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Structural Integrity Assessment Applying Ultrasonic Testing

dr Miodrag Kirić dipl. ing. The Institute for Materials Testing of Republic Serbia

Boul. V. Mišića 43, Belgrade miodrag.kiric@institutims.co.yu

ABSTRACT

European standards for ultrasonic examination of welded joints are discussed with regard to the methods for sensitivity setting. It is especialy important the frequently used method 1 based on side drilled cylindrical holes which is compared with ASME standard. For methods 1 and 2 are given expressions for received acoustical signals reflected from corresponding reflectors in far field of the normal and angle probes. The possibility of application of notch beyond the standard limits is considered, as well two concepts of ultrasonic testing of welded joints and the characterization of discontinuities as planar.

INTRODUCTION

The experiences gained in pressure vessels testing have shown that manual ultrasonic testing (UT) by using pulse-echo technique compared to other nondestructive methods is the best method for discontinuities characterization and revealing because of its versatility and possibility of quantification and memorization of results enabled by digital ultrasonic flaw detectors.

The aim of UT determines the technique to be applied, also its sensitivity and testing levels [1,4-6]. The determination of crack features may include: basic ultrasonic parameters, crack parameters and its proximity to the surface or to other discontinuities as well its basic shape and orientation.

The paper considers UT of welded joints from the European standards point of view. It is a multitask testing because it is aimed at detection of discontinuities, characterization of discontinuities and evaluation of discontinuities, by taking into account the geometry and welding technology. When a discontinuity is detected, the determination of its features is necessary for its evaluation with respect to known acceptable criteria. The differentiation between the quality control concept and the fitness-for-purpose concept should be made, [1]. In order to correctly identify the discontinuity types specified in the acceptance criteria, or to make a correct fitness-for-purpose evaluation, it may be necessary to make a more detailed sizing and assessment of the shape of the discontinuity, [2,7]. Echodynamic patterns obtained as transient echo shapes during probe movement, are used as one of criteria for the classification, as well for surface discontinuity sizing. Surface notches of equal lengths and different depths give echo envelopes whose widths can be related to actual notch depths, [3], thus it is possible to evaluate the discontinuity depth.

ACCEPTANCE LEVELS

Evaluation of indications detected by testing of welded joints, is based on reference levels, established by standard EN 1712 [4]. The setting of sensitivity and reference level for method 1 is performed by 3 mm diameter side drilled hole (SDH) perpendicular to ultrasonic beam axis. The diameter is the same for different frequencies and parent material thickness (PMT), while according to ASME Section V it is dependent on PMT. For method 2 of EN 1712 (the distance gain size system, or DGS), ultrasonic probe frequencies (f) and reference levels depend on PMT, as given in Table 1 and Table 2 for longitudinal (L) and transverse (T) waves. Reference levels for method 2 are defined by the diameter d of equivalent plane circular reflector (FBH), perpendicular to ultrasonic beam axis. Evaluation level, related to reference level, is important from two reasons: all indications equal to or exceeding evaluation level shall be evaluated and the other is that it is used to evaluate the lateral dimension of an indication [4,5].

TABLE 1. Reference levels for L-waves and normal probes for method 2 (DGS), [4, 6]

Thickness of parent material, t (mm) Nominal probe frequency (MHz)

8 ≤ t <15 15 ≤ t < 40 40 ≤ t ≤ 100

1,5 to 2,5 - d=2 mm d=3 mm

3 to 5 d=2 mm d=2 mm d=3 mm

TABLE 2. Reference levels for T-waves and angle probes for method 2 (DGS), [4, 6]

Thickness of parent material, t (mm) Nominal probe frequency (MHz)

8 ≤ t <15 15 ≤ t < 40 40 ≤ t ≤ 100

1,5 to 2,5 - d=2 mm d=3 mm

3 to 5 D=1 mm d=1,5 mm -

One remark should be given and it is that sensitivity required for T-waves is higher than for L-waves if f is from 3 to 5 MHz, because shorter wave length of T-waves compared to wave length of L-waves at the same frequency allows to detect the reflector of smaller diameter d. However, at higher frequencies of L-waves, reference levels are the same as for lower frequencies.

Similar question can be put regard to the reference level for tandem examination; it is a distance amplitude curve (DAC-curve) for flat bottom hole of diameter 6 mm, which is the same when PMT or probe frequency changes. DAC curve is at the same time the evaluation level and recording (acceptance) level for discontinuities perpendicular to the surface.

