Analysis of Molecular and Morphological Effects on Slow Crack Growth in Modern PE Pipe Grades by Cyclic Fracture Mechanics Tests

Summary: Three different polyethylene (PE) pipe grades as well as three different lots of one of the grades were investigated by cyclic tests with cracked round bar (CRB) specimens, concerning resistance to slow crack growth. To enhance the test sensibility and proof its applicability for a quick quality assurance method various molecular and morphological characterizations on compression molded plates were carried out, with special attention on the influence of molecular and morphological differences, as well as lot to lot variations on the resistance to slow crack growth. The cyclic CRB tests allowed a ranking of the different pipe grades and lots with short testing times per material and testing machine, as a function of failure time as well as of crack initiation time with further reduction of testing time of about 50%. Moreover the ranking corresponded to the expectations based on the molecular and morphological properties of the materials, where only minor changes in the molecular mass distribution and the co-monomer concentration in case of lot to lot variations were proofed reliably.     

Strain hardening test on the limits of Slow Crack Growth evaluation in high resistance polyethylene resins: Effect of comonomer type

Long term performance assessment of polyethylene pipes is an issue that has greatly increased in importance in recent years due to the incorporation in the market of high resistance to crack polyethylene grades (PE100RC), where established Slow Crack Growth (SCG) evaluation using traditional tests such as Full Notch Creep Test (FNCT) or Pennsylvania Notch Tensile (PENT) Test is insufficient. The development in recent years of fast evaluation techniques such as Strain Hardening (SH) modulus has opened an important alternative for quick SCG evaluation since it correlates well with other conventional tests such as FNCT and PENT. In this work, a large number of commercial and experimental polyethylene pipe resins with different comonomer types were evaluated in order to define their SH values to rank the resins as PE100 or PE100RC. A relationship is proposed that utilizes SH test results to estimate the SCG resistance of PE pipes. 1-Butene copolymer resins display threshold SH values of 38 and 53 MPa that have been assigned to PE100 and 100RC grades, respectively. Moreover, dependence of the SH values on comonomer type used has been demonstrated. The experimental results show that 1-hexene copolymer resins exhibit higher SH values than 1-butene comonomer based resins.     

Numerical Assessment of PE 80 and PE 100 Pipe Lifetime Based on Paris-Erdogan Equation

Summary: A novel accelerated fracture mechanics extrapolation procedure based on cyclic test with cracked round bar (CRB) specimens was verified by a correlation of real pipe failure time to simulated failure times at a temperature of 60 °C. The procedure was applied to predict the long-term failure of modern PE 80 and PE 100 pipes 23 °C. Moreover, the used stress intensity factor concept also allows to consider the impact of arbitrary additional loading situations like soil loads or point loads and to assess pipe lifetime under complex loading situations.     

A new automated method for recording and predicting failure by slow crack growth in polyethylene

The PENT test (ASTM F1473) which measures the resistance to slow crack growth (SCG) has been modified so that the crack opening displacement (COD) can be automatically recorded through a computer. From the curve of COD rate versus time, fine details of the fracture processes can be observed and a very early stage of fracture initiation can be determined. Consequently, the time for complete failure can be predicted from the initiation time sooner than has been possible by previous techniques.     

Determination of slow crack growth behaviour of polyethylene pressure pipes with cracked round bar test

In this work, a short time test method to determine the slow crack growth behaviour of samples made out of pipes was evaluated. The cracked round bar (CRB) method used provides results below 48 h with brittle fracture surfaces, which indicates the type of slow crack growth failure. To evaluate the usability of the method, the results were compared with well-known tests such as notch pipe test, 2 notch creep test and instrumented Charpy impact tests. The results indicate that the CRB test can be used to predict long term slow crack growth behaviour of PE pipes.     

Investigation of the Slow Crack Growth Behavior of Static and Cyclic Loaded Specimens of Polyethylene by 2D and 3D Optical Fracture Surface Analysis

Summary: A fracture surface investigation was conducted to study the applicability of cracked round bar (CRB) specimens for an accelerated extrapolation concept for a lifetime assessment of polyethylene (PE) pipes. Scanning electron microscopy and topography metrology with InfiniteFocus were used to study the slow crack growth behavior in CRB specimens at different loading conditions. The results confirm the compliance of the CRB test with the requirements of linear elastic fracture mechanics.     

