The generalized stress intensity factor (GSIF) criterion is suggested by many authors for the failure prediction at corners in monolithic materials. More recently, it has been successfully used to predict the failure initiation of an adhesive bonding between two steel plates under a 3-point flexion. It involves 2 related critical values. One in the far field takes into account the geometry of the structure but ignores the (small) adhesive thickness. The other in the near field takes into account the adhesive layer but ignores the global structure. Matching conditions ensure the equivalence of these two approaches. The robustness of the criterion allows using it for the single lap joint failure test as well. The peculiar failure behaviour of this structure is analysed, in a first step for initiation and next for propagation and final failure. The unexpected initiation prediction of the failure mechanism agrees fairly well with a thorough analysis of the experiments.

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The effects of Ni-YSZ cermet re-oxidation in anode supported Solid Oxide Fuel Cells (SOFCs) have been investigated. Damage mechanisms have been studied in both cases of direct oxidation in air (i.e., fuel shutdown) or by an ionic current (i.e., fuel starvation).Direct oxidation tests show that the electrolyte cracks for a conversion degree of Ni into NiO ranging between similar to 58 and similar to 71%. This failure mode has been modelled considering both the bulk expansion of the cermet induced by the transformation of the Ni phase and the change of mechanical stresses in the multilayered cell.In the case of fuel starvation, a thin layer of the cermet was electrochemically re-oxidised at 800 degrees C and then reduced under a hydrogen stream. This 'redox' cycle was repeated until the degradation of the cell. The evolution of the impedance diagrams recorded after each cycle suggests that the cermet damages in an area close to anode/electrolyte interface. The mechanical modelling states that a delamination can occur along the interface between the Anode Functional Layer(AFL) and the Anode Current Collector (ACC) substrate. This theoretical result confirms the experimental trends observed by impedance spectroscopy. (C) 2009 Elsevier B.V. All rights reserved.

La resistance au cisaillement de la liaison CFC/cuivre realisee par procede AMC est testee dans la configuration monobloc. Un dispositif specifique a ete developpe pour permettre de tester la liaison suivant des secteurs de 53° afin d'evaluer la tenue de la liaison en fonction de son orientation par rapport aux directions principales du CFC, materiau orthotrope, et du flux thermique subi par la liaison lors de tests de fatigue. Les resultats obtenus sont compares aux contraintes residuelles lors de l’assemblage, calculees par elements finis.

To understand the degradation of two thermal barrier coating (TBC) systems, we determined the adhesion energy between the bondcoat and the topcoat and its evolution during cyclic oxidation at 1,100 °C, by means of a modified 4-point bending test. An yttria stabilized zirconia (YSZ) ceramic topcoat was deposited by electron beam physical vapour deposition (EBPVD) on a Ni-based superalloy with either an intermediate β-(Ni,Pt)Al bondcoat or a newly developed Zr-doped β-NiAl bondcoat. Although a similar evolution of the adhesion energy during cyclic oxidation has been recorded for both systems, observations of the fracture surfaces combined with a microstructure study revealed different degradation mechanisms. An energetic model of spallation is applied to predict their lifetime. According to this approach, the TBC failure is induced by the accumulation of strain energy in the ceramic layers and resisted by the interfacial fracture toughness. The predicted lifetime is consistent with experiments for both systems.

The adhesion strength of ceramic/metal interface plays an important role on the reliability of many engineering components. In the present study, the cross-sectional indentation technique is used to determine the interface adhesion of a thick silver film on barium titanate substrate. The test introduces a Vickers micro-indentation at the brittle substrate close to its interface; a small substrate chip is produced to push the thick film laterally and delaminate it from the substrate. The average strength value for the Ag/BaTiO3 interface varies from 2 to 4 J m–2. This is close to the value obtained using a blister technique.

The shaft loaded blister test is a way to measure the adhesion of thin films on their substrate. It has been adapted to evaluate the mechanical resistance of brazing joints between ceramic and metal. The improved test has been used to study copper/alumina joints brazed with a CuAgTi alloy.

The fracture energy at the interface between the porous cathode and the electrolyte of a planar solid oxide fuel cell is determined by a four-point bending test. During the preparation of specimens the resin used to bond the stiffeners completely impregnates the porous cathode. The adhesion of the resin to the electrolyte is measured separately in order to extract a reliable value of the interface fracture energy (20.2 +/- 6.7 J m(-2)). The mode-mixity is found to be equal to 31 degrees. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The purpose of this work has been focused on the calculation of the stress field inside planar anode supported cells. The cell fracture has been estimated through the statistical approach of Weibull. After elaboration, a high residual compressive stress has been calculated in the thin electrolyte layer. A slight tensile stress has been pointed out in the anode in a region close the anode/electrolyte interface. For high electrolyte thickness (>20 μm), this tension leads to low survival probabilities of the anode. At SOFC operating temperature, the elaboration stress is partially relaxed. In this condition, the thermal cycling between the room temperature to the SOFC operating one should not induced any cell degradation. The first cermet re-oxidation step has been also analyzed. This study has shown that the cathode is damaged as soon as the anodic expansion reaches values between 0.05-0.09 %. The electrolyte fracture has been predicted to occur for anodic expansion ranging between 0.12-0.15 %.

