Polymer Specimens Research Articles

High Temperature Performance of Concrete Confinement by MWCNT Modified Epoxy Based Fiber Reinforced Composites.

The conventional method of fiber reinforced polymer (FRP) wrapping around concrete columns uses epoxy as the binder along with synthetic or natural fibers such as carbon, glass, basalt, jute, sisal etc. as the reinforcement. However, the thermal stability of epoxy is a major issue in application areas prone to fire exposure. The current work addressed this major drawback of epoxy by modifying it with a nanofiller, such as multiwalled carbon nanotubes (MWCNT), and reinforcing it using basalt and sisal fibers. The effect of exposure to elevated temperature on the behavior of concrete cylinders externally confined with these FRP systems was analyzed. Three types of specimens were considered: unconfined; confined with sisal fiber reinforced polymer (SFRP); and confined with hybrid sisal basalt fiber reinforced polymer (HSBFRP) specimens. The test samples were exposed to elevated temperature regimes of 100 °C, 200 °C, 300 °C and 400 °C for a period of 2 h. The compressive strengths of unconfined specimens were compared with various confined specimens, and from the test results, it was evident that the mechanical and thermal durability of the FRP systems was substantially enhanced by MWCNT incorporation. The reduction in the compressive strength of the FRP-confined specimens varied depending on the type of the confinement. After two hours of exposure at 400 °C, the compressive strength corresponding to the epoxy-HSBFRP-confined specimens were improved by 15%, whereas a 50% increase in strength corresponding to MWCNT-incorporated epoxy-HSBFRP-confined specimens was observed with respect to unconfined unexposed specimens. The MWCNT-modified epoxy-incorporated FRP-confined systems demonstrated superior performance even at elevated temperatures in comparison to unconfined specimens at ambient temperatures.

Micromechanics Modeling of Transverse Tensile Strength for Unidirectional CFRP Composite

Transverse tensile strength of unidirectional (UD) composites plays a key role in overall failure of fiber-reinforced composites. To predict this strength by micromechanics, calculation of actual stress in constituent matrix is essentially required. However, traditional micromechanics models can only give the volume-averaged homogenized stress rather than an actual one for a matrix, which in practice will cause large errors. In this paper, considering the effect of stress concentration on a matrix, a novel micromechanics method was proposed to give an accurate calculation of the actual stress in the matrix for UD composite under transverse tension. A stress concentration factor for a matrix in transverse tensile direction is defined, using line-averaged pointwise stress (obtained from concentric cylinder assemblage model) divided by the homogenized quantity (obtained from a bridging model). The actual stress in matrix is then determined using applied external stress multiplied by the factor. Experimental validation on six UD carbon fiber-reinforced polymer (CFRP) specimens indicates that the predicted transverse tensile strength by the proposed method presents a minor deviation with an averaged relative error of 5.45% and thus is reasonable, contrary to the traditional method with an averaged relative error of 207.27%. Furthermore, the morphology of fracture section of the specimens was studied by scanning electron microscopy (SEM). It was observed that different scaled cracks appeared within the matrix, indicating that failure of a UD composite under transverse tension is mainly governed by matrix failure. Based on the proposed approach, the transverse tensile strength of a UD composite can be accurately predicted.

Rapid depolymerization of poly(ethylene terephthalate) thin films by a dual-enzyme system and its impact on material properties

Enzymatic hydrolysis holds great promise for plastic waste recycling and upcycling. The interfacial catalysis mode, and the variability of polymer specimen properties under different degradation conditions, add to the complexity and difficulty of understanding polymer cleavage and engineering better biocatalysts. We present a systemic approach to studying the enzyme-catalyzed surface erosion of poly(ethylene terephthalate) (PET) while monitoring/controlling operating conditions in real time with simultaneous detection of mass loss and changes in viscoelastic behavior. PET nanofilms placed on water showed a porous morphology and a thickness-dependent glass transition temperature (Tg) between 40°C and 44°C, which is >20°C lower than the Tg of bulk amorphous PET. Hydrolysis by a dual-enzyme system containing thermostabilized variants of Ideonella sakaiensis PETase and MHETase resulted in a maximum depolymerization of 70% in 1h at 50°C. We demonstrate that increased accessible surface area, amorphization, and Tg reduction speed up PET degradation while simultaneously lowering the threshold for degradation-induced crystallization.

