This thesis investigates the influence of structural adhesive bonding on the fatigue behavior of an aviation related aluminum alloy. For this, both notched and pre-cracked CCT specimens were strengthened with Boron fiber-reinforced epoxy and Al-patches and fatigued in atmospheres of different temperature and humidity. The experimental results show that adhesive bonds need to be assessed by using statistical evaluation methods due to their distribution range. To provide a sufficient surface quality for adhesive bonding a surface pretreatment is mandatory. The objective of this procedure is to clean the surfaces and thus to increase the adhesion strength. The epoxy patches were activated by atmospheric plasma technique, while the aluminum was treated with a Nd-YAG laser. The latter causes the formation of a nanostructured oxide layer which has been found to be very effective for adhesive bonding. With a high resolution scanning electron microscope detailed investigations of these oxide layers were conducted. In fact laser treatment also increases surface roughness deteriorating the lifetime of the basic material. However, for bonded structures the stress field of the center notch or crack is diminished due to a load transfer from the basic material into the repair patch. Thus, the lifetime of bonded specimens increases. The influence of the repair procedure on the fatigue behavior of notched specimens was evaluated and discussed using continuum mechanics. It was made possible to quantify an average load transfer by applying SN curves. This consideration leads to an underestimation of load removal when crack propagation dominates the lifetime as in case of pre-damaged specimens. In this case, evaluation of the repair procedure can be conducted only with fracture mechanics. Applying the DC potential drop method it was possible to evaluate the influence of the repair on both the crack propagation rate and the load transfer. Bonded specimens were found to fail at higher crack lengths than non-repaired references. Its crack propagation rate was reduced severely. Hence, the lifetime of pre-cracked and bonded specimens increased. The strength of different interfaces involved in the bonding could lastly be assessed in terms of quality and quantity by fracture surface analysis. It became apparent that the nanostructured oxide layer evolved from laser pretreatment has the best resistance and is highly-qualified to be utilized for structural adhesive bonding. In a laboratory environment epoxy patches demonstrated interlaminar failure and thereby restricted further lifetime improvement. A noticeable weakening of the boundary layer looms in humid environment caused by diffusion paths provoking both an extensive adhesive failure of the epoxy patches and a local adhesive failure of the Al-patches.    «

This thesis investigates the influence of structural adhesive bonding on the fatigue behavior of an aviation related aluminum alloy. For this, both notched and pre-cracked CCT specimens were strengthened with Boron fiber-reinforced epoxy and Al-patches and fatigued in atmospheres of different temperature and humidity. The experimental results show that adhesive bonds need to be assessed by using statistical evaluation methods due to their distribution range. To provide a sufficient surface quali...    »