The effect of time and temperature on flexural creep and fatigue strength of a silica particle filled epoxy resin

Composite materials that use an epoxy resin as a matrix resins have superior mechanical properties over standard structural materials, but these materials exhibit time and temperature behavior when used for long periods and under high temperatures. This time and temperature behavior has not been fully explained. The purpose of this paper is to further describe this time and temperature behavior, increasing the reliability of this class of composite materials. The time and temperature dependence of flexural strength was examined by creep and fatigue testing. Flexural creep tests were carried out at various temperatures below the glass transition temperature. Flexural fatigue tests were carried out at various stress ratios, temperatures below the glass transition temperature and 2 frequencies. The time-temperature superposition principle held for the flexural creep strength of this material. A method to predict flexural creep strength based on the static strength master curve and the cumulative damage law is proposed. When the fatigue frequency was decreased while temperature and stress ratio are held constant the flexural fatigue strength decreases. The time-temperature superposition principle was also found to hold for the flexural fatigue strength with respect to frequency.     

Experimental characterization and thermoviscoelastic modeling of strain and stress recoveries of an amorphous polymer network

An acrylate polymer network was submitted to thermomechanical shape memory cycles. The set of experiments characterized the material stress-free strain recovery and the strain-constrained stress recovery in uniaxial tension. Experimental parameters like temperature of strain fixation, amount of strain and heating rate, were varied in order to provide a relatively complete set of experimental data. A model combining the amorphous polymer viscoelasticity and its time–temperature superposition property was used to predict the shape memory behavior of the acrylate polymer network. All the model parameters were characterized using classical tests for mechanical characterization of polymers, which do not include shape memory tests. Model predictions obtained by finite element simulations compared very well to the experimental data and therefore the model relevance for computer assisted application design was assessed.     

Heat treating of a ring rolled Ni–Fe–Cr superalloy

Abstract

Haynes HR-120 alloy (UNS N08120) is a nickel–iron–chromium alloy that exhibits high strength at elevated temperature and resistance to carburising and sulphidising environments. These properties make this alloy suitable for the production of components of land based turbines, including rings. Manufacture and heat treating of such rings require strict control of the processing variables, such as temperature and deformation ratios, as well as the time and temperature of the solution treatment, due to their effect on microstructure and mechanical properties. It is common practice to treat this alloy at temperatures above 1100°C to promote dissolution of undesired particles and recrystallisation of deformed structures, but it has been found that grain coarsening can occur during treatment. The present work presents the results of a series of solution heat treatments that were performed within a broad range of temperatures on industrial ring rolled pieces. It was found that the increment in time and temperature enhances the dissolution of intergranular precipitates that result in the improvement of mechanical properties, but grain coarsening is observed to occur when the material is treated for long times and high temperatures. The best combination of mechanical properties and grain size was obtained by treating the material for half an hour at 1050°C.     

Extremely Rapid Self-Healable and Recyclable Supramolecular Materials through Planetary Ball Milling and Host–Guest Interactions

The host–guest interaction as noncovalent bonds can make polymeric materials tough and flexible based on the reversibility property, which is a promising approach to extend the lifetime of polymeric materials. Supramolecular materials with cyclodextrin and adamantane are prepared by mixing host polymers and guest polymers by planetary ball milling. The toughness of the supramolecular materials prepared by ball milling is approximately 2 to 5 times higher than that of supramolecular materials prepared by casting, which is the conventional method. The materials maintain their mechanical properties during repeated ball milling treatments. They are also applicable as self-healable bulk materials and coatings, and they retain the transparency of the substrate. Moreover, fractured pieces of the materials can be re-adhered within 10 min. Dynamic mechanical analysis, thermal property measurements, small-angle X-ray scattering, and microscopy observations reveal these behaviors in detail. Scars formed on the coating disappear within a few seconds at 60 °C. At the same time, the coating shows scratch resistance due to its good mechanical properties. The ball milling method mixes the host polymer and guest polymer at the nano level to achieve the self-healing and recycling properties.     

