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Before embarking upon the manufacture, or even the final design, of any large structure subject to repeated loading, it is both reasonable and logical to carry out sufficient small scale tests on materials and sections to enable a picture to be built up of the probable behavior of the completed assembly. Although this technique may be admirably suitable for qualitative work, for quantitative results some factor, often not completely understood, has to be introduced to take care of the size effect between small and full scale testing. As an example of the magnitude of this, consider the case of stepped shafts having smooth machined fillets between the stepped portions. For a basic 3-in. diameter shaft having a ratio of fillet radius to shaft diameter of 0.06, the fatigue strength was found to be ±17,500 psi, while for a 9 3/4-in. diameter shaft of similar material having the same radius-diameter ratio, the fatigue strength was ±15,000 psi, a reduction of 14.3 per cent. It has thus become necessary to consider increasing the scope of scale fatigue testing to include specimens of a size comparable with full-scale requirements. In defining limits for large-scale fatigue tests on specimens having discontinuities, it is as well to think in terms of the mass of material behind the discontinuity, and the fact that small discontinuities become very much a surface phenomenon. To come down to much below a quarter full size, in the case of large assemblies, is to return to small scale testing, where the properties of the material, with special respect to forging and heat treatment, can vary. Thus for marine shafting of up to 24 in. in diameter, a minimum diameter of 6 in. would be acceptable for large-scale tests. As far as the upper limit of specimen size is concerned, it is largely bound up with the maximum size of testing machine that can be efficiently and economically designed. The problems of space, and of power, become rapidly more complex as the -specimen size is increased. For example, a 3-in. diameter shaft under torsional fatigue required a 2-hp motor to induce a stress of ±22,000 psi on resonance; while for a 9 3/4-in. diameter shaft, a 30-hp motor was needed to give the same stress on resonance. For shafts above 10 in. diameter, the damping increases as a power of the diameter, and there will also be increased frictional and windage forces created in the exciting mechanism. Furthermore, the question of the limiting size of forgings from which these large shafts can be made has to be carefully considered. Though large forgings give a good material with surface properties consistent with slow cooling, in practice it was found that in forming the specimens a large part of the best of the material was removed by machining. Though this also occurs in the manufacture of large crank units, it does not represent the best in marine practice, and therefore to aim at very large forgings for test specimens would not lead to a comparable increase in value of the results. It thus becomes economically impracticable to continue testing forged shafts much above 10 to 12 in. diameter as part of a fatigue program, and it becomes more attractive to proceed to tests on full-scale installations, where much valuable information may be obtained to be a guide for future modification and design.
Bunyan, T. W.
Principal EngineerEngineering Adviser, Lloyd's Register of ShippingPeninsular and Oriental Steam Navagation Company, LondonLondon,