The authors would like to acknowledge Stuart Leigh Phoenix for his helpful discussions, Mike Riley for his assistance with the mechanical test setup, and Honeywell for donating some of the materials. Funding for Amy Engelbrecht-Wiggans was provided under grant 70NANB17H337. Funding for Ajay Krishnamurthy was provided under grant 70NANB15H272. Funding for Amanda L. Forster was provided from the Department of Defense through interagency agreement R17-643-0013.

1. Cutting procedure for warp-direction specimens that are cut perpendicular to the axis of the roll
<ol>
	<li>Identify a bolt of unidirectional material to be tested. 
	NOTE: There is no warp (used to describe the direction perpendicular to the axis of the roll) and weft (used to describe the direction parallel to the axis of the roll) in the traditional textile sense, as the material used here is not woven, but these terms are borrowed for clarity.</li>
	<li>Manually unroll the bolt to expose the precursor material (i.e., the identified material unwound from the bolt but still connected to the bolt). 
	NOTE: The width of this bolt will become the material&#8217;s total length (refer to Supplemental Figure 1b), so for a 300 mm gauge length (corresponding to a 600 mm total specimen length), using the procedure and testing grips specified below, the piece of material cut from the bolt should be 600 mm wide. The length of this piece of material will be that of the width of the bolt on which the material is rolled (approximately 1,600 mm, in this case). This is depicted in Supplemental Figure 1b.</li>
	<li>Visually verify that the principal fiber direction is parallel to the width of the bolt, as shown in Supplemental Figure 1b. The fiber direction of the top layer of the material (i.e., that which a viewer sees when looking down onto the specimen) is termed the principal fiber direction.</li>
	<li>Cut a small tab in the precursor material with a scalpel, approximately 3 mm wide, with the tab&#8217;s length aligned nominally parallel with the principal fiber direction of the precursor material, as shown in Supplemental Figure 1c.</li>
	<li>Manually grasp the tab and pull it up to tear the tab away and expose the fibers on the layer underneath, running perpendicular to the tab. Keep pulling on the tab until the two layers have been separated across the whole length of the precursor material (Supplemental Figure 1d). 
	NOTE: This step will produce a region where only cross fibers are visible, as shown in Supplemental Figure 1d.</li>
	<li>Remove any loose fibers neighboring the exposed cross fibers remaining from the edge of the tab. 
	NOTE: In the current UD laminate system, it was observed that the fibers are not perfectly parallel (as shown in Figure 1) and that they may cross over neighboring fibers. Thus, fibers neighboring those being separated will frequently become separated in this process. The neighboring fibers that become loose may be as much as 1&#8211;2 mm away from the expected path of the tab used for separation.</li>
	<li>Using a medical scalpel, cut along the exposed cross fibers, thus separating the piece of precursor material from the bolt.
	<ol>
		<li>Determine the distance cut that dulls the blade, causing a less clean cut (i.e. after 400 cm of cutting this material, a scalpel could become dull and scratched, as shown in Supplemental Figure 2 and Supplemental Figure 3). Replace the blade before it becomes dull, or if it is damaged. Examine several cutting instruments when testing a different type of material to determine the best one. 
		CAUTION: Care must be taken with all sharp blades or cutting tools to avoid injury. Cut-resistant gloves may be worn in this step to reduce the risk of injury.</li>
	</ol>
	</li>
	<li>Turn over the material, so that now, the principal fiber direction is in the warp direction. 
	NOTE: Since the principal fiber direction refers to the layer that is being viewed (the top layer), turning the material over will change the principal fiber direction from weft to warp (see Supplemental Figure 1b).</li>
	<li>Mark the grip lines on the material aligned in the weft direction. 
	NOTE: These lines run from manufactured edge to manufactured edge, parallel to the cut edges and 115 mm from these cut edges. These will be further explained in step 4.4.1, but the grip lines are lines used when loading specimens (which are cut later) into the tensile testing grips.</li>
	<li>Determine the principal fiber direction for the specimen to be cut from the material, using step 1.3. 
	NOTE: Be aware that fiber orientation may not be exactly perpendicular to the manufactured edge; in that case, follow the exact fiber line. Avoid the area near the manufactured edge because it may not accurately reflect bulk material properties.</li>
	<li>Orient the material on a suitable self-healing gridded cutting mat that is large enough to fit the width of the material (between the cut edges) and a length (weft direction) of at least 300 mm, as referenced in step 1.16.
	<ol>
		<li>Carefully align the fiber direction with the gridlines on the cutting mat. Use the cut edge of the material as a guide in lining up the material; however, aligning the fiber direction of the specimen is most important.</li>
		<li>Tape the material to the cutting mat. 
		NOTE: Tape should never be placed anywhere near the center of the specimen; instead, it should be used at what will be the ends of the specimens to be cut from the material. The ends will be in the grips when a specimen is tested; therefore, any damage caused to the material by the tape is minimized. Taping only the corners of the material that are far from the cut will ensure that the material will not move and that, when cutting a specimen, the blade will not also be cutting tape. Low-tack adhesive tape (e.g., painter&#8217;s tape) works well because it adheres well enough to keep the fabric in place without damaging the material when it is removed.</li>
	</ol>
	</li>
