Environmental stress cracking (ESC) of plastics caused by non‐ionic surfactants

The susceptibility of various plastics to environmental stress cracking (ESC) was studied using a variety of agents in common use. It was found that styrenic polymers are susceptible to ESC by many kinds of agents, including detergents, that have non‐ionic surfactants as their components. The ESC of acrylonitrile‐butadiene‐styrene (ABS) copolymer caused by non‐ionic surfactants was studied intensively, and the dependence of crack propagation behavior on temperature and on the kind of non‐ionic surfactants was investigated by edge crack tension (ECT) tests. The morphology at the crack tip of ECT specimens was investigated by a transmission electron microscope (TEM). It was found that crack propagation behavior was closely related to the morphology at the crack tip and the morphology was affected by temperature and type of surfactants. The change in morphology at different experimental conditions was attributable to the difference in the solubility and diffusivity of surfactants at the crack tips.     

聚碳酸酯／弹性体合金的断裂过程形貌研究

采用扫描电子显微镜 (SEM)和长距离显微镜 (LDM)对PC/弹性体共混体系试件中的裂纹在平面应变和平面应力下发展的形貌进行观察。结果表明 :弹性体的加入影响了材料断裂的各个过程。在平面应变下 ,裂纹尖端出现超钝化现象 ;平面应力下 ,第一次观察到裂纹后岸两侧出现副裂纹的现象。采用透射电子显微镜 (TEM )观察慢速加载时裂纹前沿各区的形貌 ,从微观上解释弹性体增韧PC的原因     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

Creep-Crack Growth by Damage Accumulation in a Glass-Ceramic

The growth rate, near-tip creep response, and damage processes of creep cracks in a pyroceram glass-ceramic were studied under tensile loading at elevated temperatures. The rates of crack extension were characterized as a function of the applied stress intensity factor. The damage processes which occurred near the crack tip and led to creep crack extension were identified using a replica technique and by direct observations in a scanning electron microscope equipped with a high-temperature loading stage. The accumulated creep strains near the crack tip were measured via the stereoimaging technique. The results indicate that creep-crack growth in the pyroceram glass-ceramic occurs in both continuous and discontinuous manners, with the damage processes manifested as the nucleation, growth, and coalescence of inhomogeneously distributed cavities and microcracks. Measurements of the total accumulated creep strain near the crack tip suggest that crack extension follows a critical strain criterion. Both the microcrack density and the total accumulated creep strain show similar dependence with distance from the crack tip. These observations suggest that damage accumulation and crack extension in the glass-ceramic are controlled by the near-tip creep rates.     

Failure analysis of carbon black filled styrene butadiene rubber under fatigue loading conditions

Abstract

The present paper deals with the fatigue crack growth in a carbon black-filled styrene butadiene rubber (CB-SBR) under fully relaxing loading conditions. More precisely, it is devoted to the determination of the scenario of crack growth. For that purpose, an original ‘microcutting’ technique, previously applied by the authors on natural rubber (NR), is used to observe microscopic phenomena involved in fatigue crack growth thanks to scanning electron microscopy (SEM). Results show that the crack tip grows following a tearing line by generating ligaments; it explains the differences between fatigue responses of crystallisable and non-crystallisable rubbers during crack propagation. So, contrary to crystallisable elastomers such as NR, the microstructure of SBR is similar at crack tip and in the bulk material, and the crack tip does not resist crack propagation. Moreover, the morphology of fracture surfaces only depends on particles encountered by the fatigue crack during its propagation.     

