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Table of contents of this Page:
2. Ultrasonic NDT

2.1. Background
2.2. Work at TWI
2.2.1. Pulse-echo compression wave technique
2.2.2. Pulse-echo creeping wave technique
2.2.3. The tandem technique
2.2.4. Time-of-flight-diffraction (TOFD) technique
2.3. Comparison with other studies

2.1. Background

There are fundamental difference between the ultrasonic inspection of polyethylene and the ultrasonic inspection of metals. Previous work at TWI (6) has shown that the attenuation of ultrasound in polyethylene is a factor of ten higher than for metals. Figure 1 shows that this attenuation [dB/mm] is directly proportional to the frequency [MHz] of vibration of the ultrasound, and also shows that polyethylene attenuates ultrasonic shear waves much more rapidly than compression waves. Essentially, this means that for many practical applications, ultrasonic testing is limited to using low frequency compression waves (i.e. <4MHz) in order to achieve sufficient penetration and sensitivity on typical thicknesses (25mm) and grades of polyethylene.



2.2. Work at TWI

In (7) the authors studied the effect of material composition on ultrasonic wave velocity and attenuation. Separate studies were also included to assess the degree of anisotropy (i.e. differences in elastic properties in different directions) present in the polyethylene materials considered (see Table 1).
Table 1: Product specification of polyethylene materials examined

Product	Colour	Density(g/cm3)	Application
Dowlex	White	0.937	Heating, installatioii, irrigation pipes
Finathene 3802 B	Black	0.948	Anticorrosive coatings, cable jackets
Eltex TU B 121	Black	0.958	Gas pipe, industrial fluid transportation
Eltex TU B 124	Dark blue	0.951	Water transportation (drinking)
*Finathene 3802	Light blue		Water transportation (drinking)
Finathen@e3802 Y	Yellow	0.941	Gas distribution systems
Eltex TU B 125	Orange	0.951	Gas transportation

Details not supplied by manufacturers

It was concluded that ultrasonic wave velocity and attenuation do not vary significantly with material composition and that, when inspected ultrasonically, polyethylene exhibits very little anisotropy and no adjustment in sensitivity is required when testing in different directions. This earlier study also reports on the effectiveness of a compression wave pulse-echo technique for inspecting butt fusion welds in polyethylene pipe.

2.2.1. Pulse-echo compression wave technique

The probe configuration for the pulse-echo compression wave technique is shown in Fig.2.
A polyethylene wedge is used to couple ultrasonic energy from a 2.25MHz compression wave transducer into the polyethylene specimen. The angle of the wedge is such that compression waves are generated in the polyethylene specimen at an angle of 60'. Additional wedges, designed to generate compression waves at an angle of 45' in polyethylene, have also been evaluated. The ultrasonic procedures developed so far (6,7) have been shown to be largely insensitive to a range of flaws, including inclusions (e.g. material inserts and dust) and cold welds, in 25mm thick butt fusion welded polyethylene pipe. A cold weld can occur when the hot plate temperature is too low, or the delay time between removing the hot plate and joining the pipe ends is too great, causing the pipe ends to crystallise before coming into contact and resulting in a weld with little or no strength. One of the reasons for the poor performance of the pulse-echo technique is the weak response from vertically oriented flaws in a highly attenuative material like polyethylene. This was highlighted in (7) using three 10mm diameter flat bottomed holes drilled at different through-thickness positions at one end of a pipe section, and is illustrated here in Fig.3.

Each hole was detected by strong specular reflections from the corner between the hole and the end of the pipe, rather than from backscattered reflections from the face of each hole. This earlier work (7) did, however, identify other ultrasonic techniques such as creeping waves, time-of-flight diffraction (TOFD) and the tandem technique as being potentially more suited to the inspection of butt fusion welded polyethylene pipe.


2.2.2. Pulse-echo creeping wave technique

Creeping waves are high angle compression waves which are normally used on metals to detect surface breaking cracks and other near surface flaws. Unfortunately creeping waves are only effective over a very short range, as energy is continuously being converted into secondary shear waves (Fig.4).

TWI have considered a pulse-echo creeping wave technique to inspect the fused area immediately beneath the outer weld bead on butt fusion welded polyethylene pipe ( 8). The sensitivity of creeping waves to gross discontinuities in the weld such as a lack of fusion and large material inclusions, located close to the surface, has been demonstrated. The pulse-echo creeping wave technique however, has not been able to detect dust contamination of the fused interface or the presence of a nominal cold weld.

2.2.3. The tandem technique

The ultrasonic probe configuration for the tandem inspection technique is shown in Fig.5.

On metals, tandem inspection is most commonly applied to welded joints in thick walled vessels. The technique is particularly sensitive to flaws oriented perpendicular to the scanning surface and because of this, is potential suited to detecting lack of fusion type flaws in butt welded polyethylene pipework. By moving the transmitting probe (T) relative to the receiving probe (R) it is possible to obtain full through-thickness coverage of the welded region. Two, 1MHz transducers, designed to generate compression waves at 60' in polyethylene were found to be most appropriate for the tandem inspection of welds in 25mm thick polyethylene pipe. Figure 6 shows a typical A-scan response from a perpendicular reflector (5mm diameter Aluminium foil disc) located in the mid-thickness of such a weld. If a series of these A-scans are collected at consecutive positions around the circumference of a polyethylene pipe, it is possible to construct a composite A-scan image of the welded region.



