Three-Point Bending Fatigue Tests of Carbon Fiber Reinforced Plastic Using the SEM Servopulser

Significance of the Topic

Carbon fiber reinforced plastics (CFRP) combine exceptional strength and low weight, making them vital in aerospace, automotive and industrial structures. Evaluating fatigue performance is key to ensuring safe, reliable long-term use in critical components.

Objectives and Study Overview

This study aimed to generate the S-N curve for CFRP under three-point bending, then observe fatigue crack initiation and growth in situ using a combined fatigue testing system with scanning electron microscope (SEM Servopulser). Such insights support improved composite design and durability assessment.

Methodology and Used Instrumentation

• Test specimen: CFRP laminates (0/90)/(45/-45) alternating, dimensions 36×5×3 mm.
• Static three-point bending: displacement control at 1 mm/min, support span 30 mm, indenter radius R2, n=4.
• Fatigue tests: maximum stress levels from 377 to 535 MPa (60–85% of 628 MPa average flexural strength), stress ratio R=0.1, frequency 20 Hz, n=2 runs per stress, specimen gold-coated for SEM.
• Equipment: Shimadzu Servopulser EHF-L system, SEM Servopulser high-temperature fatigue tester, Servo Controller 4830, 10 kN load cell, three-point bending jig, Windows software for 4830.

Main Results and Discussion

Static tests yielded an average flexural strength of 628 MPa (SD 50.6 MPa, CV 8.1%). The resulting S-N curve showed specimens that survived 10 million cycles at lower stress levels. In situ SEM observations at cycles 719 and 5,500 revealed progressive damage: initial fiber rupture and matrix cracks under the indenter, followed by matrix cracking on the opposite side as loading continued, demonstrating detailed crack evolution at magnifications up to ×100.

Benefits and Practical Applications

The integration of fatigue testing with SEM enables real-time, high-resolution visualization of crack initiation and propagation. This approach aids composite material development, failure analysis, quality control and lifecycle prediction in industries relying on CFRP.

Future Trends and Possibilities

Advances may include high-temperature fatigue studies, automated image analysis for quantitative damage metrics, expansion to other composite systems, and integration with predictive modeling for accelerated durability assessment.

Conclusion

The SEM Servopulser demonstrated effective fatigue evaluation of CFRP, combining precise dynamic loading control with direct microscopic observation of crack development, offering a powerful tool for composite reliability studies.

References

• Application News No. 01-00794: Three-Point Bending Fatigue Test and Crack Observation of Cracked Specimens Using SEM Servopulser
• Application News No. 01-00792: Three-Point Bending Fatigue Tests of Welded Material Using the SEM Servopulser