The application of the Alternating
Current Field Measurement
(ACFM) technique for public safety
applications.
A. Raine
T S C Inspection Systems
6 Mill Square
Featherstone Road Wolverton Mill
Milton Keynes MK12 5RB
England
Abstract
The use of NDT is well known in the oil and gas
industry, petrochemical industry, shipping and aerospace.
Theme parks are visited by thousands of people worldwide and the rides are becoming more
adventurous by demand. This places a greater strain on the material being used and
fracture mechanics analysis is being carried out to determine the critical defect size
that can be allowed to exist. From a customer satisfaction point of view the rides must be
available when the theme park is open which in some cases is all year. This means
inspection techniques have to be used which are non-invasive but which can reliably detect
the even smaller defects required to be detected.
Bridges are even greater people carriers, rail or road, and are often coated and have
difficult access.
Interest has been recently aroused in the structural integrity of high-rise buildings
constructed with low toughness weld metal. The majority of buildings constructed in the
West Coast of America have used this material and quite a lot especially in the California
area has been subject to seismic shocks. Tests carried out have shown that these shocks
would be greater than those applied during the toughness tests which produced low values.
This has led to the thought that some of these buildings will have defects present in
their welded joints. The problem that now exists is one of inspection and determining if
defects of a significant nature are present. The Alternating Current Field Measurement
technique has been used successfully to inspect through coatings and with rope access
techniques and so can be applied to these public safety applications.
Introduction
The ACFM technique has been used since the late
1980's for the subsea and topside inspection of offshore oil and gas installations in both
its subsea and portable form. It has been further used in other applications as its use
has become better understood and the value of being able to inspect for surface breaking
fatigue cracks without removing the coating has been accepted. Initially the ACFM probes
produced were only suitable for the inspection of long node welds but as the application
became more varied additional probes have had to be produced. An early addition was the
pencil probe produced to inspect areas in welds that had been ground out to remove earlier
detected defects. This probe was profiled to fit the ground out area and could detect if
fatigue cracks had further propagated after their removal. During a trial organised by
University College London where samples were produced to reproduce some of the difficult
geometry’s and access problems located in process plants, it was found that
additional probes were required to gain access and detect and size the defects located
within the samples. A range of mini and micro pencil probes has now been produced with
straight and 90 degree access with increased sensitivity. In addition to this it was
realised that the inspection of short lengths of weld also created problems in that the
communication rate was too slow to produce a good representation of the weld result on the
VDU screen. New software has now been produced that eliminates this problem including
communication rates, which allows scanning speeds seven times faster than before. This
allows greater presentation on the screen for shorter lengths of welds and faster scanning
speeds for the inspection of long lengths of weld. This new range of probes and software
has expanded the use of the ACFM equipment into new applications including theme park
inspection, highway applications and building construction.
Alternating Current Field
Measurement
The ACFM technique is an electromagnetic non-contacting technique which has been
developed to be able to detect and size surface breaking defects in a range of different
materials and through coatings of varying thickness. The basis of the technique is that an
alternating current flows in a thin skin near to the surface of any conductor. When a uniform current is introduced into the area
under test if the area is defect free the current is undisturbed. If the area were to have
a crack present then the current would flow around the ends and the faces of the crack. A
magnetic field is present above the surface associated with this uniform current and this
will be disturbed if a surface-breaking crack is present. It was realised that if these
disturbances could be measured they should have some relationship to the defects that had
caused them. University College London carried out studies into the mathematical modelling
of these magnetic fields and their associated disturbances. A good correlation was
produced between the theoretically predicted magnetic field disturbances and those
measured and thus showed that it was possible to make quantitative measurements of the
magnetic field disturbances and relate them to the size of the defects which produced
them. Special techniques are used to induce these electric currents and the components
used are built into the ACFM probes Small detectors or sensors are also built into the
probe which measure the magnetic field disturbances .Two components of the magnetic field
are measured, the Bx along the length of the defect which responds to changes in surface
current density and gives an indication of depth when the reduction is the greatest and Bz
which gives a negative and positive response at either end of the defect caused by current
generated poles and thus gives and indication of length. Figure 1 .A physical measurement
of defect length indicated by the probe position is then used together with a software
program to determine the accurate length and depth of the defect.
In order to aid interpretation the Bx and Bz components are plotted against each other
and when a complete loop indication is produced this confirms the presence of a crack.
This is called the Butterfly plot Figure 2 and because it is not sensitive to probe speed
aids in the interpretation of the data collected and confirms defect indications. During
the application of the ACFM technique actual values of the magnetic field are being
measured in real time and these are used together with mathematical model look-up tables
so that there is no need for calibration of the ACFM instrument using a calibration piece
with artificial defects such as slots. These can be used for demonstration purposes but
they are not representative of real cracks as they do not behave electrically as a crack.
