Metal Fatigue

Metal fatigue occurs in metal structures subjected to repeated cycles of loading and unloading below the level of ultimate tensile stress. If the load is aboard a certain level, microscopic cracks may form and grow. Eventually the crack may reach a critical size and the structure may suddenly fracture.

Cycles of loading and unloading may occur during

The take off and landing of air craft

The motion of ships as they ride over the waves.

The start – stop cycle of cars and moving machinery

Cracks are slow to form but their rate of growth increases dramatically as the crack gets bigger.  This acceleration results from a localised increase in the stress at the top of the expanding crack, which comes about quite naturally because the forces on the object are supported by a diminishing cross-sectional area.

Defects present when the material is made may serve to promote the formation of cracks. In practice it is often impossible to prevent the presence of these defects. Scratches in the surface of the material and sharp edges may serve to localise stress and serve as points of crack growth. It is good practice when building machines to avoid sharp corners by using round rather than square holes though not all sharp edges can be eliminated. Screws must necessarily have sharp edges and may have defects caused by poor drilling or machining to act as points for crack formation.

Some materials – mild steel but not aluminium - have a fatigue limit so that cracks will not grow if the applied stresses are below a certain level. This is about 10% of ultimate tensile stress for mild steel. Aluminium will always allow crack growth whatever the stress level.

Crack growth is a predictable process and can be allowed for when servicing machinery. Aircraft habitually fly with cracks throughout the structure. The growth of cracks is monitored and the size is tested periodically using non destructive testing techniques to ensure they do not reach the point at which could cause catastrophic failure. The inspection interval is chosen so that a crack which is large enough to be detected cannot grow to a size which would cause a catastrophic failure before the next scheduled test. In practice a safety margin is added so that a detectable crack cannot grow to a critical size before the next but one scheduled inspection.

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