Chapter_6 - Chapter 6 Case Studies: Materials Selection...

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Chapter 6 Case Studies: Materials Selection Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby
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Materials for Oars Credit for inventing the rowed boat seems to belong to the Egyptians. The real stimulus for development of boats and oars came in 1900 with the establishment of rowing as an Olympic sport. Since then both have drawn the fullest craftsmanship and materials of their day. Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby Figure 6.11
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Materials for Oars: Translation An oar is essentially a beam loaded in bending Oars must be strong enough not to break, but they are designed on stiffness – to give a specified elastic deflection under a given load Must also be light – extra weight increases the drag on the hull Oars must also be tough enough to withstand being dropped or clashing together Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby
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Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby The material index for the oar is that for a light, stiff beam:
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Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby Figure 6.2 The appropriate selection chart plots Young’s modulus against density. The selection line for the material index has a slope of 2; it is positioned so that a small group of materials is left above it. They are the materials with the largest values of M and represent the best choice, provided they satisfy the other constraint. Woods, carbon- reinforced polymers, and certain ceramics are the best choices based on their location relative to the selection line
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Ceramics are brittle and fail to meet the toughness constraint of the design. Composite blades are lighter than wood for the same stiffness and offer greater control of properties. Until recently a CFRP oar cost more than a wooden one, but the price of carbon fibers has fallen sufficiently that the two cost about the same. Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby
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Mirrors for Large Telescopes The total cost of a large (236”) telescope is about $300 million. The mirror itself accounts for only about 5% of the overall cost; the rest of the cost is the mechanism that holds, positions and moves it as it tracks across the sky. As the mass of the mirror increases, the sections of the support structure have to increase as m 2 , and so does the cost. Materials Selection in Mechanical Design, 4th Edition © 2010 Michael Ashby Figure 6.3
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Mirrors for Large Telescopes: Translation At its simplest, the mirror is a circular disk with diameter 2R and thickness t When horizontal, it will deflect under its own weight ; when vertical, it will not deflect significantly This distortion must be small so it does not interfere with performance – this means that the deflection of
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Chapter_6 - Chapter 6 Case Studies: Materials Selection...

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