The correlation of tear deviation and resistance with the bound rubber content in rubber-silica composites

The tear strength (T_S) of rubber-silica composites is inevitably lowered by the reduction of viscoelastic dissipation imparted by the use of bifunctional silanes. It is of interest to find out whether promoting crack tip deviation represented by a slip-stick tearing can compensate for such a loss in the tear strength. Here, the phenomenon of crack growth in terms of the T_S and also the tearing type is considered for both the untreated and silane-treated silica rubber composites to figure out the microstructure parameters affecting the slip-stick tearing. It was realized that within a certain volume fraction of the reinforcing filler, deviation whether in the form of slip-stick or knotty tearing can be found for both cases. Tearing for silane-treated silica is more similar to a slip-stick tearing with an ordered pattern of deviation and re-initiation; whereas tearing in the composites with untreated silica is like a knotty one with random deviation and re-initiation. Interestingly, a dual role was identified between the bound rubber content and the tearing: on one hand, increasing the bound rubber directly augments the viscoelastic dissipation and the value of T_S, and on the other, it inversely suppresses the crack tip deviation. The second part of this work deals with applying strategies to promote crack tip deviation in treated silica systems. By increasing the degree of bonding at the rubber-silica interface and reducing the bound rubber, the tear deviation was successfully promoted. With a slip-stick type of tearing the crack had to proceed through a tortuous path, thereby enhancing the effective tear diameter and the subsequent tear strength. This roughening role of bound rubber is however insufficient to fully compete with the impact of bound rubber on the viscoelastic dissipation, and thus the decreased T_S of composites with treated silica cannot be totally compensated by this strategy.     

Improved resistance to crack growth of natural rubber by the inclusion of nanoclay

The effect of nanoclay on the fatigue crack growth behavior was investigated. Fatigue tests were carried out on edge notched specimens under cyclic tension loadings. A power–law dependency between crack growth rate and tearing energy was obtained. Natural rubber (NR) filled with 5 phr organically modified montmorillonite (OMMT) possessed the lowest value of the exponent, b, and the smallest crack growth rate at a given tearing energy, denoting the strongest resistance to crack growth. Strain‐induced crystallization was probed by synchrotron WAXD experiments, showing earliest occurrence and strongest ability of crystallization in NR with 5 phr OMMT due to the better exfoliation and orientation of clay layers. The study on the viscoelastic property by dynamic mechanical analysis indicated that NR filled with 10 phr OMMT had the largest contribution to tearing energy attributed to the viscoelastic dissipation in the viscoelastic region in front of the crack tip. This revealed that the strain‐induced crystallization played a more important role in the crack growth resistance than the viscoelastic dissipation for clay filled rubber. Copyright © 2010 John Wiley & Sons, Ltd.     

Tearing of unvulcanized natural rubber

The tear behavior of unvulcanized natural rubber has been studied by using established techniques normally adopted for the study of vulcanized rubbers. Unvulcanized rubber has been found to tear in a relatively steady manner, in contrast to the stick-slip tear behavior of the vulcanized rubber, the tearing energy being dependent on the rate of tearing. Crystallization seems to be an important factor in determining the tear behavior since it has not been found possible to tear unvulcanized SBR under the same conditions. The effect of the pronounced imperfect elastic nature of the material was studied under conditions where the driving force for tearing was solely governed by the rate of release of elastic energy. Under such conditions, it has been found that the tearing energy is determined not by the strain energy required to stretch the material but by the energy which can be recovered on retraction. The set developed in the test piece, due to imperfect elasticity, has also to be taken into account.     

Structure and properties of styrene-butadiene rubber (SBR) with pyrolytic and industrial carbon black

