The crack propagates through the material by a process called cleavage. 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. Figure 2 illustrates that as the temperature goes down, the tensile strength Curve A and the yield strength Curve B increase.
The increase in tensile strength, sometimes known as the ultimate strength a maximum of increasing strain on the stress-strain curve , is less than the increase in the yield point. At this temperature and below, there is no yielding when a failure occurs. Hence, the failure is brittle.
The temperature at which the yield and tensile strength coincide is the NDT temperature. When a small flaw is present, the tensile strength follows the dashed Curve C. At elevated temperatures, Curves A and C are identical.
Therefore, if a flaw exists, any failure at a temperature equal or below the NDT temperature for flawed material will be brittle. As discussed earlier in this chapter, brittle failure generally occurs because a flaw or crack propagates throughout the material. The start of a fracture at low stresses is determined by the cracking tendencies at the tip of the crack. If a plastic flaw exists at the tip, the structure is not endangered because the metal mass surrounding the crack will support the stress.
When brittle fracture occurs under the conditions for brittle fracture stated above , the crack will initiate and propagate through the material at great speeds speed of sound. 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. As the temperature increases, a higher stress is required for a crack to propagate. The relationship between the temperature and the stress required for a crack to propagate is called the crack arrest curve, which is shown on Figure 2 as Curve D. At temperatures above that indicated on this curve, crack propagation will not occur.
Fracture toughness is an indication of the amount of stress required to propagate a preexisting flaw. The fracture toughness of a metal depends on the following factors. The intersection of the crack arrest curve with the yield curve Curve B is called the fracture transition elastic FTE point. The FTE is the temperature above which plastic deformation accompanies all fractures or the highest temperature at which fracture propagation can occur under purely elastic loads.
The intersection of the crack arrest curve Curve D and the tensile strength or ultimate strength , curve Curve A is called the fracture transition plastic FTP point. Above this temperature, only ductile fractures occur. Figure 3 is a graph of stress versus temperature, showing fracture initiation curves for various flaw sizes.
It is clear from the above discussion that we must operate above the NDT temperature to be certain that no brittle fracture can occur. 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.
The defects are essentially stress concentrators and may take any one of the following five forms. Notches, which are discontinuities caused by abrupt changes in the direction of a free surface, are often fracture initiators.
Among the common intentional notches are sharp fillets and corners, holes, threads, splines, and keyways. Notches can also be produced accidentally by mechanical damage, such as from dents, gouges, or scratches.
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