Mechanical Properties,. Technologies and. Economic Importance

16 августа, 2015

Pokraskin

The present book is dedicated to the description of the mechanical properties, the technologies and the economic importance of adhesives.

First of all interference fitted and adhesively bonded joints has been studied as an effective means to increase the transferable load while reducing both the weight and the stress level of the joined components. In such a type of joints the influence of the assembly technique on the shear strength of the joints has been investigated. Therefore the shear strength of hybrid joints realized by ‘press fitting’, by ‘shrink fitting’ or by ‘cryogenic fitting’ were compared in order to suggest the best way of joining both for technological and economic importance.

Then the hybrid interference and some design aspects for practical application in (the automotive steel wheel) have been analyzed. An experimental evaluation of the contributions of the adhesive and the interference to the resultant resistance of the hybrid joint was carried out with particular attention to the phenomena occurring at the interface level and the effect of the adhesive nature, its curing technology and its mechanical response. The outcomes of the laboratory analyses were validated in the steel wheel system. The adhesive contribution mainly affects the static resistance of the hybrid joint, and is strongly related to the type of adhesive exploited. On the other hand, the interference seems to play an important role in the fatigue behavior, especially in the wheel system.

he static and fatigue strength properties of press fitted and adhesively bonded joints has been, also, studied extensively by considering the Engagement Ratio (i. e., the coupling length over the coupling diameter). Coupling and decoupling tests have been performed both on press-fitted and adhesively bonded specimens and on pin-collar samples, considering four

different levels for the Engagement Ratio. The study shows that the Engagement Ratio has a negligible effect on the shear strength of the adhesive and also on the relationship between the decoupling and the coupling forces. Moreover, the obtained results show that a too high interference level in press — fitted and adhesively bonded joints may have a detrimental effect on the adhesive strength.

Furthermore the assessment of the adhesive performance of two binders for reassembling fragment porous stones and, more specifically, the effect of nano-titania in the hydration and carbonation of the derived mortars, have been investigated. The nano-titania of anatase form, has been added in mortars containing (a): binders of either lime and metakaolin or natural hydraulic lime and, (b): fine aggregates of carbonate nature. The nano-titania proportion was 4.5-6% w/w of binders. The physicochemical and mechanical properties of the nano-titania mortars were studied and compared to the respective ones of the mortars without the nano-titania addition, used as reference. DTA-TG, FTIR, SEM and XRD analyses indicated the evolution of carbonation, hydration and hydraulic compound formation during a period of one-year curing. The mechanical characterization indicated that the mortars with the nano-titania addition, showed improved mechanical properties over time, when compared to the specimens without nano-titania. The results evidenced carbonation and hydration enhancement of the mortar mixtures with nano-titania. The hydrophylicity of nano-titania enhances humidity retention in mortars, thus facilitating the carbonation and hydration processes. This property can be exploited in the fabrication of mortars for reassembling fragments of porous limestones from monuments, where the presence of humidity controls the mortar setting and adhesion efficiency. The rapid discoloration of methylene blue stains applied to mortars with nano-titania supported the self-cleaning properties of mortars with nano-titania presence. Based on the physicochemical and mechanical characterization of the studied adhesive mortars with nano-titania, binders of metakaolin-lime and natural hydraulic lime, have been selected as most appropriate formulations for the adhesion of fragment porous stones in restoration applications.

Then the adhesive crack propagation has been investigated for some load bearing applications. Fatigue cracks, either induced by defects or by applied stresses, may appear and propagate, thus becoming potentially harmful for the structural integrity of a part or a whole structure. Therefore, in-situ structural health monitoring (SHM) of bonded joints is essential to maintain reliable and safe operaional life of these structures. This chapter presents various in-situ SHM techniques which are used to monitor bonded composite joints with a focus on fatigue crack monitoring based on the backface strain (BFS) technique. Case studies are presented and discussed for adhesively bonded single lap joint (SLJ). Sensors associated with this technique are also explained, with emphasis on Fibre Bragg Grating (FBG) optical sensors.

Finally a numerical method able to reproduce three-dimensionally the fatigue debonding and/or delamination evolution in bonded structures has been proposed in order to improve their performances. The cohesive zone model previously developed by the authors to simulate fatigue crack growth at interfaces in 2D geometries is extended to 3D cracks under mixed-mode I/II loading

The Editor Dario Croccolo

Department of Industrial Engineering,

University of Bologna, Italy, e-mail: dario. croccolo@unibo. it