JOURNAL OF COSMETIC SCIENCE 84 FIBER DIMENSIONAL ANALYSIS Determination of the shear modulus requires an accurate cross-sectional area and shape to be determined for each fi ber being tested. This is particularly important for natural fi bers, which exhibit large fi ber-to-fi ber variation combined with variation over the length of the sample. After sample preparation and environmental equilibration, both groups of fi bers were measured in an automated Dia-Stron FDAS765 laser scan micrometer. The Dia-Stron FDAS765 uses a Mitutoyo LSM500S laser micrometer (Mitutoyo UK Ltd., Andover, UK), which has an operating range of 5–2000 microns, with an accuracy and repeatability signifi cantly better than 1 micron. The FDAS765 has been commercially available for 15 years and is widely used for normalization of fi ber measurements to cross-sectional area (27). The samples are fi rst loaded into a linear cassette, which is then placed in the ALS1500 Automatic Loading System. A pneumatically operated pick-up head transfers the samples sequentially from the linear cassette to the laser micrometer and then back to the cassette. The entire measurement sequence is controlled by Dia-Stron UvWin software. Once loaded in the laser micrometer, a small tensioning force is applied to one end of the sample to ensure the fi ber is orthogonal to the laser beam. The fi ber is then measured at several positions (or “slices”) along its length. At each position, the fi ber is rotated by 360°, and the maximum and minimum dimensions are determined by a sensor measur- ing the time interval for which the fi ber interrupts the beam of a high-speed scanning laser. The dimensional results are stored within the UvWin program for subsequent use with the Dia-Stron Fiber Torsion Tester or can be exported for use with other instruments. TORSIONAL PENDULUM EXPERIMENTS Following equilibration, one end of a crimped fi ber was inserted into a groove close to the top of the custom-built “Single Fiber Torsion Rig” (Figure 1), and a pendulum bob Figure 1. Single-fi ber torsion rig.

NEW METHOD FOR MEASUREMENT OF FIBER TORSION 85 (mass 6.5 g) was attached to the lower end of the fi ber. The pendulum bob is composed of a groove for the fi ber crimp and a disk containing a number of equally sized and spaced slits through which light can pass. When the crimped fi ber is attached to the torsion rig and the pendulum bob, the plane in which the bob oscillates should be normal to the direction of the fi ber. When mounted on the torsion rig, the pendulum bob is orientated between a light source and sensor. Although care is taken in the preparation of fi bers of 30-mm gauge length, some fi bers may be slightly longer or shorter, and the rig is fi tted with an adjustment screw to make small adjustments to the position of the bob between the light source and sensor. When a measurement is to be made, the custom rig-operating software is initiated and experimental parameters entered. An initial oscillation of 135° at a rate of 75°/s, with a 30-s data acquisition time and a 50-Hz sampling rate, is employed. Following entry of the experimental parameters, the motor on the top of the rig is initi- ated and rotates to set the oscillation of the fi ber and bob in motion. The frequency and amplitude and the dissipation in rotation of the fi ber and bob are determined by record- ing the intermittent signal of the light incident on the sensor as the slits and fi lled sec- tions of the bob pass by the light source. The data is collected electronically and can be displayed as shown in Figure 2. The shear modulus was calculated using the method described below (28). The frequency of a torsional pendulum is given by: 1 2S C f I where f = the frequency in Hz, C = torsional rigidity, and I = the moment of inertia of the pendulum bob in kg/m2. Figure 2. Typical response produced by a fi ber in free oscillation on torsion rig following application of excitation force.