Analysis of interfacial fracture in ceramic–metal assemblies under effect of thermal residual stresses

In this study, the finite element method is used to analyze the behavior of interface cracks in ceramic–metal assemblies under the effect of thermal residual stresses. These stresses are due to the elaboration process of the bimaterial junctions. The stress intensity factor is used as fracture criterion. The effects of the temperature of elaboration and the metal thickness on the variations of the thermal SIF are highlighted. The obtained results show that the mode of fracture under thermal residual stresses for all ceramic–metal couples is the mixed mode (opening + sliding). The mode II (the sliding mode) is the dominant mode.     

Effect of residual stresses on mechanical properties and interface adhesion strength of SiN thin films

Residual stresses play a significant role in the mechanical reliability of thin films. Thus in this study, the mechanical properties and interface adhesion strengths of SiN thin films containing different residual stresses have been investigated by using nanoindentation and nanoscratch tests. With varied residual stresses from compressive to tensile, the penetration depth of nanoindentation tests shifted to a higher value. The hardness and elastic modulus decreased from 11.0 and 95 GPa, respectively, for the film containing a compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. With decreasing compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m². Compressive stresses were expected to blunt crack tips and inhibit crack propagation, while tensile stresses enlarged crack opening and facilitated crack propagation, thus changing the mechanical properties of the SiN thin films.     

Multiple cracks on the arc-shaped interface in a semi-cylindrical magneto-electro-elastic composite with an orthotropic substrate

The present work investigates the problem of multiple cracks on the arc-shaped interface of a semi-cylindrical magneto-electro-elastic layer bonded onto an orthotropic substrate. Continuously distributed dislocation is used to simulate the anti-plane interfacial cracks. The problem is formulated as a Cauchy singular integral equation by integrating the Green’s function of an interfacial point dislocation. Both the theoretical derivation and numerical computation are verified in special cases. The effects of the interface end, crack space, layer thickness, stiffness ratio and material orthotropy are surveyed, among which the fracture behavior of the interface end deserves special attention in design.     

Fatigue crack propagation of through cracks in thin sheets under combined traction and bending stresses

ABSTRACT Fatigue crack propagation tests of through cracks in 3 mm thick aluminium alloy 6013‐T6 plates, under combined membrane and bending stresses were carried out. Five different values for the ratio of bending to membrane stresses, SB, were examined: 0, 0.55, 1.25, 1.8 and 2.23. Firstly, the results were elaborated by taking into account only the membrane stress and the front dimension of the crack for the evaluation of the stress intensity factor range. The results relevant to the lower SB values, evaluated in this mode, show good agreement with the results obtained when only the membrane stress was applied, while the results obtained at the higher SB values exhibit a remarkable increase in the crack propagation rate. The results were successively evaluated on the basis of tabular stress intensity factor solutions available in the literature; the agreement between the predicted and the experimental data occurs when mean values of stress intensity factor, rather than local values, were used at the back face of the plate, i.e., the face where the bending produces a compressive stress, to counteract the high gradient of stress intensity factor present in this area.     

Analysis of multiple interfacial cracks in an orthotropic bi-material subjected to anti-plane shear loading

The present work concerns with the elasto-static problem of double interfacial cracks located between two dissimilar orthotropic plates. The dimensions of the bi-material composite, are assumed to be finite. The crack faces are subjected to anti-plane shear traction. Finite Fourier transforms are applied to reduce the problem to a triple series equations, and then to a system of singular integral equations with Cauchy type singularity. That are solved numerically using Gauss-Chebyshev integration formulae. The stress intensity factors, are determined in a closed form expressions. The obtained results agreed with the previous analytical ones. Further, a parametric study is introduced to investigate the effects of the geometric and elastic characteristics of the composite on the values of the stress intensity factors.     

Experimental evaluation of the electric pulse effect on residual stresses in a Ti-Al-Si-Ag coating

Experimental results and stress-strain state simulation by the finite element method show that a thin oxidation-resistant coating sputtered onto a titanium alloy substrate develops high-level residual stresses. The electric pulse passed through a coated specimen significantly reduces an average level of residual stresses in the coating. __________ Translated from Problemy Prochnosti, No. 3, pp. 47–52, May–June, 2008.     