Estimating the creep behavior of polycarbonate with changes in temperature and aging time

Thermoplastic resins are typically used without any kind of physical aging treatment. For such materials, creep behavior and physical aging, which depend on time and temperature, occur simultaneously. The effects of these processes are evident after quenching and are recorded in the material as a thermal history. This history strongly influences mechanical properties and creep behavior in particular. Thus, a more thorough understanding of the physical aging process is desirable. We examined the creep deformation of polycarbonate (PC) to reveal the effects of physical aging on creep behavior. The effects were dependent on both time and temperature. The relationship between physical aging and creep behavior exemplified superposition principles with regard to time and both pre-test aging time and pre-test aging temperature. The superposition principles allowed the calculation of creep deformations at a given temperature; the calculated results were corroborated by experimental data.     

Application of Kramers–Kronig relations to time–temperature superposition for viscoelastic materials

Dynamical mechanical analysis (DMA) is an experimental technique commonly used to study the frequency and temperature dependence of the mechanical properties of viscoelastic materials. The measured data are traditionally shifted by application of the time–temperature superposition principle to obtain the master curves of the viscoelastic material. The goal of this work is to present a methodology to determine the horizontal and vertical shift coefficients to be applied to the isotherms of storage and loss moduli measured. The originality lies in the calculation of the shift coefficients by a method requiring fulfilment of the Kramers–Kronig relations conveying the causality condition. The computed vertical shift coefficients are compared to the coefficients predicted by the Bueche–Rouse theory.     

A model combining structure and properties of a 160/220 bituminous binder modified with polymer/clay nanocomposites. A rheological and morphological study

The present contribution focuses on the modification of a 160/220 bituminous binder with clay and polymer/clay nanocomposites. Bitumen/polymer/clay ternary blends were prepared using styrene–butadiene–styrene, ethylene vinyl acetate and ethylene methylacrylate copolymers mixed with an organomodified montmorillonite. Dynamic mechanical analyses were performed in the extended domain of stress, temperature and frequency to analyse the thermorheological behaviour of the blends. The time–temperature superposition principle was applied to shift the experimental data recorded at different temperatures and generate master curves of the linear viscoelastic functions. For all blends, the mechanical response of the system was found to be strongly and intimately influenced by the nanocomposite modification. In some cases, a solid-like behaviour appears and delays the Newtonian transition. Morphological analyses performed with fluorescence microscopy allowed to associate the binder properties with the presence of clay silicates, which alter the colloidal equilibrium of the bitumen and enhances the compatibility between bitumen and polymers. Based on the morphological and rheological results, a structural model of the prepared blends is proposed.     

Numerical implementation of strain rate dependent thermo viscoelastic constitutive relation to simulate the mechanical behavior of PMMA

In this work, a nonlinear viscoelastic constitutive relation was implemented to describe the mechanical behavior of a transparent thermoplastic polymer polymethyl methacrylate (PMMA). The quasi-static and dynamic response of the polymer was studied under different temperatures and strain rates. The effect of temperature was incorporated in elastic and relaxation constants of the constitutive equation. The incremental form of constitutive model was developed by using Poila–Kirchhoff stress and Green strain tensors theory. The model was implemented numerically by establishing a user defined material subroutine in explicit finite element (FE) solver LS-DYNA. Finite element models for uniaxial quasi-static compressive test and high strain rate split Hopkinson pressure bar compression test were built to verify the accuracy of material subroutine. Numerical results were validated with experimental stress strain curves and the results showed that the model successfully predicted the mechanical behavior of PMMA at different temperatures for low and high strain rates. The material model was further engaged to ascertain the dynamic behavior of PMMA based aircraft windshield structure against bird impact. A good agreement between experimental and FE results showed that the suggested model can successfully be employed to assess the mechanical response of polymeric structures at different temperature and loading rates.     

