Editing Damage tolerance (section)

==Non-destructive inspections==

Manufacturers and operators of aircraft, trains, and civil engineering structures like bridges have a financial interest in ensuring that the inspection schedule is as cost-efficient as possible.  In the example of aircraft, because these structures are often revenue producing, there is an [[opportunity cost]] associated with the maintenance of the aircraft (lost ticket revenue), in addition to the cost of maintenance itself.  Thus, this maintenance is desired to be performed infrequently, even when such increased intervals cause increased complexity and cost to the overhaul.  Crack growth, as shown by [[fracture mechanics]], is exponential in nature; meaning that the crack growth rate is a function of an exponent of the current crack size (see [[Paris' law]]). This means that only the largest cracks influence the overall strength of a structure; small internal damages do not necessarily decrease the strength. A desire for infrequent inspection intervals, combined with the exponential growth of cracks in structure has led to the development of [[non-destructive testing]] methods which allow inspectors to look for very tiny cracks which are often invisible to the naked eye. Examples of this technology include [[eddy current]], [[Ultrasonic testing|ultrasonic]], [[Dye penetrant inspection|dye penetrant]], and [[X-ray]] inspections.  By catching structural cracks when they are very small, and growing slowly, these non-destructive inspections can reduce the amount of maintenance checks, and allow damage to be caught when it is small, and still inexpensive to repair. As an example, such repair can be achieved by drilling a small hole at the crack tip, thus effectively turning the crack into a [[notch (engineering)|keyhole-notch]].<ref name="notch">{{cite journal|last1= Liu | first1= M. | display-authors=etal |title= An improved semi-analytical solution for stress at round-tip notches | journal= Engineering Fracture Mechanics | year=2015 | volume=149| pages=134–143 | doi= 10.1016/j.engfracmech.2015.10.004 | s2cid= 51902898 |url= http://drgan.org/wp-content/uploads/2014/07/032_EFM_2015.pdf }}</ref>