Hybrid CFRP-Aluminum Joint Fatigue Testing for Autonomous Mobile Robot Chassis Under Variable Loads

Autonomous mobile robots (AMRs) demand lightweight, high-stiffness chassis that can withstand repeated variable loads during operation. Hybrid joints combining carbon fiber reinforced polymer (CFRP) and aluminum offer an optimal balance of weight reduction and durability. This case study presents fatigue testing of a bonded-bolted hybrid joint designed for an AMR chassis, using Toray T700S CFRP and 7075-T6 aluminum, with a focus on S-N curve generation and failure mode analysis per ASTM D7774.

Joint Design and Material Selection

The hybrid joint consists of a 4 mm thick CFRP plate (Toray T700S / Hexcel 8552, Vf = 62%, [0/90]₂s layup) bonded to a 6 mm thick 7075-T6 aluminum bracket using Hysol EA 9394 epoxy adhesive, reinforced with four M6 titanium alloy bolts (Ti-6Al-4V) torqued to 12 N·m. The overlap area is 50 mm × 80 mm. The CFRP laminate properties are: E₁₁ = 135 GPa, E₂₂ = 9.5 GPa, G₁₂ = 5.2 GPa, ν₁₂ = 0.30, tensile strength 862 MPa (ASTM D3039). Aluminum 7075-T6: E = 71.7 GPa, yield strength 503 MPa, ultimate strength 572 MPa (ASTM B209).

Fatigue Test Setup and Variable Load Profile

Fatigue testing was performed on a servo-hydraulic test frame (MTS 810) with a 100 kN load cell. The joint was loaded in tension-tension mode (R = 0.1) at 5 Hz. The variable load profile simulated AMR chassis loads during acceleration, braking, and turning: a block loading sequence with three load levels (60%, 40%, and 20% of static ultimate strength) applied in repeating cycles. The static ultimate strength of the joint was determined via quasi-static test (ASTM D7774) to be 28.5 kN (6,406 lbf). The test was run until failure defined as a 20% drop in stiffness or visible delamination/adhesive failure.

Worked Numerical Example: S-N Curve Generation

To generate an S-N curve, three load levels were tested with five specimens each. The maximum cyclic load P_max for each level was:

• Level 1: 60% of ultimate = 0.60 × 28.5 kN = 17.1 kN (3,844 lbf)
• Level 2: 40% of ultimate = 0.40 × 28.5 kN = 11.4 kN (2,562 lbf)
• Level 3: 20% of ultimate = 0.20 × 28.5 kN = 5.7 kN (1,281 lbf)

The corresponding stress amplitude in the adhesive layer (assuming uniform shear stress over bond area = 50 mm × 80 mm = 4,000 mm²) is:

• Level 1: τ_max = 17,100 N / 4,000 mm² = 4.275 MPa
• Level 2: τ_max = 11,400 N / 4,000 mm² = 2.850 MPa
• Level 3: τ_max = 5,700 N / 4,000 mm² = 1.425 MPa

The mean cycles to failure (N_f) were: Level 1: 12,500 cycles; Level 2: 85,000 cycles; Level 3: 520,000 cycles. The S-N curve follows the power law: τ_max = 14.2 × N_f^−0.15 (MPa).

Comparison of Fatigue Performance: Bonded vs. Bolted vs. Hybrid

The hybrid joint exhibited 28% higher fatigue life than the bolted-only configuration and 37% higher than bonded-only at 40% ultimate load. The synergy between adhesive bonding and mechanical fastening redistributes stress and delays failure initiation.

Failure Mode Analysis and Microstructural Observations

Post-test examination using optical microscopy and scanning electron microscopy (SEM) revealed three distinct failure zones:

• Zone A (adhesive layer): Cohesive failure within the epoxy adhesive, with voids and microcracks initiating at the bondline edges.
• Zone B (CFRP laminate): Delamination between 0° and 90° plies near the bolt holes, accompanied by matrix cracking and fiber breakage.
• Zone C (aluminum bracket): No visible plastic deformation; slight fretting wear at the bolt head contact surface.

The hybrid configuration shifted the failure mode from sudden adhesive debonding or catastrophic bolt pull-through to progressive delamination, providing warning signs before complete failure.

Implications for AMR Chassis Design

The fatigue test results demonstrate that hybrid CFRP-aluminum joints can reliably sustain variable loads typical of AMR operation. Design recommendations for chassis engineers include:

• Use a bonded-bolted hybrid joint with at least 4 bolts in a rectangular pattern to distribute load.
• Apply adhesive with a minimum bondline thickness of 0.2 mm to avoid stress concentrations.
• Torque bolts to 60–70% of proof load to optimize load sharing without over-stressing the composite.
• Design for a safety factor of 2.5 on fatigue life based on the S-N curve (e.g., limit cyclic stress to < 2.0 MPa for infinite life).

Conclusion

This case study confirms that hybrid CFRP-aluminum joints are a viable solution for lightweight, fatigue-resistant AMR chassis. By combining the high specific stiffness of CFRP with the ductility and bolt load capacity of aluminum, and using a bonded-bolted configuration, engineers can achieve both weight savings and durability under variable loads. The S-N curve and failure mode data provide a basis for design optimization. For custom hybrid joint design and manufacturing, contact our engineering team.