The Influence of Mechanical Stress Micro Fields around Pores on the Strength of Elongated Etched Membrane.

The investigation of the mechanical properties of polymer track-etched membranes (TMs) has attracted significant attention in connection with the extended region of their possible applications. In the present work, the mechanical stress fields around the pores of an elongated polyethylene terephthalate TM and around the 0.3 mm holes in model polymer specimens were studied in polarized light and with the finite element method. A break-up experiment showed the controlling role of stress field interaction in the forming of a microcrack system and the generation of a main crack with rupture of the TM (or model pattern). This interaction depended on the relative distance between the pores (holes) and their orientation. The results of the calculations of the pore distribution function over the surface of the TM via the net method and wavelet analysis are presented. The fractal character of pore distribution was established. The geometric characteristics of the TM pore system as initial defects are inherited by obtaining TM-based composites.

Mechanical Properties and Coagulation Characteristics of Flue Gas Desulfurization Gypsum-Based Polymer Materials.

To resolve problems caused by the accumulation of flue gas desulfurization gypsum (FGDG) in the environment, a polymer material was prepared using FGDG, granulated blast furnace slag (GBFS), fly ash (FA), and solid sodium silicate (SSS). The compressive strength of these polymer specimens cured for 3, 28, and 60 d was regularly measured, and their condensation behavior was analyzed. Both the formation behavior of mineral crystals and microstructure characteristics were analyzed further using X-ray diffraction and scanning electron microscopy. The compressive strength of pure FGDG polymer specimen (whose strength is generated by particle condensation crystallization) is insufficient and the condensation is slow. The addition of appropriate amounts of GBFS, FA, and SSS can continuously and considerably improve the compressive strength and shorten the setting time. The optimal proportions of FGDG, GBFS, and FA are 50%, 20%, and 30%, respectively, with the SSS addition amount of 20 g. The incorporation of GBFS, FA, and SSS can promote the polymerization of calcium, silicon, and aluminum in FGDG to form silicate and aluminosilicate minerals. Their formation is the main reason for the increased compressive strength and accelerated coagulation.

Comparative study of biomaterial surface modification due to subcritical CO2 and autoclave disinfection treatments

The decontamination of medical tools is a sustainable alternative to the use of disposable items. Sub- and supercritical fluids can be applied to decontaminate the surfaces of most biomedical materials with relative instrumental simplicity and moderate capital and operational costs, and offers a promising alternative to autoclavation. For future adaption it is important to assess the extent of material surface degradation, since CO2 has corrosion potential against metals and is soluble in polymers, and also forms a weak acid in the presence of residual humidity. Experimental results are reported for subcritical CO2 (37 oC, 50 bar, 2 h) decontamination of surgical stainless steel, Ti-6Al-4V alloy, PEEK polymer, ZrO2 ceramic and natural tooth enamel specimens seeded with E. coli, St. aureus, Pr. intermedia and C. albicans sealed inside polymer-paper packages during 1, 2, 5 and 10 cycles of treatment. Characterization by SEM and nanoindentation revealed that detectable alterations could be found only after the first cycle of treatment, whilst subsequent 10 treatment cycles did not cause further change. The findings provide a good basis for ongoing research into the potential of the supercritical fluid decontamination.

