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Surface-Breaking Flaw Detection in Mild Steel Welds usingQuantum Well Hall Effect Sensor Devices

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Citation for published version (APA):Watson, J. M., Liang, C-W., Sexton, J., Biruu, F., & Missous, M. (2018). Surface-Breaking Flaw Detection in MildSteel Welds using Quantum Well Hall Effect Sensor Devices. Paper presented at The 45th Annual Review ofProgress in QNDE Conference, Burlington, United States.

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Download date:14. Apr. 2021

Surface-Breaking Flaw Detection in Mild Steel Welds using

Quantum Well Hall Effect Sensor Devices

James M. Watson 1 a), Chen-Wei Liang 1, James Sexton 1, Firew A. Biruu 1 and

Mohamed Missous 1 b)

1 The University of Manchester, School of Electrical and Electronic Engineering, Manchester, Greater Manchester,

M13 9PL, United Kingdom

a) james.watson-6@postgrad.manchester.ac.uk

b) m.missous@manchester.ac.uk

Abstract. With growing industrial interest in Non-Destructive Evaluation (NDE) applications of Quantum Well Hall

Effect (QWHE) sensors, a study was conducted to quantitatively determine the detection capabilities and general

performances of low frequency QWHE imaging for surface-breaking flaws and their comparison with Magnetic Particle

Inspection (MPI), Eddy Current Testing (ECT) and Alternating Current Field Measurement (ACFM). In this study, a

probe consisting of a QWHE sensor, illuminating electromagnet and sensor circuitry was controlled using an automated

XY scanner with a measurement step of 250 µm to simulate an integrated array of QWHE sensors of 250 µm pitch. This

probe applied a 3 mT, 100 Hz frequency magnetic field to map the surface magnetic field and Magnetic Flux Leakage

(MFL) response of five bespoke dressed mild steel weld samples. These samples provided 15 surface-breaking flaws of

varying length from 3 to 11 mm; mainly longitudinal toe and center-line cracks which are representative of typical

requirements of our industrial partners. The same samples were also subjected to MPI, ECT and ACFM provided by

leading industrial companies using their own qualified personnel, equipment and procedures. The outcomes and

performance of each NDE technique including QWHE imaging were then compared and evaluated.

INTRODUCTION

An initial study was conducted to quantitatively determine the general performance and detection capabilities of

QWHE sensor-based magnetic imaging to compare them to existing, well established electromagnetic NDE

techniques. Samples of mild steel were synthesized by Sonaspection Ltd. with dressed welds and artificial flaws

before being subjected to MPI, ECT, ACFM and QWHE imaging inspections. These tests were performed on a

double-blind trial basis with each technique not being fully optimized to achieve “best” results. MPI was performed

by BAE Systems using a standard 50 Hz ~1.2 T yoke, contrast paint and kerosene-based media spray according to

their standard NDE procedures. ECT was performed by Rolls-Royce Nuclear with probes and inspection frequencies

available that were not optimized for mild steel welds. These ECT probes were mounted on an XY measurement

system to obtain limited imaging capabilities. ACFM was performed manually by TSC Inspection Systems using

their standard AMIGO probe, more suited to monitoring gross-sized flaws in service structures over small flaws of

this study. Their twin-field probes optimized for weld inspections were not available at the time of this study.

QWHE imaging was achieved using a probe consisting of a single QWHE sensor, sensor circuitry and an

illuminating electromagnet which applied a 3 mT, 100 Hz magnetic field. The XY movement of the probe was

controlled using a scanner of step size 250 µm to simulate an integrated QWHE sensor array of 250 µm pitch. A

significantly smaller step size of < 100 µm could have been used to obtain ever higher resolution images but would

not have been appropriate for these studies and would not represent a meaningful sensor array for millimeter size

NDE crack detection purposes. Throughout the QWHE imaging tests, an applied magnetic field frequency of 100

Hz was used. To date, there has been no research into an optimal inspection frequency or measurement steps for any

applications of QWHE imaging, including NDE crack detection.

The general performances, including main advantages and limitations of each technique are discussed in their

retrospective sections. The outcomes of each NDE inspection trial were also compared, evaluating:

The amount of flaws detected (not probability of detection).