Three acceptance levels for method 2 are illustrated in Fig. 1 according to EN 1712 (the first of them should be the level 1). It is to be noted that EN 1714 [6] specifies methods for manual UT of fusion welded joints in metallic materials equal to and above 8 mm thick and EN 1713 requires that the examined depth should be greater than 5 mm, so that the surface layer of material up to 5 mm should not be tested [7]. However, according to ASTM E 169, UT is performed if metallic material thickness is above 6 mm.

0 0,5 t t 1,5 t 2 t

Length of indications

Key: Acceptance level Evaluation level

Reference level: 0 dB

Recording levels: Acceptance level 2 = Reference level

Acceptance level 3 = Reference level + 4 dB

FIGURE 1. EN 1712 acceptance levels for method 2 and mmtmm 10015 <≤

ASME standard Section XI gives acceptance criteria for internal as well for surface flaws, based on linear fracture mechanics relations of Newman and Raju [8]. The criteria for surface planar flaws, perpendicular to the surface, are illustrated here by the diagram in Fig. 2. The acceptable flaw sizes, a and l correspond to points below the curve for given PMT. It is evident the resemblance of Fig. 1 with discrete levels and Fig. 2 with continual levels. For thick materials, the influence of l on allowable value of a is small if l > 25 mm according to Fig. 2, but according to Fig. 1 it is neglected in three intervals of flaw length dependent on PMT, with transitions between them described by step functions of length l .

Complementary examination should be carried out in the case of any doubt by using additional probes, by analysis of echodynamic pattern in the lateral movement or using other NDT as radiography, or time of flight ultrasound diffraction technique, TOFD, which is more appropriate for defect sizing in the material thickness direction. TOFD is the approach whose significance is that it is not amplitude dependent and the principles involved (energy is dissipated only from the extremities of discontinuity), enable precise through-wall size and location measurement of detectable defects. It is standardized by EN 583-6 [9].

dB 10

4 0

- 4

Relative echo amplitude

Reference level

Acceptance level 3

0 10 20 30 40 500

1

2

3

4

5

6 Surface planar flaws D=100 mm D= 65 mm D= 13 mm The boundary between significant and non-significant flaws

Unacceptable flaws

Acceptable flaws

Flaw height a, mm

Length l, mm

FIGURE 2. The dependence of allowable height a on length l for surface planar flaws according to ASME Section XI

THE INFLUENCE OF FREQUENCY AT METHODS 1 AND 2

Ultrasonic frequency is among the most important probe parameters, because it influences beam shape, resolution, wave attenuation and, as it will be shown by taking in mind the data given in Tables 1 and 2, the ratio of received and emitted acoustical signal P/P0. UT is greatly determined by area of piezoelectric plate, Sa , the distance of a reflector, r, from the entrance point of beam axis on test surface and the near field length

vfSSN aa ππλ == (1)

where λ is wave length and v is wave velocity. Half beam angle defined by the first wave amplitude minimum is

)/61,0sin()/61,0sin( afvarcaarc == λθ (2)

It is assumed that artificial reflector size 2b fulfils the condition for proportionality between echo amplitude and reflector area Sb, oriented normally to the beam axis:

Wb 4,02 <<λ (3)

where the half width of the main leaf of the ultrasonic field for a normal probe, W, depends on distance r and angle θ in radians

θrW = (4)

Thus the condition (3) at small angle θ becomes

ar

bλ

λ 244,02 << (5)

and at the end of the near field it is approximately

ab 2,02 <<λ (6)

If the reflector is flat bottom hole (FBH) in a normal probe far field, the ratio of received and emitted acoustical signal is [10]

rbafbh e

rSS

PP δ

λ2

220−= (7)

where δ is attenuation coefficient for L-waves in the material.

For a long side drilled hole (SDH) in far field, perpendicular to beam axis, the ratio is also dependent on frequency:

racyl e

rbS

PP δ

λ2

30 2−= (8)

For a FBH perpendicular to beam axis in far field of an angle probe, holds an approximative formula for 0

PPfbh

)(22

220

11

cos)(cos rr

t

baltfbh

terrSSD

PP δδ

βλα +−

+= (9)

where tλ is wave length of T-waves, ltD is the transmission coefficient for incident L-waves at the interface between an angle

beam probe and material under testing, 1δ and 1r are the attenuation coefficient and the path length for L-waves in refracting

prism, respectively, 2r is the path length for L-waves in the prism emitted by the equivalent piezoelectric plate and β is incident angle in probe and α is refracted angle in the material [10].