304不锈钢应力腐蚀的临界氯离子浓度

通过慢拉伸实验得出了304不锈钢应力腐蚀敏感性与溶液中Cl-浓度的关系,用扫描电镜对拉伸试样的断口形貌进行了分析,得出了304不锈钢发生应力腐蚀的临界氯离子浓度。采用自行设计的装置对304不锈钢试样施加拉应力,通过恒应变条件下的电化学原位测试研究了304不锈钢钝化膜破裂电位与氯离子浓度的关系。得出导致钝化膜破裂电位突变的Cl-浓度与发生应力腐蚀破裂的临界Cl-浓度是基本一致的结论。     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Piezospectroscopic measurement of the stress field around an indentation crack tip in ruby using SEM cathodoluminescence

Piezospectroscopy using stress-sensitive lines in the cathodoluminescence (CL) spectrum generated in response to excitation by the electron beam of an SEM has recently been shown to be a promising technique for submicron resolution stress measurements in alumina and other ceramics. This paper develops and applies the technique by mapping the wavelength shifts of the R₁ CL line around the tip of an indentation crack in a ruby single crystal. Accounting for crystallographic anisotropy, the shifts observed were quantitatively consistent with the classical crack tip stress field for all polar angles ahead of the crack tip and indicated a crack tip stress intensity factor of 1.0 MPa m^1/2. This is significantly lower than the fracture toughness of the crack plane (4.5 MPa m^1/2), and indicates the post-indentation development of lateral cracks and slow crack growth. The spatial resolution of the stress measurements was measured as 550 nm and the effects of specimen heating by the electron beam were shown to be negligible.     

Whisker-Reinforced Calcium Phosphate Cements

Self-setting calcium phosphate cements (CPC) consisting of hydroxyapatite (HA) and tetracalcium phosphate were reinforced with various amounts of hydrothermally synthesized HA whiskers with an aspect ratio of 10. Flexural strength (σ_b) and work of fracture (γ_wof) were calculated by four-point bending tests, and fracture surfaces were analyzed by scanning electron microscopy (SEM). σ_b and γ_wof of the CPCs that were reinforced with 30 vol% HA whiskers increased by 60% to 7.4 MPa and by 122% to 102 J/m², respectively. SEM analyses reveal that this increase could be correlated to energy-dissipating processes (e.g., whisker pull-out) at the crack tip.     

用J积分法表征聚碳酸酯／弹性体合金的韧性

采用Ｊ积分法表征聚碳酸酯（ＰＣ）／弹性体共混体系的韧性。结果表明，当加入弹性体后，体系断裂韧性ＪＩＣ明显提高，提高了约８０％。在弹性体含量为５－１５％时，断裂韧性ＪＩＣ变化不大，撕裂量Ｔｍ随着弹性含量的增加而增大，在弹性体含量为１０％时增加了１２．５倍。弹性体增韧ＰＣ的原因主要在于提高其抗 扩展能量。采用ＳＥＭ观察材料中的裂纹在平面应变下的发展过程发现，起裂前裂尖出现钝化和靴状超钝化现象，从微观上     

Role of crack tip shielding mechanisms in fatigue of hybrid epoxy composites containing rubber and solid glass spheres

The fatigue crack propagation (FCP) resistance of epoxy-based composites containing various concentrations of solid glass spheres (SGS) and/or reactive liquid rubber (CTBN) was examined. The FCP results show that the simultaneous use of rubber and solid glass spheres (hybrid composites) results in synergistic improvement in FCP resistance of composites through the entire crack growth regime. The nature of synergistic interactions was elucidated by careful examination of the fatigue fracture surfaces and the subfatigue fracture surfaces of fatigue samples. It was shown that when rubber particles cavitate in the vicinity of the glass spheres, regardless of the nature of the interface, glass particle debonding from the matrix is suppressed due to a change in the crack tip localized stress state. This, in turn, results in improved pinning/bridging efficiency of the glass spheres. Furthermore, it was shown that crack tip plastic zone-rubber particle interactions induce a transition in FCP behavior of rubber-modified epoxies. Consequently, crack tip shielding mechanisms become active when the size of the plastic zone at the crack tip becomes large compared to the size of the rubber particles. © 1995 John Wiley & Sons, Inc.     

