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Comparison of Strain Hardening Modulus and Crack Round Bar testing for new generation pipe materials


Testing long term performance of PE 100 and PE 100 RC pipe grades can be time consuming. Unipetrol researchers have compared the two most common ISO test methodologies


The Pennsylvania notched test (PENT) was developed two decades ago to enable fast testing of slow crack growth in polyethylene pipe materials. The method was developed especially for testing PE 63 and PE 80 resins and for these grades gave a good determination of the quality of the prepared materials in a short-time period. Today, however, PE 63 and PE 80 resins have been replaced by PE 100 and PE 100 RC materials, which offer far superior resistance to slow crack growth. For these new materials PENT testing times can take several years. The need for a faster test method led


to creation of two new techniques that have been transformed into the ISO 18488 and 18489. ISO 18488: Polyethylene (PE) materials


for piping systems. Determination of strain hardening modulus (SHM) in relation to slow crack growth. In this method test specimens cut from compression moulded sheet are subjected to a tensile test at 80°C. The stress-strain curve is obtained sufficiently beyond the natural draw ratio and SHM determined from the slope of this curve in the area after the natural draw ratio. ISO 18489: Polyethylene (PE) materials


for piping systems. Determination of resistance to slow crack growth under cyclic loading – Crack Round Bar Test (CRB). This method uses a cyclic tensile test with constant load range on a cylindrical specimen under suitable test conditions within the stress range where slow crack growth is achieved. A test specimen with circumferential notch is tested. The number of cycles until final failure is recorded as a function of the stress. To assess the validity of these tests


five materials were selected; one extrusion HDPE, two PE 80, one PE 100


Authors: Jir ˇí Sadílek1 , Jaroslav Kuc ˇera1


and one PE 100 RC. Comparison of both techniques was based on the time required for specimen preparation, the need for laboratory equipment, test specimen measurement and measured data evaluation. Results are shown in Tables 1 to 4. The study shows that the SHM allows test specimens to be prepared easily and two tensile tests can be completed in one hour. However, evaluation of the


measured data is complicated and time consuming and testing without appropriate equipment is not possible. This test is associated with crazing, fibril creation and fibril deformation. Specimen preparation for the CRB test


is more complicated. Testing without appropriate equipment is not possible and testing is time consuming. However, the test is associated with slow crack growth and final evaluation is easier.


Table 1: Time required for test specimen preparation Strain Hardening Modulus


Cracked Round Bar Test


• preparation via compression moulding • preparation via compression moulding • punching of 10 test specimens • additional annealing


• machining of 14 test specimens


• test specimen notching • conditioning


Procedures takes a maximum of 2 days


Table 2: Laboratory equipment Strain Hardening Modulus • tensile-testing machine • load cell


• extensometer up to 1,100% • temperature chamber


• grips • micrometer


Both test methods require specific and expensive laboratory equipment


Table 3: Test specimen measurement Strain Hardening Modulus • temperature 80°C


• test specimen put into grips • temperature equilibrium


• speed of testing = 20 mm/min • deformation = 1,100%


Two or three test specimens can be tested in one hour


Cracked Round Bar Test • temperature 23°C


• test specimen put into grips • temperature equilibrium • testing frequency = 10 Hz • breakage = end of test


Poorer materials require 100,000 cycles


(170 minutes equivalent to one test specimen in 3 hours)


Better materials may require 1,000,000 cycles (1,700 minutes equivalent to one test specimen in 28 hours)


Table 4: Measured data evaluation Strain Hardening Modulus


• measured data exported to EXCEL file • recalculation and application of mathematical model


Cracked Round Bar Test


• number of cycles automatically evaluated • microscopy for fracture surface evaluation • calculation of stress intensity factor


• evaluation of SHM Measured data evaluation is faster for CRB testing


1UNIPETROL RPA, s.r.o. - POLYMER INSTITUTE BRNO, odšte


, Martin Chytil1, Pavel Hutar , Jan Poduška2 ˇpný závod


ˇ2 2Institute of Physics of Materials of the Czech Academy of Sciences www.unipetrol.cz/en Procedures take 5 days


Cracked Round Bar Test • fatigue-testing machine • load cell


• special grips • microscope


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