Bridge Crane Main Girder Weld Crack Detection & Repair Guide
📋 Key Summary
The fillet welds joining the flange plate to the web plate on a bridge crane's main girder are continuously subjected to alternating bending stress, making them one of the most stress-concentrated areas in the entire metal structure. Industry maintenance data shows that among bridge cranes in service for more than 15 years, fatigue cracks in the main girder cover plate-to-web fillet welds can occur at a rate exceeding 20%. Once a crack penetrates the cover plate, the girder's load-bearing capacity drops significantly, and under extreme operating conditions, brittle fracture at mid-span may result. This article compares the detection capabilities of three non-destructive testing methods—magnetic particle, ultrasonic, and phased array—then breaks down the applicable boundaries of three repair paths: weld repair, stiffener plate reinforcement, and full girder replacement, providing a basis for selecting graded treatments according to crack depth and location.
Many user units treat main girder weld cracks as a minor issue—"just run a bead over it and you're done." But on-site teardown inspections often tell a different story: simply build-up welding on the crack surface is equivalent to re-encapsulating the fatigue source inside the weld, and the crack will re-initiate just a few millimeters away from the original location. A crack isn't metal "breaking"—it's the inevitable result of stress accumulating over time at a geometric weak point. If repair welding doesn't simultaneously improve the stress path, the crack is only delayed, not eliminated.
Under heavy-duty operating conditions—metallurgy, port operations, shipbuilding—A6 to A8 bridge cranes experience dozens of full-load lifting cycles on the main girder every day. Take a bridge crane with work duty A7, a span of 22.5 meters, and a rated lifting capacity of 50 metric tons: at full load, the bending stress at the mid-span section of the main girder approaches 70% of the material's allowable stress, and the flange plate-to-web fillet welds are precisely the critical stress concentration zones. Kelude has observed over years of European Standard double-girder custom projects that for heavy-duty cranes of this class in service for more than 12 years without dedicated weld inspection, the probability of cracks appearing in main girder fillet welds rises markedly.
Three Methods for Detecting Main Girder Weld Cracks: MPI, UT, and Phased Array Scanning
The first step in dealing with a main girder weld crack is not to rush into repair welding—it's to determine the nature, depth, and orientation of the crack. Choose the right inspection method, and the repair plan can be accurately defined.
Main girder weld cracks can be classified by cause into four types: fatigue cracks, cold cracks, hot cracks, and overload cracks. Among in-service cranes, fatigue cracks predominate. Fatigue cracks typically propagate perpendicular to the principal tensile stress and most often originate at geometric discontinuities such as weld toes, undercut, and porosity. TSG 51-2023 Crane Safety Technical Supervision Regulation and ISO 12480-1 safety regulations for lifting appliances both require that user units perform visual inspection and non-destructive testing of metal structure welds at specified intervals, and that upon finding cracks, the crane must immediately cease use and undergo evaluation and remediation.
Three non-destructive testing methods are commonly used in engineering practice for weld inspection: Magnetic Particle Testing (MT), ultrasonic testing (UT), and phased array ultrasonic testing (PAUT), with Penetrant Testing (PT) and Radiographic Testing (RT) as supplementary methods. MT is extremely sensitive to surface and near-surface open cracks in ferromagnetic materials and is the preferred method for surface crack screening on main girder fillet welds. UT penetrates through plate thickness to detect internal defects but has a detection blind spot of approximately 3 mm near the surface. PAUT uses electronic sector scanning to achieve multi-angle coverage, offering better detection efficiency and defect imaging capability than conventional UT.
Repair paths also fall into three categories: local weld repair, stiffener plate reinforcement, and full girder replacement. Local weld repair is suitable for shallow, short, non-through cracks. Stiffener plate reinforcement targets crack zones caused by insufficient local stiffness. Full girder replacement is reserved for heavy-duty main girders with dense cracking and multiple fatigue sources. The three differ dramatically in cost and downtime—the selection criterion is not "which is cheaper" but the crack's depth, location, and propagation trend. Weld design and stress behavior are also governed by FEM 9.353 welded joint design specification, where the geometric details of welded joints directly determine the fatigue strength grade. In manufacturing main girders for European Standard double-girder bridge cranes, Kelude incorporates non-destructive testing of welds as a fixed step in the Factory Acceptance Test—every fillet weld undergoes MPI, and critical load-bearing welds are sampled by ultrasonic testing at a specified ratio.
Detection Capability Comparison: Sensitivity, Blind Spot Depth, and Applicable Defect Types
Each inspection method has its applicable boundaries—the key is how well it matches the crack type, plate thickness, and site conditions. The following table presents a parameter comparison of five methods across key dimensions.
