Tue, 19 April, 2022
A presentation by Mike Troughton, TWI Technology Fellow and Technology Manager for Plastics, has resulted in a forthcoming revision to the international standard EN 12814-7: Testing of welded joints of thermoplastics semi-finished products - Part 7: Tensile test with waisted test specimen. The presentation was on the results of the TWI Core Research Programme (CRP) project ‘Improvement of the reduced section tensile test specimen geometry for assessing the integrity of butt fusion welds in polyethylene pipes’.
A reduced section tensile test for butt fusion welds in polyethylene (PE) pipes is specified in a number of standards, including ISO 13953 and EN 12814-7. In previous CRP projects, TWI has found this to be the most discriminating, short-term mechanical test for assessing the quality of these joint types, where either the failure mode or the energy to break is evaluated. However, as joint thickness increases, the ductility of the test specimen decreases, therefore the current specimen geometry is not as discriminating for joints in thick walled pipe.
This CRP project used both design of experiments (DoE) and finite element analysis (FEA) to assess the effect of specimen geometry on the failure mode, and the elongation of loading holes which affects the measured energy to break using crosshead displacement, in butt fusion welds in PE pipes.
It then proposed three geometries for different ranges of specimen thickness, as well as proposing that for thicknesses greater than 30mm, the specimen be cut into two or more equal slices such that the thickness of the slices is less than 30mm. Experimental results from tests using these new specimen geometries were compared with tests using the existing geometry, and they agreed closely with the DoE and FEA predictions.
Mike made the presentation to the CEN/TC249/WG16, (Welding of thermoplastics) European Standards Working Group – in his capacity as the UK Principal Expert on this Working Group and its predecessor, and a position that he has held for the last 25 years – who approved a new work item to revise EN 12814-7, based on the new proposed specimen geometries detailed in the CRP project.
Speaking of the achievement, Mike said “TWI’s Core Research Programme is designed to create value for TWI, its Industrial Members and the global engineering community, and this particular project, like all CRP projects, was instigated to address a particular, identified need. Therefore, we are delighted that the research undertaken on this project provided demonstrable evidence to inform the continuing maintenance of the international standard EN 12814-7 as up-to-date and industry-relevant.”
Participation in the TWI Core Research Programme, and access to prior results, is only available to TWI Industrial Member companies. If you are interested in exploring TWI Industrial Membership for your company or organisation, including gaining access to proprietary, in-depth, industry relevant research, please email us at ContactUs@twi.co.uk and we will be in touch to discuss your requirements.
Find out more about TWI’s Core Research Programme (CRP).