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A more efficient integrated computational approach for the design of injection and compression moulded composites which maps the results of manufacturing simulations as input to structural simulations is proposed. This methodology overcomes the limitations caused by insufficient detail provided by the second order fibre orientation tensor (FOT) output from manufacturing simulations which are needed in advanced structural finite element models that incorporate damage and failure. An artificial neural network (ANN) framework is proposed to predict a fibre orientation distribution function (ODF) from the FOT. The ANN contains the equation for calculating the FOT from the ODF such that the ODF is optimized to reproduce the input FOT. It is shown that the proposed framework is nearly as accurate as using experimentally measured ODF for predicting the stress strain response and failure of a compression moulded composite.

A high quality graphene paper has been fabricated using drop-casting of a custom developed stable ink mixture. The graphene/ethyl cellulose mixture can produce graphene paper coating on Kapton substrate at fairly large scales by finely adjusting the viscosity of the ethyl cellulose solvent blend. It was possible to produce a morphologically intact 300 nm-thick stacked structures of high purity graphene platelets after annealing at 300 °C, as characterized by SEM, Raman Spectroscopy and AFM. The electrical resistivity of a large batch of samples was measured and the results thoroughly evaluated, using a design of experiments approach. The resistivity values achieved were found to be lower or comparable to the best results reported in the literature. As expected, resistivity is inherently related to the concentration of the as-prepared ink and subsequent annealing conditions. However, prolonged annealing time can deteriorate mechanical properties as determined by cyclic bending and shear tests. It is anticipated that the present graphene paper has potential use in flexible electronics applications since the lowest resistivity and structure achieved were generally unaffected by the range of mechanical loadings studied.

Using a DGEBA/TETA epoxy filled with silver microflakes, we sought to investigate if the onset of conductivity would approximately obey the power-law as cure progresses, as reported in the open literature. We monitored the resistivity change as a function of cure and vitrification behaviour, using a set of concurrent experiments; in-situ four-wire electrical resistance measurements in a newly-developed probe-mold device, and differential scanning calorimetry. It was evident that the electrical conductivity evolved very differently depending on heating conditions, the composite glass transition temperature, and filler content. Periods were observed during cure where electrical conductivity was disrupted, attributed to stresses produced by vitrification. This produced as much as 1500-fold increases in final developed electrical resistance depending on heating conditions and composite glass transition properties, for the same conductive filler content. This discovery has far ranging implications on the industry practice of applying multi-step cure schedules that feature vitrification during cure.

The changes in specific heat capacity (C p) of a polycarbonate/acrylonitrile–butadiene–styrene (PC/ABS) blend subjected to aging have been determined using modulated differential scanning calorimetry (MDSC). MDSC experiments were conducted before and after hygrothermal conditioning of specimens under 9 different aging conditions. The effects of the two factors, hygrothermal conditions and scanning temperature, on material C p have been determined by statistical method of analysis of variance. Linear curves were fitted to experimental values of C p for the range of initial temperature of 24 °C (room temperature) up to 80 °C (glassy region of the material). The obtained linear relationship can be used to model heat transfer of PC/ABS materials exposed to physical aging and hygrothermal conditions. C p values were found to be independent of moisture content in conditioned samples within the range of conditions studied.

A repeatable and fast synthesis of one- and two-dimensional silver nanostructures with thickness of 20–25 nm, constructed from highly stable hexagonal and triangular nanoplates has been achieved. Various useful morphologies can be constructed on the gram-scale from the reduction of aqueous solutions of silver nitrate by L-ascorbic (L-ASB) acid in the presence of poly(methacrylic)acid sodium salt, within a few minutes by simple room temperature mixing. Contrary to the literature, the assembly mechanisms governing the final morphology are not kinetically controlled, and are instead selective based on concentration of the reaction mixture. These diverse silver nanostructures meet the criteria desired for feasible industrial production and incorporation into nanoscale devices.

A micromechanics model that utilizes microstructural characteristics for predicting the bending stress-strain response of a SMC composite is presented. The model integrates matrix, interphase and fibre damage models to form a framework that is able to accurately represent the damage response of the SMC using a single parameter damage model that is equivalent to the von mises interfacial stress. This is a valuable capability given the inherent random nature of SMC properties. Before being subjected to three-point bending tests, three SMC samples from different areas of a molded plate were characterized using X-ray computed tomography. The through thickness properties such as fibre orientation and volume fraction were characterized for multiple layers and then used as input data for a finite element model to predict the stress-strain behaviour. It was found that prediction accuracy increased with more layer properties. The model was able to represent the stress strain results within 2% of the experimentally measured response using 12 microstructural layers for a 2.8 mm thick specimen.