	<li>Cut the specimens from the material using the blade and a straight edge. The strips formed are the specimens. Do not let the material move in this process; otherwise, determine the fiber direction anew and reorient the material accordingly.
	<ol>
		<li>Place the straight edge at the desired location corresponding to the appropriate specimen width (i.e., 30 mm). Note that the medical scalpel is thin enough that no offset in the placement of the straight edge is necessary to account for the cutting location. Align the straight edge to the grid on the cutting mat or any other user-established reference line on the cutting mat.</li>
		<li>Clamp the straight edge in place by clamping on either end of the straight edge. Check the positioning of the straight edge after clamping, as it may have moved during the clamping process.</li>
	</ol>
	</li>
	<li>Cut the specimen away from the material along the straight edge, using the medical scalpel. Ensure a single, clean, smooth cut, with a constant velocity and pressure. 
	NOTE: Some pressure can be applied by the blade against the straight edge to keep the blade cutting precisely at the edge of the straight edge. 
	CAUTION: Care must be taken to avoid injury, so it is advisable to wear cut-resistant gloves when handling the medical scalpel. Furthermore, since the smoothest cut can be obtained while cutting toward the body, wearing a cut-resistant apron or lab coat is advised.</li>
	<li>Examine the cut edge of the strip under the microscope. Change the blade if the cut edge has significantly more protruding fibers or other defects when compared to a cut made with a new, sharp blade.</li>
	<li>Unclamp the straight edge, taking care that the material does not move in the process. If the material did move, redetermine the fiber direction and reorient the material appropriately.</li>
	<li>Repeat steps 1.12&#8211;1.15 until the maximum number of specimens that can be cut from 300 mm of material has been obtained. 
	NOTE: For specimens with a width of 30 mm, 300 mm of material is equivalent to 10 specimens, while for specimens with a width of 70 mm, this is equivalent to 4 specimens. This 300 mm limit has been determined to work well for the unidirectional laminate studied here but may vary for other laminates.</li>
	<li>Repeat steps 1.10&#8211;1.11 as needed (i.e., redetermine the principal fiber direction and reorient the material before continuing to cut more specimens). 
	NOTE: The protocol can be paused here. If specimens are not to be used immediately, store them in a dark, ambient location.</li>
</ol>
2. Cutting procedure for weft-direction specimens that are cut along the axis of the roll
NOTE: There is no warp and weft in the traditional textile sense, as the material used here is not woven, but these terms are borrowed for clarity.
<ol>
	<li>Determine the width and length of the material desired according to the number and size of the specimens to be cut. 
	NOTE: For this unidirectional laminate and for specimens with a gauge length of approximately 300 mm, two specimens placed end to end can be cut along the width of the bolt. Thus, a set of 40 specimens may be cut out in two columns of 20 specimens each, as shown in Supplemental Figure 4, prior to severing the material from the roll. If the width of the specimens is 30 mm, then the material should be cut at 20x the specimen&#8217;s width (as there are 20 specimens per column) with some extra space (i.e., 610 mm).
	<ol>
		<li>Determine the fiber direction along the weft for the width of interest, following the instructions from steps 1.4&#8211;1.6.</li>
		<li>Cut the exposed cross fibers (i.e., across the warp fibers) using a blade, thus separating the precursor material from the bolt. 
		CAUTION: Care must be taken with all sharp blades or cutting tools, to avoid injury. Cut-resistant gloves may be worn in this step to reduce the risk of injury.</li>
	</ol>
	</li>
	<li>Prepare to cut off lengths that match the desired specimen length (i.e., cut in the warp direction at the specimen length of interest). To obtain a 300 mm gauge length (corresponding to a 600 mm total specimen length), using the procedure and testing grips specified below, keep in mind that the material should now be 600 mm x 610 mm.</li>
	<li>Follow steps 1.9&#8211;1.17 to cut out the desired specimens. 
	NOTE: The protocol can be paused here. If the specimens are not to be used immediately, store them in a dark, ambient location.</li>
</ol>
3. Analysis of cutting methods by scanning electron microscopy
<ol>
	<li>Prepare the samples for an analysis by scanning electron microscopy (SEM) by cutting squares of approximately 5 mm in length and width, preserving at least two edges of the square from the cutting technique of interest. These preserved edges should be identified and are the edges that will be evaluated under the microscope.</li>
	<li>Mount the samples on the SEM sample holder by adhering them with tweezers onto suitable double-sided carbon tape.</li>
	<li>Coat the samples with a thin (5 nm) layer of conductive material, such as gold palladium (Au/Pd), to mitigate surface-charging effects under the scanning electron microscope.</li>
	<li>Load the samples into a scanning electron microscope and image them at about 2 kV of accelerating voltage and with a 50&#8211;100 pA electron current. Apply charge neutralization settings to counter charging effects where necessary.</li>
</ol>
4. Tensile testing of UD laminate specimens
<ol>
	<li>Measure the grips to determine the difference between the crosshead initial location value and the distance between where the specimen contacts the top and bottom grips under minimal tension. Read the crosshead location from the testing software. Calculate an effective gauge length from this by measuring the effective gauge length at this crosshead location. Add the offset (amount of displacement) to the crosshead location to determine the effective gauge length (the measured effective gauge length minus the crosshead location).</li>
	<li>Number the specimens prepared according to sections 1 and 2 with a soft-tipped permanent marker so the order in which they were prepared is clear. Mark other information as well, such as the date of preparation and orientation. 
