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1 S U M M A R Y Acceptance Criteria of Complete Joint Penetration Steel Bridge Welds Evaluated Using Enhanced Ultrasonic Methods This report summarizes the research and findings of NCHRP Project 14-35: Acceptance Criteria of Complete Joint Penetration Steel Bridge Welds Evaluated Using Enhanced Ultrasonic Methods, which focused on developing guidelines to evaluate complete joint penetration (CJP) welds in steel bridges based on updated acceptance criteria (developed during this research) and developing proposed modifications to the American Welding Society (AWS) D1.5:2015 Bridge Welding Code . The 2015 edition of AWS D1.5 included Annex K, which provided an inspection procedure and acceptance criteria to apply phased array ultrasonic testing (PAUT) to the inspection of steel bridge welds. These acceptance criteria were workmanship-based and were carried over from previous AWS D1.1:2010 conventional ultrasonic testing (UT) methods. Rather than implement acceptance criteria based on workmanship, this research project focused on tying the acceptance criteria to the criticality of weld flaws using fracture mechanics in a fitness-for-service-based approach. The 2015 edition of AWS D1.5 did not provide means for using alterna- tive UT methods such as time-of-flight diffraction (TOFD) UT or full matrix capture (FMC)âtotal focusing method (TFM) PAUT for inspection of steel bridge welds. These methods are suited to evaluate flaw criticality based on measurements of flaw size rather than amplitude responses. The current version of AWS D1.5 Annex K utilizes encoded line scanning using PAUT sector scans (S-scans) to provide sound coverage for detection and rejection of weld flaws. Although S-scans sweep through a range of incidence angles, line scanning with a set index offset (i.e., distance between probe and weld centerline) will only impact each point in the volume of the weld with a single incidence angle, ignoring beam spread and reflections off of the backwall. Therefore, the maximum amplitude determined during line scanning is typically not maximized compared to the maximum amplitude during raster scanning. This is a very important distinction. Based on this fact, research was performed with the objective of developing an inspection procedure that will diminish the scatter in results due to variability in probe location, flaw location (i.e., transverse and through-thickness location), flaw tilt, and flaw skew. It was determined that a two-part inspection procedure would best meet this objective. The pro- posed procedure would use (1) encoded line scanning to detect flaws above a set amplitude limit and (2) inspection of these suspect locations using manual raster scanning to maxi- mize the amplitude response for determination of acceptance/rejection. In this methodol- ogy, encoded line scanning will provide necessary sensitivity to critical flaws and provide for permanent documentation of encoded results. Manual raster scanning is used to maximize the amplitude response for indications which exceed the flaw detection amplitude limit to evaluate acceptance. This testing will maximize the amplitude response of indications and better ensure that critical flaws are rejected.
2 An analytical parametric study was performed in order to establish the critical flaw size that would be considered rejectable for typical bridge CJP welds, and the results were used to identify the critical flaw size that should be reliably detected and rejected and to establish revised acceptance and rejection criteria. The critical flaw sizes were evaluated based on fracture mechanics using a British Standards Institute BS 7910:2013  fitness-for-service (FFS) approach. It is inherently assumed that all internal flaws are cracks. The critical size of volumetric flaws was also evaluated through review of prior fatigue test results. This review highlighted that the fatigue crack growth threshold of volumetric flaws is highly variable, but when considering the lower bound, it is very similar to that of planar flaws. Therefore, FFS analysis could be applied for volumetric flaws by evaluating them as crack-like and determining the flaw size that will not undergo fatigue crack growth. FFS was performed to evaluate two different failure modes for planar flaws. The first is failure due to fatigue crack growth in order to provide infinite fatigue life. The second is failure due to brittle fracture. The analytical studies that were performed under this task included parametric studies of various plate geometries, welds, residual stress fields, flaw types, flaw sizes, and locations. The results from the analytical program have identified the target critical flaw size. A round robin testing program was performed in order to gain insight into the capabilities of the current technicians in the steel bridge industry and to identify best practices for improved flaw detection and flaw characterization. The round robin testing program was used to determine the minimum flaw size that could be reliably detected with enhanced ultrasonic methods and how the advanced methods compare with the conventional UT method. The round robin experimental testing program was performed by circulating weld flaw specimens to acquire inspection data from PAUT, conventional UT, TOFD, and radio- graphic testing (RT) technicians. The inspection results were compared with the various inspection methods; the comparison included the hit/miss rate, rejection rate, frequency of false calls, accuracy of flaw height and length sizing, accuracy of flaw type characterization, and technician variability. The results of the round robin testing program were used to improve development of future PAUT scanning procedures and acceptance criteria. These results also highlighted that moving forward with the development of acceptance criteria based solely on the mea- surement of the flaw sizeâwith the accuracy and reliability provided with flaw size measure- ments using the current PAUT workforceâis not feasible at present. Therefore, the focus of the research was on improving on the acceptance criteria based on maximum amplitude and flaw length in AWS D1.5 Annex K. An acceptance criteria based on physical measure- ments of the flaw height and length has also been provided in order to allow for alternative UT inspection methods or future technological improvements. Experimental testing of various bridge welds and base materials have highlighted that the acoustic properties may vary considerably, including acoustic attenuation and shear wave velocity. Additional calibration requirements are necessary in order to account for possible differences between the calibration block and test object. Not properly accounting for these acoustic properties could result in large variations of the amplitude of indications in the test object. The experimental results of the acoustic properties of various bridge base materials were used to generate benchmarked material models for ultrasonic inspection simulations using CIVA-UT. These simulations were used to develop requirements on the calibration block material and scanning parameters in order to limit the possible error in reference sensitivity. Simulations of PAUT inspections were performed using CIVA-UT to aid in the initial procedure development. CIVA-UT is an analytical ultrasonic simulation software which can compute ultrasonic beam properties and simulate ultrasonic inspection of various reflec- tors, including weld flaws. The modeling incorporated weld flaws that corresponded to the critical flaw sizes developed previously. These flaws serve as a âlower boundâ flaw set from
3 which an improved acceptance criteria were developed to consistently reject these flaws. As long as flaws of this size or larger are consistently rejectable, the procedure will be effective at removing critical flaws from service. Therefore, the acceptance criteria is grounded in frac- ture mechanics, although it does not use flaw height measurement for evaluation. Rather, the future acceptance criteria primarily uses amplitude for flaw evaluation. The procedure was verified through experimental testing of weld flaw specimens to verify that the detection and rejection of critical weld flaws were improved.