MT and PT are both surface inspection methods. MT works by the accumulation of magnetic particles at the leakage magnetic field of a defect to reveal cracks; PT relies on penetrant seeping into open defects and then being developed. Their shared limitation is that they can only detect surface-open or near-surface defects—they cannot reach buried defects inside the flange plate-to-web fillet welds of the main girder. This is precisely their limitation in thick-plate main girder inspection. UT and PAUT target internal defects. Conventional UT uses a single-angle sound beam and has a blind zone of approximately 3 mm near the surface. PAUT uses an array transducer to achieve multi-angle sector scanning, covering all fusion faces of the main girder fillet weld in a single pass, with intuitive defect imaging. The difference between laboratory calibration and on-site measured defect detection rates often reaches around 20%—this is the margin that must be allowed for when selecting phased array equipment.
RT is suitable for characterizing volumetric defects (porosity, slag inclusion) but is less sensitive than UT for area-type defects such as cracks, and it requires radiation protection and site clearance on-site, limiting its application in routine main girder weld inspection. For the strength and fatigue design basis of the main girder, refer to the load combination and fatigue strength grade classifications in FEM 1.001 crane design specification and FEM 9.321 main girder design specification.
| inspection method | DetectableDefectType | SurfaceCrackDetectionLSB | DetectionDepth Range | Near-Surface Dead Zone | Field Applicability |
|---|---|---|---|---|---|
| Magnetic Particle Testing (MT)（MT） | Surface and Near-Surface OpeningsCrack | Surface and Near-Surface OpeningswidthApprox.0.01mm | Approx. Below Surface2mm | None | Ferromagnetic Material，High Sensitivity，Requires Grinding/Cleaning |
| ultrasonic testing（UT） | Subsurface EmbeddedDefect、Planar TypeDefect | Notapplicable toVery Near-Surface | Full Plate Thickness | Approx.3mm | Requires Couplant，Operator Experience Dependent |
| Phased Array Ultrasonic (PAUT)（PAUT） | InternalDefect、Weld SeamAll-Position | Superior to Conventional UT | Full Plate Thickness | Approx.2mm | Electronic Sectorial Scan，Intuitive Imaging，high efficiency |
| Radiographic Testing (RT)（RT） | Internal Volumetric TypeDefect | For Planar TypeCrackInsensitive | Limited by Plate Thickness Penetration | None | Requires Radiationguards，Limited Field Operation |
| Penetrant Testing (PT)（PT） | Surface and Near-Surface OpeningsDefect | Micron-Level Opening | Surface Only | None | non-ferromagnetic materialApplicable，low cost |
Weld Crack Repair: Choosing Between Weld Repair, Stiffener Reinforcement, and Full Girder Replacement
The choice of repair method depends less on the welding procedure itself than on three variables: crack depth, crack location, and root cause. Get that assessment wrong, and even a code-compliant weld repair will see the crack return within six months.
Classifying the crack into one of three tiers simplifies the decision:
① Shallow surface crack, depth not exceeding one-quarter of the plate thickness, and outside any fatigue-sensitive zone — local weld repair with back-gouging is sufficient.
② Local through-thickness crack — gouging and re-welding is required to restore the original cross-section.
③ Crack running continuously along the fillet weld between the flange plate and web plate, with multiple fatigue initiation sites — stiffener plate reinforcement or full main girder replacement is the only long-term solution.
Standards describe ideal operating conditions, but the workshop rarely delivers them. Weld repair is only viable when the main girder material is weldable, the weld location is accessible, and the work can be performed under a qualified welding procedure — otherwise the repair itself introduces new residual stress and welding defects. Full girder replacement restores the design life, but it means extended downtime and higher disassembly and reassembly costs. Kelude Heavy Industry's repair plan recommendations for in-service equipment always begin with a root-cause analysis of the crack, followed by a balanced assessment that weighs the equipment's remaining useful life against production scheduling.
