Experience with a full notch creep test in determining the stress crack performance of polyethylenes

A laboratory method to measure the stress crack resistance of polyethylenes was developed and has since been applied in our laboratory for more than twelve years. The experience gathered since our first paper is herewith reported. The creep rupture test of circumferentially notched specimens cut from plaques or pipes has proven to be a rapid and reliable method to evaluate the stress crack performance. Surfactant-assisted stress cracking was employed to accelerate testing. The stress crack resistance of several polyethylene samples was studied with respect to its dependence on stress, temperature, and environment. The creep rupture behavior at different temperatures could be superposed by horizontal shifting when the stresses were normalized in proportion to the respective bulk yield stresses. The notch tip radius turned out not to be very crucial, and variation of the nominal concentration of the surfactants, nonylphenolpolyglycolethers, scarcely affected slow crack growth. Acceleration of testing by surfactants equalized property differences to a noticeable extent but did not influence the ranking of the materials. The activation energy of crack growth was in the expected range. Defects introduced into the line by butt joint welding were precisely modeled by the full notch creep test.     

聚乙烯管材SCG性能评价及寿命预测论述

管材的慢速裂纹萌生和扩展至管材破坏的时间是评价管材使用性能的一项重要指标。随着原材料性能的提高,聚乙烯(PE)管材抗裂纹萌生和耐慢速裂纹扩展的能力大幅提高。优异的PE100 RC管材在FNCT、PENT等实验评价方法下,测评时间均超过15000 h。PE管慢速裂纹评价方法普遍存在实验条件复杂、重复性差等问题,严重制约了PE管材的开发进程。对此,国外学者提出了更简洁快速的评价方法——循环载荷缺口圆棒(CRB)法。文章着重论述了,循环载荷CRB法对管材SCG性能及管材寿命预测的评价,同时,论述了几种管材性能评价方法的相关性。最后,指出了循环载荷CRB方法后续用作其他塑料管评价的研究方向。     

基于循环载荷法评价聚乙烯管材性能的可靠性研究

为研究循环载荷法对加速表征聚乙烯（PE）管材耐慢速裂纹扩展（SCG）性能的可靠性,对5种不同等级的PE管材采用循环载荷缺口圆棒（CRB）试验和全切口拉伸蠕变（FNCT）试验的方法对其耐慢速裂纹扩展性能进行评估,同时对其试验结果进行相关性分析。此外,通过测量PE管材的分子量及其分布、结晶度和片晶厚度等高分子材料参数进一步佐证所选PE管材耐慢速裂纹扩展性能的优劣,进而验证CRB试验方法的可靠性。结果表明,循环载荷CRB试验方法不仅能可靠有效评估PE管材的耐SCG性能,且相比FNCT试验方法,其试验所需时长更短及试验条件要更贴近实际使用工况。     

基于循环载荷试验的聚乙烯管材寿命预测研究

采用循环载荷裂纹圆棒（CRB）试验,测试了A~C 3种聚乙烯（PE）材料在载荷比（R）分别为0.1、0.3和0.5时,缺口圆柱棒试样的裂纹扩展速率（da/dt）与应强度因子（K_I）的对应关系。通过结合聚合物材料线弹性断裂力学理论,外推至R=1（静载）条件,得到含缺陷PE管材的使用寿命（t_f）,并与全缺口蠕变试验（FNCT）评价结果进行了对比分析。结果表明,当A~C 3种PE管材内壁存在a_ini=0.4 mm的初始裂纹缺陷时,CRB试验外推得的管材的t_f分别为44、53和65年;FNCT外推结果分别为53、59和79年;CRB和FNCT的寿命预测结果具有较好的正相关性,寿命预测结果偏差较小,试验结果证实了该方法的有效性。     

Structure and crack growth in gas pipes of medium-density and high-density polyethylene

The microstructure and resistance to slow crack growth of two commercial polyethylene pipe materials were studied. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to reveal the crystallite thickness and width distributions and the size of the lamellar stacks. The resistance to slow crack growth was assessed by uniaxial constant loading of notched specimens and by hydrostatic pressure testing of notched and unnotched pipes. The high-density material contained roof-lamellae, suggestive of a segregation of low molar mass species. Notched uniaxial testing revealed large differences in slow crack growth-resistance between the two PEs despite the fact that the average tie-chain concentration was similar. Hence, low-molar mass segregation, which was found to be higher for the high-density material, definitely decreases the resistance to slow crack growth. Notched uniaxial testing was a sensitive method for ranking these PEs according to their resistance to slow crack growth, and 15 times faster than that achieved in conventional unnotched pressure testing. Failure time extrapolations from higher temperatures to 20°C were made, using a multiple linear regression method (SEM-Q1), the Arrhenius equation, and universal shift-functions to investigate their applicability. The extrapolations resulted in longer life times compared with experimental data, regardless of the method used. The SEM-Q1 method (lower-confidence-limit data) gave the best fit to the 20°C experimental data followed by the Arrhenius equation.     