To evaluate the performance of pyrolytic carbon black (pCB), we filled styrene-butadiene rubber (SBR) with pCB and N330 industrial carbon black (CB). We used two ratios of pCB and N330: 1/9 and 1/1. N330 was selected because its specific surface area was close to that of pCB. The overall CB content in the mixes was 0, 30, 45 and 60 part per hundred rubber (phr). We studied the effects of types and amounts of CB on the dispersion, cure behavior, dynamic mechanical thermal behavior, tensile mechanical and fracture mechanical properties of the filled rubbers. Dispersion of pCB was poorer than that of N330 CB. With respect to tensile mechanical properties – except tear strength – N330 outperformed pCB. The tear strength and fracture mechanics characteristics (J-integral at crack tip opening, and trouser tear strength) of SBR were higher with pCB than with N330. This can be attributed to the broader dispersion of pCB than N330. The combined use of N330 and pCB resulted in intermediate values, reflecting the actual N330/pCB ratio for all measured parameters. We wanted to correlate the mechanical performance with the apparent molecular weight between crosslinks (M_c), and found reasonable correlations for the Payne effect, tensile strength and critical J-integral. On the other hand, we only found a tendency for tear characteristics; this was ascribed to additional effects of (p)CB dispersion.     

Crack propagation behavior in rubber materials

Within the linear viscoelastic theory, crack tip fields are calculated at various crack tip velocities. A transition from rubbery to glassy material behavior in the vicinity of the crack tip can be observed. Shear and bulk behavior is analyzed separately. Whereas the increase of tearing energy at higher crack tip velocities can be ascribed to the shear behavior, bulk behavior influences the fracture mechanism. The results support experimental investigations that the instability separating stable from unstable crack propagation is related to a change in the fracture mechanism. At low crack tip velocities, material separation is the result of formation, growth, and coalescence of cavities. At high crack tip velocities, cavitation is suppressed and fracture is driven by a rather brittle mechanism resulting in a decreased amount of energy to propagate the fracture process zone. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 882–891. This article was submitted by the authors in English.     

Viscoelastic effects in cutting of elastomers by a sharp object

The cutting behavior of elastomers by a sharp object was investigated using various elastomers such as acrylonitrile–butadiene rubber (NBR), styrene–butadiene rubber (SBR), and natural rubber (NR). The effects of crosslinking density, cutting rate, and temperature on the cutting energy of elastomers were investigated. The cutting behavior of swollen elastomers was also investigated. It was found that the cutting energy increased as the molecular weight between crosslinks increased. It was also found that the cutting energies of various elastomers did not yield a single line. Moreover, even in the threshold condition of cutting process, the cutting energy was much higher than the threshold fracture energy. These results suggest that the cutting behavior cannot be explained by only a C C bond rupture process, but it includes other energy dissipation processes. The curves for cutting energies obtained at different cutting rates and temperatures were well superimposed on a single master curve when they were shifted using the WLF (Williams, Landel, and Ferry) equation. Therefore, it is supposed that the cutting of elastomers by a sharp object includes viscoelastic energy dissipation process and is the viscoelastic behavior. It was also found that the variation of cutting energy over a considerable range of effective rates was smaller than that of the tear energy. It is attributed to the fact that the change of the crack tip diameter, i.e., roughening or reduction, was restricted by the diameter of razor blade. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1283–1291, 1998     

Essential work of fracture and j-integral measurements for ductile polymers

In the ductile tearing of polymers that neck before failure it is shown that the specific essential fracture work (w_e), consisting of the energies dissipated in forming and tearing the neck, is a material property for a given sheet thickness and is independent of specimen geometry. Work of fracture experiments using both double deep-edge notched (DENT) and deep-center notched tension (DCNT) geometries with different ligament lengths yielded almost identical w_e values for a grade of high-density polyethylene. These measurements for w_e are in fairly good agreement with the theoretical values based on the J integral evaluated along a contour surrounding the neck region near the crack tip. Under J-controlled crack growth conditions, it is shown that J_c obtained by extrapolation of the J_R curve to zero crack growth and the slope dJ/da are identical, respectively, to w_e and 4βw_p obtained from the straight line relationship between the specific total work of fracture (w_f) and ligament length (l).     

Solvent-induced crack growth in rubbery block polymers

The rapid cracking of lightly stressed rubbery block polymers of styrene and isoprene in certain liquids and vapors has been examined experimentally, by using model test pieces containing a single crack. Solvents which preferentially dissolve the rigid molecular end blocks rather than the rubbery center blocks are efficient cracking agents. The stress required for crack growth to occur is shown to be in accord with a simple energy criterion: the stored elastic energy must be sufficient to provide a characteristic energy for the newly formed surface. This characteristic energy ranges from values close to the surface energy of simple liquids up to about 100 times this value for thicker test pieces or slowly diffusing vapors, when some tearing of an incompletely swollen core is inferred. “Induction times,” before the initial crack starts to grow, are shown to be due to a progressive increase in stored energy under a constant stress as the material absorbs solvent and softens until the critical energy criterion is met. Thus, a timedependent fracture process is shown to be in accord with a constant energy criterion. Above the critical condition the rate of crack growth depends strongly upon stress, like tearing of amorphous elastomers, and the crack then accelerates rapidly.     

