Inspecting the quality of butt fusion welds can be conducted by either visual or volumetric methods. Visual inspection evaluates surface features, such as the external weld bead and axial alignment. The external weld bead should be smooth and have a symmetrical profile around the whole circumference. The alignment of the pipes/fittings should also be checked; the axial misalignment should not exceed 10% of the nominal wall thickness.
The external weld bead (and sometimes also the internal weld bead) of a PE pipe butt fusion weld can be removed using an appropriate de-beading tool, which removes the weld bead in one piece. The bead can then be bent back manually at a number of locations along its length. If the two halves of the weld bead split apart, this indicates that there is contamination in the weld. The technique will only detect fine particulate contamination, which is pushed out into the beads during the welding process. It will not detect embedded flaws or cold welds and may not detect coarse particulate contamination.
For more thorough quality inspections, volumetric non-destructive testing, such as Ultrasonic Testing (UT), can be used to detect embedded flaws. Common techniques are Phased Array Ultrasonic Testing (PAUT) and Time-of-flight diffraction (TOFD). PAUT, by deploying sophisticated procedure development, can detect planar embedded flaws along with contamination and cold fusions. The method gives full volumetric data of the weld that is installed as a permanent record. TWI is a world-leader in PAUT of butt fusion welds in PE pipes and has developed the PolyTest system, specifically for inspecting welded joints in PE pipes.
Further information
For further information on mechanical testing of welded joints in thermoplastics please contact us.