SYMPOSIA PAPER Published: 01 January 1976
STP33372S

Effect of Spectrum Type on Fatigue Crack Growth Life

Source

An ever-present problem in airplane design has been the accurate prediction of fatigue crack growth rate in new (never flown) aircraft. This problem has become even more critical in light of the new U. S. Air Force Damage Tolerance Requirements (MIL-A-83444). Briefly, the purpose of this specification is to protect flight structures from potentially deleterious defects which can be introduced into the structure during processing, manufacturing, etc. One of the most important things to consider when performing a damage tolerance analysis is the type of fatigue loading spectrum to be used. The purpose of this paper is to present a method of deriving a randomized flight-by-flight loading spectrum which can be used in preliminary design, and to show the effect on crack growth life of varying certain important parameters.

The random spectrum is derived using mission profiles, MIL-A-008866A or flight recorder exceedance data, design loads, and stress data. The mission mix and actual load sequence is determined randomly. The spectrum is then analyzed using a conventional crack growth computer program which calculates crack growth (a versus N) curves, and critical crack length.

The sensitivity of fatigue crack growth life to the variation in a number of parameters was investigated. These parameters include different random sequences and cycle counting methods. Random sequence effects include randomizing the loads on a mission segment or on a flight-by-flight basis. The results of this study are compared to the results obtained when the basic exceedance data was combined into a conventional block loading sequence.

Author Information

Reiman, JA
ASD/ENF, Wright-Patterson AFB, Ohio
Landy, MA
ASD/ENF, Wright-Patterson AFB, Ohio
Kaplan, MP
ASD/ENF, Wright-Patterson AFB, Ohio
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Details
Developed by Committee: E08
Pages: 187–202
DOI: 10.1520/STP33372S
ISBN-EB: 978-0-8031-4672-3
ISBN-13: 978-0-8031-0344-3