Mechanisms controlling the hydraulic conductivity of anionic polymer-enhanced GCLs

The hydraulic conductivity (k) of specimens of enhanced-bentonite geosynthetic clay liners (EB-GCLs) comprising anionic polymers permeated with two concentrated salt solutions, 500 mM NaCl and 167 mM CaCl2, was measured to determine the effects of polymer properties and specimen preparation method on the k and the associated roles of polymer retention and elution in dictating the measured k. The results of hydrogel formation tests illustrated that poly(acrylic acid) hydrogel was formed in solutions tested during EB-GCL hydration. A dry sprinkling method of specimen preparation resulted in low k (≤5.5 × 10−11 m/s) in multiple EB-GCLs, with a low fraction (≤2.5%) of retained polymer. In contrast, polymer elution from EB-GCLs prepared using a dry mixing method resulted in interaggregate seepage and an increase in k. Higher polymer retention occurred for the wet-mixed EB-GCLs, but did not directly correlate to low k. The long-term k of the EB-GCLs is dependent on several factors, including (i) formation of hydrogel, (ii) mobilization of hydrogel into and blocking of the most conductive pores, (iii) balance of seepage forces and hydrogel crosslink bond strength, (iv) kinetics of hydrogel formation, and (v) adsorption of polymer to the surfaces of the bentonite particles or aggregates of particles.

Influence of a Flat Polyimide Inlay on the Propagation of Guided Ultrasonic Waves in a Narrow GFRP-Specimen.

Structural health monitoring systems for composite laminates using guided ultrasonic waves become more versatile with the structural integration of sensors. However, the data generated within these sensors have to be transmitted from the laminate to the outside, where polyimide-based printed circuit boards play a major role. This study investigates, to what extent integrated polyimide inlays with applied sensor bodies influence the guided ultrasonic wave propagation in glass fiber-reinforced polymer specimens. For reasons of resource efficiency, narrow specimens are used. Numerical simulations of a damping-free specimen indicate reflections of the -mode at the integrated inlay. This is validated experimentally with an air-coupled ultrasonic technique and a 3D laser Doppler vibrometry measurement. The experimental data are evaluated with a method including temporal and spatial continuous wavelet transformations to clearly identify periodically occurring wave packages as edge reflections and distinguish them from possible inlay reflections. However, even when separating in-plane and out-of-plane movements using the 3D measurement, no reflections at the inlays are detected. This leads to the conclusion that polyimide inlays are well suited as substrates for printed circuit boards integrated into fiber-reinforced polymer structures for structural health monitoring, since they do not significantly influence the wave propagation.

Compressive behavior and analysis-oriented model of FRP-confined engineered cementitious composite columns

A total of 18 specimens of fiber-reinforced polymer (FRP) confined engineered cementitious composite (ECC) were tested under axial compression in this study. Both monotonic loading and cyclic loading were considered. Effects of different FRP materials, glass FRP (GFRP) tube and carbon FRP (CFRP) jacket, as well as different FRP thicknesses were investigated. All the specimens exhibited the typical strain hardening behavior, indicating the effective enhancement of compressive strength and strain of ECC under FRP confinement. Failure modes, stress–strain behavior and dilation behavior were presented and analyzed. The FRP-confined ECC showed large ultimate axial strains, demonstrating the superior deformability and ductility performance. Design equations were developed to predict the ultimate compressive strength and strain. Based on the test results obtained from this study and other test results collected in the literature, an analysis-oriented model was proposed to predict the overall compressive stress–strain behavior of FRP-confined ECC columns.

Analysis of Permeation and Diffusion Coefficients to Infer Aging Attributes in Polymers Subjected to Supercritical CO2 and H2 Gas at High Pressures.