The slot has other disadvantages as it will not be located in a characteristic
metallurgical or geometric position i.e. in the heat affected zone area between the weld
metal and the parent plate and it will probably not have a characteristic crack shape.
The ACFM technique is insensitive to permeability changes and lift off and, as it does not
rely on probe contact, it can be used to inspect through coatings of various thickness and
material.
Applications
The ACFM technique was originally developed for the inspection of carbon steel welds
on subsea structures, which were usually nodal welds. A number of probes were developed, a
general purpose weld inspection probe, a 30 degree angle probe for examining tight angle
geometry's and a pencil probe specially designed to examine welds that had been subjected
to grinding. This was used to inspect the bottom of the ground toe of the weld to
determine if defects were present and then determine their length and depth or to confirm
that the defect had been removed. The technique was also used to inspect structures that
had been coated with protective or anti fouling coatings so that the expensively applied
coatings did not have to be removed and reapplied thus avoiding costly preparation and
reinstatement. The technique was also adopted by topside inspection engineers for the
inspections of process and pressurised plant systems, structural steelwork and crane
pedestals. The system was used in conjunction with rope access teams allowing inspection
without scaffolding and proving the usefulness of two man operations and the Butterfly
plot. Inspections could be carried out up to 50 metres between the ACFM operator and the
probe pusher.
The technique has also been applied to the inspection of drill threads on casing and drill
tools. A special transportable system has been produced to automatically inspect the drill
thread ends and classify them. This provides Go-NoGo reporting. The system is based on new
ACFM array technology. A hand held probe has also been produced to inspect drill threads
with the portable ACFM system.
New materials are being used for components and coatings on offshore structures but the
ACFM system has now been successfully applied to ferritic steels, austenitic stainless
steels, aluminium, duplex, super duplex, monel and inconel. It has also been used to
inspect through the following coatings, flame sprayed aluminium, epoxy coating, standard
paints, ferrite based paints and copper coated threads.
Some inspections have to be carried out when the plant is operational and ACFM has
been used during inspections at -20 degrees centigrade and up to 500 degrees centigrade.
Because of the above advantages the ACFM technique has been used to inspect coated flare
booms, epoxy coated pig traps, painted nozzle welds, pipe butt welds, pipe and saddle
support welds and pressure vessel seam welds as well as the above mentioned inspections.
Evaluation of the ACFM technique.
Trials have been carried out with the ACFM technique to determine its ability to
detect surface breaking defects and to accurately size their length and depth. Reference
1. A library of nodes was produced at the University College London. These were fatigued to produce real fatigue cracks
of varying length and depth. About 200 fatigue cracks located in various geometries were
produced and these were inspected with the ACFM technique together with other techniques
so that a true comparison of performance could be produced. Probability of Detection
curves were produced for all of the techniques and ACFM proved to have similar detection
capabilities to that of MPI Figure (3). This figure shows that ACFM detection is
marginally better than MPI and on assessment it was shown that MPI had four times more
false calls than ACFM. Crack length comparisons showed that there was little difference
between ACFM predictions and MPI Figure (4) and that the correlation between predicted and
actual crack depths was also good. Figure (5). Reference 2. Some of the library nodes were
then coated with 0.04" and 0.08" of epoxy and the trials repeated and the
results showed that the performance was unaffected by the coating. Figures (6 ) and
(7).Reference 3.
Application to theme park inspection.
Theme park rides are made up of several component parts. The structural section of the
ride is very similar to the tubulars found in offshore structures with fairly long chord
and brace node welds in the track and support areas and thus the problems of inspection
are more of access than geometry. Figure (8) The foundation base sections have short
fillet welds with access holes similar to those found in offshore sections. Figure (9).
These samples were made to examine UK technicians using the ACFM technique to inspect
topside production plant. The carriages, axles and carts or trucks have a different
problem. Figures (10), (l1) and (12). The majority of the welds on these components are
short and have difficult access. This creates two problems one of end effect and the other
of weld presentation. To reliably inspect these welds there is a requirement to have small
probes with high sensitivity and little response to edge effect and hard wearing probe
faces. The communications rate between the ACFM instrument and the computer needs to be
fast to obtain a meaningful length of weld on the screen of the computer. The alternative
is to scan slowly.
Technical Software Consultants have addressed these problems with the introduction of the
mini and micro pencil probes. Both of these probes have either straight or 90-degree
access and have stainless steel probe faces. The mini and micro probes have slightly
different sensitivity in that one can detect defects 0.04" deep and the other
0.02" These probes are particularly suited for the detection of shallow defects in
tight access areas.
