Acoustic emission study of the TDCB test of microcapsules filled self-healing polymer

The failure process of self-healing microcapsule filled epoxy composites was investigated with Acoustic Emission (AE) measurement during tapered double cantilever beam (TDCB) tests. Unfilled, epoxy microcapsule filled and self-healing microcapsule (dual-microcapsules consisting of epoxy capsules and trifluoromethanesulfonic acid (TfOH) capsules) filled epoxy matrix specimens were compared. It was found that, with acoustic emission measurements, reliable data can be retrieved about the nature and progress of crack propagation and failure. Comparing the unfilled and the microcapsule filled specimens, considerably more AE hits were detected in the latter case due to the cracking of capsules and capsule-matrix debonding, which was confirmed by scanning electron microscopy investigations. Also, key differences were observed in the shape of the sum of AE hits curves and amplitude ranges of detected AE hits. Uniform distribution of the healing agent was demonstrated by energy-dispersive X-ray spectroscopy.     

Failure of elastomeric polymers due to rate dependent bond rupture

A new cohesive zone model is developed in order to study the mechanisms of adhesive and cohesive failures of soft rubbery materials. The fracture energy is estimated here using a strategy similar to that of Lake and Thomas (LT) by considering the dissipation of stored elastic energy followed by the extension and relaxation of polymer chains. The current model, however, departs from that of LT in that the force needed to break an interfacial bond does not have a fixed value; instead, it depends on the thermal state of the system and the rate at which the force is transmitted to the bond. While the force required to rupture a chain is set by the rules of thermomechanically activated bond dissociation kinetics, extension of a polymer chain is modeled within both the linear and nonlinear models of chain elasticity. Closed form asymptotic solutions are obtained for the dependence of crack propagation speed on the energy release rate, which are valid in two regimes: (I) slow crack velocity or short relaxation time for bond dissociation; (II) fast crack velocity or long relaxation time for bond dissociation. The rate independent and the zero temperature limit of this theory correctly reduces to the fracture model of LT. Detailed comparisons are made with a previous work by Chaudhury et al. which carried out an approximate analysis of the same problem.     

Stress dependence of optically active diamagnetic point defects in silicon oxynitride

The cathodoluminescence (CL) spectrum arising from diamagnetic point defects of silicon oxynitride lattice was analyzed to extract quantitative information on local stress fields stored on the surface of a silicon nitride polycrystal. A calibration procedure was preliminarily made to obtain a relationship between CL spectral shift and applied stress, according to the piezo-spectroscopic effect. In this calibration procedure, we used the uniaxial stress field developed in a rectangular bar loaded in a four-point flexural jig. Stress dependence was clearly detected for the most intense spectral band of a doublet arising from diamagnetic ([triple bond]Si-Si[triple bond]) defects, which was located at around 340 nm. The shallow nature of the electron probe enabled the characterization of surface stress fields with sub-micrometer-order spatial resolution. As applications of the PS technique, the CL emission from [triple bond]Si-Si[triple bond] defects was used as a stress probe for visualizing the residual stress fields stored at grain-boundary regions and at the tip of a surface crack propagated in polycrystalline silicon nitride.     

应变速率和电位对应力腐蚀裂纹扩展的影响

以滑移-溶解-再钝化模型为基础,推导出应力腐蚀裂纹扩展速率与裂尖应变速率和电位之间的理论公式.计算表明,在裂纹扩展速率与裂尖应变速率的关系曲线中有两个特征区域.裂纹扩展速率在区域I随裂尖应变速率增加而增大,而在区域II不随裂尖应变速率的改变而变化.用慢应变速率拉伸技术（SSRT）测量了304L不锈钢的裂纹增长速率.当电位控制在区域II的阳极区时,理论计算的裂纹扩展速率与实验得到的结果比较吻合.     

Nanocavitation around a crack tip in a soft nanocomposite: A scanning microbeam small angle X‐ray scattering study

We explore nanocavitation around the crack tip region in a styrene‐butadiene random copolymer filled with typical carbon black (CB) particles used in the rubber industry for toughening the rubber. Using quasistatic loading conditions and a highly collimated X‐ray microbeam scanned around the crack tip, we demonstrate the existence of a damage zone consisting of nanovoids in a filled elastomer matrix. The existence of voids near the crack tip is demonstrated by a significant increase of the scattering invariant Q/Q₀ in front of both fatigued and fresh cracks. The size of the zone where cavities are present critically depends on the macroscopic strain ε_m, the loading history, and the maximum energy release rate G applied to accommodate the crack. Our findings show that nanovoiding occurs before fracture in typical CB‐filled elastomers and that realistic crack propagation models for such elastomers should take into account a certain level of compressibility near the crack tip. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 422–429     

A finite element method based comparative fracture assessment of carbon black and silica filled elastomers: Reinforcing efficacy of carbonaceous fillers in flexible composites