Evaluation of the fracture toughness properties of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) (Dupont Tradename Teflon) is a common polymer with many structural applications including sheet, gaskets, bearing pads, piston rings and diaphragms. The interest here developed because this polymer is being considered as the major component of a newly proposed `reactive' material with a possible application as a projectile to replace common inertial projectiles. Little mechanical property data is available on this material since it is commonly used only as a coating material with the dominant properties being its low friction coefficient and high application temperature. Previous work (Joyce, 2003) on commercially available sheet PTFE material has demonstrated the applicability of the normalization method of ASTM E1820 (1999), the elastic-plastic fracture toughness standard to develop fracture toughness properties of this material over a range of test temperatures and loading rates. Additional work on the aluminum filled `reactive' derivative of the basic PTFE polymer (Joyce and Joyce, 2004) has also recently been completed. In this work, standard ASTM E1820 fracture toughness specimens machined from sintered pucks of PTFE were tested at four test temperatures and at a range of test rates to determine the J _Ic and J resistance curve characteristics of the PTFE material. The major results are that while crack extension is difficult at standard laboratory loading rates at ambient (21 °C) temperature or above, for temperatures slightly below ambient or for elevated loading rates, a rapid degradation of fracture resistance occurs and cracking occurs in a ductile or even nearly brittle manner.     

DMA testing of Ni–Mn–Ga/polymer composites

Ni–Mn–Ga/polymer composites are an interesting group of materials with new potential for applications such as vibration damping. Polymer selection is critical for Ni–Mn–Ga/polymer composites. The transition temperatures of both components should be selected carefully considering the future application, the contact between Ni–Mn–Ga and polymer is important and stiffness of the polymer matrix is one of the influencing factors. The adhesion between Ni–Mn–Ga and the polymer matrix was studied in shear stress tests. Ni–Mn–Ga adhered strongly to the tested epoxies but considerably less so to silicones. The damping properties of Ni–Mn–Ga/polymer composites were studied by dynamic mechanical analysis (DMA). Compared to the pure polymer sample, DMA measurements confirmed extra damping in the martensite–austenite transition temperature region of the Ni–Mn–Ga after a soft epoxy matrix had been applied.     

Viscoelastic behaviour of macro-defect-free cement

The viscoelastic behaviour of macro-defect-free (MDF) cement was studied by dynamic mechanical analysis. MDF specimens with various moisture contents in the range 1.70%–3.20% moisture, were measured at 1 Hz as a function of temperature from 34–96 °C and as a function of frequency from 0.05–5 Hz at 34°C. The viscoelasticity of MDF was found to be affected by moisture content through its plasticizing effect on the PVA binder. Time-moisture and temperature-moisture superposition of the shear moduli were found to be possible for MDF, and linear relationships between log time and linear moisture and between temperature and moisture were found. How the microstructure of MDF affects the viscoelastic response is also discussed through mechanical models. A comparison of the models with known experimental data suggests that the viscoelastic response arises from both direct connections between the inorganic particles and from connections through the polymer binder. Inorganic links are estimated to connect 45% of the inorganic phase.     

Estimation of biaxial tensile and compression behavior of polypropylene using molecular dynamics simulation

The polymeric materials in general exhibit strong time–temperature dependence and viscoelastic behavior. The time–temperature superposition principle is typically used to estimate the long-term viscoelastic behavior. In addition, Mises criterion and Tresca criterion have been proposed to estimate the yield or failure stresses in a multiaxial stress state and Christensen failure criterion can be applied in the case of different tensile and compressive strengths. In this study, using molecular dynamics method, uniaxial and biaxial tensile and compression test simulations were performed for polypropylene at various strain rates and temperatures. It was observed that the compressive fracture stresses were higher than the tensile fracture stresses. In addition, the fracture stress was high at a low temperature and high strain rate and these fracture stresses are in good agreement with Christensen failure criterion curves. Furthermore, the long-term viscoelastic behavior can almost be predicted from the short-term viscoelastic behaviors at three different temperatures using time–temperature superposition principle. But, the simulations at a wide range of temperatures is important to predict the more accurate long-term viscoelastic behavior.     

Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)

This study proposes a comparison between three identification methods of the mechanical properties of a shape memory polymer (Veriflex®): quasi-static tensile tests, tensile dynamic mechanical analysis (DMA) and modal tests. The Young’s modulus and the Poisson’s ratio are determined at ambient temperature using the first technique. The DMA is used to determine the evolution of the viscoelastic properties versus the temperature and the frequency under harmonic loading. The modal analysis is used to identify the viscoelastic properties of the material at higher frequencies. The purpose of this study is to check the validity of the time–temperature equivalence for the Veriflex® obtained from the DMA measurements. It is shown that the viscoelastic properties predicted through the master curve are consistent with the measurements collected using quasi-static and modal test. The aging effect on SMP properties is also quantified.     

Tensile,creep and fatigue behaviours of short fibre reinforced polymer composites at elevated temperatures: a literature survey

Polymer composites are suitable alternatives to metals in some applications as they are cost effective, lightweight and corrosion resistant. Short fibre reinforced polymer composites (SFRPCs) are typically subjected to complex loadings in applications, including static, cyclic, thermal and their combinations. These applications may also involve harsh environmental conditions such as elevated temperature and moisture which can dramatically affect mechanical properties. In this paper, a broad survey of the literature on mechanical behaviour of SFRPCs at elevated temperatures is presented. The mechanical behaviours included consist of tensile, creep, isothermal fatigue, thermo‐mechanical fatigue and creep–fatigue interaction. Environmental effects such as moisture and ageing at elevated temperatures are also included. The studies reviewed include experimental works, modelling works and failure mechanisms studies. A critical assessment of the information from the literature presented for each type of behaviour is also provided.     

Long-term creep-rupture failure envelope of epoxy

An accelerated testing methodology based on the time-temperature superposition principle has been proposed in the literature for the long-term creep strength of polymer matrices and polymer composites. Also, it has been suggested that a standard master curve may be a feasible assumption to describe the creep behavior in both tension and compression modes. In the present research, strength master curves for an aerospace epoxy (8552) were generated for tension and compression, by shifting strength data measured at various temperatures. The shift function is obtained from superposition of creep-compliance curves obtained at different temperatures. A standard master curve was presented to describe the creep-rupture of the polymer under tension and compression. Moreover, long-term creep-rupture failure envelopes of the polymer were presented based on a two-part failure criterion for homogeneous and isotropic materials. Ultimately, the approach presented allows the prediction of creep-rupture failure envelopes for a time-dependent material based on tensile strengths measured at various temperatures, considering that the ratio between tensile and compressive strengths is known.     

Time-temperature superposition and the controlling role of solvation in the viscoelastic properties of polyaniline thin films

Comprehensive exploration of the viscoelastic properties of polyaniline films exposed to aqueous perchloric acid has been made as a function of applied potential (E), temperature (T), and mechanical oscillation frequency (f = ω/2π) using high-frequency acoustic wave resonators. The outcomes are expressed in terms of storage and loss shear modulus signatures, G'(E, T, ω) and G″(E, T, ω). Surprisingly, these are barely sensitive to potential, through which both polymer charge and solvation are manipulated, and only modestly sensitive to temperature. In contrast, the response to timescale is dramatic. Using the principle of time-temperature superposition, G' and G″ at different temperatures and frequencies (time scales) can each be placed on master relaxation curves. Models developed for mechanical properties of bulk polymers at low frequency were applied to these thin film responses at high frequency. These include the Williams-Landel-Ferry model, the activation model, and the Rouse-Zimm model based, respectively, on concepts of free volume, thermal activation, and relaxation. Each of the models could be applied with physically reasonable outcomes in terms of the relevant parameters (thermal expansion coefficient, glass transition temperature, and activation enthalpy). G' and G″ values are correlated with solvent content. The enthalpy change for solvent entry is small, positive and relatively independent of polymer charge state, all of which contrast sharply with the behavior of thiophene-based conducting polymers in organic solvents.     