	NOTE: The specimens used herein have dimensions of 30 mm x 400 mm&#8212;but sample dimensions may vary for other materials&#8212;and were obtained by following either section 1 or section 2. If the specimens are not to be used immediately, store them in a dark, ambient location.</li>
	<li>If the strain will be measured using a video extensometer, manually mark the gauge points with a permanent marker, using a template for consistency, as shown in Supplemental Figure 5a, to give points for the video extensometer to track and, thus, measure strain. If the strain will be calculated from the crosshead displacement, skip this step.</li>
	<li>Load the specimen into the center of the capstan grips.
	<ol>
		<li>Insert the end of the specimen through the gap in the capstan and position the end of the specimen at the grip line drawn in step 1.9, as shown in Supplemental Figure 5b. Take care to center the specimen on the capstan grips by aligning the center of the specimen within approximately 1 mm of the center of the capstan grips.</li>
		<li>Turn the capstan to the desired position, making sure to keep the specimen centered. Use a tensioning device&#8212;for example, a magnet placed on the specimen if the grips are magnetic&#8212;to gently hold the specimen in place, and lock the capstan in place with the locking pins.</li>
		<li>Repeat steps 4.4.1 and 4.4.2 for the other end of the specimen.</li>
	</ol>
	</li>
	<li>Apply a preload of 2 N, or some other suitably small load.</li>
	<li>Record the crosshead displacement/actual gauge length.</li>
	<li>Program the instrument to perform the tensile test, at a constant rate of extension of 10 mm/min, using the video extensometer or crosshead displacement to record the strain, and press start to begin the test.</li>
	<li>Monitor the display and stop the test when the sample has broken, as evidenced by a loss of 90% in the observed load on the display. Record the maximum stress, which is the same as the failure stress due to the nature of the material, and the corresponding failure strain. Repeat steps 4.3&#8211;4.8 for the remaining specimens.</li>
	<li>Save the broken specimens for further analysis.</li>
	<li>Check for stress at failure as a function of specimen number and original specimen placement in the material, as well as other indications of problematic data, for instance, data points that deviate extremely from the Weibull18 distribution, and investigate possible causes, such as samples damaged during preparation or handling, before continuing.</li>
</ol>
5. Preparation of specimens for ageing experiments
<ol>
	<li>Beginning an ageing experiment
	<ol>
		<li>Calculate the total amount of material needed for the study per environmental condition and based on a specimen extraction plan of every month for 12 months. 
		NOTE: For this study, 40 specimens per extraction and a total of 12 extractions were used for planning purposes.</li>
		<li>Cut the total amount of material needed for each condition. Cut each strip wide enough to accommodate the required number of specimens plus at least 10 mm. 
		NOTE: An extra 5 mm of material will be trimmed from each side of the specimen before performing tensile testing. The extra material is used because the edges of the samples may be damaged due to handling during the ageing protocol.</li>
		<li>Place the cut ageing strips in trays to be placed in the environmental chamber as shown in Supplemental Figure 5c. The trays used in this study could each hold approximately 120 strips.</li>
		<li>Select exposure conditions for the environmental study based on the expected use and storage environment of the material2. 
		NOTE: In this study, nominally 70 &#176;C at 76% relative humidity (RH) was used.</li>
		<li>Program an environmental chamber for dry, room temperature conditions (e.g., about 25 &#176;C at 25% RH). Allow the chamber to stabilize at these conditions and, then, place the sample tray on a rack in the chamber, away from the walls and any locations in the chamber that appear to attract condensation.</li>
		<li>Program the environmental chamber to the desired temperature as determined in step 5.1.4, leaving the humidity about 25% RH.</li>
		<li>Once the chamber has stabilized at the target temperature from step 5.1.4, program the chamber to increase the humidity to the desired level as determined in step 5.1.4.</li>
		<li>Check the chambers daily to ensure that water supply and filtration are adequate, and note when out-of-tolerance conditions are observed. Recording deviations and interruptions in a log on the front of each chamber or in a nearby notebook is a good practice.</li>
		<li>Repeat steps 5.1.5&#8211;5.1.8 for all other specimens of interest.</li>
	</ol>
	</li>
	<li>Extracting aged material strips for analysis
	<ol>
		<li>When ready to extract the aged material strips from an environmental chamber for analysis, first program the chamber to decrease the relative humidity to approximately 25% RH.</li>
		<li>After the environmental chamber has stabilized at the low-humidity condition, program the temperature to drop to, approximately, room temperature or 25 &#176;C. This step prevents condensation when the chamber door is opened.</li>
		<li>Once the environmental chamber has stabilized at the conditions of step 5.1.5, open the chamber, remove the tray containing the aged material strips of interest, take out the desired strips, and place them in a labeled container.</li>
		<li>Return the tray to the environmental chamber.</li>
		<li>Following the procedure given in steps 5.1.6 and 5.1.7, return the chamber to the conditions of interest, if continuing the ageing study. If not, then it may remain at the nominally ambient state.</li>
		<li>Record the extraction on the chamber log, if one is being used.</li>
		<li>Cut the aged specimens from the aged material strips, following steps 1.7&#8211;1.17.</li>
		<li>Test the specimens as described in section 4.</li>
	</ol>
	</li>
</ol>

