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.     

Fracture and electric-field-induced crack growth behavior in NBT-6BT relaxor ferroelectrics

The fracture properties of 0.94(Na_0.5Bi_0.5)TiO₃-0.06BaTiO₃ (NBT-6BT) relaxor ferroelectrics were investigated using the Vickers indentation method and computation of crack tip opening displacement. It was found that an unpoled sample had a fracture toughness of around 1.35 MPa m^1/2. In contrast, an electrically poled sample exhibited anisotropy with a lower fracture toughness perpendicular to the poling direction and a higher value in the parallel direction, as compared to the unpoled sample. Upon cyclic electrical loading (with applied electric field amplitudes between 0.73E_C and 1.4E_C), the indented surface crack was found to propagate. In general, the crack grew rapidly during the initial cycles followed by crack arrest, and the principal driving force for crack growth was proposed to be residual stress around the indentation, as evidenced by the limited field dependence of crack growth. There was also a contribution from the electromechanical strain, which played a role at high cycles (>100 cycles) and high fields (>1.3 E_C). Evidence of a saturation threshold of crack propagation is an advantage for the electromechanical reliability of relaxor ferroelectrics in devices.     

管材专用高密度聚乙烯的研究进展

综述了国内高密度聚乙烯(HDPE)的生产装置及工艺。采用双峰聚合工艺使短支链更多地分布在高相对分子质量部分是HDPE管材从PE80级升至PE100级的主要原因。管材的耐环境应力开裂性能随HDPE相对分子质量减小而下降,提高短支链含量可提高管材的耐环境应力开裂性能。HDPE的相对分子质量越高,管材抗裂纹扩展性能越好,将短支链分布在高相对分子质量端可提高抗裂纹扩展性能。     

Slow crack growth resistance of electrically conductive zirconia-based composites with non-oxide reinforcements

Slow crack growth (SCG) behavior of four zirconia-based composites reinforced with 40 vol% WC, TiC, NbC or TiCN were studied by means of double-torsion testing. Compared to monolithic zirconia, the composites had a higher resistance to fast fracture, i.e., higher fracture toughness. The extent of toughening depended on the reinforcement type, shifting the V-K_I (crack velocity versus stress intensity factor) curve parallel to higher K_I values. More importantly, these composites were less sensitive to SCG. Identical V-K_I/K_IC curves with steeper slopes compared to monolithic zirconia were observed for the investigated composites, independent on the reinforcement type. No rising R-curve was measured, at least in the crack-size domain investigated by SCG. Therefore, the higher SCG resistance of the composites was due to the intrinsic stress-assisted corrosion resistance of the covalent non-oxide secondary phase.     

The correlation of slow crack growth in linear polyethylene by the J-integral

The fracture of a polyethylene (PE) homopolymer by slow crack growth was measured with a three point bending (TPB) specimen and a single edge notch tension (SENT) specimen. The crack growth rate correlated with the stress intensity for each type of specimen. However, for a given K, SENT specimens exhibited a faster crack growth rate than TPB specimens. Since PE is non-linear, the J-integral is more appropriate than K and it was found that J does correlate the SENT and TPB results. In addition, the degree of non-linearity of the PE was increased by quenching. For the quenched state it was also found that J correlated the SENT and TPB results in accordance with the dependence of J on the degree of non-linearity of the material.     

Effect of stabilization on creep crack growth in high‐density polyethylene

Polyolefins, particularly polyethylene, are known to fail via crack initiation and crack propagation when exposed to multiaxial long‐term static stresses at elevated temperatures. Using concepts of linear elastic fracture mechanics, this article describes and discusses the effects of stabilization on the kinetics of creep crack growth (CCG) in high‐density polyethylene (PE‐HD) and the failure micromechanisms involved. As for the influence of stabilization, six PE‐HD formulations (two polymer types, each with three stabilizer systems) were investigated. CCG initiation times and CCG rates were determined at 60 and 80°C in distilled water as functions of the crack tip stress field characterized by the stress intensity factor. Although no influence of the stabilizer type was found in either polymer type for CCG initiation times and CCG rates at high crack speeds, significant effects of the added stabilizer type and concentration were detected for low CCG rates. The observed phenomena were explained in terms of local aging processes in the immediate vicinity of the crack tip, which were controlled by the presence and content of various stabilizers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3191–3207, 2003     