Mechanisms of crack propagation in dry plaster

Controlled crack propagation tests were performed on single edge notched bend samples to investigate the crack growth behaviour of dry plaster. The influence of the relative notch depth on the crack resistance curve has been studied and appears to be very important. The results are discussed considering a qualitative model based on the specific microstructure of plaster and in situ observations of the crack propagation. Two mechanisms acting at different scales and undergoing complex interactions are involved: crack bridging by small gypsum crystals acting locally behind the crack tip and secondary cracking in a macroscopic frontal process zone. Interaction of the main crack with secondary ones undergoes substantial branching and crack accelerations leading to bridging destruction due to sudden crack opening.     

Fracture toughness and crack growth mechanism for multiphase polymers

The fracture behavior of acrylonitrile-butadiene-styrene (ABS) was investigated using the J-integral method under monotonic loading. Two ways were used to monitor the onset of crack growth: the dyeing method and the length of craze region ahead of initial notch. The blunting at the crack tip and crack growth mechanism for ABS Was observed using a scanning electron microscope. Before the onset of crack growth, the energy put into material was dissipated to create crazes ahead of the initial notch and to deform the material at the crack tip. A part of the energy was released to create a new crack surface after the onset, of crack growth.     

Fatigue crack growth in PE-HD pipe grades

Abstract

In this work, fatigue crack growth (FCG) experiments were used to evaluate the long term performance of pipe grade polyethylene (PE). The parameters for the ranking tests were defined based on basic investigations of the influence of test frequency, R-ratio (F _min/F _max) and specimen configuration on FCG in PE at room temperature. Eight different commercial pipe grades (PE-80 and PE-100) were included in the investigations and two different specimen configurations [Compact Type (CT) and Cracked Round Bar (CRB)] were used. A procedure was established to initiate quasi-brittle cracks that led to failure within 60 h. Crack-growth initiation times were determined from crack-opening displacement data and crack growth kinetics were analysed using linear elastic fracture mechanics (LEFM). A ranking of the materials investigated by their FCG behaviour was possible and compared with results from Full Notch Creep Tests (FNCT).     

聚乙烯管材应变硬化模量与裂纹扩展速率相关性研究

采用应变硬化试验和锥体试验分别得到不同聚乙烯100（PE100）管材试样的应变硬化模量和裂纹扩展速率,并对它们之间的相关性进行研究,以期验证应变硬化试验法评价PE管材耐慢速裂纹扩展性能的正确性。结果表明,锥体试验中PE管材试样的裂纹扩展程度随着试验时间的增加而增大;应变硬化试验和锥体试验对不同PE管材试样的耐慢速裂纹扩展性能评价结果完全相同,应变硬化试验法评价PE管材耐慢速裂纹扩展性能的正确性得到验证;应变硬化试验不仅误差较小,而且可以区分不同牌号PE100管材耐慢速裂纹扩展性能的细微差异,评价结果相对于锥体试验要更加可靠。     

Slow crack growth‐notches‐pressurized polyethylene pipes

A general method for predicting the lifetime of a polyethylene structure that fails by slow crack growth was applied to the case of externally notched pressurized pipes. An analysis of experimental data indicated that the residual stress must be taken into account. The critical notch depths associated with a given lifetime were calculated as a function of pipe size, PENT value of the resin, and temperature. The results were tabulated to serve as practical guide lines for deciding whether a pipe should be discarded if the notch is too deep. The current 10% of the thickness rule now used by industry was found to be invalid. POLYM. ENG. SCI., 47:1951–1955, 2007. © 2007 Society of Plastics Engineers     

聚乙烯承压管道及原料耐慢速裂纹增长测试标准及实验要求

聚乙烯(PE)管材在施工和使用过程中会产生裂纹并缓慢扩展,评价耐慢速裂纹增长性能包括原料测试和管材测试两部分。介绍了国际标准和国家标准中关于耐慢速裂纹增大试验的要求、原理和应用,并列举了通过切口试验加速老化评价和预测 PE 原料长期性能和使用寿命的几种方法。     

Time and temperature effects on the fracture toughness of rigid poly(vinylchloride) pipe materials

The fracture toughness of rigid poly(vinylchloride) pipe materials has been investigated over a range of temperatures and rates. Conditions are described for valid fracture toughness (K_IC) tests and notch insensitive (ductile) behavior; time-temperature effects on transitions in K_IC are defined. The modes of crack extension are characterized over a range of temperatures, and the mechanisms of crack resistance are discussed, including some quantitative data for the yielded zone at the crack tip.     

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.     

Design and pressure rating of PE fittings: stress concentrations,slow crack growth and use of ISO 9080 regression coefficients in material choice

Abstract

Some new ideas are advanced on the selection and qualification of PE materials for use in fittings for PE pipe systems. These are designed to address the problem of ensuring that the fitting does not fail by slow crack growth during the lifetime of the pipe. It has been found that FEA studies of the local stresses in fittings correlate well with observed failure behaviour and this information can be usefully employed to predict long term slow crack growth behaviour. Furthermore, a useful DSC-based procedure is proposed for characterising crack growth resistant polyethylenes.     