There is a need to understand the permeation flux behavior of polymers exposed to high-pressure and -temperature fluids continuously for long time intervals. This study investigates evidence of structural alterations in polymer specimens as indicators of material aging through the monitoring of transport coefficients at pressure steps from 10 barg to 400 barg and temperatures ranging between 30 °C and 90 °C. The continuous flow permeation methodology is a well-established technique described in the literature for applications from membrane separation processes to polymeric pressure barriers used for complex fluid containment in the oil and gas industry. In this study, a novel methodology has been used that allows the permeating flux of supercritical CO2 and H2 gas through raised-temperature polyethylene and polyvinylidene fluoride films at varying elevated temperatures and pressures to be determined, over timescales of several months using gas chromatography. During these long-term measurements, changes in the test conditions, principally in temperature and stepwise increases in differential gas pressure, were made in order to determine the activation energy for permeation along with the transport coefficients of permeation, diffusion, and sorption. At no time was the polymer film allowed to outgas during the temperature or pressure alterations. The permeation experiments are complemented by differential scanning calorimetry tests to track changes in polymer crystallinity before and after exposure of the specimen to plasticizing gases, which revealed the extent of structural alterations inflicted on the specimen due to high temperature and pressure loads. It is seen that specimens that were exposed to starting high pressures aged more than those that had gradual increases in feed pressure. Furthermore, the relationship between transport coefficients and fractional free volume in the polymer upon exposure to high pressure and temperature conditions is explored. Lastly, the benefit of using fugacity in place of feed pressure for the calculation of the permeability coefficient is discussed. This study contributes to the understanding of the effect of prolonged exposure of the polymeric specimens to CO2 and H2 gas under stepwise pressure and temperature loading on their flux behaviors and crystallinity, and to candidate polyethylene-based specimens for oil field deployment.

Surface Dielectric Characteristics of GFRP and PTFE in Cryogenic Environment Under the Switching Impulse Superimposed on DC Voltage

For insulation materials utilized in DC superconducting fault current limiter, surface discharge along the interface is more damaging to the insulation relative to penetration breakdown. Therefore, for the reliable and economical insulation design of DC superconducting fault current limiter, it is very important to investigate an insulation materials and study the surface dielectric characteristics. Additionally, DC superimposed switching impulse can cause critical stress to the insulation of DC superconducting fault current limiter and can potentially result in breakdown. Therefore, the insulation materials of DC superconducting fault current limiter must have sufficient surface distance to prevent surface discharge under DC superimposed switching impulse. In this paper, the surface breakdown tests were performed by applying DC superimposed switching impulse to glass fiber reinforced polymer (GFRP) and polytetrafluoroethylene (PTFE) specimen in cryogenic environment. Surface dielectric characteristics according to positive and negative polarities of DC and switching impulse were analyzed. In addition, the surface dielectric characteristics were analyzed by selecting the gap distance. From the experimental results, the surface dielectric strength of GFRP was higher than that of PTFE, but the difference in surface dielectric strength between GFRP and PTFE decreased as the gap distance increased. The surface breakdown voltage of PTFE was lowest at the superposition of positive DC and positive switching impulse. However, GFRP has the lowest surface breakdown voltage when negative DC superimposed positive switching impulse. Consequently, it is possible to design the optimal surface distance of solid insulation materials in DC superconducting current limiter by considering the superposition of DC and switching impulse polarity.

Flexural and fracture behaviors of functionally graded long fibrous polymeric composite beam-like specimens

From the early seventies of the twenty-centenary until now, polymeric composite materials (PCMs) have been a tremendous development in structural engineering applications such as decks and beams of bridges. However, most of the specifications still used only plate-like specimens (breadth ≫ depth) rather than beam-like specimens (depth ≫ breadth). The present work has studied the flexural and fracture behaviors of beam-like specimens made of conventional and functionally graded (FG) PCMs. The three patterns: linear, non-linear, and stepwise/layered FG-PCMs, resulting from the distribution of fiber through the depth, were manufactured from long glass fibers reinforced epoxy. A three-dimensional finite element method was employed to demonstrate the progressive damage process of smooth and matrix-cracked specimens. The present results showed that the FG-PCM beams have higher flexural strength and fracture toughness values for the same fiber volume fraction than those of the conventional beams. In the case of long fiber PCMs, the matrix cracked specimens withstand a higher applied load than the through-thickness (common) cracked specimens confirming that the fracture toughness measured from the common cracked specimens is not applicable. The fracture toughness of PCMs depends on the type of test and the specimen geometry. Numerical predictions showed good agreement with the experimental results.

