An elastic element (modulus E) and' a sliding element (yield stress σ p) are coupled in series to get a rheological model of an elasto-plastic structure and work-hardening is accounted for by coupling another elastic element (modulus Es) parallel to the sliding element. This is applied for the compression, recovery, and recompression curves obtained in copper wire buckling. Agreement between theory and experiment is excellent and it is demonstrated how yield starts on the compression and extension sides of the wire cross section and moves towards the center; a new pair of yield-limit positions appears for a new deformation (strain history). The same theory is applied to paper buckling, where the work-hardening effect mentioned must be introduced and calculated, and it is indicated that buckling involves a "weakening" of the structure compared to extension. It is also demonstrated that a model of frictional-elastic buckling developed for textile fabrics does not fit paper buckling well; this is an indication that the rhe ological coupling is significant.
The creep behavior of Lincoln wool fiber in water is studied by varying temperature (from 10°C to 97°C.), stress (from 9 to 55 x 107 dyne/cm2) and initial extension rate (from 70 to 300%/min.).There are two difficulties in the creep experiment. The first is that constant static stress could not be applied instantaneously, and the second is that the creep strain within a few seconds may not be measured accurately. In this experiment, the creep strain can be magnified by optical lever technique ( Fig. 1) and 0.001 mm, deformation is detected by the cinematograph which projects the scale. The instantaneous strain at 0.1 second can be measured.The results of this experiment are as follows :(1) The instantaneous strain depends upon the rate of loading. The higher the speed of exten sion under the same load, the lower the instantaneous strain.(2) According to the successive creep and its recovery tests, the creep curves nearly coincide with each other after several repeated tests.(3) The length of the fiber is well reversible from creep test under 2 % extension (in Hookean region) in water. The specimen is more completely recovered at a temperature higher than that of the creeping temperature. The fiber, although, contracts after creep test under low extension above 60°C. It is suggested that supercontraction and permanent set depend upon the amount of broken and re-formed hydrogen or polar bonds. The creep recovery from high extension is not sufficient during short time (15 min.) below 60°C.(4) Beyond the yield point, the instantaneous strain increases remarkably with rising of the temperature, whereas a little decrease of Young's modulus is obtained in Hookean region.
The purpose of this instalment is to obtain fundamental data on the staple length, sliver thickness and stretch-breaking force of the Turbo Stapler and devise methods to analyze them.
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