Effect of strain rate on the compression behaviour of a woven fabric S2-glass fiber reinforced vinyl ester composite

Quasi-static (˜10⁻³s⁻¹) and high strain rate (>500 s⁻¹) compression behavior of an S2-glass woven fabric/vinyl ester composite plate was determined in the in-plane and through-thickness directions. In both directions, modulus and failure strength increased with increasing strain rate. A higher strain rate sensitive modulus was found in the through-thickness direction while a higher strain rate sensitive failure strength was found in the in-plane direction. In the in-plane direction, the failure mode was observed to change from splitting followed by “kink banding” (localized fiber buckling) to predominantly splitting at increasing strain rates, while it remained the same in the through-thickness direction.     

Influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites

This paper reports the influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites. The compressive behaviors of plain woven composites along out-of-plane direction were investigated from experimental and numerical approaches. In experimental, the compressive stiffness and strength decreased as the size of plain woven composite specimens increased. In finite element analysis (FEA) model, a new microstructure model with random defect distribution was established to find the influence of inner defects and specimen size effect on the compressive behaviors under high strain rates. From the numerical results, the compressive strength, modulus and fracture strain decreased obviously with the increase of volume fraction and size of defects. We found that the good agreement existed between the testing and the FEA results. The defects size and distribution were the main factors to weaken the compressive stiffness and strength.     

Dynamic property and constitutive model of polyvinyl butaral material at high strain rates

The polyvinyl butaral (PVB) interlayer of automotive windshield plays an important role in the protection of both pedestrian and passenger, the mechanical property of PVB material should be in‐depth studied. In this article, the systematical uniaxial tensile experiments of PVB material under high strain rates are conducted, the strain rates range from 125.6 to 3768 s^?1. The results of experiments show that there exists a phenomenon of stress spurt caused by the stress hardening in the final stage of tension, and the strain rate exerts great influence on mechanical property of PVB material. Further, the data fitting basing on Mooney–Rivlin model is carried out, it is found that the fitting results are consistent with the experiment data, which means that the Mooney–Rivlin constitution model can describe the large deformation behavior of PVB material. At last, the rate‐dependent mechanical behavior of the PVB material is further investigated in this article. On the basis of the experiment results and Johnson–Cook model, a rate‐dependent constitutive model is proposed to describe the tensile mechanical property of PVB material under high strain rates. This work will be beneficial to the simulation and analysis of automotive collision safety and pedestrian safety protection, which are related to damage of automotive windshield. Copyright © 2014 John Wiley & Sons, Ltd.     

Mechanical behavior of basalt and glass textile composites at high strain rates: A comparison

The aim of this paper is to study and compare the mechanical behavior of woven basalt and woven glass epoxy composites at high strain rates, in order to assess the possibility of replacing glass fiber composites with basalt fiber composites for aircraft secondary structures, such as radomes, fairings, wing tips, etc. Both composites were produced using the same epoxy matrix, the same manufacturing technique, and with comparable densities, fiber volume fractions, and static stiffnesses. Dynamic tensile and shear experiments were performed using a split Hopkinson tension bar, in addition to reference quasi-static experiments to compare both material behaviors over a wide range of strain rates. Normalized results with respect to the material density and fiber volume fraction showed that basalt epoxy composite had higher elastic stiffness, ultimate tensile strength, ultimate tensile strain, and absorbed energy in tension compared to glass epoxy composite. This suggests a promising potential in replacing glass fibers composites with basalt fiber composites in aircraft secondary structures and, more generally, components prone to impact. However, for the basalt epoxy composite, improvements in the fiber-matrix adhesion and in the manufacturing technique are still required to enhance their shear properties compared to glass fiber composites, and fully exploit the potential of basalt epoxy composites in aeronautical applications.     

Effects of the strain rate and temperature in stress–strain tests: study of the glass transition of a polyamide-6

In this work new insights are presented on the measurement of the tangent and secant moduli from stress–strain curves in polymeric systems. Expressions for the strain-rate and strain dependence of both moduli are derived for systems characterised by a distribution of relaxation times. The equivalent frequency of the stress–strain experiments is shown to be dependent on the strain rate and on the strain at which the measurements are carried out. Such considerations enable using quasi-static tensile stress–strain tests to study relaxational processes in polymeric materials. The tensile behaviour of a 30% glass fibre reinforced polyamide 6 was characterised at different strain rates and temperatures, covering the glass transition region. A master curve of the tangent modulus as a function of strain rate was successfully constructed by simple horizontal shifting of the isothermal data. The temperature dependence of the shift factors was well described by the WLF equation. It was also possible to fit the master curve considering a polymeric system with a distribution of relaxation times, relevant parameters such as the KWW β parameter being extracted. The results were found to be consistent with dynamic mechanical analysis results.     

Effect of bication (Cu-Cr) substitution on the structure and electrochemical performance of LiMn2O4 spinel cathodes at low and high current rates

This report describes the detailed structural and electrochemical characterization of a series of low content (0.01 to 0.05) Cu-Cr bi-metal doped LiMn₂O₄ cathode material synthesized by sol–gel method. The structural and morphological features were described using XRD, SEM, TEM, EDAX and FTIR techniques. The electron transfer and its feasibility were discussed through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The charge–discharge studies were performed to evaluate the capacity fading and rate capability. It was found that the electrochemical performance is very much dependent on the amount of Cu-Cr bi-metal doping and interestingly decreased the capacity fading with high cycleability. The sample with the least amount of dopants (i.e., LiCu_0.01Cr_0.01Mn_1.98O₄) demonstrated much improved capacity, cycleability and high rate capability. The LiCu_0.01Cr_0.01Mn_1.98O₄ cathode exhibited a discharge capacity of 112 mA h g^?1 at very first cycle and retained 93 mA h g^?1 after 100 cycles at a C rate of 0.3. Further, the same material at very high current density (5 C) retained 83% of the initial discharge capacity. The Cu-Cr doping stabilized the spinel structure by suppressing the Jahn-Teller distortion effect and Mn dissolution and the resultant material showed the workability of the cathodes for devices which work at substantially high C-rate of 5C.     

