Monitoring the civil infrastructure with acoustic emission: bridge case studies.
The inspection component of contemporary bridge-maintenance policies and programs extends beyond visual inspection to include a wide range of monitoring methods. These range from inspection that detects and assesses fatigue crack growth, through methods that provide characterization over span lengths, to methods that monitor the overall bridge for displacement and settling. The combination of these diverse inspection and monitoring methods permits not only the detection of fault conditions but also assists in diagnosing the cause of the condition and in recommending follow-up maintenance actions.
Infrastructure maintenance-management programs for load-bearing structures ensure their safe, reliable and economic operation without undue risk to public health and safety. The infrastructure must be fit for service with a probability of satisfactory performance according to established performance functions under both specific service and extreme operating and environmental conditions for the duration of its design life. Accordingly, the total ownership cost of a bridge or any other infrastructure component throughout its lifecycle is affected by both the types of maintenance policies selected and the implementation schedule for these policies as well as the risk associated with unplanned or unscheduled repairs arising from contingencies such as accidental damage.
The benefits of both risk-based and risk-informed approaches used extensively in the nuclear and aerospace industries are starting to be appreciated in the civil infrastructure. Quantitative risk assessment (QRA) is the structured and systematic examination of possible hazards associated with a structure, facility or process. In the risk-based approach, decision-making is solely based on the numerical results of a risk assessment. It places a heavy reliance on risk assessment results than may be currently impracticable for bridge maintenance management due to uncertainties in probabilistic input. The risk-informed approach lies between the risk-based and purely deterministic approaches. Risk-informed input complements the traditional deterministic approach and can be used to reduce unnecessary conservatism in purely deterministic approaches and to identify areas with insufficient conservatism in deterministic analysis. The details of the issue under consideration will determine where the risk-informed decision falls within this spectrum.
The risk-informed approach uses quantitative or qualitative probabilistic risk assessment and risk insights and complements them with bridge engineering knowledge, operating experience, engineering judgment and a broad range of resources to mitigate risk. The ensemble of inspection methods available for bridge monitoring includes, but is not limited to, those in Table 1. Acoustic emission (AE) is a significant resource with a specific role to play in the spectrum of risk-informed bridge maintenance resources both in its own right and as a complement to other monitoring technologies. Acoustic emission detects and locates propagating defects and quantifies their severity. Also, using supplementary information on load and strain, the AE is correlated to the condition, under which the damage occurs. Therefore, as in risk based inspection, AE is used to identify components and areas of a large structure suspected of having active defects, which then can allow cost-effective NDT and further analysis by fracture mechanics to determine the severity of defect from the structural integrity point of view. At this point, the decision to repair, strengthen or replace the damaged component is taken or to re-monitor structure at a later time with a certain schedule or to monitor continuously. This strategy allows early detection of active defects and aids the development of cost-effective priority-based maintenance for complementary NDT depending on the actual damage and its significance for the safety of the structure.
Candidate Sites for Acoustic Emission Monitoring
Inputs into selecting candidate sites for AE monitoring from bridge engineering knowledge, operating experience and engineering judgment include the formal categorization of fracture-critical locations, experience with a specific types of bridges, for which a bridge engineer has responsibility, and bridge-specific experience including inspection and in-service bridge history. A combination of these is generally used to select the sites. Typically, bridge members susceptible to fatigue crack initiation and eventual failure due to fracture are those that receive stress ranges above the threshold stress range for a designated fatigue-detail category, as identified by AREMA-established fatigue-detail categories (A to E') .
Long bridge members that have internal or load-path redundancy and good fabrication details are least susceptible to fatigue cracking. Shorter members such as floor-system components including stringers, floor beams (including the connections) and truss hangers that receive considerably more stress cycles and are subjected to other, usually not designed for out-of-plane stresses, should be investigated more closely. Truss eye-bars, due to a low fatigue-detail category deserve special attention. Also, any bridge members that have been subjected to collision, section loss due to corrosion, fire or any other damage could be subjected to accelerated loss of fatigue life.
Beside drawings of design bridge details, records of past maintenance performed on the bridge are valuable in establishing any fatigue-prone details that may have been inherently built into the structure or introduced afterward. These include tension members that may be subject to stress concentrations, such as sharply coped or welded members, unusual or suspect repairs or those with loosely attached connectors (rivets or bolts). The type of steel used in the fabrication of the bridge is useful to know in establishing the criticality of existing fatigue cracks that may have already initiated.
The history of loading (typical train configurations, car weights and magnitude of annual tonnage) is valuable information for calculating stress ranges and the corresponding number of cycles and determining any member that may have reached its useful and safe remaining fatigue life and progressed to the crack initiation phase. For bridges with two-track loading, the incidence of the tracks being occupied at the same time is also useful for a more accurate fatigue life assessment.
