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Development of inspection techniques for an automated Non-Destructive

Evaluation (NDE) approach for testing welded joints in plastic (PE) pipes

Fredrik Hagglund, Malcolm A Spicer and Mike J Troughton

TWI Ltd,

Granta Park, Great Abington, Cambridge, CB21 6AL, UK

Phone: +44(0)1223 899000

Fax: +44(0)1223 890952

Email: malcolm.spicer@twi.co.uk

Abstract

A reliable Non-Destructive Evaluation (NDE) approach is required for the inspection of

different polyethylene (PE) pipe joints in various material grades and pipe sizes. In

February 2010 a European (FP7) funded project on the development and validation of an

automated NDE approach for testing welded joints in plastics pipes (TestPEP), involving

15 organisations from seven European countries, was started. In initial studies, the technical

problem, industry needs and the proposed approach to the solution were defined. Several

additional tasks need to be solved before approaching the final solution. The challenging

material properties of PE pipes, low speed of sound and high frequency dependent

attenuation, must be overcome. A flexible scanner with probe and wedge holder

incorporated must be adaptable for the variety of pipe joint configurations. Furthermore, a

rugged flaw detector instrument will be developed, capable of performing the advanced

procedures required for these materials. In this paper the development progress of the

inspection techniques for the different pipe joints and pipe sizes are presented. The different

joints have specific appearances and several individual techniques need to be applied to

fully cover the weld fusion zones. In a separate task, work on the insertion of artificial

flaws is being undertaken, and these pipe samples will be inspected with the developed

techniques. Initial evaluation of the capability of the inspection techniques are presented in

this paper.

1. Introduction

Plastic pipes offer significant advantages over other materials such as cast iron, steel,

copper and concrete, for the transportation of fluids such as natural gas, water, effluent and

corrosive liquids. They do not corrode; have a longer predicted service life, leading to less

frequent replacement; they are less expensive to install due to their light weight and

flexibility; and have significantly lower leakage rates due to having an all-welded system.

However, their more widespread use in safety critical environments is being restricted by

the lack of a reliable NDE method to increase confidence in the welded joints. Pipeline

leakage not only causes high repair costs but can also result in disastrous safety and

environmental consequences.

Several studies have been conducted to develop a reliable NDE method for welded plastic

joints. The studies have focussed on the two main techniques for plastic pipe welding,

electrofusion (EF) and butt fusion (BF). In recent years, phased array ultrasonic technology

(PAUT) has been considered to assess the integrity of EF-joints (1, 2)

. The main advantage

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with using a phased array probe on these joints is that the fusion zone in the pipe direction

is covered by electronic scanning within the probe and mechanical scanning is only

required in the circumferential direction. This solution provides huge advantages when it

comes to inspection time and data interpretation. However, these studies concentrated on

specific pipe sizes with outer diameters (OD) of 125 and 250mm.

Traditionally, BF-joints have been examined with several different techniques using

conventional ultrasonic transducers (3, 4, 5)

. The techniques that have been employed include

pulse-echo, tandem, creeping waves, and time-of-flight diffraction (TOFD). Due to the BF-

joint geometry a combination of several of these techniques is required to achieve sufficient

coverage of the fusion zone. In recent years, studies have been conducted to inspect BF-

joints using PAUT (6, 7). The main technique used was the pulse-echo, where a sector scan

of the fusion zone is achieved by steering the beam at different angles. Currently, the only

commercial ultrasonic inspection systems for plastics pipes are in North America and South

Korea (8, 9)

. The American system is limited to BF welds and uses conventional time-of-

flight-diffraction rather than phased array and, as a consequence it is not applicable to more

complex weld configurations such as elbows, reducers and tees. The Korean system is

limited to EF-joints and does not record data.

The TestPEP project will develop phased array ultrasonic NDE procedures, techniques and

equipment for the volumetric examination of welded joints in polyethylene (PE) pipe of

diameters from 90mm up to 1m. A key aim of the project is to develop an inspection

system that is site-rugged and simple to operate. Current phased array instruments require

ventilation and space and many have fragile viewing screens. The concept in this project is

to have a black box instrument with a simple ethernet connection to download the recorded

data, and to provide the necessary robustness of the phased array probe. Another objective

of the project is to analyse the data semi-automatically so that a red/green (yes/no) answer

can be provided and the system can be operated by pipe laying technicians. The prototype

NDE equipment, designed and built as part of this project will be assessed under both

laboratory and field conditions.

