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

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 work^1,2has 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’s (NIJ’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 (<0.05 mm) where the fibers in each layer are parallel to each other^5,6,7 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°/90° 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 humidity^1,2, 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 ageing^1,2,8. These results demonstrate the need for a thorough understanding of the material properties with ageing, in order to evaluate the material’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 sample^9,10,11. 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 armor^9,13.

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 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 standards^14,15 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 > precursor material > material > 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.