Alternative methods for derivation of safe life limits for a 7050-T7451 aluminium alloy structure

Abstract The safe-life method is widely used to ensure that airframes have an acceptably low probability of structural failure. This method uses a scatter factor to account for the variability in a metal’s fatigue performance to maintain a probability of failure below acceptable levels. However, given the empirical nature of scatter factors, the method does not enable the influence of the various factors that contribute to this scatter to be individually assessed. This paper compares alternative methods for determining the safe life of monolithic 7050-T7451 aluminium alloy structures with the traditional approach used to life fighter/attack aircraft. These methods are designed to protect the fleet by maintaining a cumulative probability of failure below 1/1000, as required by a certification structural design standard. The alternative methods are based on estimating the distribution of the most significant factor affecting the scatter noted in fatigue test results, namely the sizes of the fatigue initiating material discontinuities.

[1]  Fatigue cracking from a corrosion pit in an aircraft bulkhead , 2014 .

[2]  Lorrie Molent,et al.  The comparison of complex load sequences tested at several stress levels by fractographic examination , 2005 .

[4]  Lorrie Molent,et al.  A comparison of crack growth behaviour in several full-scale airframe fatigue tests , 2007 .

[5]  Y. Macheret,et al.  Effect of Improving Accuracy of Load Monitoring on Aircraft Probability of Failure , 2007, 2007 IEEE Aerospace Conference.

[6]  Rhys Jones,et al.  Fatigue crack growth and damage tolerance , 2014 .

[7]  R. Pippan.  THE GROWTH OF SHORT CRACKS UNDER CYCLIC COMPRESSION , 1987 .

[8]  A. J. Green,et al.  Characterisation of equivalent initial flaw sizes in 7050 aluminium alloy , 2006 .

[9]  J. Schijve.  Fatigue life until small cracks in aircraft structures: Durability and damage tolerance , 1994 .

[10]  B. Aktepe,et al.  Review of fatigue monitoring of agile military aircraft , 2000 .

[11]  Lorrie Molent,et al.  Calculating crack growth from small discontinuities in 7050-T7451 under combat aircraft spectra , 2013 .

[12]  Lorrie Molent,et al.  Experimentally derived crack growth models for different stress concentration factors , 2008 .

[13]  Rhys Jones,et al.  Implications of the lead crack philosophy and the role of short cracks in combat aircraft , 2013 .

[14]  P. Goel,et al.  The Statistical Nature of Fatigue Crack Propagation , 1979 .

[15]  P. R. Underhill,et al.  Effect of small cycles and load spectrum truncation on the fatigue life scatter in 7050 Al alloy , 2009 .

[16]  Lorrie Molent,et al.  Assessing relative spectra severity using single fatigue test coupons , 2009 .

[17]  Russell Wanhill,et al.  Typical fatigue-initiating discontinuities in metallic aircraft structures , 2012 .

[18]  Robert O. Ritchie,et al.  AN ANALYSIS OF CRACK TIP SHIELDING IN ALUMINUM ALLOY 2124: A COMPARISON OF LARGE, SMALL, THROUGH‐THICKNESS AND SURFACE FATIGUE CRACKS , 1987 .

[19]  R. Ritchie,et al.  Propagation of short fatigue cracks , 1984 .

[20]  A. Merati.  A study of nucleation and fatigue behavior of an aerospace aluminum alloy 2024-T3 , 2005 .

[21]  M. McDonald,et al.  Evaluation of spectrum fatigue crack growth using variable amplitude data , 2008 .

[22]  Lorrie Molent,et al.  An experimental evaluation of fatigue crack growth , 2005 .

[23]  Russell Wanhill,et al.  The lead crack fatigue lifing framework , 2011 .

[24]  Simon Barter,et al.  Interpreting fatigue test results using a probabilistic fracture approach , 2005 .