Emerging Applications of High Performance Steel and Ultrasound Impact by qcq15579

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									    EMERGING APPLICATIONS OF HIGH PERFORMANCE
    STEEL AND ULTRASOUND IMPACT TREATMENT FOR
       IMPROVED FATIGUE LIFE OF STEEL BRIDGES
                    Verma K. K.1; Statnikov E. S.2; and Tehini L.3
Abstract
With the advent of high performance materials, emerging advanced technologies and emphasis on the Federal
Highway Administration’s (FHWA) Strategic Goals to mitigate congestion during construction as well as
during the service life of a structure, the national perspective in the highway industry is changing towards
rapid construction of highways and bridges; as well as to the deployment of maintenance techniques that can
be performed quickly, economically, and effectively to achieve long lasting structures to safely transport
people and goods.

This paper highlights recent advances in High Performance Steels (HPS) and maintenance technique
employing Ultrasonic Impact Treatment (UIT) for steel bridges. These bridges will last longer, resist
corrosion and fatigue cracking, and can be maintained efficiently and economically compared to those built
and maintained in a traditional manner using conventional bridge steels. This material (steel) development
effort has led to high strength 50-100 ksi (345 to 690 MPa yield strength), improved weldability, high
toughness, and improved weathering (atmospheric corrosion resistant). Currently, high performance steel
grades: HPS 50W, HPS 70W and HPS 100W are available. For the reasons explained above, this paper will
address (a) Construction of New Structures Using High Performance Steels (HPS) and, (b)
Maintenance of Structures Using Ultrasonic Impact Treatment (UIT).


Key words: Steel Bridges, Steel Plates, Weldability, and Fatigue Cracking.



Construction of New Structures Using High Performance Steels (HPS)

HPS Introduction:
HPS-70W is the first HPS product that was developed under a cooperative research program of the FHWA,
the U.S. Navy, and the American Iron and Steel Institute (AISI). The related research and development effort
has been a model partnership among government, industry, and academia to improve the cost-effectiveness of
highway bridge construction. The three partners, working with universities and State transportation
departments, have contributed key pieces of this program to develop high strength steels with high toughness
and improved weldability, referred to as HPS (4). HPS grades are shown in Figure 1. Their improved
weldability is demonstrated in Figure 2. More details of these grades are reviewed below. Though the
developmental activities relate to Navy shipbuilding and steel bridge industries, this paper will be limited to
the results from the HPS research and development applicable to bridges. New grades of HPS are now
1
  Principal Bridge Engineer, United States Department of Transportation, Federal Highway administration, Office of
bridge Technology, Washington, DC 20590 USA Krishna.verma@fhwa.dot.gov
2 Vice President (Research) Applied Ultrasonics, PO Box 100422 Birmingham, Al 35210
estatnikov@appliedultrasonics.com
3 Former President Applied Ultrasonics, PO Box 100422 Birmingham, AL 35210 lethini@tw-technologies.com
                                                   Page 1 of 16
commercially available for highway bridge construction. Results are said to be “the fastest ever technology
transfer within the bridge construction industry in North America.” The pay-off is realized through cost
savings.

This paper will focus on HPS with improved weldability and toughness. Microalloying additions were an
important part of the alloy design of these grades. The development of these new grades started with a
requirement of low sulfur levels, as low as 0.002% maximum, with calcium inclusion shape control. This
helped provide a basis for improved toughness in all orientations with resistance to lamellar tearing during
welding. Plate production of these improved grades also utilized the optimum processing for high strength
steel plates using either/or/both Quenching and Tempering (Q&T) and Thermo-Mechanical-Controlled-
Processing (TMCP). The prime focus in the research was to develop the following for steel bridges:


ASTM A709 Grades
         HPS 50 W
     - HPS 70W – TMCP to 2” (51 mm); Q&T to 4” (102 mm)
     - HPS 100W – Q&T to 4” (102 mm)

HPS 70W
HPS 70W is the most widely used of the HPS grades(1). The chemistry of ASTM A709 HPS 70W (3) is shown
in Figure 4. HPS 70W is produced by Q&T or TMCP. Grade HPS 70W is available from a number of steel
producers. Because Q&T processing limits plate lengths to 50 ft. (15.2 m) in the U.S., TMCP practices have
been developed on the identical HPS 70W chemistry to produce longer plates to 2” (50 mm) thick. HPS 70W
plates produced by TMCP are available to 1500” (38 m) long depending on weight.

