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Vinson Kurian

Metal fatigue restricts durability of structures and machinery. It was one of the major structural problems with aircraft and what really restricts its life.

Thiruvananthapuram , Nov. 10

PIONEERING research work by BiSS Research, the in-house R&D unit of the Bangalore Integrated System Solutions (BiSS), and the US Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio, has broken new ground demystifying the phenomenon of `metal fatigue'.

Metal fatigue is a progressive localised damage due to fluctuating stresses and strains on any material, which restricts durability of structures and machinery. Painstaking research is being carried out by leading labs on ways to improve fatigue performance since much is at stake for high-value industries such as aircraft and shipbuilding.

A major area of focus has been on `inducing' compressive residual stress to prevent fatigue cracking.

Speaking to Business Line here, Dr R. Sunder, Research Director at BiSS, said metal fatigue restricts durability of structures and machinery. It was one of the major structural problems with aircraft and what really restricts its life. In airplanes, it's the cabin pressure cycles on landing and takeoffs that develop fatigue cracks.

"The problem is as old as the industrial revolution. Ever since Wohler's early research on the failure of railway axles in the mid nineteenth century, it has been well known that compressive residual stress enhances fatigue life, while tensile residual stress reduces it".

Residual stress has since become a billion dollar industry, with manufacturers implementing processes that enhance fatigue life through `short peening', stress coining and cold-working.

Short peening is the process in which metals balls are fired into a crack and hammered thereon to induce compressive residual stress. While the effect has been well characterised and extensively studied, its cause was not entirely clear. After all, metal fatigue is associated with cyclic slip, or repeated cycling of the load, which is not known to be sensitive to residual stress.

But, recent research appears to finally throw light on what may be the primary operative mechanism of residual stress in metal fatigue. Collaborative research by BiSS Research and the US Air Force Research Laboratory appear to show that compressive residual stress works the same way as protective coatings do.

Compelling evidence comes from the country's first fracture mechanics test facility at BiSS that permits comparative testing in a variety of environments including air, salt water and vacuum.

Crack growth tests performed under specially programmed load sequences appeared to suggest that the residual stress effect virtually vanishes in vacuum. The BiSS experiments seem to debunk the long held view that residual stress works by opening up or closing the fatigue crack.

According to Dr Sunder, the results of the study will be presented as a lead paper at the forthcoming joint annual symposium of the American Society for Testing and Materials and the European Structural Integrity Society.

The most effective method of improving fatigue performance is improvements in design: eliminate or reduce stress raisers by streamlining the part, avoid sharp surface tears resulting from punching, stamping, shearing, or other processes, prevent the development of surface discontinuities during processing, reduce or eliminate tensile residual stresses caused by manufacturing and improve the details of fabrication and fastening procedures.

Discovery of electron microscopy made the crucial difference by enabling much better understanding of metal fatigue. Till then it was more of an event - something keeps breaking for sometime, and one fine day, it breaks. Electron microscopy revealed that metal fatigue was all about formation of a fatigue crack developing and slowly growing over a period of time.

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