Design and modeling of adhesively bonded repairs

The use of composite materials in load-bearing applications is continuously increasing in various industries, especially in the railroad, automotive, aerospace and wind energy sectors. Even the smallest damage, such as cracks in load-bearing structures, requires repairs to prevent so-called fatigue crack growth, which in the worst case can lead to structural failure. Bonding solutions are promising in this respect. Patch adhesive repairs have also proven to be extremely effective for lightweight aluminum structures. Compared to conventional repair methods such as riveting or welding, bonded repairs offer several advantages.

Advantages of adhesively bonded repairs

In order to prevent crack propagation in the event of damage and to restore the mechanical properties of a structure, such as the fiber composite outer skin of an aircraft, surface repairs using adhesively bonded CFRP patches are recommended. Bonded repairs are not only characterized by their lower weight, but also offer an attractive appearance and improved aerodynamic properties compared to conventional repair methods.

At Fraunhofer IFAM, the mechanical performance of various repair geometries was successfully tested under dynamic load. Tests using shafted, bonded repair specimen carried out in the accredited testing laboratory enabled validated lifetime predictions.

Modeling and prediction of the performance of repair patches

Predicting the behavior of the patch repair after curing and when failure will occur is essential for a correct design. Fraunhofer IFAM has developed various failure models that are capable to predict the failure behavior of bonded joints under different loading conditions and thus simulate potential damage mechanisms.

Patch adhesive repairs are also suitable for the repair of aluminum aircraft fuselages. Due to the cyclical change in pressure between take-off, flight operations at a corresponding altitude and landing of an aircraft, the smallest material defects and cracks in the outer skin are subject to a cyclical growth process known as fatigue crack growth. The fact that this growth can be stopped by patch repair bonding has been proven by a combination of mechanical tests and simulations at Fraunhofer IFAM.

We support you in your adhesive bonding projects with our know-how and our testing laboratory, e.g.

• Support in the development of safe, economical and sustainable repair concepts using adhesive bonding Mechanical testing of adhesives and adhesive bonds
• Mechanical testing of bonded joints and fibre composite repairs
• Experimentally supported lifetime prediction as requirements for the approval of fibre composite repair bonds

Our experts are also developing bio-based adhesives that can be used both for patch adhesive repairs and as a matrix material for fiber composites.