Four basic types of fracture mechanisms: Part II

Intergranular fracture and fatigue fracture surfaces among mechanisms of failure that are observed in actual parts.

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In the previous article, we discussed the fracture surfaces that are apparent for ductile or overload type fractures, and the fractures that occur with brittle cleavage fracture. In this article, we will discuss intergranular fracture, as well as fatigue fracture surfaces.

Intergranular Fracture or Decohesive Rupture

In intergranular fracture (Figure 1), there is decohesion at the grain boundaries. The fracture surface often has the appearance of “rock candy” and there is little bulk plastic deformation. This type of fracture is usually related to either environmental or specific microstructures.

Figure 1: Intergranular fracture of 300M alloy steel, exhibiting the typical “rock-candy” features.

Grain boundaries in most engineering materials are stronger than a single grain of material (assuming properly processed). The grain boundaries interrupt the lattice of individual grains and provide for increased strength by pinning dislocation movement. Fine grain materials have more surface area than do larger grained materials, so more dislocations are pinned, increasing ductility and strength.

Grain boundaries are also locations where voids and crystal faults gather. They are also an easy path for diffusion. Impurities often segregate to grain boundaries. Failure along grain boundaries can occur by either environmental or by specific microstructures where precipitation has occurred.

Intergranular fractures can be divided into two different categories — those that have a dimpled intergranular fracture and those that have a brittle intergranular fracture. In dimpled intergranular fracture, microvoid coalescence occurs on the grain boundaries. In brittle intergranular fracture, the grain boundaries show no evidence of microvoid coalescence but show a more “rock candy” type of fracture [1].

In dimpled intergranular fracture, the grain boundaries have a low ductility. Often this is due to the formation of microvoids at precipitates decorating the grain boundaries. It can also occur in aluminum alloys where significant precipitate free zones (PFZ) have occurred. The grain boundaries in this case are nearly pure aluminum and are weaker (and more ductile) than the interior of the grains. This is most common in 7XXX (Al-Mg-Zn) alloys [2].

Brittle intergranular cracking is usually associated with grain boundary strengthening. It is easily recognized because of its highly faceted appearance. Causes can include brittle second phase particles at grain boundaries, segregation of impurities to grain boundaries, or environmentally induced embrittlement [3].

In either brittle or ductile intergranular fracture, the mode of fracture is readily apparent. However, the mechanism or cause of fracture is not so readily deduced.

The causes of intergranular fracture are usually associated with improper processing or a specific environment that weakens the grain boundaries. Generally, the causes of intergranular fracture are:

  • Grain boundary precipitates.
  • Segregation of impurities to grain boundaries by thermal processing.
  • Elevated temperatures and stress (creep).
  • Environmentally attack or weakening of the grain boundaries (usually specific systems).
  • Examples of causes of intergranular fracture are listed in Table 1.
Table 1: Typical causes of intergranular fracture, and examples.

Fatigue-Induced Fracture

Parts are subjected to varying stresses during service. These stresses are often in the form of repeated or cyclic loading. After enough applications of load or stress, the components fail at stresses significantly less than their yield strength. Fatigue is a measure of the decrease in resistance to repeated stresses.

Fatigue failures are brittle appearing, with no gross deformation. The fracture surface is usually normal to the main principal tensile stress. Fatigue failures are recognized by the appearance of a smooth rubbed type of surface, generally in a semicircular pattern. The progress of the fracture (and crack propagation) is generally suggested by beach marks. The initiation site of fatigue failures is generally at some sort of stress concentration site or stress riser.

Three factors are necessary for fatigue to occur. First, the stress must be high enough that a crack is initiated. Second, the variation in the stress application must be large enough that the crack can propagate. Third, the number of stress applications must be sufficiently large that the crack can propagate a significant distance. The fatigue life of a component is affected by several variables, including stress concentration, corrosion, temperature, microstructure, residual stresses, and combined stresses.

The fracture surface of fatigue is divided into four distinct regions:

1. Crack initiation, the early development of fatigue damage.

2. Slip band crack growth, the early stages of crack propagation, often called stage I crack growth.

3. Stable crack growth, which is usually normal to the applied tensile stress. This is called stage II crack growth.

4. Unstable crack growth with final failure from overload. This is called stage III crack growth.

Fatigue usually occurs at a free surface, with the initial features of stage I growth, fatigue cracks, initiated at slip-band extrusions and intrusions [4]. Cottrel and Hull [5] proposed a mechanism for the formation of these extrusions and intrusions (shown schematically in Figure 2) that depends on the presence of slip, with slip systems at 45-degree angles to each other, operating sequentially on loading and unloading. Wood [6] suggested that the formation of intrusions and extrusions was the result of fine slip. The notch created on a microscopic scale would be the initiation site of stable fatigue crack growth.

Figure 2: Schematic representation of the mechanism of fatigue intrusions and extrusions after [5].

In stage II, stable fatigue crack growth, striations (Figure 3) often show the successive position of the crack front at each cycle of stress. Fatigue striations are usually detected using electron microscopy and are visual evidence that fatigue occurred. The absence of fatigue striations does not preclude the occurrence of fatigue, however.

Figure 3: Fatigue striations evident in a 7XXX aluminum. Each striation represents one stress cycle.

Striations are formed by a plastic-blunting process [7]. At the end of the stage I crack tip, there exist sharp notches due to the presence of slip. These sharp notches cause stress to be concentrated at the crack tip. The application of a tensile load opens the crack along slip planes by plastic shearing, eventually blunting the crack tip. When the load is released, the slip direction reverses, and the crack tip is compressed and sharpened. This provides a sharp notch at the new crack tip where propagation can occur.

Fatigue is influenced by many factors, including the stress cycle, amount of residual stress present, surface finish, stress concentration factors, as well as many other variables.

Conclusion

In this article, a brief description of intergranular fracture and fatigue has been discussed. This article is not intended to be a thorough examination of these modes of failure, but to introduce the reader to the different mechanisms of failure that are observed in actual parts.

Should you have any questions regarding this article, or have suggestions for further articles, please contact the editor or myself. 

References

  1. P. G. Shewmon, “Grain Boundary Cracking,” Metall. Mater. Trans. B, vol. 29, no. June, pp. 509-518, 1998.
  2. S. Kuramoto, G. Itoh and M. Kanno, “Intergranular Fracture in Some Precipitation Hardened Aluminum Alloys at Low Temperatures,” Metall. Mater. Trans. A, vol. 27 (10), no. October, pp. 3081-3088, 1996.
  3. S. P. Lynch, “Mechanisms of Intergranular Fracture,” Mater. Sci. Forum, vol. 46, pp. 1-24, 1989.
  4. P. J. Forsyth and C. A. Stubbington, “The Slip-Band Extrusion Effect Observed in Some Aluminum Alloys Subjected to Cyclic Stresses,” J. Inst. Metals, vol. 83, p. 395, 1954.
  5. A. H. Cottrell and D. Hull, “Extrusion and Intrusion by Cyclic Slip in Copper,” Proc. Roy. Soc. (London), vol. A242, p. 211, 1957.
  6. W. A. Wood, “Some Basic Studies of Fatigue in Metals,” in Fracture, B. L. Auerback, Ed., Cambridge, MA: MIT Press, 1959, p. 412.
  7. C. Laird, “Fatigue Crack Propagation,” in ASTM STP 415 Fatigue Crack Propagation, Conshohocken, PA, ASTM, 1967, p. 136.