This study is focused on numerical investigation on fracture behaviors of carbon black (CB) and silica filled elastomeric composites. Finite element analysis (FEA) in compliance with multi-specimen method is used to calculate J-integral and geometry factor of the rubber composites up to a displacement of 20 mm for single edge notch in tension (SENT) and double edge notch in tension (DENT) specimens. An empirical relationship between crack tip opening displacement (CTOD) and crack advancement is established depending on notch to width ratio (NWR). The stress contours across the notches for SENT and DENT specimens is discussed briefly. It is found that fracture propagation resistance of CB filled elastomer is 125% more than that of silica filled elastomer. Although, Silica filled elastomer have good tensile strength and crosslink density but it fails to replace carbon black in terms of fracture properties. The critical J-integral for CB filled elastomer is 18.7% and 32.2% more than silica filled elastomer for SENT and DENT specimens respectively. The effect of specimen type on various fracture properties is also explored. The factor of safety is found to be significantly more in case of CB filled elastomers making them less vulnerable to crack propagation and catastrophic failure.     

Crystal growth of polyethylene from dilute solution: Growth kinetics of {110} twins and diffusion-limited growth of single crystals

We have studied the growth kinetics of {110} twins and single crystals of polyethylene in dilute solution of tetrachlorethylene. In terms of {110} twins, we succeeded in obtaining twins without {100} sectors, using a relatively high molecular weight fraction M_w > 10⁴. It is confirmed that the growth is enhanced at the reentrant corner of the twins, and the enhanced growth face inclines to the {110} face because of consecutive generation of steps at the corner. These facts are strong evidence for nucleation-controlled growth of single crystals. The growth rates and obliquity are measured at various supercoolings and concentrations. From consideration of kinetics of steps on the growth face, the following rates and velocity are independently determined from the experimental data: nucleation rate on a flat face, velocity of step propagation, and generation rate of steps at the reentrant corner. The supercooling dependence strongly supports regime II growth. The results on concentration dependence show that the velocity of steps is proportional to concentration over the whole range examined, and the nucleation rate is independent of it in the usual range and becomes proportional to it in the lower range. This concentration dependence of nucleation rate is attributed to the density of adsorbed polymer on the growth face. From this evidence, it is suggested that the rate of travel of steps is limited by volume diffusion of solute polymer, whereas the growth face is saturated with adsorbed polymer at ordinary concentrations. This contradictory situation could be explained by the hypotheses that the saturation density is rather low and that surface diffusion of adsorbed polymer is much slower than volume diffusion of solute polymer. The lower limit of the rate of folding is also determined for the first time from the velocity of step propagation. As regards the single crystals, it is found that the habit maintains a lozenge shape with sharpened points, even at very high supercooling (δT < 50°C) if the concentration is very dilute. Diffusion-limited growth is verified for the first time at the higher supercoolings, where the growth rate is almost independent of supercooling. The growth rate becomes almost equivalent to the velocity of steps determined in the experiments with twins, and this fact will support the accuracy of the evaluation of the step velocity. The order of magnitude of the growth rate obtained agrees with the value which is calculated from the balance between the flux of solute polymer to the growth face and the rate of growth of single crystals.     

Decompression wave speed and crack velocity measurements during S4 test in water pressurized plastic pipes: Part 1. Experimental methods

In a Small Scale Steady State (S4) test apparatus of ISO 13477, instrumentations were designed and successfully adapted to determine decompression wave speed and crack velocity during rapid crack propagation event in water-filled plastic pipes. The basic design for decompression wave speed measurement involved the use of high-frequency dynamic pressure transmitters, located external to the water-filled pipe and connected to pressure measurement positions inside the pipe, by means of stainless steel tubes. For the crack velocity measurements, timing wire system with the required circuitry capable of giving the precise temporal indication of the propagating crack was designed and employed. In this paper, detailed design of instrumentations adapted to the S4 test apparatus and the assessment techniques used to obtain decompression wave speed and crack velocity are described. It was also demonstrated that the methods developed were viable for these measurements which are known to affect the rapid crack propagation behavior in water-filled plastic pipes.     

有机玻璃断裂韧性断面形态组份结构关系的研究

测定了有机玻璃的抗裂纹增长因子K_(1C)与材料组份、拉力机夹头速度的关系。发现增塑剂和交联剂对抗裂纹性能的影响,可从聚合物分子运动能力对裂纹端点塑性屈服过程的影响说明。在对断面形态、断面层厚度、折光指数以及K_(1C)值与拉力机夹头速度关系研究之后,认为用“撕布”模式和次级断裂模式分别解释慢裂纹、快裂纹扩展过程中的实验现象是合适的。     

Acid–Base‐Responsive Intense Charge‐Transfer Emission in Donor–Acceptor‐Conjugated Fluorophores

Herein we report on the synthesis and acid‐responsive emission properties of donor–acceptor (D–A) molecules that contain a thienothiophene unit. 2‐Arylthieno[3,2‐b]thiophenes were conjugated with an N‐methylbenzimidazole unit to form acid‐responsive D–A‐type fluorophores. The D–A‐conjugated fluorophores showed intense intramolecular charge‐transfer (ICT) emission in response to acid. The effect of the substitution on their photophysical properties as well as their solvent‐dependence indicated non‐twisting ICT emission in protonated D–A molecules. The quinoidal character of 2‐arylthienothiophene as a donor part is discussed, as it is assumed that it contributes to suppression of the molecular twisting in the excited state, therefore decreasing the nonradiative rate constant, thereby resulting in the intense ICT emission. Acid–base‐sensitive triple‐color emission was also achieved by the introduction of a base‐responsive phenol group in the donor part.     

