Mechanical response and failure of an Ultra High Molecular Weight PolyEthylene at cryogenic temperature

International audienceThe increasing use of natural gas has led to the development of supply technologies. Associated with a decrease of the volume by a factor of 600, a temperature reduction to its liquefaction point (-163 °C), is favoured. Flexible pipes allowing Liquefied Natural Gas (LNG) supply, are being developed for offshore use. The pipe is subjected to complex loading, such as internal pressure or bending due by sea swell. Furthermore, the pipe is composed of a multimaterial structure with a convoluted geometry that could lead to stress concentrators. This work aims at a better understanding of the mechanical response and failure of the sheath, the prior tightness barrier of the pipe. The sheath material is an Ultra High Molecular Weight PolyEthylene (UHMWPE), which has an estimated glass transition temperature of approximately -119 °C.H2 samples were cut out from the thin sheath in two orientations, i.e. longitudinal and circumferential. In order to compare the mechanical properties at the undulation scale three positions were investigated: the two extremums and in between these positions. Tensile tests were carried out at 3 different temperatures especially at -163 °C and at 3 different crosshead speeds. For each test, load applied and crosshead displacement were measured. Digital Image Correlation (DIC) was used to obtain local displacement fields. All the fracture surfaces were observed post-mortem, thanks to S.E.M..Data from the stress-strain curves and from fracture surfaces, were utilized to plot the evolution of toughness, the ductile-brittle surface ratio with respect to the test temperature. A ductile-brittle transition temperature (DBTT) was then identified close to the operating temperature. Therefore, there is a risk of brittle failure. The experimental database so far obtained, allows the FE modelling of the pipe to be undertaken