Notch Sensitivity of Ceramic Composites with Rising Fracture Resistance

A theoretical framework is developed for the notched strength of ceramic composites that exhibit rising fracture resistance. It is based on established concepts of crack stability under stress-controlled loadings. On using a linear representation of the resistance curve (expressed in terms of an energy release rate), straightforward analytical solutions are obtained for the strength as well the amount of stable crack growth preceding fracture and the associated fracture resistance. Calculations are performed for several test configurations commonly used for material characterization, including single- and double-edge-notched tension, center-notched tension, and single-edge-notched bending. The results reveal salient trends in strength with notch length and specimen geometry. An assessment of the theory is made through comparison with experimental measurements on an all-oxide fiber composite. Transitions in the degree of notch sensitivity with notch length are identified and explored. The utility of the theoretical results both for rationalizing the trends in measured notched strength and for guiding experimental studies of notch sensitivity is demonstrated.     

Fracture prediction in tough polyethylene pipes using measured craze strength

In this study, an empirical model is developed that predicts the time to failure for PE pipes under combined pressure and deflection loads. The time‐dependent craze strength of different PE materials is measured using the circumferentially deep‐notched tensile (CDNT) test. In agreement with previous research, results indicate that bimodal materials with comonomer side‐chain densities biased toward high‐molecular‐weight PE molecules exhibit significantly higher long‐term craze strengths. A comparison of currently available PE materials with CDNT samples taken from a PE pipe that failed by slow crack growth in service clearly indicates the superior performance of new‐generation materials. Using measured craze strength data from the CDNT test, a simplified model for predicting failure in buried PE pipes is developed. Extending previous research, the reference stress concept is used to calculate an equivalent craze stress for a pipe subjected to combined internal pressure and deflection loads. Good agreement is obtained between the model predictions and observed failure times in an experimental test‐bed study of pipes under in‐service loading conditions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

The acoustic emission analysis of the crack processes in alumina

The crack processes in single-phase alumina specimens have been investigated by acoustic emission (AE) analysis with regard to the increase of the crack resistance. During loading of a notched specimen, individual AE signals are observed at first, which are probably due to the generation of microcracks in a process zone around the notch. At higher loads signal clusters are found, which should be due to the coalescence of microcracks. By these coalescence events the main crack is formed. At macroscopic crack propagation most AE events are located within the crack tip zone. However, up to about 20% of all events are located within the crack flank zone behind the crack tip. Thus, it can be concluded that there is an energy dissipation at the crack tip at beginning of the loading, which determines the starting value of the crack resistance. At macroscopic crack propagation crack flank interactions contribute to the increase of crack resistance, too. However, it cannot be decided from AE if the contribution of the process zone at the crack tip or of the crack flanks in the wake of the crack tip play the major role in increasing the crack resistance.     

Lifetime prediction for ABS pipes subjected to combined pressure and deflection loading

As part of an investigation into the performance of acrylonitrile‐butadiene‐styrene (ABS) systems for water transportation, this paper presents a method for predicting the service lifetimes of buried pipes under in‐service loading conditions. A linear fracture mechanics approach was used to analyze brittle failure initiating from adventitious flaws located at the bore surface of pipe. Failure criteria were determined using the time‐dependent, quasistatic, plane strain fracture toughness of the ABS material, combined with empirical parameters that describe slow, steady crack growth. The expected operating conditions of a buried pipe were then separated into static loading contributions from internal pressure, diametrical deflection and residual stress. Idealized stress intensity factors associated with mode‐I crack opening under each of these components were determined using a finite element analysis and superposed to describe the general case in service. The computed nett stress intensity factor was then combined with the previously determined fracture toughness and slow crack growth data in an algorithm to simulate incremental radial crack growth from the pipe bore. Predicted failure times compared well with an experimental model of expected operating conditions, which combined hydrostatic pressure and parallel‐plate deflection loading of an internally notched pipe. The prediction method was also used to identify the factors that control the lifetime of a pipe in service. The influence of material visco‐elasticity was investigated by simulating variations in fracture toughness and slow crack growth resistance. It was proposed that, in practice, these variations are governed by opposing changes in visco‐elasticity. The effect of changing diametrical deflection and residual stress distribution were also simulated, allowing recommendations on pipe manufacture and installation conditions to be made.     

Crack propagation in biaxially oriented polypropylene at low temperature

The crack growth behavior of polypropylene biaxially oriented by cross-rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at ?40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.     

Fracture of fiber reinforced composites

Characteristics of the fracture of fiber reinforced plastic composites are described in terms of the elastic stress distribution at the crack tip, the mechanism of crack tip damage, and the modes and conditions of final fracture. The three-dimensional, stress field at the tip of a sharp crack in a laminate is presented and contrasted to traditional two-dimensional models. The response of the material in the form of inter- and intraply damage formation and growth under increasing load is characterized, and its effect in blunting the main crack is examined. The final fracture conditions, which may range from quasi-brittle to notch insensitive, are discussed and related to the damage zone extension. Observed and anticipated effects of various material and geometric parameters are also discussed.