Publications

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Polymers used in hydrogen transportation, production, storage, and dispensing operations of the hydrogen infrastructure are subject to demanding performance temperatures (-60°C to +140°C) and pressures (0.9 MPa to 87 MPa), under static and cycling conditions of hydrogen exposure. Cycling exposures which include pressurization and depressurization stages can particularly affect properties of these soft materials. Other factors such temperature of exposure in hydrogen environments can also play an influential role on polymer degradation behaviors. In this work, we evaluated the influence of varying rates of depressurization (1, 10, 20, 40 MPa/min and uncontrolled) with model elastomer compounds exposed to high-pressure hydrogen cycling (17 MPa to 87 MPa) at ambient temperature. The goal was to develop an understanding of factors that affect rapid gas decompression in elastomers, which is a phenomenon common in hydrogen fueling operations. Cycling was followed by ex-situ characterization for changes in properties. Dynamic Mechanical Thermal Analysis (DMTA), density, compression set, Attenuated Total Reflectance-FTIR (ATR-FTIR), nanoindentation, and X-ray computer tomography were characterization techniques used to compare polymers before and after cycling. Polymer degradation in the form of internal damage was found to increase with rate of depressurization. EPDM showed the most dependence on rates of depressurization, compared to FKM and HNBR formulations. Additionally, filled, and unfilled model compounds of EPDM, FKM, HNBR, and NBR were tested in high-pressure (17 MPa to 87 MPa) and low-pressure (10 MPa to 31 MPa) cycling conditions at -40°C and +85°C. These experiments were performed at a fixed depressurization rate. The goal of these experiments was to better understand temperature effects under pressure cycling conditions for elastomeric polymer seals. Filled formulations of EPDM, HNBR, and NBR exhibited increased compression set and decreased storage modulus under cold cycling exposures to a greater extent than when cycled at ambient. For filled polymers cycled at low pressures at 85°C and -40°C, FKM showed the most resistance to blistering. HNBR and NBR showed heavy swelling and blistering under both these conditions. Micro CT imaging of one of the polymers (EPDM) subjected to high-pressure cycling at 85°C showed great damage in the form of cracks in the center of the sample. Filled formulations exhibited decreased compression set and storage modulus under hot cycling exposures to a greater extent than with cold and ambient cycling. The findings from these studies will help build a strong understanding of polymer behaviors in cycling hydrogen under rapid gas decompression and thermal conditions encountered in fueling operations and storage. Proper material selection for appropriate use-conditions within components is also enabled.

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Due to challenges in generating high-quality 3D speckle patterns for Digital Volume Correlation (DVC) strain measurements, DVC experiments often utilize the intrinsic texture and contrast of composite microstructures. One common deficiency of these natural speckle patterns is their poor durability under large deformations, which can lead to decorrelation and inaccurate strain measurements. Using syntactic foams as a model material, the effects of speckle pattern degradation on the accuracy of DVC displacement and strain measurements are assessed with both experimentally-acquired and numerically-generated images. It is shown that measurement error can be classified into two regimes as a function of the percentage of markers that have disappeared from the speckle pattern. For minor levels of damage beneath a critical level of damage, displacement and strain error remained near the noise floor of less than 0.05 voxels and 100 με, respectively; above this level, error rapidly increased to unacceptable levels above 0.2 voxels and 10,000 με. This transition occurred after 30%–40% of the speckles disappeared, depending on characteristics of the speckle pattern and its degradation mechanisms. Furthermore, these results suggest that accurate DVC measurements can be obtained in many types of fragile materials despite severe damage to the speckle pattern.

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A sample of natural quartzite rock was submerged in deionized water and illuminated with 1.8-J pulses of 527- nm light with 15-ns duration over an area of 3 cm² [fluence = 0.6 J/cm²]. This relatively low fluence and intensity [40 MW/cm²] were far below the threshold needed for direct ablation via plasma formation or thermal evaporation. With each laser pulse, a small cloud of sub-μm particles was released from the surface and dispersed into the submerging water, forming a long-lived suspension. After one hundred laser pulses, the processing was terminated and the surface of the originally colored quartzite was rendered colorless. The quartzite rock was cut in cross section and the colorless material on the surface was examined with X-ray fluorescence. We report that the transition element Fe was found to be significantly depleted in this colorless layer. This supports the hypothesis that the laser exposure lead to a transient hydrothermal dissolution of the material, followed by a recrystallization process of the SiO₂ that preferentially released iron oxides into the submerging water.

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