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  1. Abstract

    The ballistic performance of edge-clamped monolithic polyimide aerogel blocks (12 mm thickness) has been studied through a series of impact tests using a helium-filled gas gun connected to a vacuum chamber and a spherical steel projectile (approximately 3 mm diameter) with an impact velocity range of 150–1300 m s−1. The aerogels had an average bulk density of 0.17 g cm−3with high porosity of approximately 88%. The ballistic limit velocity of the aerogels was estimated to be in the range of 175–179 m s−1. Moreover, the aerogels showed a robust ballistic energy absorption performance (e.g., at the impact velocity of 1283 m s−1at least 18% of the impact energy was absorbed). At low impact velocities, the aerogels failed by ductile hole enlargement followed by a tensile failure. By contrast, at high impact velocities, the aerogels failed through an adiabatic shearing process. Given the substantially robust ballistic performance, the polyimide aerogels have a potential to combat multiple constraints such as cost, weight, and volume restrictions in aeronautical and aerospace applications with high blast resistance and ballistic performance requirements such as in stuffed Whipple shields for orbital debris containment application.

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  2. null (Ed.)
    We report a mechanical metamaterial-like behavior as a function of the micro/nanostructure of otherwise chemically identical aliphatic polyurea aerogels. Transmissibility varies dramatically with frequency in these aerogels. Broadband vibration mitigation is provided at low frequencies (500–1000 Hz) through self-assembly of locally resonant metastructures wherein polyurea microspheres are embedded in a polyurea web-like network. A micromechanical constitutive model based on a discrete element method is established to explain the vibration mitigation mechanism. Simulations confirm the metamaterial-like behavior with a negative dynamic material stiffness for the micro-metastructured aerogels in a much wider frequency range than the majority of previously reported locally resonant metamaterials. 
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  3. null (Ed.)
  4. Debonding at the core–skin interphase region is one of the primary failure modes in core sandwich composites under shear loads. As a result, the ability to characterize the mechanical properties at the interphase region between the composite skin and core is critical for design analysis. This work intends to use nanoindentation to characterize the viscoelastic properties at the interphase region, which can potentially have mechanical properties changing from the composite skin to the core. A sandwich composite using a polyvinyl chloride foam core covered with glass fiber/resin composite skins was prepared by vacuum-assisted resin transfer molding. Nanoindentation at an array of sites was made by a Berkovich nanoindenter tip. The recorded nanoindentation load and depth as a function of time were analyzed using viscoelastic analysis. Results are reported for the shear creep compliance and Young’s relaxation modulus at various locations of the interphase region. The change of viscoelastic properties from higher values close to the fiber composite skin region to the smaller values close to the foam core was captured. The Young’s modulus at a given strain rate, which is also equal to the time-averaged Young’s modulus across the interphase region was obtained. The interphase Young’s modulus at a loading rate of 1 mN/s was determined to change from 1.4 GPa close to composite skin to 0.8 GPa close to the core. This work demonstrated the feasibility and effectiveness of nanoindentation-based interphase characterizations to be used as an input for the interphase stress distribution calculations, which can eventually enrich the design process of such sandwich composites. 
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  5. Abstract

    Resin uptake plays a critical role in the stiffness‐to‐weight ratio of wind turbine blades in which sandwich composites are used extensively. This work examines the flexural properties of nominally half‐inch thick sandwich composites made with polyvinyl chloride (PVC) foam cores (H60 and H80; PSC and GPC) at several resin uptakes. We found that the specific flexural strength and modulus for the H80 GPC sandwich composites increase from 82.04 to 90.70 kN · m/kg and 6.03 to 7.13 MN · m/kg, respectively, with 11.0% resin uptake reduction, which stands out among the four core sandwich composites. Considering reaching a high stiffness‐to‐weight ratio while preventing resin starvation, 32% to 38% and 40% to 45% resin uptakes are adequate ranges for the H80 PSC and GPC sandwich composites, respectively. The H60 GPC sandwich composites have lower debonding toughness than H60 PSC due to stress concentration in the smooth side skin‐core interphase region. The ailure mode of the sandwich composites depends on the core stiffness and surface texture. The H60 GPC sandwich composites exhibit core shearing and bottom skin‐core debonding failure, while the H80 GPC and PSC sandwich composites show top skin cracking and core crushing failure. The findings indicate that an appropriate range of resin uptake exists for each type of core sandwich composite, and that within the range, a low‐resin uptake leads to lighter blades and thus lower cyclic gravitational loads, beneficial for long blades.

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  6. Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability.

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