The material properties of compression moulded composites depends on the fibre orientation within a component, which changes as a function of position due to the flow of the material during the manufacturing process. Existing analytical tools are incapable of capturing all of the underlying physics of the problem involving long fibres which deform and interact with one another during the flow process. Artificial neural networks are used to predict the fibre orientation within a compression moulded composite by training the network on fibre orientation data. The fibre orientation data was produced using X-ray micro computed tomography measurements. The proposed artificial neural network framework is able to predict the orientation of the in plane orientation tensor normal components within 0.103 and the through thickness orientation tensor normal component within 0.0095 which is lower than the variability orientation of neighbouring microstructural units. Once trained the artificial neural network framework allows for near instantaneous predictions of fibre orientation presenting an opportunity for increased efficiency of the virtual design process of compression moulded composite materials.

Using silver nanobelts and silver microflakes in a DGEBA/TETA epoxy matrix, we sought to investigate the relationship between the evolving electrical resistivity of formulations of hybrid nanocomposites during the curing process. This was characterized using three methods: (i) in situ four-wire electrical resistance measurements; (ii) differential scanning calorimetry, and (iii) dilatometry. In a previous work we reported that the resistivity of microcomposites was strongly affected by partial vitrification during curing. In this study, the reported vitrification effect is observed again, further validating the concern of far-ranging implications on the industry practices. The addition of silver nanobelts greatly improved conductivity of the composites, though it was observed that the improvements are often lost during subsequent heating and cooling cycles. Resistivity observations indicate that the sensitivity may be due to insufficient nanobelt–nanobelt contacts in the composite, and thus further increasing the nanobelt fraction of the filler content can maximize conductivity.

Analysis of the available published data calls into question some of the reported effects of silver, graphene, and boron nitride nanoparticles on the cure behavior and glass transition temperature (Tg) of epoxy-based conductive nanocomposites. The usefulness of most studies is limited, as the final Tg value is only reported for a single composition. More useful studies provide a comparison of Tg across compositions and/or to the Tg of the neat matrix material. However, the main focus is on nanoparticle effect, while the presence of residual solvent is generally overlooked. Although the data are too sparse for firm conclusions, reductions in Tg seem to correlate with a known residual solvent or mild degas conditions (low temperature, unagitated), while increases are more likely when no solvent is used or after rigorous degassing. Using solvent control groups to differentiate solvent effects from filler effects, we conducted our own differential scanning calorimetry experiments. It was found that silver microparticles have no statistically significant effect on Tg, but appeared to when solvent content was not accounted for. The erroneous apparent effect is very similar to reported effects for nanoparticles in the literature. If a solvent must be used, we recommend that the residual solvent content be quantified and provide the Tg of a control sample with a representative solvent content without nanoparticles. Analysis of the present literature also suggests that (i) nanoparticle surface chemistry has a significant effect and (ii) poor dispersion/aggregation results in a reduction in Tg.

The second order fibre orientation tensor has been used to predict the mechanical response of injection and compression moulded composites with the capability to couple mould filling with structural finite element simulations to account for the microstructure. In order to accurately model these materials the mesh size used simulations must be representative of the actual microstructure of the composite. Herein a method to determine the microstructure size required to achieve a near constant value of the orientation tensor based on its derivative is presented. The method is demonstrated with the microstructure of a sheet moulding compound composite which has been evaluated using X-ray micro computed tomography. The average values of the derivative of the orientation tensor and its standard deviation approach constant values with increasing microstructure size. For the material considered a microstructure size of 5 mm square is sufficient to achieve a near constant value of the fibre orientation tensor, which is called a microstructural unit.

This paper offers an approach to model the coupled physics of structural mechanics, moisture diffusion, and heat conduction together with physical aging. The modeling is performed on a glassy polymer blend and hence incorporates the material viscoelastic behavior. Determination of the coupling coefficients, which link the governing equations of the mechanisms to each other, is particularly challenging. In this work, an estimation procedure has been used to determine the coupling coefficients, based on the experimental data reported in our recent papers. The effects of physical aging on mechanical stress–strain are also included using the shift factors, which were specified experimentally. The effect of physical aging on moisture diffusion is also modeled using a time-varying boundary condition. Experimental verification of the model shows that the developed model is capable of predicting the deflection of plastic parts subjected to hygrothermal conditions; i.e., conditions where moisture diffusion and physical aging phenomena are influential.

Silver nanobelts are demonstrated here to undergo inter-particle joining at relatively low temperatures of less than 180 °C. For surface-coated networks of nanobelts this joining reduced the network sheet resistance by 95%. The joining mechanism appears to be non-diffusional oriented attachment, caused by the thermal reactivation of the halted oriented attachment mechanism that occurred originally at room temperature during the rapid nanobelt synthesis. This self-assembly mechanism was explored by in situ electrical and calorimetric experiments, and supported by electron microscopy. Unlike pentagonal silver nanowires, silver nanobelts do not rely on diffusional instability to achieve workably low joining temperatures. The oriented attachment displayed by nanobelts represents a new approach to achieving valuable reductions in network resistance, disentangled from the instability and diffusion-driven failure by nanoparticle degradation displayed by competing silver nanoparticles.