Any additional indications.

The approximate indication signal amplitude to background floor level.

The accuracy of flaw sizing.

The approximate inspection times.

Samples

Fifteen flaws representative of those encountered in carbon-steel butt-welds were manufactured by Sonaspection

Ltd and distributed over 5 mild steel weld samples were used in this study. The flaws were predominantly center-

line and toe cracks of various lengths ranging from 3 mm to 11 mm, longitudinal to the direction of the welds, as

shown in Figures 1(a) to 1(e) below. Each sample measured 150 mm x 150 mm with a thickness of 20 mm and each

containing a full penetration weld with caps and roots ground flush. The parent metal was a generic mild steel with

electrical and magnetic properties comparable to those o AISI 1010.

(a) (b)

(c) (d)

(e)

FIGURE 1. Official validated sketches provided by Sonaspection Ltd. of each sample and used in this study. Flaws present on

cap side unless otherwise stated. (a) Sample 6441-01 with 11 mm toe crack, 13 mm linear porosity (root side) and 10 mm toe

crack. (b) Sample 6441-02 with 8 mm toe crack, 8 mm center-line crack (root side) and 8 mm toe crack. (c) Sample 6441-03 with

5 mm toe crack, 5 mm linear porosity (root side) and 5 mm toe crack. (d) Sample 6441-04 with 5 mm toe crack, 4 mm center-line

crack (root side) and 5 mm toe crack. (e) Sample 6441-05 with 3 mm toe crack, 3 mm center-line crack (root side) and 3 mm toe

crack.

Magnetic Particle Inspection

Arguably the easiest and most common NDE technique for ferromagnetic materials, MPI is a fast and efficient

method particularly for inspecting large surface areas whilst providing instantaneous results as an image [1]. It has

low start-up and running costs as well as being comparatively easy to use and qualify in industry. In practice, it

suffers from a low detection limit of ~3 mm and a poor probability of detection, depending on the specific industrial

application [2]. Unlike other electromagnetic NDE methods, it cannot provide any flaw depth measurements or

estimates, its flaw length measurements are inaccurate with false indications being common, particularly when

sample geometry acts as a natural MFL site (e.g. filet welds). It requires surface preparation, including pre-cleaning

and is invasive as coatings must be removed [3]. It is also a messy technique which typically requires post-test

cleaning and can contaminate / disrupt some production line stages.

MPI was performed on the samples (Figures 2(a) and 2(b)) by BAE Systems using their industry standard

approved 50 Hz ~1.2 T yoke, standard white contrast paint and kerosene-based black magnetic ink spray media. A

3-lines “Burmah-Castrol strip” magnetic flux indicator was used to validate the strength and performance of the

equipment, as standard to their procedures.

(a) (b)

FIGURE 2. Photographs of MPI indications of two flaws from this study. (a) MPI indication of 4 mm center-line crack on

sample 6441-04 root side. (b) MPI indication of 5 mm linear porosity cluster on sample 6441-03 root side.

These MPI tests were able to detect all of the flaws with no false indications, as well as being able to size and

position them within 1 mm error. It must be stated that these were manufactured as MPI samples, meaning that the

flaws were designed to provide the most realistic MPI indications and the “correct” positions and lengths were

validated by Sonaspection Ltd.’s own MPI tests. Consequently, the indications obtained by other methods may not

be as clear or as realistic and are subject to some reasonable tolerance.

Eddy Current Testing

Although ECT is a group of different electromagnetic NDE techniques, in this study only conventional eddy

current testing was used (i.e. single frequency, absolute, phase and amplitude measurements). As such, it is this

method which ECT will be referred as. ECT is known to be a very sensitive electromagnetic NDE technique where

modern equipment and software allow inspections to be set-up and performed easily. It also provides instantaneous

results usually through the real-time plotting of a complex impedance plane [4]. Compared to other techniques, it

has a good detection limit and probability of detection in practice [2], with a sensitivity enabling reliable flaw sizing

estimates based on pre-test calibrations [5]. In most applications, ECT is non-invasive meaning non-conducting

coatings do not have to be removed, depending on the coating thickness, sample material properties and test

parameters. However, flaw characterization is difficult with ECT, particularly when using complex impedance plane

analysis. As with other sensor-based electromagnetic NDE methods, it is sensitive to lift-off and sample geometry,

the main drawback being a comparatively slow inspection time based on the small footprint of ECT probes [6]. This

makes it ineffective at inspecting large surface areas, with false indications common in practice due to its sensitivity.

