AAE552-Session5

AAE552-Session5 - AAE 552 Spring 2009 – A F Grandt 1 1...

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Unformatted text preview: AAE 552 Spring 2009 – 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 Session 5 – 23 January 2009 2 Contact Information Alten F. (Skip) Grandt, Jr. Email: [email protected] Telephone: • Office: 765-494-5141 • Home: 765-463-4276 FAX: 765-494-0307 Course webpage accessed at: http://www.itap.purdue.edu/tlt/blackboard 3 AAE 552 Session 5 23 January 2009 Last Time: • Failure modes and inspection • Damage tolerance Today: • Fatigue/NDE design criteria • NDT collaboration overview • Ch 4 NDE overview 4 Assignment Please read textbook : • Chapter 4 Please Submit : • Session 9 (2 February) – Abstract for Paper #1 – see webpage for more details 5 • Most failure modes susceptible to damage • Inspection required to detect damage before it can cause failure • Must know what to find • Must know how often to inspect Summary 6 Damage Tolerance The ability of a structure to resist prior damage for a specified period of time . Initial damage • material • manufacturing • service induced • size based on inspection capability, experience, . . . Key attribute for “critical” components Time (cycles N) Crack size Desired Life AAE 552 Spring 2009 – A. F. Grandt 2 7 Fatigue Design Criteria See AFG chapter 1 (Section 1.5) • Several ways to treat fatigue • Criteria differ in their view of cracking and implementation of inspection – Infinite life – Safe-life – Fail-safe – Slow crack growth – Retirement for cause 8 S-N Approach to Fatigue Goal: • Relate cyclic stress amplitude with fatigue life • Link coupon tests with structure performance • Historical approach first developed by Wohler • Considered a “crack nucleation” approach 9 Stress-life (S-N) Approach (Section 9.2.2) • Test specimens at different constant ∆ S • ∆ S = ∆ P/A or ∆ My/I • Measure life N (usually total cycles to failure) • Life increases as load amplitude decreases • Considerable scatter in data • “Run-outs” suggest “infinite life” possible (mainly steels) • Does not directly consider crack growth P P Log cycles N ∆ S/2 S time ∆ S M M 10 Endurance Limit (Fatigue Strength) • Maximum amplitude without fatigue failure • S f ≡ S e ≡ ∆ S/2 for “infinite” life • “Infinite” > 10 6 or 10 7 cycles • For steels S e 2245 S ult / 2 • Not all materials have endurance limit • Define quasi limit = ∆ S/2 at 2245 10 7 cycles • Highly dependent on specimen condition, prior load history, residual stresses, etc. Log life ∆ S/2 Endurance limit 11 Fatigue Design Criteria Infinite Life • Prevent fatigue damage from ever developing • Based on endurance limit, threshold K concepts ==> very low design stresses • Used for simple components/loading (e.g....
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This note was uploaded on 11/04/2010 for the course AAE 552 taught by Professor Longuski during the Spring '10 term at IUPUI.

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AAE552-Session5 - AAE 552 Spring 2009 – A F Grandt 1 1...

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