Dynamic properties of crosslinked epoxy resin at large cyclic and static deformation

Measurement of dynamic properties of crosslinked epoxy resin have been performed under torsional cyclic deformation with different amplitudes and frequencies and with extensional creep under different loads. It is found in both cases that the dynamic modulus decreases above a certain critical value of deformation. Truncation of the glassy state region and shifting of the transition zone to lower temperatures and higher frequencies have been observed as effects of large amplitude deformation. The maximum reduction in the modulus value and the minimum in the critical amplitude both occur in the region of T_g Shear fatigue of the material has been observed in the glassy state with a frequency- and temperature-dependent fatigue life. It is found that the loss modulus under extensional creep depends upon the values of the deformation and stress whereas the storage modulus depends solely upon the deformation. The ratio of energy expended during static and cyclic deformations is shown to depend only upon the extensional deformation.     

Cyclic fatigue—crack growth along polymer/glass interfaces

Delamination of polymer/glass interfaces was studied under cyclic and monotonic loading using an interfacial, four-point flexure sandwich specimen. Specifically, crack growth rates along epoxy acrylate/glass interfaces were characterized over a range of velocities from 10⁻⁹ to 10⁻⁶ m/s as a function of low (10–20% RH) and high (75–80% RH) humidities. For low humidities, interfacial crack growth rates under cyclic loading are almost two orders of magnitude greater than those under monotonic loading with energy release rates G = G_max of the cyclic loading. At high humidities, interfacial crack growth rates under monotonic loading are approximately equal to cyclic fatigue crack growth rates at low humidity.     

Effect of temperature on the compressive stress-strain properties of polystyrene

The compressive stress-strain behavior of a commercial polystyrene has been studied and the effect of deformation temperature on modulus, yield stress, percent yield strain and yield energy was determined. Yield energy is the only one of these parameters that is linear with temperature in the ductile region. A change in the mode of failure from ductile to brittle occurs between 5–30°C at a strain rate of O.1/in./in./min. At all temperatures studied, the yield or fracture stress varied linearly with the rate of deformation for strain rates ranging from 0.1 to 1.0 in./in./min. The yield data as a function of temperature were analyzed via a rate expression modified to incorporate the Coulomb-Navier yield criterion, Activation energy was found to be a function of deformation temperature with a change in slope occurring near the β transition. Activation volume increased linearly with deformation temperature, for the range studied. Agreement of dynamic mechanical and yield activation energies imply that the type of motion and the height of the energy barrier are similar for both. However, an increase in activation volume for stressed vs unstressed conditions suggests that a greater number of chain segments move as a result of stress biasing. Also the increase of both activation volume and activation energy with temperature implies that the correlated length of chain movement increases as temperature is increased. Similar to activation energy, yield stress exhibits a change in temperature dependence near the β transition. Data on other glassy polymers suggest that the highest temperature sub-T_g, transition is related to the change in the temperature dependence of yield stress.     

A micro-macro correlation of ozone-induced fracture in rubber

Rate of molecular bond rupture is successfully correlated by a Griffith-type energy balance to the strain energy release rate during ozone cracking of rubber. Rate of bond rupture is determined from electron paramagnetic resonance (EPR) measurements. The rate of strain energy release is determined from stress–elongation measurements during stress relaxation, creep, and cyclic loading tests. To compare with macroscopic crack studies, it was assumed that each ruptured bond created a given amount of fracture surface. Numerical agreement could be obtained by assuming each broken bond results in the production of an area of approximately 10^?13 cm². Using the surface energy density determined from stress relaxation tests in an energy balance gives surprisingly accurate predictions of expected behavior in creep and cyclic loading tests. There is a one-to-one correspondence between the rate of crack growth (bond rupture) and rate of energy release from strain and external work in all cases. It is proposed that such correlations give credence to a Griffith-type approach to environmental cracking which it did not have previously.     

