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Unformatted text preview: AAE 552 Spring 2011 A. F. Grandt 1 1 AAE 552: Nondestructive Evaluation of Structures and Materials A. F. Grandt, Jr. Professor of Aeronautics and Astronautics Purdue University W. Lafayette, IN 47907 Spring 2011 2 AAE 552 Session 13: 9 February 2011 Last time Finish proof testing Today: Fatigue crack growth life Inspection intervals/strategies Start visual inspection 3 Assignment Please read : Sections 16.5 (Damage Tolerant Configurations) in textbook Chapter 10 Visual and Liquid Penetrant Inspection Due Dates: Abstract #2 due Monday, 21 Feb Paper #1 due Monday, 28 Feb Note: some examples of the hybrid (powerpoint notes) format for paper #2 are posted on blackboard will discuss this format in more detail in a later class 4 Advantages Proof Testing Structure readily screened for gross defects Previous inspections (ultrasonic, X- ray, etc.) are reliably checked Upper limit for the initial crack size is readily determined Proof test overload may retard subsequent fatigue crack growth (see Chapter 7). 5 Disadvantages Proof Testing May fracture during proof test (i.e. the method is not nondestructive) Can be expensive for large components Difficult to proof all components to same level, or to accurately determine applied loads in certain areas Difficult to load structure similar to service Existing cracks may extend subcritically during proof cycle 6 Fatigue Crack Growth Criterion Objective : Determine a criterion that specifies cyclic growth of pre-existent cracks Relate cyclic load, crack size, geometry, material Correlate lab tests with structure Evaluate materials Approach : Assume that the cyclic stress intensity factor controls fatigue crack growth rate Verify experimentally AAE 552 Spring 2011 A. F. Grandt 2 7 Remote vs Wedge Load Constant amplitude P fatigue test Measure crack length vs cycles 2a W B P P P = constant P time 2a W B P P a K = a B P K = = P/BW 8 Measure Crack Growth da dN Crack Length (a) Number of Cycles (N) Crack Length (a) Number of Cycles (N) da dN a* K = a 2a Remote Load 2a P Crack Face Load = K P B a 9 Correlate Rate da/dN vs K 2a 2a Log K Log da/dN K th K c = K P B a K = a Crack Length (a) Number of Cycles (N) da dN a* da dN Crack Length (a) Number of Cycles (N) 10 da/dN - K is Material Property Log K Log da/dN K th K c K = ( a) 1/2 (a) geometry stress crack length Material da/dN = F( K) 11 Compute Fatigue Life N f a o , a f = initial, final crack sizes F(K) = function of: cyclic stress: , R, . . ....
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This note was uploaded on 03/23/2011 for the course AAE 552 taught by Professor Grandt during the Spring '11 term at Purdue University-West Lafayette.
- Spring '11