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Artificial Thermal Ageing of Polyester Reinforced and Polyvinyl Chloride Coated Technical Fabric
Chapters
Summary January 29th, 2020
Here, we simulate accelerated thermal ageing of technical fabric and see how this ageing process influences the mechanical properties of the fabric.
Transcript
This protocol facilitates the performance of simplified accelerated aging tests on textile fabrics and the evaluation of mechanical properties on tested materials. This procedure makes it possible to determine to determine the future material properties in a relatively short period of time, which is essential for structural designers specializing in textile fabric roof structures. To assess viscoplastic constitutive relations, a testing machine with a software rope is require to perform constant strain rate tests controlled by an extensometer.
Before beginning the experiment, confirm that a testing machine, equipped with an extensometer and the appropriate software is available and that a thermal chamber is accessible. Unroll the technical fabric AF9032 bale and draw at least 42, 300 by 50 millimeter shapes on the fabric surface parallel to the warp direction and at least 42 parallel to the fill direction. Use a permanent marker to indicate the warp direction on each specimen before cutting out the specimens.
Use a slide caliper to measure the specimen's thickness and count the number of threads at the short edge of the specimen. Next, set the thermal chamber to a constant temperature of 80 degrees Celsius. When the temperature almost reaches 80 degrees Celsius, open the chamber door and place at least seven sets of specimens into the chamber, closing the door as soon as possible to avoid a temperature drop.
After one hour, use thermal gloves to remove the reference set of specimens, removing the successive experimental sets once a week every two weeks for the next 12 weeks. After their removal from the chamber, leave the specimens at room temperature for one week. Before the test, use a permanent marker to make two black dots 50 millimeters apart in the middle of each specimen and install two 60 millimeter flat inserts into each grip of the testing machine.
When all four inserts are in place, open the software in the machine and select the program dedicated to the tensile tests. Then select the starting position with a 200 millimeter grip to grip separation in the software and click the Starting Position button to execute the 200 millimeter grip to grip separation. To set up the video extensometer, move the camera along the supporting bar until the lens is situated at the middle of a specimen.
Check whether the lens of the camera will provide a clear view of the specimen markers during the entire experiment. Place the calibration device in front of the camera and select the proper brightness and focus for the lens. Clamp the device with the grips and select the proper type of markers in the Targets window in the video extensometer software.
Use the Scale option to select the calibration procedure and select the calibration distance in the Scale window. Then change the marker type to Pattern in the Targets window to allow the extensometer to follow the marks on the specimen. When all of the materials and equipment are ready, set the test parameters in the testing machine software and place the specimen in the grips in the vertical and horizontal directions along the main vertical axis of the machine.
Use a tabular spanner to close the grips and perform the tests at the selected constant strain rate until each specimen breaks. Then repeat the test every two weeks with each subsequent set of samples. After 80 degrees Celsius experiments, repeat the procedure weekly at 90 degrees Celsius.
When all of the specimens have been tested, use graphing software and the cross-section area of the samples to recalculate the registered force and elongation increments according to elementary strength of materials equations to the stress strain relations. Then, plot a graph of the obtained data for the warp and fill samples. To set up a piecewise linear model for non-linear elastic modeling, for each sample curve find the strain ranges detecting the linear or close to linear stress strain relation.
Use the Fit Regression option in the graphing software and the least square method to identify the best fit line in the chosen region. Denote the tangent as the longitudinal stiffness value for which the I index corresponds to the current direction of the material and the J index is a consecutive number of the identified line. When all of the parameters have been delineated, identify the intersection points between the lines.
To perform an Arrhenius extrapolation, assign the reference temperature according to the average value based on the results of local meteorological station and assign the thermal chamber temperature as that which was used in the aging test. Then, calculate the reaction rate constant from equation to extrapolate the aging time expressed in weeks to years. In this image, the stress strain curves for the warp and fill directions of AF9032 fabric obtained at different aging times in the 80 degree Celsius temperature level for a strain rate of 0.0011 per second are shown.
As observed, the difference between the reference one hour aging period test and the rest of aging periods is typically clear. The aging time does not seem to substantially affect the material response in the warp direction. In contrast, the ultimate tensile strength in the fill direction samples is much lower in the artificially aged samples than in the unaged specimens.
Moreover, for the fill direction, the achieved stress strain curves have divergent trajectories when the strains exceed 0.06. The first linear part of the experimental stress strain curve of a simple tensile test corresponds to the stiffness of the technical fabric covering made of PVC. The results obtained at 90 degrees can be extrapolated to real years using the Arrhenius simplified relation.
The hardening parameter in the warp direction and the viscosity parameter in the fill direction can also be calculated. It should be emphasized that for the Bodner-Partom constitutive relations, the experiments with constant strain rates are required. The machine is bound to perform this type of operation.
The video extensometer can be replaced by a mechanical extensometer and various constitutive models can be incorporated to reflect the textile fabric performance. This technique can be used for the lifetime prediction of different composite materials. The operation of a high temperature thermal chamber requires the use of thermal gloves and the complex testing machine should be always operated with care.
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