Evolution of mechanical properties of polypropylene separator in liquid electrolytes for lithium‐ion batteries

Knowledge of the mechanical behaviors of polymeric separators immersed in liquid electrolytes is of great significance for predicting the long‐term performance of lithium batteries with high performance and safety. In terms of tensile tests, heating shrinkage, and dynamic mechanical analysis as well as the essential work of fracture method, the study reported here encompasses a systematic investigation of the mechanical properties of a typical commercial polypropylene separator in mixtures of ethylene carbonate and dimethyl carbonate and lithium hexafluorophosphate (LiPF₆), comparing with the results in ionic liquid (IL) electrolyte composed of lithium tetrafluoroborate (LiBF₄) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF₄) and dry condition. It has been found that liquid electrolytes have obvious negative effect on the dimensional stability at elevated temperature and mechanical properties, especially on crack resistance of the polymer separator. LiBF₄‐BMIBF₄ has much smaller damage on the strength, Young's modulus and fracture toughness of separator than the organic solution except the dynamic modulus at high temperature. Notably, the maximum tensile stress, Young's modulus and the reciprocal of relaxation time of the polymer separator are linearly dependent with strain rate under quasi‐static condition, and the relaxation time has clarified the coupling effect mechanism of liquid electrolyte and loading rate. Moreover, the non‐dimensional viscoelastic constitute equation could perfectly track the tensile behavior of wet and dry separators at different strain rate, and a property model could well characterize the temperature‐dependent storage modulus of polymer separators from rubbery to viscous state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46441.     

The Fracture Toughness of Adhesive-Bonded Joints

Fracture mechanics is related to adhesion theory and the testing of adhesive-bonded joints in the lap-shear configuration. The complexity of the stress field necessitates the strain energy release rate approach, which is followed to derive the relation for a lap-shear sample: G_c = P_c ²/4b (dC/da). G_c is the fracture toughness (critical strain energy release rate), P_c is the breaking or crack instability load, a and b are crack lengths and widths, respectively, and C is the sample compliance for the Tap-shear sample with a crack of these dimensions at each loading edge. It was found that G_c ranged from 1.18 to 1.42 with an average value of 1.34 in.-lb./in.² for epoxy bonded aluminum strips (EPON 934 and Alcald 2024-T3). Evidence, in the form of photoelastic stress patterns, suggesting that crack extension occurs in the opening mode in lap-shear samples is presented and discussed.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.     

Tensile yield in poly(chlorotrifluoroethylene) and poly(vinylidene fluoride)

Uniaxial tension tests to the yield point were performed on poly(chlorotrifluoroethylene) (PCTFE) and poly(vinylidene fluoride) (PVF2) from room temperature to near the melting point at a strain rate of 2 min^?1. At room temperature and at least two elevated temperatures, measurements were also made at strain rates from 0.02 to 8 min^?1. The properties of these polymers were found to be similar to those of other semicrystalline polymers. In the absence of other transitions, yield energy was found to be a linear function of temperature extrapolating to zero near the melting temperature. The ratio of thermal to mechanical energy to produce yielding is smaller than for glassy polymers. Yield stress is a linear function of log strain rate. The ratio of yield stress to (initial) Young's modulus is about 0.03 at room temperature for both polymers. Yield stress is a linear function of unstrained volume. Yield strain, elastic, and plastic strain all initially increase with temperature, but PCTFE shows a decrease with temperature starting at about 100°C, thus behaving like a glassy amorphous polymer in this region.     

Influence of the dynamic stress intensity factor on cyclic crack propagation in polymers

A relationship of the form developed for the evaluation of cyclic crack propagation in tensile/tensile fatigue was used to investigate the effects of frequency on fatigue crack growth. In order to establish the correlation between stress intensity K and dK/dt at 21°C, dynamic fracture toughness tests were performed on a range of polymers. It was shown that, in general, fracture toughness increased with the strain rate applied. Consequently, a decreasing trend in the crack growth rate was observed in the fatigue tests performed at higher frequencies. The occurrence of other localized peaks of fracture toughness recorded at various temperatures and strain rates is described. The fractography of fatigue surfaces is also discussed.     

