【国外标准】 Standard Practice for Ultrasonic Testing of Polyethylene Butt Fusion Joints

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 classification6 as that examined. This shall be confirmed by measuring the acoustic velocity of the pipe being examined as described in Practice E494. 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).75.7 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.FIG. 1 Typical Bead Profile for Polyethylene Butt Fusion Joint5.9 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. For applications on relatively thin wall thickness, for example, < 15 mm [0.6 in.], one-sided TOFD (or the quasi-chord technique) may be considered to help reduce the extent of these dead zones. See Annex A2 for details on this option.5.10 PAUT can be used to address the near surface dead zone that occurs with TOFD.1.1 This practice establishes procedures for ultrasonic testing (UT) of butt fusion joints in polyethylene pipe. Although high density polyethylene (HDPE) and medium density polyethylene (MDPE) materials are most commonly used, the procedures described may apply to other types of polyethylene.NOTE 1: The notes in this practice are for information only and shall not be considered part of this specification.NOTE 2: This practice references HDPE and MDPE for pipe applications as defined by Specification D3350.1.2 This practice does not address ultrasonic examination of electrofusion joints (coupling joints), socket joints, or saddles.1.3 This practice provides two ultrasonic examination procedures. Each has its own merits and requirements for examination and shall be selected as agreed upon in a contractual document.1.3.1 Examination Procedure A, Time of Flight Diffraction (TOFD), uses a pair of probes, one transmitting and the other receiving. The procedure usually, although not necessarily, requires access to both sides of the joint from one surface. Provided that position encoding is used, the procedure can be conducted by semi-automated or automated means that provide recoded imaging.1.3.2 Examination Procedure B, Phased Array Ultrasonic Testing (PAUT), uses low velocity refracting wedges or water gaps to produce angled compression mode pulses. The procedure can be applied where access is limited to one side of the joint from one surface. Provided that position encoding is used, the procedure can be conducted by semi-automated or automated means that provide recoded imaging.1.4 The practice is intended to be used on thicknesses of 9 to 60 mm [0.375 to 2.4 in.] and diameters 100 mm [4 in.] and greater. Greater and lesser thicknesses and lesser diameters may be tested using this standard practice if the technique can be demonstrated to provide adequate detection on mockups of the same wall thickness and geometry.1.5 This practice does not specify acceptance criteria.1.6 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.8 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.