Representative AE Monitoring Locations
Maintenance history complemented by extensive bridge inspection has identified the following bridge structure locations where AE monitoring has been applied most extensively:
Link pin connection
Copes and stringers
Stiffener to weld connection.
These locations are shown in Fig. 1. Other areas where AE has been applied include intermittent welds on cover plates, riveted connections in high-stress zones, cracks in restraint connectors where the web is rigidly fixed to the web of another girder and collision damage. Figure 2 shows a complex hanger eye-bar and link-pin location and an instance of impact damage monitored using AE.
AE Monitoring Procedure
Resonant AE transducers are used in linear or planar arrays to detect the presence and the location of defects and to monitor their activity under normal service loads. In the case of railroad bridges that are normally subject to high loads relative to their design loads, the loading is provided by normal rail traffic. For highway bridges, normal traffic can be used. In addition, proof loads with a loaded truck have been used to apply static and a variety of dynamic loads. Also, traffic management including stopping traffic and allowing it to accumulate, allowing various lanes to proceed and similar controls over the traffic during testing provides load management. In certain areas such as remote communities where special vehicles, such as logging trucks, are active, management of the passage of these vehicles provides for loading management as does the case where other large trucks such as concrete mixers and similar vehicles are present. The sensors are positioned to optimize the detection of the signals coming from the area of interest. An example is shown in Fig. 3.
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Areas of interest, such as long welds, can be monitored using linear location. While the AE data, the severity and location, are valuable inputs to condition assessment, correlations to other sensed data significantly enhance the value of the AE data for maintenance decision-making. Two complementary sensing streams that can provide useful correlations are strain and temperature. In Fig. 4, the results of testing an active crack in a floor-beam web at its connection to a main girder show the location of the source of emission, its correlation to strain and representative AE waveform.
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Standard AE guidelines  for AE testing under controlled stimulation can be adapted to this type of bridge inspection. In Fig. 5, AE activity is defined by the relationship between a stimulus that is conventionally a controlled monotonically increasing load and an activity metric, such as AE counts. Under normal bridge load traffic, the stimulus is the number of fatigue cycles introduced by the passing traffic. The correlations between strain and AE in Fig. 4 provide a basis for assessing activity using the fatigue cycles as the stimulus.
AE activity and intensity are measured to generate an AE source index shown in Fig. 6. This is further complemented with information from other non-destructive testing information to define the Fatigue Assessment Index that, in turn, provides the series of recommendations in Fig. 7 for follow-up action based on the Fatigue Assessment Index. Correlations between AE and laboratory studies of crack growth rates on bridge members that have been removed for repair and from data available in the literature provide a basis for calculation of crack growth rates.
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AE monitoring with strain correlation was applied to a 376-m long open deck bridge carrying two tangent tracks on separate spans located in central Canada. The 20 north track spans of riveted construction were built in 1910 and designed to American Railway Engineering Association specifications of 1908. The 20 south track spans each ranging in length from 16.7 m to 22.6 m were designed according to Canadian Standards Association (CSA)-1950 and CN-1972 specifications and were of welded girder construction fabricated by various contractors during the period from 1963 to 1974.
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Presently, the south track receives roughly 33 million gross tons of annual traffic, (476,000 freight cars and 16,000 locomotives), which is approximately a 115% increase since its construction. Most loaded cars have a gross weight of 120 t but the bridge has recently been subjected to an increasing number of 130 t cars. Fatigue cracks were first detected on these spans approximately 10 years ago at the bottom of the welded end stiffener to web connections. Figure 8 shows a typical fatigue-prone location at the bottom connection of a vertical end stiffener welded to the interior girder web. These cracks were not considered threatening as they initiated at the bearing areas and drilling the crack tips seemed to contain them in the same vicinity.
However, as time progressed, this same type of crack was now being detected at the bottom of intermediate stiffeners where they connect to the transverse brace frames at the middle of the spans. This disturbing situation quickly initiated procedures to monitor the growth of these cracks by CN inspectors at increasing frequencies. AE monitoring supplemented visual inspection with quantitative data on crack activity. The replacement of these spans, estimated at approximately $10 million CDN, appeared to be the inevitable recourse. To avoid a possible premature replacement, an intensive effort was undertaken to research available methods of safely extending the useful life of the south track spans. Operating speeds were reduced and AE monitoring assessed the crack activity levels of critical areas. This AE information was essential for the development of manageable risk strategies required to maintain existing and projected levels of safe train operations to date over the bridge.
Another case study of a 97.5-m long open deck bridge located in Western Canada consisted of one 46-m deck truss, and three deck plate girders measuring 21.8 m, 15 m and 11.7 m in length, two of which are suspended and connected to stringers projecting beyond the end floor beams at each end of the deck truss. All spans were of riveted construction built in 1913 and designed to Dominion Government Class Heavy Specifications of 1908.
