BONDING OF CERAMICS: AN ANALYSIS OF THE TORSION HOURGLASS SPECIMEN

There is a recent and growing interest in joining ceramic parts due to their increased use in several fields such as next-generation nuclear plants, aeronautic engine parts and aerospace components. For high temperature applications, glass-ceramics are used as an “adhesive” for ceramic parts, this generates the need for test methods suitable to assess their bond strength. Unfortunately, the various test procedures currently used lead to different results. One recent test is based on torsion of hourglass shaped joined ceramics, originated from a modification of the ASTM F734-95 standard, with the aim of obtaining failure under a pure shear state in the bondline subjected to torsion. However, results obtained from different versions of the hourglass geometry show differences which are still difficult to compare. Moreover, due to the brittle nature of the materials and especially when the adhesive strength is comparable to that of the substrates, the failure is not confined in the bond and propagates also in the substrates. In this case, the results are still of arguable application for design purposes. The aim of this paper is to give an insight on torsion of hourglass-shaped joined ceramics and on the interpretation of the obtained results, by means of detailed analytical and numerical studies of the stress distribution in the specimen, and taking into account the brittle nature of the materials. The main findings are: i) the stress state in the bondline is not singular; ii) a non negligible stress concentration arises out of the bondline

Torsion Test vs. Other Methods to Obtain the Shear Strength of Elastic-Plastic Adhesives

Nowadays adhesive joints are more and more used; therefore, a precise and reliable shear strength measurement of these joints is necessary to design and predict a final components’ performance. This work aimed to assess the shear strength value of adhesively joined ceramics (SiC, Si3N4) and steel in the case of an elasto‐plastic (ductile) joining material (Loctite EA 9321 AERO) by an experimental campaign and associated analytical modelling. The joined samples were tested using a single lap offset test in compression (SLO), an asymmetrical 4‐point bending test (A4PB, ASTM C1469), and by torsion on fully joined hourglass shaped samples (THG). A simple model based on the elastic‐plastic response in shear was proposed to fit the torque‐rotation curve measured in the torsion tests. The results showed that, with the adopted test methods and conditions, and by using the model, consistent values of shear strength could be obtained by torsion tests

Ricerca sulle PMI piemontesi del settore automobilistico

Sistema informativo delle attività produttive- Indice #3- Introduzione #5- Crisi, ma di chi? Il contesto del settore auto #11- Le mutazioni del sistema automotive piemonese #17- L'aggiornamento dell'indagine diretta #25- Un approfondimento dell'indagine diretta. Il futuro è un'ipotesi #47- Le politiche regionali a sostegno del settore automotive in Europa #65- Appendice: incentivi alla rottamazione nei principali paesi europei #117- Bibliografia e sitografia #12

Failure Criteria for Adhesive Bonds and their Relevance to Design: A Review

A unique, generally accepted, methodology to predict the failure conditions and load of adhesive bonds is still lacking. The primary distinction that needs to be done is between: i) rupture conditions, which inevitably involve material non-linearity; ii) design conditions, within linearity limits. Typically, the research is focussed on i), and this excludes the numerous solutions or numerical models based on linear elasticity. A first research line on bond failure relies on adhesive plasticity. Hart-Smith [1] obtained design charts admitting that at the ends of the overlap the adhesive yields plastically. Later, Crocombe and co-authors [2] assumed global yielding of the adhesive to be the key phenomenon. A second research line, appeared since early 1970‘s [3], is based on fracture mechanics; the related tests are focussed on the adhesive strength by using ad hoc specimens (double cantilever beam, end notched flexure). In a design perspective, the problem in applying fracture mechanics is that the existence of a crack must be assumed somehow. A synthesis of these two approaches, likely the most popular today, is given by damage mechanics, in particular by the cohesive zone used to describe the failure of the adhesive [4]. Clearly, all these methodologies require numerical modelling. On the other hand, in a simpler perspective oriented to joint design, criteria based on adhesive elastic stresses have been also proposed [5], to help dimensioning the joints. These are based on conventional stresses, for which a limit combination can be easily assessed experimentally, under both static and dynamic (impact) conditions. The proposed presentation aims at giving an overview of the available approaches, trying to identify merits, lacks and needs for future developments. [1] L.J. Hart-Smith, Tech. Rep. CR-112235,112236 (1973). [2] A.D. Crocombe, Int. J. Adhes. Adhes., 9, 145 (1989). [3] E.J. Ripling, S. Mostovoy, H.T. Corten., J. Adhesion, 3, 107 (1971). [4] J.W. Hutchinson, A.G. Evans, Acta Mater., 48, 125 (2000). [5] L. Goglio, M. Rossetto, E. Dragoni, Int. J. Adhes. Adhes., 28, 427 (2008)

Chapter 5. Continuum Mechanics Modelling by Finite Elements

The chapter reports in brief the principles and several cases of application of the FEM to the analysis of bonded joints. The topics are presented under the light of selected papers of the related scientific literature. The main topics treated are failure prediction based on continuum analysis, model size reduction (efficient modelling), stress singularities and geometric non-linearity