The
number of highway, railway, and mass transit steel bridges with
welded details in the U.S. has been estimated at 123,000 with
between 2,500 and 5,000 of these bridges having low fatigue
resistant details . Fatigue cracks that go undetected can lead
to larger cracks and in some cases cause structural failure.
Cracks that can be detected and accurately measured can be repaired
or retrofitted. Typically, bridge inspectors will conduct a
visual inspection to determine if a structure is experiencing
detrimental cracking. However, detecting and sizing cracks cannot
always be done with a purely visual inspection.
Many
non-destructive testing (NDT) methods can be used to supplement
the visual inspection of structures. However, ultrasonic testing
is one of the most versatile NDT methods. It can be used to
detect defects beneath the surface of a material and is relatively
inexpensive. The Time of Flight Diffraction (TOFD) method is
an ultrasonic technique that has promise in the area of steel
bridge inspections. In this method, one transducer transmits
an ultrasonic signal through the material being inspected. This
signal is then received by a second transducer. This produces
a single waveform or plot of signal amplitude versus time. If
there is a defect in the material between the transmitter and
the receiver, the ultrasonic signal will be diffracted and this
will alter the waveform. The time of arrival of diffracted ultrasonic
signals is used to calculate crack location and depth. Since
identifying a flaw from a single waveform can be difficult,
multiple waveforms are "stacked" together to create
what is called a D-scan. A D-scan is a three dimensional colorscale
plot showing location of the transducer-receiver pair on the
test surface and the waveform collected from that location.
Additionally, the time scale of the waveform can be converted
to a depth scale through the thickness of the test piece. This
allows flaws to be readily identified and sized directly from
the D-scan. The advantages of the TOFD method over other ultrasonic
methods are that it allows large volumes of material to be inspected
efficiently, produces permanent records of data, and reduces
operator misjudgment and subjectivity.
A
previous study at the University of Wisconsin-Madison showed
that the TOFD technique was successful in both locating and
sizing cracks accurately . In that study however, a limited
number of samples with simple geometries (flat plates with implanted
fatigue cracks) were available for testing and a larger set
of data is required before the procedure can be recommended
for the use in bridge inspections. One objective of the current
research is to extend the use of the TOFD method to the inspection
of complex geometries such as changes in flange thickness, welded
cover plates, and welded T-sections, all typical of what might
be encountered in an in-service steel bridge. Another objective
is to determine if surface conditions typical to what might
be encountered in the field, such as paint or corrosion, affect
the accuracy of the method. The final goal in this research
project is to develop a set of guidelines for the use of the
TOFD method in field inspections.
The
results that have been obtained to date indicate that the TOFD
method could be a very useful tool in the inspection of steel
bridges. A series of flat plates with saw-cuts and implanted
fatigue cracks was inspected to test the setup and equipment.
The lengths and depths of these cuts were determined to within
3% of the reported values. The shallowest crack that was detected
extended approximately 2 mm into a plate from the surface. This
has been defined as the limit of method since specimens with
smaller cracks were inspected but the cracks were not detected.
These results indicated that the setup and equipment are capable
of producing acceptable 
results.
The next step was to inspect several plates with light corrosion
on the surface and implanted fatigue cracks in blind tests.
All the cracks were detected using the TOFD method and the length
was measured to within 5% of the reported value. For comparison,
these plates were also inspected using another nondestructive
testing method, magnetic particle testing (MT). While MT was
capable of detecting the flaws, the depth of the flaws could
not be measured with this method. The figure at the left shows
the colorscale image produced from a TOFD scan of one of the
cracks and a photograph of the MT test of the same crack. The
length of the flaw was found to be approximately the same using
both methods.
To
meet the objective of extending the TOFD method to more complicated
geometries and surface conditions, several specimens were taken
from girders of a formerly in-service bridge that was being
replaced. One of these was a portion of the girder flange with
a change in thickness. This piece was also painted on one side.
While there were no known cracks in this specimen, it was inspected
with the TOFD method to determine if the paint or change in
thickness affected the method. Neither the paint nor the varying
thickness seemed to inhibit the use of the method. In addition
to this specimen, a T-section and a rolled section with a welded
cover plate are also being inspected. This type of geometry
presents a particular challenge because relatively few other
studies have attempted to use the TOFD method for inspection
of these types of sections. This work is currently being conducted
and a major part of the work is to identify the paths that the
ultrasonic waves travel in these geometries. Once this has been
completed satisfactorily, specific transducer locations will
be recommended for inspection of each type of geometry.
In
addition to locations of the transducers, spacing between the
transducers is important in acquiring accurate test results.
A series of tests were done on flat plates with cracks in order
to optimize the setup to get the most accurate results for any
thickness of steel and for any depth of crack, within the detectable
limits. The main conclusion which could be drawn from these
tests was that optimum spacing of the two transducers was a
function of flaw depth, thickness of material, and the angle
at which the ultrasonic signal enters the material. The two
figures at the right show scans of the same crack, one with
a non-optimum spacing and the other with an optimum spacing
between the transducers. The crack is shallow, only about 2
mm deep, so it is difficult to identify. However, it is much
easier to see the crack in the scan that was performed with
the optimum spacing.
The
TOFD method has so far proven to be a useful tool in detecting
and sizing cracks in simple geometries. Through this research
more complex geometries will be tested to determine the capabilities
of this method. By accurately detecting and sizing cracks, engineers
can make better decisions about how to deal with the cracking.
While this research focuses on the inspection of bridges, this
method could be used to inspect many types of steel structures.
Fisher,
J.W., The Evolution of Fatigue Resistant Steel Bridges,
Transportation Research Board, 76th Annual Meeting Lecture Preprint.
Zippel, W.J., Pincheira, J.A., and Washer, G., "Measurement
of Cracks in Steel Elements Using the Ultrasonic Time of Flight
Diffraction Method," ASCE Journal of Performance of Constructed
Facilities, May 2000, pp. 75-82.
