38 N. Manoharan and S. Gururaja to improve the visibility of subtle temperature variations. Pristine specimens and specimens with bondline defect exhibited similar surface temperature behavior throughout the test, where the initial stages of loading showed a decrease in the surface temperature due to the thermoelastic effect, followed by a gradual increase as an indication of damage initiation, and a sudden spike (approximately 20◦C in both configurations) during final failure. The speckle pattern shown clearly distinguishes the adhesive layer from the adherend on either side. The transverse strain (εxx) field showed a uniform distribution across each layer in pristine specimens, whereas a high strain concentration around the location of defect was seen in the other configuration indicating stress localization. Similar trends were seen when comparing the longitudinal strain (εyy) contours, where a uniform strain distribution with high values of strain at the location of crack initiation could be seen in pristine specimens, suggesting a more progressive failure, while early stress redistribution around the location of the defect was observed in the defective specimen, leading to a sudden failure [10]. Shear strains (εxy) showed high values at the interface region where the defect was placed, aligning with a complete interface debond with catastrophic failure at the same location (as seen in the thermograms). A relatively uniform strain field, with uniform gradual damage accumulation leading to final failure with crack progression from the interface through the adhesive layer causing final failure via crack propagation was observed in pristine specimens. In contrast, specimens with bondline defects showed a non-uniform strain field with localized damage, resulting in sudden, catastrophic failure, highlighting the detrimental effects of even a small defect in the bondline. Conclusions and Future Work 2D-DIC and surface temperature behavior of adhesively bonded UD-GFRP specimens under quasi-static tensile loads were analyzed. A clear distinction in the full-field strain and temperature behavior was observed, and the combined data provided insight into failure mode in the presence and absence of process defects. The severity of bondline defects was evident, leading to sudden failure and a reduction in final failure load. Efforts are ongoing to perform cyclic loading on adhesively bonded UD-GFRP specimens. Damage accumulation under fatigue is being studied and the fatigue behavior is evaluated with IRT. The temperature signals throughout the tests are recorded and analyzed in the frequency domain to observe the behavior of the first and the second harmonic components, which have been shown to provide information regarding the damage state of the material [3]. References 1. Borges, C. S. P., et al. ”From Fundamental Concepts to Recent Developments in the Adhesive Bonding Technology: A General View.” Discover Mechanical Engineering, vol. 2, no. 1, 8, 2023. 2. Mishnaevsky, L. Jr. ”Root Causes and Mechanisms of Failure of Wind Turbine Blades: Overview.” Materials, vol. 15, no. 9, 2959, 2022. 3. Zaeimi, M., et al. ”Fatigue Limit Estimation of Metals Based on the Thermographic Methods: A Comprehensive Review.” Fatigue & Fracture of Engineering Materials & Structures, vol. 47, no. 3, 611-646, 2024. 4. Vergani, L., et al. ”A Review of Thermographic Techniques for Damage Investigation in Composites.” Frattura e Integrita` Strutturale, vol. 8, no. 27, 1-12, 2014. 5. Pathak, P., et al. ”Examining Infrared Thermography-Based Approaches to Rapid Fatigue Characterization of Additively Manufactured Compression Molded Short Fiber Thermoplastic Composites.” Composite Structures, vol. 351, no. 118610, 2025. 6. Janeliukstis, R., et al. ”Review of Digital Image Correlation Application to Large-Scale Composite Structure Testing.” Composite Structures, vol. 271, no. 114143, 2021. 7. Wang, X., et al. ”Characterisation of Composite-Titanium Alloy Hybrid Joints Using Digital Image Correlation.” Composite Structures, vol. 140, no. 702-711, 2016. 8. Kuhn, M., et al. ”Fatigue Properties of a Structural Rotor Blade Adhesive Under Axial and Torsional Loading.” Fatigue & Fracture of Engineering Materials & Structures, vol. 46, no. 3, 1121-1139, 2023. 9. Pothnis, J. R., et al. ”Open-Hole Fatigue Testing of UD-GFRP Composite Laminates Containing Aligned CNTs Using Infrared Thermography.” Composite Structures, vol. 324, no. 117557, 2023. 10. Chen, J., et al. ”Nondestructive Testing and Evaluation Techniques of Defects in Fiber-Reinforced Polymer Composites: A Review.” Frontiers in Materials, vol. 9, no. 986645, 2022.
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