========================================== It is an ancient Mariner, And he stoppeth one of three. 'By thy long gray beard and glittering eye, Now wherefore stoppst thou me?
'The Bridegroom's doors are opened wide, And I am next of kin; The guests are met, the feast is set: May'st hear the merry din.' He holds him with his skinny hand, 'There was a ship,' quoth he. 'Hold off! unhand me, graybeard loon!' Eftsoons his hand dropt he. ========================================= Continuing the Ancient Mariner's tale of the concrete prism stressed by the steel prism...... [Think of this as a serial in parts, like the Sherlock Holmes stories in the nineteenth century Strand Magazine. Appropriately enough, my director with whom I had a grand falling out was named Dr Watson <g> ] The next job is to get rid of the testing machine and to have a closed system which consists merely of the steel block load cell (stress = alias for strain) and the concrete prism (strain). In this case the load cell will be in tension and the concrete prism in balancing compression. So we can look at this either as the steel stressing the concrete in compression or the concrete stressing the steel in tension. Over the "elastic" range we can write, Ps = k1.dLc/Lc or Pc = k2.dLs/Ls Better still, and far more revealing we can write k1.dLc/Lc = k2.dLs/Ls In short we can forget about stress altogether and see this as a relationship between strains. Now the beauty of strain as a variable is that it is DOMOMETRIC variable. I'm not fond of neologisms myself but in this case I don't know an existing word which conveys the meaning I want, so I had better define domometric variable. Domo is Latin for home or house, so a domometric variable is a variable where the metric, the measuring scale in other words, is an in-house scale, i.e. one which relates to a characteristic measure of the object under consideration. In this case the metric is the length of the specimen. This is an analogous situation to that pertaining to temperature and the three water vapour power phases. In that case the domometric variables were temperature. The huge advantage of the domometric variable of strain is that it can be measured with any other metric one cares to think of - feet, centimeters, sachine, verst, vara or light years. It doesn't matter what foreign scale one uses, the value for strain will always be the same. Put another way, strain is dimensionless. --------------------------------------------------------- A small digression. When I was a young Scientific Officer I was having a conversation with Dr Randall Wood on the subject of dimensional analysis. He said, when he was working at NPL. there was a chap who has the knack of solving the most amazingly intractable problems in heat transfer, etc. by using dimensional analysis techniques. I am confident that the reason he was so successful is that dimensional analysis implicitly bypasses the jerry built conceptual variables, like FORCE, etc., of traditional physics. As Clayton remarks in a recent e-mail where he comments on Buchanan's UBIQUITY ======================================== >He observes that "the world is simpler >than it seems" (p72). As you always >said, it is just that we look at it in >the wrong terms. >Nigel ======================================= If I did say that I think I must have pinched the phrase from Stafford Beer. -------------------------------------------------- BALANCING TENSILE AND COMPRESSIVE STRAINS. Now let us balance the tensile and compressive strains by arranging the load cell as the tensile flange of a beam and the concrete as the compressive flange of a beam. Let compressive strain be positive, i.e. strain energy input to the material. Thus the concrete has positive strain energy relative to ambient Beta-atmosphere pressure. Let tensile strain be negative, i.e. strain energy output from the material. Thus the steel has negative strain energy relative to ambient Beta-atmosphere pressure Now when we load such a beam two strain energy changes take place. The steel stretches. Its negative strain energy is increased arithmetically but decreased algebraically. The concrete shrinks. Its positive strain energy is increased arithmetically, and also increased algebraically. If the stiffnesses of the two flanges is virtually the same (in other words when subjected to an external standard strain, a one percent change in length of the standard metre, say, they deform the same amount) then the strain energy input needed by the concrete will be provided almost wholly by the strain energy output of the steel. I don't know if any Vortexians have ever walked across a small prestressed concrete bridge, the single beam type that is often used to span rural streams. But if they have they might have noticed how flexible the beams were. Indeed my Screwtape devil often tempts me to resonate the beam to destruction by jumping up and down. Fortunately, my guardian angel manages to talk me out of it <g>. Though he didn't realise it, by inventing prestressed concrete Eugène Fressinet provided us with a synthetic model for understanding the behaviour of all materials whatsoever. For all materials can be partitioned into two phases. A phase in a state of negative strain relative to its ambient strain outside the material, and a phase in a state of positive strain relative to its strain outside the ambient material. But what about all those bonds and thingees? Forget 'em. As Buchanan so correctly observes on page 72 of UBIQUITY, ======================================== All in all, science took nearly four centuries to consume the iron magnet. Yet in unravelling the mystery physicists learned a profound lesson: the world is simpler than it seems. And that when it comes to understanding some things the details most certainly do not matter. ======================================== For a structural engineer these two phases are best thought of as struts (the positive strain phase) and ties (the negative strain phase). As a materials scientist I find the best mnemonic for the two phases is to see one as a quasi solid and the other as a quasi fluid. Each phase in turn may be partitioned into quasi-solid and quasi-fluid phases, and this process may be carried on indefinitely (yes, its elephants all the way down) giving us a binary hierarchy of strains. These iterative hierarchical strains provide us with a binary conceptual structure to replace what one might describe as the imperial conceptual structure of second millennium physics, the analogue of the imperial weights and measures structure of our forebears. If a century ago I had prophesied the in 100 years the most important machine on earth would simply be taking in and pumping out long strings of ones and zeros I'm sure I would have got some very funny looks, so I don't really expect any but a tiny minority to take my Iterative Hierarchical Mechanics manifesto any more seriously than early 20th century man would have taken the proposal for a binary digital computer. Fortunately Parkie doesn't have the power to close down Vortex and to ship me off to Siberia in a closed train. 8-) ============================= As I was going to St. Ives, I met a man with seven wives. Each wife had seven sacks, Each sack had seven cats, Each cat had seven kits. Kits, cats, sacks, and wives, .......................... ============================= Since my grandchildren have no difficulty in grasping the nature of a septenary system, Vortexians should have no problem with binary. 8-) And if I don't stop there, I wont get any tea. Cheers Grimer ================================================== Who is she that cometh forth as the morning rising Fair as the moon Bright as the sun Terrible as an army set in battle array - King Solomon - ===================================================