Analysis of the distribution of thermal residual stresses in bonded composite repair of metallic aircraft structures

In this study, the distribution of the thermal residual stresses due to the adhesive curing in bonded composite repair is analysed using the finite element method. The computation of these stresses comprises all components of the structures: cracked plate, composite patch and adhesive layer. In addition, the influence of these residual stresses on the repair performance is highlighted by analysing their effect on the stress intensity factor at the crack tip. The obtained results show that the normal thermal stresses in the plate and the patch are important and the shear stresses are less significant. The level of the adhesive thermal stresses is relatively high. The presence of the thermal stresses increases the stress intensity factor at the crack tip, what reduce the repair performance.     

Numerical simulation of residual stresses in diamond coating on Ti-6Al-4V substrate

In this paper, we present numerical simulations of the residual stresses developed between diamond coatings and Ti-6Al-4V substrates when using chemical vapour deposition technique. The large difference in thermal expansion coefficients of diamond and titanium alloys results in high residual stresses in the diamond film. This could lead to interfacial cracking and material failure. The finite element method was used to simulate the cooling process of diamond films at various thicknesses and deposited at temperatures ranging from 600 °C to 900 °C. The influence of different parameters such as temperature, film thickness, material characteristics, geometry and edge effects are investigated for different case geometries. The film debonding and cracking is discussed and numerical results are compared with existing experimental and numerical results. Finally, some propositions are made to enhance the experimental process in order to reduce the residual stress intensities and the possible material degradation.     

Interaction of welding residual stresses and warm pre-stressing on brittle fracture of a pipe containing an internal semi-elliptical crack

Residual stresses (RS) due to welding process, may change the load bearing capacity of cracked components. These stresses can also affect the benefit of warm pre-stressing (WPS) cycles which are used for improving structure behaviour. RS are obtained from a two-passes welding simulation of a pipe and verified by experiments. A semi-elliptical internal crack at the weld line is considered. Redistribution of RS field after introducing the crack shows a significant tensile RS are remained at the crack tip. Two common WPS cycles, load-cool-fracture (LCF) and load–unload-cool-fracture (LUCF), are applied using the model at room and low temperature subjected to axial loading. Using local approach to fracture shows that welding RS dramatically raise the fracture probability. LCF has more influence on reducing the fracture probability in comparison with LUCF. The interaction of welding RS and WPS cycles still improves the fracture properties, however, welding RS cause to decrease the benefit of WPS. Comparing RS distributions on crack-tip shows that applying WPS cause to release a significant amount of welding RS and therefore, WPS can be very useful for welded structures. The near crack-tip opening stresses at a same fracture load are further studied for all cases.     

Equilibrium depth and spacing of cracks in a tensile residual stressed thin film deposited on a brittle substrate

The depth and spacing of cracks in a tensile residual stressed thin film bonded on a brittle substrate are analyzed thermodynamically using the minimum energy theorem on the basis that the film has the same mechanical properties as the substrate. The results show that the cracks penetrate into the substrate. Simple and approximate relationships between three dimensionless parameters, i.e., the normalized crack depth and spacing, and the cracking resistance number, are derived, which determine the fracture behavior of the film.     

The role of carbon nanofibers on thermo-mechanical properties of polymer matrix composites and their effect on reduction of residual stresses

In this research, the effects of carbon nanofibers (CNFs) on thermo-elastic properties of carbon fiber (CF)/epoxy composite for the reduction of thermal residual stresses (TRS) using micromechanical relations were studied. In the first step, micromechanical models to calculate the coefficient of thermal expansion (CTE) and Young's modulus of CNF/epoxy and CNF/CF/epoxy nanocomposites were developed and compared with experimental results of the other researchers. The obtained results of the CTE and Young's modulus of modified Schapery and Halpin-Tsai theories have good agreement with the experimental results. In the second step, the classical lamination theory (CLT) was employed to determine the TRS for CNF/CF/epoxy laminated nanocomposites. Also, the theoretical results of the CLT were compared with experimental results. Finally, reduction of the TRS using the CLT for different lay-ups such as cross ply, angle ply, and quasi-isotropic laminates were obtained. The results demonstrated that the addition of 1% weight fraction of CNF can reduce the TRS that the most reduction occurred in the unsymmetric cross-ply laminate by up to 27%.     

The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.     

Analysis of the influence of the level of residual stresses on the ultimate stresses in a cycle for welded structures according to the results of testing of small-size specimens

We propose a procedure for the determination of the diagrams of ultimate stresses in a cycle for welded joints with preliminarily induced steady-state residual stresses according to the results of testing of small-size specimens without residual stresses. Translated from Problemy Prochnosti, No. 3, pp. 107–115, May–June, 2009.     

