John J. Lesko

Characterization of fatigue and combined environment on durability performance of glass/vinyl ester composite for infrastructure applications

Abstract As composite materials find increased use in infrastructure applications, where design lives are typically much longer than those in aerospace, the issue of durability becomes more critical. The tolerance of composites to damage induced by cyclic loading and moisture ingress is of utmost importance. This study highlights the effects of short-term cyclic moisture aging on the strength and fatigue performance of a glass/vinyl ester pultruded composite system. In particular, it addresses the change in quasi-static properties and tension–tension (R=0.1) fatigue behavior of a commercial glass/vinyl ester system in fresh and salt water. The quasi-static tensile strength was seen to reduce by 24% at a moisture concentration of 1% by weight. This reduction in strength was not recoverable even when the material was dried, suggesting that the exposure to moisture caused permanent damage in the material system. Even though the fatigue damage process of the unaged or ‘as-delivered’, fresh-water- and salt-water-saturated material was similar, the cyclic moisture absorption–desorption experiments altered the fatigue performance of the composite system tested. Results were consistent with Mandells postulate that fatigue failure in glass-fiber-reinforced polymeric composites is a fiber-dominated mechanism with a characteristic slope of 10% UTS/decade.

Fatigue performance of carbon fibre/vinyl ester composites : the effect of two dissimilar polymeric sizing agents

Abstract Carbon fibre/vinyl ester composites were made from sized carbon fibres. The carbon fibres were sized with an in-house sizing process and then formed into a unidirectional fabric. This fabric was processed into composite panels by a resin film infusion (RFI) technique. The effects of two dissimilar sizing agents—a brittle thermoplastic K-17 poly(vinyl pyrrolidone) (PVP) and a ductile thermoplastic polyhydroxyether (phenoxy resin)—on notched fatigue, short-beam shear, transverse flexure properties and compression properties were studied. The fatigue properties of carbon fibre/vinyl ester composites were influenced drastically by the type of sizing agent used. A 20-fold increase in lifetime was demonstrated at a loading level of 207 MPa for the ductile phenoxy-sized composite compared with the unsized composite. The brittle PVP-sized composite panel showed a sixfold increase in lifetime compared with the unisized case. The phenoxy-sized composite panel showed a 40% increase in flexural modulus and the PVP-sized panel showed a 20% increase compared with the unsized composite panels. Negligible differences in the shear strength, flexural strength and static compressive strength were observed for the different interphase agents. The implications of these results for tailoring polymeric interphases in vinyl-ester-matrix composites are discussed.

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.

Post-Curing Effects on Marine VARTM FRP Composite Material Properties for Test and Implementation

Structural composites are increasingly being utilized in many large naval and civil structures where it is vital that their long-term performance be predictable and their variability definable over the life of the structure. However, these properties may be influenced by the degree of cure of the resin, particularly for room-temperature-cured systems. Thus, this investigation defines the postcure effects on E-glass/vinyl-ester fiber-reinforced polymer (FRP) composites manufactured using the vacuum-assisted resin transfer molding (VARTM) method, which are typical of those used by the US Navy for ship structures. The composites are differentiated by varying levels of postcure temperature and duration, and examined for the effects of advancing cure at various points in the time after postcure. Pseudo-quasi-isotropic [0/+45/90/-45/0]s and angle ply laminate [±45]2s samples from each level of postcure are examined at 1, 10, 30, 100, and 300 days after postcure in order to track strength, stiffness, failure strain, creep, and fatigue performance as functions of time. In parallel, the matrix polymer is inspected using FTIR (Fourier transform infrared spectroscopy) to directly assess the degree of conversion. Dynamic mechanical analysis and shrinkage measurements are also undertaken to assess the Tg and the amount of shrinkage undergone during post-curing, as well as the advancing of the level of cure during the prescribed aging time. Results suggest that the degree of conversion is limited to 80% for the vinyl-ester oligomer and 90-95% for styrene following a postcure of 93°C. It is observed that after 300 days of ambient storage the nonpostcured samples approach the degree of conversion exhibited by those postcured at 93°C, as measured by FTIR. Resin dominated quasi-static properties are greatly affected by the degree of cure, whereas fiber dominated properties are not. Where the degree of cure is comparatively low, viscoelastic properties cause greater changes in creep response as well as influencing fatigue performance.

