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ULTRASONIC METHODS TO DETECT AND EVALUATE DAMAGE IN STEEL a

a

S. Hirsekorn , P.W. Van andel & U. Netzelmann

a

a

Fraunhofer-Institut für zerstörungsfreie Prüfverfahren (IZFP), Universität, Gebäude 37 , D-66123, Saarbrücken, Germany b

Shell International Oil Products B.V. , Amsterdam, The Netherlands Published online: 27 Apr 2007.

To cite this article: S. Hirsekorn , P.W. Van andel & U. Netzelmann (1998) ULTRASONIC METHODS TO DETECT AND EVALUATE DAMAGE IN STEEL, Nondestructive Testing and Evaluation, 15:6, 373-393, DOI: 10.1080/10589750008952880 To link to this article: http://dx.doi.org/10.1080/10589750008952880

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ULTRASONIC METHODS TO DETECT AND EVALUATE DAMAGE IN STEEL S. HIRSEKORNa,*, P.W. VAN ANDELb.t and U. NETZELMANN a a Fraunhofer-Institut

fur zerstorungsfreie Prufverfahren (IZFP), Universitdt, Gebdude 37, D-66I23 Saarbrucken, Germany; b Shell International Oil Products BY, Amsterdam, The Netherlands (Received 4 October 1999) Porosity, microcracks, or secondary phase inclusions alter the sound velocities of materials, Ultrasonic scattering at grain and phase boundaries in polycrystalline, multi-phase, or porous media causes attenuation of sound waves and dispersive sound velocities. These effects and also the amplitudes of the scattered waves can be exploited for materials characterization if their dependence on the structure properties is known quantitatively. In this paper the usefulness of sound velocity, attenuation, and backscattering measurements to detect and value damage in steel e.g. caused by hydrogen attack is discussed. Numerical evaluations of theoretical results and experimental investigations are presented. It turns out that in most cases velocity measurements do not yield sufficient quantitative information about damage because the variations in sound velocity caused by slight differences of the undamaged material are often larger than those caused by damage. Sound attenuation and backscattering amplitude measurements are more appropriate to estimate damage in steel. Keywords: Attenuation; Cracks; Scattering; Steel; Ultrasound; Velocity

I. INTRODUCTION

Theoretical studies [1-9] have shown that voids or microcracks in a material will reduce the velocities of compressional and shear waves. The dependence between velocity and porosity (volume fraction of • Corresponding author. Present address: Zevenaar Electronica & Sensoren, Shellenkrans 23, 6904 PS Zevenaar.

t

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voids and/or cracks) can be used for damage evaluation if the value for zero porosity of the specific material considered is known accurately enough [9]. The relation is linear for small volume fractions of porosity. It was also predicted that the decrease in velocity will be larger for compressional than for shear waves. The relative changes ofthe sound velocities are caused by changes of bulk and shear modulus produced by microcracking because of the missing restoring force across the crackwalls. Independently of each other Hasegawa [10]and Birring et at. [II] found by experiments that steel is attacked beyond the acceptable level by hydrogen if its velocity ratio of shear to compressional waves, which normally has the value 0.54, is higher than 0.55. Both papers contain no quantified specifications of the damage. To detect and evaluate damage from hydrogen attack in pipeline steel an automated ultrasonic equipment has been developed [12]. Correlating the number and amplitude of indications and their density in ultrasonic C-scans of a few thousand steel samples with metallographic investigations yielded the threshold value of unacceptable damage. It is not based on theoretical derivations but a matter of experience. There is no statement about detectability in connection with a quantified specification of damage. Measurements clearly demonstrated that attenuation and backscattering amplitude increase with hydrogen attack [10,11], e.g., for 10 MHz compressional waves the attenuation turned out to be 1.6 dB/cm in damage-free and 4.2-5.8 dB/em in attacked samples, the backscattered signal amplitude increased by 16-21 dB because of hydrogen attack in the material tested [11]. In practice, it is more convenient to use lower frequencies (about 4 or 5 MHz) even if attenuation and backscattering amplitudes decrease with frequency. Attenuation and backscatter methods are hampered if the sample surfaces are corroded. Corrosion additionally causes ultrasonic scattering [II], but becomes a significant contribution only for higher frequencies. In each of the papers [10,1 I], quantified statements about the damage and its advancement derived from the measured quantities are missing. In our paper, the theoretical-physical basis to solve this problem is given by ultrasonic scattering theories which describe ultrasonic propagation in materials with microscopic inhomogeneities such as polycrystalline structure, secondary phase inclusions, voids, or microcracks.

