The aviation industry faces the challenge of offering aircraft that are lighter, more economical, and safer. One of the solutions is to increase the use of composites. For these materials, adhesive bonding has proven to be the appropriate joining technology. To check these adhesive joints, costly and time-consuming maintenance measures are carried out. An intelligent Structural Health Monitoring (SHM) system can extend these intervals and allow the use of a predictive maintenance system. This paper describes the method of Ultrasonic Lamb Waves for monitoring a adhesively bonded Carbon Fiber Reinforced Polymer (CFRP) aircraft fuselage. Prior to this, the production of a segment of a fuselage and the characterization of the materials (CFRP and adhesive) is shown. Afterwards the method of Ultrasonic Lamb Waves with the use of piezoelectric transducers and signal processing based on the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) algorithm are explained. At the end, the experimental evaluation of an undamaged and a damaged fuselage structure is done. The results have shown the possibility of RAPID algorithm for damage detection on adhesive. An outlook on future work is given.

Hybrid adhesive bonding is widely applied in industry nowadays. Whereas combinations of mechanical fastening and structural adhesive bonding are being used for a long time, hybrid bonds, which combine (resistance spot) welding and adhesive bonding, are a quite recent development. Although this technique of hybrid joining yields structures with excellent mechanical characteristics concerning static and dynamic stiffness and strength, it is hard to numerically predict these characteristics accurately. This originates from the fact that usually only some data is known on the stiffness and strength of the adhesive and spot welds but very few accurate and relevant information is available on the fracture toughness of both joining types. The presented research work consists of three main parts. - Main theoretical background on stiffness, strength and fracture characteristics of the considered joint types. - Mode I and II testing of hybrid adhesive bonded/spot welded specimen to retrieve force-displacement data and virtual testing based on finite element (FE) modelling. - Uncertainty quantification between experimental and virtual work. In general, the mode II tests correspond well with real test data. This cannot be concluded for the mode I tests. The flexibility of the double U–type samples (KS2) is difficult to simulate accurately, particularly from the point when first damage occurs. For both, the experimental and virtual data, the hybrid adhesive/spot welded joints showed the highest maximum force.

The use of adhesively bonded joints is increasing in the lightweight design; however, their fatigue performance in safe-life design is challenging. Therefore, a combination of damage-tolerant design and Structural Health Monitoring (SHM) is often considered. The present work deals with a new approach for detection of crack initiation in adhesively bonded single lap joints (SLJ) subjected to constant and variable amplitude fatigue loading. For this purpose, the so-called backface strain method is used, in which a strain gauge for damage monitoring is placed at a highly sensitive point in the overlap area of SLJs. A striking converging-diverging trend in strain amplitude curves was attained in the tests subjected to constant amplitude loading, which was in a good agreement with the onset of crack initiation observed with video-microscopy. Additionally, this methodology was demonstrated on the SLJs subjected to variable amplitude loading using an additional farfield (outside of the overlap area) strain gauge, which showed a high potential for detection of crack initiation.

Fiber-reinforced plastics (FRP) are widely used in the automotive industry due to their lightweight construction potential. Especially, injection-molded components are very popular because of their short cycle times. Unfortunately, this manufacturing process results in some disadvantages for delayed adhesive bonding. On the one hand, such thermoplastics have a very low surface energy (Polyamide 6 (PA6) 36 mJ/m), on the other hand residues of release agents on the surface hinder adhesion between product and adhesive. These disadvantages have to be overcome by the right surface pre-treatment process. Currently, a solvent-containing process is usually used for this purpose. If necessary, the use of primer is common. In the presented paper, the pre-treatment process of an injection-molded component made of polyamide 6 reinforced with glass short-fibers is presented. First, material-specific properties are measured. The influence of laser pre-treatment at 355 nm and 1064 nm wavelength is investigated. Measurements of surface free energy and surface roughness are conducted. The bonding strength is validated by peel test and tensile shear tests. The shear strength was doubled by laser pre-treatment at a medium energy level, partially cohesive failure of the adhesive was achieved. Both investigated wavelengths led to a great improvement of bonding strength at suitable parameters.

Adhesively bonded pipe-to-socket joints are used in numerous industrial applications. One example is in the transport of media, e.g., in the gas industry or in structural joints for such sectors as construction, wind energy, and the vehicle industry. To monitor such load-transmitting bonded joints, this study investigates a method based on the integration of polymer optical fibers into the adhesive layer. Previous methods for monitoring the condition of pipes, such as acoustic or ultrasonic methods or the use of glass fiber optic-based sensors (FBG or OTDR), are very complex in methodology and require cost-intensive (opto-) electronic devices to generate and evaluate the sensor signals; they are therefore unsuitable for large-scale use. The method investigated in this paper is based on the measurement of integral optical transmission with a simple photodiode under increasing mechanical stress. When tried at coupon level (single-lap joint), the light coupling was varied to obtain a significant load-dependent sensor signal. Based on an angle-selective coupling of 30° to the fiber axis, a drop of 4% of the optically transmitted light power by a load of 8 N/mm² can be detected for the adhesively bonded pipe-to-socket joint with the structural adhesive Scotch Weld DP810 (2C acrylate).