13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Figure 2. Single yarn specimens with (a) 25mm and (b) 50mm gage length for dynamic tensile tests 2.2. Quasi-static tensile testing Quasi-static tensile tests of Kevlar fabric were performed on a 90 kN INSTRON machine operated under closed-loop displacement control with a displacement rate of 2.5 mm/min. Digital data acquisition was used to collect data at a sampling rate of 2 Hz. The test was continued until complete failure of the specimen. The overall specimen deformation was measured by stroke movement. The two plates were held together within the hydraulic grips to ensure uniform pressure application and prevent any fabric slippage (more details in Naik et al. [9]). Five replicates were tested for each specimen size in both warp and fill directions, and the deformation and failure behaviors of the specimens were recorded by a CCD monochrome camera. In order to measure the mechanical properties of the yarn, single yarns were extracted from fabrics and only the warp yarns were tested. A low capacity load cell (275N) was used to record the force. A universal joint was connected to the testing frame to allow rotation of the grip and remove any potential bending moment. The universal joint also helped in the alignment of yarn during test. To avoid any slipping of test sample during testing, the yarn was wrapped around the upper and lower mandrels and aligned using a laser beam alignment level [14]. Ten replicate samples of six different gage lengths (50, 125, 200, 275, 350, 425 mm) were tested on an MTS test frame under displacement control at a strain rate of 4.2 × 10-4 s-1 (quasi-static). 2.3. Dynamic tensile testing Dynamic tensile tests were conducted on an MTS high rate servo-hydraulic testing machine operated in open-loop control [3]. The speed of the stroke is controlled by the opening and closing of the servo-valve of hydraulic supply. By manually turning the servo-valve, the rate of flow of hydraulic fluid can be controlled, resulting in different stroke speeds. However there are some differences in the actual stroke speeds between individual tests although the opening of servo-valve is not changed, especially at higher stroke speeds. The actual stroke speeds of individual tests are determined by the slopes of real-time displacement-time curves obtained from the tests. A description of the high strain rate testing system, test setup and data processing procedure are (b) (a)
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