Roughness of oxide glass subcritical fracture surfaces

An original setup combining a very stable loading stage, an atomic force microscope and an environmental chamber, allows to obtain very stable subcritical fracture propagation in oxide glasses under controlled environment, and subsequently to finely characterize the nanometric roughness properties of the crack surfaces. The analysis of the surface roughness is conducted both in terms of the classical root mean square roughness to compare with the literature, and in terms of more physically adequate indicators related to the self‐affine nature of the fracture surfaces. Due to the comparable nanometric scale of the surface roughness, the AFM tip size and the instrumental noise, a special care is devoted to the statistical evaluation of the metrologic properties. The roughness amplitude of several oxide glasses was shown to decrease as a function of the stress intensity factor, to be quite insensitive to the relative humidity and to increase with the degree of heterogeneity of the glass. The results are discussed in terms of several modeling arguments concerning the coupling between crack propagation, material's heterogeneity, crack tip plastic deformation and water diffusion at the crack tip. A synthetic new model is presented combining the predictions of a model by Wiederhorn et al (J Non‐Cryst Solids, 353, 1582‐1591, 2007) on the effect of the material's heterogeneity on the crack tip stresses with the self‐affine nature of the fracture surfaces.     

Crack-Tip Structure in Soda–Lime–Silicate Glass

The topology of crack tips in soda–lime–silicate glass was investigated using atomic force microscopy (AFM). Studies were conducted on cracks that were first propagated in water and then subjected to stress intensity factors either at or below the crack growth threshold. Exposure to loads at the crack growth threshold resulted in long delays to restart crack growth after increasing the stress intensity factor to higher values. After breaking the fracture specimen in two, the "upper" and "lower" fracture surfaces were mapped and compared using AFM. Fracture surfaces matched to an accuracy of better than 0.5 nm normal to the fracture plane and 5 nm within the fracture plane. Displacements between the upper and lower fracture surfaces that developed after a critical holding time were independent of distance from the crack tip, and increased with holding time. Despite the surface displacement, crack tips appeared to be sharp. Results are discussed in terms of a hydronium ion–alkali ion exchange along the crack surfaces and corrosion of the glass surface near the crack tip by hydroxyl ions.     

Characterization of the AIN–W Interface in a Cofired Multilayer AIN Substrate

The AIN–W Interfaces in a cofired multilayer AIN substrate were observed using an optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Optical and SEM observations showed an intricate intricatelocking AIN-W grain structure at the interface. After the W pad was removed from the substrate with a NaOH etchant, the surface morphology of the W metal at the interface side was found to be very rough, with a small-grain microstructure compared with that at the free surface side. Electron microprobe analyses using SEM revealed that there was no diffusion of either W or Al at the interface at the order of a few micrometer's resolution. Bright-field images, dark-field images and selected area electron diffraction (SAD) patterns using TEM indicated there was no secondary phase between AIN and W. However, scanning transmission electron microscopy using an energy dispersive X-ray detector revealed that there was a 200-nm thick W diffusion layer from the interface into the AIN ceramics. It was concluded that the high adhesion strength between the W conductor and the AIN substrate (>20 MPa) was not due to any secondary phase but to mechanical interlocking of AIN and W during cofiring.     

Environmental stress cracking of poly(butylene succinate)/cellulose triacetate blend films

Blend films of poly(butylene succinate) (PBS) with cellulose triacetate (CTA) were prepared using a solvent‐cast technique. The environmental stress cracking (ESC) behavior of commercial PBS films and that of as‐prepared PBS/CTA blend films were investigated in an aqueous NaOH solution. It was found that the fracture behavior of PBS films can be analyzed by linear fracture mechanics, that is, the stress‐intensity factor, K, is found to control the crack‐growth rate, da/dt, in PBS films. The enhancement of da/dt induced by increasing the normality of NaOH(aq) is more marked at a high value of K rather than at a low value of K. Furthermore, the ESC behavior of the PBS/CTA blends was studied in NaOH(aq). It was found that the behavior cannot be analyzed by linear fracture mechanics and that the addition of CTA into PBS increases the ESC resistance of the blends. Large‐scale yielding of the material around the crack tip seems to be responsible for the high ESC resistance of the blends. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 510–515, 2003     

