Acceptance Criteria for Defects in Polyethylene Welds, Coupling Phased Array Ultrasonic Testing and Destructive Tests

Abstract

As a promising reference technique for non-destructive evaluation of both electrowelded and butt-fused polyethylene (PE) assemblies, Phased Array Ultrasonic Testing (PAUT) is still studied extensively in several laboratories worldwide and is supported by the technical standard ISO TS 16943. During the last 10 years, several joint projects have been completed aiming at evaluating the acuity of PAUT applied to both pipes and electrofused assemblies either exhumed from the field or prepared in laboratory. More recently, a focus has been made on fixing some acceptance criteria combining PAUT data and long term resistance of the laboratory joints. This paper presents the updated data obtained on electrofused assemblies—63 mm saddles and 110 mm sockets—containing different types of defects such as: insufficient heating time, pipe under-penetration in the socket, excessive localized scraping, pollutants and calibrated thin strips, in both mass and cross configuration, put at the interface pipe-saddle. PAUT scanning on the different specimens, both during the welding phase and after cooling, confirms the capability of the technique to visualize and size the Heat Affected Zone (HAZ), which can be revealed and compared afterwards on sample sections. Moreover, most of the defects are detected and sized, confirming the fairly good performance of PAUT, except for the smallest strips which are located in non accessible zones, due to the particular design of the saddle. Long term resistance of the welds is then evaluated by Hydrostatic Pressure Tests (HPT) followed by a decohesion test after rupture, according to the requirements of both the ISO 13956 and NF EN 1555 standards. Under such test conditions, every joints comply with the requirements of the standards (rupture time greater than 1000 h at 80 °C and 5 MPa), even those violating the critical proportions of non-welded zones.