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As fiber-reinforced polymer (FRP) composites find application in highway bridge structures, methodologies for describing their long-term performance under service loading will be a necessity for designers. The designer of FRP bridge structures will be faced with out-of-plane damage and delamination at ply interfaces. The damage most often occurs between hybrid plys and dominates the lifetime response of a thick-section FRP structure. In this paper a model is developed to address these issues. The methodology employs the quadratic delamination initiation failure criteria, in conjunction with a delamination growth law to describe the out-of-plane damage under bending. These phenomena are combined with the critical element residual strength life prediction tool to determine remaining bending stiffness and moment capacity of a pultruded and hybrid FRP 20.3-cm-deep structural shape. The model successfully describes the onset of delamination prior to fiber failure and suggests that out-of-plane failure controls the life of the structure.

Pinhole formation in proton exchange membranes (PEM) may be caused by a process of flaw formation and crack propagation within membranes exposed to cyclic hygrothermal loading. Fracture mechanics can be used to characterize the propagation process, which is thought to occur in a slow, time-dependent manner under cyclic loading conditions, and believed to be associated with limited plasticity. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric material with limited viscoelastic and plastic dissipation, and has been found to be associated with long-term durability of polymeric materials. Insight into this limiting value of fracture energy may be useful in characterizing the durability of proton exchange membranes, including the formation of pinhole defects. In an effort to collect fracture data with limited plasticity, a knife slit test was adapted to measure fracture energies of PEMs, resulting in fracture energies that were two orders of magnitude smaller than those obtained with other fracture test methods. The presence of a sharp knife blade reduces crack tip plasticity, providing fracture energies that may be more representative of the intrinsic fracture energies of the thin membranes. Three commercial PEMs were tested to evaluate their fracture energies (G(c)) at temperatures ranging from 40 to 90 degrees C and humidity levels varying from dry to 90% relative humidity (RH). Experiments were also conducted with membrane specimens immersed in water at various temperatures. The time temperature moisture superposition principle was applied to generate fracture energy master curves plotted as a function of reduced cutting rate based on the humidity and temperature conditions of the tests. The shift with respect to temperature and humidity suggests that the slitting process is viscoelastic in nature. Also such shifts were found to be consistent with those obtained from constitutive tests such as stress relaxation. The fracture energy is more sensitive to temperature than on humidity. The master curves converge at the lowest reduced cutting rates, suggesting similar intrinsic fracture energies; but diverge at higher reduced cutting rates to significantly different fracture energies. Although the relationship between G, and ultimate mechanical durability has not been established, the test method may hold promise for investigating and comparing membrane resistance to failure in fuel cell environments. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 333-343, 2010

Pinhole defects that form in proton exchange membranes (PEMs) due to the cyclic hygrothermal stresses induced during the operation of a fuel cell and cause gas crossover may be interpreted as a result of crack formation and propagation. The goal of this study is to employ a fracture test to approach the intrinsic fracture energy of a perflourosulfonic acid proton exchange membrane. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric materials with minimal plastic dissipation and the in absence of viscous dissipation, and has been associated with the long-term durability of polymeric materials where subcritical crack growth Occurs under slow time-dependent or cyclic loading conditions. Insights into this limiting value of fracture resistance may offer insights into the durability of PEMs, including the formation of pinhole defects. in order to achieve this goal, a knife slit test which significantly reduces the plastic deformation during the test by limiting the plastic zone size with a sharp blade is conducted. Additionally, double edge notched tension tests and trouser tear tests are conducted to obtain the essential work of fracture and tear energy, respectively. It has been found that although the fracture energy obtained with the knife slit test is still several times larger than the intrinsic fracture energy of regular polymer materials, it is several orders of magnitude lower than those obtained with the other two methods, where process-dependent viscous and plastic dissipation dominate over the intrinsic material property.

The widely used yttria-stabilized zirconia (YSZ) electrolyte is subjected to thermal and external stresses under operation, so that the enhancement of the mechanical properties is an important issue in planar solid oxide fuel cells. Fracture strengths of 8 mol % YSZ electrolytes as 100Ã--100 mm squire plates, 23 mm disks, and 17 mm disks were evaluated using plate tensile, ball-on-ring, and pressure-on-ring testing methodologies, respectively. Finite element analysis (FEA) was validated and used to calculate the stress distribution and peak stress for the biaxial strength tests. A Weibull analysis was carried out on the test/FEA-predicted peak stresses, and Weibull strength, modulus, and material scale parameters were found for each test methodology. The methodologies were compared and evaluated based on the results of the Weibull analysis; the pressure-on-ring test is preferred for brittle thin-film fracture strength testing. ©2005 American Society of Mechanical Engineers

The structural response of continuous fiber, polymer matrix composites to fire exposure is presently of interest to the construction and marine industries. This paper addresses experimentally measured lifetime of E-glass vinyl ester composite laminates subjected to combined centric compression and one-sided simulated fire exposure. Under such conditions, these laminates (having a nearly quasi-isotropic stacking sequence) support a 10 MPa compressive stress under low heat fluxes (20-30 kW/m2) for approximately 102 s. Thermally modified micromechanics and laminate mechanics are successfully used to describe the observed life times when limited to thermal reversible effects. In these cases, the glass transition temperature controls the life the composite.

