NDT of butt fusion welds in PE pipe

TWI Bulletin, November/December 1995

Ian Munns graduated from the University of Hertfordshire in 1992 and joined TWI later that year. Since then he has been widely involved with the NDT of plastics, composites, ceramics and adhesively bonded joints. Ian is currently working in the Numerical Modelling Section in the Structural Integrity Department.

George Georgiou graduated from Imperial College in Mathematics in 1972 and stayed on a further year as a research student studying theoretical fluid dynamics. He was a full-time mathematics lecturer at Tottenham College of Technology until 1983, gaining his PhD in 1982. Since 1990 he has worked at TWI on a variety of NDT related problems. In particular he is working on NDT of plastics, sprayed coatings and adhesives and is currently involved with BSI and CEN committees which are drafting standards for ultrasonic inspection of welds.

Use of polyethylene in the gas, water and chemical process industries has increased dramatically over the past two decades. I J Munns and G A Georgiou explain why.

Polyethylene pipes with a diameter less than 250mm are normally joined using an electrofusion process, but for areas where the application is critical or the pipes are of larger diameter (>250mm) and of thicker section, the hot plate butt fusion process is preferred. ^[1]

Quality of butt fusion joints in polyethylene pipe systems is primarily governed by control of the process parameters during welding. However, as new polyethylene materials are introduced ( eg PE100) and increased demands are placed on existing polyethylene materials, there is an additional need to monitor quality through reliable non-destructive testing (NDT) methods. ^[2,3] There are no nationally accepted standards for the NDT of welds in polyethylene. A guideline standard (ASTM F600-78) for the manual ultrasonic inspection of butt fusion welds in polyethylene pipe was introduced, but the results obtained were so heavily dependent upon the skill of the operator that it was withdrawn in 1991.

Ultrasonic NDT

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

Work at TWI

An earlier study at TWI ^[5] considered the effect of material composition on ultrasonic wave velocity and attenuation. Separate studies were also included to assess the degree of anisotropy ( ie differences in elastic properties in different directions) present in the polyethylene materials considered, see Table. It was concluded that ultrasonic wave velocity and attenuation do not vary significantly with material composition and, when inspected ultrasonically, polyethylene exhibits very little anisotropy, and no adjustment in sensitivity is required when testing in different directions.

Product specification of polyethylene materials examined

Pulse-echo compression wave technique

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 transducer into the polyethylene specimen. TWI has evaluated wedges designed to introduce compression waves at an angle of either 60° or 45° in the polyethylene under inspection.

Ultrasonic procedures developed so far ^[4,5] are largely insensitive to a range of flaws, such as inclusions, 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 poor performance of the pulse-echo technique is the weak response from vertically orientated flaws in a highly attenuative material like polyethylene. Vertically orientated flaws tend to produce a weak backscattered signal in the direction of the ultrasonic probe, when inspected using an angled pulse-echo technique. This weak response is so heavily attenuated in polyethylene that it is unlikely to be detected by this form of inspection.

Pulse-echo creeping wave technique

The Tandem technique

The ultrasonic probe configuration for the Tandem inspection technique is shown in Fig.4. This technique is particularly sensitive to flaws orientated perpendicular to the scanning surface, so is potentially 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. The most appropriate arrangement for the tandem inspection of welds in 25mm thick polyethylene pipe was found to be using two 1MHz transducers, designed to generate compression waves at 60° in polyethylene. Figure 5 shows a typical A-scan response from a perpendicular reflector (5mm diameter aluminium foil disc) located in the weld mid-thickness.

If a series of these A-scans is 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 6 shows a composite A-scan image of a 2mm aluminum foil disc insert in 25mm of polyethylene. The signal to the right of the response from the disc insert is due to 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.

Time-of-flight diffraction (TOFD) technique

The TOFD technique is especially sensitive to flaws orientated perpendicular to the scanning surface and, for this reason, its use for butt weld inspection of polyethylene pipes has been explored. Probe configuration for TOFD is shown in Fig.7, where two, 2.25MHz, 60° compression wave probes are 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.8. Each signal on the A-scan is separated 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.7). The presence of a flaw will result in two diffracted signals, from the top and bottom edges of the flaw, being detected after the lateral wave and before the backwall echo.

In TOFD inspection of metals, it is possible to measure accurately 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 signals diffracted from the top and bottom of a flaw. Accurate sizing of flaws in polyethylene is generally more difficult. 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.9.

To summarise, the tandem and TOFD techniques are sensitive to gross planar discontinuities, down to at least 2mm in diameter at the fused interface, but they are not sensitive to the presence of a cold weld. However, it is reported ^[6] that the presence of chalk dust at the fused interface causes a variation in the tandem and TOFD responses. Further work is needed to optimise these techniques for this flaw type.

Radiographic NDT

Work at TWI

A range of IQIs was manufactured from polyethylene ^[5] . These included an ASME plaque (ASME V, Article 22), a British step wedge with holes (BS EN 462-2:1994), and the previously mentioned British wire type.

The whole range in common use is illustrated in Fig.10. These were assessed and compared for their ability to provide a reliable measure of radiograph quality and sensitivity, and for their practical handling. All IQI types were able to achieve sensitivities better than 2%.

a) ASME plaques with holes (ASME (V), Article 22 1989);
b) European Wire type (BS EN 462-1 1994);
c) European step wedge with holes (BS EN 462-2 1994);
d) European duplex wire type (BS EN 462-5: To be published)

The same programme of work ^[5] considered the effect of X-ray absorption through different grades of polyethylene. It was concluded that the very low absorption by polyethylene of X-ray radiation can vary significantly from one polyethylene type to another. Although, for the range of polyethylene materials shown in the Table, only two exposure charts are necessary to produce radiographs in the density range specified by BS 2600 ^[9] . Figure 11 illustrates a typical exposure chart. These exposure charts have been used to produce prototype radiographic procedures for the inspection of butt fusion welds in polyethylene pipe. ^[5,6] The procedures make use of two double-wall single image radiographic inspection techniques. When the X-ray source is in the plane of the weld ( Fig.12) the technique is referred to as the 'straight' technique, and the weld fusion face appears as a straight line on the radiograph. When the source is offset from the plane of the weld it is referred to as the 'throwing' technique and the weld fusion face appears as an elipse on the radiograph. The straight technique was used as a primary method. The throwing technique was used on occasions to provide additional information regarding the shape and character of the flaw.

a) Straight technique;
b) Throwing technique

Prototype procedures discussed here have been used to detect reliably a range of manufactured and natural flaws in polyethylene pipes, including; dust contamination of the fusion face, planar discontinuities in the fusion zone, lack of fusion and a thinning of the weld bead corresponding to the presence of a cold weld.

Conclusions

• Ultrasonic creeping waves can be used to inspect the fusion area immediately beneath the outer weld bead in polyethylene pipes.
• Ultrasonic tandem and TOFD techniques can be used to detect and image perpendicular planar flaws in butt fusion joints in polyethylene pipes.
• Ultrasonic techniques are currently unable to detect nominally cold welds.
• Radiographic intensities of between 16kV and 26kV are optimum pipe inspecting thicknesses of polyethylene pipe between 5mm and 50mm.
• With radiography it is possible to infer the presence of a cold weld from local radiographic density measurements.
• Polyethylene wire type IQIs have been proven to achieve radiographic sensitivities at least as good as polyethylene plaques and step wedges with holes, and have other practical handling advantages with regard to pipe inspection.

References