Mold replication in injection molding of high density polyethylene

In order to deepen the mechanisms at the basis of mold surface replication onto the molded plastic surface, a novel experimental approach is proposed. Up to 20 different mold surface textures were made by machining with repetitive patterns of peaks and valleys. Mold replication tests were performed by over-molding of high density polyethylene (HDPE) on steel inserts. The surface morphology of inserts and injection molded parts was acquired by surface analyzer, and all the main roughness parameters were extracted and compared as well as the geometrical profiles. Surface morphology was also measured on molded samples after thermal relaxation at 100°C. As expected, a strong correlation was found between the roughness of mold insert and molded part over the full experimented range. Profiles on the molded surface have the same repetitive pattern of the corresponding insert surface but with lower peaks, higher valleys, and a horizontal shrinkage. Comparing molded HDPE surface profiles before and after thermal relaxation, it was observed a similar change to the one highlighted between mold insert and molded part. This occurrence suggests that the final surface appearance of the molded part is also a function of the relaxation mechanism during or immediately after injection molding.     

Post-yield behaviour of low density polyethylenes: 2. Plastic fracture

Diamond-shaped cavities have been observed to grow in a stable fashion through oriented low density polyethylenes, linear and high pressure, and account for the ultimate fracture characteristic of these polymers. Their growth has been examined by the application of fine grids on the specimen surface, and (adjacent) elements of material at the diamond tip deform in simple shear in turn, as the maximum shear is attained. Simple shear tests on drawn material confirm that the angle between the linear faces of the diamond cavities is determined by the onset of strain softening followed by strain hardening. The characteristics of these determine the extent of yielding in each element.     

A stable porous superhydrophobic high‐density polyethylene surface prepared by adding ethanol in humid atmosphere

A stable porous superhydrophobic high‐density polyethylene (HDPE) surface with water contact angle of 160.0 ± 1.9° and sliding angle of 2.0 ± 1.6° was obtained by adding ethanol in humid atmosphere at 5°C. Soaked in water with temperatures ranging from 5 to 50°C for 15 days, even suffering compressive forces, and the water contact angles were still higher than 150°. After water flowed through the surface continuously for 30 min, even water droplets with a diameter of 4 mm dropped onto the HDPE surface from 30 cm high for 10 min, the water contact angles were also higher than 150°. A brief explanation to the formation of the porous superhydrophobic HDPE surface was put forward. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

聚乙烯管材发生慢速裂纹增长的影响因素及对策

耐慢速裂纹增长性能是提高聚乙烯（PE）管材耐用性的关键因素。文章综述了PE管材发生慢速裂纹增长的机理和影墒因素,最后给出了提高PE管材耐慢速裂纹增长性能的改进措施。     

Measurement of Very Slow Crack Growth in Glass

The rate of very slow crack growth in glass is measured by inducing small, controllable changes in the direction of propagation of Hertzian cone cracks at known times. After completion of a growth sequence, the sample is sectioned to reveal the fracture surface. The stress intensity factor at each stage of crack growth is calculated by using finite-element modeling of the stresses near the crack tip. Data are presented for crack growth velocities as low as 10⁻¹⁴ m/s in soda–lime glass. These data provide strong evidence for the existence of a subcritical limit for crack growth in this material.     

