Fig. C-3. Ductile-Brittle Transition Typical of Steels steels lowers the transition temperature. Not all metals display a ductile-to-brittle transition. Those having an FCC structure, such as aluminum, remain ductile down to even cryogenic temperatures.

Since the grain boundaries are usually stronger than the grains themselves, ductile fractures normally occur in a transgranular manner (i.e., through the grains) in metals having good ductility and toughness. Brittle failure modes, which exhibit little or no plastic flow, can occur along the grain boundaries, or intergranularly. A comparison of these two failure modes are shown schematically in Fig. C.4. The occurrence of intergranular failures at room temperature often implies some embrittling behavior, such as the formation of brittle grain boundary films, or the segregation of impurities or inclusions that cluster at the grain boundaries. However, at temperatures high enough for creep to become dominate, the reverse is true, with intergranular failures being more common than transgranular failures.

Fatigue failures generally initiate at a surface stress concentration, followed by relatively slow crack growth through the part until the remaining cross section becomes too small to support the stress, and final failure occurs. The final overload zone can be either a ductile or a brittle failure. Under the right types of light and magnification, the growth of a fatigue crack through a metal (Fig. C.5) can often be detected by observing fatigue striations, which are lines that essentially represent one fatigue cycle per striation.

C.3 Fracture Toughness3,4

Fracture toughness is a measure of the ability of a metal to resist fracture in the presence of a flaw. The fracture mechanics approach assumes that all real structures contain one or more sharp cracks, either as a result of manufacturing or due to material defects. The problem then becomes one of determining the level of stress that may be safely applied to a crack, before it grows to a critical size and causes failure.

Transgranular Failure Intergranular Failure

Through-the-Grains Along-the-Grain Boundaries

Transgranular Failure Intergranular Failure

Through-the-Grains Along-the-Grain Boundaries

Fig. C-4. Transgranular and Intergranular Failures

When the material behavior is brittle rather than ductile, the mechanics of fracture are quite different. Instead of the slow growth coalescence of voids associated with ductile rupture, brittle fracture proceeds by high velocity crack propagation through the loaded member. However, normally ductile materials can also fail in a brittle manner, in the presence of a crack, if the combination of crack size, part geometry, temperature, and/or loading rate lies with certain critical regions. The use of higher strength materials, the wider use of welding, and the use of thicker, highly loaded structural members have reduced the capacity of structural members to accommodate local plastic deformation without failure.

The elastic stress field in the vicinity of a crack tip can be described by a parameter called the stress intensity factor K, which is a function of the crack geometry and the applied stress in the immediate vicinity of the crack. As shown in Fig. C.6, a fracture toughness test is performed by applying a tensile stress to a standard specimen, with a crack of a known size and geometry. The stress intensity factor K can be calculated using the general formula:

K = aa(aw)05 in ksi (in.)0 5 where a = geometry factor for the specimen and flaw a = applied stress in ksi a = flaw or crack size in in.

There are three crack opening modes that can be evaluated (Fig. C.7). However, Mode I, the tensile opening mode, is usually measured because it is the limiting value for K, producing the lowest values.

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