A theoretical linear-elastic model is used to study the effect of embedded sensors and actuators on a composite's compression strength. An existing analytical model is employed to predict the local undulations in the plies which result from the presence of the embedded devices. Using these results in conjunction with a micromechanical model for compression strength, predictions are made for the reduction in this property due to sensors or actuators. Comparisons are made with experimental data for graphite epoxy systems, and the model is shown to accurately predict the trends in such data. Parametric studies are performed to investigate the effect of embedded sensor/actuator diameter, laminate thickness, laminate stiffness, and laminate shear strength on the compression strength of composites containing these embedded sensors/actuators. It is shown that degradations in a composite's compression strength is as severe for sensors/actuators embedded at 30-degrees as those embedded at 90-degrees to the structural fibers.

The objective of this work is to investigate the crack growth properties of a natural rubber vulcanizate. This work utilizes a modified double cantilever beam (DCB) for measuring the crack propagation in natural rubber. Results of the crack growth in the plane strain dominated DCB are presented and compared to results from plane stress single edge notch test specimens. It is found that the crack growth rate in natural rubber under plane strain conditions is one hundred times faster than under plane stress. The failure surface of the DCB is examined for cavitation occurring within the rubber due to high hydrostatic pressure. No evidence of cavitation was found. (C) 2002 Elsevier Science Ltd. All rights reserved.

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. (C) 2002 Elsevier Science Ltd. All rights reserved.

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). (C) 2000 Elsevier Science Ltd. All rights reserved.

This work addresses issues of long-term durability of hydrogen-air proton exchange membrane fuel cells (PEMFCs) under cyclic current loading conditions, simulating the real road driving conditions for automotives. The same type of membrane-electrode assembly (MEA) was also aged under constant current mode as a control and the results were compared with those of the cyclically aged MEA. Both MEAs were characterized for cell polarizations, impedance spectra, Tafel plots, hydrogen crossover rates as well as electrochemical active surface areas at intervals of 100 h of aging. It was demonstrated that hydrogen crossover increased dramatically after 500 h of current cycling due to pinhole formation and was the most dominant degradation source. The fuel cell approached the end of its useful lifetime after 1000 h of operation. On the other hand, the hydrogen crossover rate remained approximately constant for the MEA under constant current operation. Mass transport limitations were identified as the major source of decreased performance during the constant current operation. This decrease in performance was partially reversible when cathode flooding was resolved by setting the cell at lower current densities. At the end, a phenomenological durability model was established successfully to describe the aging processes and cell performance at different time nodes.

The focus of this research was to investigate the effect of thermal degradation upon the mechanical properties of a natural rubber compound. We examined both the quasi-static and dynamic mechanical properties of a natural rubber vulcanizate which had been subjected to isothermal, anaerobic aging. The thermal aging was conducted between the temperatures of 80 degreesC and 120 degreesC for times ranging from 3 to 24 days. The effect of thermal degradation was measured using the changes in the crosslink distribution of the vulcanizates as functions of time at temperature. A master curve relationship between the crosslink distribution of the vulcanizates due to thermal degradation and the static and dynamic mechanical properties has been developed. It was found that the both the quasi-static and dynamic mechanical properties correlated with the percentage of poly and monosulfidic crosslinks, where in general higher levels of polysulfidic crosslink gave rise to the highest mechanical properties.

The ability to determine the state of strain within a plane is of great importance to the health monitoring of composite materials and structures. In this preliminary study, fiber optic strain sensors are arranged in a rosette configuration to study their effectiveness in monitoring arbitrary strain states within a neat resin and a graphite/epoxy laminate. The extrinsic Fabry Perot Fiber Optic Strain Rosette Sensor (FP-FOSRS) is very capable of measuring axial (0-degrees) and transverse (90-degrees) strains. However, discrepancies are noted between strain measured by the embedded Fabry Perot Sensors and externally mounted strain gages at 45-degrees. Further, it is believed that the compressive strength of FP-FOSRS embedded composites is most affected by the sensors' presence. A compressive strength reduction of 300% is measured. This reduction is compared with the result predicted by a micromechanical model.