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The full description of the procedures used in this paper requires the identification of certain commercial products and their suppliers. The inclusion of such information should in no way be construed as indicating that such products or suppliers are endorsed by NIST or are recommended by NIST or that they are necessarily the best materials, instruments, software or suppliers for the purposes described.

Proper determination of the fiber direction is critical. The advantage of the method described in steps 1.4&#8211;1.6 of the protocol is that there is complete control over how many fibers are used to start the separation process. However, this does not mean that there is a complete control over the final separated region&#8217;s width, as the fibers are not fully parallel and can cross over each other. In the process of separating one batch of fibers, frequently, fibers neighboring those being separated will also be sep...

The National Institute of Standards and Technology (NIST) helps law enforcement and criminal justice agencies ensure that the equipment they purchase and the technologies that they use are safe, dependable, and highly effective, through a research program addressing the long-term stability of high-strength fibers used in body armor. Prior work1,2has focused on the field failure of a body armor made from the material poly(p-phenylene-2,6-benzobisoxazole), or PBO, which led to a major revision to the National Institute of Justice&#8217;s (NIJ&#8217;s) body armor standard3. Since the release of this revised standard, work has continued at NIST to examine mechanisms of ageing in other commonly used fibers such as ultra-high-molar-mass polyethylene (UHMMPE)4 and poly(p-phenylene terephthalamide), or PPTA, commonly known as aramid. However, all of this work has focused on the ageing of yarns and single fibers, which is most relevant for woven fabrics. However, many body armor designs incorporate UD laminates. UD laminates are constructed of thin fiber layers (&#60;0.05 mm) where the fibers in each layer are parallel to each other5,6,7 and the armor is constructed by stacking the thin sheets in alternating orientations, as depicted in Supplemental Figure 1a. This design relies heavily on a binder resin to hold the fibers in each layer generally parallel, as seen in Supplemental Figure 1b, and maintain the nominally 0&#176;/90&#176; orientation of the stacked fabrics. Like woven fabrics, UD laminates are typically constructed out of two major fiber variations: aramid or UHMMPE. UD laminates provide several advantages to body armor designers: they allow for a lower-weight armor system compared to those using woven fabrics (due to strength loss during weaving), eliminate the need for woven construction, and utilize smaller diameter fibers to provide a similar performance to woven fabrics but at a lower weight. PPTA has previously been shown to be resistant to degradation caused by temperature and humidity1,2, but the binder may play a significant role in the performance of the UD laminate. Thus, the overall effects of the use environment on PPTA-based armor are unknown8.
To date, only very preliminary work has been performed to characterize the aging of the binder resins used in these UD laminates and the effects of binder aging on the ballistic performance of the UD laminate. For example, during the development of the conditioning protocol used in NIJ Standard-0101.06, UD laminates showed visual signs of delamination and reductions in V50 after ageing1,2,8. These results demonstrate the need for a thorough understanding of the material properties with ageing, in order to evaluate the material&#8217;s long-term structural performance. This, in turn, necessitates the development of standardized methods to interrogate the failure properties of these materials. The primary goals of this work are to explore methods and best practices for accurately testing the mechanical properties of UD laminate materials and to propose a new test methodology for these materials. Best practices for ageing UD laminate materials are also described in this work.
The literature contains several examples of testing the mechanical properties of UD laminates after hot-pressing multiple layers into a hard sample9,10,11. For rigid composite laminates, ASTM D303912 can be used; however, in this study, the material is approximately 0.1 mm thick and not rigid. Some UD laminate materials are used as precursors to make rigid ballistic protective articles such as helmets or ballistic-resistant plates. However, the thin, flexible UD laminate can also be used to make body armor9,13.
The objective of this work is to develop methods for exploring the performance of the materials in soft body armor, so methods involving hot pressing were not explored because they are not representative of the way the material is used in soft body armor. ASTM International has several test-method standards relating to testing strips of fabric, including ASTM D5034-0914 Standard Test Method for Breaking Strength and Elongation of Textile Fabrics (Grab Test), ASTM D5035-1115 Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method), ASTM D6775-1316 Standard Test Method for Breaking Strength and Elongation of Textile Webbing, Tape and Braided Material, and ASTM D395017 Standard Specification for Strapping, Nonmetallic (and Joining Methods). These standards have several key differences in terms of the testing grips used and the specimen size, as mentioned below.
