m19_ps06_spring04 - operations which provides a margin for...

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Unified Engineering Spring 2004 Problem M19 For wing spars bending loads are the major design driver. For engine disks the power density (and hence thrust) scales with the blade tip speed (V) squared. The centrifugal stress in a spinning disk is proportional to the density of the material used ( r ) and the tip speed squared, and to a good approximation this is given by: s ª 3 r V 2 . Thus engines are designed to 8 develop as much thrust as the material strength will allow. . a) If wing spars and engine disks are to be designed to a criterion such that there is no permanent deformation at the limit stress (i.e. yield does not occur). Determine the material selection criterion for (i) wing spars and (ii) turbine disks. b) Rank the materials below according to their suitability for each application. c) Airframe structures typically operate at about 40% of the limit load during normal
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Unformatted text preview: operations, which provides a margin for unexpected maneuver loads and gust loading. Engine disks regularly experience about 90% of their design limit load, as their loads are much more predictable due to the dominance of the centrifugal loading. Determine the critical crack size in your top ranked materials for each application under normal operating loads. d) Hence comment on the feasibility of using a damage tolerant design approach in each case. Material Density Young's Yield Stress Fracture Kg/m 3 Modulus (GPa) (MPa) Toughness MPa√m Al Alloy 2024 (T3) 2800 70 345 44 Al Alloy 7075 (T651) 2800 70 495 24 Ti Alloy Ti-6Al-4V 4510 110 910 50 Steel 17-7 PH 8000 193 1435 77 (Stainless) Medium Carbon 7800 208 260 54 Steel...
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