Scott W. Case

Modeling of moisture diffusion in the presence of bi-axial damage in polymer matrix composite laminates

Abstract It is now well known that Fick’s Law is frequently inadequate for describing moisture diffusion in polymers and polymer composites. Non-Fickian or anomalous diffusion is likely to occur when a polymer composite laminate is subjected to external stresses that could give rise to internal damage in the form of matrix cracks. As a result, it is necessary to take into account the combined effects of temperature, stress (or strain), and damage in the construction of such a model. In this paper, a modeling methodology based on irreversible thermodynamics applied within the framework of composite macro-mechanics is extended to the case of a bi-axially damaged laminate. The model allows characterization of non-Fickian diffusion coefficients as well as moisture saturation level from moisture weight gain data for laminates with pre-existing and non-evolving damage. A symmetric damage tensor based on continuum damage mechanics is incorporated in this model by invoking the principle of invariance with respect to coordinate transformations. For tractability, the diffusion governing equations are simplified for the special case of a laminate with bi-axially oriented matrix cracks that is subjected to a uniaxial tensile stress. The final equations obtained from this derivation indicate that both effective diffusivity and maximum saturation level for this particular case can be expressed as quadratic functions of crack density. Comparisons with test data for a bi-axially damaged AS4/PR500 graphite/epoxy woven composite are provided for model verifications.

The mechanics of composite strength evolution

Abstract The present paper investigates the concept of evolution of strength in fibrous composite systems. In particular, the conceptual foundations of the ‘critical-element method,’ developed by the authors, are defined and systematically combined to construct a philosophy for the prediction of remaining strength and life of composite materials in the presence of combined mechanical, thermal, and environmental applied conditions that may produce concomitant fatigue, creep, and stress rupture. This philosophy, developed and applied over a fifteen year period, has not been presented in collected form in previous publications. Since Dr. Pagano was a contributor to some of the micromechanical concepts used in the philosophy, the present volume was judged to be an appropriate place for a statement of the philosophy.

The Effect of Mechanical Fatigue on the Lifetimes of Membrane Electrode Assemblies

Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) fuel cells is one of the major technological barriers to the commercialization of fuel cell vehicles. The cracks in the electrode layers of the MEA, referred to as mud-cracks, are potential contributors to the failure in the PEM. To investigate how these mud-cracks affect the mechanical durability of the MEA, pressure-loaded blister tests are performed at 90°C to determine the biaxial fatigue strength of Gore-Primea® series 57 MEA. In these volume-controlled tests, leaking rate is determined as a function of fatigue cycles. The failure is defined to occur when the leaking rate exceeds a specified threshold. Postmortem characterization using bubble point testing and field emission scanning electron microscopy (FESEM) was conducted to provide visual documentation of leaking failure sites. The analysis of the experimental leaking data indicates that the MEA has much shorter lifetimes at the same nominal stress levels than membrane samples without the electrode layers. FESEM photomicrographs of leaking locations identified via the bubble point testing show cracks in the membrane that are concentrated within the mud-cracks of the electrode layer. These two pieces of information indicate that the mud-cracks within the electrode layers contribute to the leaking failures of the MEA assembly. For the fuel cell industry, this study suggests there is an opportunity to reduce the likelihood of membrane pinhole failures by reducing the size and occurrence of the mud-cracks formed during the MEA processing or by increasing the fatigue resistance (including the notch sensitivity) of the membrane material within the MEA.

On the Use of Pressure-Loaded Blister Tests to Characterize the Strength and Durability of Proton Exchange Membranes