The bridge traffic serviced a new intermodal terminal that led to a dramatic increase in total annual tonnage over this bridge. Last year, the bridge received approximately 20 million gross tons of annual traffic--an increase of over 80% from just 2 years ago. The majority of this traffic is comprised of articulated double-stack container cars, which can have axle loads up to 35 tons.
The link-pin connection between the webs of the top stringer and the suspended deck plate girder consisted of a rectangular plate on each side of the main member webs connected by top and bottom pins (see attached photo). The photo shows a typical fatigue-prone location on the link plate at the center of the pin hole.
This connection detail is subjected to considerable out-of-plane bending stresses caused by lateral sway from wind and train motion, which were not considered in the design. In addition the connection is also susceptible to high impact stresses from rail joints and out-of-round and flat rolling-stock wheels. A crack initiating behind the pin nut would not be detected by visual inspection and could propagate quickly and without warning before detected by a visual inspection. Crack propagation in this area could lead to a catastrophic failure before detection by the next scheduled inspection. Signs of distress include unusual sway, wear in the pin connection and rust stains on the surface area of the link-pin connection.
The potential for catastrophic failure at this location was assessed and in light of increased and more frequent heavy axle traffic, measures to mitigate the possibility of any failure was taken beyond the normal inspection procedures in place. Monitoring of these locations by TISEC Inc. using AE and the results enabled the development of a suitable risk management strategy for this location including the projected timely scheduling for the retrofit of the link-pin connection to include a redundant load system.
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A third example is a 1050-m long bridge in Western Canada consisting of 107 spans of various types ranging from through trusses to timber pile trestles. The spans in question are five 48.5-m riveted through trusses built in 1904 by the Dominion Bridge Company and designed to Dominion Government Specifications of the early 1900's. The traffic includes cars from four railroads accumulating an annual gross tonnage of approximately 54 million tons (610,000 cars and 20,000 locomotives). The spans were originally designed for a Cooper's E40 loading and were rehabilitated in the mid-1990's to include a new/strengthened floor system and truss members to accommodate heavier axle cars weighing up to 130 tons having a rating of up to E65.
Related in-service observations include the fact that truss hangers have been known to be the weak link members of through truss spans (several failures of these members have been noted in the railroad industry). These members are particularly susceptible to fatigue distress due to the additional action caused by the floor beams subjecting the hangers to bending stresses along with the designed axial stresses. Bending stresses attributed to floor beam connections were typically not accounted for in the design of hangers. With the introduction of heavier car loading these members have been subject to stress ranges that exceed the threshold stress range for the riveted hanger connection detail. Coupled with increased cyclic loading from more frequent car loading, the fatigue life of the hanger connection at this location was substantially reduced. Figure 10 shows the top hanger connection where cracks have been known to initiate leading to a failure scenario.
As this bridge is a vital link for various railroad loadings destined to overseas markets from international ports, disruption of this traffic due to bridge component failure could not be tolerated. Hanger failure at this location could potentially result in a derailment situation serious enough to put the bridge out of service for many weeks and cause millions of dollars in damage. As the top hanger connections are difficult to inspect and identify initiating cracks, on-going inspection-based maintenance was implemented by TISEC at the hangers at this location. Results of the monitoring substantiated a need for the retrofit of the hangers and provided the Bridges and Structures Department vital information required to schedule and complete the retrofit within the time frame provided by establishing a manageable level of risk.
Multiple data streams of bridge monitoring data, AE and strain provide a basis to assess the condition of a bridge in terms of the fatigue-related defects, to quantify the effect of the condition on structural integrity and to generate recommendations for follow-up maintenance actions.
The authors wish to recognize the contributions of their respective companies in supporting the development and application of AE monitoring over the last 20 years and bringing it to its important role in risk-informed inspection-based maintenance. They also recognize the support of Precarn Inc. for development of much of the intelligent systems technology for data interpretation.
[1.] AREMA Manual for Railway Engineering, Article 1.3.13i, (2004), American Railway Engineering and Maintenance-of-Way Association, Washington, DC.
[2.] ASTM E569-07 Standard Practice for Acoustic Emission Monitoring of Structures During Controlled Simulation, ASTM International, West Conshohocken, PA, www.astm.org, 2002.
D. ROBERT HAY , JOSE A. CAVACO  and VASILE MUSTAFA 
} TISEC Inc., 2755 Pitfield Boulevard, Montreal, QC, H4S 1T2 Canada; ) Canadian National Railways, Walker Operations Bldg, Floor 2, 10229 127 Avenue, Edmonton, AB, T5E 0B9 Canada
Table 1 Bridge monitoring technologies. Strain Vibration Resistance Acoustic Emission Fiber Optic Satellite Imaging Bragg Inclination Long gage Alignment Brillouin Settlement Temperature
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|Author:||Hay, D. Robert; Cavaco, Jose A.; Mustafa, Vasile|
|Publication:||Journal of Acoustic Emission|
|Article Type:||Case study|
|Date:||Jan 1, 2009|
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