A new range of control software QFM 2 has also been produced which has additional features
such as a faster communications rate allowing scanning speeds of up to 2"/ second. This can be used for scanning long welds
faster or producing longer images on the computer screen for short weld inspection. This
software also allows automatic centralisation of the data display and the ability to
select and print single scans of data. Different values of lift off can also be selected
in order to inspect through different thicknesses of coating.
The combination of these developments will allow the experience gained from critical
offshore inspection to be applied to the inspection of the theme park components so that
they can be carried out more efficiently and reliably.
In one theme park the track of one of the rides is made up of 300 ties each one having 70
welds of varying length and geometry. During the annual shutdown of this ride a number of
these ties are cleaned, inspected using magnetic particle inspection techniques and then
the ties are repainted. This normally takes three weeks, one for the cleaning, one for the
inspection and one for refurbishment and repainting. This is one of the major problems as
the paint has to be matched as closely as possible with the original colours. During one
inspection 30 ties were inspected with magnetic particle inspection. During the next
inspection the ACFM technique was used. No prior cleaning was required and 64 ties were
inspected in four days and one day was used for repairs and re-inspection. No additional
painting was required except for the localised painting where the repairs had taken place.
In an industry where the customer expects all of the rides to be available when they visit
the theme park the reduction in down time is very important.
Highway related inspection.
There are 240,000 welded steel bridges in the USA with an average age of 45 years and
of these there are 58.9%, which are structurally defective. The major form of failure in
these bridges is fatigue. Reference (4) In 1967 one steel bridge collapsed and 49 people
were killed. The initial failure was a 1/8th long fatigue crack in an eye bar.
A second bridge failed in 1980, which was also caused by fatigue. In the USA there are
27,000 bridges classed as fracture critical and because of this there is a need to have an
efficient and reliable NDT technique. In the opinion of the Federal Highways Authority NDT
is still not used efficiently during operations and maintenance. Reference (5) In the UK
there are similar problems with the increasing use of heavy road transport and the Euro
regulations allowing heavier axle weight. The combination has caused not only fatigue
problems but also bridge deck problems.
There are a number of road bridges produced from box girder construction which have
longitudinal as well as transverse cracking. Unfortunately the majority of these welds are
coated and to clean and inspect would be very expensive and labour intensive. These box
sections are about 40' long with both horizontal and vertical welds present. Figure (13)
The problem of inspecting for and detecting fatigue cracks through coatings has been well
known in the offshore industry for over thirty years and is now being tackled with the use
of electro magnetic techniques such as the ACFM technique. The results obtained following
inspection are a major factor in calculating the structural integrity of these welds and
determining the valid life of the welded joint in terms of Probability of Failure and
Reliability Index. The offshore industry required a technique, which could inspect and
detect surface breaking fatigue cracks through coatings. Several of these companies funded
the development of the ACFM technique and also the trials, which proved its reliability to
not only, detect but also to be able to characterise cracks in terms of length and depth.
The technique has since been approved by certifying authorities such as Det Norske
Veritas, Bureau Veritas, OCB, Germanischer Lloyds and Lloyds Register.
The problems of inspection of road bridges are not unrelated to that of the inspection of
offshore structures in that the material is steel which is coated, the welds have
difficult access and geometry and the inspection has to be reliable and repeatable. These
were the same problems and background with which ACFM technique was presented. The ACFM
technique has since successfully overcome these problems using the portable unit, two man
rope access, specially developed probes and communication techniques and has been used to
carry out inspection of coated steel structures such as offshore structures and bridge
sections.
One other problem which has arisen is the failure of overhead signs and light supports
which have been subject to high cycle fatigue. This has caused fatalities in one county in
the UK and also in one of the Northern States of the USA
and has caused the inspection and design of these structures to be re-examined.
Some of these designs such as the flagpole design where the weld is on the elbow may have
to be changed. Because of their location it is not easy to remove the protective coating
inspect and re-coat without causing some disruption to the traffic flow. The use of the
ACFM technique would eleviate some of these problems. Figure (14) shows one overhead sign,
which still has the white background paint, used during magnetic particle inspection
present. One side of this highway was closed to allow inspection to take place.
Comparative trials has shown that there can be a 60% saving in time and cost when changing
from magnetic particle inspection to a non contacting technique such as ACFM.
Building Construction.