The new grade HPS 70W was an improvement upon a former grade 70W. Lower carbon and sulfur levels, as
well as tighter chemistry ranges are required, including a significant vanadium addition to help with strength.
The chemistry is used to achieve properties to plate thickness of 2” (51 mm) using TMCP and to 4” (112 mm)
using Q&T. The mechanical properties using either processing routes are remarkably similar and robust as
noted in Figures 3 and 5. The only major property challenge has been achieving strength for thicker plates as
shown in Figure 5. This led to an increase in the allowed manganese (Mn) level in thicker plates.

The Charpy-V-Notch (CVN) impact properties for HPS 70W have been consistently well above even the
more demanding fracture critical requirements of 35 ft-lb @-10oF (48J @ -23C). The minimum CVN
requirement is shown in Figure 1. According to steel specification requirements, if the yield strength is over
85 ksi (586 MPa), the CVN test temperature is dropped to –25oF (-32C). Even at this lower test temperature
and with a higher strength level, the minimum CVN requirement is met consistently, as shown in Figure 3.
The data shown in Figure 3 represent quenched and tempered production. Similar CVN results are achieved
for up to 2 in. (51 mm) thick plates produced by TMCP.
The improved weldability of HPS 70W has been demonstrated to allow welding with limited preheat
requirements to 2-1/2 in. (64 mm) thick, whereas the former grade 70W required preheat at thickness
over 0.75 in. (19 mm). To take advantage of this improved weldability, new welding consumables
were developed for submerged arc welding with low hydrogen conditions. The fabrication practices
for HPS 70W were summarized in a document now available from American Association of State
Highway and Transportation Officials (AASHTO)(6).




                                                Page 2 of 16
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HPS 50W
With the successful introduction of HPS 70W, a number of bridge owners requested an HPS version for the
more common 50W grade. They were interested in improved weldability and toughness. Steel companies
established that an HPS 50W could be provided from the exact chemistry of HPS 70W when processed using
conventional hot rolling or controlled rolling. Initial production of HPS 50W has shown excellent CVN
toughness levels, easily meeting the specification requirements.

HPS 100W
The HPS 100W grade is the most recent HPS that has become available. The development of the HPS 100W
grade was again an industry joint activity (5). The development was based upon the Navy work on HSLA-100
using Cu-Ni Copper-Nickel (Cu-Ni) alloying with other alloy additions. HPS 100W has a columbium (Cb)
addition for grain refinement and Vanadium (V) addition to smooth out the tempering curve in this Q&T
alloy. Figure 6 shows the chemistry of the specification. A range of microstructures can be found in the alloy
ranging from martensite in thin/surface locations and acicular ferrite in the thicker locations. It has been
demonstrated that good properties are achievable through 4” (102 mm). The excellent properties on an initial
application for the grade are summarized in Figures 7 and 8. To achieve higher toughness in thicker plates
would require increasing the nickel content. The HPS Steel Advisory Committee is considering evaluation of
the chemistry for bridge applications to 4 in. (102 mm) thick plates.




                                                Page 5 of 16
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Summary
The microalloyinig additions of Cb, V and Ti (titanium) has been beneficial in achieving microstructures with
improved strength in both TMCP and Q&T produced grades of steel. HPS Grade 70W and HPS Grade 50W
steel are being widely used today throughout the U.S.A. in steel bridges; however, HPS Grade 100 steel,
though promising, has had only limited applications for bridges. Since 1997, there has been steady growth in
the use of HPS in bridges. The use of HPS has also been broadening throughout the U.S.A., as noted in
Figure 10.
Initial plate girder bridges were made entirely of HPS 70W. In some cases, this was justified based on
clearance issues. Most current uses of HPS higher strength grade 70W steel plates are in locations subject to
high stresses in flange plates, with HPS grade 50W plates being used in webs and other flange locations
(hybrid design). It is anticipated that HPS 100W will also see its application in special high stressed
locations.