Analysis of fracture behaviour of fibre reinforced polypropylene using R-curve concept

Experimental results for investigation of dynamical crack resistance curves in the instrumented Charpy impact test on polypropylene (PP)/glass fibre composites are presented. For this purpose the multiple specimen R-curve method, stop-block technique is used. With the aid of J-integral versus stable crack growth (δa) curves the influence of a special coupler system on crack toughness as resistance against stable crack growth is discussed. It is shown that it is possible to quantify different energy dissipative processes with the new fracture mechanical material value J × T₇ (T₇ - tearing modulus). The problems of determining physical crack initiation values for short fibre composites are discussed. The physical material background for using the ‘plastic hinge’ model to describe the deformation behaviour of PP/glass fibre composites is shown, using the example of selected crack opening displacement (σ) versus δa curves.     

Characterizing fracture energy of proton exchange membranes using a knife slit test

Pinhole formation in proton exchange membranes (PEM) may be caused by a process of flaw formation and crack propagation within membranes exposed to cyclic hygrothermal loading. Fracture mechanics can be used to characterize the propagation process, which is thought to occur in a slow, time‐dependent manner under cyclic loading conditions, and believed to be associated with limited plasticity. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric material with limited viscoelastic and plastic dissipation, and has been found to be associated with long‐term durability of polymeric materials. Insight into this limiting value of fracture energy may be useful in characterizing the durability of proton exchange membranes, including the formation of pinhole defects. In an effort to collect fracture data with limited plasticity, a knife slit test was adapted to measure fracture energies of PEMs, resulting in fracture energies that were two orders of magnitude smaller than those obtained with other fracture test methods. The presence of a sharp knife blade reduces crack tip plasticity, providing fracture energies that may be more representative of the intrinsic fracture energies of the thin membranes. Three commercial PEMs were tested to evaluate their fracture energies (G_c) at temperatures ranging from 40 to 90 °C and humidity levels varying from dry to 90% relative humidity (RH). Experiments were also conducted with membrane specimens immersed in water at various temperatures. The time temperature moisture superposition principle was applied to generate fracture energy master curves plotted as a function of reduced cutting rate based on the humidity and temperature conditions of the tests. The shift with respect to temperature and humidity suggests that the slitting process is viscoelastic in nature. Also such shifts were found to be consistent with those obtained from constitutive tests such as stress relaxation. The fracture energy is more sensitive to temperature than on humidity. The master curves converge at the lowest reduced cutting rates, suggesting similar intrinsic fracture energies; but diverge at higher reduced cutting rates to significantly different fracture energies. Although the relationship between G_c and ultimate mechanical durability has not been established, the test method may hold promise for investigating and comparing membrane resistance to failure in fuel cell environments. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 333–343, 2010     

Fracto-emission from neat epoxy resin

Fracto-emission is the emission of particles (e.g. electrons, photons, ions, neutral species) due to crack growth in materials. These emissions can be related to a number of fracture related phenomena including microcracking, crack speed of dynamic crack growth, locus of fracture (in filled materials), and potentially the extent of crack branching. Here, we examine the emission of electrons, positive ions, and photons during and following the fracture of a neat epoxy resin subjected to tensile and tensile impact loading in vacuum. Experiments which detect correlations of crack tip position and emission intensity show that the emissions occur during and following fracture. We also illustrate that observed variations of the fracture surface morphology under different loading conditions correlated with characteristics of the photon and charged particle emission. For example, regions of the surface exhibiting the highest degree of surface roughness resulted in more intense emission.     