Electron and positron pair emission by low energy positron impact on surfaces

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.     

Automated monitoring of the crack propagation in mode I testing of thermoplastic composites by means of digital image correlation

One of the main challenges of the mode I double cantilever beam (DCB) test is the simultaneous determination of the applied load and displacement with the developing delamination length. The present work addresses this issue by side-view tracking the crack propagation by means of digital image correlation (DIC). Two different reduction methods were developed to determine the crack length from the DIC data. On the one hand, the crack tip position was defined by the high strain concentration in the immediate vicinity of the crack tip, and on the other hand, by crack tip opening displacement (CTOD). The data obtained enabled the calculation of the energy release rate of carbon fibre reinforced thermoplastic specimens with either run-arrest or stable crack extension. For reasons of comparability, top surface analysis (TSA), as recently reported, was also carried out. Following this approach, the crack propagation was tracked applying DIC to the top specimen surface. The methods developed showed a good correlation with both the standardised procedure and TSA. It was shown that DIC can be used as an alternative to the conventional optical measuring tools to follow the crack propagation in the mode I DCB test.     

Circular dichroism in the angle-resolved C 1s photoemission spectra of gas-phase carvone enantiomers

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.     

Alternating crack propagation during directional drying

The propagation of fractures during the drying of a colloidal silica suspension confined in a vertical microtube is investigated. During the drying process, the particle concentration increases until gel formation. In the gelled region, the tensile stresses increase and lead to the formation of two vertical perpendicular cracks propagating in the drying direction, dividing the tube into four equivalent regions. Throughout the drying process, these two fractures do not propagate at the same velocity. The top crack inhibits the propagation of the crack that is left behind, and the slow propagation of a fracture is followed by the rapid propagation of the other.     

稀土对9Cr2Mo钢抗淬裂性能的影响

利用电镜和金相等手段研究了稀土元素对 9Cr2Mo钢抗淬裂性能的影响。9Cr2Mo钢中加入稀土元素可以提高出现裂纹的淬火次数 ,延缓淬火裂纹的扩展速度 ,改变淬火裂纹的扩展途径 ,从而提高 9Cr2Mo钢的抗淬裂能力。     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

Suppression of atomic radiation in a cylindrical nanocavity

An excited atom placed inside a cylindrical nanocavity cannot radiate if the frequency of emission is below the cutoff frequency for electromagnetic wave propagation in the cavity. However, we demonstrate that it can be de-excited through the emission of surface plasmons. The observable effect will be a substantial enhancement in the lifetime of dipole-allowed transitions. The recently discovered carbon nanotubules will be explored as potential nanometer scale cylindrical cavities and the lifetimes of excited states of Helium (2p → 1s) in the earbon nanocavities will be calculated as an example of this nonradiative decay mechanism. The dependence of the transition rate as a function of the radial distance from the tube surface is studied by varying the initial radial quantum number of the center-of-mass wavefunction for an atom confined within the nanotube. The results of the calculation are used to explore the possible application of concentric carbon nanotube structures as TEM waveguides.     

Auger spectrum of a water molecule after single and double core ionization

The high intensity of free electron lasers opens up the possibility to perform single-shot molecule scattering experiments. However, even for small molecules, radiation damage induced by absorption of high intense x-ray radiation is not yet fully understood. One of the striking effects which occurs under intense x-ray illumination is the creation of double core ionized molecules in considerable quantity. To provide insight into this process, we have studied the dynamics of water molecules in single and double core ionized states by means of electronic transition rate calculations and ab initio molecular dynamics (MD) simulations. From the MD trajectories, photoionization and Auger transition rates were computed based on electronic continuum wavefunctions obtained by explicit integration of the coupled radial Schro?dinger equations. These rates served to solve the master equations for the populations of the relevant electronic states. To account for the nuclear dynamics during the core hole lifetime, the calculated electron emission spectra for different molecular geometries were incoherently accumulated according to the obtained time-dependent populations, thus neglecting possible interference effects between different decay pathways. We find that, in contrast to the single core ionized water molecule, the nuclear dynamics for the double core ionized water molecule during the core hole lifetime leaves a clear fingerprint in the resulting electron emission spectra. The lifetime of the double core ionized water was found to be significantly shorter than half of the single core hole lifetime.     

几种高聚物拉伸过程中的声发射现象

研究了４种塑料ＰＳ、ＰＯＭ、ＡＢＳ和ＰＣ拉伸过程中的声发射现象，发现声发射信号主要产生在屈服前和断裂时，说明高聚物的声发射信号主查由于分子链的解缠结，晶体的滑移裂纹的亚临界扩展和分子链的断裂等诸因素产生的，ＡＢＳ拉伸时大面积银纹的生成也可能是信号源，拉伸应变速率，材料的结构和不同的屈服过程等对塑料的拉伸声发射规律有较大影响。