Improving the consistency of environmental stress cracking resistance testing for polyethylenes

This paper explores a novel modification to the Bell Test (ASTM-D1693) for minimizing its variability in failure time by the introduction of heat notching. The new notching technique for assessing environmental stress cracking resistance was evaluated with four different grades of polyethylene and compared to results using the ASTM standard for notching. Differences in both the failure time and the associated variability were witnessed with this technique, evaluated at various annealing conditions between the cold crystallization temperature and crystal melting temperature of a polyethylene. Best results were found using the midpoint temperature between these two conditions for the heated blade. Localized elastic moduli for the notched samples were obtained by force mapping via atomic force microscopy, showing decreased crystallinity content at the point of concentrated stress was favored for more consistent failure times. The work highlights the critical nature of local phase morphology at the notch in the assessment of environmental stress cracking resistance.     

A critical evaluation of indentation crack lengths in air

An extensive overview is presented of Vickers indentation crack lengths in ceramics in air. Measurement of such crack lengths is one of the most common and powerful assessments of the fracture properties of ceramics and the overview provides a critical evaluation of observed behavior as functions of material type and indentation load, and an extensive basis for comparison of results from new materials and analyses. The overview considers single crystals, polycrystals, transforming materials, glasses, and multiphase materials, including cermets, glass-ceramics, and tooth enamel. The coverage extends over structural and electronic ceramics, including oxides, carbides, nitrides, and titanates. The data are presented in a single format for ease of interpretation in terms of idealized indentation fracture and for inter-material comparisons; most data are unique to this work, but the results of selected studies from the published literature are included. The overview considers the precision and accuracy of crack length measurements and demonstrates a simple quantitative evaluation and ranking scheme for ceramic fracture based on load-adjusted crack length and cracking susceptibility. Indentation hardness and cracking threshold are also determined and related to the susceptibility. Material toughness is related to cracking susceptibility by fracture mechanics analyses: typical crack length measurements in air are shown to provide estimates of inert toughness with a relative uncertainty of ±50%.     

基于应变硬化模量方法评价聚乙烯管材耐慢速裂纹增长性能研究进展

概述了聚乙烯（PE）管材3种典型失效破坏模式（韧性破坏、脆性破坏、热氧老化破坏）,简述了慢速裂纹增长的分子机理和断裂力学机理,详细介绍了应变硬化模量（SH）方法的测试原理和试验过程,介绍了目前SH法测试与材料微观结构相关性的研究进展,综述了国内外关于SH法评价管材耐慢速裂纹增长性能的可靠性研究以及应用SH法评价材料耐环境应力开裂性能的研究进展,并对应用SH法在控制产品质量方面提出展望。     

Craze morphology and molecular orientation in the slow crack growth failure of polyethylene

An optical microscopy study and a micro‐Raman spectroscopy study were carried out on polyethylene samples subjected to an environmental stress crack resistance (ESCR) test. The aim was to elucidate the molecular deformation mechanisms associated with the failure process. It has been shown that in the early stages of the ESCR test, in a regime of low local stress, failure in the craze occurs via a brittle process with limited ductility and with molecular orientation being detected. As the experiment progresses, however, extensive fibrillation takes place. The molecular orientation in these fibrils was found to be comparable to that measured in cold‐drawn samples. Moreover, the fibril molecular orientation decreased from the crack to craze tip and was found to be higher in the midrib part of the fibril (fibril failure point). As a consequence, fibril creep is the most likely mechanism of failure in the craze. Microscopy and Raman measurements showed that the extent of the brittle process is molecular weight‐dependent, that is, the brittle process seems to operate longer at higher molecular weights. These observations are in agreement with a previous work which showed that the molecular stress per macroscopic strain/stress decreases with increasing molecular weight, therefore holding the high molecular weight craze in a regime of low local stress for longer testing times. Fibrils spanning the craze are envisaged as the anchor points that hold the structure during the process of failure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 283–296, 2000     

Influence of the molecular structure on slow crack growth resistance and impact fracture toughness in Cr-catalyzed ethylene-hexene copolymers for pipe applications

