occurs, in most cases this is the worst type of fracture because the visible damage cannot be repaired in a part or structure before it breaks. Brittle fractures display either transgranular or intergranular fracture.

This depends upon whether the grain boundaries are stronger or weaker than the grains:. Subscribe to our newsletter to get expert advice and top insights on corrosion science, mitigation and prevention. By: Della Anggabrata. Department of Energy, Jan Personnel need to understand brittle fracture. This type of fracture occurs under specific conditions without warning and can cause major damage to plant materials. Metals can fail by ductile or brittle fracture. Metals that can sustain substantial plastic strain or deformation before fracturing exhibit ductile fracture.

Usually a large part of the plastic flow is concentrated near the fracture faces. Metals that fracture with a relatively small or negligible amount of plastic strain exhibit brittle fracture. Cracks propagate rapidly. Brittle failure results from cleavage splitting along definite planes. Ductile fracture is better than brittle fracture, because ductile fracture occurs over a period of time, whereas brittle fracture is fast, and can occur with flaws at lower stress levels than a ductile fracture.

Figure 1 shows the basic types of fracture. Brittle cleavage fracture is of the most concern in this module. Brittle cleavage fracture occurs in materials with a high strain-hardening rate and relatively low cleavage strength or great sensitivity to multi-axial stress. Many metals that are ductile under some conditions become brittle if the conditions are altered. The effect of temperature on the nature of the fracture is of considerable importance.

Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the Nil-Ductility Transition NDT temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It is determined by some form of drop-weight test for example, the Izod or Charpy tests.

Ductility is an essential requirement for steels used in the construction of reactor vessels; therefore, the NDT temperature is of significance in the operation of these vessels. Small grain size tends to increase ductility and results in a decrease in NDT temperature. Grain size is controlled by heat treatment in the specifications and manufacturing of reactor vessels.

The NDT temperature can also be lowered by small additions of selected alloying elements such as nickel and manganese to low-carbon steels. Of particular importance is the shifting of the NDT temperature to the right Figure 2 , when the reactor vessel is exposed to fast neutrons. The reactor vessel is continuously exposed to fast neutrons that escape from the core. Consequently, during operation the reactor vessel is subjected to an increasing fluence flux of fast neutrons, and as a result the NDT temperature increases steadily.

It is not likely that the NDT temperature will approach the normal operating temperature of the steel. Crack propagation cleavage in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.

Brittle Fracture. In this section Ductile to Brittle Transition. Materials that do not fail in a ductile manner will fail in a brittle manner.