Mechanical hysteresis of a polyether polyurethane thermoplastic elastomer

The mechanical hysteresis of a polyether polyurethane thermoplastic elastomer was studied as a function of temperature, percent strain, and deformation energy. Hysteresis values remained small at low temperatures when the extent of the sample deformation did not disrupt the glassy matrix. This was readily evident at temperatures below the glass transition temperature, T_g of the polymer where the material did not formally yield. At temperatures above the T_g of the polymer, hysteresis remained small even at substantial strains levels and demonstrated the capabilities of the hard segment domains to act as physical crosslinks. At elevated temperatures, percent hysteresis increased as the hydrogen-bonded hard segment domains weakened. When mechanical hysteresis was considered on the basis of constant deformation energies, hysteresis values reached a maximum in the vicinity of the T_g of the polymer. These maxima existed as a consequence of two opposing trends: the decreasing resiliency of the polymer as it becomes a glass and the increase in the resistance of that glass to undergo deformations sufficient to cause plastic flow. Finally, a hysteresis response surface constructed as a function of deformation energy and temperature was found to be sensitive to both the strain-induced crystallization of the rubbery soft segment matrix and to the strain-induced yielding of the glassy soft segment matrix.     

An experimental study of uniaxial fatigue behavior of an epoxy resin by a new noncontact real‐time strain measurement and control system

Uniaxial fatigue behavior of an epoxy resin was investigated with a recently established non‐contact real‐time strain measurement and control system. A relation of strain amplitude vs. fatigue life for fully‐reversed strain‐range‐controlled uniaxial fatigue tests was obtained. Quantitative analyses of evolutions of various mechanical properties (including stress range, elastic modulus, nonlinear stress‐strain relation, dissipated strain energy density, etc.) during entire fatigue life period were carried out based on recorded stress‐strain data. From the evolution of the stress‐strain hysteresis loops, a gradual degradation of modulus and a decrease of nonlinear effect in stress‐strain response were observed. It was also found that these two phenomena were independent of the loading control mode (stress‐control or strain‐range‐control) and the mean stress/strain values in the cyclic loading. Fractographic analysis was also performed and the mechanism of crack initiation and propagation of the epoxy material under cyclic loading was investigated. POLYM. ENG. SCI., 47:780–788, 2007. © 2007 Society of Plastics Engineers     

Crack growth behavior of natural rubber influenced by functionalized carbon nanotubes

In this work, the silane coupling agent bis‐(triethoxysilylpropyl) ‐tetrasulfide (TESPT) is used to modify the carbon nanotubes. After modification, carbon nanotubes can be well dispersed in the natural rubber (NR) matrix and form a strong and flexible network. Based on the original real‐time crack tip morphology monitoring, crack propagation and scanning electronic microscopy tests, it is revealed that modified carbon nanotubes filled NR samples (NR/F‐CNTs) have better crack resistance. It is found that modified carbon nanotubes can resist the cavitation process during cyclic loading. Crack tip morphology monitoring tests indicate that the crack tip of NR/F‐CNTs is rougher and the ligaments are thinner and densely distributed. A crack branching phenomenon is also observed. It proves that F‐CNTs increase the energy consumption of NR during cyclic loading. It is concluded that the F‐CNTs used in this work improve the crack resistance of NR in two ways: the one is cavitation resistance and the other is the increase of energy consumption for crack propagation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44527.     