适用于针尖增强拉曼技术的Au针尖的研制

采用针尖增强拉曼光谱技术(TERS)研究了刻蚀电位对Au针尖刻蚀效果的影响, 初步探讨了刻蚀过程的机理. 通过监控刻蚀过程中的振荡电流-时间曲线并与扫描电镜得到的结果比较, 发现可以直接利用电流-时间曲线简单地判断刻蚀后针尖的可能形状, 而无需再借助扫描电镜进行表征. 这不但提高了实验效率, 而且还可以避免针尖在转移和电镜表征过程中可能引入的污染. 研究结果表明, 在体积比为1:1的发烟盐酸和无水乙醇的刻蚀液中, 于2.2 V的电压下, 结合电化学方法控制终点可以得到形状对称尖锐的针尖. 这种针尖不但适合于TERS研究, 而且可用作STM针尖和微纳电极并用于其它针尖增强光学技术.     

Comparison of tear test methods

Tear strength is an important property of a rubber vulcanisate, particularly when considering it for applications such as cables, hoses and tyres. In the present study four test methods, as specified for vulcanised rubbers in British Standards (BS 903 Part A3 and BS 6899), have been investigated on five different compositions based on a nitrile natural rubber, an ethylene acrylic rubber, a polyurehane and undisclosed chloroprene elastomer. The objective of the study was to examine the influence of thickness, grain effects and strain rate on the tear strength. It was also intended to establish if the different tear test methods would rank materials in the same order and whether a constant ratio exists between the tear strength values obtained using the different methods for different materials.

Sample thicknesses in the range 0·5–3 mm and strain rates in the range 100–500 mm/min all produced small but significant variations in the tear strength, confirming that standardisation of these variables is very desirable. Grain effects were found to produce even larger variations in the test results, particularly for the trouser test pieces. It was found that the tests could be divided into two groups:

• (a) trouser and cable tests;

• (b) angle and crescent tests. The tests in each group ranked materials in the same order and the ratios between tests on different materials within each group was approximately the same.

     

Fracture energies of styrene-butadiene-styrene block copolymers (I). Effects of rate,temperature, and casting solvent

Specimens of styrene-butadiene-styrene (SBS) block copolymers, Kraton D-1102, were prepared by solution casting using three different solvents: toluene, cyclohexane, and a mixture of tetrahydrofuran and methyl ethyl ketone (THF/MEK). Measurements of fracture energies of SBS specimens were carried out at various temperatures and rates using two methods (i.e., a conventional tear test and a recently developed cutting test). Effect of casting solvent on the fracture energy was investigated as well. It was found that stick-slip tearing dominates at low temperatures (−50 ∼20°C). Tear strength increased with decreasing temperature. Eventually, a value of 180 kJ/m² was reached at −70°C, close to the glass transition temperature of polybutadiene phase. At temperatures higher than 20°C, however, steady tearing was found and the tear strength gradually decreased with increasing temperature. Tear strength was virtually zero at 100°C, above the glass transition temperature of polystyrene phase. Effect of temperature on tear strength is more pronounced than that of tearing rate. In contrast, the intrinsic strength of SBS block copolymers determined from cutting test remains unchanged, about 570 J/m², over a wide range of temperatures and rates. Specimens cast from THF/MEK solution exhibit yielding phenomena when stretched. Moreover, they possess a relatively larger tear strength, compared to those cast from either toluene or cyclohexane solution. A more continuous polystyrene phase is expected to develop in THF/MEK as-cast specimens which is believed to account for the large tear strength. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2003–2015, 1997     

The effects of crosslink density and crosslink type on the tensile and tear strengths of NR,SBR and EPDM gum vulcanizates

The effects of crosslink density and crosslink type on the tensile and tear strengths of gum NR, SBR and EPDM vulcanizates have been studied. Over a wide range of crosslink densities, sulphur vulcanizates for these rubbers have higher strengths than the peroxide vulcanizates. These results show that crystallizability of the rubbers is not an important factor in producing separate curves in the strength vs crosslink density plots. Tear strengths appear to be more sensitive to crosslink structures than tensile strengths. A composite plot shows that tensile strengths are approximately proportional to tear strengths for all three rubbers.     