ULTRASONIC DETECTION OF STEEL DAMAGE

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2. NUMERICAL ESTIMATIONS OF THE ULTRASONIC EFFECTS INDICATING DAMAGE


2.1. Calculation of Scattering Coefficients In hydrogen attacked steel, grain scattering and scattering at cracks are superposed. To detect and evaluate the damage from attenuation orvelocity dispersion measurements, the part of the attenuation caused by scattering from cracks and its related velocity dispersion must be known separately. The frequency independent velocity changes due to the presence of cracks may also be exploited for damage estimation. If the experiments are aimed at backscattering measurements, attenuation is to be evaluated. In what follows the scattering coefficients due to the different kinds of scatterers are estimated numerically for some typical examples. The scattering coefficients in transmission due to grain scattering only are given in an analytical form in the literature [13-18]. Concerning the scattering by cracks some further considerations are necessary. We assume the cracks caused by hydrogen attack to be penny-shaped, so that they can be described by oblates with diameter d and a crackopening of din, where n = 10 is a realistic value [19]. The crack length I may be defined as the averaged line length of all possible cuts perpendicular to the crack plane. This definition relates crack length and crack diameter by I = Ld]«, and the volume ofacrack is given by Ve = 47r(dI2)31 (3n) = 7r4 p1(48n). If N is the number of cracks per unit area, for pennyshaped cracks the porosity (volume fraction of cracks) follows to be Vp = NVel1 = (7r4 /48)(Nln)P . Furthermore, if the orientation distribution of the penny-shaped cracks is isotropic, the ultrasonic scattering by the cracks can be described as scattering by pores with an effective diameter of

deff

""

d

l'

5 dx{1 + (nz - l) xZfo. ,

(I)

which yields deff "" 0.30 I d if n = 10 [20]. The calculation of the effective diameter follows from a scattering theory which describes ultrasonic propagation in short fibre reinforced materials [20,21]. This model includes the description of ultrasonic propagation in homogeneous and isotropic materials with porosity or penny-shaped cracks (oblates) because pores and cracks can be considered as a secondary phase.


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The considerations and assumptions above are the basis to estimate ultrasonic attenuation caused by scattering from cracks in comparison to attenuation by grain scattering. For grain and for crack scattering, the scattering coefficients of compressional waves in transmission, O'SLgrain and O'SLcracks, are calculated. Within the Rayleigh-region, they also describe the attenuation of backscattering [22-33]. Beyond the Rayleigh-region the attenuation of backscattering O'SB is experimentally found to be significantly smaller than the attenuation in transmission O'ST [34]. A theoretical relation between the scattering coefficients O'SB and O'ST in this regime is not yet known. The interaction between grain and crack scattering cannot be estimated till now because of the lack of theoretical background. There only exists a phenomenological description which linearly superposes the scattering at grain boundaries and at cracks in the same material [35]. In this model, grain scattering follows from [13] while scattering at cracks is calculated using the results from [36]. The different parts are weighted by the volume fractions of grains and cracks, respectively. This model does not take into account multiple scattering effects between scatterers of different kind. As pointed out above, 4- 5 MHz compressional waves are convenient to test hydrogen attack in steel. For 4 MHz compressional waves, the normalized frequencies (that is wavenumber of compressional waves k times radius of the scatterers) are given by kdg/2 = 0.097 (d g = grain diameter = 451lm) for grain scattering and with the crack length / = 20-100 urn (d = crack diameter, deff = diameter of effective pore equivalent) k/= 0.085-0.426, kd/2 = 0.067-0.334, kderr/2 = 0.022-0.108 for scattering by cracks. Therefore, the Rayleigh approximation is valid for both, scattering by grains and by cracks. Within the Rayleigh-region, the ratio ofenergy backscattered by cracks and by grains, respectively, is equal to the ratio of the corresponding scattering coefficients [27]. The Rayleigh approximation yields relatively simple analytical expressions for the scattering coefficients [13,18,37]: Grain scattering:

(2)


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Here, A = C I I - C I2 - 2C44 is the anisotropy factor, K = W/VT is the wavenumber of shear waves, «[k = VL/VT is the ratio of compressional to shear wave velocity, and Sg is the scattering factor [27]. Crack scattering:


OSLeraeks

V

= 2:I VerrP "

'V I

=

1r

(d2err) 2 ~9 Sc

(k d2err)

4

(3)

Here, , is the total scattering cross section, V p is the porosity (volume fraction of penny-shaped cracks) as defined above, and Veff is the volume of the pore equivalent in the case of isotropic orientation distribution of the cracks. S; is the scattering factor given by

From this, the ratio of back scattered energy within the Rayleigh-region follows to be OSLeraeks = OSL Q of crack to grain scattering in sample 3 follows from an A-scan in radial direction (similar to that one of sample I given in Fig. 5) to be about 0.84. As in sample I crack and grain scattering is of the same order of magnitude. Theoretical values for C>SL Q are given in Table X as function of volume fraction and length of the cracks and the grain diameter. Metallographic investigations yielded a crack length of about 50 urn and a crack area fraction of about (4000 ± 1000)/63700 = 0.063 ± 0.016 in the highly damaged region of sample 3. If we assume that the volume fraction of the cracks is equal to its area fraction and use the grain diameter of 741lm determined by ultrasonic attenuation, the ratio of crack to grain scattering is C>SLQ = 0.11 for randomly oriented cracks and C>SL Q = 0.27 for exactly parallel aligned cracks and sound incidence perpendicular to the cracks. That is, the value estimated from the experiment is in the same order of magnitude as the theoretical estimation (factor 3.1-7.6 in dependence whether the cracks are randomly oriented or somehow aligned). The larger ratio of crack to grain scattering in experiments than in theoretical estimations may be caused by a broad crack-length distribution. Large cracks dominate the backscattering behaviour. The model calculations are based on a single fixed crack length which is set equal to the mean crack length, and so, the influence of larger cracks is neglected. Experimental investigations at a 0.5 Mo steel sample of 451lm grain diameter obtained from a pipeline of 72 mm wall thickness confirm this assumption. The pipeline has been 18 years in service at 150 bar partial hydrogen pressure and at a temperature of 285°C. Hydrogen attack is present in large areas of this pipeline. Backscattering was just detectable from a region with a crack density of N = 200/mm 2 and an average crack length of 20 urn. At this region the


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crack length distribution is very broad including cracks up to 100 urn length, which may have caused the detectability. Metallographic investigations showed that the large cracks consist of two or more small interlinked cracks. The typical length of these large cracks is of the same order of magnitude as the grain diameter or even larger. The detection limit in ultrasonic backscattering contains approximately 10% interlinked cracks, i.e. 20jmm 2 . Crack interlinking causes a rapid increase of the upper limit in the size distribution. Backscattering increases strongly with crack size. Therefore, interlinking causes a rapid increase in backscattering which, according to the model calculations, is sufficient to reach the detection level. The model can be adapted to include a crack size distribution [40].

4. SUMMARY

Theoretical estimations show a decrease of sound velocities and an increase of the ratio of shear to compressional wave velocity with increasing volume fraction of cracks. The ultrasonic velocity measurements on hydrogen attacked 0.5 Mo steel samples (tube sections) confirm this behaviour. But the theoretical as well as the experimental investigations show that quantitative statements are very difficult. Experimentally, for the samples at our disposal the order of magnitude of the effect turned out to be at the limit of measurement accuracy using standard ultrasonic velocity measuring techniques. The non-monotonous behaviour of the sound velocities in the radial direction of the tubes may be caused by non-monotonous increase of damage from the outer to the inner surface. For some samples, the area fraction ofcracks turned out to be a non-monotonous function of depth. As can be seen from the experimental investigations, it is more suitable to detect and quantify damage in steel from attenuation and backscattering than from velocity measurements. C-scans of two tube sections perpendicular to the tube axis clearly show a decrease in the backwall echo amplitude at the inner surface of the tube (Figs. 2 and 3) which indicates that the attenuation clearly increases (as stated in Section 3.1 more than 10dB) in the damaged region. In accordance to the attenuation measurements, A-scans of the specimens I and 3 in the radial direction of the tubes show an increase


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in backscattering amplitude towards the damaged region near the inner surface (Sections 3.3 and 3.4). The increase amounts to about a factor 1.5-2.0 (Fig. 5). That is, the ratio of scattering amplitudes caused by cracks and grains, respectively, is about 0.5-1.0, which corresponds to an intensity ratio of 0.25-1.0.


Acknowledgements

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