Methods described in ASTM D5034-0914 and ASTM D5035-1115 are very similar and focus on testing standard fabrics rather than high-strength composites. For the tests in these two standards, the jaw faces of the grips are smooth and flat, although modifications are allowed for specimens with a failure stress greater than 100 N/cm to minimize the role of stick-slip-based failure. Suggested modifications to prevent slipping are to pad the jaws, coat the fabric under the jaws, and modify the jaw face. In the case of this study, the specimen failure stress is approximately 1,000&#160;N/cm, and thus, this style of grips results in excessive sample slippage. ASTM D6775-1316 and ASTM D395017 are intended for much stronger materials, and both rely on capstan grips. Thus, this study focused on the use of capstan grips.
Further, the specimen size varies considerably among these four ASTM standards. The webbing and strapping standards, ASTM D6775-1316 and ASTM D395017, specify to test the full width of the material. ASTM D677516 specifies a maximum width of 90 mm. In contrast, the fabric standards14,15 expect the specimen to be cut widthwise and specify either a 25 mm or 50 mm width. The overall length of the specimen varies between 40 cm and 305 cm, and the gauge length varies between 75 mm and 250 mm across these ASTM standards. Since the ASTM standards vary considerably regarding specimen size, three different widths and three different lengths were considered for this study.
The terminology referring to specimen preparation in the protocol is as follows: bolt &#62; precursor material &#62; material &#62; specimen, where the term bolt refers to a roll of UD laminate, precursor material refers to an unwound amount of UD fabric still attached to the bolt, material refers to a separated piece of UD laminate, and specimen refers to an individual piece to be tested.

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			<td height="65" style="height:66px;width:247px;">Capstan Grips</td>
			<td style="width:95px;">Universal grip company</td>
			<td style="width:136px;">20kN wrap grips</td>
			<td style="width:388px;">Capstan grips used in testing</td>
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		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Ceramic knife</td>
			<td style="width:95px;">Slice</td>
			<td align="right" style="width:136px;">10558</td>
			<td style="width:388px;"></td>
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		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Ceramic precision blade</td>
			<td style="width:95px;">Slice</td>
			<td style="width:136px;">00116</td>
			<td></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:247px;">Clamp</td>
			<td style="width:95px;">Irwin</td>
			<td style="width:136px;">quick grip mini bar clamp</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Confocal Microscope</td>
			<td style="width:95px;"></td>
			<td style="width:136px;"></td>
			<td></td>
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		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Cutting Mat</td>
			<td style="width:95px;">Rotatrim&#160;</td>
			<td style="width:136px;"></td>
			<td style="width:388px;">A0 metric self healing cutting mat</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Denton Desktop sputter coater&#160;</td>
			<td style="width:95px;"></td>
			<td style="width:136px;"></td>
			<td>sputter coater</td>
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		<tr height="23">
			<td height="23" style="height:23px;">FEI Helios 660 Dual Beam FIB/SEM</td>
			<td style="width:95px;">FEI Helios</td>
			<td style="width:136px;"></td>
			<td>Scanning electron microscope</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Motorized rotary cutter</td>
			<td style="width:95px;">Chickadee</td>
			<td style="width:136px;"></td>
			<td></td>
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		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Rotary Cutter</td>
			<td style="width:95px;">Fiskars</td>
			<td style="width:136px;">49255A84</td>
			<td></td>
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		<tr height="23">
			<td height="23" style="height:23px;">Stereo Microscope</td>
			<td style="width:95px;">National</td>
			<td style="width:136px;">DC4-456H</td>
			<td></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:247px;">Straight edge</td>
			<td style="width:95px;">McMaster Carr</td>
			<td style="width:136px;">1935A74</td>
			<td></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:247px;">Surgical Scalpel Blade</td>
			<td style="width:95px;">Sklar Instruments</td>
			<td style="width:136px;"></td>
			<td style="width:388px;"></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:247px;">Surgical Scalpel Handle</td>
			<td style="width:95px;">Swann Morton</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Universal Test Machine</td>
			<td style="width:95px;">Instron</td>
			<td align="right" style="width:136px;">4482</td>
			<td>Universal test machine</td>
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		<tr height="23">
			<td height="23" style="height:23px;width:247px;">Utility knife</td>
			<td style="width:95px;">Stanley</td>
			<td style="width:136px;">99E</td>
			<td></td>
		</tr>
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cutting procedures, tensile testing, and ageing of flexible unidirectional composite laminates

