Mechanical failures of metals can be broadly classified as those in which the part is: a) deformed to such an extent that it was no longer serviceable; or b) fractured. Since the fracture of a part is often more disastrous than excessive deformation, emphasis is placed upon failures by fracture. However, rarely does a structure or machine fail in normal service simply because loads are greater than the strength of the parts. Occasionally this will happen if service requirements are underestimated. Much more common, however, are failures where factors other than static loading of the parts are involved, such as fluctuating stress, corrosion, and rubbing between supposedly fixed members.

The nature of the stresses, and other factors, have a marked in­fluence on the nature of a fracture, so a knowledge of the several types of fractures provides a powerful tool in determining the causes of failures in service.

Static or Overload Fracture. When a metal member fails because of a single application of a load greater than the strength of the member, there is usually considerable plastic deformation prior to fracture. This deformation is apparent on inspection of the fractured parts. This ability of a member to withstand plastic deformation is a function not only of the properties of the material but also of the geometry of the member, the type of stress to which it is subjected, the rate of loading, and, in many materials, the temperature.

Although there are very few applications of metals in which it is necessary for the part to deform appreciably in service some ductility is essential to permit yielding of the metal at sharp notches. Such notches are almost universally present in commercial metals due to inclusions, boltholes, welding defects, scratches, corrosion pits and other reasons. The stress at the bottom of a severe notch may be several times as large as the average stress on the section; a ductile material will deform plastically in the small highly stressed region and so redistribute the stress that its peak value will be reduced. If conditions are such that the material behaves in a brittle manner, a crack will be formed before plastic deformation can take place. Fre­quently, this crack will propagate at high velocity through the rest of the section. This type of fracture is characterized by very little deformation adjacent to the fracture and often by markings known as “herringbone” or “chevron” patterns on the fracture surface.

The strength of metals generally decreases as the temperature in­creases. Consequently, parts that have adequate strength at room temperature may fail if the operating temperature becomes high and the fracture can occur after many hours under constant load. If the temperature is high enough, the strength of the grain boundaries in the metal will be reduced to such an extent that intergranular cracking will occur, and the resulting fracture will show very little ductility.

Fatigue Fractures. When a metal is subjected to repeated or fluctuating loads, it may eventually fracture at stresses much lower than those, which would be required to cause fracture under static conditions. This phenomenon is known as fatigue and is the most common cause of primary failures of metals in service. Laboratory tests have estab­lished the fact that a fatigue fracture is progressive in nature; after a number (often many million) of cycles of stress, a small crack forms in the region where the stress is highest. Under continued stressing, this crack grows in a direction generally perpendicular to the tensile stress until the cross section of the member is reduced to such an extent that the remaining area fractures from overload. Because of this mechan­ism, the surface of such a fracture shows two characteristic regions that are usually quite different in appearance.

Because the stress required to initiate a fatigue crack is usually less than that needed to cause plastic deformation of the metal, a fatigue fracture is characterized by its brittle nature. In most fatigue fractures that occur in service the fatigue crack extends only part way across the section, and there is a considerable area broken by overstress. This part will show some ductility, so fractures of this type will not fit together well when the parts are replaced in approximately their original relationship (the fatigue crack portion will be held apart by the deformed overstress portion).

The surfaces of fatigue fractures frequently show characteristic markings, which represent the outline of the fatigue crack at various points in its growth. When these markings are present, they provide a means of locating the origin of the fracture accurately.

It is generally agreed that the primary factor responsible for the fatigue failure of a part is much more often its geometry than the material of which it is made. A large percentage of the failures inves­tigated could have been avoided by more careful design to eliminate points of stress concentration. The stress necessary to cause fatigue failure is usually less than that required for plastic deformation of the metal, so there is no opportunity for redistribution of the load in the neighborhood of the notch.

Fretting caused by rubbing between closely fitting members is a frequent source of fatigue fractures which does not seem to be prop­erly appreciated. When fretting is severe between steel members it can often be detected by the presence of a fine brown powder. This should always be regarded as a danger sign and corrective action should be taken if the parts are subjected to fluctuating stress. A pow­der, usually black, is formed by fretting between aluminum members.

As most fatigue fractures start on the surface, the condition of the surface is of primary importance if fatigue failures are to be avoided. Decarburization of the surface of a steel member, for example, reduces the fatigue strength to that of a similar member having low carbon content throughout. Defects of the surface such as tool marks, scratches, corrosion pits, are far more damaging to the fatigue strength of a part than to its static strength.

Fatigue fractures are often erroneously blamed on “crystallization” of the metal. Since all structural metals are crystalline from the time they solidify from the molten state, the term “crystallization” in con­nection with fatigue is meaningless and should be avoided.

Stress Corrosion Cracking. When a metal is exposed to a corro­sive media and is subjected to a load, fracture may occur even though the stress is less than that required to cause fracture in the absence of corrosion. The corrosion necessary to initiate stress corrosion cracking need not be severe, and frequently will not cause noticeable corrosion on the surface. In many instances this type of fracture can occur in parts that are subjected to no external load whatsoever, due to the internal stress developed in the part during fabrication.

This is also a progressive type of fracture, beginning with a small crack and progressing across the section perpendicular to the tensile stress. The resulting fracture is brittle, as stress corrosion cracking can occur under stresses less than those required to cause plastic deformation. It differs from a fatigue fracture in that there are usually numerous cracks that tend to “wander” through the metal, giving a coarse appearance to the fracture surface. A polished section through the metal damaged by stress corrosion cracking usually shows a network of fine cracks.

Corrosion is frequently a contributing factor to mechanical failures. A small corrosion pit can concentrate the stress in the surface of a part and be dangerous if the part is subjected to fluctuating loads. Even corrosive agents that do not appreciably corrode the metal can affect its fatigue strength. Some light metals and copper alloys are found to give trouble under mildly corrosive conditions and care should be taken to avoid the possibility of stress corrosion in the use of these materials. Preventive measures in the selection of materials will minimize these effects.

The precautions, which must be observed by everyone concerned with the design, fabrication or use of metal parts in order to avoid failure in service, are:

 1. Design: Elimination of sharp reentrant angles at fillets, key ways and other changes of section. Accurate stress analysis at points of stress concentration and verification of the analysis by experiment or by fatigue testing of sample parts. Careful selection of material and fabricating methods to produce parts of adequate transverse strength if significant stresses are to be applied in that direction. Proper combination of ma­terial and static design stress to eliminate the possibility of stress corrosion cracking in the environments to be met. Specification of materials that will provide adequate resistance to brittle fracture, particularly under shock loading and low ambient temperatures.

2. Fabrication: Careful workmanship to avoid points of stress con­centration caused by welding defects, tool marks, improperly formed fillets, grinding cracks, quenching cracks and others. Control of forging practice to avoid folds, seams, and internal fissures. Protective atmo­spheres during heat treatment to avoid decarburization. Proper control or elimination of residual stress caused by pressfits or cold working in parts subject to corrosive environments. Rigid inspection of highly stressed parts to eliminate surface imperfections.

3. Use: Control of operations to restrict the loads to those for which the machine or structure is designed. Protection of parts from corrosion that can cause damage. Periodic inspection of highly stressed members by personnel acquainted with the nature of fatigue failures. Prompt repair or replacement of parts damaged by unusual conditions, such as fire, collision, overloading or unusual corrosion. Periodic disassembly of clamped members to inspect for evidence of fretting.