Effect of Polyacrylate Interlayer Microstructure on the Impact Response of Multi-Layered Polymers Joshua S. Stenzler and N. C. Goulbourne Aerospace Engineering Department University of Michigan Soft Materials Research Laboratory Ann Arbor, MI 48109 jstenz@gmail.com Abstract Soft polymers are widely used as interlayers and adhesives in multi-layered systems for defense applications. This research focuses on the large deformation response of several polyacrylate adhesives with varying microstructures. Understanding the mechanical response of these materials under numerous dynamic loading conditions is instrumental in deriving high fidelity constitutive models and optimizing performance when integrated into a multi-layered structure. In recent years, there has been an emphasis on rate dependence research of soft materials, such as thermoplastic polyurethanes/polyureas (moduli order of 3–5 MPa), at quasi-static and very high rates (SHPB), but not intermediate. The goal is to characterize ultra-soft polyacrylates (moduli on the order of 100–600 kPa) through a wide range of strain rates (0.001–1 /s) using different methods and modes of loading. Methods utilized are Instron (servohydraulic and ElectroPULS) tensile and compression testing, and intermediate instrumented impact on multi-laminates. Characteristics of the large, non-linear deformation testing response include: rate dependence, hysteresis, cyclic effects and strain softening. The microstructure and mechanical behavior of the polyacrylate adhesives is related to the capability to modulate impact loads for loading times of approximately 2 milliseconds. 1. Introduction Transparent armor is utilized in military defense applications such as personnel protection (visors/shields/goggles) and air/ground vehicle windows. Typically, these optically clear, multi-layered polymer laminates are required to defeat incoming threats, withstand multiple impacts, and maintain optical clarity with minimal fragmentation and visual distortion for the user [1-3]. A typical configuration of transparent armor consists of multiple tough thermoplastic polymer outerlayers with rubbery interlayer adhesives. For many years, the component materials have consisted of the glassy polymers polycarbonate (PC) and poly(methyl methacrylate) (PMMA) with thermoplastic polyurethane (TPU) adhesives [4-5]. The transparency, toughness, and rate dependence of these three materials are the main reasons they are excellent choices for use in transparent armor to withstand projectile impact. Previous research by this group has compared the impact mechanics of multi-layered, impact resistant, polymer structures with two TPU interlayers and an optically clear polyacrylate [6]. It was shown that when utilized as an interlayer, the substantially more compliant polyacrylate had similar (if not less) fracture than the two TPU adhesives tested. The polyacrylate examined in the previous work is VHB 4905, which is an optically clear, solid acrylic adhesive with no discernible microstructure [6]. Therefore, the goal of this paper is to investigate the effect of three polyacrylates with varying microstructures and mechanical responses. VHB 4905 is included in this study for comparison purposes. Despite these new materials being opaque, a correlation is sought between microstructure and quasi-static material properties of the interlayers to impact performance and energy absorption mechanisms. Relationships of this sort would be useful in tailoring future polymers with superior impact performance that could be optically clear and utilized in transparent armor. Furthermore, the purpose of this research is to examine the role of soft, pressure-sensitive polyacrylate adhesives in the impact response of three-layered PMMA/PC multi-laminates. Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 241 for Experimental Mechanics Series 15, DOI 10.1007/978-1-4419-9794-4_35, © The Society for Experimental Mechanics, Inc. 2011 T. Proulx (ed.), Time Dependent Constitutive Behavior and Fracture/Failure Processes, Volume 3, Conference Proceedings of the Society
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