电子显微镜在橡胶研究中的应用

综述了电子显微镜在弹性体共混、橡胶断裂形态及炭黑结构等方面的应用。     

Grwoth rate versus temperature relations characteristic of environmental stress cracks in low-density polyethylenes and the underlying thermodynamic mechanism

Growth rate versus temperature relations of environmental stress cracks (ESC) in low-density polyethylenes (PE) are investigated. In the experiments, bent PE specimens are immersed in dimethylsiloxanes (DMS) of various molecular sizes. The growth rates are increased from 10^?9 to 10^?4 m s^?1 by a few tens of degrees increase in temperature. The relations behave in apparently complicated manners depending on DMS molecular size, PE molecular weight and amount of bending deformation. Their Arrhenius type plots, however, fall into either of the two characteristic shapes as in PE ESC induced by several non-ionic surfactants and n-propanol. They are consistently interpreted in terms of the authors' thermodynamic theory. The active liquids, though incompatible with a stress-free polymer, migrate into its matrices in a restricted region at the crack tip. The change in the paths of the state shifts leading to failure in the relevant thermodynamic potential diagrams is essential to the ESC kinetics.     

聚芳醚酮改性苯并恶嗪树脂动态冲击性能研究

以聚芳醚酮改性苯并恶嗪树脂为原材料,用动态数字化冲击仪测试了改性树脂浇铸体的冲击性能,获得了树脂体系的冲击力-位移-能量曲线、冲击功和冲击韧性,并用扫描电子显微镜对冲击断面进行了观察。结果表明,聚芳醚酮共混改性苯并恶嗪树脂体系在冲击破坏时的能耗以裂纹形成功为主,体系的冲击韧性值与冲击功的变化趋势相似;冲击断面整体上呈现河流状纹路,裂纹扩展路径上能有效阻隔裂纹发散的点或吸收能量的长条状碎小断面少,对整体韧性贡献较小。     

Mode-l Fracture Behaviour of Adhesive Joints. Part I. Relationship Between Fracture Energy and Bond Thickness

The fracture properties of adhesive joints of aluminium were investigated using a rubber-modified tough epoxy resin system (G_IC = 2.76 kJ/m²) as adhesive material. Compact tension (CT) adhesive joints were manufactured for a wide range of bond thickness t (from 0.05mm to 10mm) and fracture tests conducted under static load. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology. A large deformation elastic- plastic finite element model was developed to evaluate the J-integral value for different bond thickness. The fracture energy, J_c , was found to be highly dependent on the bond thickness and was lower than that of the bulk adhesive. As the bond thickness was increased J_c also increased, though not monotonically, towards the fracture energy of the bulk adhesive. This result was caused by the complicated interactions between the stress and strain fields, plastic deformation of the adhesive around the crack tip, constraint from the adherends and the failure path. It was shown that values of J_c as a function of bond thickness correlated well with the variation of plastic zone height. Scanning electron micrographs from the fracture surfaces of the CT adhesive joints illustrated that the failure path was mainly cohesive through the centre-plane of the adhesive layer. Brittle fracture mechanisms were observed for thin bonds (0.04mm < t< 0.5 mm) but tough fracture mechanisms were identified for thick bonds (t > 1 mm).     

Reinforcement of precursor-derived Si–C–N ceramics with carbon nanotubes

Nanocomposites consisting of precursor-derived Si–C–N ceramics incorporated with carbon nanotubes (CNTs) were successfully prepared by casting of a mixture of CNTs and a liquid precursor polymer followed by cross-linking and thermolysis. The effect of CNTs on the fracture toughness of these nanocomposites was investigated by a thermal loading technique. The results reveal a dependence of the fracture toughness on the type of the CNTs. One type shows a significant increase of the fracture toughness at CNT contents of only 1–2 mass%, whereas the other one exhibits no effect. The microstructural effects of CNTs observed at the fracture surfaces of the nanocomposites by scanning electron microscope (SEM) and transmission electron microscope (TEM) can be correlated with the observed fracture toughness behavior.