Comparison of the developed thermal stresses in Al2O3–SG, ZrO2–12%Si+Al and ZrO2–SG coating systems subjected to thermal loading

In this investigation, thermal and structure finite element analysis has been employed to analyse the thermal stresses developed in Al₂O₃–SG, ZrO₂–12%Si+A1 and ZrO₂–SG.coatings subjected to thermal loading. Systems with 0.4 mm coating thickness and 4 mm substrate material thickness were modelled. Zirconia–spherical cast iron (SG) coatings with NiAl, NiCrAlY and NiCoCrAlY interlayers were also modelled. Nominal and shear stresses at the critical interface regions (film/interlayer/substrate) were obtained. The results showed that the lowest stress levels are in ZrO₂–SG coatings. Furthermore, the interlayer thickness and material combinations have a significant influence on the level of the developed thermal stresses. It is also concluded that the finite element technique can be used to optimise the design and the processing of ceramic coatings.     

Effect of thermal cycling on the damping behavior in alumina borate whisker with and without Bi2O3 coating reinforced pure aluminum composites

Alumina borate whiskers with and without Bi₂O₃ coatings reinforced pure aluminum composites were fabricated by squeeze casting. The damping behavior of the composites was described. Samples had undergone thermal cycling during the damping measurement. For the Al₁₈B₄O₃₃w/Al composite, two damping peaks are present either in the heating or in the cooling process. For the Al₁₈B₄O₃₃w/Al composite with whisker coatings, there are also two damping peaks in the heating process, but there are three damping peaks in the cooling process. The research results indicate that a distinct difference presents in the damping-temperature curves for the Al₁₈B₄O₃₃w/Al composite with and without whisker coatings. The damping behavior and damping mechanism in the composites during thermal cycling are explained through dislocation motion, interfacial and grain boundary slip.     

Studies of surface and interface phonon polariton characteristics of wurtzite ZnO thin film on wurtzite 6H-SiC substrate by p-polarized infrared attenuated total reflection spectroscopy

The surface and interface phonon polariton characteristics of a wurtzite ZnO thin film grown on a wurtzite 6H-SiC substrate are investigated by p-polarized infrared attenuated total reflection spectroscopy. Two absorption dips corresponding to the leaky surface phonon polariton (SPP) mode of ZnO and the interface phonon polariton (IPP) mode of ZnO/6H-SiC are clearly observed at 558 and 916 cm^− 1, respectively. The observations are in good agreement with the results determined from the theoretical surface polariton dispersion curve simulated by means of an anisotropy model. Finally, the effects of the ZnO thin film thickness on the SPP and IPP modes are deduced from the theoretical calculations.     

Analysis of damage formation and propagation in metallic thin films under the action of thermal stresses and electric fields

Summary A quantitative analysis is presented of void formation, morphological stability, and evolution in metallic thin films, which are problems that underlie serious reliability issues in interconnections used in integrated circuits. The analysis is based on the coupling of bulk and interfacial mass transport phenomena with elastic deformations and current stressing. The formation and growth of intergranular voids in bamboo-structure conductor lines due to stresses that develop during processing is investigated. The investigation is aided by self-consistent simulation of bulk and grain-boundary diffusional processes using bicrystal models with elastic grains. A systematic analysis is presented of the morphological evolution of transgranular voids in passivated and unpassivated aluminum lines under current densities that are typical of electromigration testing. The effects of the electric field and surface properties on the morphological stability of voids are examined and morphologies that bifurcate from rounded or wedge-like void shapes are predicted. The theoretical results are discussed in the context of experimental data of void propagation under electromigration conditions.     

Analysis and simulation of cracking patterns in coating under biaxial tensile or thermal stress using analysis/FEM strain-accommodation method

The cracking patterns in coatings under biaxial tensile or thermal stress are analyzed by the “analysis/FEM strain-accommodation method” that combines the strain of the substrate with a coating obtained from thermo-elastic analysis with the strain of the substrate calculated by a finite element method. The simulation using this method is effective not only for expressing the cracking patterns observed in punch press tests of disk specimens with WC-Co cermet and Al₂O₃-TiO₂ ceramic coatings but also predicting the cracking process for the coating deposited on a part with a complex shape under thermal stress.     

Effects of oxide thickness, Al2O3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al₂O₃ interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.