Welded steel bridges consist of many fracture critical members (FCMs) that are require periodic non-destructive testing (NDT) for fatigue crack detection. Fatigue cracks develop for several reasons including [1]:
- Welded details that are susceptible to fatigue cracks
- distortion-induced fatigue cracking
- Many bridges are over 90 years old
- Corrosion fatigue cracks, and
- Increases in the number of heavy vehicles leading and d allowable leading to significant accumulation of loading cycles.
The article summarizes the most used conventional and advanced non-destructive testing technologies for bridge fracture critical member inspections.
Visual Inspection of Steel Bridges
Visual inspection of steel bridges is by far the most common and cost-effective method to examine bridge steel for visible fatigue cracks. The bridge inspections are carried out mainly in accordance with American Welding Society (AWS) D1.5, Bridge Welding Code (BWC), Chapter 6 Inspection and the National Bridge Inspection Standards (NBIS) require that bridge FCM be inspected every 24 months. NBIS Bridge Safety Inspectors are required to attend and pass two and one-half week bridge safety inspection training courses prior to leading any inspections.
Many state DOTs publish Bridge Safety Inspection Manuals that detail the minimum recommended inspection reporting requirements and supporting non-destructive testing techniques. PennDOT’s bridge inspection manual can be reviewed here:
An example fatigue crack indication on a steel girder is shown in Figure 1. Wet visible magnetic particle testing (WVMT) was used on an area over which paint cracking was observed along the weld toe line. Upon removal of the paint and proper surface preparation, a light coating of white contrast paint was applied to the welded area. Black wet bath was applied while magnetizing the area and a reproduceable indication from the crack was reproduced.
Figure 1: Wet visual magnetic particle indication along toe line of longitudinal stiffener to web fillet weld.
Ultrasonic Testing of Fracture Critical Members
The majority of FCMs are tested non-destructively with straight beam ultrasound, shear wave ultrasound, and phased array ultrasound. As defined by the Federal Highway Administration (FHWA) Bridge Inspector’s Reference Manual, the more commonly FCM tested with ultrasound include:
- Steel girders
- Steel trusses tension members
- Steel box girders
- Steel hangars
- Suspension cables
- Steel ties of tied arches or trusses
- Pin-and-hanger assemblies
- Steel floor beams or cross girders
A steel girder may have various welds that require inspection including complete joint penetration (CJP) flange splice welds, stiffener CJP welds, and partial joint penetration (PJP) stiffener to web and/or flange welds. A example of flange transition weld is shown below in Figure 2.
Figure 2: Flange transition weld may be inspected visually, with ultrasound, radiography, magnetic particle testing and alternating current field measurement (ACFM) methods.
Phased array testing (PAUT) has emerged as a leading technology for CJP and even PJP joints where radiography is not practical or a thick paint coating prevents visual or magnetic particle inspections. Prior to inspection, a phased array scan plan for is created to visualize inspection coverage in the specified joints. In many cases, CAD drawings are imported into the NDT visualization software to optimize ultrasonic testing results.
Bridge Weld Testing with Alternating Current Field Measurement
Steel bridge fillet welds may crack along the fusion zones of the weld-to-stiffener and weld-to-web. Fatigue cracks may propagate directly through the throat of the fillet weld. Fatigue cracks may also initiate at the toes of the weld and propagate into the web and/or stiffener as shown in Figure 3.
Alternating current field measurement (ACFM) was developed to detect surface and near surface fatigue cracks on uncoated and coated steel structures. The main advantage compared to visual testing or magnetic particle testing is that minimum surface preparation is required. This is a major asset during steel bridge tests since surface preparation is often performed from an aerial lift device and additional safety factors must be considered. In many cases surface preparation is more time consuming than the actual visual or magnetic particle testing which has also cost implications on traffic control. While ACFM will never completely replace magnetic particle and liquid penetrant non-destructive testing techniques, there are many suitable applications for which is an excellent alternative. Compared to eddy-current testing, ACFM is less less sensitive to lift-off variations. Due to the benefits of 1) non-contact testing, 2) minimal cleaning, and 3) quantitative detection, the ACFM technique is now widely used for steel weld inspections.
Figure 3: Likely fatigue crack locations in bridge girder stiffener fillet welds.
Guided Wave Ultrasonic Testing of Bridge Cable
Guided wave ultrasonic testing (GWUT) is used as a screening tool for oil and natural gas pipelines, bridge pile, and railroad track. The technology provides real-time feedback on structural condition without the need for direct access to the area of interest. Cable inspection typically takes about 30 – 60 minutes or up to 90 minutes if analysis is performed on-site. The sensor is installed on bottom end of cable. A low frequency ultrasonic wave transmitted sensor to top end of cable – typically the main cable or upper socket. Similarly, a guided wave is transmitted downwards to the suspender cable socket.
The ultrasonic wave is reflected from wire breaks and other cross-sectional area losses. The analysis identifies %CSA loss by location and severity: minor, moderate, and advanced. Follow-up visual and/or magnetic flux leakage inspections may be recommended based on inspection results. Example GWUT data is shown in Figure 4 to familiarize the reader with the process. A primary objective before analyzing the data is to establish adequate signal-to-noise (SNR) ratio and accurate distance range. High SNR is established by sweeping frequency to find the natural frequency of the ultrasonic wave in the cable. SNR is confirmed through observation of high amplitude reflections from the lower and upper sockets. Range accuracy is established through fine tuning the wave velocity in the cable.
Figure 4: Example GWUT bridge cable data.
Magnetic Flux Leakage Testing of Bridge Cables
MFL testing of cable and steel cables introduces a magnetic field along the primary axis of the cable using a magnetizing measurement head. CSA losses, cause a disruption in the magnetic field causing it to leak out from the cable. The MFL is detected by a Hall sensor in the measuring head which converts the signal to a voltage for digitization. The measuring head is generally equipped with an encoder wheel to accurately track wire break locations.
The cable testing data are presented as milli-voltage (mV) versus distance correlated to the encoder wheel. The measured mV on the vertical axis is proportional to the magnetic flux leakage caused by cable breaks. The upper data (OUT) is more sensitive to breaks in the outer strands. The bottom data (INN) reports problems towards the inner strands.
Figure 5. Magnetic flux leakage (MFL) testing of bridge cable.
[1] Fatigue cracking in welded steel bridges, JW Fisher, CC Menzemer - Transportation Research Record, 1990.