Shear Fatigue Testing of Adhesives

Significance of the Topic

Adhesive bonding is a critical joining technique across industries such as automotive, aerospace, electronics and civil engineering. The ability to reliably predict long-term fatigue performance under shear loading is essential for ensuring structural integrity and safety, particularly where adhesive bonds replace or augment traditional welding methods.

Study Objectives and Overview

This study investigates the shear fatigue behavior of four commercial adhesives in accordance with JIS K 6864 (ISO 9664). Using a tabletop dynamic and fatigue testing system, the work aims to generate static shear data and S-N curves, and to characterize failure modes under cyclic loading.

Methodology and Instrumentation

The adhesives tested (TB1160, TB2049/2149, TB2237J and TB3953) were bonded to SUS304 stainless steel substrates with a 25 × 12.5 mm overlap area. Specimens were cured under specified time and temperature conditions for each adhesive type. Static shear tests were conducted at a crosshead speed of 1.2 mm/min (n = 3) to determine baseline shear strength (τR). Fatigue tests were performed at a frequency of 30 Hz, with mean stress set to 35 % of τR and stress amplitude varied by adhesive (e.g., 20–30 % of τR for TB1160). Three replicates were run per condition.

Main Results and Discussion

Static shear strengths ranged from 1.07 MPa (TB1160) to 23.64 MPa (TB2237J). Fatigue testing produced S-N curves that revealed:

• TB1160 and TB3953 exhibited mixed interfacial and cohesive failure modes; low-cycle failures often involved interfacial debonding, while high-cycle tests showed cohesive fracture within the adhesive layer.
• TB2049/2149 and TB2237J displayed higher fatigue endurance with predominantly cohesive failure behavior at elevated cycle counts.

Failure mode analysis underscored the impact of substrate surface condition on fatigue life; cohesive fractures reflect intrinsic adhesive durability, while interfacial failures indicate surface interaction effects.

Benefits and Practical Applications

The methodology enables quality control and comparative evaluation of adhesive systems under standardized shear fatigue loading. It provides insights for material selection, joint design optimization and lifetime prediction in demanding applications such as vehicle body assembly and bonded structures in aerospace components.

Future Trends and Potential Applications

Emerging directions include:

• Integration of surface treatment and environmental conditioning (temperature, humidity) to simulate real-world service conditions.
• Implementation of in situ monitoring (e.g., acoustic emission, digital image correlation) for early damage detection.
• Development of predictive models and digital twins for adhesive joint fatigue life estimation.
• Exploration of novel adhesive formulations with tailored viscoelastic or self-healing properties.

Conclusion

The Servopulser EHF-L tabletop dynamic and fatigue testing system, combined with Servo Controller 4830, effectively executes shear fatigue tests per JIS K 6864. The protocol yields reproducible static and fatigue data, enables failure mode differentiation, and supports adhesive performance assessments for industrial bonding applications.

Instrumentation Used

• Tabletop fatigue testing machine: Servopulser EHF-L
• Load cell: 10 kN
• Test jig: Non-shift screw-type grips
• Control device: Servo Controller 4830
• Software: Windows-based application for Servo Controller 4830