For a SDH perpendicular to beam axis in far field of an angle beam probe, the ratio of received and emitted acoustical signal is

)(23

20

11

)(cos2cos rr

t

altcyl

terr

bSDPP δδ

βλα +−

+= (10)

The ratio of received and emitted acoustical signal for a FBH in far field is proportional to the second power of frequency for a normal probe as well for an angle probe, eq. (7) and eq. (9).

The ratio of received and emitted acoustical signal for a SDH in far field is proportional to frequency for a normal probe as well for an angle probe, eq. (8) and eq. (10).

The interpretation of these relations is that if frequency increases, the ratio of received and emitted acoustical signal for both probes should increase proportional to f for SDH and proportional to f2 for FBH. If other parameters and reflector to probe distance are the same and if attenuation change is not considered, the effect of increased frequency is the same as it was the effect of increased b2, or Sb for SDH. The effect when FBH is used is more pronounced, because it is proportional to f2. It is shown that the change of Dlt is not significant if β is between 380 and 530, corresponding to α from about 450 to 700 [10,11].

It is a simple task to find out the relation between sizes of equivalent FBH and SDH at the same distances in far field. For an angle beam probe it is shown in [12] that the diameter of an SDH, equivalent to the FBH respect to the acoustical signal is

28 2

22

bS

r rb

t

=+λ ( )

(11)

The diameter 2b of equivalent SDH depends on Sb, the surface of FBH, but also on f2. Similar relation holds for a normal probe and L-waves, too. It can be concluded that if different sizes of FBH, dependent on frequency, are used for sensitivity setting, according to the relation (11), the diameter 2b of equivalent SDH should change with Sb and independently, also with frequency as f2. However, according to EN 1712 ultrasonic probe frequencies and reference levels (diameters of FBH) for method 2 depend on PMT, but for method 1 diameter of SDH does not.

NOTCH AS REFLECTOR OF ULTRASONIC WAVES

Standard EN 1712 method 3 for sensitivity setting if a probe angle ≥ 700 is used for the thickness range 8 mm ≤ t<15 mm is based on a rectangular notch with a depth of 1 mm.The reference level is equal to a DAC curve for the notch. Notch as calibration reflector is defined also by ASME standard. Here will be discussed the use of notch for UT with regard to EN 1712 and EN 1713.

The research of notches as reflectors for distance scale and sensitivity calibrations proved that they are well defined and easy applicable reflectors. This can be illustrated by nothces made using electric-discharge machine in a 30 mm thick plate, perpendicular to its surface, made of low-carbon HSLA steel ČSN 11484.1 for a spherical tank and cutted from its wall. The notch depths are 1, 2 and 3 mm, lengths are 12 mm and the notch width is 0,3 mm. The A-scan and echodynamic pattern from 1 mm notch depth are given in Figs. 3 and 4 respectively. The echoes are obtained using the double traverse technique and a miniature 70°, 4 MHz probe. Echo height is given in percents of the full screen height (FSH) at the top of the screen. Thus, UT can detect the notch of depth 1 mm from the distance 175,8 mm, simulating a shallow surface crack, when the material thickness is 30 mm. The A-scan and echodynamic pattern from 2 mm notch depth and the same angle beam probe are also given in Figs. 5 and 6 respectively.

Figure 3. Echo from the 1 mm notch depth Figure 4. Dynamic echo from the 1 mm notch depth (70°, 4 MHz probe, distance 175,8 mm) (70°, 4 MHz probe, echo beginning at 172,6 mm)

Figure 5. Echo from the 2 mm notch depth Figure 6. Dynamic echo from the 2 mm notch depth (70°, 4 MHz probe, distance 175,8 mm) (70°, 4 MHz probe, echo beginning at 178,9 mm)

The notches are compared to SDH of 3,2 mm diameter by comparing their echos, Figs. 7 and 8. According to the ASME Section V, the reference calibration reflector for material thickness equal to 30 mm is the SDH 3,2 mm in diameter. The DAC curve is constructed for the same miniature angle probe. The test instrument calculated and displayed the projection distance PR=71,8 mm and the notch depth D=29,1 mm in Fig. 7 because the direct scan technique is used. From Figs. 7 and 8 it is seen that notch echos are higher than DAC curve for 2 dB for 1 mm notch and for 6,5 dB for 2 mm notch, thus the SDH 3,2 mm echo at the same distance is the smallest one. It should be noted that notch lengths are 12 mm.