Variation of the strain rate during CSM nanoindentation of glassy polymers and its implication on indentation size effect

The indentation strain rate is currently assumed to remain unvaried during continuous stiffness measurement (CSM) nanoindentation where is imposed to remain constant. To probe the validity of this assumption for the nanoindentation of glassy polymers, a series of experiments have been performed at different set values on poly(methyl methacrylate) and polycarbonate using CSM technique. It is firstly shown that the actual value changes drastically at shallow indentation depths and it takes a considerable depth, which is material independent, for this parameter to attain a stabilized value. Furthermore, the strain rate is measured directly as the descent rate of the indenter divided by its instantaneous depth ( ), and indirectly via considering the variations of the load and hardness during the test. Both of these approaches reveal that the strain rate is considerably larger at shallow depths, and the depth beyond which it becomes constant is material and ratio dependent. Finally, by considering the relationship between the hardness and strain rate, it is observed that although the strain rate variation alters the hardness, its contribution is not able to justify the observed indentation size effect; hence, other contributing factors for this phenomenon are discussed for their possible effects. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2179–2187     

Methyl methacrylate emulsion polymerization at low monomer concentration: Kinetic modeling of nucleation,particle size distribution,and rate of polymerization

The results of a mathematical model developed in the authors' previous work are discussed and compared against final number (N) and size distribution of particles (PSD) and the rate of polymerization (RP) experimental data of methyl methacrylate (MMA) emulsion polymerization above the critical micelle concentration (cmc) of the surfactant. On the basis of the model results, the hypothesis that the observed bimodal PSD can be ascribed to secondary nucleation as proposed in the literature is questionable. It is discussed that this PSD can also be caused by differences in the growing rate of different‐size particles as predicted for styrene emulsion polymerization. Because of the small particle size obtained at low initial monomer concentration, the high rate of free‐radical desorption reduces the accumulation of these species; therefore, the autoacceleration effect is less pronounced for the conditions under study compared with the usual behavior of the RP during MMA emulsion polymerization above cmc. Similarities and differences between model predictions and experimental data are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2547–2556, 2001     

The temperature window of minimum flow resistance in melt flow of polyethylene. Further studies on the effect of strain rate and branching

The existence of a narrow temperature window (150–153°C) of smooth extrudability coupled with a minimum in flow resistance (extrusion pressure) in high-molecular weight polyethylene (>4 × 10⁵ g mol^?1) was the subject of a previous article where it was associated with strain-induced formation of the mobile hexagonal mesophase. The new findings of this note show that this minimum in flow resistance only sets in above a critical strain rate; this is interpreted in terms of the requirement of a critical strain rate in order to stretch molecules to their fully extended configuration. Furthermore, this critical strain rate is shown to be higher for lower molecular weight materials, in agreement with a priori considerations. Additionally, the temperature at which the pressure minimum occurs in a polyethylene containing methyl branches shifts to a significantly lower value than that for the linear material. This is interpreted in terms of the ? CH₃ groups raising the crystal free energy, thereby lowering the temperature at which the transition to the hexagonal phase occurs.     

Influence of the nature of ligands in iridium complexes with C60 fullerene and ·P(O)(OPri)2, ·CMe3, and ·CCl3 radicals on regioselectivity of addition and stability of spin-adducts formed: an EPR study

The addition of ^·P(O)(OPrⁱ)₂ (R¹), ^·CMe₃ (R²), and ^·CCl₃ (R³) radicals to metallofullerenes (η²-C₆₀)IrH(CO)(CNBu^t)₂(o-HCB₁₀H₉CCH₂PPh₂-B,P) (1), (η²-C₆₀)IrH(CO)(DIOP) (DIOP is (4R,5R)-(+)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane, 2), and (η²-C₆₀)IrH(CO)(PPh₃)₂ (3) was studied by EPR spectroscopy. A stability study of spin adducts (SAs) of R¹ radicals with complexes 1 and 2 showed that when the reactions are initiated by illumination with 366-nm light, the EPR spectra exhibit only signals of those isomers that are formed upon attack of the R¹ radicals on the carbon atoms of the cis-1 and cis-2 bonds (i.e., carbon atoms of the fullerene hemisphere to which the metallofragment is attached). Investigations of the reactions of R² and R³ radicals with complexes 1–3 initiated with 366-nm light made it possible to detect (i) regioisomers formed by adding these radicals to carbon atoms of the cis-n bonds and (ii) SAs formed by adding the radicals to carbon atoms of other bonds in complexes 1–3. The hyperfine structure of the EPR spectrum essentially depends on the spatial structure of substituents at the metal atom and allows individual regioisomers of not only phosphoryl radicals, but also carbon-centered radicals R² and R³ with metallofullerenes 1–3 to be identified. The rate constants for addition of R² and R³ radicals to complexes 2 and 3 were determined. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1302–1309, July, 2007.