Introduction

Ultrasonic Non-Destructive Testing (NDT) techniques are successfully used for detection of defects in various metallic constructions and inspection of a quality of welded joints.

Recently, the ultrasonic NDT methods have been used in a field of testing of different synthetic products. Ultrasound has found numerous applications in characterization of various polymers. A novel problem arising in this field is non-destructive testing and evaluation of multi-layer plastic pipes. The purpose of such a testing is detection and sizing of various defects, which are arising during manufacturing process.

The main goal of this investigation was to examine the possibilities of detection of standard artificial defects in multi-layered plastic pipes by means of ultrasonic imaging techniques.

Measuring method

For ultrasonic non-destructive testing two main methods may be used - a transmission method and a pulse-echo method. The transmission method involves only measurement of signal attenuation. The defects in the test object are detected by comparing the intensity of ultrasound transmitted through the test piece without defects with the intensity transmitted through the test piece with defects.

The pulse-echo method may be used to measure the delay time and signal attenuation. The defects in the test object are determined from the time-of-flight between the initial pulse and the echo produced by the defects. Defects sizes are estimated by comparing the signal amplitudes between reference signals reflected by the known big size reflectors.

In this investigation the ultrasonic pulse-echo immersion method was used (Fig. 1).

Fig 1: Measurement set-up used for investigation of plastic pipe samples

The ultrasonic transducer in a water tank is excited by a short electrical pulse. The ultrasonic pulse travels in the plastic pipe and is reflected by its’ surfaces, layers, internal surfaces and artificial defects. The transducer can be positioned under water surface at any position with respect to the front surface of a pipe, using two-dimensional stepwise coordinate scanner. The ultrasound propagation direction is along z-axis, the ultrasonic transducer is scanned along x and y-axes.

The reflected signal is presented on the graphical screen in form of A- and B-scans. A-scan presents the amplitude of the electrical signal on the receiving transducer versus z-axis. A-scan can give detailed information about discontinuities in the pipe under investigation. The size of discontinuities may be estimated from the amplitude of the reflected signals. B-scan is the set of the A-scans at some particular coordinate; lets say y, when the other coordinate x is varied. Then the image is called B-scan by X. When the coordinate y is varied, then the image is called B-scan by Y. The B-scans produces the image of a pipe cross section.

If to assume, that t₁ is the delay time of the ultrasonic pulses reflected from the external surface of the pipe, t₂ - delay time of ultrasonic pulses reflected from the internal surface, t₃ – delay time of ultrasonic pulses reflected from the artificial defects, then the distance between external surface of the pipe sample and artificial defect can be calculate:

(1)

where D is the wall thickness of the pipe sample.

For estimation of accuracy with which the position of defects may be found, the artificial defects at the known distance z_o in a plastic pipe were exploited.

Experimental results

Experimental investigation of plastic pipe defects was carried out by the imaging system "IZOGRAF", developed in Ultrasound institute. The system has all necessary units and software for automated ultrasonic data acquisition, imaging and data processing. Experimental set-up used for investigation of defects in plastic pipes is presented in Fig.2.

Fig 2: Experimental set-up for plastic pipe defects detection with ultrasonic NDT system "Izograf".

As an ultrasonic transmitting/receiving transducer the Panametrics transducer V308 (frequency 5 MHz, aperture 19 mm) was used. The ultrasonic transducer and plastic pipes were immersed in a water tank. The distance from the transducer till samples was equal to the Panametrics transducer focal distance (48,8 mm). The surfaces of the samples were aligned perpendicular to the axis of the ultrasonic transducer. The ultrasonic transducer was excited by 140 V amplitude and 80 ns duration electrical pulse.

Experimental investigations of plastic pipes were performed with one, two and three layers plastic pipe samples. In pipe samples artificial defects – side-drilled holes (SDH) and flat bottom holes (FBH) – were made (Fig.3). The holes in pipe samples were at known positions, determined by mechanical measurements. The results of ultrasonic measurements were compared with the known coordinates of the artificial defects.

Fig 3: Artificial defects (side-drilled holes (SDH) and flat bottom holes (FBH)) in plastic pipe sample.

As one layers pipe was a polypropylene (PP) pipe with 50-60 % chalk (the wall thickness D=7.4 mm) used. The distance, between testing holes and the front surfaces were z₁=1.4 mm, z₂=3.7 mm, z₃=6.1 mm, z₄=5.0 mm, z₅=3.5 mm, z₆=2.0 mm. The diameter of all holes was 0.5 mm. The results of measurements are presented in Fig.4. In this figure A-scan, B-scan by X and B-scan by Y are shown. Detection of SDH defects is presented in Fig.4a and detection of FBH defects – in Fig.4b. All artificial defects were successfully detected.

a)	b)
Fig 4: A-scans, B-scans by Y and B-scans by X with artificial SDH (a) and FBH (b) defects in one layer pipe.

a)	b)
Fig 5: A-scans, B-scans by Y and B-scans by X with artificial SDH (a) and FBH (b) defects in two layers pipe.

The second pipe was two-layer pipe, the total wall thickness of which was D=7.5 mm. The distances of holes from the front surfaces were z₁=0.7 mm, z₂=3.8 mm, z₃=5.4 mm, z₄=4.7 mm, z₅=3.4 mm, z₆=1.5 mm. Diameters of the holes were 0.5 mm. Detection of SDH defects is presented in Fig.5a, detection of FBH defects – in Fig.5b. All holes were also detected.

The third pipe sample was three layer pipe: PP layer - fibreglass layer – PP layer. The wall thickness of this pipe sample was D=10.8 mm. The distance of holes from the front surfaces were z₁=1.7 mm, z₂=5.0 mm, z₃=9.0 mm, z₄=9.0 mm, z₅=5.0 mm, z₆=2.0 mm. The diameter of the holes SDH No.1 and FBH No.4 was 0.5 mm. The diameter of others holes was 1.5 mm. Detection of SDH defects is presented in Fig.6a. The artificial defect SDH No.1 in the first layer is detected reliably. In the second layer the defect SDH No.2 is not reliably and in the third layer the defect SDH No.3 is not detected at all. How it is seen from Fig.6b, detection of the FBH in different layers is analogous. The main problem in the second layer is caused by fibreglass.

a)	b)
Fig 6: A-scans, B-scans by Y and B-scans by X with artificial SDH (a) and FBH (b) defects in III layers pipe.

The results of measurements of position of the artificial defects are presented in Table 1. Mechanical measurement results and measurement results with the NDT system "Izograf" are in this table are compared. The position of the defects was calculated using Eq.1. From the results presented in Table 1 follows that the absolute error of the defects position is not bigger than 0.7 mm.

Defects

SDH No.1

SDH No.2

SDH No.3

FBH No.4

FBH No.5

FBH No.6

Pipe

Mechanical Measurement

Detected with "Izograf"

Mechanical Measurement

Detected with "Izograf"

Mechanical Measurement

Detected with "Izograf"

Mechanical Measurement

Detected with "Izograf"

Mechanical Measurement

Detected with "Izograf"

Mechanical Measurement

Detected with "Izograf"

I layer D=7.4 mm, hole diameters®

z1 1.4mm 0.5 mm

z1 1.1mm

z2 3.7mm 0.5 mm

z2 3.7mm

z3 6.1mm 0.5 mm

z3 5.5 mm

z4 5.0mm 0.5 mm

z4 5.5mm

z5 3.5mm 0.5 mm

z5 3.5mm

z6 2.0mm 0.5 mm

z6 1.9mm

II layers D=7.5 mm, hole diameters®

z1 0.7mm 0.5 mm

z1 0.7mm

z2 3.8mm 0.5 mm

z2 3.1mm

z3 5.4mm 0.5 mm

z3 5.7mm

z4 4.7mm 0.5 mm

z4 5.3mm

z5 3.4mm 0.5 mm

z5 3.9mm

z6 1.5mm 0.5 mm

z6 1.8mm

III layers D=10.8mm, hole diameters®

z1 1.7mm 0.5 mm

z1 1.4mm

z2 5.0mm 1.5 mm

Bad detected

z3 9.0mm 1.0 mm

Not detected

z4 9.0mm 0.5 mm

z4 9.2mm

z5 5.0mm 1.5 mm

Not detected

z6 2.0mm 1.5 mm

Not detected

Table 1:

Conclusions

The experiments carried out with the imaging system "Izograf" proved that it is possibly to defect reliably SDH and FBH in one and two layers plastic pipes. Detection of holes in the fibre-reinforced layer and under this layer is a serious problem. For solution of this problem new testing techniques and signal processing procedures should be developed.

References

1. Ultrasonic Testing. The Nondestructive Testing Handbook, second edition. Vol.7. A.S. Birks, R.E. Green, P. Mclntire. American Society for Nondestructive Testing. 1991. 893 p.
2. Nondestructive Evaluation and Quality Control. ASM Handbook. Vol.17. ASM International, USA. 1994. 795 p.
3. Ravi-Chandar K., Schneider E. Ultrasonic Detection and Sizing of Plastic Zones Surrounding Fatigue Cracks. Res. Nondestr. Eval. 1994. P. 191-209.
4. Saka M., Schneider E., Holler P. A New Approach to Detect and Size Closed Cracks by Ultrasonics. Res. Nondestr. Eval. 1989. P. 65-75.
5. Gendron R., Tatibouet J., Guevremont J., Dumoulin M. M., Piche L. Ultrasonic Behavior of Polymer Blends. Polymer Engineering and Science. 1995. Vol.35. No.1. P. 79-91.

R. Kažys, D. Pagodinas, O. Tumšys

Daugiasluoksniu cilindriniu strukturu defektu nustatymo ultragarsiniu metodu galimybiu tyrimas
Reziume
Atliktu eksperimentiniu tyrimu tikslas – išsiaiškinti daugiasluoksniu plastikiniu vamzdžiu defektu fiksavimo ivairiuose gyliuose galimybes. Bandymams pasirinktos polipropileno (PP) ir polietileno (PE) sluoksniu vamzdžiu atkarpos ir jose išgrežtos standartines šonines (SDH) ir plokšciadugnes (FBH) kiaurymes. Tyrimu metu nustatyta, kad 0,5 mm standartines kiaurymes gerai fiksuojamos vieno ir dvieju sluoksniu vamzdžiu atkarpose. Tuo tarpu triju sluoksniu vamzdžio atkarpoje, kai vidurinis sluoksnis poretas bei akustiškai neskaidrus, nepavyko užfiksuoti defektu nei antrame, nei treciame sluoksniuose. Šiai problemai spresti reikia ieškoti nauju defektu nustatymo bei signalu apdorojimo metodu.

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