The development progress of the inspection techniques for the EF and BF joints for

different pipe sizes are presented here. To assess the joints, several individual techniques

need to be employed to fully cover the weld fusion zones. Development of the techniques

has been undertaken, and these have been evaluated on test samples. Detection results from

the test samples and initial evaluation of the capability of the inspection techniques are

presented.

2. Material

2.1 The welded joint configuration

The two different joints that will be investigated in the project have very little in common

except that both have a fusion zone between two plastic materials.

2.1.1 The Electro fusion joint

The design of an EF-joint comprises two pipe ends, which are attached inside a coupler

sleeve called a fitting. The fitting has wires around the bore of the sleeve on the inside close

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to the surface. When a current is applied to the fitting the wires heat the surrounding pipe

and fitting material and the fusion zone is created. An EF joint is shown in Figure 1(a).

2.1.2 The Butt-fusion joint

The BF-joints are created by using a heating plate to melt the ends of two pipes which are

then fused together by a pressure applied for a certain time. The process then creates a weld

bead of the excess pipe material on both the inner and outer surface. An image of a BF-joint

can be seen in Figure 1(b).

(a) (b)

Figure 1. (a) An EF plastic pipe joint. (b) A BF plastic pipe joint showing the weld

bead on the outer surface.

2.2 Test samples

For the development of the inspection techniques for BF-joints, test samples with artificial

flaws were created, covering a range of pipe sizes between 180mm and 710mm OD. Flat

bottom holes (FBHs) and slots were considered sufficient to evaluate the performance of

the proposed techniques. The FBHs were used to evaluate the tandem and the sector pulse-

(a) (b)

Figure 2 (a) Arrangement of FBHs in the pipe end. (b) Arrangement of slots in the

middle of the pipe.

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echo scans for the BF-joints. The slots were used to evaluate the creeping waves and the

TOFD technique. The FBHs were machined at the pipe ends and the slots were machined in

the middle of the pipe. The arrangement of the FBHs and slots are shown in Figure 2. For

the EF-joints the fitting itself was considered sufficient to evaluate the technique. The 0-

degree normal linear scans that will be used on the EF-joints were evaluated on the EF-

fitting. With sufficient resolution achieved in detecting the wires, it is believed that the

fusion zone, which is located just below the wires, can be inspected.

3. Equipment

3.1 Probes

For the evaluation of the inspection technique on the EF-joint two different 1D linear 128

element probes were used; a 5MHz probe and a 7MHz probe. The demand on the probe is

low since no steering and only focussing at the fusion zone is required.

The BF-joints have a smaller fusion area and high angles are required to fully cover the

fusion zone. The probes can then be physically smaller, enabling the usage of a smaller

pitch and increasing the steering capabilities. For the inspection of BF-joints an angled

wedge was used. The angle of the wedge needs to be optimised for the inspection angle

inside the pipe. By doing this, the physical steering is optimised and the electronic steering

can be minimised, hence, improving the overall performance. For the evaluation of the

inspection techniques on the BF-joints two pairs of identical 1D linear 32 element probes

were used; a pair of 2MHz probes and a pair of 4MHz probes. By using these probes on the

plastic pipes, the pitch p, will be larger than half the wavelength, p > λ/2. This implies that

the steering capabilities are limited. To overcome this issue, an appropriate physical angle

of the wedge must be used to minimize the steering angles by the transducer elements. The

refracted angles for all the techniques and pipe sizes to be used in these inspections are

between 55-90 degrees. By the application of Snells law, the incident angles for these two

extreme refracted angles can then be calculated as 32.8 degrees and 41.5 degrees

respectively. By constructing the water wedge with an angle of 35 degrees, the array only

needs to steer the beam to a maximum of 6.5 degrees.

