To: |
"[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>, "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx> |
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From: |
David Leal <david.leal@xxxxxxxxxxxxxxxxxxx> |

Date: |
Sun, 10 Jul 2011 21:47:13 +0100 |

Message-id: |
<20110710205938.1AEED138CC4@xxxxxxxxxxxxxxxxx> |

Dear Azamat and others, (01) A theory of properties is interesting to me, and it may help to look in detail at a property which mentioned by Azamat. (02) A small region of solid matter can have a property that is its stress tensor. A stress tensor is a single thing, and should not be thought of as a "compound property" or a "property of properties". The following points should be noted about a stress tensor: (03) 1) Inevitably stress varies from position to position within a solid object. Sometimes when we choose a small enough neighbourhood of a point P within a solid object, the stress within that neighbourhood varies by only a small amount, so that we can say "Body B in the neighbourhood of point P has stress S". But if the neighbourhood is too small, then there is no concept of stress and instead only molecular bonds. Hence sometimes, we cannot say that "Body B in the neighbourhood of point P has stress S", because by the time we have shrunk the neighbourhood sufficiently to get a stress value that does not vary too much within the neighbourhood, the concept of stress has disappeared. This situation can occur at a crack tip - a location of particular interest to engineers. (04) Density is also a property that can only apply to "Body B in the neighbourhood of a point P", if the neighbourhood is of a sufficient size. If some neighbourhoods have one molecule in them and others have two, then the variation of density from point to point is somewhat extreme. :) (05) 2) A stress tensor can be thought of as bilinear function from pairs of direction vectors to force per unit area. The function can be encoded as a matrix of force per unit area values. However, this encoding is not the thing itself. The encoding depends upon the coordinate system used to express the directions. If the coordinate system is not Cartesian, then there are alternative co-variant and contra-variant matrix encodings. (06) 3) A stress tensor has "invariants". These are quantities that are derived from the tensor, where the quantity is not dependent upon the coordinate system chosen to encode the stress tensor. An example is von Mises's equivalent stress. (07) It is not clear to me, how a stress tensor would be included in an ontology for properties. Is a stress tensor regarded as a "quantity", like a mass? Is a von Mises's equivalent stress regarded as a "property" of a stress tensor? (08) Best regards, David (09) At 19:44 07/07/2011, AzamatAbdoullaev wrote: > |

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