Three-Point Bending Fatigue Tests of Welded Material Using the SEM Servopulser

Importance of the Topic

Welded joints are widely used in critical sectors such as construction, transportation and electronics. Fatigue failures of welds can lead to catastrophic outcomes. Reliable assessment of weld durability under cyclic loading is therefore essential for ensuring long-term safety and performance.

Objectives and Study Overview

This study aimed to evaluate bending fatigue behavior of welded specimens composed of Type 304 stainless steel and SS400 structural steel. Static three-point bending tests established baseline flexural strength. Fatigue tests spanning 46 % to 80 % of static strength were conducted. Post-test fracture surfaces were analyzed by EPMA for element distribution and in situ crack propagation was observed using a SEM-integrated Servopulser.

Methodology and Instrumentation

Test specimens (36×4×3 mm) were TIG-welded using TG308 rod with a C1 chamfer around the joint. Equipment configuration:

• Servopulser EHF-L tabletop dynamic and fatigue testing system
• SEM Servopulser high-temperature fatigue testing machine with scanning electron microscope
• EPMA-8050G electron probe microanalyzer
• Servo Controller 4830 with Windows software

Static three-point bending used a 10 kN load cell and 30 mm support span. Fatigue tests ran at 20 Hz with stress ratio R=0.1.

Main Results and Discussion

Static tests yielded a mean flexural strength of 881 MPa (SD 63.8, CV 7.2). S-N curves from 405 to 700 MPa showed fatigue lives decreasing with higher stress. Two specimens under identical peak stress exhibited different failure sites: specimen A fractured at the weld, specimen B in the SS400 parent metal, indicating weld strength exceeded base material in the latter.
EPMA element mapping before testing delineated interfaces between Type 304, TG308 weld and SS400. Post-test analysis revealed Ni and Cr depletion at the fracture center of specimen A, while specimen B showed negligible Ni/Cr signals, confirming failure in SS400.
In situ SEM observation captured crack initiation near the specimen midspan at 1 000 cycles and progressive extension monitored at magnifications up to ×1 000. After 8 000 cycles, crack length increased significantly, demonstrating real-time visualization capabilities.

Benefits and Practical Applications

This integrated approach enables:

• Quantitative S-N characterization of welded joints
• Fractographic and elemental insight via EPMA
• Non-destructive monitoring of crack initiation and growth in SEM
• High-precision dynamic control for reproducible loading

Such capabilities support failure analysis, quality control and design optimization for welded structures.

Future Trends and Opportunities

Advances may include coupling in situ SEM with acoustic emission or digital image correlation for multi-modal fatigue studies. High-temperature fatigue, micro-scale welded joints and automated image analysis hold promise. Integration with machine learning could enable predictive maintenance models based on real-time crack data.

Conclusion

The combination of Servopulser and EPMA provides a comprehensive toolkit for fatigue assessment of welds. Static and fatigue testing paired with elemental mapping and in situ SEM observation delivers deep understanding of crack behavior and weld integrity.

Reference

• Three-Point Bending Fatigue Tests of Carbon Fiber Reinforced Plastic Using the SEM Servopulser Application News No. 01-00793
• Three-Point Bending Fatigue Test and Crack Observation of Cracked Specimens Using SEM Servopulser Application News No. 01-00794