Many body armor designs incorporate unidirectional (UD) laminates. UD laminates are constructed of thin (&#60;0.05 mm) layers of high-performance yarns, where the yarns in each layer are oriented parallel to each other and held in place using binder resins and thin polymer films. The armor is constructed by stacking the unidirectional layers in different orientations. To date, only very preliminary work has been performed to characterize the ageing of the binder resins used in unidirectional laminates and the effects on their performance. For example, during the development of the conditioning protocol used in the National Institute of Justice Standard-0101.06, UD laminates showed visual signs of delamination and reductions in V50, which is the velocity at which half of the projectiles are expected to perforate the armor, after ageing. A better understanding of the material property changes in UD laminates is necessary to comprehend the long-term performance of armors constructed from these materials. There are no current standards recommended for mechanically interrogating unidirectional (UD) laminate materials. This study explores methods and best practices for accurately testing the mechanical properties of these materials and proposes a new test methodology for these materials. Best practices for ageing these materials are also described.

Many iterations of cutting and testing were performed to investigate several different variables. Some variables that were examined include the cutting technique and cutting instrument, the testing rate, the specimen dimension, and the grips. One critical finding was the importance of aligning the specimens with the fiber direction. Data analysis procedures (consistency analysis, Weibull techniques, outlier determination, etc.) are discussed below, as are considerations for ageing.
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Cutting Procedures, Tensile Testing, and Ageing of Flexible Unidirectional Composite Laminates

The goal of the study was to develop protocols to prepare consistent specimens for accurate mechanical testing of high-strength aramid or ultra-high-molar-mass polyethylene-based flexible unidirectional composite laminate materials and to describe protocols for performing artificial ageing on these materials.

Many body armor designs incorporate unidirectional (UD) laminates. UD laminates are constructed of thin (&#60;0.05 mm) layers of high-performance yarns, ...

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8.1K Views.  National Institute of Standards and Technology. The goal of the study was to develop protocols to prepare consistent specimens for accurate mechanical testing of high-strength aramid or ultra-high-molar-mass polyethylene-based flexible unidirectional composite laminate materials and to describe protocols for performing artificial ageing on these materials.

Video: Cutting Procedures, Tensile Testing, and Ageing of Flexible Unidirectional Composite Laminates

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Ultrasonic Welding of Thermoplastic Composite Coupons for Mechanical Characterization of Welded Joints through Single Lap Shear Testing

This paper presents a novel straightforward method for ultrasonic welding of thermoplastic-composite coupons in optimum processing conditions. The ultrasonic welding process described in this paper is based on three main pillars. Firstly, flat energy directors are used for preferential heat generation at the joining interface during the welding process. A flat energy director is a neat thermoplastic resin film that is placed between the parts to be joined prior to the welding process and heats up preferentially owing to its lower compressive stiffness relative to the composite substrates. Consequently, flat energy directors provide a simple solution that does not require molding of resin protrusions on the surfaces of the composite substrates, as opposed to ultrasonic welding of unreinforced plastics. Secondly, the process data provided by the ultrasonic welder is used to rapidly define the optimum welding parameters for any thermoplastic composite material combination. Thirdly, displacement control is used in the welding process to ensure consistent quality of the welded joints. According to this method, thermoplastic-composite flat coupons are individually welded in a single lap configuration. Mechanical testing of the welded coupons allows determining the apparent lap shear strength of the joints, which is one of the properties most commonly used to quantify the strength of thermoplastic composite welded joints.

A straightforward procedure for ultrasonic welding of thermoplastic composite coupons for basic mechanical testing is described. Key characteristics of this ultrasonic welding process are the use of flat energy directors for simplified process preparation and the use of process data for the fast definition of optimum processing conditions.

This paper presents a novel straightforward method for ultrasonic welding of thermoplastic-composite coupons in optimum processing conditions. The ...