Figure 7. DAC curve and the echo from the 1 mm notch Figure 8. DAC curve and the echo from the 2 mm notch

By regression analysis it was obtained the dependence of ultrasonically determined notch depth from known notch depth. According to ASME standard, as the measure of the notch depth was taken the dynamic echo width at the half of the echo amplitude. For small depths, up to 3 mm, it was obtained a linear regression given in Fig. 9. The linear regression is explained so that the echo amplitude is a linear function of notch size that reflects the beam, because notch lengths are equal. It should be noted that correlation coefficient is high (R=1) and it supports linear character of the relationship between measured and known notch depths.

The conclusion may be that ultrasonic examination can reveal and evaluate surface notches of limited length even at PMT equal to 30 mm. Thus the restriction of EN 1713 standard concerning the surface layer which is not tested is too conservative.

Figure 9. The linear relationship between the echo width and notch depth [3]

TWO CONCEPTS OF ULTRASONIC TESTING OF WELDED JOINTS

UT of welds for structural integrity assessment is a multitask examination because it is aimed at detection of discontinuities, characterization of discontinuities and evaluation of discontinuities. UT of welded joints has one of two functions:

1. The control of weld quality based on flaw acceptance criteria. This is the quality control concept. 2. To give a reasonable guarantee that the weld under examination contains no discontinuities which can bring into danger its

structural integrity in normal exploitation. This is the fitness-for-purpose concept.

The acceptability of a welded joint is assessed from one of two concepts. Standards for acceptance or rejection must take into account the safety reasons, the engineering aspects of the structure, its cost and required service performance. The second concept allows greater discontinuities then the quality control concept. Cracks can be allowed under some conditions, too [13]. Some details of two concepts are compared in Table 2.

TABLE 3. Comparison of two concepts of UT [1] WELD HOMOGENEITY

Weld quality Structural integrity ACCEPTABILITY STANDARDS

Quality control criteria based on engineering experience

Fitness-for-purpose criteria based on fracture mechanics calculations

ULTRASONIC EXAMINATION 1. Defect evaluation Measurable defect parameters: Geometrical defect characteristics: 1.1 Maximum echo indication 1.1 Height (through thickness size) 1.2 Probe movement along and perpendicular to weld 1.2 Length 1.3 Number of echos when scanning 1.3 Proximity to scanning surface 1.4 Discontinuity orientation and nature 1.4 Orientation and nature 2. Procedure characteristic 2.1 Directed towards most serious discontinuities and expected (manufactured) discontinuities

2.1 Directed towards most serious discontinuities and expected (service-induced) discontinuities

2.2 Sensitivity setting which determines the ability to detect certain (the smallest allowable) discontinuities

2.2 Sufficiently high sensitivity

2.3 Ussually limited number of probes (it increases with quality class)

2.3 Several probes and beam directions; the use of additional creeping wave probes.

2.4 Thick welds are usually considered divided into thickness zones and examined independently

2.4 In some cases it may be sufficient to examine outer (surface) layers to demonstrate fitness-for-purpose

2.5 Detection probability depends on quality class; modest operator’s skill is necessary

2.5 Detection probability is high; good operator’s skill is necessary

4 6 8 10 12 14 160,5

1,0

1,5

2,0

2,5

3,0

3,5

The

dept

h of

the

notc

h, m

m

The width of dynamic echo, mm

Y = - 0,625+0,25 X R=1,000 SD=0

Experiment

The criteria for a discontinuity to classify as planar or non-planar are of primary importance. The criteria according to EN 1713 are :

- welding technique, - location of the discontinuity, - maximum echo height, - directional reflectivity, - A–scan, - echodynamic pattern.