Modelling the effects of temperature and loading rate on fatigue properties of hot mixed asphalt

Fatigue cracking is one of the primary distresses of asphalt pavement. Critical strain energy density (CSED) has shown great potential to be a material parameter for fatigue cracking prediction. For the CSED to be used in future fatigue model and pavement design, a model is needed to predict the CSED as a function of the loading rate and the temperature, analogous to dynamic modulus. In this study, indirect tensile (IDT) tests were conducted to determine the properties of hot mixed asphalt at different loading rates and temperatures. It was found that time–temperature superposition principle is valid for IDT strength at both low and intermediate temperatures; and valid for failure strain and for the CSED at intermediate temperatures only. The shift factors for dynamic modulus were close to those of IDT strength and CSED, respectively. However, there was a discrepancy between shift factors of dynamic modulus and those of failure strain.     

The effects of atmospheric aging on a hybrid polymer matrix composite

The effect of thermal exposure in an atmospheric environment for up to 1 year on the flexural performance, under both static and fatigue loading, of a glass fiber/carbon fiber hybrid polymer matrix composite material was evaluated. It was found that exposure to a temperature near, but below, the glass transition temperature resulted in diminished flexure strength as well as reduced fatigue performance. The magnitude of property reduction was, in general, proportional to the amount of aging time, and was found to be dictated by the dominant aging mechanism. Scanning electron microscopy revealed that the modest reduction in mechanical properties at intermediate aging times was predominantly attributed to thermal oxidation, while for longer aging times thermal aging (dimensional relaxation) was the primary cause for the substantial reduction. Dimensional relaxation of the composite was measured at several isothermal aging temperatures, from which, the activation energy of the aging process was determined. This work provides insight into the evolution of mechanical properties as a function of aging time in an atmospheric environment for a hybrid polymer matrix composite.     

Mechanical and thermal expansion properties of a participate filled polymer

The use of particulate filled polymer formulations over wide temperature ranges has resulted in a need to understand their mechanical behaviour better. In this investigation, a crosslinked epoxy-urethane polymer filled with Al₂O₃ particles was studied. Mechanical and thermal expansion properties were determined at ambient and liquid nitrogen temperatures and compared to theoretical predictions. Other parameters under consideration were volume fraction and adhesion between filler and matrix. The theoretical equations employed for predicting mechanical properties appear fairly reliable at ambient temperature but unreliable at liquid nitrogen temperatures. The degree of bonding between filler and matrix influences mechanical properties at ambient temperature but at liquid nitrogen temperature no difference in properties owing to matrix filler bonding was evident. This result is attributed to compressive stresses on the filler particles resulting from the lower thermal expansion of the filler.     

Mechanochemically prepared reactive and energetic materials: a review

Reactive and energetic materials are typically metastable and are expected to transform into thermodynamically favorable reaction products with substantial energy release. Preparation of such materials by mechanical milling is challenging: They are easily initiated by impact or friction. At the same time, milling offers a simple, scalable, and controllable technology capable of mixing reactive components on the nanoscale. In most cases, for reactive materials milling should be interrupted or arrested to preserve the metastable phases. Arrested reactive milling was exploited to prepare many inorganic reactive materials, including nanocomposite thermite, metal–metalloid, and intermetallic systems. Prepared materials are fully dense composites with unique properties, combining high density with extremely high reactivity. Different milling devices were used to prepare reactive materials and an approach was developed to transfer the process conditions between different mills. Different milling protocols, such as milling at cryogenic temperatures or staged milling can be used to prepare hybrid reactive materials with different components mixed on different scales; it was also used to tune the particle size distributions of metal-based reactive material powders. Metal–halogen composites were prepared, with metal matrix stabilizing a halogen (e.g., iodine) at temperatures substantially exceeding its boiling point. Mechanochemically prepared reactive materials can be classified based on the energy of reaction between components and the energy of oxidation of the bulk material composition. Work on mechanochemical preparation of reactive and energetic materials is reviewed with the focus on unique properties and ignition and combustion mechanisms of the mechanochemically prepared reactive materials. An ignition mechanism for nanothermites involving preignition reaction leading to a gas release preceding rapid temperature rise is discussed. A combustion mechanism is also discussed, in which the nanostructure of the mechanochemically prepared material is preserved despite the very high combustion temperatures.