BS ISO 18488:2015 pdf download – Polyethylene (PE) materials for piping systems — Determination of Strain Hardening Modulus in relation to slow crack growth — Test method.
4 Principle 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. The strain hardening modulus is determined from the slope of this curve in the area after the natural draw ratio. 5 Apparatus 5.1 Tensile-testing machine, complying with ISO 527-1 and capable of maintaining a test speed of (20 ± 2) mm/min. 5.1.1 Load cell, which shall comply with Class 1, as defined in ISO 7500-1. The load cell shall be able to accurately measure forces in the range of 40 N for 0,30 mm thick samples and 120 N for 1,0 mm thick samples. 5.1.2 Extensometer, which shall comply with Class 1, as defined in ISO 9513. The traverse displacement shall not be used as a measure of strain. For a 0,30 mm thick specimen, a non-contact extensometer is preferred. 5.1.3 Temperature chamber, to control the temperature at (80 ± 1) °C. 5.1.4 Clamps, remote operation to enable closing and opening without the need to open the temperature chamber is recommended. 5.2 Devices for measuring the thickness and width of the test specimens. 5.2.1 Thickness shall be measured with a device with an accuracy of 0,005 mm and a device with a contact dimension of less than the width of the parallel specimen section (4,0 mm). 5.2.2 Width shall be measured with a device with an accuracy of 0,01 mm. Care shall be taken not to change the width of the test specimen by deformation. Therefore, it is recommended to use a microscope to measure width to avoid deforming the test specimen. 5.3 Punch knife, in accordance with 6.1 shall be used.
7 Test procedure — Measure the exact dimensions of width (b) with an accuracy of 0,01 mm and of thickness (h) with an accuracy of 0,005 mm of each individual test specimen. — Condition the test specimens for at least 30 min in the temperature chamber at a temperature of (80 ± 1) °C prior to starting the test. — Clamp the test piece in the upper clamp. The clamps shall be chosen to avoid damage and slippage of the test piece. — Close the temperature chamber. — After reaching the temperature of (80 ± 1) °C, clamp the test piece with the lower clamp — The sample shall remain between the clamps for at least 1 min before the load is applied and measurement starts. — Apply a pre-stress of 0,4 MPa with a speed of 5 mm/min. — During the test, measure the load and elongation sustained by the specimen. — Extend the test specimen at a constant traverse speed of 20 mm/min Data points shall be collected from λ = 8,0 until λ = 12,0 or breakage for analysis of the data. If the sample breaks before λ = 8,5, the sample shall be rejected. At least five specimens shall be tested. If slippage of the test bar takes place in the clamps, the test result shall be discarded and the test repeated.