Refinement of Steel Bridge Ultrasonic Testing Techniques
research project 14-35 discovered issues with ultrasonic testing (UT),
including phased-array ultrasonic testing (PAUT) that need to be addressed. The
issues relate to calibration and to how acceptance criteria should be related
to the possibility of cracks.
calibration, 14-35 discovered that the current calibration methods are lacking
because they sometimes fail to adequately account for sound velocity and
attenuation. Current methods were developed in the 1960s and rely on
calibration blocks made of A36 steel. Project 14-35 found differences in sound
velocity and attenuation between A36 and A709 steels, particularly for steels
made using thermo-mechanical controlled processing (TMCP). It makes sense to
account for these differences in calibration, but it is not known how much
correction is needed, such as whether calibration can be addressed by using a
block made from the same grade of steel or if calibration to the heat or plate
acceptance criteria, 14-35 found that current methods may not find some cracks.
From decades of practice, it is known that RT and UT as are effective in
discovery of workmanship discontinuities, and work by Florida DOT demonstrated
that PAUT acceptance criteria of D1.5 Annex K are comparable to RT and UT. However,
project 14-35 discovered that current criteria will not reject cracks that are
of a critical size. D1.5 does not allow any cracks, but when indications are
found in welds, it is not always clear whether the indication is a crack. The
14-35 investigator proposed acceptance criteria intended to preclude cracks,
but it was not within the scope of the 14-35 study to try these out in the
shop, so it is not known how effective they would be. More work is needed to
better correlate Code acceptance criteria for RT, UT and PAUT with critical
crack sizes and establish how methods should be adapted to ensure such cracks
objectives of these study are
how calibration should be improved for UT and PAUT to effectively account for
velocity and attenuation differences in bridge steels
how UT, PAUT, and RT acceptance criteria should be adapted to ensure successful
performance of bridge welds, including the suitability of the performance
criteria proposed in study 14-35.
project will improve nondestructive evaluation of welds, improving the accuracy
of test results and increasing confidence in NDE methods and bridge welds.
Improved confidence will facilitate the replacement of RT and traditional UT
with PAUT. PAUT is a better method for evaluating welds because
to traditional UT, it facilitates evaluation at a broad variety of angles,
improving the characterization of discontinuities. Further, with encoding, PAUT
creates a permanent electronic file of the test results that can be reviewed at
any time; by contrast, with traditional UT only the UT technician’s reports and
notes are available.
does a better job of evaluating the weld than RT because RT can only provide a
flat-plan view of the weld; this means that RT lacks information about
discontinuity depth, and it is likely to miss thin planar discontinuities that
are parallel or near parallel to the X-ray or gamma ray propagation such as
incomplete fusion or tight straight cracks. These types of discontinuities are
considered more detrimental to the welded member and cracks regardless of size
are not permitted.* PAUT
is less intrusive and time consuming in the shop than RT. Thus, the move to
PAUT will make fabrication more efficient.
following are found in the existing body of knowledge:
described above, the NCHRP studied the use of PAUT for bridge welds under
project 14-35 at Purdue University. The study 1) provided recommendations for
critical flaw sizes, including recommendations for addressing stress
concentrations in thickness transitions; 2) raised concerns about the skill of
PAUT technicians based on round robin testing and provided recommendations for
addressing skills; 3) discovered calibration issues related to sound velocity
and attenuation (described above); and 4) provided recommended changes for
scanning to D1.5 Annex K (described above). The results of this study air
published in NCHRP Report 908.
DOT conducted a comparative study of RT, UT and PAUT (described above) and
found good agreement, including what they describe as promising results
FHWA studied four welds with built-in defects using PAUT. Calibration was
performed on a calibration block made of A1018 steel. They found that scanning
from two index points would probably be necessary to get weld coverage and plan
a second phase to evaluate the 6-db drop method of evaluating welds
(Development of Phased-Array Ultrasonic Testing Acceptability Criteria (Phase
1), FHWA Publication No. HRT-14-074).
done by the Indian Institute of Technology found that weld crown ripples can
compromise evaluation when using the third leg but that use a dual-matrix array
with a transmit-receive longitudinal wave is very effective for far-side bevel
evaluation without removing weld reinforcing.
DOT studied the use of PAUT for corrosion evaluation and found it to be very
effective for mapping and loss-of-section evaluation but that rough surfaces
make evaluation difficult.
research tasks include
literature review and light synthesis of current practice, including which
fabricators and owners have used PAUT on bridge projects
the velocity and attenuation of a broad sample of the types of steel used in
bridge fabrication, including the variety of processing. From this sampling,
establish how much variation in velocity and attenuation can be expected in
round-robin testing using PAUT on bridge steels including both D1.5 Annex K and
NCRHP Report 908 criteria as well as RT and UT. Then compare the results and
assess the criteria for their effectiveness in evaluation bridge welds.* Develop
proposed changes to D1.5 based on the results of the study.
the report and recommendations.
results of this study will be presented to the AASHTO/AWS Bridge Welding Code
committee as well as AASHTO CBS committee T-17 for consideration of adoption
|Sponsoring Committee:||AKB20, Steel Bridges
|Research Period:||12 - 24 months|
|RNS Developer:||Ronnie Medlock, Member AKB20|
|Source Info:||Ronnie Medlock|
VP Technical Services
High Steel Structures, LLC
1915 Old Philadelphia Pike
Lancaster, PA 17602
|Index Terms:|| |
Bridges and other structures