The viscosity of a set of N-succinyl-chitosan (NSC) solutions was characterized near the 0.2Pas rheological injectability limit. This is believed to be the first such report in the open literature. Viscosity was characterized at physiological pH and ionic strength as a function of NSC degree of substitution, NSC concentration, temperature, and shear rate. NSC was synthesized via Yamaguci's method and characterized using H-NMR, membrane osmometry, TGA and isothermal vacuum drying. NSC synthesis results were shown to fit a reproducible log-linear correlation and both optimum drying temperature and thermal decomposition temperature were found to be a function of NSC degree of substitution. Viscosity results were explained using Katchalsky's full model for polyampholyte ionization combined with a charge induced excluded volume model proposed by Higgs. The model predicted a polyelectrolyte/polyampholyte transition which agreed well with experimental data. For minimally injectable formulations a maximum in primary amine concentration is expected at 32sub% amine NSC.

This work has studied the competing effects of physical aging and moisture absorption on the relaxation behaviour of a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) polymer blend after being subjected to both thermal and hygrothermal aging. Physical aging was simulated by thermal aging the blend at temperatures up to 80 ∘C which is below the glass transition temperatures of ABS and PC in the blend, respectively. Nine different combinations of relative humidity and temperature were investigated for hygrothermal aging. In both cases, two sets of aging times were applied: “short” term (≤16 hour aging) and “long” term (@168 hour aging). The short term tests were used to generate momentary master curves by applying time/aging-time and time/moisture superposition principles. Using the Kohlrausch–Williams–Watts (KWW) expression, the model parameters determined from curve fitting master curves were used to predict longer term stress relaxation behaviour. The predictions, verified against experimental results, showed reasonable agreement for all hygothermal exposure conditions except for full immersion. Moreover, comparisons of KWW parameters for both short and long term tests suggested that physical aging processes dominate absorbed moisture effects in terms of influencing viscoelastic behaviour, especially when the aging temperature approaches 80 ∘C.

Novel injectable hydrogels have been formed at 37 � C under physiological pH using a polymer-polymer cross- linking reaction. Three different formulations were tested. Af- ter 1-day cure time at body temperature, the elastic modulus of unswollen samples ranged between 3 and 5 kPa but after 11 additional days at 4 � C exceeded the target modulus of 10 kPa. Modulus data showed good agreement with theoretical crosslink density, enabling the prediction of stiffer/faster cur- ing gel formulations. Rubber elasticity theory provided a good fit to the experimental data up to 73% compressive true strain. Based on an analysis of modulus results, it was inferred that despite the presence of two aldehyde functional groups, only one mechanically significant crosslink can form per oxidized repeat unit on the alginate chain. V C 2011 Wiley

As an extension of an earlier study on aging of a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blend, this paper has explored the roles of two competing phenomena, physical aging and moisture diffusion, during hygrothermal aging. Moisture absorption experiments at four different relative humidity conditions were compared with full immersion condition. Uptake curves show two different diffusion behaviour including Fickian moisture absorption and anomalous peak overshoot. The equilibrium moisture uptake in the polymer system, the coefficient of diffusion of moisture and the dual glass transitions were characterized. In general, the uptake behaviour could be explained using free volume expansion and contraction concepts that are associated with diffusion and structural recovery theories, respectively. However, using statistical methods, it was found that the relationship between the effects of aging temperature and relative humidity is both complex and interactive, with either diffusion or relaxation processes dominating depending on the extent of hygrothermal exposure.

A novel load–displacement sensing instrument has been designed and fabricated to characterize the fracture properties of brittle thin films at low temperature (approximately −30 °C) and pressure (1.6e-4 Pa) environments. In this study, the instrument was used to investigate the effects of harsh environments on the fracture behaviors of organosilicate glass (OSG) and silicon carbonitride (SiCN) thin films under four-point bend loading. Experimental results showed that the fracture strengths of film stacks are the highest when the environment contains a very low water molecule concentration. This condition can be achieved by purging the testing chamber with pure nitrogen or reducing the chamber pressure to less than 1 Pa. In contrast, cracks propagated readily along OSG/SiCN interfaces when experiments were performed in deionized water. The effects of low temperature (approximately −30 °C) and pressure on thin film fracture were also studied, and the results demonstrated that there is no observed degradation of the OSG fracture properties. X-ray photoelectron spectroscopy (XPS) technique was used to identify the chemical composition of the fracture surfaces.

This work is part of a larger study aimed at characterizing the viscoelastic–viscoplastic behavior of a continuous fiber glass mat thermoplastic composite. The purpose of this paper is to experimentally and numerically decouple the viscoplastic strains from total creep response. This enabled the characterization of the evolution of viscoplastic strains as a function of time, stress and loading cycles. The separation also allowed viscoplastic strain development to be corresponded with the progression of failure mechanisms such as interfacial debonding and matrix cracking which were captured in situ . This was achieved by performing creep tests at seven stress levels between 20 and 80 MPa. For each stress level, a series of creep-recovery tests were performed on single specimen for increasingly longer durations from 1 to 24 h. Stress and time dependence of viscoplastic strains were determined experimentally. Using part of the data generated, a viscoplastic model was developed following a method proposed by Nordin. The model had excellent agreement with experimental results for all stresses and times considered. In multiple loading cycles, the viscoplastic strain development is accelerated with increasing number of cycles at higher stress levels. The results further verify the technique for numerical separation of viscoplastic strains proposed in an earlier work. Finally, it was found that the development viscoelastic strains during creep are affected by the previous viscoplastic strain history.

