Editing Damage tolerance (section)

==History==

Structures upon which human life depends have long been recognized as needing an element of [[fail-safe]]ty.  When describing his flying machine, [[Leonardo da Vinci]] noted that "In constructing wings one should make one chord to bear the strain and a looser one in the same position so that if one breaks under the strain, the other is in the position to serve the same function."<ref name="riddick">{{citation
  | title = Safe-life and damage-tolerant design approach for helicopter structures applied technology laboratory
  | author = Riddick, H. K.
  | year = 1984
  | publisher = US Army Research and Technology Laboratories (AVRADCOM), Virginia
  | url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830025690.pdf
}}</ref>

Prior to the 1970s, the prevailing engineering philosophy of aircraft structures was to ensure that airworthiness was maintained with a single part broken, a redundancy requirement known as [[fail-safe]]ty.  However, advances in [[fracture mechanics]], along with infamous catastrophic fatigue failures such as those in the [[de Havilland Comet]] prompted a change in requirements for aircraft.  It was discovered that a phenomenon known as [[Widespread fatigue damage| multiple-site damage]] could cause many small cracks in the structure, which grow slowly by themselves, to join one another over time, creating a much larger crack, and significantly reducing the expected time until failure <ref>{{citation
  |author1=Brett L. Anderson |author2=Ching-Long Hsu |author3=Patricia J. Carr |author4=James G. Lo |author5=Jin-Chyuan Yu |author6=Cong N. Duong  |name-list-style=amp | title = Evaluation and Verification of Advanced Methods to Assess Multiple-Site Damage of Aircraft Structure
  | publisher = Office of Aviation Research, US Department of Transportation, Federal Aviation Administration
  | year = 2004
  | access-date = June 1, 2016
  | url = http://airportaircraftsafetyrd.tc.faa.gov/programs/agingaircraft/structural/reports/04-42-Vol-I.pdf
  | archive-url = https://web.archive.org/web/20111018161217/http://airportaircraftsafetyrd.tc.faa.gov/Programs/agingaircraft/Structural/reports/04-42-Vol-I.pdf
  | archive-date = October 18, 2011
}}</ref>