Interpreting the Lempel–Ziv complexity of acoustic emission signals for identifying damage modes in composite materials

Lempel–Ziv (LZ) complexity has been used predominantly in solving the information theoretic problems. However, its application is way beyond information theoretics. It has been used successfully in different biomedical applications. In this research work, this LZ complexity is used for identifying the damage modes in a plain weave fabric carbon fibre-reinforced polymer (CFRP) specimens tested at an elevated temperature. The acoustic signals generated during the tensile test of CFRP specimens at an elevated temperature are recorded. Their LZ complexity values are extracted and compared with different acoustic descriptors. When LZ complexity is used along with peak amplitude of the acoustic signal and its weighted peak frequency, it is able to distinguish the different damage sources of the signal. The acoustic signals with LZ complexity above 0.6 generally indicate their source as delamination or debonding of the fibres in the CFRP specimen. This multiparameter approach is more efficient in distinguishing the different damage modes in the CFRP specimens.

An Experimental Approach for the Determination of the Mechanical Properties of Base-Excited Polymeric Specimens at Higher Frequency Modes

Structures made of the thermoplastic polymer polyether ether ketone (PEEK) are widely used in dynamically-loaded applications due to their high-temperature resistance and high mechanical properties. To design these dynamic applications, in addition to the well-known stiffness and strength properties the vibration-damping properties at the given frequencies are required. Depending on the application, frequencies from a few hertz to the ultrasonic range are of interest here. To characterize the frequency-dependent behavior, an experimental approach was chosen and applied to a sample polymer PEEK. The test setup consists of a piezoelectrically driven base excitation of the polymeric specimen and the non-contact measurement of the velocity as well as the surface temperature. The beam’s bending vibrations were analyzed by means of the Timoshenko theory to determine the polymer’s storage modulus. The mechanical loss factor was calculated using the half-power bandwidth method. For PEEK and a considered frequency range of 1 kHz to 16 kHz, a storage modulus between 3.9 GPa and 4.2 GPa and a loss factor between 9 × 10−3 and 17 × 10−3 were determined. For the used experimental parameters, the resulting mechanical properties were not essentially influenced by the amplitude of excitation, the duration of excitation, or thermal degrad.ation due to self-heating, but rather slightly by the clamping force within the fixation area.

Portraiture and Double Bond Conversion of a Monomethacrylate-based Oral Prosthetic Resin Substituted with a Novel Tri(azine-acrylate) Cross-linker

To formulate, design, and chemically characterize a novel denture base resin (DBR) copolymer containing triazine-based antimicrobial comonomer and also to evaluate the double bond conversion (DC) in the copolymer with various concentrations of the comonomer by fourier transform infrared (FTIR) spectroscopy. The study groups comprise a control group G0 in which the specimens (n = 10) were polymerized without the triazine comonomer and trial groups G10 and G20 where the polymerized specimens (n = 10 each) contained 10 and 20% triazine comonomer, respectively. FTIR was employed to ascertain and evaluate copolymerization (CP) and DC. The obtained DC values were subjected to statistical analysis. A new denture base copolymer containing antimicrobial triazine comonomer was formed with ascertained copolymerization and higher DC than the control group. Twenty percent triazine comonomer in the copolymer exhibited the maximum DC. Incorporation of the antimicrobial comonomer copolymerized with DBR to form a novel denture base copolymer exhibiting high DC. The novel denture base copolymer may prevent the microbial adhesion on the denture surface thereby preventing denture-induced stomatitis in the edentulous patients. Nonetheless, this novel copolymer may enhance the other necessary properties of the DBR and would ameliorate the living quality of the senile geriatric population with good in vivo serviceability.