In Part I of this article, the short-term tensile creep of a 3-mm-thick continuous long-fibre glass mat thermoplastic composite was characterized and found to be linear viscoelastic up to 20 MPa. Subsequently, a nonlinear viscoelastic model has been developed for stresses up to 60 MPa for relatively short creep durations. The creep response was also compared with the same composite material having twice the thickness for a lower stress range. Here in Part II, the work has been extended to characterize and model longer term creep and recovery in the 3-mm composite for stresses up to near failure. Long-term creep tests consisting of 1-day loading followed by recovery were carried out in the nonlinear viscoelastic stress range of the material, i.e., 20–80 MPa in increments of 10 MPa. The material exhibited tertiary creep at 80 MPa and hence data up-to 70 MPa has been used for model development. It was found that viscoplastic strains of about 10% of the instantaneous strains were developed under load. Hence, a non-linear viscoelastic–viscoplastic constitutive model has been developed to represent the considerable plastic strains for the long-term tests. Findley's model which is the reduced form of the Schapery non-linear viscoelastic model was found to be sufficient to model the viscoelastic behavior. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the nonlinear model has been developed. The model predictions were found to be in good agreement with the average experimental curves. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

This work is part of a larger experimental program aimed at developing a semi-empirical constitutive model for predicting creep in random glass mat thermoplastic (GMT) composites. The tensile creep response of a long-fiber GMT material has been characterized for 3- and 6-mm thick material. Tensile tests showed that the variability within and between plaques are comparable with an overall variability of about 6% and 8% for the 3- and 6-mm thick materials, respectively. The thicker material exhibited slightly higher variability and directional dependence due to greater flow during molding of the plaques. Short-term creep tests consisting of 30 min creep and recovery, respectively, were performed over the stress range between 5 and 60 MPa. Three tests for determining the linear viscoelastic region were considered which showed that the 3- and 6-mm thick GMT are linear viscoelastic up to 20 and 25 MPa respectively. The 6-mm thick GMT consisting of a higher fiber weight fraction was linear over wider stress range. Furthermore, it was found that plastic strains were accumulated during creep, which suggests that a nonlinear viscoelastic–viscoplastic model would be more appropriate for long-term creep at relatively high stresses, which will be presented in our companion paper. The magnitude of the plastic strains developed in the creep tests presented here was lower because a single specimen was loaded at multiple stress level over short durations. Hence, a nonlinear viscoelastic constitutive model has been developed for the two thickness materials. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

The effects of temperature on the tensile creep of continuous random fiber glass mat thermoplastic composite (GMT) have been studied following an accelerated characterization procedure. The objectives of this work are twofold. First, is to obtain a long-term creep model using time–temperature superposition (TTS) that can represent behavior within the linear viscoelastic regime (up to 20 MPa) at room temperature. The second is to develop a non-linear viscoelastic model that accounts for a wide range of stresses and temperatures. Creep and recovery tests were carried out for a stress range between 20 and 60 MPa over a temperature range of room temperature to 90 °C. TTS was applied to obtain a master curve which was curve fitted to a nine-term Prony series. It was found that material generally behaved non-linearly for all stresses and temperature. For stresses up to 50 MPa, the non-linear viscoelastic behavior due to temperature can be reasonably modeled by only the time–temperature shift factors from TTS. At 60 MPa, however, the non-linear parameters have to be modeled as a product of stress and temperature dependent functions. The model predictions are in good agreement with the experimental results at most stress and temperature levels. The creep curves predicted at higher temperatures especially at 60 MPa tend to underestimate at longer times.

An inherent challenge with polymer blends is the difficulty in resolving the glass transition, T g, for the smaller mass fraction component. The objective of this work was to determine the practical scanning conditions for identifying the dual T g’s for a 75:25 polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blend using a thermomechanical analyzer (TMA). Scanning rates up to 20°C min−1 using dilatometer and expansion modes were studied. Heating and cooling rates were found to affect both T g values but the effects were not simple relationships. T g values could either increase or decrease depending on the scanning rate applied. Higher rates resulted in large thermal lags which opened the accuracy of measurements to question. Generally, higher rates tended to display only one T g but the duality of T g’s can be detected with scanning rates between 0.5 and 5°C min−1 for both modes.

