THE “NOTCHED CYLINDRICAL BARS UNDER CONSTANT LOAD TEST” (NCBT) FOR ASSESSING THE RESISTANCE TO CRACK INITIATION OF PE100 AND PE100 RC: DETERMINATION OF THE INITIATION TIME

Papers # 2016 Berlin

A specific methodology has been designed by ENGIE’ R&T Division to allow an accurate determination of the initiation (incubation) time of the NCBT data. This methodology is based on two different data processing procedures. The first data processing proceeds from searching the minimum of the second derivative of both a 3rd order and a 6th order polynomial fitting of the curve. Such approach is not well suited to very flat curves or to unsteady curves on the one hand, and needs to define a low limit of the strain for data processing, in order not to emphasize the curvature at the beginning of the test on the other hand. The second data processing proceeds from searching the minimum of the strain rate all along the test. Such an approach is pertinent and does not require any low strain limit to process the data. Moreover, processing the data in real time allows one to stop the test a short time after the minimum strain rate has been detected. This preferred procedure gives way to an accurate determination of the initiation (incubation) time, hence reducing drastically the NCB Test duration for a quick evaluation of a PE grade.

In the last five years some new and innovative tests have been designed to assess for Slow Crack Growth (SCG) behavior of PE100 and PE100 RC with the objective of reducing drastically the test time duration. Three tests have been focused on by the WG20 of the ISO TC138-SC5 committee. These tests are the Strain Hardening Test (SHT), the Cyclic Crack Round Bar Test (CCRBT) and the Notched Cylindrical Bars under constant load Test (NCBT). These three tests allow to assess for SCG resistance of PE100 and PE100 RC in a reasonable test duration less than 1,000h with the major advantage to avoid the utilization of any surfactant suspected of modifying the SCG process. The two former tests were the subject of an intensive evaluation through round robin tests managed by the WG20 and finally became ISO standards in 2015. The NCBT was first evaluated in 2013on PE100 and PE100 RC by ENGIE’R&T Division and then proposed to the WG20. The promising results presented during the Plastic Pipes XVII opened the way to the standardization process and to a CD document in 2015. One of the advantages of the NCBT is the possibility to detect in a very accurate way the crack initiation (or incubation) time and thus to shorten the test time duration in a very effective proportion. The crack initiation time corresponds to the “trigger” for crack to progess and can be modelled by means of an elasto-visco-plastic theoretical approach as presented formerly as, for instance, during the Plastic Pipes XVI. A first method was designed by R&T Division in order to detect the initiation time. This method, based on the fitting of the NCBT curve by means of polynomial laws, required approximately 2/3 of the up-to-failure curve and was very sensitive to the flatness of the curve. In order to avoid this, a refined method is proposed which focuses on the strain rate. The method was successfully applied on four PE100 and five PE100 RC. Then the “live” detection of the minimum strain rate allows a very accurate determination of the crack initiation time and a drastic test time reduction for a quick evaluation of a PE grade.Furthermore, the true stress applied to the specimen can be calculated – for comparison purposes with the target one -once the initiation time has been determined by provoking the failure of the material in liquid nitrogen.

Related keywords : Polyethylene; PE; Notched Cylindrical Bars under constant load Test (NCBT), Crack Initiation (Incubation) Time, Slow Crack Growth, PE100 RC, ISO TC138-SC5-WG20.
Please note that the whole article content is available on PPCA website onlySource : 2016 Berlin


Related papers

ACCEPTANCE CRITERIA FOR VOLUME DEFECTS IN WELDED ASSEMBLIES, DETECTED AND SIZED USING THE PHASED ARRAY ULTRASONIC TECHNIQUE

Author(s) : Dominique GUEUGNAUT, Manuel TESSIER, Romuald BOUAFFRE 2018 Las-Vegas

Given the virtual absence of available up-to-date results regarding the acceptance criteria for defects at electro-welding interfaces, a study has been launched by GRTgaz RICE with the two-fold aim of confirming the accuracy of the PAUT technique for the detection and sizing of calibrated pit-shaped defects implanted...

LABORATORY DEVICE FOR CHECKING THE THERMAL ENERGY DELIVERED BY THE ELECTROFUSION FITTINGS

Author(s) : Dominique GUEUGNAUT, Adil BOUJLAL, Aymeric LOPITAUX, Bertrand SEVRE 2018 Las-Vegas

A special concept has been developed by GRTgaz Research and Innovation Center for Energy (RICE) for temperature mapping at the interface of an electrowelded assembly.The technical solution consists of a tubular piece in place of the real PE pipe, made of a non-weldable polymer with thermal parameters close to those of...

