Hello Richard, I have not had the time to study all of the RC vs CC posts to follow the line of thought there but the question of compressive stress in soundboards and the deleterious, ubiquitous effects so direly lamented and commented upon in articles by Fandrich in the Journal and defended vociferously here on the list by Nossman are predicated on very questionable assumptions, number one and, number two don't appear to be as generalized a fault as is frequently claimed. Some of the reasons for these discrepancies, I would suggest, lie in certain erroneous assumptions made in their analyses which I have intimated before. The tests used by labs to develop testing data for mechanical properties in wood are based upon simple, clear specimens, a point noted in cautionary commentary made by Hoadley and in other references which are frequently referred to here by this school of thought even though the general cautions and limitations of these tests themselves appear to have been disregarded, or misunderstood. The tests themselves are uniaxial, that is the compressive force or tensile force, as the case may be, is applied in one dimension; for example the determination of the fiber stress at proportional limit in perpendicular to grain compression which is so often used to assert that soundboards experience disabling damage. Uniaxial stresses in wood in situations such as the testing for mechanical properties normally quoted are greatly different from stress distributions occuring in soundboards. Such distributions preclude the kind of simplied approach taken in extending such things, for example, simply calculating that cross grain compression will reach such and such a value, and that a soundboard has, in fact, been damaged in such a case. This is practically a case of admiring the Emperor's new clothes in my mind. This may or may not have happened and, in fact, it is unlikely that stresses will have reached damaging levels due to the fact that the interactive stresses in actual, real soundboards, invalidate applying, without at least some discretion, simple unaxial moduli. In a real soundboard the stresses encountered would be more accurately characterized as tri-axial due to the loading of the strings, the bridges, crown, etc. This is an absolutely critical point. Triaxial stress in wood is not as well studied and, where such studies have been undertaken they demonstrate that it is entirely unwarranted to make generalizations about complicated structures and stress distributions proceeding from the results of simple uniaxial tests. Elastic moduli change depending upon the complexities of the stress distribution itself. To disregard, or be unawares of such things and proceed to develop a complicated rationale for failure without taking them into account is produce nothing but questionable generalizations as I have maintained for several years now. Take a look at: http://www.ndt.net/article/v04n11/mascia/mascia.htm#5 for one such study. The uniaxial tests themselves are made under certain simplifying assumptions which are also questionable. Again, this is part of the rationale, in my opinion, for the cautions found here and there in such references. A second, also highly important point of criticism concerns the assumptions made for the uniaxial tests of mechanical properties themselves which assume isosotropy and the applicability of Saint Venant's principle which asserts that the intermediary cross section must have the same distribution as the end cross section. Such assumptions limit their applicability to a complicated composite structure such as a soundboard in which it is necessary to take into account the effect of numerous boundary conditions such as bridges, the rim, the ribs, soundboard buttons and holes, not to mention alien stresses such as the downbearing load, having been forced down on the rim in certain areas during installation, the effect of the bridge and crown itself. etc. Another useful site on this point: http://www.nd.edu/~ame/announcements/Horgan.html I include part of the abstract of the site given just above for emphasis: "A proper assessment of end or edge effects in composite structures is of fundamental technological importance. The extent to which local stresses can penetrate structural elements must be understood by the designer. Thus a distinction must be made between global domains (where Strentgh of Materials or other approximate theories may be used) and local domains which require more detailed (and more costly) analyses based on exact elasticity. The neglect of end effects is usually justified by appeals to some form of Saint-Venant's principle and years of experience with homogenous isotropic elastic structures has served to establish this standard procedure. Saint-Venant's principle also is the fundamental basis for static mechanical tests of material properties. Thus property measurements are made in a suitable gage section where uniform stress and strain states are induced and local effects due to clamping of the specimen are neglected on invoking Saint-Venant's principle. Such traditional applications of Saint-Venant's principle require major modifications when strongly anisotropic and composite materials are of concern. For such materials, local stress effects persist over distances far greater than is typical for isotropic materials. In this lecture, we provide a survey of situations in linear elasticity where anisotropy and material inhomogeneity induce such extended Saint-Venant end zones. The implications for the analysis and design of structures using advanced composite materials are addressed." It should be easy, although I don't know why it would require my posts to make them aware of it, for a certain school of thought which frequently condemns soundboards based upon what can be seen to be questionable calculations, to see the utility of caution for those that claim to have fully understood soundboard behavior when making sweeping generalizations about such a "strongly anisotropic and composite material" such as a soundboard most eminently is. Yet, no such caution appears to be evident. Regards, Robin Hufford Richard Brekne wrote: > Part 1.1 Type: Plain Text (text/plain) > Encoding: 7bit
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