3.2 Wedges

To perform the inspection on plastic pipes, novel water wedge prototypes have been

designed and manufactured. The advantage of using a water wedge is low attenuation and a

velocity ratio enabling the steering of angled beams to the fusion zone. The main

challenges with a water wedge are possible air bubbles and maintaining the water between

the elements and the PE material.

For the probes to be used on the EF-joints, 0-degree water wedges have been manufactured

to be able to perform the inspections required. The wedge with the probe and the sealing

skirt attached is shown in Figure 3(a). The sealing material is flexible and the sealing skirt

can also be customised to match the specific profiles of the fittings.

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(a) (b)

Figure 3. (a) The normal 0-degree water wedge with probe and water sealing skirt. (b)

The angled water wedge with probe and water sealing skirt.

For the probes to be used on the BF-joints, angled water wedges have been manufactured.

The angle of these probe wedges is 35° to minimize the electronic steering on the inspected

pipe dimension. The wedge and sealing skirt with the probe mounted on it is shown in

Figure 3(b).

3.3 Instruments

The variety of ultrasonic techniques used for the inspections in this project set high

demands on the ultrasonic instrument. A number of different instruments have been

evaluated according to their performance in relation to the demands of the techniques to be

used; however, this is outside the scope of this paper. The data presented in this paper have

been acquired using; the Omniscan MX PA by Olympus with its Tomoview software; the

Dynaray from Zetec with their Ultravision 3 software: and the Multi2000 32x128 from

M2M with its CIVA based software.

4. Development of inspection technique for EF-joints

For the EF-joints a normal water wedge will be used with a water column between the

transducer and the pipe fitting surface. A normal focusing linear scan will be used, focused

on the fusion area between the fitting and the pipe, see Figure 4. The heating wires are

located just above the fusion area and sufficient resolution to be able to see both the wires

and between the wires is required. The resolution is generally dependent on the frequency,

higher frequencies give higher resolution. However, PE is a highly attenuating material and

attenuation increases exponentially with frequency. Thus, the frequency needs to be

reduced for larger pipes to be able to get sufficient propagation of sound.

Taking the limitations of coverage and resolution mentioned above into consideration it is

necessary to use probes with different frequencies; higher frequency for smaller pipes and

lower frequency for larger pipes. The pipes are divided into different categories depending

on fitting thickness/pipe diameter/wall thickness. This means that probes with different

frequencies need to be manufactured for each category. The coverage can then be

optimized by choosing optimal aperture sizes for each probe, e.g. a larger pitch for the

larger pipes.

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Figure 4. Schematic drawing of the inspection technique for the EF-joints.

5. Development of inspection techniques for BF-joints

Four techniques have been investigated as shown in Figure 5 and described below:

Figure 5. Schematic drawings of the inspection techniques used on BF-joints.

5.1 Tandem

The tandem technique covers approximately two thirds of the fusion face towards the inner

surface of the weld. It shows good performance on planar flaws within the weld. The

tandem technique is challenging to implement due to a long propagation distance inside the

highly attenuating material combined with several internal reflections which will decrease

the signal-to-noise ratio. In conventional ultrasonic inspection, the tandem technique is

implemented using a transmitter and a receiver probe, one in front of the other. However,

when using a single phased array probe, a self-tandem technique can be achieved using one

part of the array as the transmitter and one part as the receiver.

A 32 element probe was employed. The last 16 elements (17 to 32) were used for the

transmitter and first 16 elements (1 to 16) were used for the receiver. The transmitter is set

to transmit with start on element 17 and sweep upwards to element 32 and the receiver was

set to start on element 16 and sweep downwards to element 1. The active elements and the

sweeping direction are illustrated in Figure 6 for the transmitter and receiver.

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Figure 6. (a) The elements for the receiver and transmitter. (b) The active elements

used for sweeping in both the transmitter and receiver.

The standoff between the front of the probe/wedge and the weld centreline and the

thickness of the pipe wall determine the appropriate angle to be used for the self-tandem

technique. The coverage of the self-tandem technique is restricted by the angle and the

number of beams used.

