Michael Ashby about cellular glass as a new engineering material

logo_smallMichael F. Ashby is a Cambridge University professor about new (and old) materials. By accident, I found his textbook about engineering materials Vol. 1  on research gate.

The textbook describes the important physical properties and physcis behind these properties. But also the availability and price are given some attention.

ashbyThe book is of course illustrated with case studies. Hereunder we cite the case study about the support of a large telescope mirror.

Introduction
The worlds largest single-mirror reflecting-telescope is sited on Mount Semivodrike,
near Zelenchukskaya in the Caucasus Mountains. The mirror is 6m (236 inches) in
diameter, but it has never worked very well. The largest satisfactory single-mirror
reflector is that at Mount Palomar in California; it is 5.08 m (200 inches) in diameter. To
be sufficiently rigid, the mirror (which is made of glass) is about 1 m thick and weighs
70 tonnes.*
bookThe cost of a 5m telescope is, like the telescope itself, astronomical – about UKE120 m or US$180 m. This cost varies roughly with the square of the weight of the mirror so it rises very steeply as the diameter of the mirror increases. The mirror itself accounts for about 5% of the total cost of the telescope. The rest goes on the mechanism which holds, positions and moves the mirror as it tracks across the sky
(Fig. 7.1). This must be so stiff that it can position the mirror relative to the collecting system with a precision about equal to that of the wavelength of light. At first sight, if you double the mass M of the mirror, you need only double the sections of the
structure which holds it in order to keep the stresses (and hence the strains and deflections) the same, but this is incorrect because the heavier structure deflects under its own weight. In practice, you have to add more section to allow for this so
that the volume (and thus the cost) of the structure goes as M2. The main obstacle
to building such large telescopes is the cost. Before the turn of the century, mirrors were made of speculum metal, a copper-tin alloy (the Earl of Rosse (1800-18671, who lived in Ireland, used one to discover spiral galaxies) but they never got bigger than 1 m because of the weight. Since then, mirrors have been made of glass, silvered on the front surface, so none of the optical properties of the glass are used. Glass is chosen for its mechanical properties only; the 70 tonnes of glass is just a very elaborate support for 100 nm (about 30 g) of silver. Could one, by taking a radically new look at mirror design, suggest possible routes to the construction of larger mirrors which are much lighter (and therefore cheaper) than the present ones?

At the end, it is summarized in a material index, which is calculated for a few materials.

telescopetable

The following conclusions are taken:

The optimum material is CF’RP. The next best is polyurethane foam. Wood is obviously
impractical, but beryllium is good. Glass is better than steel, aluminium or concrete
(that is why most mirrors are made of glass), but a lot less good than beryllium, which
is used for mirrors when cost is not a concern.

But at the end, we find the inventive step:

The most obvious obstacle is the lack of stability of polymers – they change dimensions with age, humidity, temperature and so on. But glass itself can be foamed to give a material with a density not much larger than polyurethane foam, and the same stability as solid glass, so a study of this sort can suggest radically new solutions to design problems by showing how new classes of materials might be used.

Indeed, with low density cellular glass, we arrive at a material index = 0.1, which is the best value. However, with the usable dimensions of cellular glass (60 x 40 cm), the idea of building a large mirror cannot be realized with cellular glass. But the continous glass foaming technique as developed by GLAPOR allows dimensions up to 3.2 m. Even wider up to 6m (like for float glass) is possible in case there should  a market for this large product. Indeed, light structures built from foamed recycled glass have a certain future.

telescope

book2Also Engineering Materials vol2 is available in pdf on nanotech. This book contains a nice chapter about glass and the production of glass articles. Also ceramics and more particular concrete are mentioned with a lot of data to be used for the calculation of an order of magnitude.

 

 

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