Observation of Crack Propagation Process in Aluminum Alloy Specimen Using Air-Servo Microfocus X-Ray CT System

Observation of Crack Propagation in Aluminum Alloy Specimen Using an Air-Servo Microfocus X‑Ray CT System — Summary

Significance of the topic

Fatigue damage from repeated loading is a primary cause of structural failure across many industries. Understanding where cracks initiate, how they propagate in three dimensions, and how deformation evolves prior to final fracture is crucial for improving materials design, predictive maintenance, and safety assessments. Non‑destructive 3D imaging during fatigue testing provides unique insight into subsurface crack growth and deformation mechanisms that surface-only inspections cannot capture.

Objectives and study overview

This study demonstrates integrated fatigue testing with time‑lapse microfocus X‑ray computed tomography (X‑ray CT) to observe crack initiation and propagation in an Al‑5052 notched specimen. The workflow combined static tensile testing to determine maximum load, generation of an S–N curve, selection of fatigue test conditions, and repeated CT imaging during cyclic loading to capture crack growth and specimen deformation over time.

Methodology

- Specimen: Al‑5052 with a central longitudinal notch (dimensions provided in the original study).
- Static testing: Quasi‑static tensile tests (1 mm/min) on five specimens to determine mean maximum test force (~486.9 N).
- S–N testing: Fatigue tests performed across maximum load levels corresponding to 40–90% of the static maximum to establish the S–N curve. Fatigue test parameters included stress ratio R = 0.1, frequency 10 Hz, up to 1×10^6 cycles, typically two repeats per condition.
- Time‑lapse imaging: Based on the S–N results, a fatigue test at 70% of the static maximum load was selected for in‑situ CT observation. After 30,000 cycles imaging was performed every 1,000 cycles; imaging frequency increased when displacement changes indicated accelerated damage.
- Data collection: Cross‑sectional CT slices and 3D reconstructions were used to track crack nucleation, three‑dimensional propagation, and global displacement fields relative to a reference cycle.

Used instrumentation

Key equipment and configuration items used in the study included:

• Servopulser EHF‑LV020k1A for S–N diagram acquisition (table‑top dynamic/fatigue testing machine).
• Air‑Servo Microfocus X‑Ray CT System (inspeXio SMX‑225CTS) integrated with an Air‑Servo Mini fatigue testing stage for in‑CT static/fatigue loading and imaging.
• Load cells: 2 kN for initial S–N tests; 1 kN for in‑CT fatigue experiments.
• Controller: Servo Controller 4830 with Windows software for waveform control and synchronization.
• Manually tightened tensile jig for S–N testing; specimen remained mounted for transport into CT to enable imaging without remounting.

Main results and discussion

- S–N characterization established baseline fatigue behaviour and informed the selection of 70% of static maximum load for the in‑CT experiment.
- Crack initiation occurred between 30,000 and 31,000 cycles; the first observable crack appeared at 31,000 cycles. Final fracture occurred at 36,392 cycles under the chosen loading condition.
- Time‑lapse CT cross sections revealed progressive crack growth: no detectable crack at 30,000 cycles, first appearance at 31,000 cycles, gradual propagation thereafter, and rapid displacement increase near failure.
- 3D reconstructions showed asymmetric crack growth across the specimen width (left side propagated more than right), which correlated with measured surface displacements and localized necking between opposing cracks immediately before fracture.
- Static loading applied within the CT enabled opening of the crack tip for clearer imaging, enhancing visibility of internal crack morphology.
- The servo controller achieved a high‑quality sinusoidal control waveform during cyclic loading, supporting precise synchronization between mechanical loading and imaging.

Contributions and practical applications

- Demonstrates a practical workflow for combining fatigue testing and non‑destructive 3D imaging to capture initiation and growth of internal cracks in metallic specimens.
- Enables direct observation of subsurface damage evolution without specimen sectioning, useful for materials development, failure analysis, and validation of computational fatigue models.
- The integrated setup supports imaging with the specimen maintained in the test jig, reducing handling‑induced variability and preserving crack state between mechanical testing and imaging.

Future trends and potential uses

- Increased automation of time‑lapse imaging scheduling driven by real‑time mechanical signals (e.g., sudden displacement changes) to capture key events more efficiently.
- Combining X‑ray CT with higher frame rate or phase‑contrast imaging to resolve finer crack tip details and early microvoid coalescence.
- Integration with digital volume correlation (DVC) and advanced image processing to quantify local strain fields and correlate them with crack growth mechanisms quantitatively.
- Application to a broader set of materials (composites, high‑strength alloys, additive manufactured components) and multiaxial or variable amplitude loading protocols to expand design‑relevant fatigue data.

Conclusion

The Air‑Servo Microfocus X‑Ray CT System coupled with precise servo control allows nondestructive, time‑resolved 3D observation of crack initiation, asymmetric propagation, and pre‑fracture deformation in an Al‑5052 notched specimen. The approach provides detailed morphological and displacement information that enhances understanding of fatigue processes and supports improved material evaluation and failure prediction.

Reference

1. Nishikawa Y., Yamamoto T., Iguchi S. Observation of Crack Propagation Process in Aluminum Alloy Specimen Using Air‑Servo Microfocus X‑Ray CT System. Shimadzu Corporation Application News. First Edition: Jul. 2026.