Figure 7 shows a composite A-scan image of a 2mm aluminium foil disc insert in 25mm of polyethylene. The signal to the right of the response from the disc insert is due to the divergence of the ultrasonic beam and corresponds to ultrasound reflected from the inner weld bead. This signal should always be present as the probes are moved around the entire circumference of the pipe, and can be used as a check on whether ultrasound is being coupled into the polyethylene pipe or not. From the results presented in a recent TWI report (8), the use of the tandem technique to detect planar discontinuities, such as isolated areas of lack of fusion or large planar inclusions, has been demonstrated down to 2mm in through-thickness size. It should be noted that these flaw types were simulated by introducing Aluminium foil inserts at the pipe section interface during welding. Also, the technique is sensitive to dust contamination of the welded interface. Areas known to contain chalk dust show some correlation to regions on the composite A-scan results where the amplitude of the signal reflected from the inner weld bead is reduced. The tandem technique has not been able to detect the presence of a nominally cold weld.

2.2.4. Time-of-flight diffraction (TOFD) technique

In weld inspection of metals, TOFD is used to determine the through-thickness height of flaws. The technique is especially sensitive to flaws oriented perpendicular to the scanning surface and, for this reason, its use for butt weld inspection of polyethylene pipes has been explored. The probe configuration for TOFD is shown in Fig.8.

Here two, 2.25MHz, 60' compression wave probes were used. Ultrasound is generated in a broad beam which isonifies the complete thickness of the welded interface. Displayed on a time-trace, the amplitude signals detected by the receiving probe would look similar to Fig.9. Each signal on the A-scan is separation in time according to the distance ultrasound has to travel between the transmitting and receiving transducers. The first ultrasonic signal to be detected is the lateral wave, which travels the most direct route between transmitter and receiver. The final signal detected by the receiver is the reflection from the inner wall of the pipe, termed the backwall echo (Fig.8).

A perpendicular flaw in the weld line will diffract ultrasound from its top and bottom edges and some of this diffracted energy will be detected by the receiving transducer. Diffracted signals appear as low amplitude responses, between the lateral and backwall echo signals on the A-scan. In TOFD inspection of metals, it is possible to accurately measure the through-thickness size of a flaw by scaling the separation between the diffracted signals. Unfortunately, the lower frequencies required for polyethylene inspection mean that it is not always possible to identify the signals diffracted from the top and bottom of a flaw, and accurate sizing of flaws in polyethylene is generally problematic. A-scans collected by the TOFD technique are commonly displayed in composite form as D-scan images. One such image, showing the TOFD response from a 2mm planar discontinuity in 25mm of polyethylene, is shown in Fig.10.

A flaw will only be detected by TOFD if it is capable of diffracting incident ultrasound from its edges. For this reason it is unlikely that TOFD inspection would be sensitive to a cold weld in polyethylene. A nominally cold weld is synonymous with poor fusion across the entire welded interface and there is no local discontinuity from which diffracted signals can originate. The inability of the TOFD technique to detect the presence of a cold weld has been confirmed experimentally (8). To summarise, the TOFD technique is sensitive to gross planar discontinuities, down to 2mm in diameter, at the fused interface but, as expected, it is not sensitive to the presence of a cold weld. Also, it is reported (8) that the presence of chalk dust at the fused interface causes a Variation in the TOFD response detected, although further work is needed before this can be verified.

2.3. Comparisons With Other Studies

One of the difficulties in comparing the TWI results with other research work is the lack of published material on the NDT of large diameter polyethylene pipes.

The notable work carried out by the American Gas Research Institute (GRI) uses a pulseecho technique with an array of probes and has concentrated solely on testing gas distribution pipes between 50-100mm in diameter, and up to approximately 12mm thick. Attenuation is less of a problem here than for the kind of diameters and thicknesses considered by the work at TWI. The GRI study claims to have developed a technique which is sensitive to a range of manufactured flaws in polyethylene pipe, such as lmm diameter end-drilled holes but, most importantly, is able to detect the presence of cold welds.

It is important to mention some of the major differences between the work undertaken at TWI and that at GRI- The GRI work uses an array of 18 transducers linked together in a ring, which fastens directly on to the circumference of the pipe to be inspected. Each transducer

is angled to interrogate a particular region of the weld in the through-thickness direction. The pulse-echo response from each transducer is monitored automatically and a decision on joint quality is provided by the computer software. The reliability of the GRI system has been proven on polyethylene pipes less than 100mm in diameter and 12mm thick (9), but no reference is made to the suitability of the technique for inspecting pipes of larger diameter and thicker section. The GRI work is based on a pulse-echo technique and experience at TWI on using this technique to inspect pipes of thicker section (25mm) is questionable. TWI have concentrated their efforts on developing alternative ultrasonic inspection techniques, such as tandem and TOFD, suitable for inspecting welds in polyethylene pipe of thicker section. |Top|

3.0. Radiographic NDT

3.1. Background
3.2. Work at TWI
3.3. Comparison with other studies

4. Conclusions

5. References

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