Ultra-high molecular weight polyethylene (UHMWPE) composites are used in a wide array of protective armor systems. Design of these systems is largely performed through empirical studies which can be costly and time consuming. Modeling tools that enable enhanced performance through exploitation of the design of laminated composite architectures are desired. In this effort, we present a multi-scale, finite element-based representative volume element (RVE) approach that uses laminate mechanics to capture the ply-level material nonlinearity and strain-induced fiber reorientation of UHMWPE composite laminates subjected to low-velocity impact (LVI) loading. The effects of strain rate on the ply-level material response and predicted LVI response of UHMWPE composites are explored. An LVI methodology is developed to characterize the impact performance of thick-section UHMWPE composite materials and applied with the RVE finite element model to calibrate material and delamination properties for Honeywell (R) SpectraShield (R) II SR-3136 and DSM Dyneema (R) HB210. The multi-scale RVE approach accurately captures the peak back-face deformation and extent of delamination for laminates of these materials across three impact energies. The LVI methodology provides a means to evaluate and rank the impact performance of various UHMWPE composite materials, laminate architectures, and processing condi-tions. The LVI methodology coupled with the multi-scale RVE approach produces ply-level behavior and delamination properties linked to materials and processing conditions, ultimately creating a modeling tool that is capable of exploring the design of laminate architectures for UHMWPE composite structures.

In this work, we present a novel approach for improving the delamination resistance and durability of structural composite laminates using compliant thermoplastic polyurethane (TPU) interlayers that allow decoupling of the structural plies within the laminate thus preventing catastrophic delamination. Laminates with different compliant interlayer thicknesses are fabricated and characterized under multiple low velocity impacts (LVI) to demonstrate the significant improvement in delamination resistance that can be achieved. Finite element analysis and experimental data on samples of the interlayer laminates subjected to three-point bending is presented to provide additional insight into the unique decoupling phenomena enabling significant improvements in delamination resistance of structural composite laminates.

The ballistic performance of polydicyclopentadiene (pDCPD) was investigated and compared to two epoxy resins that a have similar glass transition temperature (Tg) to pDCPD. The ballistic performance of these materials (at an effective stain rate of 104–105 s−1) was characterized by determining the kinetic energy of the projectile where there is a 50 % probability that the projectile will penetrate a witness foil behind the sample (KE50). The ballistic performance of pDCPD showed a 300–400 % improvement over the structural epoxy resins. Typical, highly crosslinked epoxy networks become brittle at low temperatures, but pDCPD has a superior ballistic performance over a broad temperature range from (−55 to 75 °C), despite having a glass transition temperature of 142 °C, which characteristic of structural resins. pDCPD also exhibited a room temperature glassy storage modulus of 1.7 GPa, making pDCPD a potential structural resin that can overcome the structural vs. energy dissipation trade-off that commonly exists with some conventional crosslinked polymers. Quasi-static measurements of pDCPD when compared to epoxy resins suggested that the performance of pDCPD relates to higher fracture toughness and lower yield stress relative to typical epoxies, while molecular dynamics simulations comparing pDCPD to epoxy resins suggest that the performance of pDCPD is due to the lack of strong non-covalent interactions and the facile formation of nanoscale voids.

Abstract : A survey of mechanical and impact properties for glass-reinforced polydicyclopentadiene (p-DCPD) matrix composites is undertaken to assess the suitability of the new resin system as a replacement for existing epoxy-matrix systems. The p-DCPD resin system has superior thermal stability and intrinsic fracture-toughness properties that are desirable for Army-unique applications. However, challenges in conventional processing techniques and chemical-sizing compatibility were encountered during the study. Various novel glass fibers and commercially available fabrics were infused with both p-DCPD resin and SC-15 epoxy and mechanical and impact testing performed. The key results indicate p-DCPD composites have lower mechanical and impact performance compared to SC-15 composites due to chemical-sizing incompatibility at the fibermatrix interface. Despite efforts to tweak processing and to develop compatible sizings with industry partner Pittsburgh Plate Glass Industries, the p-DCPD composites had poor interfacial bonding that manifested as significantly lower interlaminar-shear strength. The main conclusion of this study is this: Given the desirable properties of p-DCPD, a program effort should be undertaken to develop a chemical sizing that will be 100 compatible with the p-DCPD resin and allow for processing of composites that have improved or comparable structuralas well as ballisticproperties to epoxy-matrix composites.