Cyclic tests on cracked round bars as a quick tool to assess the long term behaviour of thermoplastics and elastomers

The aim of this work is to demonstrate the applicability of the cracked round bar test recently developed for PE-HD to other polymeric materials. The main advantage of this new test method are rather short testing times for PE-HD materials used in long-term applications such as piping. Therefore, this test is of high interest for other polymers used in similar applications.Five thermoplastic materials used for plumbing (PE-HD, PP-B, PB, PVC-U, PA12), a technical polymer (POM) and an elastomeric material (H-NBR) have been tested. Scanning electron microscopy has been applied to investigate fracture surfaces.Results show that the test method seems to be basically applicable to all tested materials. Most materials showed similar fracture behaviour as postulated in literature, despite the high acceleration factor of the cyclic CRB test.     

Influence of specimen geometry on the slow crack growth testing of HDPE for pipe applications

The majority of field failures in piping are attributable to slow crack growth (SCG) fractures. These fractures are characterized by the stable growth of a crack with little deformation in the plastic material. Slow crack growth (SCG) testing involves accelerating the growth mechanism through elevated temperature, concentrated stress, constrained geometry, surfactants or some combination of these factors. Some of these accelerated tests, Pennsylvania Edge-Notch Tensile (PENT) and Full Notch Creep Tests (FNCT), have been designed specifically to promote the SCG failure by stress concentration. However, the development of new polymeric materials used in pipes for transportation, has greatly increased the time required to happen SCG failure through these accelerated tests. Recently, new specific geometries for specimens to promote the failure by SCG have been analyzed, such as the Circumferentially Deep Notched Tensile (CDNT) sample.In this work, the reliability of the CDNT specimen to promote SCG failure on two types of ethylene copolymers was studied. The ligament surfaces after failure were analyzed to identify the SCG. The failure times were compared with those obtained on the same materials tested with a PENT geometry.     

Estimation of Slow Crack Growth Behavior in Polyethylene after Stepwise Isothermal Crystallization

The use of analytical methods to estimate the SCG resistance of PE pipe materials has been suggested due to the fact that the slow crack growth (SCG) behavior of polyethylene (PE) is governed by structural parameters such as molecular weight and side chains. In the research project presented here, the molecular structure of several commercially available polyethylene-high density (PE-HD) materials was analyzed using a modified stepwise isothermal segregation (SIS) technique and compared with results from fracture mechanics experiments. A good correlation between the SIS results and SCG rates was found. It turned out that the modified SIS technique is a fast and simple method and could be used to assess SCG resistance in PE.     

Abnormality in using cyclic fatigue for ranking static fatigue induced slow crack growth behavior of polyethylene pipe grade resins

Slow crack growth behavior in polyethylene pipe grade resins were studied using both static fatigue (stress-rupture) and cyclic fatigue tests. This was done to better understand the applicability of cyclic fatigue in the prediction of slow crack growth ranking determined from the static fatigue test. In all polyethylene pipe grade resins tested at 80 °C, reduced crack growth failure times were exhibited when the cyclic fatigue test was employed. However, when applied to rank the resins through their slow crack failure times, the cyclic fatigue results did not always confirm those obtained from the static fatigue test. That is, in some cases, a resin with higher slow crack resistance ranking (longer failure times) than another resin in static fatigue exhibited lower ranking (shorter failure times) in the cyclic fatigue test. This abnormality of reversal in ranking is not a general observation but does occur. Based on the data obtained so far, when resins with smaller differences between static fatigue and cyclic fatigue slow crack growth failure times are compared with those resins having larger differences, the chances of correctly predicting the ranking obtained from static fatigue using cyclic fatigue tend to decrease. Hence, it is suggested that one needs to practice caution when using cyclic fatigue to predict the static fatigue ranking of resins for slow cracking resistance. Some insight into the cause of such abnormality is discussed with reference to creep-fatigue interactions.     