RESEARCH PROGRAM FOR ASSESSING SEISMIC RISKS ON POLYETHYLENE GAS PIPELINES

Author(s) : Dominique GUEUGNAUT, Charles FERNANDEZ, Romuald BOUAFFRE, Bertrand SEVRE, Florent ANGELINI 2016 Berlin

Representative earthquake loadings have been reproduced in the ENGIE Lab-CRIGEN laboratories on three PE grades, including an excavated old PE63 pipe operated during more than 35 years. Dynamic loading does not alter the pipe residual performances for the three PE families tested, as shown by both the hydrostatic...

ULTRASONIC PHASED ARRAY INSPECTION OF POLYETHYLENE DAMAGED PIPES WITH CALIBRATED FAULTS

Author(s) : Dominique GUEUGNAUT, Florent ANGELINI, Romuald BOUAFFRE, Manuel TESSIER 2016 Berlin

A test campaign has been carried out by ENGIE Lab-CRIGEN in order to evaluate the acuity of the Phased Array Ultrasonic Technique (PAUT) for detecting and sizing calibrated defects implanted at the inner surface of a PE pipe.These calibrated defectshad four different geometries, called “V-shaped”, “half-V shaped...

The “Notched Cylindrical Bars Under Constant Load Test” (NCBT) as a Means to Assess the Resistance to Crack Initiation and to Slow Crack Growth of PE100 and PE100 RC

Author(s) : GUEUGNAUT Dominique, BOUJLAL Adil, BOUAFFRE Romuald, LOPITAUX Aymeric 2014 Chicago

The emergence of a new generation of PE100, named PE100 RC, with a significant enhanced Slow Crack Growth (SCG) resistance enlights the need for new testing methods in order to be able to test such resins in reasonable test durations. Consequently in the last five years some new tests have been designed to assess for...

Ultrasonic Phased Array Inspection of Polyethylene Pipelines & Fittings Used for Gas Distribution

Author(s) : GUEUGNAUT Dominique, ANGELINI Florent, BOUJLAL Adil, BOUAFFRE Romuald, SEVRE Bertrand, LOUET Patrick 2014 Chicago

Polyethylene (PE) is a widespread material for the manufacturing of gas and water distribution pipelines and a large number of PE electrofusion pipe joints can be found over the thousands of kilometres of PE pipeplines all over the world, thus generating an increasing interest in checking the quality of these joints....

3D FINITE ELEMENT SIMULATION OF ELECTROFUSION POLYETHYLENE JOINTS FOR GAS DISTRIBUTION

Author(s) : Chebbo Z., Boujlal A., Gueugnaut D., Vincent M., Tillier Y. 2012 Barcelona

(Electrofusion, Finite Element, Heat transfer, Interdiffusion, crystallisation and melting Kinetics, Micro-sampling) Electrofusion (EF) is widely used to join polyethylene (PE) components in the gas distribution networks. This paper provides experimental and 3D-finite element tool capable of reproducing the major...

Analysis of slow crack initiation of old polyethylene resins by means of an elasto-visco-plastic rheological model: experimental and numerical approach

Author(s) : Boujlal Adil 2012 Barcelona

(Polyethylene, Long term behavior, Rheology, Slow Crack initiation time) Predicting PE failure mechanisms needs suitable models in order to describe accurately both material behavior and damage processes. In this area GDF SUEZ’ Research & Innovation Division carries out an in-depth study to determine on the one hand...

IDENTIFICATION OF THE PE RESINS OF THE DISTRIBUTION NETWORKS BY MEANS OF THE DSC-SIS TECHNIQUE APPLIED ON MICRO-SAMPLING

Author(s) : Gueugnaut Dominique, Boujlal Adil, Rousselot Denise, Bouaffre Romuald 2012 Barcelona

(Polyethylene, SIS technique,Micro-sampling)The use of polyethylene(PE) since the early seventies on the GDF SUEZ’ distribution networks has lead to the coexistence of a very wide variety of PE resinswith significant differences in their long term performances. Nevertheless, in order tojustify a possible delayed or...

THE CAPABILITY OF THE ULTRASONIC PHASED ARRAY TECHNIQUE FOR DETECTING AND MEASURING DEFECTS AT THE INNER SURFACE OF A POLYETHYLENE PIPE THROUGH AN OUTSIDE ANALYSIS

Author(s) : Gueugnaut Dominique 2012 Barcelona

(Polyethylene, Non destructive technique, Ultrasonic Phased Array, Slow Crack initiation time) Since a few years it has been proved that - among the available concepts - the Ultrasonic Phased-Array technique (PA UT) was one of the most promising candidate for Non Destructive and Non Intrusive analysis of PE and...

Members of the Association

BOREALISBOROUGEINEOS O&PIRPCKorea Petrochemical IND. Co., LTDLyondellBasellPetroChina Dushanzi Petrochemical CompanyPRIME POLYMERSABICSCG Chemicals & Thai PolyethyleneSinopecTASNEE
TOP