Rolls-Royce conducted the ECT trials using Zetec absolute probes of frequency 2.5 MHz and 6 MHz, both

probes and frequencies were not necessarily optimized for mild steel weld inspection. A standard XY measurement

stage was used to provide limited imaging capabilities (Figures 3(a) and 3(b)) using Magnifi software. A standard

ECT calibration block was used to provide flaw depth estimates.

(a) (b)

FIGURE 3. ECT phase shift images of complete weld scans of two samples from this study. (a) ECT indications of two 8 mm toe

cracks on sample 6441-02. (b) ECT indications of 5 mm linear porosity cluster on sample 6441-03.

Unfortunately, only 4 of the samples were tested due to time constraints. However, from the samples tested all of

the flaws were detected, including the smallest 3 mm cracks. This method did produce 4 additional indications

relating to microstructure, 2 with comparable magnitude to flaw signals, the other 2 with lower magnitudes. The

noise from one such microstructure, suspected to be lack of fusion, is shown in Figure 3(a). ECT was able to size

63% of flaws within 1 mm error, the remainder within 2 mm error. Likewise, 82% were positioned within 1 mm

error, the remainder within 2 mm error.

Alternating Current Field Measurement

ACFM is a strongly growing and emerging electromagnetic NDE technique that is mainly used for the detection

and accurate sizing of gross flaws of in-service structures, typically in inaccessible places such as offshore rigging

[7]. Most notably, compared to other techniques, ACFM equipment has been fundamentally designed with the end

user in mind. Consequently, ACFM inspections are very easy to set-up and perform, with instantaneous results in

real time through the ASSIST software. In practice, it is very accurate at sizing flaw lengths and depths using the

ASSIST algorithms and indication database. There is also a large coverage from a single probe (compared to

conventional ECT) with inspection frequencies used that require none/minimal surface preparation. This technique

is non-invasive as non-conductive coatings do not have to be removed [8], dependent on the sample material

properties and coating thickness. However, as with other sensor-based electromagnetic NDE methods, ACFM is

sensitive to lift-off and sample geometry; and can this can add to difficulties in characterizing flaws. The AMIGO

probes have a poor detection limit as their main applications are for gross flaw detection and accurate sizing. As

such, it must be noted that the ACFM technique could be optimized to have a much better detection limit, with

limited industrial/commercial demand preventing this.

For this study, ACFM was performed by TSC Inspection Systems, the leading manufacturer of ACFM

equipment, using their standard AMIGO probes which were not optimized for mild steel welds. Manual

measurements were taken using a standard 50 kHz AMIGO probe (Figure 4(a)) with magnetic field component

measurement plots and flaw sizing estimates (Figure 4(b)) autonomously compiled into a report after the testing was

complete.

(a) (b)

FIGURE 4. (a) ACFM manual measurement lines drawn on sample 6441-02 with flaw measurements. (b) ACFM indications of a

11 mm toe crack on sample 6441-01.

As expected, 3 of the smallest flaws (one 4 mm and two 3 mm cracks) were undetected by the ACFM AMIGO

probe, as the typical applications are much different. However, 90% of the flaws detected were sized correctly or

within 1 mm of error, the remainder within 2 mm error. None of the flaws were positioned correctly due to the

measurements being entirely manual. However, they were all within 2 mm of error.

Quantum Well Hall Effect Sensor Imaging

Developed and tested by The University of Manchester, an XY scanner and QWHE sensor probe (Figure 5(a))

were used to apply a 3 mT, 100 Hz magnetic field to each sample under test, using a QWHE sensor to measure the

MFL [9] caused by each flaw (Figure 5(b)). The effective active sensing area of the QWHE sensor was 160 x 160

µm2.

