If a specimen is subjected to high stress beyond its withstand strength, it fails and fracture of material occurs in two or more portions of the test specimen.
After considerable plastic deformation, ductile fracture occurs and shows a characteristic reduction in the cross-sectional area of the specimen near the fractured portion. A brittle fracture occurs suddenly when a small crack in the cross-section area of the material grows resulting in a complete fracture. But brittle fracture does not show much plastic deformation.
Actually, by a careful examination of the fractured surface area and the macro and micro metallurgical examination of the fractured specimen, much interesting information as to the probable cause of its failure can be deduced by an experienced metallurgist.
Apart from the ductile and brittle types of fractures, we also have fractures on specimens caused by fatigue and creep of material. Failures caused by fatigue and creep of material are known as fatigue failure and creep failure.
A specimen of the materials often fractures at a stress level far below their strength, if the stress is either
- alternating type or
- it is varying periodically.
An example of alternating stress: Consider an axle fitted with two wheels. The axle bears the weight of the vehicle and at the same time, it rotates along with wheels. Because of weight, the axle undergoes a little deflection causing compressive stress in the top half of the cross-section and tensile stress in the bottom half of the cross-section area.
But since the axle is rotating, after every 180° of rotation, the bottom half becomes the top half and vice versa. Thus the nature of stress at any point in the axle keeps alternating between compression and tension due to its rotation.
A varying stress cycle means that the magnitude of the stress keeps reducing and increasing periodically although its sign does not change. If the material is subjected to several million cycles of either alternating or varying stress, it gets fatigued and fails even though the magnitude of such stresses may be far lower as compared to its strength.
Fortunately, there is a level of alternating and varying stress, which the material is able to withstand without failure even if it is subjected to an infinite number of cycles. This is called the endurance limit. A designer ensures that a component subject to fatigue in service is so designed that its actual stress level remains below the endurance limit.
The visual examination of a fatigue fracture shows three distinct zones. They are:
- The area of crack propagation during service. This area is usually characterized by circular ring-like scratch marks with the point of crack initiation as the center.
- The point of crack initiation, it is the point from where the crack may have originated example a notch like a keyway or some materials defect like an impurity or even a surface blemish.
- The remaining area of a cross-section showing signs of sudden breakage. As a result of crack propagation with time, a stage comes, when the remaining cross-sectional area becomes too small to sustain the stress and fractures suddenly.
Failure of material can take place even under steady loads within the material strength. This happens if the subjected components remain under steady loads for a very long time especially when they are subjected to high-temperature conditions. Some common examples are blades of the steam turbine, stays in boilers, parts of a furnace etc. Such failures are termed creep failures due to the fact the material continues to deform plastically under such loads although at a very slow rate. But over long periods of time, the effect of creep can become appreciable resulting in the ultimate failure of the specimen.