Characterization and modeling of slow crack growth behaviors of defective high-density polyethylene pipes using stiff-constant K specimen

In this study, slow crack growth (SCG) resistances of defective and normal high density polyethylene (HDPE) pipes were measured using the stiff-constant K (SCK) specimen, where the stress intensity factor (SIF) was maintained at a constant value within a certain crack length range. A significantly reduced SCG resistance was observed in the defective pipe; a detailed procedure for evaluating SCG kinetics using the SCK specimen has been provided herein. The results of a fracture surface analysis indicate that the white window patterns, resulting from poor carbon black dispersion, are the main reason for poor SCG performance. In addition, a crack layer (CL) model was derived for the SCK specimen geometry and was compared with experimental results. It was observed that the crack and process zone growth resistance parameters were significantly lower in the case of the defected pipe than those in the case of the normal pipe.     

Notched Ring Test for Measuring Slow Cracking Resistance in Plastics Pipes and Fittings

It is known, that the lifetime of polyethylene pipes is essentially limited by slow crack growth (SCG). For state of the art PE materials common SCG testing methods have reached their limits with respect to extension of testing times. A comparatively new method is the Notched Ring Test (NRT) as developed by Choi et al.^[1] Pipe rings notched at the inner wall are used. The test is carried out in 80 °C water under constant bending load. The arrangement of the notch at the inner wall reduces testing times using the residual stress of extruded pipes. A disadvantage of this method is that there is no clearly defined failure time because SCG takes place between two phases of creeping. The output of this test is an “on-set slow cracking time” (crack initiation), obtained by analysis of the displacement curve. In this work it has been shown that the NRT method yields to brittle fracture within acceptable time frames.^[2] Methods for data analysis are presented. This test could be very useful applied in research and development for resin evaluation and as a tool in quality control in pipe production for evaluating the process conditions.     

The effect of γ-irradiation on slow crack growth in polyethylene

HDPE was γ-irradiated at room temperature. The resistance to slow crack growth (SCG) was measured in single edge notched tensile specimens under constant load as a function of the dose. The resistance to SCG initially decreased to a minimum value at a dose between 0.05 and 0.10 Mrd. The minimum value was 45% less than for the undosed state. For doses greater than 0.10 Mrd, the resistance to SCG increased up to a dose of 50 Mrd, where its value had increased by a factor of 10². The gel point occurred at 1–3 Mrd. MI and the crack opening displacement exhibited maximum values at a dose of 0.1 Mrd. The behaviors of SCG, MI and crack opening displacement were consistent with the explanation that chain scission dominated for doses less than 0.1 Mrd, and cross-linking dominated at the higher doses. For doses beyond 50 Mrd, the resin became so brittle that it cracked during the loading of the specimen. Beyond the gel point the density increased from 0.9694 to 0.9716 g/cm³ at a dose of 160 Mrd. ©1995 John Wiley & Sons, Inc.     

Effect of γ-irradiation on slow crack growth in a copolymer of polyethylene

The effect of γ-irradiation on slow crack growth (SCG) in a medium density polyethylene (MDPE) was measured and compared with behavior of high density polyethylene (HDPE) and a recrystallized HDPE (RCHDPE). The three materials exhibited the same dependence on dose up to 3 Mrd. The HDPE became brittle above 50 Mrd. The resistance to SCG of MDPE and RCHDPE increased very rapidly with dose above 3 Mrd, until at 50–80 Mrd their resistance to SCG became extraordinarily high. This high resistance to SCG was accompanied by a transition from crazing to shear deformation at the root of a notch. It was found that for the same concentration, crosslinks are more effective than short chain branches for increasing resistance to SCG. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2349–2354, 1998     

Durability study of flexible bonded joints: The effect of sustained loads in mode I fracture tests