In a recent issue of the Los Angeles Times the front page headlines was " Weld
metal tests stir steel building concern" and the leading sentence was "Engineers
do not allow this material to be used for building bridges or oil pipelines. Reference (6)
They know it’s not strong enough." The basis of the story was that trials had
been carried out on welds manufactured from a well-known weld electrode and they had
produced low fracture toughness values. The impact values were only 5 foot-pounds at 0
degrees Fahrenheit whereas for the majority of structural uses fracture toughness values
of 20 foot-pounds at 0 degrees Fahrenheit are required. Reference (7) In the UK even
structural material classed as "mild steel" has toughness values of 30
foot-pounds at 0 degrees Fahrenheit. Reference (8) The worrying part of the story was that
this material is and has been used for the construction of buildings throughout the West
Coast of America. The weld metal is used to join beams and columns used in major
constructions and following an earthquake in 1994 it was found that there were ruptured
weld connections in 150 buildings, which had been constructed using this weld metal.
Building officials in Los Angeles banned the use of this material in July of 1996 for all
new construction but there are 1500 steel structures in the area, which have been
constructed using this material. The whole of the West Coast is known for its
susceptibility to earthquakes and a lot of the area has been subject to tremors at some
time or another. It could be quite easy to assume that as the majority of buildings were
constructed from this low toughness material they could now include defective welded
connections. The scenario for the buildings that have suffered cracking has been that the
cracks have initiated in the weld metal during a tremor then have propagated into the
adjoining beams and columns. The stress levels, produced by the earthquakes, that these
welds would be subjected too would be much higher than those at which the above fracture
toughness tests were carried out suggesting even more that this steel would perform
inadequately during a seismic shock resulting in cracking of varying severity.
Two points have been made by several engineers in discussions about this potential
problem. One is the problem of retrospectively carrying out inspection because the
majority of the buildings are covered by claddings of some sort and the other is the level
of inspection that was carried out and the level of inspection that should be carried out.
It is accepted that all of the external cladding of a building cannot be removed for
inspection purposes but one possibility is to expose areas where it is possible such as on
internal walls and utility areas and carry out a percentage inspection which will give an
indication of the integrity of the overall structure. The beams, columns and welds will
have been coated with a protective paint to deter the onset of corrosion and the
inspection technique chosen will have to have a proven track record in the detection of
cracking through coatings. The ACFM technique has been used in trials where samples have
been coated in epoxy coatings as stated earlier. The technique was able to detect and size
in terms of length and depth the defects beneath the coatings. The ACFM technique has a
proven track record for its use in the offshore, oil petrochemical and structural
steelwork industries as a detection and sizing tool and could be usefully deployed for
this application. Any cracks produced during past tremors could now be propagating because
of the fatigue process until critical lengths and depths are reached when failure would
occur. The ACFM technique could be used to detect and size these cracks and then the
structural engineers could calculate the integrity of the welded joint as they now do on
offshore structures. Depending on the outcome of these calculations decisions could be
made whether to repair, strengthen or monitor the welded connection. This could prove an
economic solution to a possible potential problem.
Comments
Theme park inspection with the emphasis on safety of personnel and integrity of
inspection requires a technique that can reliably detect defects and size them through
coatings of various types. Trials carried out in the past have shown that the ACFM
technique can detect and size defects as well and better than conventional surface
breaking detection techniques and its performance will not deteriorate when inspecting
through paint or other forms of coating. Therefore with the added benefit of not requiring
paint removal or refurbishment the application of ACFM should provide an economic benefit
to the inspection of theme parks.
The same arguments can be used for the inspection of road bridges and overhead traffic
signs and large light fittings. With the susceptibility of certain constructional steels
used in the production of high rise buildings producing cracks at low levels of stress
there has been an increasing interest in the use of a least disruptive inspection
technique to be used to determine the integrity of major construction welds. ACFM is a
non-contacting technique, which can be used to inspect through coatings and can be used to
inspect for and characterise any cracks located. These results can then be used to
calculate the structural integrity of the welds being inspected and determine their
fitness for purpose.
References
1. W. D. Dover, J. R. Rudlin, Results of Probability of Detection Trials {Uncoated
Welds}
IOCE Conference Proceedings, 1992.
2. W. D. Dover, J. R. Rudlin. Crack Sizing Trials. IOCE Conference Proceedings, 1992.
3. G. A. Raine, W. D. Dover, J. R. Rudlin. Trials on Coated Nodes. IOCE Proceedings, 1992
4. Inspection of Steel Bridges - C. Frank, University of Texas, April 1992
5. The role of Nondestructive Evaluation in Managing our Aging Highway Infrastructure. S.
Chase Plenary Address,
1997 ASNT Spring Conference, Houston, March 1997.
6. Los Angeles Times, D. Willman, December 30th, 1966
7. University of Texas, M. D. Englehardt, May 1994
8. BS 7191 Mild steel for offshore structural use.
Back to Amigo