5.2 Sector pulse-echo

The sector pulse-echo technique aims to cover most of the weld fusion zone, except for the

upper ¼ of the weld close to the outer surface. A sector scan, using all the elements in the

array to create an aperture, sweeping the beam from the lower angle to the higher angle was

used. The transmitted beams were focussed at the inner surface distance and Dynamic

Depth Focussing (DDF) was used when receiving the beams. The coverage of the sector

pulse-echo technique is approximately the lower ¾ of the weld, however, the beam spread

will contribute to the coverage in this technique, increasing the coverage further. This

technique benefits from having both direct pulse echo signals and tandem signals reflecting

from the inner wall before reflecting from a defect at the weld centreline and back to the

array. This increases the detected signals from the defects, but careful interpretation of all

detected signals must be undertaken.

5.3 Creeping waves

The creeping wave technique aims to cover the region close to the outer surface. The

technique covers the upper part of the weld that the sector pulse-echo and the tandem

technique cannot see. Creeping waves are compression waves propagating immediately

beneath the inspection surface, to detect surface-breaking and near-surface defects. As

creeping waves propagate, mode conversions at the surface cause secondary shear waves to

be emitted. However, shear waves do not travel any significant distance in PE and are

effectively cancelled out. This continuous transfer of energy results in high attenuation of

the waves and inspection is only effective over a relatively short range.

Creating creeping waves or near creeping waves with a phased array probe are achieved

using a sector scan between the angles of 78 degrees and 90 degrees in a similar manner to

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the sector pulse-echo technique. The creeping waves are effectively only produced by the

higher angles, and the response will be a combination of creeping waves and high angle

pulse-echo signals. A sector scan with all elements is created. The beam spread will

contribute to receive sound outside the range of angles used in the configuration. The

creeping waves combined with high angled sector pulse-echo covers from the outer surface

to about 5mm down.

5.4 Time-of-flight diffraction (TOFD)

The TOFD technique aims to cover the entire weld fusion zone although there is a

possibility that a couple of millimetres close to the outer surface will be missed, depending

on how the technique is implemented. The conventional technique utilises forward

diffraction from the flaw tips and is sensitive to flaws perpendicular to the pipe surface.

Using the steering and focussing capabilities of phased array ultrasonic testing, several

different configurations can be considered.

1) Imitating conventional TOFD. The aperture is used to transmit one beam with beam

spread and the sound is received in the same way. The limitation is that it produces

weak signal amplitude, as the focussing capabilities of the phased arrays are not used.

2) Pitch-catch with two sector scans. With this technique, the transmitter uses a large

aperture to transmit focused beams at the weld centreline. Angles that cover the entire

pipe through wall thickness are used. The receiver is set up in the same way. The main

advantage is that good signal amplitude is achieved at each location due to the

focussing. The main limitation is that the mechanical setup is highly important to be

able to receive the transmitted sound. The transducers have to be positioned carefully at

the same distance from the weld centreline.

3) A combination of conventional TOFD and sector scan. This technique transmits one

beam with beam spread like technique 1. The sound is then received with focused

beams using a large aperture like technique 2. The advantages with this technique are

that the mechanical setup is less important and that it has high flexibility. The main

limitations are that the technique is demanding for the instrument to perform and that

the received signal must be weighted according to the transmitted sound, which has to

be carefully implemented.

The configuration evaluated at this stage of the project is the second technique. It provides

a compromise between instrument demands, software development and inspection results.

6. Results

Results for the development of the techniques for EF-joints and BF-joints are shown below.

Sector scans providing results at one position around the pipe on two pipe sizes, together

with 360° circumferential scanning results for a BF-joint, are shown.

6.1 EF results

The electronic scan on the EF-fitting was performed over one of the fusion zones at one

position around the circumference of the pipe fitting. In the scans the top surface of the

fitting is visible, along with the bottom surface and the wires located just above. Some parts

of the bottom surface are masked by the wires. Scans on a 225mm OD and a 710mm OD

pipe fitting are shown in Figure 7. In Figure 7b the first repeat of the top surface is shown

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just under the bottom surface of the fitting. These reflections show the irregular outer

surface of the 710mm fitting. To avoid this disturbing reflection, an appropriate length of

the water path inside the wedge will be required.