The effects of physical aging of a 75 : 25 PC/ABS blend have been studied using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). From DSC, two distinct peak endotherms at about 90°C and 110°C, which are associated with the glass transition of ABS (Tg,ABS) and PC (Tg,PC) components, respectively, were observed. When progressive aging was monitored at 80°C for over 1000 h, the changes in enthalpic relaxation, glass and fictive temperatures for the blend followed similar trends to those already seen in the literature for PC aged between 125 and 130°C. The rate of enthalpy relaxation was also comparable. The plot of peak endotherm against logarithmic aging time for the PC blend constituent, however, behaved quite differently from the linear relationship known for highly aged PC. The ABS peak component also appeared to be insensitive to aging. Both observations were confirmed to be statistically significant using analysis of variance methods. Using temperature modulated-DSC, there is evidence that aging increases the blend miscibility as the Tg,PC shifts toward the stationary Tg,ABS during aging. Parallel FTIR investigations found oxidation of butadiene during aging to be even at this relatively low temperature, forming hydroxyl and carbonyl degradation products. The presence of ABS in the blend also appeared to have prevented the shifting from the trans-cis to trans-trans arrangement of the carbonate linkage, which is a well-known phenomenon during elevated temperature aging of PC alone. Moreover, the carbonate linkage appears to have been at the lower energy, trans-trans, arrangement prior to the aging process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

The effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140°C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Non-Fickian or anomalous diffusion is frequently observed when the absorption of moisture by a polymer is being studied. Different models have been presented in the literature that can accurately predict the trends of the weight-gain curves. However, it is not always clear which of these models yield good predictions of moisture distribution. This article presents a time-resolved moisture distribution study of an epoxy sample immersed in deuterated water (D2O) at 70°C over a period of 2.5 months. The moisture distribution was measured during that period with a novel high-resolution magnetic resonance imaging technique that is well adapted to the imaging of thin plates. The experimental results showed that the concentration of D2O at the surface of the sample increased with time, even after 2.5 months. These results were used to evaluate the performance of several standard diffusion models. Although this study is phenomenological, it appears that a model featuring time-varying boundary conditions yields the best representation of moisture absorption for these samples. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

The analysis of adhesively bonded single-lap joints has largely been concerned with the prediction of the adherend bending moment at the overlap edge and corresponding adhesive stresses due to tensile loading. The most well-known models in this area are those developed by Goland and Reissner (1944), Hart-Smith (1973), and Oplinger (1991, 1994), which are based on beam theory. Of the three, the Hart-Smith solution has been considered the most deficient, since it does not account for the effect of the overlap deflection on the edge moment as pointed out by Oplinger. In this first part of our paper, it will be demonstrated that the differences in the edge-moment predictions between the Hart-Smith model and those by Goland and Reissner and Oplinger, respectively, are mainly due to mathematical errors in Hart-Smith's original derivation. The errors have been overlooked for over 30 years because researchers consistently focused on the simplified version rather than the original expression. In this work, the Har...

The influence of cooling rate on the stress relaxation response of prestrained polyvinylidene fluoride (PVDF) film cooled from 60°C, which is the upper service temperature limit, has been investigated. The film was heated to 60°C, strained at 0·5% and then cooled to ambient between 1 and 5°C min−1. Subsequently, the heat treated films were subjected to stress relaxation tests at 0·5% strain (50 μm extension) and the relaxation curves were fitted using the Kohlraush–Williams–Watts (KWW) expression. Modulated differential scanning calorimetry (MDSC) showed that crystallinity changes were only detectable for the highest cooling rate. When the crystallinity level dropped by ∼2% for the sample cooled at 5°C min−1, the relaxation time constant was reduced by nearly two orders of magnitude. The results indicate the film relaxation behavior is highly sensitive to thermomechanical loading near the onset of the α c transition. In practical terms, small mechanical straining can dramatically alter PVDF film p...

Water ingress inside honeycomb sandwich panels during service has been linked to in-flight failure in some aircraft. There is an ongoing effort to develop nondestructive testing methods to detect the presence of water within the panels. Magnetic resonance (MR) represents an attractive approach in that it is sensitive to moisture. Using a unilateral MR sensor, testing can be applied directly to the surface of the panel. The viability of MR is demonstrated through laboratory imaging of both water within sandwich panels, as well as the adhesive itself. The detection of water using a one-sided handheld MR sensor is presented. It is shown that simple detection, as well as spatial localization of water within sandwich panels is possible.

The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔHa) and fictive temperature (Tf) as a function of aging temperature (Ta) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔHa and Tf changes were only appreciable for (Tg -70 K)

The results of a magnetic resonance imaging (MRI) investigation concerning the effects of an aluminum honeycomb sandwich panel on the B 1 and B 0 fields and on subsequent image quality are presented. Although the sandwich panel structure, representative of an aircraft composite material, distorts B 0 and attenuates B 1 , distortion-free imaging is possible using single point (constant time) imaging techniques. A new expression is derived for the error caused by gradient field distortion due to the heterogeneous magnetic susceptibility within a sample and this error is shown not to cause geometric distortion in the image. The origin of the B 0 distortion in the sample under investigation was also examined. The graphite-epoxy `skin' of the panel is the principal source of the B 0 distortion. Successful imaging of these structures sets the stage for the development of methods for detecting moisture ingress and degradation within composite sandwich structures.