The use of pressurized blister specimens to characterize the biaxial strength and durability of proton exchange membranes (PEMs) is proposed, simulating the biaxial stress states that are induced within constrained membranes of operating PEM fuel cells. PEM fuel cell stacks consist of layered structures containing the catalyzed PEMs that are surrounded by gas diffusion media and clamped between bipolar plates. The surfaces of the bipolar plates are typically grooved with flow channels to facilitate distribution of the reactant gases and water by-product. The channels are often on the order of a few millimeters across, leaving the sandwiched layers tightly constrained by the remaining lands of the bipolar plates, preventing in-plane strains. The hydrophilic PEMs expand and contract significantly as the internal humidity, and to a lesser extent, temperature varies during fuel cell operation. These dimensional changes induce a significant biaxial stress state within the confined membranes that are believed to contribute to pinhole formation and membrane failure. Pressurized blister tests offer a number of advantages for evaluating the biaxial strength to bursting or to detectable leaking. Results are presented for samples of three commercial membranes that were tested at 80°C and subjected to a pressure that was ramped to burst. The bursting pressures exhibit significant time dependence that is consistent with failure of viscoelastic materials. Rupture stresses, estimated with the classic Hencky’s solution for pressurized membranes in conjunction with a quasielastic estimation, are shown to be quite consistent for a range of blister diameters tested. The technique shows considerable promise not only for measuring biaxial burst strength but also for measuring constitutive properties, creep to rupture, and cyclic fatigue damage. Because the tests are easily amenable to leak detection, pressurized blister tests offer the potential for characterizing localized damage events that would not be detectable in more commonly used uniaxial strength tests. As such, this specimen configuration is expected to become a useful tool in characterizing mechanical integrity of proton exchange membranes.

GNOSIS: the first instrument to use fiber Bragg gratings for OH suppression

The near-infrared is an important part of the spectrum in astronomy, especially in cosmology because the light from objects in the early universe is redshifted to these wavelengths. However, deep near-infrared observations are extremely difficult to make from ground-based telescopes due to the bright background from the atmosphere. Nearly all of this background comes from the bright and narrow emission lines of atmospheric hydroxyl (OH) molecules. The atmospheric background cannot be easily removed from data because the brightness fluctuates unpredictably on short timescales. The sensitivity of ground-based optical astronomy far exceeds that of near-infrared astronomy because of this long-standing problem. GNOSIS is a prototype astrophotonic instrument that utilizes “OH suppression fibers” consisting of fiber Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47 and 1.7µm. GNOSIS was commissioned at the 3.9m Anglo-Australian Telescope with the IRIS2 spectrograph to demonstrate the potential of OH suppression fibers, but may be potentially used with any telescope and spectrograph combination. Unlike previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion and in a manner that depends purely on wavelength. We present the instrument design and report the results of laboratory and on-sky tests from commissioning. While these tests demonstrated high throughput (� 60%) and excellent suppression of the skylines by the OH suppression fibers, surprisingly GNOSIS produced no significant reduction in the interline background and the sensitivity of GNOSIS+IRIS2 is about the same as IRIS2. It is unclear whether the lack of reduction in the interline background is due to physical sources or systematic errors as the observations are detector noise dominated. OH suppression fibers could potentially impact ground-based astronomy at the level of adaptive optics or greater. However, until a clear reduction in the interline background and the corresponding increasing in sensitivity is demonstrated optimized OH suppression fibers paired with a fiber-fed spectrograph will at least provide a real benefit at low resolving powers. Subject headings: atmospheric effects – infrared: diffuse background – instrumentation: miscellaneous

Relaxation of Proton Conductivity and Stress in Proton Exchange Membranes Under Strain

The stress relaxation and proton conductivity of Nafion 117 membrane (N117-H) and sulfonated poly(arylene ether sulfone) copolymer membrane with 35% sulfonation (BPSH35) in acid forms were investigated under uniaxial loading conditions. The results showed that when the membranes were stretched, their proton conductivities in the direction of the strain initially increased compared to the unstretched films. The absolute increases in proton conductivities were larger at higher temperatures. It was also observed that proton conductivities relaxed exponentially with time at 30°C. In addition, the stress relaxation of N117-H and BPSH35 films under both atmospheric and an immersed (in deionized water) condition was measured. The stresses were found to relax more rapidly than the proton conductivity at the same strains. An explanation for the above phenomena is developed based on speculated changes in the channel connectivity and length of proton conduction pathway in the hydrophilic channels, accompanied by the rotation, reorientation, and disentanglements of the polymer chains in the hydrophobic domains.