Damage development and lifetime prediction of cross-ply ceramic-matrix composites subjected to cyclic loading at room and elevated temperatures

ABSTRACT

In this paper, the damage development and lifetime prediction of fibre-reinforced ceramic-matrix composites subjected to cyclic loading at elevated temperatures in oxidising atmosphere have been investigated. Considering the damage mechanisms of matrix cracking, interface debonding, interface wear and interface oxidation, the damage evolution of fatigue hysteresis dissipated energy, fatigue hysteresis modulus, fatigue peak strain, interface shear stress and broken fibres fraction have been analysed. The relationships between damage parameters and internal damage of matrix cracking, interface debonding and slipping, and fibres fracture have been established. The experimental fatigue hysteresis, interface slip lengths, peak strain, and the fatigue life curves of cross-ply CMCs under cyclic loading at elevated temperature have been predicted. The different fatigue behaviour in unidirectional and cross-ply CMCs at room and elevated temperatures subjected to low-cycle and high-cycle fatigue has been discussed.     

A constitutive model for the nonlinear viscoelastic viscoplastic behavior of glassy polymers

Two features of the glassy state of an amorphous polymer, which play a key role in determining its mechanical properties, are the distributed nature of the microstructural state and the thermally activated (temporal) evolution of this state. In this work, we have sought to capture these features in a mechanistically motivated constitutive model by considering a distribution in the activation energy barrier to deformation in a thermally activated model of the deformation process. We thus model what is traditionally termed the nonlinear viscoelastic behavior as an elastic-inelastic transition, where the energetically distributed nature of inelastic events and their evolution with straining is taken into account. The thermoreversible nature of inelastic deformation is modeled by invoking the notion of strain energy stored by localized inelastic shear transformations. The model results are compared to experimental data for constant true strain rate uniaxial compression tests (nonmonotonic) at different rates and temperatures; its predictive capabilities are further tested by comparison with compressive creep tests at different stress levels and temperatures.     

Thermal and strain rate sensitive compressive behavior of polycarbonate polymer - experimental and constitutive analysis

The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 ^?3 to 10³ and temperature range of 213 to 393 K. By conducting these experiments, the true stress–strain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress–strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.     

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

Polyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔK_eff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode-I intensity factor range ΔK_I, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y-shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out-of-phase biaxial loading and uniaxial loading.     

Damage Development and Moduli Reductions in Nicalon—Calcium Aluminosilicate Composites under Static Fatigue and Cyclic Fatigue

Unidirectional and cross-ply Nicalon fiber-reinforced calcium aluminosilicate (CAS) glass-ceramic composite specimens were subjected to tension–tension cyclic fatigue and static fatigue loadings. Microcrack densities, longitudinal Young's modulus, and major Poisson's ratio were measured at regular intervals of load cycles and load time. The matrix crack (0° plies) density and transverse crack (90° plies) density increased gradually with fatigue cycles and load time. The crack growth is environmentally driven and depends on the maximum load and time. Young's modulus and Poisson's ratio decreased gradually with fatigue cycles and load time. The saturation crack densities under fatigue loadings were found to be comparable to those under monotonic loading. A matrix crack growth limit strain exists, below which matrix cracks do not grow significantly under fatigue loading. This limit coincides with the matrix crack initiation strain. Linear correlations between crack density and moduli reductions obtained from quasi-static data can predict the moduli reductions under cyclic loading, using experimentally measured crack densities. A logarithmic correlation can predict the Young's modulus reduction in a limited stress range. A fatigue crack growth model is proposed to explain the presence of two distinct regimes of crack growth and Young's modulus reduction.     

Fatigue crack growth in polymers. I. Effect of frequency and temperature

The effects of frequency, from 0.1–100 Hz, and temperature, ?60°C to +21°C, on fatigue crack propagation in poly (methyl methacrylate) and polycarbonate were investigated. A cyclic crack propagation law proposed by Arad-Radon-Culver, namely where λ is (K_max²-K_min²) and K_max and K_min are the respective values of maximum and minimum stress intensity factor, was applied to describe a relationship between crack growth and cyclic life. Cyclic tests performed in tension between zero load and K_max showed a linear relationship between the crack lengths and the number of cycles for all temperatures and frequencies tested. It was found that, in general, the cyclic crack growth decreased with decreasing temperature and increasing frequency. However, important exceptions to this rule have been noted.     

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54°) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66° than for 5°, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth at polymer adhesive interfaces.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.