Table of contents
The central challenge in hybrid lightweight construction
In modern design concepts for automotive, aerospace and electronics, hybrid assemblies made of various lightweight materials such as aluminum, steel and carbon fiber reinforced plastics (CFRP) are increasingly being joined using adhesive bonding technology. This joining technology enables two-dimensional force transmission and design-optimized geometries, but faces a central technical challenge: different thermal expansion coefficients between the joining partners and adhesives. This so-called delta-alpha problem (delta-a problem) can lead to internal stresses and critical failure mechanisms, particularly under cyclical or process-related temperature loads (European Aluminum Association 2015; Dietrich 2018).
Material-specific thermal expansion coefficients and their effects
Each material has a characteristic coefficient of thermal expansion (α)which describes the change in length as a function of temperature. The range of these values varies considerably between the materials used in mixed construction joints: steel has values of α ≈ 11.5-13.1 × 10-⁶ K-¹, while aluminum has significantly higher expansion coefficients of α ≈ 22-25 × 10-⁶ K-¹. Epoxy resin adhesives are in the range of α ≈ 45-200 × 10-⁶ K-¹, and CFRP composites show strongly anisotropic expansion properties with values between -1.0 and 1.5 in fiber orientation and up to 65 × 10-⁶ K-¹ transverse to the fiber orientation.
fiber (Dietrich 2018).
The resulting relative displacements and stresses that occur during temperature cycles – for example during production processes or during operation in the temperature range from -40 °C to +200 °C – place considerable strain on the bonded joint. Particularly critical are areas close to the glass transition temperature (Tg), in which adhesives change from viscoelastic to elastic or plastic properties, which can have a significant impact on the joint strength and service life (DFR Solutions n.d.).
Damage mechanisms and their technical consequences
The damage to bonded joints caused by different thermal expansion coefficients manifests itself through various mechanisms. Different α-values result in shear and tensile stresses, which can lead to both interface failure and cohesion fracture in the adhesive itself. Adhesive gap thickness and component dimension are decisive factors for the stress distribution. (European Aluminum Association 2015; NPL 1999).
Particularly with CFRP joints, the anisotropy of the expansion must be taken into account, so that the laminate structure and fiber orientation also significantly influence the stress development. This must be taken into account when designing lightweight structures and composites, as the thermal stress is caused both by the mismatch of the joining partners and by the contraction of the adhesive during curing (Dietrich 2018).
Moisture as an additional influencing factor
In combination with temperature, moisture is a decisive additional factor influencing the joint strength. It can significantly change the mechanical properties of adhesives, weaken adhesion to the substrate and accelerate age-related damage such as delamination, cracking or adhesive layer deformation. The interaction with temperature increases diffusion and the hydrolytic degradation process in the adhesive, especially in outdoor applications and electronic components.
Service life prediction and test methods
The service life of bonded joints under alternating thermal loads can be reliably estimated using a combination of accelerated ageing tests, cyclic temperature tests and modern prediction models. Time-lapse tests simulate long-term loads under realistic temperature cycles in order to simulate the failure behavior and the development of cracks in the adhesive. Modern short-cycle prediction methods such as the Stepped Isothermal Method (SIM) or Stepped IsoStress Method (SSM) allow the rapid determination of creep behavior and relaxation effects in connection with the thermal mismatch of different joining materials (NPL 1999).
Fatigue and thermal shock tests record the number of fatigue cycles and the occurrence of damage mechanisms, which are crucial for service life assessment. Experimental results are increasingly being combined with numerical simulations and established test methods such as climate change tests in order to enable practical service life predictions.
Optimized material combinations and adhesive systems
Combinations of adhesive and substrate materials that have similar thermal expansion coefficients are particularly effective in minimizing the risk of thermally induced cracking, delamination or stress formation. Epoxy resin adhesives in combination with metallic substrates such as aluminium or steel are particularly recommended if the adhesive formulation is specifically modified with fillers or flexibilizers in order to reduce the coefficient of expansion. expansion coefficient to that of the metal (European Aluminium Association 2015; Dietrich 2018).
Due to their low modulus of elasticity or high elasticity, silicone adhesives and polyurethane systems offer advantageous properties with widely varying thermal expansion coefficients and reduce thermal cracking and fatigue.
Practical solutions and design recommendations
Several factors are crucial for the successful implementation of reliable bonded joints in hybrid lightweight construction. Optimizing the adhesive system with suitable adhesives and flexibilization helps to reduce stresses. The choice of CFRP laminate structure and the optimization of overlap length, adhesive gap thickness and joining geometry are decisive factors. The process control and temperature management should be selected in such a way that critical areas of the glass transition temperature are avoided (DFR Solutions n.d.; NPL 1999).
Implications for practice
There are concrete practical implications for development engineers in automotive and aerospace, material scientists and quality teams. The analysis and simulation of the delta-alpha problem is essential for the design of reliable, durable bonded joints in hybrid lightweight construction. Test methods such as dilatometry, thermomechanical analysis (TMA) and DSC are key tools for benchmarking and process optimization. The validation of thermal cycles is an integral part of any quality and validation strategy (European Aluminium Association 2015; DFR Solutions n.d.).
Conclusion
The varying degrees of thermal expansion of adhesives and joining parts is a critical factor for the mechanical integrity of modern mixed construction bonded joints. By specifically modifying the adhesive properties, optimizing the component and joining geometry and using established test methods, development engineers can influence the mechanical performance in a targeted manner and minimize the risk of failure (Dietrich 2018; NPL 1999; DFR Solutions n.d.).
List of sources
Dietrich, R. (2018). Analysis of the thermal expansion incompatibility of FRP-metal hybrid structures. Technical University of Munich. Available at: https://mediatum.ub.tum.de/1393107
European Aluminum Association (2015). Joining Dissimilar Materials. Available at:
https://european-aluminium.eu/wp-content/uploads/2022/11/11-joining-dissimilar-materials_2015.pdf
NPL (1999). Cyclic Fatigue Testing of Adhesive Joints. Available at: https://www.researchgate.net/publication/237635154
DFR Solutions (n.d.). Temperature Cycling and Fatigue in Electronics. Available at:
https://www.ekwb.com/wp-content/uploads/2020/05/1-Temperature-Cycling-and-Fatigue-in-Electronics-White-Paper-1.pdf