As part of a wider research program related to polycarbonate embrittlement, the effects of heat-aging on mechanical relaxation behavior have been studied by examining the relationship between secondary transitions and stress relaxation behavior. In this Part I, the differences in response between two molecular weight polycarbonates (PC) are compared for injection molded samples. Dynamic mechanical spectra showed that the presence of an intermediate β transition (∼ 80°C) is strongly dependent on molecular weight and heat-aging. However, the β 1 (35°C) and the γ (-100°C) peaks are generally insensitive to either effect. The study also attempted to interpret the similarities and differences in relaxation response using free volume and conformational change arguments, which have been subject to much scrutiny. Using the KWW stretched exponential to characterize stress relaxation, it appeared that bulk free volume recovery concepts could explain differences in stress relaxation response but not the corresponding losses in toughness. Hence, it is proposed that changes in relaxation response are most likely due to an interplay of relatively large scale molecular volume and molecular conformation processes that affect intermolecular cooperativity. These high-activation processes are related to the broad β region (β 1 < T < T g ).

The significance of heat-aging effects on low-molecular-weight polycarbonate has been studied by performing a two-factor Analysis of Variance (ANOVA). Although this work was primarily motivated by the large experimental scatter observed in stress relaxation results for LMW 2608 (Part I), the effect of heat-aging on the characteristics of secondary transitions (γ and β 1 ) generated by dynamic testing was also investigated. Both types of tests were performed using a dynamic mechanical analyzer. The statistical analysis verified an earlier suggestion that both the secondary transitions were insensitive to heat-aging. In the quasi-static stress relaxation tests, the curve-fitted KWW parameters (τ, E o , β') were evaluated using ANOVA for increasing heat-aging time and test temperature. Two other statistical techniques were also applied to test repeatability-the power of each aging time/test temperature combination and the number of observations needed to achieve 90% repeatability. In conclusion, both τ and β' could describe the self-retarding nature of volume recovery although the repeatability of β' was substantially higher. However, the unrelaxed modulus, E o , was found to be an unreliable indicator of whether heat-treatment had caused changes in the intrinsic structure. Overall, the study showed that the repeatability of the stress relaxation test results is generally very poor for the confidence levels tested.

The purpose of this work was to develop a cure kinetics model for a commercially available high temperature cure epoxy adhesive commonly used in the aerospace industry. While there are several phenomenological cure kinetic models commonly used in the literature for describing the rate of conversion of thermosetting epoxy adhesives as a function of degree of conversion, none of these models adequately depicts the adhesive used in this work over the entire range of conversion. Hence, by curve fitting empirical data collected using differential scanning calorimetry and refinement of existing models, an alternative model is proposed. The form of the present model suggests that chemical curing is the result of the combination of two autocatalytic reactions. The model is able to account for both the chemically controlled and diffusion controlled regimes of the cure. This paper also describes a novel iterative approach for predicting kinetics parameters as a function of isothermal cure temperature. Excellent agreement between experimental measurements and model predictions has been demonstrated over the entire range of conversion.

Fourier transform infrared spectroscopy was used to detect changes or degradation in chain structure of PC/ABS specimens after being exposed to three hygrothermal conditions. The two main absorbance peaks of interest are associated with carbonyl and hydroxyl groups due to the oxidation of butadiene in the ABS component. The changes to these groups were evident in earlier work during thermal aging of this blend but were not found in this hygrothermal study. However, the increase in intensity within the hydroxyl region for two of the conditions was attributed to bulk water diffusion in the material. When viewed together with results published earlier by our research group, the changes driven by bulk moisture diffusion are reversible. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41316.

The stress relaxation profiles across the skin-core-skin layers of injection molded polycarbonate and extruded polyethylene sheet are presented. The profiles were obtained by progressively removing the layers followed by stress relaxation tests using a Dynamic Mechanical Analyzer (DMA). The machined layers were characterized using the elastic modulus, Eo, and the relaxation time, o. It was found that the DMA was able to distinguish the differences in viscoelastic response across the thickness of the polymer samples. The variation in behaviour was consistent with the expected morphology and molecular orientation developed due to the processing method.

Adhesively bonded balanced single-lap joints have been studied using finite element analysis and verified either experimentally or theoretically. In the finite element method, a geometrically nonlinear two-dimensional method was applied using ANSYS code Version 5.3. The effects of (i) plane strain and plane stress conditions; (ii) simply supported and fully fixed boundary conditions at the adherend ends; (iii) filleted and unfilleted overlap end geometries; and (iv) two different adhesive materials on the bending moment factor, k, and the adhesive stresses have been evaluated. The finite element simulations were compared with experimental results only for the filleted joint bonded with a relatively flexible adhesive. The results for unfilleted joints bonded with flexible and rigid adhesives were compared with the analytical solution proposed by Oplinger.

