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Detection and Localizing of Defects in Welded Joints of Items from Plastic Using Ultrasound V.P. Rad'ko, V.A. Troitskij, E.O.Paton Electric Welding Institute of the NAS of Ukraine Contact

Ultrasonic testing (UT) of welded joints of polyethylene pipes with artificial and real defects was performed with series-produced ultrasonic flaw detectors operating in the frequency range of 1 to 10 MHz, namely USN-50 (52), Zipscan-3, etc. In view of very high damping of transverse ultrasonic waves, longitudinal waves had to be used for control of the welds on polyethylene pipes and other plastics, the velocity of longitudinal waves in polyethylene (of the order of C = 2340 m/s) being much lower than in steel (C = 5850 m/s). In this connection, in sounding at an angle in polyethylene, compared to steel, ultrasonic probes have to be used, with prisms of appropriate materials exciting longitudinal waves in the item being controlled at the frequencies of 1 to 2.5 MHz with high incidence angles of ultrasound (65 - 70⁰). This provides a comparatively widely open ultrasonic beam, so that the entire cross-section of the weld can be covered at once at a fixed distance from the weld.
     Control sensitivity should be set using a reference block made of a pipe of the appropriate size and type. As in hot tool butt welding all the defects lie normal to the pipe axis, flat bottom holes of 2 mm diameter and 20-30 mm depth are drilled in the reference block over the entire cross-section, they serving as reference reflectors. The echo-signals exceeding the threshold level from the flat bottom drilled holes, are classified as echo-signals from inadmissible defects. The registration threshold, according to the current experience, is set to be by 6 to 10 dB lower than the indications from reference defects. Depending on the depth of their location, the defects are detected at different time of flight and amplitude. With thicker walls (above 10 to 15 mm), in order to cover the entire weld section, control is performed in two passes with different distances from the probe to the weld, and with different signal gain. If the wall thickness is 20 to 60 mm, it is rational to perform control by the tandem method. In case of reflection from a thickened portion in upsetting (flash), signals due to the flash geometry come from the lower side, which, however, do not create great problems for an experienced operator and often are even desirable for control of acoustic contact. Echo-signals from defects should come before, as the defects are located closer to the acoustic probe.
The critical (most dangerous) defect in flash-butt welding of plastics (polyethylene) is the so-called lack-of-penetration (lack-of-fusion), similar to defects formed in flash-butt welding. These defects, in view of the tight pressing together of the surfaces, usually are transparent for ultrasound. Investigations showed that such a transparency is not constant along the entire length of the weld and that spot reflectors are found in individual areas of the weld. The initial parameters for UT control are the incidence angle of ultrasound and its frequency. The incidence angle is selected from the condition of probing of the maximal possible area of the welded joint section by the direct beam, and, at least, so that the beam axis passed through the wall center.
    When the frequency is selected, the following factors should be taken into account: wave length l = C/f is reduced with the increase of frequency, as in this case the dimension of the minimal defect detectable by ultrasound, becomes smaller, ultrasound damping, however, increasing with increase of frequency. Therefore, with the increase of the wall thickness, when damping becomes considerable, it is necessary to decrease the ultrasound frequency in order to ensure its passage, as shown in Table 1.

Type	Incidence angle, degr.	Frequency, MHz	Range of wall thickness
Inclined	60	1.8	10-60
Transmitter-receiver (duet)	60	1.8	10-20
Transmitter-receiver (tandem)	45	1.8	20-60
Transmitter-receiver	60	2.5-5	6-10
Table 1: Specifications of the developed and manufactured probes

Control of welded joints of polyethylene pipes is performed by echo-pulse method, namely with a direct beam using transmitter-receiver (T-R) ultrasonic probes with high incidence angles of 65 to 70⁰; T-R probes with incidence angles of 40 to 50⁰ and T-R probes with incidence angles of 65 to 70⁰ connected in a tandem.
    A series of investigations were performed on detection and localizing of defects in welded joints of polyethylene pipes by the diffracted wave method (TOFD) implemented in Zipscan-3 equipment. The TOFD method is a method of a more accurate (compared to echo-pulse method) sizing and identifying of defects. Zipscan-3 equipment which implements this method, displays the data in the vertical section of the weld, thus representing the real relative location of defects and their dimensions. Unlike the traditional UT methods which are based on measurement of reflected energy, TOFD method uses the energy of ultrasonic waves diffracted at the defect "edges". Short ultrasonic pulses, wide angle ultrasonic probes (transmitter and receiver), receiving duct with a high gain for receiving and recording signals of a small amplitude, of defects diffracted in the screen, are used for the purpose.
Experimental studies were performed taking into account the information from production. The main attention is given to defects which cause failure of the welded joint in service. Such defects are lacks-of penetration and cracks, not detectable in visual examination. These defects have the feature of being located in the central part of the pipe wall. Investigations were conducted in two stages, namely first on standard test blocks which allowed determination of the general regularities which are found in UT of welded joints of polyethylene pipes, and then on the actual welded joints with defects which develop in the welded joint in the case of violation of the technology requirements.
As shown by investigations, UT is the most effective in detection of lacks-of-penetration as they are vertically oriented. With the wall thickness of up to 30 mm, the size of the detected defects is 1 to 2 mm by height for extended intervals, and 3 to 10 mm² in area for local lacks-of-penetration. Ultrasonic probes of various types were made during performance of this work. These probes are designed for control of welded joints on polyethylene pipes of different diameters from 100 to 600 mm and different thicknesses of 6 to 60 mm, using series-produced flaw detectors of UD2-12, USN-52 type for control under production conditions.
Specifications of some of the developed and made probe mock-ups are given in Table 2. Specifications of acoustic probes of the best foreign analogs are given in Table 3.

Defect type	Defect dimensions Iavis average extent	Control validity R
Crack,	1 - 2	0.85-0.95
Lack-of-penetration Blowhole,	1 - 2	0.8-0.9
Pore Lack-of-fusion (cold welding)	1 - 2	0.85-0.95
Table 2: Validity of control results

Type	Incidence angle, degr.	Frequency, MHz	Probe, dia., mm	Range of wall thicknesses, mm
70 V 2 ST-	70	2	10	15
65 V 2 ST-	65	2	10	15-30
45	45	2	8x9	30-60
45	45	2	8x9	30-60
Table 3: Specifications of foreign acoustic probes

The validity of control was evaluated on welded joints of samples made with violation of the welding technology, and welded joints of polyethylene pipes which failed in service. Welded samples with loss of tightness were also studied.
    By their appearance and dimensions of flash, these joints in the majority of the cases met the requirements made in visual inspection. As shown by experiments, UT is effective in detection of cracks which lead to loss of the welded joint tightness in service. Thus, ultrasound is used to perform an indirect check of the tightness of welded joints on polyethylene pipes.
 As shown by investigations (Table 2) lacks-of-penetration and cracks, as well as blowholes and pores are the most effectively detected in UT. The validity of detection of lacks-of-fusion which are characterized by the presence of "mirror" surface areas after fracture (called "cold welding" in foreign publications) is much lower.

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