A statistical analysis based on weakest link theory is employed to describe the brittle fracture induced at singularities in ceramic materials. Relationships are stated between the Weibull probability and the notch geometry. For low Weibull's modulus, survival probabilities only depend on the generalised stress intensity factor, whereas for high Weibull's modulus, probabilities also depend on the notch tip radius. The effect of the tip radius on the failure probabilities is equivalent to the exclusion of a small volume surrounding the notch tip. From these results, a numerical methodology based on the finite element analysis is proposed to state if singularity is harmful for a ceramic structure. Elsewhere, for a notch with high stress singularity order and symmetrically loaded, the Batdorf's theory gives the same results than the Weibull one. In the case of a low stress singularity order, the prediction of failure can strongly depend on the multi-axial criterion.

The present paper deals with mechanical adhesion energy determination of thermal oxide scales on metallic alloys. AISI 430Ti alloy (Fe–18Cr–0·4Ti–0·5Si, wt-%) was isothermally oxidised at 900°C in Ar–20%O2 atmosphere to grow thermal oxide scales of various thickness. These oxide/metal systems were subjected to room temperature in situ tensile test in the SEM chamber allowing continuous observation of surface failure during the test. Characteristic observations were oxide transverse cracking and scale spallation. A theoretical model was developed to quantify stress and strain evolution in oxide during the test, allowing to derive adhesion energy from strain and stress values at spallation onset. It was found that oxide scales on AISI 430Ti exhibited high mechanical adhesion energy, decreasing from 163 to 38 J m−2 with increasing oxide thickness in the range from 0·3 to 2·2 μm. Excellent agreement was observed with adhesion energy values obtained using the inverted blister test on the same alloy.

In order to understand the degradation of thermal barrier coating (TBC) systems better, we determined the adhesion energy (Gc) between the bond coat and the top coat and its evolution during cyclic oxidation. This energy was evaluated by means of a modified four-point bending test. The systems tested consist of a YPSZ (7 wt.%Y2O3–ZrO2) EB-PVD ceramic top coat deposited on a single-crystal AM1 superalloy substrate protected with either a β-(Ni, Pt)Al bond coat or a newly developed Zr-doped β-NiAl bond coat. Although the results show a similar adhesion evolution for both systems, different microstructural degradations are involved.

The purpose of this paper is to show the role of the sidewall barrier on the nucleation / growth of electromigration induced voids in Cu interconnects. A comparison is made between an anisotropic PVD process and a conformal CVD process. Standard electromigration (EM) and scanning electron microscopy (SEM) give a clear picture of the failure mechanisms. SEM shows the steps of void growth in 120 nm linewidth. EBSD (electron backscattering diffraction) shows that voids are nucleated at grain boundary

Adhesion energy values for thermal oxide scales cyclically grown at 850 and 950°C in air on ferritic and austenitic stainless steels were obtained using an inverted blister test and a tensile test working in the SEM chamber. The blister test used water pressure for debonding the metalscale interface, whereas the tensile test led to transverse compression generating scale spallation by buckling. Adhesion energy, defined by energy for interface crack propagation by unit area, was shown to be in the range 10 to 650 J.m–2 for the chromia-rich scales with thickness in the micrometer range. Ferritic grades gave less adherent scales than austenitic ones, and a great influence of titanium was evidenced, greatly increasing scale adhesion; niobium was less operative. Adhesion was well connected with nature and morphology of Ti and/or Nb-containing precipitates at the metal-scale interface.

The aim of this Study was to compare three adhesion tests carried out on plasma-sprayed copper coatings on aluminium Substrates. The first test, the bond pull test, designated EN 582 or ASTM C633. involves a uniaxial static stress and is commonly used in the coating industry. The second test, the LASAT (LASer Adhesion Test). is a recently developed technique based on spallation phenomenon due to laser induced shock waves. In this test, the coating delamination results from spallation at the coating/substrate interface due to uniaxial tensile stress. The last test, the bulge and blister test, involves a quasi-static measurement of the crack propagation energy at the coating/substrate interface. These three techniques have been used to evaluate the influences of different process parameters involved in the coating adhesion such as aluminium surface roughness, substrate pre-heating and plasma spray conditions.