A review of the procedures that should be performed when materials are fatigued by aging.
by Juan Manuel Álvarez*
Regarding the issue of materials, fatigue is closely related to aging, considering the concept of fatigue to "the accumulation of work cycles" measured as loads and directed under the criterion of the efforts fulfilled in the life of a structure in fulfillment of its function, determined by the design and selection of materials according to specifications.
Regardless of the type of fatigue of the materials, aging itself indicates the impairment of the material's ability to meet the mechanical specifications with which it was properly selected for use.
For this reason it is considered that it will not comply with the reliability that it should have under the parameters of structural safety, and therefore there is a greater operational risk and of affectation to people and even the environment, according to the structure and the function it fulfills.
Meanwhile, another aspect that directly affects the aging of materials is their repair. The limitations to the concept of repair are very great when the material ages, because the material will not have the capabilities to fulfill the jobs for which it is intended (as cited under the topic of reliability in the previous article).
If the philosophy and objective of the repair is taken into account, such as "returning to the initial design conditions", an aged material will be limited to repair, or put another more bluntly, an aged material should not be "repaired", because it will certainly not recover its initial design conditions in terms of the natural mechanical specifications that it must meet.
Going into the subject of the procedure that must be done to carry out the repair, the structure can be repaired by placing reinforcements or jacketing, but that does not mean that the properties that due to aging it has already lost are being improved or returned.
From another point of view, maintenance is made difficult by the effect of low reliability and limitation in repair. If the maintenance of the structure is affected, the maintenance programs will limit its remaining useful life, which in turn implies that corrective maintenance cannot be carried out and therefore it will be necessary to move to another level of maintenance such as recovery, which is determined by the change of parts, of components or structure.
Fatigue itself determines the service life of the material:
- For use in terms of reliability.
- For repair in terms of maintenance.
- So it is worth noting that aging must be understood as part of a natural process in the life of the service of a material, a product or a structure.
How long? The answer to this question must be related to fatigue.
This remaining life of the material will be affected to the extent that in the course of its service life it has been demanded even above the thresholds of mechanical requirement for which it was selected. Similarly, frequency variations in the demand of the material in alternating or fluctuating mode accelerates the aging process by shortening the fatigue life of the material.
It is important to determine in advance, from the design period, the loads and stresses that the material will withstand and thus foresee its service life under the concepts of reliability and repairability, in such a way that for the purposes of good maintenance and use the aging of the material can be controlled as much as possible.
Age control
In relation to the control of aging, it should be noted that for this purpose there is and it is recommended to comply with a program, which aims to monitor the reliability and repairability of the product in order to estimate its remaining time of service life.
The good management and compliance with maintenance programs are fundamental aspects, which is why it is definitive to comply with the aging program from the beginning of the start-up of the service of the material, component or structure.
Thus, the issue of material fatigue is an important part of the aging process and otherwise around or in addition to the issue of fatigue, other effects on the material that affect aging itself happen and that will be treated in future articles of this cycle called "Criteria and considerations related to aging".
The following article will deal with the topic "Corrosion fatigue" with which the reactive affectations will be conceptualized in the best way.
As always I wish you a good aging and many successes in your work
* Juan Manuel Álvarez is a Consulting Member of the technical committees of NACE International, Member of A.S.T.M, of ACICOR (Colombian Association of Corrosion Engineers), of ASCOR (Colombian Association of Corrosion and Protection), of ACOSEND (Colombian Association of Welding and Non-Destructive Testing), of ICONTEC (Colombian Institute of Technical Standards), of the technical committees of END, Welding, Coatings, Fuels and others. He is also a university professor, lecturer and specialist in metal structures and composite materials. Advisory Services, Audits, Training and Contracts for the Control and Prevention of Corrosion. You can write to the e-mail: [email protected]
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