The effects of contact stresses and instrument compliance on the storage modulus, E′, measurements for a rectangular sample with span-to-thickness ratio of about 17 is presented. Using finite element analysis, it has been found that contact stress effects are negligible even when the applied force is near the maximum limit. Machine compliance, however, appears to affect the stiffness, and hence E′ values, particularly at static loads less than 4 N. An analytical model describing machine compliance effects has been developed and can be used for correcting the stiffness measurements of the geometry studied. The model has been verified for three different rigid thermoplastics.

An experimental study of the effects of oscillation amplitude on the elastic storage modulus, E′, for a rigid thermoplastic measured using three point-bending is presented. The results are part of a wider investigation on the operating conditions needed to improve repeatability and reproducibility of E′ measurements for the recently commercialized TA Instruments 2980 Dynamic Mechanical Analyzer. It has been found that sample alignment is required to achieve reliable E′ values. Based on experiments performed on polycarbonate at 1 Hz using a span-to-thickness ratio of 11, 17 and 33, respectively, the optimum amplitude for obtaining the lowest standard deviation under ambient conditions is found to be about 100 μm. It is also found that a span-to-thickness ratio of

Experimental testing of composite-to-aluminum single-lap joints has been performed to verify the theoretical model proposed in Part I. Using specimens bonded with a flexible and a rigid adhesive, tests were conducted at room temperature and −40°C. Results show that the joint stiffness is more affected by the response of the adherends to the test temperature than by the modulus of the thin adhesive layer. It is found that the theoretical model is able to predict the joint stiffness and the rate of stiffness loss with crack growth. The best correlation is found for the rigid adhesive joint tested at −40°C in which the fracture mode is pure adhesive cracking. Since the model does not account for other failure modes such as delamination and interfacial tearing, the stiffness loss due to such failures is underestimated.

In this part of the paper, a simple generalised model for predicting the stiffness of a single-lap joint has been developed. The model considers each component of the joint as separate spring elements and, therefore, each sustains different levels of deflection. The individual deflections due to tension loading are found using basic mechanics analysis and by applying the Adams–Peppiat stress equation for adhesive bonded lap joints. The model can also predict the stiffness loss due to adhesive bond fracture by accounting for the shorter overlap length due to cracking.

The activation energy associated with the glass transition relaxation of an epoxy system has been determined by using the three-point bending clamp provided in the recently introduced TA Instruments DMA 2980 dynamic mechanical analyzer. A mathematical expression showing the dependency of modulus measurements on the sample properties and test conditions has also been derived. The experimental results showed that the evaluation of activation energy is affected by the heating rate and test frequency, as well as the criterion by which the glass transition temperature (Tg) is established. It has been found that the activation energy based on the loss tangent (tanδ) peak is more reliable than on the loss modulus (E2) peak, as long as the dynamic test conditions do not cause excessive thermal lags.

A comprehensive test protocol, described in Part I of this paper, has been used to evaluate the adhesive strength, viscoelastic properties, and shear fatigue behavior of two polyurethane joint sealants for use in undoweled concrete highway joints. The results of these tests showed that the protocol was able to assist in selecting the more suitable sealant for the expected service conditions in New Brunswick, Canada. Sealant A, a coal-tar modified polyurethane with a glass transition temperature of −20.7°C, showed a significantly reduced flexibility at a severe low temperature of −40°C. This low temperature induced loss in flexibility was supported by the relatively low peel force required to initiate cohesive failure and the relatively high resistance to shear deflection noted with the shear fatigue test. Sealant B, on the other hand, was found to have a glass transition temperature of −43.2C and maintained its flexibility at extreme low temperatures for all dynamic tests performed. Using dynamic mechanic...

Sealants used to seal the transverse contraction joints used in joining concrete highway sections without dowels must be flexible enough to withstand excessive conditions such as large temperature fluctuations, severe pavement deflection due to heavy truck traffic, and prolonged moisture exposure. Choosing the most appropriate sealant for the expected service condition from the wide variety of sealants available on the market today can be a daunting task. In Part I of this paper, a comprehensive test protocol that correlates sealant viscoelastic properties with the sealant resistance to deflection is presented. It is proposed that the selection of a sealant should be based on a complete evaluation consisting of three test procedures: (1) the ASTM C794 adhesion-in-peel test to assess concrete/sealant adhesion; (2) dynamic mechanical analysis (DMA) to evaluate sealant flexibility by determining the glass transition temperature, Tg; and (3) the shear fatigue test developed by the writers to analyze sealant p...

Gamma-ray Compton scattering is used for the detection of debonding in adhesively bonded composite-aluminum joints. A collimated narrow beam of monochromatic photons, generated by a 137 CS source, is directed towards the joint and scattered photons are recorded, using a detector located on the same side as the source. The energy of the scattered radiation is measured and related to the angle of scattering. The occurrence of debonding is indicated by a change in the count rate at an energy corresponding to its location. The performance of the technique is successfully demonstrated experimentally for joints of different adhesive-bonding thicknesses and for artificially induced debonds.

