CompositesWorld
Issue link: https://cw.epubxp.com/i/944580
CompositesWorld.com 13 other measurements in tensile, compression, flex, shear and peel tests. e Universal Video Extensometer (UVX) can deliver resolutions to better than 3 microstrain and calibrate to unpar- alleled accuracies, with ISO 9513 class 0.5/ASTM E83 B-1, even at low strains and gauge lengths. Under controlled laboratory conditions, the UVX system will track the x and y coordinates of a target box used to define the position of a feature in the video stream to a resolution of 1/200 th of a pixel or better. is level of resolution makes the system effective in situations where conventional extensometric measurements are difficult because they are carried out • on a very small scale. • at very high temperature. • on very delicate specimens. • in high-speed testing. • in complex loading regimes (such as those tested in ASTM D 5656). • in remote sensing of large civil engineering structures. In this demonstration, two 5-megapixel cameras and suitable lenses were selected to maximize the adhesive bond line in camera view. e large number of pixels representing this bond line ensured a high measurement resolution. In this case, 0.1µm displacement resolution was achieved. e cameras were positioned front and back and synchro- nized to replicate the setup of the KGR extensometer (Fig. 2, top right). A measurement plane was calibrated, using the known dimension of the overlapping bondline for scale. is enabled extension measurements to be made in millimeters, in accor- dance with the standard. Measurement points were applied according to the positions defined in the ASTM standard. Comparing test results Test results were compared with data collected using a conven- tional extensometer (Fig. 3, bottom right). e mean value of shear modulus (81.6 ksi) was found to be within 1% of KGR- determined data (82.2 ksi). Shear modulus determined using the KGR data had a standard deviation of 27.8%, but the shear modulus determined using the UVX data had a standard devia- tion of only 14.0%. Additional testing suggested that using six measurement points, rather than three, could potentially more than halve this uncertainty to 6%. In addition to providing KGR equivalent point displacement data for manual calculation of the shear stress/shear strain curve (Fig. 4, p. 14, top), the video extensometer can measure shear strain directly, using groups of targets. e shear strain gauge is shown in Fig. 4, bottom). e gauge can be extended to cover the entire bond length and aligned with the bond line for precise measurement. A sufficiently small field of view would allow the shear strain to be measured directly in the adhesive layer, avoiding uncertainty that comes from measuring on the adherends. Video Extensometry FIG. 2 This photo of the experimental setup shows the UVX video metrology system's two cameras positioned in front and in back of the test coupon, which is mounted in the test frame. Source | Imetrum IMETRUM DATA ksi Front Back Ave Sample 1 original 83.4 90.9 87.2 rotated 113.4 71.3 92.4 Sample 2 original 59.9 68.9 64.4 rotated 79.5 47 63.3 Sample 3 original 84.8 90.5 87.7 rotated 109.5 79.8 94.7 Mean 81.6 StDev 14.0 %COV 17.2 CUSTOMER DATA ksi Sample 1 47.7 Sample 2 78.3 Sample 3 114.3 Sample 4 105.9 Sample 5 64.8 Mean 82.2 StDev 27.8 %COV 33.9 FIG. 3 A comparison of the test results obtained using the UVX video metrology- equipped extensometer and a conventional KGR extenso- meter. Source | Imetrum Although the current method, using a KGR, provides average shear strain between the adherends, it does not provide all the information that would be of benefit in a research environment. UVX can provide additional data, including changes in the thick- ness of the adhesive layer, relative rotations of the adherends, point-to-point differences in the shear strain in the adhesive layer, shear and tensile strains in the adherends and tracking of the development of cracks within the adhesive. e UVX tech- nology enables a simpler, more robust and reliable approach to the capture of the basic shear strain, as required by ASTM D 5656.