Testing of Nanoparticle Release from a Composite Containing Nanomaterial Using a Chamber System

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products containing nanomaterials is also increasing. Evaluating the nanomaterial release from products containing nanomaterials is a crucial step in assessing the safety of these products, and has resulted in several international efforts to develop consistent and reliable technologies for standardizing the evaluation of nanomaterial release. In this study, the release of nanomaterials from products containing nanomaterials is evaluated using a chamber system that includes a condensation particle counter, optical particle counter, and sampling ports to collect filter samples for electron microscopy analysis. The proposed chamber system is tested using an abrasor and disc-type nanocomposite material specimens to determine whether the nanomaterial release is repeatable and consistent within an acceptable range. The test results indicate that the total number of particles in each test is within 20% from the average after several trials. The release trends are similar and they show very good repeatability. Therefore, the proposed chamber system can be effectively used for nanomaterial release testing of products containing nanomaterials.

Nanoparticle release is tested using a chamber system that includes a condensation particle counter, an optical particle counter and sampling ports to collect filter samples for microscopy analysis. The proposed chamber system can be effectively used for nanomaterial release testing with a repeatable and consistent data range.

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products ...

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify their influence on void formation in composite parts. Wicking in a fibrous medium described by the Washburn equation was considered equivalent to a flow under the effect of capillary pressure according to the Darcy law. Experimental tests for the characterization of wicking were conducted with both carbon and flax fiber reinforcement. Quasi-unidirectional fabrics were then tested by means of a tensiometer to determine the morphological and wetting parameters along the fiber direction. The procedure was shown to be promising when the morphology of the fabric is unchanged during capillary wicking. In the case of carbon fabrics, the capillary pressure can be calculated. Flax fibers are sensitive to moisture sorption and swell in water. This phenomenon has to be taken into account to assess the wetting parameters. In order to make fibers less sensitive to water sorption, a thermal treatment was carried out on flax reinforcements. This treatment enhances fiber morphological stability and prevents swelling in water. It was shown that treated fabrics have a linear wicking trend similar to those found in carbon fabrics, allowing for the determination of capillary pressure.

An experimental method to measure geometrical parameters and the apparent advancing contact angles describing capillary wicking in unidirectional synthetic and natural fabrics is proposed. These parameters are mandatory for the determination of the capillary pressures that must be taken into account for liquid composite molding (LCM) applications.

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify ...

Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector&#39;s spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

This method aims at locating vertical subsurface defects. Here, we couple a laser with a spatial light modulator and trigger its video input to heat a sample surface deterministically with two anti-phased modulated lines while acquiring highly resolved thermal images. The defect position is retrieved from evaluating thermal wave interference minima.

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering ...

Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional component that should have high electrical conductivity, high mechanical properties, and high productivity.
In this regard, a carbon fiber/epoxy resin composite can be an ideal material to replace the conventional graphite bipolar plate, which often leads to the catastrophic failure of the entire system because of its inherent brittleness. Though the carbon/epoxy composite has high mechanical properties and is easy to manufacture, the electrical conductivity in the through-thickness direction is poor because of the resin-rich layer that forms on its surface. Therefore, an expanded graphite coating was adopted to solve the electrical conductivity issue. However, the expanded graphite coating not only increases the manufacturing costs but also has poor mechanical properties.
In this study, a method to expose fibers on the composite surface is demonstrated. There are currently many methods that can expose fibers by surface treatment after the fabrication of the composite. This new method, however, does not require surface treatment because the fibers are exposed during the manufacture of the composite. By exposing bare carbon fibers on the surface, the electrical conductivity and mechanical strength of the composite are increased drastically.

A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional ...

Preparation of Aligned Steel Fiber Reinforced Cementitious Composite and Its Flexural Behavior

The aim of this work is to present an approach, inspired by the way in which a compass needle maintains a consistent orientation under the action of the Earth&#39;s magnetic field, for manufacturing a cementitious composite reinforced with aligned steel fibers. Aligned steel fiber reinforced cementitious composites (ASFRC) were prepared by applying a uniform electromagnetic field to fresh mortar containing short steel fibers, whereby the short steel fibers were driven to rotate in alignment with the magnetic field. The degree of alignment of steel fibers in hardened ASFRC was assessed both by counting steel fibers in fractured cross-sections and by X-ray computed tomography analysis. The results from the two methods show that the steel fibers in ASFRC were highly aligned while the steel fibers in non-magnetically treated composites were randomly distributed. The aligned steel fibers had a much higher reinforcing efficiency, and the composites, therefore, exhibited significantly enhanced flexural strength and toughness. The ASFRC is thus superior to SFRC in that it can withstand greater tensile stress and more effectively resist cracking.

This protocol describes an approach for manufacturing aligned steel fiber reinforced cementitious composite by applying a uniform electromagnetic field. Aligned steel fiber reinforced cementitious composite exhibits superior mechanical properties to ordinary fiber reinforced concrete.