The effects of PET film interleaving on delamination initiation in centrally notched (±θ/0)s composite laminates with fiber directions θ = 15°, 30°, 45° and 60° respectively have been investigated. A pair of interleaves were selectively placed at delamination prone interfaces in each laminate. The circular holes were cut in glass-epoxy laminates using high-speed drilling (HSD) and abrasive waterjet cutting (AWC) in order to compare the influence of hole surface finish with interleaving. Surface profilometry was used as a nondestructive technique for detecting delamination onset during tensile testing. It was found that although HSD interleaved holes were the roughest, the delamination onset stresses were significantly increased particularly for 30° < θ < 60°. For ±θ = 60°, however, delamination onset was unaffected by either hole surface finish or interleaving.

Helical polymeric stents have been proposed as a suitable geometry for biodegradable drug-eluting polymer-based stents. However, helical stents often experience nonuniform local expansion (dog boning), which can prohibit full stent expansion using conventional methods. The development of stents and deployment methods is challenging and can be supported by numerical analysis; however, this complex problem is often approached with simplified boundary conditions that may not be appropriate for helical stents. The finite element method (explicit and implicit) was used to investigate three common stent expansion approaches with a focus on helical stent geometry, which differs from traditional wire mesh stent expansion. Although each of the three methods considered provided some insight into the expansion characteristics, common displacement controlled, and uniform expansion methods were not able to demonstrate the characteristic local deformations observed in expansion. A coupled stent-balloon model, although computationally expensive, was able to demonstrate the expected nonuniform deformation. To address nonuniform expansion, a progressive expansion approach has been investigated and verified numerically. This method may also provide a suitable solution for nonuniform expansion in other stent designs by minimizing loading and potential damage to the artery that can occur during stent deployment.

Helical coil polymeric stents provide an alternative method of stenting compared to traditional metallic stents, but require additional investigation to understand deployment, expansion, and fixation. A bilayer helical coil stent consisting of PLLA and PLGA was investigated using the finite element model to evaluate performance by uniform expansion and subsequent recoiling. In vitro material characterization studies showed that a preinsertion water-soaking step to mimic body implantation conditions provided the required ductility level expansion. In this case, the mechanical contribution of the outer PLGA layer was negligible since it softened significantly under environmental conditions. The viscoelastic response was not considered in this study since the strain rate during expansion was relatively slow and the material response was primarily plastic. The numerical model was validated with available experimental expansion and recoiling data. A parametric study was then undertaken to investigate the effect of stent geometry and coefficient of friction at the stent-cylinder interface on the expansion and recoiling characteristics. The model showed that helical stents exhibit a uniform stress distribution after expansion, which is important for controlled degradation when using biodegradable materials. The results indicated that increasing stent width, pitch value, and coil thickness resulted in a larger diameter after recoiling, which would improve fixation in the artery. It was also noted that a helical stent should have more than five coils to be stable after recoiling. This work is part of a larger research study focused on the performance of a balloon-inflated polymeric helical stent for artery applications.

The sand-water erosion behaviour of notched holes in glass/epoxy composites was investigated. Using through-the-thickness stitching, the amount of hole damage caused by impact testing prior to wear tests was varied. Erosion resistance was evaluated based on the amount of mass loss and the percentage increase in hole diameter. Results showed that the holes in unstitched laminate which incurred greatest impact damage, eroded the least. The holes in stitched laminates, however, were more susceptible to erosive wear even though the internal damage appeared less severe. Scanning electron micrographs showed that the lower half of the holes in the unstitched laminates remained intact while those in the stitched laminates were eroded substantially. Cut sections away from the hole were observed with an optical microscope. A comparison of cut sections showed that heavy stitching results in severe fibre impalement damage, thereby reducing the reinforcing capability of the fibres around the hole. For the unstitched composite, in spite of more widespread impact-induced delamination damage, the inherent presence of protruding fibres in the backplies provided additional resistance against erosion.

Intermetallic γ-TiAl has been processed using relatively inexpensive elemental titanium and aluminium powders which are randomly mixed in a stoichiometric ratio. Two powder metallurgy routes involving (i) direct hot isostatic pressing (HIP) and (ii) cold isostatic pressing followed by containerless HIPping (CHIP) were investigated. In both cases, the specimens were hot isostatically pressed at 1200°C for 2 hours at a maximum pressure of 200 MPa. X-ray diffraction analysis performed on the final products showed that the reaction was completed with direct canned HIPing and equiaxed structure of TiAl was formed. In CHIP processing, however, aluminium diffused in titanium but the incomplete dissolution led to the formation of [ γ −TiAl and α 2 −Ti 3 Al] lath structure instead. The sluggish interparticle diffusion and Kirkendall pore formation resulted in an alloy with high porosity and microstructural inhomogeneity.