·Table of Contents
·Materials Characterization and testing
Detection and Localizing of Defects in Welded Joints of Items from Plastic Using UltrasoundV.P. Rad'ko, V.A. Troitskij,
E.O.Paton Electric Welding Institute of the NAS of Ukraine
|Type||Incidence angle, degr.||Frequency, MHz||Range of wall thickness|
|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|
|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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