International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 6

Number 2

June 2012

231

1 INTRODUCTION / TASK DEFINITION

In ship structural design and assembly standard non-

destructive testing methods include visual, eddy-

current, liquid penetrant, ultrasonic and x-ray testing

as well as leakage tests. Prevalent techniques for

welded joint inspection are x-ray and ultrasonic test-

ing as the most cost-effective and efficient in respect

for flaw detection.

Non-destructive testing operations are performed

in the scope of quality assurance arrangements ac-

cording to classification instructions, specifications

of ship design and regulations of the manufacturing

process [1]. Thus a written procedure for welded

joint testing has to be established for any new ship

design where the inspection areas have to be de-

fined. Through classification instructions the mini-

mum requirements to be fulfilled for non-destructive

testing are defined. Those have to be implemented in

the written procedures for particular construction el-

ements.

The non-destructive testing of welded joints is

described in the currently generally admitted Euro-

pean codes and standards DIN EN 12062, DIN EN

25817, DIN ISO 5817, ISO 6520.

Since many years x-ray testing is considered as a

proven technique for inspection of welded joints in

ship construction. Generally speaking its advantages

and disadvantages can be seen as follows [2]:

Table 1: Advantages and disadvantages of x-ray testing

___________________________________________________

Advantages Disadvantages

___________________________________________________

Sensitive to both surface Limitations for thick-walled

and volume flaws components

Direct documentation of Inspection sensitivity is related

the inspection results to the wall thickness

by film Crack orientation must be

Flaw size and shape can be known for optimal flaw

directly seen and detection

evaluated Defect height normally can’t

No direct access to the be defined

component is required Time consuming technique with

significant equipment

expenses

Radiation hazard

___________________________________________________

Ultrasonic Sampling Phased Array Testing as a

Replacement for X-ray Testing of Weld Joints

in Ship Construction

A. Bulavinov, R. Pinchuk, S. Pudovikov & C. Boller

Fraunhofer IZFP, Saarbrücken, Germany

ABSTRACT: According to European Standard EN 1712 ultrasonic testing of thin-walled welded joints is

mandatory for wall thicknesses of more than 8 mm only. Any thinner components do have to undergo X-ray

inspection.

Besides various advantages of X-ray testing viz. high sensitivity to smallest inclusions, high acceptance in the

ship building sector and “automatic” documentation of inspection results, there are also several deficiencies

(like radiation protection issues, inspection time expenditure etc.) creating a reasonable request for more cost-

effective alternatives.

Sampling Phased Array technology introduced by Fraunhofer-IZFP provides significant improvement of flaw

detectability also in thin-walled welded joints due to its tomographic approach in processing of signals ob-

tained by ultrasonic phased array transducers. It allows high-quality imaging of welded joints and detection of

relevant material flaws. Real-time ultrasonic imaging with tomographic quality offers a great alternative to X-

ray testing with respect to inspection speed and modern documentation of inspection results.

The basic principles of Sampling Phased Array are presented in the paper and several application results ob-

tained on welded joints of marine objects are presented.

232

Though, this method requires significant operat-

ing effort for adherence of radiation protection a

spatial separation of inspection area is required.

Hence no further work can be simultaneously con-

ducted in the neighborhood. This can lead to signifi-

cant decrease in manufacturing productivity.

Ultrasonic testing of welded joints is a significant

alternative to x-ray testing that can be applied, which

in general has the following advantages and disad-

vantages [2].

Table 2: Advantages and disadvantages of ultrasonic testing

___________________________________________________

Advantages Disadvantages

_________________________________________________

Testing of thick-walled Acoustic coupling (surface

components is possible contact) is required.

without limitations Limitations due to surface

Evaluation of flaw size, type, roughness are possible

orientation can be obtained. High requirements on

Fast, cost-effective testing with inspection staff due to

immediate conclusion about rather complex calibration

indication of UT instrument

Automated or half-automated Limitations on flaw

inspection and evaluation can detectability due to

be implemented suboptimal insonification

NEW!: Imaging techniques like position or flaw

phased array allow orientation

documentation and

quantitative evaluation of

inspection results

_________________________________________________

For being able to replace x-ray by ultrasonic test-

ing the following tasks must be solved:

− Equal or better flaw detectability of relevant

flaws compared to x-ray

− Fast representation and evaluation of inspection

results

− Cost-efficient implementation of inspection sys-

tem and inspection procedure

− Mobile inspection system for in-situ applications

2 SAMPLING PHASED ARRAY TECHNIQUE

The novel ultrasonic inspection technique rapidly

coming into industrial application is phased array [3,

4]. Phased array testing offers significant advantages

for ultrasonic testing (UT) of welded joints due to its

extended information content provided by beam

steering capability. Hence the combination of me-

chanical scanning and electronic beam steering in-

creases flaw detectability, since it is being insonified

from various angles of incidence.

Phased array techniques may also have their limi-

tations in certain applications with respect to spatial

resolution in the far field of phased array transducers

or inspection speed while beam steering over a big

angle range and finite signal-to-noise ratio of the

system can be seen as an advantage.

Sampling Phased Array (SPA) technology devel-

oped by Fraunhofer IZFP is a next step in Phased

Array technology. On the one side the technique is

capable of fast synthesis of phased array ultrasonic

signals for arbitrary angles of incidence with focus-

ing in all the depths within the probe near field. On

the other hand the back projection and overlapping

in the volume the elementary wavelets obtained by

SPA according to synthetic aperture focusing tech-

nique (SAFT) principles offers the best possible im-

age reconstruction quality.

