Observations of compression and fracture in polymer networks subjected to impact loading
Introduction
Epoxy resins are used in a wide variety of structural applications such as fiber reinforced composites and adhesives. The fracture behavior of these materials is relevant across many orders of magnitude in rate, from long term creep behavior, to the high rates associated with impact. The mechanisms associated with the inherent toughness [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], as well as methods to improve the toughness [11], [12], have been studied extensively. These studies typically involve measurements of the quasi-static fracture behavior, or an analysis of fractured materials post-impact in the case of high rate fracture. Several studies have investigated the dynamic crack propagation behavior in single-edge notch bending-type geometries in poly(methyl methacrylate) [13], [14], [15], [16] and other polymers [17], as well as metals [18], finding a complex time dependence on the dynamic stress intensity factor and overall crack propagation. Failure in glass has also been visualized under conditions of plate impact [19], [20]. Due to the rates of deformation and crack propagation associated with ballistic impact as well as the experimental difficulty, relatively few studies have quantified fracture behavior during an impact event.
Recent investigations have explored the use of optically transparent polymer thermosets as standalone materials for protective applications [11], [21], [22], [23]. In addition to traditional epoxy/amine thermosets, a relatively new class of materials based on the ring opening metathesis polymerization (ROMP) of select monomers have shown promise as replacements for traditional polymers used in transparent protective systems such as polycarbonate [24], [25], [26]. These materials have the benefit of being thermosets (with the exception of poly(5-ethylidine-2-norbornene) [poly(ENB)], which allows additional processing methods for manufacturing into complex shapes as well as fiber reinforced composites [27], something not readily achievable with thermoplastics. As improvements in impact performance are realized in thermosetting polymer networks, a detailed understanding of their fracture behavior is necessary for protective systems designers.
In this study, a series of optically transparent polymer networks with well-characterized and systematically varied materials properties (fracture toughness, modulus, ductility, Tg, etc.) were synthesized. The response of these materials to high rate impacts was examined with high speed videography. High speed videography enabled quantification of the formation and propagation of compression fronts and cracks during the ballistic event. As previously predicted [7], the performance of these materials suggests a dependance, in part, on the quasi-static fracture toughness (KIC). Unlike quasi-static fracture toughness measurements, however, these dynamic measurements enabled characterization of the crack propagation in multiple directions. This revealed that a nanoscale phase-separated epoxy, which has previously been shown to exhibit high performance due to its heterogeneity, exhibits an unexpectedly low crack velocity inwards towards the projectile given its KIC. These experiments also suggest the presence of a deformation zone directly behind the projectile which appears to exhibit enhanced fracture toughness during the ballistic event, as evidenced by a reduced crack length in this region.
Section snippets
Experimental
A series of epoxy-amine networks were synthesized by mixing stoichiometric amounts of DGEBA (diglycidyl ether of bisphenol-A) from Miller-Stephenson with a series of Huntsman Jeffamine diamines, along with 4,4′-Methylenebis(cyclohexylamine) (PACM), which was purchased from Sigma Aldrich. The chemical structures are shown in Fig. 1. A mixed-amine system: DGEBA/PACM/D2000 used 50 vol% amines (PACM + D2000) with a stoichiometric amount of DGEBA. Additionally, two ring-opening metathesis
Results and discussion
In order to better understand the material responses and their physical origins as related to our impact problem CTH modeling tracked the material coordinates and the pressure throughout the impact event. The results, shown in Fig. 3, Fig. 4 are intended only to convey material response generalities as parameters such as material strength and failure threshold are spanned across a broad range. The simulations are not expected to be predictive because substitute material models were used. The
Conclusion
In this article, high speed videography was used to visualize and quantify the failure modes in a series of amine-cured epoxy resins, as well as two relatively new materials based on Ring Opening Metathesis Polymerization. Careful sample preparation and the inherent optical transparency of the materials enabled highly temporally resolved quantification of the ballistic event through high speed videography. The propagation of compression fronts at the early stages of impact were observed,
Acknowledgement
Funding for this project was provided by the U.S. Army Combat Capabilities Development Command Army Research Laboratory, AH-80.
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