For autonomous mobile robots (AMRs), every kilogram saved directly translates to increased payload capacity, longer battery life, and improved agility. This case study details how Dongguan Flex Precision Composites partnered with a leading robotics OEM to redesign their AMR chassis from welded steel to a carbon fiber reinforced polymer (CFRP) monocoque, achieving a 60% weight reduction while maintaining structural integrity. We present the engineering methodology, material selection rationale, and a worked numerical example comparing bending stiffness and mass.
The Challenge: Steel Chassis Limiting AMR Performance
The client's existing AMR used a welded AISI 1018 steel frame (density 7,870 kg/m³, Young's modulus 205 GPa, yield strength 370 MPa) weighing 18.5 kg. The robot's total weight was 85 kg, leaving only 35 kg payload capacity. The steel chassis contributed 22% of the total mass but only provided bending stiffness margins of 3.5x, far exceeding requirements. The goal was to reduce chassis weight to under 8 kg while maintaining a minimum safety factor of 2.0 under static and dynamic loads per ISO 10218-1.
Material Selection: Toray T700S / Hexcel 8552 Prepreg
We selected Toray T700S carbon fiber (4,900 MPa tensile strength, 230 GPa modulus) in a quasi-isotropic layup [0/±45/90]₂s with Hexcel 8552 epoxy resin (Tg > 190°C, Vf > 62%). The cured laminate properties (per ASTM D3039) are:
| Property | Value | Test Standard |
|---|---|---|
| Tensile Modulus (0°) | 135 GPa | ASTM D3039 |
| Tensile Strength (0°) | 2,100 MPa | ASTM D3039 |
| Density | 1,580 kg/m³ | ASTM D792 |
| Interlaminar Shear Strength | 95 MPa | ASTM D2344 |
| Glass Transition Temperature | 195°C | ASTM E1640 |
Weight Reduction Calculation: Worked Numerical Example
Consider a simply supported chassis beam of length L = 1.2 m, width b = 60 mm, and required bending stiffness EI ≥ 4,500 N·m² (to limit midspan deflection under a 500 N load to < 2 mm). For steel (E = 205 GPa, ρ = 7,870 kg/m³):
Required I = (EI) / E = 4,500 / (205 × 10⁹) = 2.195 × 10⁻⁸ m⁴ = 21.95 cm⁴.
For a rectangular cross-section, I = b h³ / 12 → h = (12 I / b)^(1/3) = (12 × 21.95 × 10⁻⁸ / 0.06)^(1/3) = 0.0356 m = 35.6 mm.
Mass per unit length m' = ρ b h = 7,870 × 0.06 × 0.0356 = 16.81 kg/m. For L = 1.2 m, mass = 20.17 kg.
For CFRP (E = 135 GPa, ρ = 1,580 kg/m³):
Required I = 4,500 / (135 × 10⁹) = 3.333 × 10⁻⁸ m⁴ = 33.33 cm⁴.
h = (12 × 33.33 × 10⁻⁸ / 0.06)^(1/3) = 0.0408 m = 40.8 mm.
Mass per unit length m' = 1,580 × 0.06 × 0.0408 = 3.87 kg/m. For L = 1.2 m, mass = 4.64 kg.
Weight reduction: (20.17 - 4.64) / 20.17 = 77% for this simplified beam. The actual chassis design achieved 60% reduction due to additional features like inserts and local reinforcements.
Design and Manufacturing Approach
The CFRP chassis was designed as a monocoque structure with integrated mounting bosses for motors, batteries, and sensors. Key steps:
- Layup: Quasi-isotropic [0/±45/90]₂s with local doublers at high-stress areas (motor mounts, payload attachment points).
- Tooling: Match-molded aluminum tool with CTE compensation for autoclave cure at 135°C under 6 bar pressure.
- Post-processing: 5-axis CNC machining (DMG Mori DMU 50) to achieve ±0.05 mm tolerance on critical interfaces.
- Inspection: Zeiss Contura CMM for dimensional verification; ultrasonic C-scan per ASTM E2580 for void content (< 2%).
Performance Validation and Results
The final CFRP chassis weighed 7.4 kg (60% lighter than the 18.5 kg steel original). Static load testing to 1,200 N (2.4x design load) showed a maximum deflection of 1.8 mm, well within the 2 mm limit. Fatigue testing per ASTM D3479 (10⁶ cycles at 500 N) showed no stiffness degradation. The robot's total weight dropped to 73.9 kg, increasing payload capacity from 35 kg to 46.1 kg — a 32% improvement.
| Parameter | Steel Chassis | CFRP Chassis | Improvement |
|---|---|---|---|
| Mass | 18.5 kg | 7.4 kg | -60% |
| Bending Stiffness (EI) | 4,500 N·m² | 4,500 N·m² | 0% |
| Max Deflection @ 500 N | 1.6 mm | 1.8 mm | +12% (still within spec) |
| Safety Factor (Yield/UTS) | 3.5 | 2.8 | -20% (acceptable) |
| Payload Capacity | 35 kg | 46.1 kg | +32% |
Conclusion and Recommendations
This case study demonstrates that a properly designed CFRP chassis can achieve significant weight reduction while meeting structural requirements. For engineers considering similar conversions, we recommend:
- Perform a stiffness-driven design (CFRP's lower density outweighs its lower modulus in bending).
- Use quasi-isotropic layups for isotropic-like behavior; tailor layups for load paths.
- Account for insert pull-out strength and local stress concentrations with doublers.
- Partner with an experienced composites manufacturer to optimize tooling and process control.
Key Takeaways
- Replacing steel with CFRP in an AMR chassis reduced weight by 60%, from 18.5 kg to 7.4 kg.
- A quasi-isotropic Toray T700S/Hexcel 8552 laminate with Vf > 62% provides 135 GPa modulus and 2,100 MPa tensile strength per ASTM D3039.
- Worked example: for a given bending stiffness, CFRP beam mass is 77% less than steel; actual chassis achieved 60% due to design features.
- CFRP chassis increased payload capacity from 35 kg to 46.1 kg, a 32% improvement.
- Proper tooling, autoclave cure, and 5-axis CNC machining ensure ±0.05 mm tolerance and < 2% void content.
Ready to reduce weight in your robotic systems? Contact Dongguan Flex Precision Composites at +86 130 2680 2289 or sales@flexprecisioncomposites.com to discuss your application.
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