Visualization of Progress of Internal Damage in Carbon Fiber Composite Materials and Mechanism of Impact Strength

Significance of the Topic

Fiber-reinforced plastics (FRP) surpass metals in strength-to-weight ratio, leading to widespread use in structural components of aircraft and automobiles. Carbon fiber reinforced plastics (CFRP) are complex due to anisotropy and resin-fiber interfaces, making detailed understanding of their damage mechanisms critical for reliable design and broader adoption of high-performance composites.

Objectives and Study Overview

This work aims to:

• Visualize internal damage progression in discontinuous thermoplastic CFRP under bending.
• Quantify strain rate and temperature effects on flexural strength via high-speed impact tests.
• Correlate flexural behavior with resin viscoelasticity using time-temperature superposition.

Methodology and Instrumentation

A custom three-point bending fixture was integrated into a Shimadzu inspeXio SMX-100CT X-ray CT system to nondestructively capture internal damage at 0.7 mm stroke increments. CT parameters: 17.5 mm FOV, 512×512 pixels, 0.034 mm voxel size, ~10 min scan.
A Shimadzu hydraulic high-speed impact tester (HITS-P10) with thermostat was fitted with a novel bending jig. Tests covered temperatures from –30 °C to 100 °C and strain rates from 0.01 to 10 /s per JIS K 7074 standard. A high-speed camera recorded surface failure.

Key Results and Discussion

• CT imaging revealed damage initiating at the compressive surface beneath the indenter, deepening with displacement and coinciding with nonlinearity in force–displacement curves.
• Flexural strength increased as temperature decreased and strain rate increased. High-speed imaging showed sequential compressive-layer cracking followed by tensile delamination.
• DMA of the resin produced a master curve and Arrhenius-based time-temperature superposition, enabling prediction of resin modulus at arbitrary conditions.
• Assuming resin modulus controls CFRP flexural strength, predicted strength using time-temperature superposition aligned closely with measured values across conditions.

Benefits and Practical Applications

This integrated approach clarifies dynamic damage mechanisms in discontinuous CFRP, guiding:

• Optimized composite design to mitigate failure modes under bending and impact.
• Accurate prediction of performance across service temperatures and load rates.
• Enhanced reliability in structural applications and feedback for material and process development.

Future Trends and Opportunities

• Combining in situ CT with real-time mechanical testing for other loading modes (tension, shear).
• Extending superposition-based models to continuous fiber composites and different matrix chemistries.
• Integration with finite element simulations to predict damage progression at component level.
• Machine learning analysis of CT datasets to accelerate defect quantification and design optimization.

Conclusion

Advances in X-ray CT visualization and dynamic mechanical analysis enable quantitative elucidation of internal damage and flexural behavior in discontinuous CFRP. Correlating resin viscoelasticity with composite strength through time-temperature superposition offers reliable performance predictions, supporting robust design and wider use of high-performance composites.

Instrumentation Used

• Shimadzu inspeXio SMX-100CT 3D X-ray CT with custom bending fixture
• Shimadzu HITS-P10 hydraulic high-speed impact testing machine with temperature chamber
• VGStudio MAX (Volume Graphics) for 3D image analysis
• Dynamic Mechanical Analyzer for resin viscoelastic properties
• High-speed camera for surface failure observation

References

1. JIS K 7074, Testing Methods for Flexural Properties of Carbon Fiber Reinforced Plastics (1998)
2. Internal observation of CFRTP bending via 3D X-ray CT, Application Note LAAN-C-XX-E031, Shimadzu Corporation (2018)
3. Nakada M. & Miyano Y., Formulation of time- and temperature-dependent strength of unidirectional CFRP, J Compos Mater 47 (2012): 1897–1906
4. Kunio T., Materials System, 6 (1987): 7–19
5. Matsuo T., Nakada M. & Kageyama K., Prediction of fiber-directional flexural strength based on time-temperature superposition, J Compos Mater 52 (2018): 793–805
6. Yano F. et al., Evaluation of strain rate and temperature dependence in 3-point bending impact tests, J Compos Mater (2018, in press)
7. NEDO Innovative Structural Materials R&D Project report
8. Shimadzu Corporation, Application Note LAAN-C-XX-E031 (2018)