Abstract

Thermoplastic composite sandwich structures are increasingly adopted in lightweight vehicle architectures due to their high specific energy absorption and rapid manufacturing potential. However, their application in crash-critical components remains limited by unstable failure modes, particularly under dynamic loading conditions. This study investigates carbon fiber reinforced polyamide 6 (CF/PA6) sandwich structures designed for controlled crash energy absorption in automotive applications. Two structural concepts are evaluated: a classical foam-type core system and a load-bearing corrugated composite core. Experimental and component-level crash results reveal that failure behavior is governed not only by material strength but by the interaction between laminate architecture, core stability, and interface strength. Adhesive bonding is identified as a critical limitation, while thermoplastic welding methods demonstrate superior performance. The findings establish that stable energy absorption requires a transition from passive core stabilization to active structural participation, redefining sandwich design as a co-engineered system. The study provides a framework for designing thermoplastic sandwich structures that achieve controlled progressive crushing and meet system-level crash requirements.

Keywords

Thermoplastic composites; Sandwich structures; Crash energy absorption; CF/PA6; Progressive crushing; Structural core; Ultrasonic welding; Failure modes; Automotive lightweighting

1. Introduction

The demand for lightweight yet crash-safe structures in modern vehicle design has intensified with the integration of new energy systems such as hydrogen storage. Unlike conventional metallic structures, thermoplastic composites offer high specific energy absorption (SEA), rapid processing, and recyclability. However, their behavior under crash loading differs fundamentally from metals. Instead of plastic deformation, composite structures dissipate energy through progressive fibre fracture, delamination, and controlled crushing.

In sandwich structures, this behavior becomes more complex. While the skin layers are responsible for load carrying, the core stabilizes the structure against buckling. Under crash conditions, this classical division of roles is insufficient. Instability, delamination, and brittle failure modes can lead to catastrophic loss of energy absorption capability.

This study investigates thermoplastic composite sandwich structures designed for crash applications, focusing on the transition from strength-based design to failure-mode-controlled design. The objective is to identify how structural configuration, bonding method, and core architecture influence crash performance at both coupon and component levels.

2. Literature Review

Thermoplastic composite crash structures have been widely studied in monolithic configurations, where high SEA values (60–70 kJ/kg) have been achieved through fibre crushing mechanisms. However, extending this behavior to sandwich structures introduces additional failure modes:

• Core crushing instability
• Skin-core delamination
• Laminate splitting under lateral loads
• Sudden buckling and collapse

Traditional sandwich designs rely on foam or honeycomb cores that act as passive stabilizers. While effective under static loading, these cores often fail to maintain structural integrity under dynamic crash conditions.

Bonding methods further complicate performance. Adhesive bonding, commonly used in thermoset systems, exhibits limited shear strength and poor compatibility with thermoplastic matrices. Recent studies have shown that thermoplastic welding methods, such as ultrasonic welding, can significantly improve interface strength and durability.

Despite these advances, there remains a lack of system-level understanding of how core architecture, laminate design, and bonding interact to determine crash performance. This study addresses that gap.

3. Methodology

3.1 Structural Concepts

Two sandwich configurations are investigated:

1. Foam-type core system
   • CF/PA6 skins
   • Low- to medium-density core (wood-based or foam equivalent)
   • Adhesive bonding
2. Structural corrugated core system
   • CF/PA6 skins
   • Corrugated CF/PA6 organo-sheet core
   • Adhesive or welded interface

3.2 Design Requirement Derivation

Crash performance targets are derived from system-level energy requirements:

• Total crash energy: ~67 kJ
• Target absorption per structural element: ~12 kJ
• Required mean crushing force: ~67 kN
• Crushing stroke: ~180 mm

This establishes a direct link between vehicle-level safety requirements and material/structure design.

3.3 Experimental Approach

The evaluation includes:

• Quasi-static compression tests
• Dynamic impact testing
• Component-level crash validation

Key variables:

• Core density and geometry
• Laminate layup orientation
• Bonding method (adhesive vs. welding)

4. Results

4.1 Foam-Type Core Performance

At coupon level:

• Stable crushing forces of 80–110 kN achieved
• Progressive crushing observed under controlled conditions

At component level:

• Catastrophic failure occurred
• Longitudinal cracking and laminate peeling dominated
• Loss of load-carrying capacity

4.2 Failure Mode Analysis

Failure mechanisms include:

• Lateral instability due to insufficient transverse reinforcement
• Delamination driven by weak interface strength
• Core collapse leading to loss of structural support

The mismatch between laboratory and real-world behavior highlights the importance of system-level validation.

4.3 Bonding Performance

Measured interface strengths:

• Adhesive bonding: <7.6 MPa
• High-performance adhesive: ~25 MPa
• Ultrasonic welding: 27–37 MPa

Simulation indicates that stable crushing requires interface strength exceeding 20–30 MPa. Adhesives fail to meet this threshold consistently.

4.4 Corrugated Core Performance

The corrugated core system demonstrates:

• Stable crushing behavior
• Mean force levels of 60–70 kN
• Increased energy absorption under inclined loading

At component level:

• Total energy absorption exceeds 40 kJ
• No catastrophic failure observed
• Controlled progressive crushing achieved

5. Discussion

5.1 Failure is a System Property

Crash performance is not governed by a single parameter but by the interaction of:

• Laminate architecture
• Core behavior
• Interface strength

Optimizing one element in isolation does not guarantee system stability.

5.2 Adhesive Bonding as a Limitation

Adhesives introduce:

• Insufficient strength
• Inconsistent performance
• Sensitivity to loading rate

Thermoplastic welding methods overcome these limitations by creating material-continuous interfaces.

5.3 Structural Core as a Design Shift

The transition from passive to structural cores represents a fundamental change:

• Core participates in load carrying
• Energy absorption is distributed
• Stability is improved

This reduces dependence on interface strength and enhances robustness.

5.4 Implications for Industrial Design

For thermoplastic sandwich structures:

• Design must start from energy absorption requirements
• Failure modes must be engineered, not avoided
• Core architecture must be co-designed with skins

This represents a shift from material selection to system engineering.

6. Conclusion

This study demonstrates that thermoplastic composite sandwich structures can achieve stable crash energy absorption when designed as integrated systems. While foam-type cores provide initial feasibility, their limitations under dynamic loading restrict their application in high-performance structures.

The introduction of structural cores, combined with high-strength thermoplastic bonding methods, enables controlled progressive crushing and meets system-level energy requirements. The findings confirm that future developments in thermoplastic sandwich technology will depend not on interface improvements alone, but on the co-design of structure, material, and failure behavior.

References

1. David, C., Vohrer, S. (2022). CF/PA6 Sandwich Structures for Crash Energy Absorption. ITHEC Conference.
2. Grünewald, J. (2018). Thermoplastic Sandwich Structures for Helicopter Applications.
3. Farley, G. (1983). Energy absorption of composite materials.
4. Mamalis, A.G. (2001). Crashworthiness of composite structures.
5. ASTM D7136 – Impact Testing of Composite Materials.