                           FIGURE 10                                            HPS Scoreboard
                                  States with High Performance Steel Bridges
                                                            November 8,,2007
                                                      In process Emile Troup update


                                 1-0-4             0-0-2                 0-0-1       7-4-0

                                                                        0-0-1                                                       3-2-1
                                                                                               1-11-0
                                6-0-0      1-0-0           3-0-0                                                                            VT 1-0-0
                                                                                                          0-0-1            20-2-4
                                                                          4-0-3        3-5-1                                                NH 1-0-1
                                                                                                                        6-3-1
                                        0-0-1                                                   5-0-1 ? 1-0-11                              MA 33-4-0
                                                1-0-1         8-0-0        12-1-1       5-1-4                                               CT 4-0-3
                                                                                                        4-0-1           8-2-3               NJ 7-0-6
                                  2-0-0
                                                                                  1-0-1 0-0-1        14-5-6             5-1-0               DE 0-0-1
                                                   1-0-0                                         0                3-0-3                     RI 0-5-0
                                                                                                 3                                          MD 5-1-4
                                                                          12-0-3                 1          2-0-0
                       Hawaii                                                            0-3-0                                              WV 20-27-34
                                                                                                                                            DC 0-0-1
                                                                                                                0-1-4
               0-0-1   Alaska



                                                                      States with many bridges in service
                                                                      States with a few bridges in service
                                                                      States with bridges in planning or design
                                                               x-x-x In Service - In Fabrication or Construction - In planning or Design



References - High Performance Steels (HPS)
1.   E. J. Czyryca, et.al., “Development and Certification of HSLA-100 Steel for Naval Ship Construction,” Naval
     Engineers Journal, May 1990, pp. 63-82.
2.   Graville, B.A., “Cold Cracking in Welds in HSLA Steels,” Welding of HSLA (Microalloyed) Structural Steels,
     ASM, Nov. 9-12, 1976.
3.   American Society of Testing and Materials, 2007 Annual Book of ASTM Standards, Vol. 01.04, A709, West
     Conshohocken, 2007.
4.   Wilson, A. D., “Properties of Recent Production of A709 HPS 70W Bridge Steels,” International Symposium on
     Steel for Fabricated Structures, ASM International, 1999.
5.   Wilson, A. D., Gross, J. H. Stout, R. D., Asfahani, R. L., Manganello, S. J., “Development of an Improved HPS
     100W Steel for Bridge Applications,” International Symposium on Microalloyed Steels, ASM International, 2002.
6.   American Association of State Highway and Transportation Officials (AASHTO), Guide Specifications for
     Highway Bridge Fabrication with HPS 70W Steel, September 2000, Washington, D.C. (see current edition)




                                                                      Page 8 of 16
Acknowledgments
The authors would like to acknowledge the assistance and contributions of Mr. Alex Wilson of the Arcelor Mittal USA
to reproduce information on development of HPS for steel bridges from papers ( 4 and 5) mentioned above.

Maintenance of Structures Using Ultrasonic Impact Treatment (UIT)
UIT Introduction

In the maintenance area, bridge owners and highway engineers now consider using UIT to increase the safety
and useful life of steel bridges, light poles and traffic sign structures, and components of other structures
subject to fatigue. The UIT method and technology was first suggested by E. S. Statnikov and developed in
the former Soviet Union for shipbuilding under his leadership, with a team from his laboratory, in the early
1970’s. During this time, the principal benefits of this method were demonstrated by achieving much higher
resistance to fatigue cracking than could be accomplished using conventional methodology. The first
demonstration in the United States of the UIT method was in 1995. Research work conducted and/or
evaluated by the authors of this paper have conclusively demonstrated that among strengthening treatment
techniques, UIT has the most beneficial effect on service life improvement of welded structures.
Many in-service steel bridges and traffic structures have fatigue-prone details, such as cover plates, stiffeners,
and socket joints. Operators and owners of these structures monitor these details and often have to retrofit
them mechanically. This is generally costly and not always effective. When UIT is applied, it minimizes the
need for retrofits and extends the fatigue life. UIT helps in arresting and preventing fatigue cracking in welds
and base metals of members by enhancing the geometry of the weld, introducing favorable compressive
stresses, and relieving residual tensile stresses (internal to the weld) due to the welding. By increasing the
resistance of the welded joint material to fatigue cracking, UIT not only increases service life but reduces the
lifetime cost of maintenance, while improving the performance of existing structures.

The results presented in this paper were carried out using the Esonix UIT technology and equipment of
Applied Ultrasonics developed under the guidance of Dr. Statnikov. Though this paper focuses on HPS and
UIT applications for steel bridges, the presentation will briefly identify other industries where HPS material
and UIT technology are being used for improved strength, toughness, and durability for fatigue prone
structures.