In this paper we present an analytical model to predict the compression strength of a composite containing coated, circular fibers. The theoretical development is based on the assumption that shear failure in the matrix governs the compression strength of a unidirectional composite material. The stress concentrations in the matrix are dependent upon the geometry of the fiber and the interphase or coating region around the fiber. Analytical results suggest that an optimum interphase might exist to maximize compression strain to failure. However, in general stiffer coatings appear to provide larger compression strength than do compliant coatings. To evaluate the analytical model, composites containing glass fibers coated with 5 different materials were manufactured. Tests on these samples support the trends predicted by the model and the strong dependence of compression strength predictions on coating properties.

Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEMs) of a fuel cell stack. The strength of the PEM, and its ability to withstand cyclic environment-induced stresses, plays an important role in membrane integrity and consequently, fuel cell durability. In this study, a pressure loaded blister test is used to characterize the biaxial strength of Gore-Select(R) series 57 over a range of times and temperatures. Hencky's classical solution for a pressurized circular membrane is used to estimate biaxial strength values from burst pressure measurements. A hereditary integral is employed to construct the linear viscoelastic analog to Hencky's linear elastic exact solution. Biaxial strength master curves are constructed using traditional time-temperature superposition principle techniques and the associated temperature shift factors show good agreement with shift factors obtained from constitutive (stress relaxation) and fracture (knife slit) tests of the material. (C) 2009 Elsevier B. V. All rights reserved.

A 5-harness satin woven graphite/epoxy composite was studied in both the as-fabricated and a hydrothermally aged condition. The fatigue damage mechanisms for both unaged and aged specimens were characterized using dynamic stiffness loss monitoring during fatigue loading,. The major damage mechanism,, of transverse yarn cracking, inter-yarn debonding, and delamination also were modeled using simple mechanistic models. It as possible to use the dynamic stiffness loss curves along with the simple damage mechanism models to characterize the evolution of transverse yarn cracking and delamination during fatigue, for both the as-fabricated and the aged specimens at room temperature (RT) and at 121 degreesC (250 degreesF). The present analysis predicted the trends in the matrix crack evolution and the delamination growth reasonably well. Based on the experiments and the analysis, the unaged material tested at RT and had the lowest rate of increase in the crack density with fatigue cycles. The unaged material tested at 121 degreesC had the highest rate of crack density increase with cycles. Crack density for the 12,000 hour hygrothermally aged material increased at a rate of 95% high er than the unaged material tested at RT. Delaminations were computed to initiate at higher fatigue cycles and also reach higher delamination levels with decreasing cyclic stress levels. For the same stress level, elevated temperature and aping led to more rapidly increasing and larger delamination lengths. The hydrothermally aged material tested at 121 degreesC had a very low threshold for delamination, which initiated below 1.000 cycles. The aged material tested at 121 degreesC also had the largest delamination length, The models developed in this paper provide a simple means to derive information about fatigue damage mechanisms (which are often difficult to characterize), using stiffness loss measurements, which are quite easy to make.

In this study, three commercially available proton exchange membranes (PEMs) are biaxially tested using pressure-loaded blisters to characterize their resistance to gas leakage under either static (creep) or cyclic fatigue loading. The pressurizing medium, air, is directly used for leak detection. These tests are believed to be more relevant to fuel cell applications than quasi-static uniaxial tensile-to-rupture tests because of the use of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test, in which a bare PEM or catalyst coated membrane is clamped with gas diffusion media and flow field plates and subjected to cyclic changes in relative humidity, because of the flexibility in allowing controlled mechanical loading and accelerated testing. Nafion (R) NRE-211 membranes are tested at three different temperatures and the time-temperature superposition principle is used to construct stress-lifetime master curve. Tested at 90 degrees C, 2%RH extruded Ion Power (R) N111-IP membranes have a longer lifetime than Gore (TM)-Select (R) 57 and Nafion (R) NRE-211 membranes. (C) 2009 Elsevier B.V. All rights reserved.

In this paper, an analysis is developed to predict the stress redistribution in the presence of single and multiple fractured fibers in a unidirectional composite material. This analysis includes the effects of constituent properties, fiber volume fraction, and crack size on the strain concentrations experienced by the adjacent fibers. These effects are not included in other predictions such as shear lag or those of Hedgepeth and Van Dyke. In addition, the predictions are compared with direct experimental measurements obtained from model composite tests.

