Fatigue Resistance of Welds

Ensuring the welds of a large steel bridge meet the requirements for a 100-year fatigue life is an extremely challenging task. This requires meticulous, unconventional control across the entire chain, from design concepts and materials science to process engineering and on-site management.Fatigue Resistance of Welds

The following are specific measures to be taken in welding consumable development, welding procedure qualification, and on-site quality control:

一. Welding Consumable Development and Selection

The goal is to obtain welding consumables with properties matching or even exceeding those of the base metal, and possessing extremely high fatigue resistance.

❶ High-toughness, low-hydrogen, and even ultra-low-hydrogen welding consumables:

▶Core objective: To prevent hydrogen-induced cracking (cold cracking), the starting point for fatigue crack initiation.

▶Measures: Develop and use ultra-low-hydrogen electrodes, fluxes, and flux-cored wires that conform to international standards (such as H4, H5, or even H8 levels in AWS A5.1/A5.5). Strictly test and control the diffusible hydrogen content of the welding consumables.

❷ Precise Matching of Strength and Toughness (Equal Strength or Moderately Super-Strength Matching):

▶Core Objective: To ensure that the strength of the weld metal is not lower than that of the base metal (equal strength or moderately super-strength), while its toughness (especially low-temperature impact toughness) is significantly higher than that of the base metal.

▶Measures: In the design of welding consumable chemical composition, optimize the alloy system (such as the ratio of Ni, Cr, and Mo) to obtain a combination of high strength and high toughness. Materials with high toughness can better prevent crack propagation.

❸ Excellent Metallurgical Quality and Purity:

▶Core Objective: To reduce stress concentration points inside the weld (such as slag inclusions and porosity).

▶Measures: Use advanced smelting technologies (such as vacuum degassing) to produce welding wire and flux, and strictly control the content of harmful impurities such as S and P. For gas shielded welding, use high-purity shielding gas.

❹ Targeted Welding Consumable Design:

▶Core Objective: To improve the microstructure of the weld metal.

▶Measures: By controlling the composition of welding materials and cooling, the weld metal achieves a fine, acicular ferrite structure, which possesses an optimal combination of strength and toughness.

二. Welding Procedure Qualification

This is not merely a “pass” qualification, but rather a systematic testing process to identify and solidify the optimal process window.

❶ Fatigue Performance Qualification Exceeding Standards:

▶Core Objective: To directly verify the fatigue life of the weld.

▶Measures: In addition to routine tensile, bending, and impact tests, fatigue tests on actual or simulated weld joints must be conducted. The test load spectrum should simulate the actual stress conditions of a bridge, applying a sufficient number of cycles (far exceeding the minimum requirements of the specification) to obtain reliable S-N curves (stress-life curves) and fatigue limits.

❷ Strict Control of Heat Input and Interpass Temperature:

▶Core Objective: To control the welding thermal cycle and avoid microstructural deterioration and excessive residual stress.

▶Measures: Through procedure qualification, the allowable heat input range and interpass temperature range for each joint and each weld pass are determined and strictly defined. Excessive heat input leads to coarse grains and decreased toughness; insufficient heat input may result in a hardened microstructure.

❸ Evaluation of Residual Stress Control Processes:

▶Core Objective: To reduce tensile stress in the weld joint area, a key factor affecting fatigue performance.

▶Measures: Evaluate and adopt post-weld stress improvement processes such as hammering (preferably pin hammering), ultrasonic impact, and explosive methods. During process evaluation, stress testing (e.g., X-ray diffraction) is used to verify the effectiveness of these measures in reducing residual tensile stress and even introducing beneficial compressive stress.

❹ Establishment of Weld Shape Control Standards:

▶Core Objective: To minimize stress concentration.

▶Measures: Evaluation should not only focus on internal quality but also specify detailed shape parameters such as weld reinforcement height, transition angle, and undercut depth. For example, a smooth transition between the weld and the base metal is required, the weld reinforcement height should not be excessive, and grinding is encouraged to create a smooth concave profile. Finite element analysis is used to help determine the optimal weld shape.

三. Fatigue Resistance of Welds,On-site Quality Control

This is the ultimate guarantee for transforming R&D and evaluation results into a century-old engineering entity, requiring extremely high execution and consistency.

❶ Extreme Management of Welders and Welding Operators:

▶Qualification: Only welders who pass extremely rigorous examinations simulating the most demanding on-site conditions are allowed to work. Regular reviews and retraining are conducted.

▶Discipline: Ensure that every welder strictly adheres to the qualified welding process parameters (current, voltage, speed), welding sequence, and interpass temperature control.

❷ Full-Process, High-Precision Parameter Monitoring and Traceability:

▶Measures: For critical load-bearing welds on bridges, the use of digital welding equipment with data logging capabilities is mandatory. Current, voltage, speed, heat input, and other parameters for each weld segment are recorded and stored in real time, achieving traceability of quality issues.

❸ Upgraded and Refined Non-Destructive Testing:

▶Standard Enhancement: Adopt acceptance standards that are more stringent than conventional standards (such as GB/T 11345). For example, raising the defect assessment level by one level.

▶Technology Upgrade: Widespread application of Phased Array Ultrasonic Testing (PAUT) to replace or supplement traditional ultrasonic testing. PAUT offers higher defect detection rates, positioning accuracy, and imaging capabilities, more reliably detecting planar defects (such as lack of fusion and cracks), which are the core origins of fatigue cracks.

▶100% Inspection: 100% non-destructive testing is performed on all critical welds and stress concentration areas.

❹Fine On-Site Treatment of Weld Shape and Stress Concentration:

▶Measures:

⑴ Forced Grinding: Fine grinding is performed on all weld initiation and extinguishing points, weld reinforcement, and transition zones with the base metal to eliminate all geometric discontinuities.

⑵ Visual Inspection: Quantitative inspection of weld shape is conducted using weld inspection rulers and even 3D scanners to ensure conformity with the optimized shape established in the process qualification.

⑶ Post-Weld Stress Improvement: Systematically applying qualified hammer or ultrasonic impact techniques on-site to treat fatigue-sensitive areas such as weld toes.

❺ Introduction of Health Monitoring and Fracture Mechanics Assessment:

▶Measures: After bridge construction, long-term health monitoring systems (such as fiber optic sensors and strain gauges) are installed at critical weld locations to monitor stress changes in real time.

▶Concept Update: Based on the “damage tolerance” design concept, assuming the existence of minor defects in the structure, regular non-destructive testing is conducted, and fracture mechanics theory is used to assess the expansion of these defects under 100-year loads, ensuring that they do not expand to critical dimensions within the design life. This is a more scientific and proactive safeguard.

四: Fatigue Resistance of Welds,Summary

Ensuring a 100-year fatigue life requires a shift in mindset from “ensuring no defects” to “proactively improving fatigue performance.” These three aspects are interconnected:

❶ Welding material development provides high-performance “ammunition.”

❷ Process qualification is the “battle manual” for finding the right “ammunition” to use and maximize its effectiveness.

❸ On-site control involves strictly and meticulously executing this “battle manual” and conducting the most rigorous “battlefield monitoring” throughout the process.

A lapse in any of these aspects can potentially compromise the 100-year lifespan. Therefore, this requires a consensus among the owners, designers, construction contractors, supervisors, and material suppliers, as well as the investment of corresponding resources and technology, to jointly achieve this ambitious goal.