Ballistic performance, at effective strain rates of (104–105 s−1), for polymeric dicyclopentadiene (pDCPD) was compared with two epoxy resin/diamine systems with comparable glass transition temperatures. The high rate response was characterized in terms of a projectile penetration kinetic energy, KE50, which describes the projectile kinetic energy at a velocity with a 50% probability of sample penetration. pDCPD showed superior penetration resistance, with a 300–400% improvement in ballistic energy dissipation, when compared with the structural epoxy resins. In addition, unlike typical highly crosslinked networks that become brittle at low temperatures, the improved pDCPD performance occurred over a very broad temperature range (−55 to 75 °C), despite exhibiting a glass transition temperature characteristic of structural resins (∼142 °C). In addition to the high Tg, pDCPD exhibited a room temperature glassy storage modulus of 1.7 GPa, offering the potential to circumvent the common structural versus energy dissipation trade-off encountered with conventional crosslinked polymers. Quasi-static measurements suggested that the performance of pDCPD is phenomenologically related to higher fracture toughness and lower yield stress relative to typical epoxies, while molecular dynamics simulations suggest the origin is the lack of strong non-covalent interactions and the facile formation of nanoscale voids to accommodate strain in pDCPD.

: For the past decade, the U.S. Army has invested a considerable amount of effort into assessing the suitability of plain-weave S2-glass fabric/SC-15 epoxy composites to replace conventional, monolithic materials (e.g., metals) in Army structural and ballistic applications. One performance-limiting material response repeatedly encountered with this composite system is interlaminar delamination under both ballistic and low-velocity impact (LVI). Strategies for improving the delamination resistance of thick-section composites are currently being developed and include the use of thicker composite plies combined with compliant films in the inter-ply region. Specifically, the LVI performance of 3WEAVE S2-glass/SC-15 composites toughened with thermoplastic urethane film inter-layers of various thicknesses are being investigated. This report documents the significant improvements in delamination resistance and durability performance that have been realized by following this strategy.

: Research was conducted on 3-D (three-dimensional) carbon and glass hybrid epoxy composites toughened with thermoplastic polyurethane (TPU) film inter-layers. The study focused on the effect of sample processing methods on impact degradation rates. The newly developed low velocity impact, four quadrant multi-hit testing protocol described in other works by Emerson et. al (2010 and 2011) and Boyd et. al (2011), which are referenced in this report, and has been successfully used to rank thick-section (13 mm) composite materials and is used here. Samples were successfully processed using room temperature, standard molding techniques, and post-cures for the toughened SC-15 epoxy with little to no warpage and defects. The use of thicker sub-layers and strengthening of the composite interfaces with TPU inter-layers arrested damage progression, reduced overall delamination, and lowered stiffness degradation per impact. However, a mismatch in carbon and glass fiber properties in the hybrid samples produced extensive localized damage per impact and had the net effect of reducing sample stiffness.

: In this work, we tested 2-D and 3-D woven S2-glass/SC-15 epoxy composite laminates with compliant interlayer and needling interlaminar enhancements for single impact performance, compression-after-impact strength, and bend and shear fatigue life. The interlaminar enhancements decreased the strength and stiffness of the material but improved the durability. Needling had a larger improvement in compression-after-impact strength than the compliant interlayer, while the compliant interlayer had a larger improvement in stress-versus-fatigue-life slope than the needling.