20fatigue

# 20fatigue - AAE 352 Lecture 20 Fatigue AAE 352 1 Unfinished...

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AAE 352 1 AAE 352 Lecture 20 Fatigue

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Unfinished issues Materials problems • Write the mass equation • Write the constraint equation (yield, deflection ….) • Write the equation for area (or thickness) (e.g. A=bh) • Use the constraint equation to solve for area (or thickness) – will contain materials and geometry • Substitute area or thickness into mass equation • Materials index will be combinations of materials prperties AAE 352 2
Fatigue failures account for 80% of stress failures AAE 352 3

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Fatigue Material that is subjected to a repeated application of a stress may fail even if the stress it is subjected to is lower than the yield strength. Such failure is referred to as Fatigue, and results from the accumulation of very small amounts of deformation in the nominally elastic range. The typical fatigue test results plot the failure stress versus the number of cycles (on a logarithmic scale); S-N curve Fatigue is an important cause of failure in many cases because it can form cracks that grow until they are large enough to cause a fracture toughness failure. Fatigue can result from cyclical application of tension and compression (bending, as in springs or the flexing of aircraft or other structures), tension only (springs, elevator cables), or compression only (bearing contacts, railroad rails). AAE 352 4
AAE 352 5 Fatigue is a danger in primary aircraft structural elements - definitions What is it? Fatigue is the process of progressive, localized, permanent crack growth created by cyclic stresses that result in fracture after a sufficient number of loading cycles. Fatigue occurs at stress levels well below the yield stress. What elements? Wing, horizontal stabilizer, vertical fin, canard, winglets, fins – Fittings – Splices – Spar caps – Spar webs Pressurized cabins – Pressure bulkheads – Cockpit window posts – Skin and stiffeners around cutouts such as doors and windows – Door and window frames

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Fracture toughness failure modes 1, 2, 3
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## This note was uploaded on 01/18/2012 for the course AAE 352 taught by Professor Chen during the Fall '08 term at Purdue.

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20fatigue - AAE 352 Lecture 20 Fatigue AAE 352 1 Unfinished...

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