Suppression of the near-infrared OH night sky lines with fibre Bragg gratings - first results

The background noise between 1 and 1.8 ?mu m in ground-based instruments is dominated by atmospheric emission from hydroxyl molecules. We have built and commissioned a new instrument, the Gemini Near-infrared OH Suppression Integral Field Unit (IFU) System (GNOSIS), which suppresses 103 OH doublets between 1.47 and 1.7?mu m by a factor of 1000 with a resolving power of 10?000. We present the first results from the commissioning of GNOSIS using the IRIS2 spectrograph at the Anglo-Australian Telescope. We present measurements of sensitivity, background and throughput. The combined throughput of the GNOSIS fore-optics, grating unit and relay optics is 36?per cent, but this could be improved to 46?per cent with a more optimal design. We measure strong suppression of the OH lines, confirming that OH suppression with fibre Bragg gratings will be a powerful technology for low-resolution spectroscopy. The integrated OH suppressed background between 1.5 and 1.7 mu m is reduced by a factor of 9 compared to a control spectrum using the same system without suppression. The potential of low-resolution OH-suppressed spectroscopy is illustrated with example observations of Seyfert galaxies and a low-mass star. The GNOSIS background is dominated by detector dark current below 1.67 mu m and by thermal emission above 1.67 mu m. After subtracting these, we detect an unidentified residual interline component of 860 +/- 210 photons s-1 m-2?arcsec-2?mu m-1, comparable to previous measurements. This component is equally bright in the suppressed and control spectra. We have investigated the possible source of the interline component, but were unable to discriminate between a possible instrumental artefact and intrinsic atmospheric emission. Resolving the source of this emission is crucial for the design of fully optimized OH suppression spectrographs. The next-generation OH suppression spectrograph will be focused on resolving the source of the interline component, taking advantage of better optimization for a fibre Bragg grating feed incorporating refinements of design based on our findings from GNOSIS. We quantify the necessary improvements for an optimal OH suppressing fibre spectrograph design.

Enhanced vibration damping of carbon fibers-ZnO nanorods hybrid composites

In this study, ZnO nanorods are grown on the surface of polyacrylonitrile based carbon fibers using a low temperature hydrothermal synthesis technique. Bi-layered carbon fiber-ZnO nanorod hybrid composite with epoxy matrix is prepared and tested for vibrational attenuations using dynamic mechanical analysis. Results revealed that the growth of ZnO nanorods on top of carbon fiber increases the damping performance by 50% while causing a slight decrease (∼7%) on the storage modulus. The enhanced damping of the hybrid composites can be related to the frictional mechanisms between the ZnO nanorod/epoxy and nanorod/nanorod interfaces combined with piezoelectric effect of ZnO.

Durability of hygrothermally aged graphite/epoxy woven composite under combined hygrothermal conditions

Abstract The objective of this study was to examine the effects of hygrothermal aging on the durability of a graphite/epoxy woven composite material system. The study was part of a larger project in which the objective was to evaluate and model the effects of moisture, temperature, and combined hygrothermal conditions on the strength and life of a graphite/epoxy woven composite material system. The results presented here represent an extension of the work by Patel and Case (Int. J. Fatigue 22 (2000) 809). The hygrothermal aging consisted of cyclical temperature and moisture variations which were meant to simulate mission conditions for an advanced subsonic aircraft. Durability studies were carried out on the aged material system in the form of fatigue and residual strength testing under humid and elevated temperature environments. Damage mechanisms and failure modes were determined through fatigue testing, residual strength testing, and nondestructive evaluation. Changes in physical appearance, thermal analysis results, fracture surfaces, and moisture diffusion behavior all supported the idea that the material was affected by the aging process. However, experimental testing also showed that the initial and residual tensile properties of the aged material were virtually unaffected by the imposed environmental aging (as compared to unaged material testing results), except when tested at elevated temperature. At elevated temperature, both the dynamic stiffness and residual strength were noticeably reduced from that at room temperature.

Durability of a graphite/epoxy woven composite under combined hygrothermal conditions

The objective of this effort was to evaluate and model the effects of moisture, temperature and combined hygrothermal conditions on the strength and life of a graphite/epoxy woven composite material system. The material system specified is a candidate for use in an advanced subsonic aircraft engine, and the imposed environmental conditions were considered to be representative of engine service conditions. Fatigue and residual strength data showed that initial and residual tensile properties and fatigue life of the material were only minimally affected by any of the imposed environmental conditions for the fatigue stress levels considered in this study. Based on this data, it was shown a residual-strength-based life prediction approach could be used to model strength and life with reasonable results. Fatigue damage progression and accumulation were found to be dependent on testing environment, suggesting that more adverse effects of environment on strength and life may be manifested for other types of loading (i.e., off-axis loading).