(a) (b)

Figure 7. (a) The electronic scan at one position around a 225mm OD pipe using a

7MHz probe. (b) The electronic scan at one position around a 710mm OD pipe using a

5MHz probe.

6.2 BF results

The techniques for BF-joints were initially evaluated at individual positions around the

pipe. In Figure 8 the sector pulse-echo and the tandem techniques at the position with the

2mm FBH close to the inner surface are shown. In Figure 8a, the sectorial scan using the

4MHz probe is shown, and in Figure 8b the tandem scan at the same position on the same

pipe with the same probe is shown. Reflections from the 2mm FBH are achieved with both

techniques.

(a) (b)

Figure 8. (a) The sectorial scan at one position around a 220mm OD pipe using a

4MHz probe. (b) The tandem scan at one position on the same pipe with the same

probe.

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6.3 Circumferential scanning results

To assess the inspection zone circumferential scans were performed. Figures 9 and 10

present the data from the scans on the 220mm OD pipe with FBHs and slots. Table 2

summarises the detection capabilities of the techniques evaluated on the 220mm OD

specimen.

Table 2 Detection capabilities of the techniques on a 220mm OD test specimens with

FBHs and slots Flat bottom holes (FBH) Slots

6mm

centre

4mm

centre

4mm

inner

4mm

outer

1.5mm

centre

2mm

centre

2mm

inner

2mm

outer

8mm 2mm 6mm 4mm

Sector PE x x x x x x x x

Tandem x x x x x x

Creeping x x x x

TOFD x x x x

The top portion of Figure 9 shows a schematic drawing of the FBHs location on the

220mmOD pipe. The bars to the left of the drawing show the theoretical coverage of the

techniques. The red bar shows the coverage with the sector pulse-echo technique and the

blue bar shows the coverage of the tandem technique. The lighter areas in the bars show the

contributions of the beam spread. In the centre part, the B-scan end view of the sectorial

pulse-echo scan on the 220mm OD pipe using a 4MHz probe is shown. The axis on the left

Figure 9. Developed view of sample FBHs with associated sectorial PE and tandem B-

scans respectively.

reveals at what depths the indications are found. The 2mm FBH close to the inner surface

of the pipe wall is marked. In the lower part the B-scan side view of the tandem scan on the

same pipe using the same probe is shown. The 6mm FBH in the centre of the pipe wall is

marked.

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The top portion of Figure 10 shows a schematic drawing of the location of the slots in the

220mm OD pipe. The bars to the left of the drawing show the theoretical coverage of the

techniques. The red bar shows the coverage with the TOFD technique and the blue bar

shows the coverage of the creeping wave technique. In the centre part, the B-scan end view

of the creeping wave scan on the 220mm OD pipe using a 4MHz probe is shown. The 78°

beam is used for the B-scan and the 8mm slot is marked. In the lower part the B-scan end

view of the TOFD scan on the same pipe using two identical 4MHz probes is shown.

Figure 10. Developed view of sample slots with associated creeping wave and TOFD

B-scans respectively.

7. Discussion

The techniques developed in this paper are part of a project aiming to incorporate the

procedures in a flaw detector to be used on a scanner for on-site inspections. This means

that a range of PE grades and pipe sizes need to be covered. The approach is therefore more

quantitative, and performance on individual material and pipe sizes could be optimised. A

part of the project is to optimise the parameters for probes and wedges and then to design

and manufacture them for specific joint configurations and pipe sizes. For the development

work of optimising the inspection techniques, currently available probes have been used. In

many cases these probes are not optimal for the specific inspection.

8. Conclusions

Techniques for inspection of both EF- and BF-joints have been developed. The detection

capabilities of the techniques for BF-joints on one pipe size have been evaluated. All

defects were detected except two FBHs with the tandem technique. However, weak

indications from their locations can be found. The important conclusion is that all defects

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can be detected by one or more techniques. The techniques will need to be verified on all

pipe sizes and joint types.

Acknowledgement

The TestPEP consortium is made up of 15 organisations from seven European countries.

The research leading to these results has received funding from the European Union's

Seventh Framework Programme managed by REA-Research Executive Agency (FP7/2007-

2013) under grant agreement no [243791-2]

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