In this study we correlate parameters describing molecular structure (molar mass distribution, short chain branching content, intermolecular heterogeneity) of different ethylene-hexene Cr-catalyzed copolymers, with slow crack growth and rapid crack propagation resistances, respectively measured with Bent Strip and Charpy tests. The PTREF technique, coupled with classical techniques, was used. Two new indices were proposed to correlate mechanical properties and molecular structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 916–928, 2001     

Crack growth mechanism in some polyamides

Fracture toughness was measured with single-edge notched three-point bend specimens. The materials used were semicrystalline polymers, polyamide-66, polyamide-1010 and polyamide-610. Their fracture surface was investigated with the help of scanning electron microscopy. The crack growth mechanism is revealed and a model suggested. The stable crack growth feature of the fracture surface is the dimples. The stable crack growth includes: the crack tip blunting, the voids initiating, coalescing and extending, the fibrillated polymer rupturing and contracting, then the formation of dimples on the fracture surface. The unstable crack growth formed a fan cleavage zone. The crack growth passed along the boundaries of spherulitic crystals in which many subcracks have occurred. The mechanism of discontinuous subcrack propagation in ductile polyamide-610 was discovered. In the compressive zone of the bend specimen, the fracture mechanism is similar to the cleavage zone and exhibits shear failure response.     

Application of a new constant G load-jig to creep crack growth in adhesive joints

A novel constant energy release rate load-jig, capable of applying loads in the full range of mode mixes from pure mode I to pure mode II, was developed for studying creep crack growth in structural adhesive joints. Since the load-jig applies only pure bending moments to uniform double cantilever beam (DCB) specimens, the expressions for the energy release rate and mode ratio are both simple and accurate. The new load-jig was used to study mixed-mode creep crack growth in DCB specimens which had either an intact fillet or a steady-state failure zone. Both a rubber-toughened and a mineral-filled epoxy adhesive were tested at room temperature, which is far below the glass transition temperature for either structural adhesive. In all cases, crack speeds were observed to decelerate, indicating that the adhesive were self-toughening over time. In addition, crack growth was observed to propagate by the initiation and coalescence of microcracks, rather than by the continuous advancement of a crack tip.     

Crack growth angle prediction of an internal crack under mixed mode load for unfilled elastomer using the strain energy density factor

This article investigates the prediction of the crack growth angle of an existing internal crack under mixed mode loading at the crack tip for an unfilled ethylene propylene diene terpolymer rubber (EPDM). For the realization of mixed mode loading, the cracks of the uniaxial loaded specimens were oriented with different angles to the loading direction. The energy density factor was used as a potential criterion for determining the crack growth angle. The determination of the strain energy density factor was carried out simulatively in Abaqus. The second-order Ogden model was used to describe the rubber-like material behavior. The relative local minimum of the strain energy density factor provides the possible growth angle. The experimental investigations show that the initial cracks grow orthogonally to the loading direction for the different crack orientation angles. For the crack orientation angle parallel to the load direction, the crack growth was observed because the strong stretching of the specimen caused strong necking in the crack region. The crack growth for the remaining crack orientation angles were induced due to shear loading at the crack tip. The predictive angle of different crack orientation angles shows very good accordance to the measured crack growth angles.     

Systematic investigations of fatigue crack growth behavior of a PE‐HD pipe grade in through‐thickness direction

Fatigue crack growth (FCG) experiments were performed on a commercial high‐density polyethylene (PE‐HD) pipe grade. To investigate the influence of different specimen types on FCG results, tests were conducted using compact tension (CT) specimens and cracked round bars (CRB). The effects of frequency and R‐ratio on FCG behavior were also studied. Furthermore, FCG tests were interrupted in the region of stable crack propagation. The crack front and the front of the process zone ahead of the crack were systematically characterized via microscopic methods in the thickness direction of the specimen. The experimental data are employed to study the mechanisms of process zone development and to determine the effective crack length by compliance relationships. This detailed information allows modeling FCG in PE‐HD at various positions in the thickness direction of the specimen. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103:1745–1758, 2007