(a) (b)

FIGURE 5. QWHE sensor probe used in this study. (a) Photograph of probe taken whilst scanning sample 6441-03. (b) Diagram

of probe showing general layout of components.

A standard depth of penetration of 0.6 mm was approximated based on the sample material properties and

inspection frequency of 100 Hz. A measurement step of 250 µm was used which simulates a representative

integrated QWHE sensor array of 250 µm pitch. The differential of the magnetic field was then calculated and

normalized against the measurement step by calculating the magnetic field gradient. These steps are mathematically

described in Equations (1) and (2), where 𝑩𝑥𝑦 represents the magnetic field measurement taken at coordinate (𝑥, 𝑦),

with a measurement step of 𝑑.

𝑩′𝑥𝑦 = 𝑩𝑥𝑦 − 𝑩(𝑥−𝑑)𝑦 (1)

𝑨′𝑥𝑦 = 𝑩′𝑥𝑦

𝑑 (2)

This value 𝑨′𝑥𝑦 was mapped in real time whilst the scan was ongoing and was plotted to produce QWHE MFL

images of each sample to be used in this study. An example of these images is given as Figure 6(a), along with

annotations explaining the various features and artefacts in Figure 6(b).

(a)

(b)

FIGURE 6. QWHE images of sample 6441-01. (a) Unprocessed QWHE image of sample 6441-01 in greyscale. The two toe

cracks appear as bright white indications within the weld boundary line. (b) Annotated version of Figure 6(a) highlighting the

indications of the toe cracks and other weld features. Blue rectangle is used in Figure 8.

A basic hill shading image processing technique was used to identify further areas of research (i.e.

microstructure). This technique visually identified the contours produced by all the MFL responses of each sample

by creating shadow-like artefacts on each image. An example of this is shown in Figure 7.

FIGURE 7. Processed QWHE image of sample 6441-01 in greyscale using basic 45° hill shading to highlight microstructural

information contained within QWHE images.

Figures 6 and 7 show that low frequency QWHE MFL imaging can not only detect the presence of representative

NDE flaws within mild steel welds but is also sensitive to microstructural changes using a 100 Hz inspection

frequency. As such, an intensity plot (Figure 8) is given showing the relative intensity differences between the MFL

responses from a crack and those from microstructures.

FIGURE 8. An intensity plot showing the relative magnitudes of MFL response from a crack and microstructures from sample

6441-01.

Figure 8 was created by compiling the data within the blue rectangle slice on Figure 6(b) onto a single intensity

vs. position graph. It shows that the intensity from MFL responses of cracks are far larger than those of

microstructures, emphasizing that QWHE sensors are not “over-sensitive” for NDE crack detection applications.

This promising initial work suggests that gross microstructural changes (such as lack of weld fusion) give maximum

MFL responses that are 1

3 the magnitude of cracks, giving this specific technique and test parameters an approximate

Signal to Noise Ratio (SNR) of 3 (~4.8 dB) at this stage.

As with the other NDE techniques employed in this study, QWHE imaging was not optimized for crack

detection in mild steel welds as the effects of numerous inspection parameters have yet to be explored, determined

and hence optimized. These include but are not limited to: the frequency of the applied magnetic field (i.e.

inspection frequency) and illumination set-up (electromagnet and/or coils, their design, etc.). As previously

mentioned, the measurement step of 250 µm used simulated an integrated QWHE sensor array of 250 µm pitch,

representative of realistic future applications of QWHE NDE sensor technology [10][11][12]. A measurement step

of < 100 µm could have been used to obtain “better” higher resolution results but would not have best represented

the future aims of technology (larger pitch arrays needed to maximize inspection area / footprint).

QWHE MFL images were obtained for each sample’s cap and root side. Using these, all of the flaws were

detected using the unprocessed images, similar of those in Figure 6. Several additional indications were found and

are believed to be MFL responses from microstructural variations within the samples, including lack of weld fusion.

As shown by Figure 8, these additional indications had magnitudes far below the magnitudes of MFL from a crack

and created a noise floor from which the MFL from cracks (and the MFL from the flaw coupon boundaries) were

clearly visible. In this trial, it was found that using the unprocessed images grossly undersized the flaw lengths.

