Development of a Computerized Method to Measure Cotton Tenacity at Different Extension Rates

Abstract

Published research has indicated that cotton strength increases as the specimen loading rate increases when measured by Stelometer and Pressley methods. This report describes an instrument designed to test 1/8 gauge bundles at constant rates of extension from 0.05 to 50 cm/min. A computer analyzes voltage signals from force and displacement transducers. Pressley jaws hold the fiber bundles, and a Stelometer vice is used to load them in the jaws. Specimens are selected with a mechanized sampling and combing procedure. The method provides measurements of strength, elongation, modulus, and two forms of toughness.A new class of fiber testing instruments is evolving to meet the demand for increased information and better precision while measuring the quality of cotton enroute to market. The quality evaluation procedure involves removing or cutting two small samples from each cotton bale, which are delivered to a USDA marketing service office for testing in a controlled laboratory. At the laboratory, they are opened and exposed to circulating air for a time sufficient to assure proper temperature and moisture content. In ofhces where the new high volume instruments (HVI) are being used, fiber length, length uniformity, strength, fineness, lint color, and trash content are measured.There are two manufacturers of HVI systems, Motion Control, Inc. (MCI) of Dallas, Texas, and Spinlab/ Uster Inc., of Knoxville, Tennessee. Both measure cotton fiber length with tapered bundles (a collection of fibers each grasped at a random position along its length with its ends free to extend parallel to each other from a clamp). Prepared specimens (5.1 to 6.4 cm wide) are measured at several distances from the clamp to form fibrograms for length analyses. A point (some distance from the clamp) is selected for strength testing.HVI strength measurements are made at a jaw displacement speed much faster than conventional laboratory instruments, but laboratory instruments are used to check strength values obtained with the HVI methods. The Cotton Division of the Agricultural Marketing Service (AMS) has reported higher HVI strength values for some cottons when compared to Stelometer values [8]. Brown [2] demonstrated the speed problem by modifying an HVI to operate at two extension speeds. He reported no average strength difference when the HVI was recalibrated at each speed, but there was a 9% loss in the strength of his calibration cottons when tested at the increased extension speed.Hertel and Phillips [3,5], on the other hand, reported strength gains with an increasing rate of loading for Stelometer and Pressley methods.The precise extension speed of HVIs is loosely controlled, and jaw displacement is not routinely calibrated. Taylor [7] measured the extension speed of a research model (MCI) at 19.8 cm/min, and Brown [2] reported the speed of an MCI 3000 system at 13.6 cm/ min. Others have studied the effect of testing time on cotton strength for Stelometer and Pressley methods [4, 5], but the literature does not reveal sufficient informat...

Development of the correction procedure for High Volume Instrument elongation measurement

“…27 In addition, the Stelometer has an estimated instrument breaking speed of 0.3 cm/min, while the HVI presents an estimated breaking speed of 12.3 cm/min. 28 Since the HVI lines and the Stelometer correlated well with each other, it was necessary to determine if the HVI provided the same statistical rankings as the Stelometer for cultivar, location, harvest method, and their interactions.…”

Cotton fibre quality

“…30 Aside from increased speed, due to faster equilibrating strain gauges and the development of high-speed measurements of specimen linear density using light, the success of the HVI strength measurement has arisen from the change in specimen loading to a constant rate of extension rather than a constant rate, which eliminated force measuring errors caused by hardware accelerations. 63 Godbey et al 64 varied the extension rate of the clamping jaws in an HVI over a wide range, and showed tensile strength increased with rates of extension at moderately high speeds and then declined at near-ballistic speeds of load. Since the measurements of specimen linear density do not provide sufficient accuracy, strength calibrations are performed by testing calibration cottons whose bundle strength value has been established.…”

Cotton Testing

“…The former is all the more remarkable as the photoelectric 'amount' has been used in a standard test of maturity. The pneumatic instrument is adjustable for a cotton's micronaire value MIC by the factor [152]: …”

“…7 is due to different rates of extension, the writers [152] varied the extension rate over a wide range -noting the rates given by commercial instruments -and showed how the increase of strength with rate of extension peaks at moderately high speeds, then declines at near-ballistic speeds. The paper also introduced a mechanised sub-sampling and sample combing procedure that gives a precisely tapered specimen.…”

“…Experts remain divided about the significance of extensibility in processing and about whether it varies directly or inversely with cotton tenacity. The need to allow for crimp, hence the uncertainty about zero strain, was stressed [152], and may be an explanation for between-instrument variation in extensibility. Extensibility was found to be the only property in which HVI results correlated poorly with single-instrument -in this instance, Stelometerresults [167].…”