Fracture morphology and mechanical properties of ethylene/vinyl acetate rubber vulcanizates reinforced by in situ prepared sodium methacrylate

Ethylene/vinyl acetate rubber (EVM) was reinforced by sodium methacrylate (NaMAA) that was in situ prepared through the neutralization of sodium hydroxide and methacrylic acid in EVM during mixing. The mechanical properties of EVM vulcanizates with different NaMAA loadings and at different crosslink densities were studied and compared with those of high abrasion furnace carbon black (HAF) filled EVM vulcanizates. The fracture surfaces of gum and filled EVM vulcanizates were observed with scanning electron microscopy. The results showed that NaMAA‐reinforced EVM vulcanizates had better mechanical properties than HAF/EVM vulcanizates. When the NaMAA loading was 50 phr, the tensile strength of the NaMAA/EVM vulcanizate was 30 MPa, the tear strength was 102 kN/m, and the elongation at break was over 400%. Fourier transform infrared analysis confirmed that NaMAA formed in the compounding process and underwent polymerization during vulcanization. Scanning probe microscopy analysis revealed that nanoscale particles dispersed in the NaMAA/EVM vulcanizates. The mechanical properties were correlated with the fracture morphology of all the vulcanizates. The tensile rupture of NaMAA‐filled EVM vulcanizates occurred through tearing from a crack in the bulk of the samples. Tear deviation occurred with the addition of NaMAA and resulted in a rough surface, leading to an improvement in the tear strength of NaMAA‐filled EVM vulcanizates. The micrographs of the tear surfaces of the vulcanizates indicated that the different fracture modes depended on the NaMAA loading and the crosslink density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1715–1724, 2004     

Fatigue life determinations of rubber springs

A method for the determination of fatigue lives of rubber springs is described which is based on a fracture mechanics approach. Fatigue experiments have been performed on model components consisting of solid rubber cylinders bonded between metal endpieces of the same diameter. Crack growth in uniaxial compression followed an approximately parabolic locus. Tearing energies were determined both experimentally and theoretically. The results could be plotted on a unique curve of tearing energy, T. versus crack growth rate, dc/dN, as could results from tests on thin strips of rubber fatigued in simple extension. Application of the approach is described to two case histories. The first is the design of a large spherical rubber joint to support an offshore oil-production platform, and the second is the fatigue analysis of a rubber antivibration mounting for a rail car after 2·5 years service.     

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。     

Mechanisms and micromechanics of fatigue crack propagation in glassy thermoplastics

An iterative approach is used to estimate, from interference optics measurements, the variation of refractive index and, hence, extension ratio along the length of a craze at the tip of a fatigue crack. The finite element method is used to compute craze surface stress distributions which are found to be similar to those obtained for static loading. High extension ratios, in the range 6 to 8 for retarded fatigue crack growth in poly(vinylchloride), are attained in the craze fibrils at the crack tip before crack jump occurs. The craze thickens primarily by surface drawing during the early stages of its growth but in the later stages the fibril creep mechanism predominates. The critical fibril extension ratio is not reached in a single cycle, as in normal fatigue crack propagation, and crack jump does not occur until, typically, after several hundreds of cycles during which the fibrils accumulate considerable damage.Presented in part at the 7th Int. Conference Deformation, Yield and Fracture of Polymers, Cambridge, UK, 11–14 April 1988.     

Influences of different dimensional carbon-based nanofillers on fracture and fatigue resistance of natural rubber composites

The dimensions of reinforcing filler is a key factor in influencing the fracture and fatigue of rubbers. Here, the fracture and fatigue resistance of natural rubber (NR) filled with different dimensional carbon-based fillers including zero-dimensional spherical carbon black (CB), one-dimensional fibrous carbon nanotubes (CNTs) and two-dimensional planar graphene oxide (GO) were explored. To obtain equal hardness, a control indicator in the rubber industry, the amounts of CB, CNTs, and GO were 10.7 vol%, 1.2 vol%, and 1.6 vol%, respectively. J-integral and dynamic fatigue tests revealed that NR filled with CB exhibited the best quasi-static fracture resistance and dynamic crack growth resistance. The much higher hysteresis loss of NR filled with CNTs weakened its fatigue resistance. The planar GO played a limited role in preventing crack growth. Furthermore, digital image correlation revealed that NR filled with CB had the highest strain amplification level and area at the crack tip, which dissipated the most local input energy and then improved the fracture and fatigue performance.