Bonded structures

Adhesive bonding technology for new material combinations and load-bearing structures

Modern lightweight construction methods have revolutionized vehicle manufacturing in the recent past. Whether ships, aircraft, automobiles or rail vehicles, or heavy steel construction, the use of the most suitable materials has led to remarkable improvements in performance, weight, shape, safety and comfort. At the same time, cost savings have been achieved. Adhesive bonding technology now enables new combinations of materials and allows previously unfeasible combinations of materials – in transport technology, for example, the bonding of glass and steel, aluminum and magnesium, or fiber composites with metal.

Furthermore, additional functions can be integrated via the adhesive, such as vibration damping, electrical insulation or corrosion protection. This extremely flexible joining technology also leads to a significant increase in the rigidity of vehicles. Today, bonded joints meet the most stringent requirements and can be excellently combined with mechanical joining methods such as clinching, riveting, bolting, and also spot welding. Fraunhofer IFAM provides comprehensive advice on the various possible combinations and has the expertise and equipment to test such load-transmitting bonded joints.

Numerical and experimental verification of bonded structures

 

The verification of bonded structures requires bond-specific procedures due to the specific material properties of the adhesives and the particular material strain in the bonded joint. We use both numerical and experimental methods for this purpose. An accredited materials testing laboratory with facilities for simulating climatic and environmental influences is available for determining material, adhesive and composite parameters.  

Simplified mathematical models and the finite element method (FEM) are used for computational verification. Adhesive-specific material models and modeling techniques are available that take into account special features of bonded joints. For the design of the adhesive joint, methods are used that take into account different types of loading, such as vibration and creep loading, as well as relaxation in the adhesive joint with humidity and temperature. With the support of the affiliated materials testing laboratory, material cards can be created for FE programs (ABAQUS, LSDyna, etc.).

 

Combined joining processes (hybrid joining)

 

Adhesive bonding is often combined with a second joining process for so-called hybrid joining. Examples here are adhesive bonding and spot welding or adhesive bonding and riveting. When more than one joining technique is combined, each joining technique should be assigned a clearly defined subset of the functions to be fulfilled in order to be able to check their fulfillment. Fraunhofer IFAM uses various methods and procedures for this purpose. For example, an automatic riveting machine is available for the production of combined metal/plastic/plastic composite riveted joints. It can be used to experimentally simulate manufacturing influences and determine process parameters. For combined joining processes, numerical methods are also used to simulate the mechanical behavior and manufacturing influences.

Accredited materials testing laboratory

 

Fraunhofer IFAM has a materials testing laboratory certified to DIN EN ISO 9001:2000-12, in which tests accredited to DIN EN ISO/IEC 17025:2005 are carried out.

Electromechanical testing machines up to 120 kN are available with facilities for simulating climatic and environmental influences to determine material, adhesive, and bonding characteristics. Single-axis and multi-axis servo-hydraulic testing machines up to 400 kN enable the measurement of the vibration and fatigue strength of components as well as bonded and hybrid joined joints and parts.

Simultaneous exposure to temperature, humidity and corrosive media is also possible. For the measurement of fine strain, the laboratory has incremental displacement transducers, clip-on strain transducers and strain gauges as well as optical, non-contact 1D - 3D measuring systems, with which spatially resolved measurement is also possible.

Test equipment for test speeds up to 10 m/s enables the testing of adhesive and hybrid joints under high load speeds (crash) in the temperature range from -60 °C to 120 °C.

The facilities are used for the experimental simulation of temperature-stress time histories of adhesive and combined joints during production.

Adhesive bonding of structures in the construction industry

 

After the packaging industry, the construction industry is the second largest user of adhesives. No modern building can be realized without the use of adhesives. In addition to the numerous applications in interior construction, such as carpets, tiles, or drywall, bonding in the area of windows, bonding of wood and glass in facade construction, and thermal insulation are necessary for the functional design of a building.

Fraunhofer IFAM researches the expansion of the current applications of adhesive bonding in the construction industry into areas previously reserved for other joining techniques or materials. For example, it has been shown that the extremely high loads of heavy steel construction can be advantageously transferred via adhesive bonds, although transferable forces reach into the meganewton range. Similarly, Fraunhofer IFAM is working on load-bearing bonded joints for wood – also in combination with other materials. In addition to static loads, influences such as temperature and humidity are also taken into account.

In addition to the selection of suitable adhesives and the development of bonding processes suitable for construction sites, Fraunhofer IFAM is also working on the design and standard-compliant calculation of bonded connections and structures.