The major types of failures likely to be encountered by metals in service are:
B. Brittle, and
C. Fatigue fractures
Wear, Fretting, Elevated Temperature and Corrosion are other important causes of failure which will be covered in a future publication in this series.
A. Ductile Fracture
Ductile fractures are characterized by tearing of metal accompanied by appreciable gross plastic deformation. The microstructure of the fracture surface is quite complex and may include both transgranular and intergranular fracture mechanisms. Ductile fractures in most metals have a gray fibrous appearance and may be flat-faced (tensile overload) or slant-faced (shear). The specimen usually shows considerable elongation and possible reduction of cross-sectional area as well. Whether a part fails in a ductile or brittle fashion depends on the thickness of the part, temperature, strain rate and the presence of stress-raisers. Most commonly seen characteristics of ductile failures are:
• Lateral contraction, or necking;
• Fracture path in the interior following a generally flat plane perpendicular to the principal stress direction, and
• Tensile stress.
Cylindrical specimens will have a “cup and cone” configuration, as shown above on the right, while the fracture surface on thick specimens will be generally perpendicular to the principal stress direction, as seen in the bolt in the illustrations above.
B. Brittle Fracture
Brittle fractures are characterized by rapid crack propagation without appreciable plastic deformation. If brittle fractures occur across particular crystallographic planes they are called Tran crystalline fracture. If along grain boundaries they are called intergranular fracture. Brittle fracture is promoted by:
• thicker section sizes,
• lower service temperatures, and
• increased strain rate.
A material’s tendency to fracture in a brittle mode can be determined by measuring its notch ductility. The most common test for this is the Charpy V-notch test. Failure under test condition can exhibit energy and fracture transitions. Shear fracture occurs under the notch and along the free surfaces. Cleavage fracture occurs in the center characterized by a bright, shiny, faceted surface. 50% cleavage is the fracture transition point. Cleavage fracture is caused by inability of the crystal structure to cross-slip. Yield strength loading is required to initiate a brittle fracture; however, only much lower stress may be needed to propagate it. Generally speaking, body-centered cubic metals exhibit a ductile to brittle transition over a relatively narrow temperature range.
The Drop Weight Test defines the nil-ductility transition temperature and is very useful for determining the brittle fracture susceptibility of low-strength steels. Linear elastic fracture mechanics evaluates structural reliability in terms of applied stress, crack length and stress intensity at the crack tip.
C. Fatigue fracture
Fatigue is a progressive localized permanent structural change that occurs in a material subjected to repeated or fluctuating stresses well below the ultimate tensile strength (UTS). Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain, all three of which must be present. Cyclic stress initiates a crack and tensile stress propagates it. Final sudden failure of the remaining cross-section occurs by either shear or brittle fracture. Striations on the crack surface are the classic sign of fatigue fracture.
High Cycle Fatigue Low Cycle Fatigue Fatigue cracks may start because of tool marks, scratches, indentations, corrosion pits and areas of high stress. At the crack tip, the material is plastic. At a small distance from the crack tip, in the material is elastic.
Low cycle fatigue cracks occur under conditions of high strain amplitude (with failure in less than about 104 cycles) whereas high cycle fatigue occurs with low strain amplitude with failure after a large number of load fluctuations. In low cycle fatigue, striations, if visible at all, tend to be rather broad, widely spaced, and discontinuous in places. Areas without striations may appear to be rubbed or may be quite featureless, except for the area of final fracture. In high cycle fatigue, the striations will be well defined and more closely spaced, with propagation evident in many flat plateaus that are joined by narrow regions of tensile tearing. The investigator should be aware, however, that in heat-treated steels striations are absent from fatigue fractures more often than they are observed, and the stronger (harder) the steel, the less likely it is that striations will be observable. Thus, suspected fatigue striations must be studied carefully to ensure that they are not artifacts of some other process. Striations should be parallel to one another along their lengths and perpendicular to the fracture direction at the region being examined.
Thermal Fatigue cracking is caused by cycling the temperature of the part in the presence of mechanical constraint, e.g., rigid mounting of pipe. It could also be caused by temperature gradients in the part.
Contact Fatigue - Elements that roll, or roll and slide against each other under high contact pressure are subject to the development of surface pits or fatigue spalls after many repetitions of load.
Corrosion pit acting as stress concentrator for fatigue crack (on left at low magnification. Higher magnification of crack tip on right.
Corrosion-Fatigue is caused by the combined action of repeated or fluctuating stress and a corrosive environment to produce failure. It frequently initiates at a corrosion pit on the surface. A very aggressive environment may actually slow the fatigue fracture process increasing the number of stress cycles to failure. The environment affects the crack growth rate, or the probability of fatigue crack initiation, or both. Test data show that for high strength steels, the fatigue strength at 10 million cycles in salt water can be reduced to as little as 10% of that in dry air. Carbon steels exhibit transgranular fracture. Copper and its alloys fail by intergranular fracture.
12. Synthesize and summarize the data, determine and report the root-cause of the failure. Proposed root causes of a failure must be based primarily on observed facts. These facts, combined with the experience, skill and knowledge of the analyst will lead to sound conclusions.
All the observed data should be reported, even if some of it seems peripheral. In the future, with additional data, it may turn out to be possible to use what seemed peripheral at first to make an even more sound interpretation.