Fatigue / Fracture Mechanics
What Is Fatigue?
Fatigue in engineering components and materials refers to the initiation and development of cracks in the component, as a result of cyclic loads, each one of which is well below the materials plastic limit. Despite popular misconceptions, fatigue has nothing to do with a material getting old or crystalline or going off in any way fatigue is simply concerned with crack development. Fatigue of engineering components is, however, extremely common, ultimately accounting for over 75% of all structural and component failures.
Fatigue is dangerous as it is often insidious in nature in that it is often manifested by catastrophic failure.
Why Does Fatigue Occur?
Stress levels required to prevent fatigue initiation are often low - which forces components designed to withstand fatigue to be large, heavy and expensive. Modern light weight components are often designed with a defect tolerant ('living with cracks') design methodoloy which allow the component to operate safely in a cracked condition for some defined lifetime. Amongst numerous others the Nuclear, Marine and Offshore Oil, Aeronautical and Motor Vehicle industries all employ these defect tolerant design methodologies.
In other cases the component was either simply not designed for fatigue or the loading has been changed due to misuse or in an attempt to extend the useful life of the component.
How To Prevent Fatigue?
There are two critical aspects in fatigue considerations, concerning (i) fatigue crack initiation, as well as (ii) crack propagation. Fatigue cracks readily initiate at stress concentrations or imperfections in the component, such as key- ways, lubrication holes or poor welding and detailing (e.g. sharp fillets).
Even when a component is well designed and fabricated, to be relatively free of such stress concentrating features, fatigue cracking can still develop if the cyclic loads are sufficiently high. The movement of dislocations and persistent slip bands, induced by the cyclic loads, leads to an effective stress concentration and a fatigue crack develops. Conventional fatigue design and careful attention to detail is the best way to combat this sort of fatigue at the initiation level.
Once a crack has developed, however, and starts to propagate through the component usually with disastrous effects, its progress is well characterised by fracture mechanics techniques. Through this fracture mechanics understanding and appropriate material parameters, as well as a knowledge of the cyclic loading spectrum, it is possible, and nowadays common, to obtain quantitative estimates of the likely remaining fatigue life of the component. This is extremely valuable for safe operation of the plant, and in specifying intelligent inspection intervals, as well as providing a methodology for scheduled repair and maintenance, and avoiding sudden and catastrophic failure.
What Is Fracture Mechanics?
Of all the ways in which engineering components and systems can fail, the most catastrophic tend to be those in which there is no obvious prior warning. There are numerous cases where a structure or engineering component has been operating perfectly satisfactorily, sometimes for years, and then suddenly fails catastrophically, often through the development of stress corrosion cracking (SCC) or the propagation of fatigue cracks. There is a modern trend to lighter and more slender structures, greater loading of existing ones and demands for higher performance. These trends often bring the stresses in the structure closer to its material limits, and thus closer to failure. Indeed, there is an increase in the number of failures, especially as an engineering plant ages or exceeds its design life.
Furthermore - all structures contain flaws. These may range from metallurgical defects through inherent inclusions and porosity, to damage introduced in service, from wear, corrosion or fatigue. In addition many structures today are welded and such welds are often a source of defects as well as high residual stresses, and a crack, once initiated, readily propagates from one plate member through the weld to another.
What is required therefore, for structures in which flaws are inevitably present and which are subject to operational stresses, is a methodology whereby the structural integrity of engineering systems can be assessed for their fitness for purpose. Fortunately the discipline of Fracture Mechanics provides exactly this quantitative relationship, between material, design and fabrication, or more simply between stress, flaw size and toughness. This interrelationship is best represented in terms of a so called Triangle of Integrity.
Why Use Fracture Mechanics?
Fracture mechanics provides quantitative answers to such structural integrity questions as:
- What is the critical crack size at service loads?
- How safe is the system if it contains a crack?
- How long might it take for a crack to grow from initial to critical size?
- How often should a particular structure be non destructively inspected?
The correct answer to these, and related questions, are of fundamental importance for the safe operation and maintenance of plant and equipment, and enable unexpected catastrophic failure to be almost completely prevented.
How To Use Fracture Mechanics To Its Best Advantage?
Fracture mechanics analysis, coupled with appropriate inspection procedures, provides a rational and quantitative method for enabling a component to be kept in service safely, at least until a scheduled inspection or maintenance outage, when repair can be effected, with minimal loss of production. Fracture mechanics is not only a powerful tool for rational evaluation of NDT flaw indications, but is also invaluable in design, materials selection and failure analysis.
A fracture mechanics evaluation of a particular flaw requires accurate knowledge of
- the size and shape of the flaw,
- the loading conditions/stress levels in the region of the flaw (Finite Element Analysis or Direct Measurement),
- operating environment, and
- the fatigue/fracture mechanics properties of the material.
Although extensive databases of material properties are available for general engineering, materials fatigue and fracture data is often not easily available and have to be derived from tests.