Experimental Setup for Thermal and Cyclic Axial Loading of District Heating Pipes

Abstract

In Europe, pre‑insulated district heating (DH) pipes are typically evaluated for quality control in accordance with EN 253; however, the standard does not include testing under simultaneous thermal and axial cyclic loads. The present work advances beyond the standard by introducing a test setup that superimposes thermal loads and cyclic axial shear stresses on pre-insulated DH pipes, thereby reflecting more realistic stress conditions in DH pipes. The configuration extends the EN 253 methodology by integrating welded connections for heat-carrier circulation, a pendulum‑bearing for controlled cyclic axial loading, and a pipe casing fixation system. A structured multi-stage test procedure—comprising thermal conditioning, cyclic axial loading, and quasi‑static axial shear testing—was developed, incorporating precise temperature control and 3D‑digital image correlation for displacement tracking. The contribution of this work lies in the conceptual design, rationale, and realisation of a flexible thermo‑mechanical test configuration that remains compatible with existing standards while enabling testing beyond their current scope. The proposed setup provides a methodological foundation for future investigations into ageing mechanisms, adhesion performance, and load interaction effects in pre‑insulated DH pipes under realistic operational conditions. As the experimental campaign is still ongoing, this paper presents the test methodology and setup, while final test results will be reported elsewhere.