Background
The fatigue strength of components with as-welded details has been studied for decades (2, 3, 4). Design S-N
curves exist in most structural codes wherein the fatigue resistance of welded joints has been defined as a
function of detail type and stress range. In this classification system, welded attachments can limit the
serviceability limit state. To avoid unacceptable limits on the design capacity, it is desirable to enhance the
fatigue resistance of common attachment details such as transverse stiffeners, cover plates, gusset plates and
other welded details that experience crack growth from a weld toe and have their resistance defined by
AASHTO Categories C, D, E or E's.(14) Enhancement of fatigue resistance of welded joints by plastic
deformation of the surface and by improvement of weld toe characteristics is well established. It is known that
conventional improvement techniques such as grinding, shot peening, air hammer peening, gas tungsten arc
(TIG) re-melting and welding consumables with improved weld toe characteristics can improve fatigue
resistance of welded details(5). However, these procedures do not provide for a means or method for in-
process control of parameters and, hence, treatment quality, while being manpower intensive, not always
efficient and less environmentally friendly. UIT treatment of the weld toe, on the other hand, is easy to use
and offers an alternative means to rationally deal with these issues. Extensive research and validation testing
have clearly demonstrated that UIT is a very effective means for improving fatigue life of welded members. (6,
7, 8, 9, 10, and 11)


                                                  Page 9 of 16
In 1997 the UIT effectiveness for fatigue enhancement was verified on a full size steel beam at the Turner
Fairbanks Test facility of the FHWA, under the guidance of Dr. W. Wright(11). Based on this successful study
an extensive validation testing and research program was initiated and funded by the FHWA, and undertaken
at the University of Lehigh in Bethlehem, Pennsylvania, under the leadership of Prof. John W. Fisher. During
this study, over 27 full size girder specimens, Figure 1 below, were tested including those fabricated from
HPS 50W and HPS 70W steel. The results of these studies have demonstrated a dramatic increase in fatigue
life of the welded members.(12,13)




                                  Figure 1 – Full Size Girder Specimen
        Note: Cover plates are welded to Tension Flange Plates (Tension Flange Plates are Actually Bottom
        Flange Plates when Assembled and Erected at the Bridge Site)

The fatigue critical detail is at the stiffener fillet weld joint adjacent to the tension flange (see Figure 1).
According to the AASHTO specifications, this welding detail has a category C fatigue classification. All end
welds joining the 1.4 m long cover plates welded to the tension flange at beam ends are fatigue prone and
classified by AASHTO Standard Specifications for Highway Bridges (14) as being a fatigue category E’ detail.

UIT was performed along the toe of the weld using the equipment as shown in Figure 2 with indenters having
a diameter of 3 mm. As a result a smooth uniform groove transition was formed along the toe between the
weld material and the base material as depicted in Figure 3. The beams were tested on a dynamic test stand
with 4 point loading and dead load present. Amsler jacks driven by means of two twin pulsators were used
(sinusoidal loading, frequency 4.3 Hz, R = 0.1 – 0.5). Load measurement and testing criteria were controlled
using strain-gages. In the fatigue prone tension area of stiffeners, the stress range was measured on the tension
girth inside surface, and also for cover plates – on the tension girth exterior surface of the flange plate. The
extent of testing and the fatigue failure criterion were dependent on initiation of fatigue cracks in the tested
detail.




                                      Figure 2. Esonix UIT equipment

                                                Page 10 of 16
                                                                            UIT area
                                                4-7
                weld
                                                                               0,1 - 0,6

                                                               3-6



               Base metal

                                       Figure 3. UIT Treated Area

S-N curves of details treated with UIT are shown in figures 4 and 5 together with the previous test results.
Data for all details show significant enhancement in fatigue life when UIT is performed in comparison with
as-welded details without UIT.




                   Figure 4. S-N Fatigue Curves – Welded Cover Plates




                                              Page 11 of 16
                  Figure 5. S-N Fatigue Curves-Welded Transverse Stiffeners

In 2002, the Texas Department of Transportation also sponsored research at the University of Texas-Austin
under the supervision of Dr. Karl Frank that examined the fatigue life improvement by UIT of socket joints,
which are found on thousands of fatigue-prone traffic sign and light pole structures. The results of this study
once again proved conclusively that through the use of UIT the life expectancy and fatigue characterization of
components such as mast arms in traffic signal poles and the area at the base of light and sign structures are
substantially improved, and in so doing there is a significant increase in the safety and reliability of these
structures. The results are shown in the S-N curve in Figure 8. These tests were conducted with a mean
stress load (Smean) of 20 ksi that is representative of the dead load found on a typical mast arm for a traffic
signal in the field. Subsequent to these tests, traffic signal poles and mast arms have been treated in the field
in Texas as shown in Figures 6 and 7.




