Bridge Pin Nondestructive Testing

TECHKNOWSERV CORP. (TKS) provides bridge pin non-destructive testing services to quantitatively assess cross-sectional area loss at hidden shoulders, barrels, and shear planes that are difficult or impossible to inspect visually.  Our non-destructive testing engineering team has inspected bridge pins on over 100 bridges and other civil infrastructure  assets over the last decade.  This blog post summaries from of the insight gained for bridge pin nondestructive testing.


Bridge pin inspection is an important part of bridge fracture critical inspection programs.  Bridge pins are steel cylinders produced by forging, casting, or cold-rolling processes.  The most common bridge pin connection configurations include; bridge pin connected eyebars, bridge pin hangers, bridge bearing connections, hinge and bridge pin connections, bridge pin connected truss cord connections, and others.  


The bridge pin connection can range in complexity from a simple system with two eyebar links or a multi-eyebar system with many links as shown in Figure 1.  Due to the complexity of the bridge pin connection(s) is it absolutely essential to study the engineering drawing prior to bridge pin inspection and to identify the depth of shear planes, potential wear surfaces, shoulders and other changes in cross-sectional area.  The pre-inspection effort is required to improve probability of detection of bridge pin defects while minimizing false positives bridge pin geometric features. 


figure1 Figure 1  Steel bridge pin showing multiple connections points. 


The objective of bridge pin testing is to inspect the volume of the bridge pin for cross-sectional area loss due to wear, corrosion, and in the worst case scenario fatigue cracks.  Bridge pin non-destructive testing focuses particularly on the pin shear planes.  Ultrasonic testing is accepted as the best non-destructive testing technique for bridge pins.  X-ray testing has been used in the past but is rarely deployed these days, compared to ultrasonic bridge pin testing, due to safety issues associated with the radiation and the high cost.


These days there are four ultrasonic bridge pin testing options for the bridge owner:

  • Straight beam ultrasonic pin testing 
  • Angle beam ultrasonic pin testing
  • Phased array L-wave ultrasonic pin testing
  • Phased array shear wave ultrasonic pin testing


Nondestructive Testing of Bridge Pins with Straight Beam Ultrasound

This is the fastest and most economical nondestructive testing method for bridge pins and is typically performed in the 2.25 to 7.5 MHz range.  This method provides a cross-sectional view of the pin and allows the user to size out internal reflectors in 2-dimensions to be passed on for engineering stress analyses.  For a long bridge pin with a small diameter, it is the recommended method.    Usually, the pin is inspected from a single side but to improve probability of detection (POD) it is recommended to inspect the bridge pin from both sides.  Example data from a straight beam non-destructive bridge test is shown in Figure 2.  On the left the bridge pin shown.  The scanning face was 4” diameter.  The steel bridge pin diameter beyond the shoulder was 5”.  The shoulder was approximately 1.5” from the end of the bridge pin.  The A-scan shown on the right displays the internal reflectors from the bridge.  Starting from left to right, the main bang, shoulder, and pin end are observable.  No bridge pin defects are observable.



Figure 2  Bridge pin and bridge pin nondestructive test data.


Nondestructive Testing of Bridge Pins with Angle Beam Ultrasound 

Straight beam ultrasound will have a blind spot equal to the depth of the ultrasonic near field which is dependent on the ultrasonic transducer frequency and diameter.  Therefore, if the bridge pin shoulder threads are required to be inspected, it is often necessary to use angle beams, or shear waves, to target the bridge pin threads directly underneath the scanning face.  Bridge pin inspection with shear waves is typically performed at 45, 60, and in some cases 70 degrees to target near surface relfectors.  Additionally the bridge pin shoulder to main diameter transition is also targeted. 


Phased Array Testing of Bridge Pins with L-waves 

TKS utilizes 16:128 and 32:128 phased array units for bridge pin inspection.  High quality data with excellent resolution is achievable with both units.  For L-wave scanning,  a 2.25 to 5 MHz L-wave probe is commonly used with a water bladder wear plate.   The major advantage of bridge pin testing with phased array ultrasound is the multi-angle inspection compared to straight beam single angle inspection.   Detection of bridge pin defects, oriented non-perpendicularly to the long axis of the pin,  is enhanced as L-waves from 0 to 30 degrees are striking the flaw versus 0 degrees only (not including beam spread).  


The bridge pin phased array (PAUT) display is broken up into a A-scan (left side of the display) and S-scan (right side of the screen).  An examples of the PAUT screen display for and L-wave bridge  inspection is shown is shown in Figure 3.   The purple axis is the depth axis.   The sectoral scan on the right shows the results of scanning and L-wave from -10 to +10 degrees with 1 degree resolution.    The left hand side of the sectoral scan are the negative angles.  The left hand side of the sectoral scan are the negative angles.  The blue line on the S-scan is the current focal law or ultrasonic beam.  The A-scan always displays the current A-scan.  Note that in this case reflector span across the entire S-scan.    The scanning range for L-waves in steel can be increased to +/-25 degrees and in some cases 30 degrees.  



Figure 3  Bridge pin phased array scan of -10 to +10 degrees


Phased Array Testing of Bridge Pins with Shear Waves 

Shear waves in steel can be generated in steel over the 45 to 70 degree range efficiently.  Typically, angular ranges exploit the maximum steerable range of the transducer-wedge-material system.  However in some cases, a more limited range is desirable depending on pin length and diameter.  Phased array shear wave testing is particularly useful for imaging shoulder threads and potential root cracks.  The upper angles, 60-70 degrees, are used to inspect the threads directly underneath the scanning surface. The lower angles, 45-60 degrees, are used to inspect the deeper threads.  An example shear wave S-scan is shown in Figure 4.   The angular range is from 45 to 70 degrees in 1 degree increments.  The 45 degree and 70 degree beams are on the lower and upper side of the S-scan, respectively.  The current shear wave A-scan selected in the S-scan is shown on the left.

Figure 4 For S-wave scanning an S-scan of +45 to 70 degree was used


Bridge Pin Testing Experience

The TECHKNOWSERV team has over 20 years of bridge non-destructive testing experience in visual, magnetic particle, liquid penetrant, eddy current, ultrasound and phased array methods.  Our inspectors are trained in confined space entry for box girder inspections, bucket truck and man-lift operation, and as rope access technicians.   Our team has participated on many signature bridge inspections across the United States with over 100 bridge and civil infrastructure assets.  TECHKNOWSERV provides bridge non-destructive testing research and development, bridge non-destructive testing level 3 services, and AWS D1.1 Structural Welding Code and D1.5 Bridge Welding Code inspections.