Adhesives in bonded structures are exposed both to external loads and environmental conditions; durability studies are currently needed to assess their service lifetime. Conditioning strategies differ in considering external load conditions (such as stressed or not stressed) for the durability analysis of double cantilever beam (DCB) bonded joints. Different test procedures such as ASTM D3762 (wedge testing) or ISO-25217 (DCB testing) exist to characterise the evolution of the fracture strength and toughness found in bonded joints. These methods depend on crack-length measurements, however, and achieving an accurate visual determination may be difficult due to the large fracture process zones (FPZs) that develop in the adhesive layer, especially in flexible or degraded bonded joints. To compensate, crack-length-independent data-reduction methods such as the compliance-based beam method (CBBM) or the J-integral method can be used, but experimental research is lacking on the suitability of these methods in ageing tests. A lack of consensus also exists in testing methodologies to evaluate the durability of bonded joints, especially when examining flexible bonded joints. The present work evaluates the influence of damage on fracture toughness within flexible bonded joints exposed to service conditions. Wedge tests and DCB tests are conducted using DCB specimens bonded with a flexible structural adhesive, proving that the degradation of flexible bonded joints exposed to environmental conditions is significantly accelerated when external loads act on them. The findings show that crack length estimation is affected due to environmental effects and thus, that crack-length-dependent test methods are not applicable in ageing tests.     

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.     

Alternative accelerated and short-term methods for evaluating slow crack growth in polyethylene resins with high crack resistance

The current market has widely adopted the new polyethylene pipe grade PE 100 RC (resistant to cracks) for pipe applications. However, the main drawback of this material is the long test period (∼10,000 h) required for ranking the resins. This paper proposes a modified Pennsylvania edge-notch tensile (PENT) test with higher load and temperature conditions (2.8 MPa and 90 °C). With the modified PENT test, failure time is six times shorter but slow crack growth is maintained. Additionally, it evaluates and finds an unexpected relationship between the strain hardening modulus and specimen thickness. These results suggest that the 0.30-mm thickness recommended by ISO 18488 is not optimal. Therefore, thicker specimens are proposed for accurate strain hardening modulus determination. Both methods are viable alternatives for evaluating the failure resistance of the new polyethylene pipe grades.     

Prediction of the remaining lifetime of polyethylene pipes after up to 30 years in use

Plastics pipes made of polyethylene (PE) play an outstanding role in gas and water supply. While for modern pipe grades typical lifetimes of 50 years are taken for granted and service times of 100 years are discussed, pipes made of PE with a lower performance have been used for decades. As the repair and rehabilitation of existing pipe systems involve immense costs, the question of their qualitative condition has to be considered. In this paper, four different pipes used in the gas and water distribution in Austria with an age up to 30 years have been investigated. After a morphological and mechanical study, particular attention was paid to material stabilization, which is essential for long-term applications. Fracture mechanics tools have been used to gain information on the resistance to crack initiation and slow crack growth. Furthermore, a fracture mechanics extrapolation procedure has been applied to predict the remaining lifetime of the pipes. The results have indicated that all the pipes investigated are still in a very good condition and are likely to be sufficiently safe to remain in use.     

Quantitative Structure�CRetention Relationship Study on Nitrogen-Containing Polycyclic Aromatic Compounds by Using Molecular Electronegativity Interaction Vector

The molecular structures of 117 nitrogen-containing polycyclic aromatic compounds (N-PACs) were described by a method of molecular structural characterization (MSC) called molecular electronegativity interaction vector (MEIV). The samples were divided into a training set and a test set. For the training set, a quantitative structure?Cretention relationship (QSRR) model was built up by multiple linear regression (MLR) and the model was evaluated by performing the cross validation with the leave-one-out (LOO) procedure. The correlation coefficient (R) and the cross-verification correlation coefficient (R _CV) of the model were 0.992 and 0.991, respectively. Moreover, the model was evaluated by the test set and satisfactory results with a correlation coefficient (R _test) of 0.993 were obtained. The results suggested good stability and predictability of the model.     

Slow crack growth in blends of HDPE and model copolymers

The resistance to slow crack growth (SCG) was measured in binary blends of high density polyethylene (HDPE) and 5–10% concentrations of model ethylene-butene random copolymers by measuring the time to failure (t_f) under a constant stress intensity. An increase of t_f with the addition of the copolymer if the copolymer could crystallize and the increase was greater the higher branch density. The copolymer with 117 branches/1000C could not crystallize and therefore its blend had a t_f that was less than that of the HDPE. The fracture energies of the blends as determined by their resistance to SCG were compared with the energy by rapid fracture, J_c, as previously measured by Rhee and Crist. It is concluded that SCG is more sensitive to variations in the microstructure than is rapid fracture and that the differences in SCG behavior can be qualitatively explained in terms of the differences in microstructure of the blends. ©1995 John Wiley & Sons, Inc.