The classification requires an examination each of the parameters against all the others. The procedure of the classification is stopped as soon as one of the above criteria is fulfilled and can be given by a flow chart diagram. High echo amplitude is characteristic for a planar discontinuity. It is compared to the reference level increased by 6 dB. A discontinuity is classified as planar if there is high directional reflectivity and a high echo in the A-scan. It means that there is a diference in echo amplitudes of at least 9 dB between two angles of incidence of the discontinuity examination using shear waves. If testing is carried out with shear waves from one direction and with longitudinal waves from the other direction, the signal difference of at least 15 dB is indicative of a planar discontinuity. If directional reflectivity is low, the next step is the examination of A-scan. If it doesn`t show a single smooth echo, the echodynamic pattern is examined.

Evaluation of discontinuities may include discrimination between planar and non-planar discontinuities as the primary discrimination of an acceptable or rejectable indication. In this case all discontinuities above the evaluation level shall be characterized and if characterized as planar, shall be rejected [4].

CONCLUSIONS

From relations and illustrations given in this paper, it is seen that UT is an efficient and useful method for structural integrity assessment, because it provides high sensitivity and versatility in manual technique, but, additionally it is, with much greater investments, very useful when automated. The reliability of UT depends on acceptance criteria and testing levels, but also on operator′s skill and attention. The application of European standards criteria requires their good knowledge and training. They allow the application of two concepts, one based on criteria for quality assurance and the other based on fracture mechanics.

The discussable is the setting of sensitivity for method 1 of EN 1712 by 3 mm diameter SDH, because received acoustical signal depends among others on SDH diameter, frequency and refracted angle chosen regarding PMT. In addition, there is a simple relation derived between diameter of FBH in far field and the diameter of an equivalent SDH at the same distance. Also, the echograms with A-scans and echodynamic patterns given here illustrate the possibility for the use of rectangular notch as calibration reflector beyond the limits determined by EN 1712.

REFERENCES

1. Kirić, B.M. and Fertilio M.A., Ultrasonic Assessment of Welded Joints Against Quality Control and Fracture Mechanics Criteria (in serbian and english), Zavarivač, Vol. 40, pp. 209-223, 1995. 2. Kirić, B.M., The approaches of European standards for ultrasonic testing of welded joints of ferritic steels – Part II: Characterization of indications (in serbian), Zavarivanje i zavarene konstrukcije, Vol. 47, 239-245, 2002. 3. Kirić, B.M., The Ph.D. thesis (in serbian), The Faculty for mechanical engineering, Belgrade, 1-251, 2000. 4. EN 1712 Non destructive examination of welds – Ultrasonic examination of welded joints – Acceptable levels, CEN, Brussels, aug. 1997. 5. Kirić, B.M., Zavarivanje i zavarene konstrukcije, The approaches of European standards for ultrasonic testing of welded joints of ferritic steels – Part I: Acceptance levels (in serbian), Vol. 47, 167-174, 2002. 6. EN 1714 Non destructive examination of welds – Ultrasonic examination of welded joints, CEN, Brussels, aug. 1997/ nov. 2002. 7. EN 1713 Non destructive examination of welds – Ultrasonic examination of welded joints – Characterization of indications, CEN, Brussels, may 1998. 8. ASME Boiler and Pressure Vessel Code, Section XI Division 1 Rules for Inservice Inspection of Nuclear Power Plant Components, 1980 Edition, SI Edition 1983 and later editions. 9. EN 583-6 Non destructive testing–Ultrasonic examination–Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities, CEN, Brussels. 10. Gurvič, A.K., Ermolov, I.N., Ultrazvukavoj kantrolj svarnih švov (in russian), Tehnika, Kijev, 1972. 11. Ermolov, I.N., Progress in the Theory of Ultrasonic Flaw Detection. Problems and Prospects, Russian Journal of Nondestructive Testing, Vol. 40, No. 10, 2004, pp. 655–678. Translated from Defektoskopiya, Vol. 40, No. 10, 2004, pp. 13–48. 12. Kirić, M., Analysis of ultrasonic testing of smaller thicknesses butt welded joints with regard to the possibilty of defects revealing (in serbian), International Symposium - Nondestructive testing in insurance of quality of materials and equipment for chemical, petrochemical and power supply plants, Sarajevo, 1978. Proceedings, Book II, pp. 5-13. 13. Kirić, M., Testing for detection and evaluation of cracks, International Fracture Mechanics Summer Schools - ′New Trends in Fracture Mechanics Applications′ (IFMASS 9), editor S. Sedmak, Varna (Bulgaria), 19-24. sept. 2005. (CD)