       Figure 6. Treatment of the Mast Arm                       Figure 7. Treatment of the Base Pole



                                                Page 12 of 16
                            Figure 8. S-N Fatigue Curves-Welded Socket Joints

UIT Field Application on Steel Bridges
Fatigue cracking on four bridges, built in 1979, on Interstate 66 in Virginia, USA, occurred at the end of the
fillet welds (connection plate fillet weld to the web) at the top end of the connection plates where the
connection plate was not positively connected to the flange. Also, it was observed that the connection plates
for the diaphragms were cut short of the bottom tension flange a distance of 4tw to 6tw.

Some of the particulars concerning each bridge are:

               Structure No.              A                B               C                D
             Year Built                 1979             1979             1979            1979
             Br. Length                 288'             288'             286'            312'
             Max Span                   105'             105'             118'            118'
             No. Spans                     3                3               3                3
             Skew Degree                  58               53              35               36
             No. Diaphragms               68               63              47               52
             No. Con. Pl.                136              126              94              104
             No. Cracks                   35               23               8               21

Structures C and D have one end span that has rolled beams with cover plates instead of plate girders.

Due to the fatigue cracking at the bridge site at the top of the connection plates, retrofitting was done at both
ends of the connection plates. Also, it was decided to carry out retrofitting of the cover plates while contract
work was ongoing on the connection plates.

Two retrofit alternatives were investigated – the conventional retrofit and ultrasonic impact treatment (UIT).
Some of the considerations for using a conventional retrofit technique were as follows:

                                                Page 13 of 16
        1.    The conventional retrofit of the connection plate would require bolting an angle or tee
              to the bottom tension flange and connection plate to make a positive connection. This
              would require field drilling two 7/8" diameter holes in the flange, and possibly two in the
              connection plate, if existing holes could not be used.
        2.    Since the connection plates were a tight fit to the top compression flanges, and cracking was
              observed at these ends of the connection plates, connection plates would require to be field
              welded to the compression flanges.
        3.    For the cover plate retrofit, a splice plate detail would require field drilling of 12 to 16 holes of
              7/8" diameter.
        4.    The time required to complete conventional retrofit details would vary from 1.5 hours per
              connection plate with angle retrofit to 4 or 5 hours per cover plate for the cover plate retrofit.
        5.    The complexity and time required for doing the work made this alternative very expensive
              compared with UIT retrofit application (see below).

In comparison, the UIT option offered the following benefits:

        1.    UIT introduces compressive stress that replaces tensile stress caused by welding and enhances a
              uniform residual stress profile across the weld heat affected zone through residual stress
              redistribution and relaxation.
        2.    The UIT could be applied in minutes (Figure 9) and did not require bridge traffic to be detoured
              or altered except for workers’ access for retrofitting.
        3.    These benefits would save substantial time and reduce traffic disruptions.
        4.    The cost of doing the above treatments per stiffener detail (Figure 10) and cover plate (Figure
              11) would be considerably less than the cost for doing the conventional retrofit.

Due to the above considerations the decision was made to weld the diaphragm connection plates to the
compression flanges, and use UIT to treat the existing vertical fillet weld details at the bottom of the
diaphragm connection plates as well as ends of the cover plates. Weld details were UIT treated for 3"+ on
each side of the connection plates and around the end of the connection plates at the bottom of the diaphragm.
The ends of the cover plates were UIT treated for a 6"+ length on each side of cover plate and across the entire
end of the cover plate. All weld details were treated at the weld toe as depicted in Figures 10 and 11 below.




            Figure 9                        Figure 10 Treated cover Plate                    Figure 11
   Applying UIT on the Bridge                                                          Treated Stiffener Detail

As of this writing, in the USA, UIT has been applied for retrofitting bridges in Colorado, Georgia, Kansas,
Oklahoma, Texas and Virginia. These applications as well as those on traffic signal poles in Texas have
demonstrated that UIT could easily be applied not only in a manufacturing environment in a shop but also in


                                                 Page 14 of 16
the field for routine standard maintenance and repairs on bridge components and traffic signal poles and other
structures.