The SPA technique offers the following practical

advantages:

1 Ultra-fast virtual beam sweep for arbitrary angle

range

2 Improved sensitivity and resolution in the near

field of the transducer

3 Fast 2D / 3D imaging

Unlike conventional Phased Array technique in-

sonifying the inspection volume by directed sound

fields under different angles of incidence, Sampling

Phased Array performs data acquisition by exciting

cylindrical or spherical waves that propagate in all

directions. This can be implemented by firing single

array elements or applying defocusing delay laws

(Figure 1). Hence a very wide angle range can be

covered after a single shot.

Figure 1: Defocused transmission and sector image reconstruc-

tion by SPA

The ultrasonic signals acquired and saved in each

probe position for every single array element serve

as an input data for image reconstruction. The recon-

struction occurred according to the SAFT algorithm

[5]. Since the sound field of array elements is very

divergent, every time signal (A-scan) received con-

tains overlapped echo-signals from available reflec-

tors in different volume positions. The reconstructed

image in one position of linear array visualizes a cut

plane perpendicular to insonification surface, the so

called sector-scan. For every point within this plane

the propagation times from the transmitting elements

and back to the receiving elements are calculated.

The amplitude values from all A-scans with match-

ing propagation times are added up in each image

point [6].

233

Thus all angles of incidence and focal depths

within the near field of the transducer can be real-

ized even after one single transmitting/receiving act.

Since the sound beam steering at each volume point,

i.e. for all angles of incidence and focal depth, is

performed not physically but virtually through the

computer, a significant increase in inspection speed

can be achieved by implementation of the SPA prin-

ciple [7]. Furthermore the synthetic focusing in the

near field of the UT transducer by the SAFT princi-

ple improves sensitivity and resolution (Figure 2).

Figure 2: Principle of image reconstruction by SPA

Thus for weld inspection the material flaws can

be represented in tomographic quality that allows

their exact sizing (Figure 3).

Figure 3: Tomographic Image of an inclined lying crack

3 INSPECTION SYSTEMS FOR INDUSTRIAL

APPLICATIONS

Modern instrument engineering, e.g. latest signal

processors and computers, offer sufficient computa-

tion power for performing SPA image reconstruction

and processing, that outmatches conventional phased

array systems in speed and quality. Versatile recon-

struction techniques [8] can be implemented in a

portable manual flaw detector (Figure 4).

Figure 4: Manual ultrasonic tomograph A1550 IntroVisor by

ACSYS

One of the main advantages of ultrasonic testing

is its ability to be implemented in automated or

semi-automated way, providing fast and cost-

effective inspection solutions for industrial applica-

tions.

4 ULTRASONIC IMAGING SYSTEMS AS

REPLACEMENT FOR X-RAY IMAGING

While position related data acquisition provided by a

manipulator or encoder wheel the ultrasonic image

can be reconstructed in such a way that it represents

the inspected area similar to the X-ray film (Figure

6). The Sampling Phased Array technique with its

improved image processing capabilities, e.g. eRDM

technique [9], can provide especially sharp and high-

contrast images for detecting relevant welding de-

fects. The evaluation of inspection results can be

performed based on equivalent flaw size, e.g. by cal-

ibrating on artificial defects like notch or side drilled

hole or by use of novel image processing algorithms

for fast quantitative flaw sizing.

Especially for thin-walled welded joints the Sam-

pling Phased Array technique offers specific ad-

vantages due to improved sensitivity and resolution

in the near field of array transducer.

Figure 5: Semi-automated SPA systems with 2D and 3D imaging capabilities

234

Figure 6: Ultrasonic inspection results on the weld seem with a

wall thickness of 6 mm with an elongated cavity in conven-

tional and Sampling Phased Array mode

The current state of standardization of ultrasonic

Phased Array testing in Europe is significantly be-

hind schedule when compared to state of the art

technology. Codes required like ISO DIS 13588 are

in preparation phase.

Fraunhofer IZFP has gathered positive experience

in introducing novel phased array techniques to the

industrial market as a replacement for X-ray testing

of heat exchanger pipes in power plants [9]. The

technique was especially developed for testing thin-

walled pipes. The ultrasonic testing procedure ap-

plied is in accordance to a TÜV Süd specification.

Despite novelty of the testing method it could be

shown that the ultrasonic imaging provides equiva-

lent performance and reliability like established test-

ing procedures.

5 CONCLUSION

The Sampling Phased Array technique is an en-

hancement to conventional phased array which does

processing of the single shot data sampling taken

off-line but in principally real time. Enhanced sens-

ing and data sampling rates allow large data samples

to be taken which again lead to 3D images to be

generated at comparatively high resolution. En-

hanced introduction of SPA into areas where X-ray

testing is currently dominating will lead to:

− Comparable or better flaw detectability

− Higher cost-effectiveness

− Prompt evaluation of inspection results

− No radiation protection

− Mobile and stationary inspection set-ups

This may be achieved through handheld as well

as automated inspection systems on site, being a

considerable advantage in qualifying large ship hull

structures.

REFERENCES

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Schiffbau, DGZfP-Jahrestagung 2005, 2.-4. Mai, Rostock,

DGZfP-Berichtsband 94-CD

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Schweißnahtprüfung, Skript zur Vorlesung, Technische

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