When a proton exchange membrane (PEM) based fuel cell is placed in service, hygrothermal stresses develop within the membrane and vary widely with internal operating environment. These hygrothermal stresses associated with hygral contraction and expansion at the operating conditions are believed to be critical in membrane mechanical integrity and durability. Understanding and accurately modeling the viscoelastic constitutive properties of a PEM is important for making hygrothermal stress predictions in the cyclic temperature and humidity environment of operating fuel cells. The tensile stress relaxation moduli of a commercially available PEM, Gore-Select (R) 57, were obtained over a range of humidities and temperatures. These tests were performed using TA Instruments 2980 and Q800 dynamic mechanical analyzers (DMA), which are capable of applying specified tensile loading conditions on small membrane samples at a given temperature. A special humidity chamber was built in the form of a cup that encloses tension clamps of the DMA. The chamber was inserted in the heating furnace of the DMA and connected to a gas humidification unit by means of plastic tubing through a slot in the chamber. Stress relaxation data over a temperature range of 40-90 degrees C and relative humidity range of 30-90% were obtained. Thermal and hygral master curves were constructed using thermal and hygral shift factors and were used to form a hygrothermal master curve using the time temperature moisture superposition principle. The master curve was also constructed independently using just one shift factor. The hygrothermal master curve was fitted with a 10-term Prony series for use in finite element software. The hygrothermal master curve was then validated using longer term tests. The relaxation modulus from longer term data matches well with the hygrothermal master curve. The long term test showed a plateau at longer times, suggesting an equilibrium modulus.

As civil engineers seek to improve buildings and bridges, they are turning to composite materials for some structural components. Thus the long-term life of these materials in damp highly acidic conditions (from concrete pore solutions) is critical. This article presents a study of moisture ingression and damage in a pultruded compression element under exposure to pore solution. The goal was to experimentally find the orthotropic diffusion coefficients for pore solution in the composite material and to evaluate the damage in the composite using SEM. A method for calculating 3D diffusion coefficients based on weight measurements by selectively sealing some surfaces against moisture ingression was successfully employed. This research extends the application of the 3D diffusion solution developed by Pierron et al. [3] to selectively sealed specimens of fixed dimension. The Arrhenius equation was then used to model the diffusion coefficients with respect to temperature. For the first time, sequential SEM images were performed in the same location before and after specimen exposure in an attempt to identify damage development separately from initial damage. These images showed little if any change in specimens exposed at room temperature over the first 19 days: however, characteristically different damage was notable at elevated temperature and for a specimen exposed for 650 days.

A life prediction method for ceramic matrix composites is developed. This model is based upon damage mechanics concepts included in the framework of the critical element model. One unique feature of the model is its ability to include general variations of temperature and applied loads as functions of time. A detailed description of application of the model to elevated temperature fatigue is examined. In addition, a validation example is presented that includes the combined effects of rupture and fatigue. The comparison with the experimental data is shown to be good, although the result does appear to be dominated by the rupture effect.

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. (C) 2000 Elsevier Science Ltd. All rights reserved.

In this paper, the problem of a penny-shaped crack in the center of multiple concentric cylinders is considered. By making appropriate choices for the stress functions in each of the constituents and making the standard assumptions of linear elasticity, it is possible to reduce the problem to the solution of a Fredholm integral equation of the second I:ind which may be solved numerically. This solutionis used in conjunction with a geometry approximation to model the stress concentrations due to a broken fiber in a unidirectional composite material. The results suggest that models of such a shear lag may be missing important features of the stress state including surrounding broken fibers in composite materials. Copyright (C) 1996 Elsevier Science Ltd.

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 biaxially 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. (C) 2001 Elsevier Science Ltd. All rights reserved.

One scheme for reliability-based design that is growing in popularity for civil and naval applications is the load and resistance factor design (LRFD). Our goal in this research is the development of a simulation to predict the remaining strength of structural composites subjected to variable fatigue loading and environmental exposure. The results of this simulation can then be used in LRFD to determine appropriate material "knockdown" factors for use in engineering design applications. The work so far focuses on modeling the response of the material to fatigue damage only. The proposed Monte-Carlo style simulation combines a phenomenological residual strength based life prediction model for composites materials with an empirical stiffness based damage accumulation model. This model is demonstrated using data for two glass reinforced vinyl-ester polymer composite systems. The analysis of these results has led to new insight into how the changes in mechanical properties of these materials exposed to fatigue loading can be modeled.

In this manuscript we use a concentric cylinder model to determine the stresses in a composite material subjected to transverse loading. After selecting a failure criterion, it is possible for us to determine a fiber coating which optimizes the strain to failure of the composite material. Experimental results are presented for a model composite system and the results are compared to the analytical predictions. The model is shown to give reasonable agreement with the experimental data.

A key need for the commercial acceptance and viability of composite structural material systems is a methodology for reliability and lifetime predictions. In this paper, we present a philosophy in which the residual strength of the composite material is calculated as a function of applied loading conditions, environment, and resulting failure mode. Two specific example applications are considered: the assessed and verified performance of fiber wound composite tubes and pultruded shapes. We attempt to demonstrate the understanding of the controlling damage mechanisms and the degradation processes that contribute to the loss in stiffness and strength under simulated service conditions.