Toughness can also be defined with respect to regions of a stress–strain diagram (for low-strain rate). Toughness is related to the area under the stress–strain curve. The stress-strain curve measures toughness under gradually increasing load. Tensile toughness is measured in units of joule per cubic metre (J·m−3) in the SI system. In order to be tough, a material must be both strong and ductile. The following figure shows a typical stress-strain curve of a ductile material and a brittle material. For example, brittle materials (like ceramics) that are strong but with limited ductility are not tough; conversely, very ductile materials with low strengths are also not tough. To be tough, a material should withstand both high stresses and high strains.
A fracture is the separation of an object or material into two or more pieces under the action of stress. Engineers need to understand fracture mechanisms. There are fractures (e.g., brittle fracture), which occur under specific conditions without warning and can cause major damage to materials. Brittle fracture occurs suddenly and catastrophically without any warning. This is a consequence of the spontaneous and rapid crack propagation. However, for ductile fracture, the presence of plastic deformation gives warning that failure is imminent, allowing preventive measures to be taken. A detailed understanding of how fracture occurs in materials may be assisted by the study of fracture mechanics.
In the tensile test, the fracture point is the point of strain where the material physically separates. At this point, the strain reaches its maximum value and the material actually fractures, even though the corresponding stress may be less than the ultimate strength at this point. Ductile materials have a fracture strength lower than the ultimate tensile strength (UTS), whereas in brittle materials the fracture strength is equivalent to the UTS. If a ductile material reaches its ultimate tensile strength in a load-controlled situation, it will continue to deform, with no additional load application, until it ruptures. However, if the loading is displacement-controlled, the deformation of the material may relieve the load, preventing rupture. It is possible to distinguish some common characteristics among the stress–strain curves of various groups of materials. On this basis, it is possible to divide materials into two broad categories; namely:
- Ductile Materials. Ductility is the ability of a material to be elongated in tension. Ductile material will deform (elongate) more than brittle material. Ductile materials show large deformation before fracture. In ductile fracture, extensive plastic deformation (necking) takes place before fracture. Ductile fracture (shear fracture) is better than brittle fracture, because there is slow propagation and an absorption of a large amount energy before fracture. Any fracture process involves two steps, crack formation and propagation, in response to an imposed stress. The mode of fracture is highly dependent on the mechanism of crack propagation. Cracks in ductile materials are said to be stable (i.e., resist extension without an increase in applied stress). For brittle materials, cracks are unstable. That means crack propagation, once started, continues spontaneously without an increase in stress level. Ductility is desirable in the high temperature and high pressure applications in reactor plants because of the added stresses on the metals. High ductility in these applications helps prevent brittle fracture.
- Brittle Materials. Brittle materials, when subjected to stress, break with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. In brittle fracture (transgranular cleavage), no apparent plastic deformation takes place before fracture. In crystalography, cleavage is the tendency of crystalline materials to split along definite crystallographic structural planes. Any fracture process involves two steps, crack formation and propagation, in response to an imposed stress. The mode of fracture is highly dependent on the mechanism of crack propagation. For brittle materials, cracks are unstable. That means crack propagation, once started, continues spontaneously without an increase in stress level. Cracks propagate rapidly (speed of sound) and occurs at high speeds – up to 2133.6 m/s in steel. It should be noted that smaller grain size, higher temperature, and lower stress tend to mitigate crack initiation. Larger grain size, lower temperatures, and higher stress tend to favor crack propagation. There is a stress level below which a crack will not propagate at any temperature. This is called the lower fracture propagation stress. For brittle fracture, the fracture surface is relatively flat and perpendicular to the direction of the applied tensile load. In general, brittle fracture requires three conditions:
- Flaw such as a crack
- Stress sufficient to develop a small deformation at the crack tip
- Temperature at or below DBTT
See also: Ductile-brittle Transition Temperature
See also: Stress-corrosion Cracking
See also: Hydrogen Embrittlement