Conclusion
    1. The results of studies done to date show that the UIT technique successfully enhances the fatigue
       strength of all welded details.
    2. The results on the girder test program conducted at Lehigh University demonstrated that through the
       use of UIT the category E’ cover plate weld details can achieve category C or better fatigue strength.
       The category C transverse stiffener details on the tension flange and web can achieve category B
       fatigue resistance or better.
    1. The results of the pole test program conducted at Texas University demonstrated that through the use
       of UIT the traditionally category E’ socket weld detail can achieve a category C or better.
    2. UIT is an effective means of improving the fatigue life of welded structures not only in new
       manufacturing/fabrication but also in maintenance and repairs.
    3. UIT is an economically viable solution for use in the retrofit and maintenance of steel bridges, traffic
       poles, and other traffic, lighting and sign structures.

References - Ultrasonic Impact Treatment (UIT)
    1.   Inventor’s Certificate 472782. Ultrasonic Head for Deformation Strengthening. Statnikov E.Sh., Zhuravlev
         L.V., Alekseev A.F., Bobylev J.A., Shevtsov E.M., Sokolenko V.I., Kulikov V.F. Inventors Bulletin, 1975, No.
         21.

    2.   Fisher, JW, Frank, KH, Hirt, MA, and McNamee, BM (1970), Effect of the Fatigue Strength of Steel Beams,
         NCHRP Report 102, Highway Research Board, Washington DC.

    3.   Fisher, JW, Albrecht, PA, Yen, BT, Klingerman, DJ and McNamee, BM (1974), Fatigue Strength of Steel
         Beams with Transverse Stiffeners and Attachments, NCHRP Report 147, Highway Research Board, Washington
         DC.

    4.   Gurney, TR, and Maddox, SJ (1971), A Re-analysis of Fatigue Data for Welded Joints in Steel, The Welding
         Institute Report R/RB/E44/7 1, Abington Hall, Abington, Cambridge, UK.

    5.   Fisher, JW, Hausammann, H, Sullivan, MD, and Pense, AW (1979), Detection and Repair of Fatigue Damage
         in Welded Highway Bridges, NCHRP Report 206, Transportation Research Board, Washington DC.

    6.   Lopez-Martinez M, et al., Fatigue behavior of steels with strength levels between 350 and 900 MPa. - Influence
         of post-weld treatments under spectrum loading. Paper D in Fatigue Behavior of Welded High-Strength Steels,
         Report No. 97-30, Royal Institute of Technology, Stockholm, Oct. 1997.

    7.   Janosch, JJ, Koneczny, H, Debiez, S, Statnikov, ES, Troufiakov, VJ, and Mikeev, PP (1995), Improvement of
         Fatigue Strength in Welded Joint (in HSS and in Aluminum Alloy) by Applied Ultrasonic Hammer Peening,
         IIW, Doc XIII-1594-95.

    8.   Haagensen, PJ, et al. (1998), Introductory Fatigue Tests on Welded Joint in High Strength Steel and aluminum
         Improved by Various Methods Including Ultrasonic Impact Treatment (UIT), IIW, Doc. XIII-1748-98.

    9.   Statnikov, E (1997), Applications of Operational Ultrasonic Impact Treatment (UIT) Technologies in
         Production of Welded Joints, IIW/IIS, Doc.XII-1667-97.

    10. Statnikov, E (1997), Comparison of Post Weld Deformation Methods for increase in Fatigue Strength of
        Welded Joints, IIW/IIS, Doc.XIII-1668-97.

    11. Wright, W (1996), Post-Weld Treatment of A Welded Bridge Girder by Ultrasonic Hammer Peening, FHWA,
        Turner-Fairbanks Test Report, 9/29/96.

                                                   Page 15 of 16
12. Fisher J., Statnikov E.S., Tehini L., Fatigue Strength Improvement of Bridge Girders by Ultrasonic Impact
    Treatment (UIT). IIW – Doc. XIII-1916-02.

13. Sougata R., Fisher J., Yen B. (2003), Fatigue Resistance of welded details enhanced by ultrasonic impact
    treatment (UIT). International Journal of Fatigue 25 1239 – 1247.

14. AASHTO, Standard Specifications for Highway Bridges, 17th ed. The American Association of State Highway
    and Transportation Officials; 2002




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