However, the basic 45° hill shading image processing technique was able to size all the flaw lengths within 2 mm of

error which is a promising start to future research in flaw profiling using QWHE images. The basic XY scanner used

in this study was unable to provide reliable positioning.

Overview

As expected, MPI appeared to perform more favorably based on the nature of the samples, how the sample

sketches and flaw sizes/positions were validated, as well as the sheer versatility and performance of the technique in

controlled settings. Also, since the ACFM was performed manually using an AMIGO probe which was designed for

completely different industrial applications, these results are not representative of the optimized performance of

ACFM for small crack detection in mild steel welds.

However, Table 1 shows that in this initial study QWHE imaging performed comparably well to conventional

ECT in this controlled setting. The additional indications from microstructure can be ignored using future post-

processing techniques and a more optimized applied magnetic field frequency (i.e. inspection frequency) at least for

these types of inspections; in others, microstructures would be prime targets as in large grain oriented electrical steel

materials.

TABLE 1. An overview of the performance of each NDE technique used in this study.

MPI ECT ACFM QWHE

Flaws found 100% 100% 100% 100%

False / additional

indications 0 4 1 9

Indication signal

amplitude to

background ratio

n/a ~4.8 dB

(SNR of ~3)

~4.8 dB

(SNR of ~3)

~4.8 dB

(SNR of ~3)

Flaw length sizing

accuracy ± 1 mm ± 2 mm ± 2 mm ± 2 mm

Total inspection

time

~ 1 hour

includes pre and

post cleaning

~ 6 hours ~ 4 hours

~ 6 hours

based on an

integrated 8 x 1

250 µm sensor

array – could be

further optimized

Notes

Official sample

drawings based

on manufacturer’s

MPI validation

tests

2 microstructural

additional

indications of

similar intensity

to flaw signal

AMIGO probe

not designed for

flaw detection of

this scale

All microstructural

additional

indications have

much smaller

intensity than flaws

It is believed that there are several near-surface flaws (within 1 mm of surface) which have been detected using

QWHE imaging at 100 Hz but not detected by ECT or ACFM due to their much smaller depths of penetration based

on their much higher inspection frequencies. The limited sensitivity of MPI would also explain why these were not

detected in the MPI tests performed by the sample manufacturers or as part of this study. In addition, since these

samples were manufactured as MPI samples, any validating ultrasonic NDE performed would have deemed these

flaws negligible for MPI use. As such, in the near future Ultrasonic Phased Array (Total Focusing Method), other

available NDE methods and finally destructive techniques will be performed to further investigate these claims of

microstructural anomalies and sub-surface flaws.

Conclusions

From this initial study, the detection capabilities and general performance of MPI, ECT and ACFM were

compared to low frequency QWHE MFL imaging, for the specific application of surface-breaking flaws in dressed

mild steel welds. This study showed that QWHE imaging can be used to detect surface-breaking flaws and also

evaluate microstructures. It was found that in this controlled setting, QWHE imaging performed comparably well to

ECT in terms of detection capability and SNR, with QWHE sensors providing full imaging capabilities technology

[10].

Future work as part of this research will be focused on developing the maturity of the QWHE imaging technique:

Frequency optimization for crack detection in mild steel welds.

Measurement step (sensor array pitch) optimization for crack detection with largest inspection area possible.

Development of image processing and enhancement techniques.

Development of automated flaw detection algorithms.

Development of database of MFL responses from cracks of known dimensions for flaw reconstruction (flaw

length and depth measurements).

Development of multi-frequency / pulsed applied magnetic field technologies for QWHE imaging for

different materials and NDE applications (such as corrosion mapping).

ACKNOWLEDGEMENTS

Industrial involvement of BAE Systems Submarines, Rolls-Royce Nuclear and TSC Inspection Systems for

conducting their parts of this trial.

EPSRC-EP/P006973/1 “FUTURE COMPOUND SEMI-CONDUCTOR MANUFACTURING HUB”)

EPSRC-EP/LO22125/1 “UK RESEARCH CENTRE IN NON-DESTRUCTIVE EVALUATION(RCNDE3)”

iCase PhD Studentship through RCNDE (BAE Systems Submarines)

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