Manifold learning and segmentation for ultrasonic inspection of defects in polymer composites

Non-destructive ultrasonic testing is beneficial for monitoring the structural health of polymer composites. However, owing to scattering and other factors, ultrasonic data often appear as noisy signals or images containing artifacts. The analysis of ultrasound signals highly depends on the expertise of trained human inspectors. Hence, the development of ultrasonic data analysis methods, particularly unsupervised methods, is necessitated. In this study, a novel unsupervised method is developed for the ultrasonic inspection of defects in polymer composites, named manifold learning and segmentation. In a uniform manifold approximation and projection model, nonlinear dimensionality reduction is first performed on high-dimensional ultrasound data for extracting and visualizing defect features. Subsequently, semantic segmentation is performed to predict/discriminate between defects and backgrounds. Consequently, subsurface defects in the composites can be effectively detected. Experimental results and comparisons on two carbon fiber reinforced polymer specimens demonstrate the effectiveness of the proposed method.

Size Effect on Mode I Fracture Resistance of Polymeric Rounded-Tip V-Notched Specimens Using the Modified Point Stress Criterion

Size Effect on Mode I Fracture Resistance of Polymeric Rounded-Tip V-Notched Specimens Using the Modified Point Stress Criterion

Photoinitiator Selection and Concentration in Photopolymer Formulations towards Large-Format Additive Manufacturing.

Photopolymers are an attractive option for large-format additive manufacturing (LFAM), because they can be formulated from structural thermosets and cure rapidly in ambient conditions under low-energy ultraviolet light-emitting diode (UV LED) lamps. Photopolymer cure is strongly influenced by the depth penetration of UV light, which can be limited in the 2–4 mm layer thicknesses typical of LFAM. Photoinitiator (PI) systems that exhibit photobleaching have proven useful in thick-section cure applications, because they generate a photoinitiation wavefront, but this effect is time-dependent. This study investigates the light transmission and through-thickness cure behavior in (meth)acrylate photopolymer formulations with the photobleaching initiator bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (BAPO). Utilizing an optical model developed by Kenning et al., lower concentrations (0.1 wt% to 0.5 wt%) of BAPO were predicted to yield rapid onset of photoinitiation. In situ cure measurements under continuous UV LED irradiation of 380 mW/cm2 showed that a 0.1 wt% concentration of BAPO achieved peak polymerization rate within 2.5 s at a 3-mm depth. With only 1 s of irradiation at 1.7 W/cm2 intensity, the 0.1 wt% BAPO formulation also achieved the highest level of cure of the formulas tested. For an irradiation dose of 5.5 J/cm2 at a duration of 3.7 s, cured polymer specimens achieved a flexural strength of 108 MPa and a flexural modulus of 3.1 GPa. This study demonstrates the utility of optical modeling as a potential screening tool for new photopolymer formulations, primarily in identifying an upper limit to PI concentration for the desired cure depth. The results also show that photobleaching provides only a limited benefit for LFAM applications with short (1.0 s to 3.7 s) UV irradiation times and indicate that excess PI concentration can inhibit light transmission even under extended irradiation times up to 60 s.

Damage identification for functionally graded materials using improved experience-based learning algorithm based on sparse regularization

This article proposes a new objective function based on sparse regularization for damage identification on functionally graded materials using an improved experience-based learning algorithm. The new objective function is developed to improve the accuracy and robustness of optimization analysis and thus to obtain better performance for damage identification. Functionally graded beam specimens of poly(methyl methacrylate) polymers with different damage situations are fabricated to reveal the effectiveness and robustness of the proposed method. Moreover, the influence of artificial noise on the identification performance is investigated. The damage identification results from both numerical and experimental validation on poly(methyl methacrylate) polymer specimens demonstrate the superiority of the new objective function for damage identification of functionally graded materials compared with the traditional objective function.

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends