ADVANCED ULTRASONIC INSPECTION IN SLOVENIA

dr. Uros Zupanc,1 dr. Darjo Zuljan, dr.-Ing Miro Uran

INSTITUT ZA VARILSTVO d.o.o., Ljubljana, SLOVENIJA 1Phone: 00386 41 312 038; E-Mail:uros.zupanc@i-var.si

ABSTRACT

Phased Array (PA) ultrasonic testing is an advanced method of ultrasonic non-destructive testing

(NDT) with growing applications in industrial and medical fields. Phased Array probe consists of

many small ultrasonic elements, each of which can be pulsed individually by advanced electronics.

Electronic focusing permits optimizing the beam shape and size at the expected defect location and

thus significantly increases probability of defect detection. The data from multiple beams are put

together to make a visual image showing various projections through the inspected object.

The conference paper presents practical experience of PA inspection in five typical industrial

applications in Slovenia: (a) airplane wing corrosion inspection, (b) steam pipeline tubes inspection

in thermal power plant with thermal cracks, (c) weld inspection of water pipeline in hydro power

plant, (d) inspection of bolts on old ski-lifts and (f) railway inspection of train axes.

Key words: Ultrasonic testing, NDT, Phased array, TOFD, Olympus Omniscan

1. INTRODUCTION

Phased Array (PA) ultrasonic testing is an advanced method of ultrasonic non-destructive testing

(NDT) with growing industrial applications. Ultrasonic phased array systems can potentially be

employed in almost any inspection where conventional ultrasonic flaw detectors have traditionally

been used. Weld, casting and forging inspection, crack detection and corrosion evaluation are the

most important applications in a wide range of industries including aerospace, power generation,

petrochemical, pipeline construction, maintenance, structural metals and general manufacturing.

The phased array testing offers significant advantages for conventional ultrasonic testing of

components due to its extended informational content.

A PA probe consists of many small ultrasonic elements, each of which can be pulsed individually

by advanced electronics controlled by a PA machine. The benefits of phased array technology over

conventional UT come from its ability to use multiple elements to steer, focus and scan beams with

a single transducer assembly. Beam steering, commonly referred to sectorial or linear scanning, can

be used for mapping components at chosen angles and chosen focal laws. Electronic focusing

permits optimizing the beam shape and size at the expected defect location to increase probability

of defect detection in volumetric inspection. Focusing can significantly improve signal-to-noise

ratio. The data from multiple beams are put together to make a visual image showing various

projections through the inspected object. This can greatly simplify the inspection of components

with complex geometry. More about the PA ultrasonic technique can be read in [1-2]. Additional

evaluation of diffraction signals by TOFD (Time-of-flight diffraction) technique in weld

applications allows fast inspection, accurate sizing of material flaws by means of corresponding

projection images. The TOFD technique relies on the interaction of ultrasonic waves with the tips of

discontinuities. This interaction results in the emission of diffracted waves over a large angular

range. Detection of the diffracted waves makes it possible to establish the presence and sizing of the

imperfection. Detail of TOFD technique can be found in [3-5].

The conference paper presents some practical experience of PA and TOFD inspection Slovenia. The

advanced ultrasonic inspection was performed to evaluate potential corrosion damage in complex

wing structures made of aluminium high-strength alloy. The NDT inspection in duration of just few

hours made it possible to highly reduce cost of alternative visual inspection with prior wing

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dismantling in time of few hounded hours. Corrosion damage was found and was properly repaired.

PA inspection of a part of steam pipeline in thermal power plant is presented. In second application,

the water injection system in steam control unit was evaluated using PA system where thermal

cracks perpendicular to back wall of pipeline were found. Indications of potential thermal cracks

were evaluated using linear and sectorial scans. In part three the results of combined automated PA

and TOFD inspection of welds are presented. Complex weld geometry made conventional UT

inspection very difficult, so automated PA-TOFD inspection was proposed and performed. Typical

indications of found imperfections are presented. On ski-lifts mechanism corrosion and worn level

detection of surface of bolts using phased array was performed. Evaluation with use of PA

technique makes it possible to inspect all bolts in the mechanism fast, without dismantling and with

very high level of material corrosion and/or wear detection. Conventional inspection of wagon axes

in present for quite a long time. But due to complex inspection geometries and thus need for

custom-made ultrasonic probes, PA inspection shows great future potential. Visualization of crack

indications helps inspector for faster and much more reliable inspection. All records of axe

inspections can be easily recorded and stored for any later re-evaluation and comparison.

2. RESULTS AND DISCUSSIONS

2.1 Corrosion inspection on aeroplane wings - PA inspection

Corrosion deterioration of aluminium structure in chloride atmosphere combined with material

fatigue indicated high potential danger. Critical areas at multiple bolting locations and reinforced

spars were inspected in total length around 30 meters per each wing. The scanning was performed

on individually marked areas of 500 mm in length and 40 mm in width and than detailed evaluated.

For inspection the Olympus Omniscan MX16-128 machine was used with specialized 5 MHz 64

elements probe with special wheel encoder presented on Figure 1. In the inspection corrosion was

indicated by a reduction of backwall echo of more than 4 dB with, i.e. corresponding thickness

reduction of more than 10%. Calibration was performed using reference block with three thickness

steps (5, 10 and 15 mm) with machined spotfaces representing 10 % and 20 % thickness reduction

on each reference thickness (Fig 1).

Fig. 1. Equipment used for airplane wings inspection (left). The calibration reference block with

machined spotfaces at different depths (right)

Figure 2 presents typical corrosion indication recorded in corrosion inspection by Omniscan MX

software. The Omniscan display was divided in three reference scans known as A-C-C view.

Amplitude A-Scan waveform presented backwall echo signal and two phased array plan-type C-

Scans presenting top view of inspected area. C-Scan in monochrome contrast was selected as

amplitude reference (A%). Bottom coloured C-Scan was selected as thickness change reference.

Yellow colour indicated 10% thickness reduction and green colour indicated approximately 20%

thickness reduction.

Fig 2: Typical corrosion indication presented in A-C-C view mode. Corrosion indication area of

approximately 40 x 100 mm was found around the bolts.

Fig. 3: Corroded area after dismantling of wing structure. Pen on the right photo served as a

reference dimension. The corroded areas are marked with an arrow.

2.2 Inspection in steam pipeline – PA inspection

The water injection pipeline system in steam control unit was evaluated using PA system where

cracks due to thermal shocks may occur. Inspection was performed with linear and sectional

scanning using standard 5 MHz 64 element Olympus IMS probe (5L64) with zero degrees wedge

and attached wheel encoder. In water injection control system cracks due to thermal shock were

found. Ultrasonic indications of potential thermal cracks were evaluated first using linear scan,

where quite scattering backwall echoes were noticed due to position of thermal cracks

perpendicular to inner wall surface (Fig 4).

Corroded area

Corroded area

Fig. 4: Cross-sectional side view B-Scan image (left) and top view C-Scan (right) of inspected

pipeline indicating rough surface and thus possible thermal cracks.

With the additional PA inspection using sectorial scan it was possible to confirm crack indications

and also to measure the depth of individual crack indications. In the water injection unit with mean

wall thickness between 42 – 44 mm thermal cracks measured up to 10 mm. Due to highly cracked

pipeline inner surface the complete steam control unit pipeline was replaced. Section of eliminated

pipeline material is presented on Figure 5.

Fig. 5: Highly cracked inner surface due to thermal shocks (left), detailed side view of eliminated

pipeline material (right)

2.3 Weld inspection – Combined TOFD and PA inspection

Weld inspection of underground water pipeline is quite complex as around 100 pcs of welds to be

evaluated with diameter of ca 3 meters. Furthermore weld geometry makes inspection even more

complex - welded on backing plate, inspection possible just from inner side of pipeline, on some

sections different thicknesses of joined pipes are used. Due to increased danger of cracks in weld

root (pipeline already ca. 30 years in service) combined PA-TOFD inspection was chosen to

maximize probability of defect detection. Special calibration blocks were prepared for crack

inspection sensitivity and precise positioning. Two common calibration blocks are presented in

Figure 6. Acceptance criteria indicated that no crack indications were allowed. The newest PA

machine was used Olympus IMS Omniscan MX2 32-128 with flex-scanner, wheel encoder, 5L16

PA probes and 5 MHz TOFD probes with 60º wedges.

Fig. 6. The calibration block design for two typical expected cracks in the pipeline welds

PA-TOFD inspection procedure is setup with help of using special software ES Beam Tool from

Eclipse Scientific [6]. The program enables us visualisation of beamset for proper index position of

both PA probes and TOFD tandem probes and setup of appropriate focusing depths. As presented

on Figure 7a beamset of PA probe on left side is focused on weld root and backing plate (blue),

opposite beamset of PA probe on right side is focused on weld toe (red). TOFD tandem probes are

setup to maximal sensitivity of volumetric inspection in the middle thickness of the weld (Fig 7b).

a.) Position of both PA probes according to weld geometry

b.) Position of TOFD probes according to weld geometry

Fig. 7. Inspection procedure for PA and TOFD inspection using ES Beam Tool [6].

Result of weld inspection can be directly evaluated on Olympus Omniscan, but can also be detailed

inspected on notebook or PC with Olympus Tomoview software. Figure 8 presents PA inspected

weld section of 700 mm in length with some comments written. Results are presented in rectified

A-scan waveform, cross-sectional B-scan profile and sectorial S-scan (A-B-S view) of one PA

probe. Interpretation of results by individual PA probe can be made and further one compared with

TOFD images (Figure 9).

Fig. 8. Inspection results in A-B-S view with comments of one PA probes evaluated in Tomoview

software. PA probe is focused on weld toe.

Fig. 9. TOFD inspection results focused on the middle part of the weld

2.4 Inspection of bolts on old ski-lifts ion

On ski-lifts mechanism corrosion and worn level detection of surface of bolts using phased array

was performed. The evaluation of bolt surface is possible just by disassembly of the mechanism and

100% visual inspection, but with long time inspection and high cost. Evaluation using PA technique

makes it possible to inspect all bolts in the mechanism fast and with very high level of corrosion

detection. Comparison of corroded/worn vs. smooth surface of bolts was compared. The diameter of

bolt was 45 mm. The tests were as follows: (a) longitudinal waves using a probe 5L16-A10 with

direct contact at the top surface of the bolt and (b) sectorial scanning using angle beam wedge N60S

for shear waves and 5L16-A10 PA probe.

Fig. 10: Ski-lift mechanism with location of a bolt (left). Worn and corroded surfaces of

disassembled bolts (right).

On Figure 11 are presented sectorial scans at angles between ± 60° using longitudinal waves of non-

corroded bolt on left picture, corroded and worn on right picture. The difference of inspected

surfaces can be easily evaluated.

Fig 11: Sectorial scan with longitudinal waves of non-corroded surface of bolt (left) compared to

bolt with corroded surface (right)

2.5 Railway inspection of train axes

Conventional ultrasonic inspection in combination with magnetic particle inspection of wagon axes

in present for quite a long time. But due to complex inspection geometries with potential cracks on

axes hidden in the wagon wheels assembly, few thousand inspected axes annually, need for

experienced NDT inspectors and custom-made conventional ultrasonic probes (3, 5 and 7 degrees

longitudinal probes and 37-39 degrees and 54-60 degrees angle probes), PA inspection with

automated clamping system shows great future potential. Daily inspection records can be easily

recorded and stored for potential later re-evaluation and comparison.

Corroded areas

Fig. 12: Sectorial scan with shear waves with angle PA probe of non-corroded surface of bolt (left)

compared to bolt with corroded surface (right)

Fig. 13: Sample with artificial crack for ultrasonic calibration (left) and real axe sample inspection

(right)

Fig. 14: Total 360 degrees sectorial phased array scan of an axe with diameter of 152 mm. At angle

of 55 degrees hidden crack under the wheel assembly was found with the length of around 55 mm.

Corroded area

Worn area

Crack

3. CONCLUSIONS

Portable PA machines are commercially available for expanding range of applications due to their

high speed testing, multiple beamsets and visualization of inspection results. With increasing

number of machines sold worldwide or especially in Europe more inspectors and experts in NDT

community are aware of advances of using advanced NDT methods. Also the cost of investment in

equipment is decreasing. The phased array solutions are not only designed for high-tech industries

such as nuclear, aerospace, petrochemical, military, composites etc. but also for manufacturing and

thus also for applications in weld, casting and forging sectors. Particularly for weld applications

Olympus software “Ray tracing” was developed to visualize the inspection to unskilled inspectors.

But on opposite side inspectors will need some more time to understand and master the advance UT

techniques. Standardization of PA procedures, techniques, inspection criteria, qualification and

certification of personnel is slowly being implemented in manufacturing industry. First prEN and

ASTM standard are already available, but higher progress is expected in coming years.

4. REFERENCES

1. Olympus IMS Phased array tutorial. Accesed at: http://www.olympus-ims.com/en/ndt-

tutorials/phased-array. Web: 2.8.2013

2. Dube N., Phased Array Technical Guidelines, Useful Formulas, Graphs and Examples,

Advanced Practical NDT Series, R/D Tech Corp., Canada, 2005, ISBN 0-9735933-1-8

3. EN 583-6, Non-destructive testing – Ultrasonic examination – Part 6: Time-of-flight diffraction

technique as a method for detection and sizing of discontinuities

4. EN ISO 10863, Non-destructive testing of welds - Ultrasonic testing - Use of time-of-flight

diffraction technique (TOFD)

5. EN 15617, Non-destructive testing of welds – Time-of-flight diffraction technique (TOFD) –

Acceptance levels

6. Software ES Beam Tool, Eclipse Scientific Accesed at:

http://www.eclipsescientific.com/Software/software.html. Web: 2.8.2013