📋 Crack Location Assessment
Identify whether the crack is in the cover plate, web plate, fillet weld, or end carriage — location determines the level of intervention
📋 Crack Depth Measurement
Distinguish shallow surface cracks from through-thickness cracks to decide between weld repair and gouge-and-repair
📋 Root Cause Identification
Fatigue cracks and welding defects require different repair strategies
📋 Material Weldability Check
Preheating and welding consumables matching for Q235B (≈S235JR), Q355B (≈S355JR), and other main girder materials
📋 Welding Procedure Qualification
Repair welding must follow a qualified welding procedure
📋 Post-Repair NDT Re-inspection
Re-inspect 24 hours after welding to confirm no new cracks
📋 Static and Dynamic Load Test
Verify the repair at rated load with static and dynamic load tests
📋 Follow-Up Inspection Schedule
First inspection at 3 months, then every 6 months thereafter
| Repair Path | ApplicableCrackDegree | DirectcostOrder of Magnitude | After Repairservice lifeand Limitations |
|---|---|---|---|
| Local Weld Repair（Back-Gouging and Rewelding） | Shallow SurfaceCrack，Depth Not Exceeding One-Quarter of Plate Thickness | Relatively Low | RequiresWelding procedure qualification record (WPQR)，FatigueCaution in Sensitive Zones |
| Excavation RepairWelding | Local Through-ThicknessCrack，Controllable Area | Moderate | Restore Originalcross-section，Requiredcontrol systemresidual stress |
| AddStiffener plateReinforcement | LocalStiffnessAccompanied by InsufficientCrack | Medium-High | AlterationStressDistribution，SuppressionCrackRe-Initiation |
| Full Girderreplacement/Section Replacement | CrackDensely Distributed、FatigueMultiple Source | High Sensitivity（subject to the actual contract） | Restorationdesign life，downtimeRelatively Long |
📖 Related reading: Static Stiffness Limits for Gantry Crane Main Girders: How Positioning Accuracy and Span Determine Deflection | From Millimeter-Level Positioning to Predictive Maintenance: How Smart Cranes Are Rewriting Workshop Safety Standards | Early Warning Signs of Rail Wear and Deformation: The Safety Baseline for RGV Track Inspection
FAQ
Q: What are the respective limits of Magnetic Particle Testing (MT) and ultrasonic testing for weld crack detection on main girders?
A: Magnetic Particle Testing (MT) works only on ferromagnetic material. It detects open cracks on the surface and within roughly 2 mm below it, offering high sensitivity but no penetration into the interior. Ultrasonic testing, by contrast, penetrates the full plate thickness to find buried defects inside the fillet welds between the flange plate and web plate, though a conventional probe has a blind zone of about 3 mm near the surface. The two methods are not interchangeable. In practice, MT is used first to scan the surface, then ultrasonic testing to inspect the interior — only used in conjunction do they cover the full range of defect types in main girder weld seams.
Q: Why do fatigue cracks in the fillet welds between the main girder flange plate and web plate concentrate in the quarter-span zone?
A: The mid-span region of the main girder carries high bending moment, and the fillet welds between the flange plate and web plate are subjected to combined longitudinal shear stress and bending normal stress — a location with a high degree of stress concentration. Following the fatigue strength verification approach of FEM 1.001 Crane Design Standard, the load spectrum factor near the quarter-span point is high and cycle accumulation is rapid, making the weld toe in this zone the most likely site for fatigue crack initiation. This is why weld seams in the mid-span zone are a priority for spot checks during periodic inspection.
Q: What does main girder weld crack detection and repair typically cost?
A: The inspection fee depends on the detection method and scope. Magnetic Particle Testing (MT) of main girder fillet welds is priced per meter, while ultrasonic or phased-array testing is priced by weld length and number of measurement points. Non-destructive testing of an entire main girder usually runs into the low thousands of dollars. Repair costs vary far more widely — local weld repair versus full girder replacement can differ by a factor of several dozen. Full girder replacement involves disassembly, transport, and reinstallation, making it a project in the hundreds of thousands of dollars. The exact figure depends on crack location, depth, and site conditions, and is subject to the actual contract.
Q: How can you tell whether a main girder weld crack is a fatigue crack or an original welding defect?
A: Fatigue cracks propagate as fine, jagged lines perpendicular to the principal tensile stress, typically initiating at the weld toe or undercut, with characteristic beach marks on the fracture surface. Original welding defects — porosity, slag inclusion, lack of fusion — are usually located inside the weld seam, can be pinpointed by ultrasonic or radiographic testing, and have relatively regular shapes. The key distinguishing factor is timing: cracks that appear after years in service are most likely fatigue cracks, while those found shortly after commissioning are more likely welding defects. Only by cross-referencing service life, operating condition, and inspection images can a reliable conclusion be reached.
Kelude specializes in custom manufacturing of 50-ton to 300-ton European Standard (EN) double-girder bridge cranes. Main girder welds are completed in-house under the FEM and EN 13001 Crane Safety Standard systems, with non-destructive testing performed on every weld seam before factory shipment, accompanied by an inspection report. For in-service equipment, we also provide on-site inspection, crack cause analysis, and repair process support for main girder weld evaluation and repair. Specific plans and quotations are subject to the actual contract. For equipment maintenance and structural assessment needs, call 400-086-9590.