Sizing of carbon fibres with aqueous solutions of poly(vinyl pyrollidone)

Abstract A sizing apparatus to coat carbon fibres with aqueous water-dispersible polymeric interphases was developed. Theapparatus utilizes a high level of fibre spreading achieved through control of fibre tension and the application of rollers. A statistical process model was developed to relate the mean sizing level to the process independent variables. Pyrolysis in a nitrogen atmosphere was developed as a quantitative sizing level determination technique for the polymer poly(vinyl pyrollidone) (PVP). Scanning electron microscopy demonstrated that the PVP was deposited on to the fibres in a highly uniform manner. The sizing process along with the statistical process model and the characterization techniques allow for the precise development of interphase materials which are tailored for optimal composite performance.

Fiber Fracture in Unidirectional Composites

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.

Interdiffusion at the interface between poly(vinylpyrrolidone) and epoxy

The study of polymer-polymer interfaces is recently attracting great interest. So far, most studies have focused on the interface between thermoplastic polymers, even though the interface between thermoplastic and thermoset polymers is also very important in numerous areas such as adhesion and composites. In the present study, bilayer films of thermoplastic poly(vinylpyrrolidone) and a thermoset epoxy were prepared and their compositional profiles at the interface were examined by electron microprobe analysis.

An Evaluation of Chemical Aging/Oxidation in High Performance Composites Using the Vickers Micro-Indentation Technique

Vickers micro-indentation was utilized to follow the changes in micro-hardness and thereby evaluate the chemical aging/oaidation phenomena in two different high performance polymeric composite systems. The two systems under study were1M8/954-2, a carbon fiber-reinforced thermoplastic-toughened thermoset composite, and IM8/ITX, a semicrystalline thermoplastic composite. Oxidation profiles with aging, were obtained on [0°]8s (unidirectional) composite specimens by indenn across the thickness of the samples in the fiber direction. Samples were aged up to 6 months in environmental chambers at 150°C in three different environments: an inert nitrogen environment, a reduced air pressure of 13.8 kPa (2 psi air), and ambient air pressure. The experimental results show a considerable drop in overall hardness of both IM8/954-2 and 1M8/ITX composites after a week of aging in all of the three environments, possibly attributable to a thermal stress-reieving effect. Very little additional change was observed after 6 weeks aging. Thereafter, the hardness profiles show drops in hardness starting at the specimen edges initially, and then gradually spread to the specimen interior with time. The above observations were indicative of diffusion controlled oxidation in the samples. Also, the hardness drops in the composites appeared to be sensitive to oxygen partial pressure in the aging environment. Samples aged in ambient air were most affected, those in nitrogen least affected, and specimens aged at the reduced pressure were intermediate. A similar diffusion controlled oxidative process was also seen from Vickers indentation results on aged 1M7/5260 thermoset (bismaleimide) composites. This study is a preliminary effort and one of the first of its kind to study the effect of chemical aging in composites using the micro-indentation technique. This work has left some questions unanswered, nevertheless it has shown potential for utilizing micro-indentation as a tool for studying composite behavior in the future. Some suitable modifications to this test technique could enhance its usefulness, thereby gaining greater insights in the area of composite durability.

Investigation of 3D Moisture Diffusion Coefficients and Damage in a Pultruded E-glass/Polyester Structural Composite

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.

Latent nucleophilic initiators for melt processing phenolic–epoxy matrix composites

Abstract Phenolic–epoxy matrix compositions have been investigated for preparing tough, flame retardant fiber reinforced composites. Melt composite fabrication with these materials requires latent initiators for the curing reaction due to the high viscosities of the matrix resins. The objective is to provide a means for ensuring stability (i.e. no reaction) of the phenolic–epoxy matrix resins at ∼140°C while the matrix is applied to the fiber preforms, then to effect rapid reaction at the cure temperature of ∼180–200°C. We have investigated the strategy of embedding the initiators for matrix cure within the fiber sizing to achieve this goal. The cure times can be significantly reduced since high initiator levels can be employed with this approach. Reaction kinetics were investigated by differential scanning calorimetry to predict composite cure times. Monomeric initiators such as tris(2,4,6-trimethoxyphenyl)phosphine encapsulated in thermoplastic polyimide fiber sizings yielded promising results. Composite toughness and fatigue properties of these flame retardant composites are excellent.