CFP of High-Strength Bolts: Micro to Macro Analysis

 Study on Corrosion Fatigue Performance of High-Strength Bolts: From Microstructure to Macro Failure

High-strength bolts, as critical connectors in steel structures, directly determine the reliability of the overall structure. Under cyclic loading, fatigue damage in bolts gradually accumulates. Over time, corrosion inevitably occurs, and the localized stress increase caused by corrosion accelerates the fatigue failure process of the bolts.

Currently, most research focuses on the fatigue performance of uncorroded bolts. Although the issue of corrosion fatigue in high-strength bolts is gradually gaining attention, studies primarily concentrate on the analysis of corrosion products and the degradation of static mechanical properties after corrosion. Research on the axial fatigue performance of high-strength bolts after corrosion is urgently needed. Addressing these issues, based on neutral salt spray corrosion tests of high-strength bolts, this study investigates the macro and micro morphology of the rust layer. Additionally, based on fatigue tests of corroded high-strength bolts, the fatigue fracture surfaces, failure mechanisms, and fatigue life are analyzed.

### 1. Neutral Salt Spray Corrosion Test and Rust Layer Morphology Analysis

To simulate the corrosion conditions of bolts in real engineering environments, high-strength bolts were subjected to accelerated corrosion using a neutral salt spray test. The test conditions were: 5% NaCl solution, temperature of 35°C, relative humidity of 95%, and a spray duration of 24 hours. By comparing different corrosion durations (24h, 48h, 72h, 96h), changes in the surface rust layer morphology of the bolts were observed.

Using scanning electron microscopy (SEM), the micro-morphology of the corroded bolt surfaces was examined. It was found that as corrosion time increased, the bolt surfaces transitioned from uniform corrosion to localized corrosion, with the appearance of distinct pitting corrosion. The formation of pitting corrosion was attributed to the accumulation of corrosive media at surface defects, leading to accelerated local corrosion rates. Additionally, energy-dispersive X-ray spectroscopy (EDS) analysis of the rust layer composition revealed that the primary components were Fe2O3 and FeOOH, indicating that the bolts primarily underwent oxidative corrosion in the salt spray environment.

### 2. Fatigue Test and Fracture Analysis of High-Strength Bolts After Corrosion

To investigate the impact of corrosion on the fatigue performance of high-strength bolts, axial fatigue tests were conducted on bolts corroded for different durations. The tests were performed under stress-controlled mode with a stress ratio R=0.1 and a loading frequency f=10Hz. By recording the fatigue life of bolts under different stress amplitudes, S-N curves were plotted to analyze the effect of corrosion on bolt fatigue strength.

The results showed that as corrosion time increased, the fatigue strength of the bolts significantly decreased. After 96 hours of corrosion, the fatigue strength of the bolts decreased by approximately 30% compared to uncorroded bolts. Furthermore, SEM observations of the fatigue fracture surfaces revealed that uncorroded bolts exhibited typical fatigue striations, while corroded bolts showed secondary cracks and corrosion products, indicating that corrosion accelerated crack initiation and propagation.

### 3. Analysis of Corrosion Fatigue Failure Mechanisms

Combining the analysis of rust layer morphology and fatigue fracture surfaces, the mechanisms by which corrosion affects the fatigue performance of high-strength bolts can be summarized as follows:

1. **Stress Concentration Effect:** Corrosion leads to surface defects such as pitting corrosion on the bolts. Under cyclic loading, stress concentration at these defects accelerates crack initiation.

2. **Hydrogen Embrittlement Effect:** Hydrogen atoms generated during the corrosion process penetrate the bolt interior, reducing material toughness and promoting crack propagation.

3. **Wedge Effect of Corrosion Products:** The accumulation of corrosion products at crack tips creates a wedge effect, accelerating crack propagation.

### 4. Conclusions and Future Perspectives

This study investigated the impact of corrosion on the fatigue performance of high-strength bolts through neutral salt spray corrosion tests and fatigue tests, and analyzed the failure mechanisms. The results indicate that corrosion significantly reduces the fatigue strength of high-strength bolts, with the failure mechanisms primarily related to stress concentration, hydrogen embrittlement, and the wedge effect of corrosion products.

Future research could further explore the effects of different corrosive environments (e.g., acidic, alkaline) on the fatigue performance of high-strength bolts and develop effective protective measures, such as surface coatings and corrosion inhibitors, to enhance the service life of high-strength bolts in corrosive environments.

### 5. Application Recommendations

In practical engineering, to ensure the connection reliability of high-strength bolts, the following measures are recommended:

1. **Material Selection:** Choose high-strength bolt materials with excellent corrosion resistance, such as stainless steel bolts or bolts with surface treatments.

2. **Enhanced Protection:** Apply surface coatings, such as galvanization or Dacromet, to isolate the bolts from corrosive media.

3. **Regular Inspection:** Conduct regular visual inspections and ultrasonic testing of bolts to promptly identify and replace severely corroded bolts.

By implementing these measures, the safety and reliability of high-strength bolts in corrosive environments can be effectively improved, ensuring the overall safety of steel structure engineering.