The aim of this work is to present an approach, inspired by the way in which a compass needle maintains a consistent orientation under the action of the ...

Magnet Assisted Composite Manufacturing: A Flexible New Technique for Achieving High Consolidation Pressure in Vacuum Bag/Lay-Up Processes

This work demonstrates a protocol to improve the quality of composite laminates fabricated by wet lay-up vacuum bag processes using the recently developed magnet assisted composite manufacturing (MACM) technique. In this technique, permanent magnets are utilized to apply a sufficiently high consolidation pressure during the curing stage. To enhance the intensity of the magnetic field, and thus, to increase the magnetic compaction pressure, the magnets are placed on a magnetic top plate. First, the entire procedure of preparing the composite lay-up on a magnetic bottom steel plate using the conventional wet lay-up vacuum bag process is described. Second, placement of a set of Neodymium-Iron-Boron permanent magnets, arranged in alternating polarity, on the vacuum bag is illustrated. Next, the experimental procedures to measure the magnetic compaction pressure and volume fractions of the composite constituents are presented. Finally, methods used to characterize microstructure and mechanical properties of composite laminates are discussed in detail. The results prove the effectiveness of the MACM method in improving the quality of wet lay-up vacuum bag laminates. This method does not require large capital investment for tooling or equipment and can also be used to consolidate geometrically complex composite parts by placing the magnets on a matching top mold positioned on the vacuum bag.

A new technique for applying consolidation pressure on the vacuum bag lay-up to fabricate composite laminates is described. The goal of this protocol is to develop a simple, cost-effective technique to improve the quality of laminates fabricated by the wet lay-up vacuum bag method.

This work demonstrates a protocol to improve the quality of composite laminates fabricated by wet lay-up vacuum bag processes using the recently developed ...

Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, but several challenging points remain, such as a low absorption efficiency due to a thin active layer and low flexibility. We present an advanced method to fabricate a flexible phototransistor array with an improved electrical performance. The outstanding electrical performance is driven by a low dark current owing to deep impurity doping. Stretchable and flexible metal interconnectors simultaneously offer electrical and mechanical stabilities in a highly deformed state. The protocol explicitly describes the fabrication process of the phototransistor using a thin silicon membrane. By measuring I-V characteristics of the completed device in deformed states, we demonstrate that this approach improves the mechanical and electrical stabilities of the phototransistor array. We expect that this approach to a flexible phototransistor can be widely used for the applications of not only next-generation imaging systems/optoelectronics but also wearable devices such as tactile/pressure/temperature sensors and health monitors.

We present a detailed method to fabricate a deformable lateral NIPIN phototransistor array for curved image sensors. The phototransistor array with an open mesh form, which is composed of thin silicon islands and stretchable metal interconnectors, provides flexibility and stretchability. The parameter analyzer characterizes the electrical property of the fabricated phototransistor.

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, ...

Operation of the Collaborative Composite Manufacturing (CCM) System

The automated tape placement and the automated fiber placement (AFP) machines provide a safer working environment and reduce the labor intensity of workers than the traditional manual fiber placement does. Thus, the production accuracy, repeatability and efficiency of composite manufacturing are significantly improved. However, the current AFP systems can only produce the composite components with large open surface or simple revolution parts, which cannot meet the growing interest in small complex or closed structures from industry.
In this research, by employing a 1-degree of freedom (DoF) rotational stage, a 6-RSS parallel robot, and a 6-DoF serial robot, the dexterity of the AFP system can be significantly improved for manufacturing complex composite parts. The rotational stage mounted on the parallel robot is utilized to hold the mandrel and the serial robot carries the placement head to mimic two human hands that have enough dexterity to lay the fiber to the mandrel with complex contour.
Although the CCM system increases the flexibility of composite manufacturing, it is quite time-consuming or even impossible to generate the feasible off-line path, which ensures uniform lay-up of subsequent fibers considering the constraints like singularities, collisions between the fiber placement head and mandrel, smooth fiber direction change and keeping the fiber placement head along the norm of the part&#39;s surface, etc. Moreover, due to the existing positioning error of the robots, the on-line path correction is needed. Therefore, the on-line pose correction algorithm is proposed to correct the paths of both parallel and serial robots, and to keep the relative path between the two robots unchanged through the visual feedback when the constraint or singularity problems in the off-line path planning occur. The experimental results demonstrate the designed CCM system can fulfill the movement needed for manufacturing a composite structure with Y-shape.

Operation of the Collaborative Composite Manufacturing CCM System

A collaborative composite manufacturing system is developed for robotic lay-up of composite laminates using the prepreg tape. The proposed system allows the production of composite laminates with high levels of geometrical complexity. The issues in the path planning, coordination of the robots and control are addressed in the proposed method.

The automated tape placement and the automated fiber placement (AFP) machines provide a safer working environment and reduce the labor intensity of ...