Local Yield Stress Distribution of HDPE Pipe Butt Fusion Joints Measured by a Small Punch Test

Butt fusion welding is the primary connection technology for high-density polyethylene (HDPE) pipelines, yet welded joints are the most vulnerable location for pipeline service failures. Conventional uniaxial tensile tests can only obtain the average strength of the whole weldment and fail to capture the spatial variation of local yield stress across narrow joint zones. In this study, small punch tests (SPTs) combined with finite element parametric simulations were adopted to quantitatively characterize the position-dependent yield stress of HDPE pipe butt fusion joints. Simulations were performed to establish a correlation formula between SPT peak load-displacement parameters and HDPE yield stress, and the effects of material properties and contact friction on empirical coefficients were systematically analyzed. The calibrated formula under a friction coefficient of 0.5 was verified against uniaxial tensile data of base HDPE, showing acceptable prediction accuracy. For joints fabricated following a ISO 21307 standard, the yield stress of the fusion zone was higher than that of the base pipe material. The maximum yield stress appeared at 3 mm offset from the weld center plane, approximately 4% higher than the base matrix. This study validated the feasibility of using SPT to realize micro-sampling localized mechanical measurement of HDPE welded joints, offering a convenient testing method for on-site quality assessment of polyethylene pipeline welds.