Significance and Use
5.1 This practice is intended primarily for the automated or semi-automated ultrasonic examination of butt fusion joints used in the construction of polyethylene piping systems.
5.2 Polyethylene piping has been used in lieu of steel alloys in the petrochemical, power, water, gas distribution and mining industries due to its reliability and resistance to corrosion and erosion. Recently, polyethylene pipe has also been used for nuclear safety-related cooling water applications.
5.3 Two ultrasonic techniques have proven useful to provide examination of fusion joint integrity; Ultrasonic time-of-flight-diffraction (TOFD) and phased array ultrasonic testing (PAUT). These techniques are often considered complementary but may be used independently of each other. The choice of the technique used may depend on a variety of parameters including diameter, thickness, surface access, detection capabilities near surfaces, and quality level required.
5.4 The joining process can be subject to a variety of flaws including, but not limited to: lack of fusion, particulate contamination, inclusions, and voids.
5.5 Polyethylene material can have a range of acoustic characteristics that make butt joint examination difficult. Acoustic velocity of the material is similar to that commonly used for ultrasound wedge materials, making it difficult to use these materials to achieve appropriate refraction of sound at the interface. Polyethylene materials are highly attenuative, which often limits the use of higher ultrasonic frequencies. It also exhibits a natural high frequency filtering effect. An example of the range of acoustic characteristics is provided in Table 1. The table notes the wide range of acoustic velocities reported in the literature. This makes it essential that the reference blocks are made of the same cell classification as that examined. This shall be confirmed by measuring the acoustic velocity of the pipe being examined. When using PAUT as the examination technique, the acoustic velocity of the reference block shall be within ±50 m/s of the examined pipe material being examined.
(A) A range of velocity and attenuation values have been noted in the literature (1-9). The boldface numbers in parentheses refer to the list of references at the end of this standard.
5.6 Polyethylene is reported to have a shear velocity of 987 m/s. However, due to extremely high attenuation in shear mode (on the order of 5 dB/mm (127 dB/inch) at 2 MHz) no practical examinations are carried out using shear mode (6).6
5.7 Due to the wide range of applications, joint acceptance criteria for polyethylene pipe are usually project-specific.
5.8 A typical butt fusion joint in polyethylene pipe has a pronounced bead profile similar to that illustrated in Fig. 1 where the bead is shown on the outer and inner surface of the pipe.
5.9 TOFD, when used on polyethylene, is simplified in that mode-converted signals are virtually eliminated due to the high attenuation of the shear mode. However, the near surface and far surface dead zones associated with TOFD may be considered limitations if determined to be excessive for the detection requirements.
5.10 PAUT can be used to address the near surface dead zone that occurs with TOFD.