Test Speed Dependence Evaluation of Shear Strength of Adhesive and Fracture Observation

Importance of the Topic

Adhesives play a critical role in modern automotive and industrial applications by enabling lightweight assembly, improving seal integrity, and reducing vibration and noise. Evaluating their mechanical performance under varying impact and loading speeds is essential to ensure reliability and safety in components that may experience dynamic forces.

Study Objectives and Overview

This study aims to quantify how shear strength of four commercial adhesives responds to different test speeds and to visualize crack initiation and propagation during high-speed shear fracture. The adhesives examined are ThreeBond TB1160, TB2237J, TB2049/2149, and TB3953, all bonded to stainless steel specimens representative of automotive joint configurations.

Methodology and Instrumentation

Shear tests were conducted at three loading rates: 0.0002, 0.02, and 2 m/s using a high-speed impact testing machine. Load and displacement data were recorded, and shear strength was calculated. High-speed video imaging captured fracture events at up to 500,000 frames per second over a sequence of 128 frames.

Used Instrumentation

• High-Speed Impact Testing Machine HITS-TX with 10 kN load cell and flat-plate grips
• Chuck displacement gauge for real-time displacement measurement
• Hyper Vision HPV-X2 high-speed video camera with stroboscope illumination and macro lens

Main Results and Discussion

• Shear strength systematically increased with test speed for all adhesives.
• At 2 m/s, TB1160, TB2049/2149, and TB3953 exhibited approximately double the shear strength measured at 0.0002 m/s, indicating pronounced rate sensitivity.
• TB1160, a modified silicone, showed greater maximum displacement with increasing speed compared to TB3953, reflecting higher ductility under dynamic loading.
• High-speed imaging revealed distinct crack propagation patterns: horizontal front advancement and vertical crack initiation in the bonded interface.

Benefits and Practical Applications

• This approach allows rapid characterization of adhesive performance under realistic impact conditions relevant to automotive and transport engineering.
• High-speed fracture imaging provides detailed insights into failure mechanisms, guiding material selection and joint design.

Future Trends and Potential Applications

• Extending speed ranges beyond 2 m/s for deeper understanding of extreme impact scenarios.
• Integration of digital image correlation for full-field strain mapping during fracture.
• Application to emerging adhesive chemistries and multi-material assemblies.
• Development of automated data analysis using machine learning to predict rate-dependent behavior.

Conclusion

The high-speed impact testing methodology demonstrated clear test speed dependence of shear strength in automotive adhesives. Combined with ultrahigh-speed video, it offers a powerful tool for evaluating dynamic adhesive performance and elucidating fracture processes.