This protocol can help answer key questions in the field of body armor development by defining a repeatable procedure to test the mechanical properties of flexible unidirectional composite fabrics. A straight, clean cut well oriented with the fiber direction is critical for the accurate measurement of failure stress by preventing additional sources of error. This technique fills a gap in current testing by allowing interrogation of a material's properties without hot pressing, which is more representative of its use in soft body armor.
Unroll a bolt of unidirectional material to be tested to a work surface. The unwound portion is the precursor material. Visually verify that the principle fiber direction is parallel to the width of the bolt.
Next, get a scalpel to isolate the precursor material. Create a tab by making a cut parallel to the fiber direction. Then make another parallel cut about three millimeters away.
Manually grasp the tab and tear it away to expose the fibers on the layer underneath. Pull until the two layers have been separated across the whole length of the precursor material. Remove any loose fibers from the edge of the tab that neighbor the exposed cross fibers.
These may be up to one to two millimeters away from the tab path. Continue by separating the piece of precursor material from the bolt. Turn over the material so the new principle fiber direction is the warp direction.
Now, mark the grip lines on the material aligned in the weft direction. These lines are a set distance from and parallel to the cut edges of the material. Once again, cut a tab in the material, then pull the tab to determine the principle fiber direction for the specimen to be cut from the material.
The fiber direction should be perpendicular to the grip lines. Take the material to an appropriate self healing gridded cutting mat. Carefully align the fiber direction with the grid lines on the mat.
Ensure that the fiber direction in the material is aligned with the grid lines. Avoid damaging the material and secure it to the cutting mat by taping its corners. Use a medical blade and a straight edge to cut specimens from the material.
Place the straight edge at the desired specimen width and align it with the grid on the cutting mat. Clamp the ends of the straight edge in place and check that it is still correctly positioned. Cut the specimen along the straight edge with a constant velocity and pressure.
Unclamp the straight edge, taking care not to move the material. Then, remove the specimen. These are all of the specimens obtained after repeating the cutting steps to maximize specimen production.
Prepare to mark points to track on a specimen for use with a video extensometer. For consistency, use a template placed over the specimen. Mark the gauge points with a permanent marker.
Ensure the marks are visible on the specimen to allow measurement of strain. Next, take the specimen to the universal testing machine. There, insert the end of the specimen through the gap in the capstan and bring it around to the grip line, forming a loop.
Center the specimen on the capstan grips. While keeping the specimen centered, turn the capstan to the desired position. Use a tensioning device to hold the sample in place before locking the capstan in place.
Complete mounting the specimen by repeating the same steps with the other end. When done, apply a small preload of two newtons and record the actual gauge length. Now set up the instrument to perform a tensile test at a constant 10 millimeters per minute and begin the test.
For the test, the testing machine applies a constant rate of extension to the sample while the load and displacement are recorded. The test continues until a break occurs in the sample. This plot is a typical load versus displacement curve for these experiments.
When monitoring the display, a 90%reduction in the observed load indicates a break in the specimen. Record maximum stress and corresponding failure strain. After making a plan for the study, cut the material needed for each condition to be tested.
The strips should be wide enough to yield the required specimens and have an additional 10 millimeters or more. Place the strips for the environmental exposure in a tray. At the environmental chamber, set its program for dry room temperature conditions and allow it to stabilize.
Once the chamber has stabilized, place the sample tray inside away from the walls or any points of condensation. Next program the chamber to reach the target temperature for the environmental study. When the temperature is stable, program the chamber to reach the target humidity.
When it is time to extract aged material, program the chamber to decrease the relative humidity to approximately that of the lab. After the humidity has stabilized, program the temperature to drop to room temperature, or about 25 degrees celsius. To retrieve the material, be prepared with an appropriately labeled container.
With the humidity and temperature stable, open the chamber, remove the required material, and place it in the labeled container. Return any remaining material to the environmental chamber and reestablish the chosen conditions. Next, take the aged material to create aged specimens.
This aged material is aligned on a self healing cutting surface. Its grip lines have been marked as before. Set up the straight edge to remove five millimeters from one side of the specimen.
Trim this amount to remove any damage from handling during the aging process. Continue to create the specimens in the same manner as the unaged material. When done, the specimens are ready for use in tensile testing experiments.
For load tests performed with a fixed capstan, the load versus extension plots of several samples show the samples slipped. This reflects the difficulty of securing the material. By contrast, with a rotating capstan the curves are smooth and consistent.
The material is an ultra-high molecular mass polyethylene unidirectional laminate. This failure stress distribution is plotted using Weibull scaling. The data are for PPTA unidirectional laminate specimens which a gauge length of 300 millimeters and width of 30 millimeters cut along the warp direction with a load of 10 millimeters per minute.
There is a low strength outlier that should be investigated. For comparison, the same data are plotted except with the outlier excluded. Note the change in the horizontal axis.
Following this procedure, molecular spectroscopy could be employed to answer questions about the chemical changes in the material due to the exposure to heat and humidity. Please remember that the medical scalpel is extremely sharp and appropriate care must be taken to avoid injury.
