Effect of residual stresses to the crack path in alumina/zirconia laminates

Laminates with alternating layers are well known from nature. The strongly bonded alumina/zirconia (Al₂O₃/ZrO₂) layers can combine high fracture resistance with high strength and stiffness when properly tailored. The presence of compressive residual stresses formed in Al₂O₃ layers can suppress and deflect cracks propagating through the layers. The crack path is governed by both the elastic properties and the internal stress field of individual layers. The laminates with various layer-thickness ratios ranging from 0.1 to 3 were used to investigate the effect of residual stresses and influence of crack formation pattern on the crack path development. The indentation surface cracks observed in various alumina-zirconia laminates exhibit the same crack deflection independently on the level of internal stresses. The crack deflection observed on the fracture surfaces of bending specimens was related to the indentations cracks. The complicated crack path was explained experimentally by 3D reconstruction with the support of numerical simulations.     

Residual stresses in alumina–zirconia laminates

Significant residual stresses can arise in hybrid ceramic laminates during the densification and cooling processing cycles. The densification stresses in alumina–zirconia laminates were calculated assuming the layers to be linear viscous with data obtained by cyclic loading dilatometry. These stresses placed the zirconia layers in biaxial tension and even at 1 MPa or less, they were sufficient to cause a type of linear cavitation damage. The methodology was also applied to asymmetric laminates, successfully predicting their observed curling behaviour. Thermal expansion mismatch stresses arise during cooling, again placing the zirconia layers in residual biaxial tension and leading to the formation of transverse (channelling) cracks. The stresses were calculated using both elastic and viscoelastic formulations and were confirmed with indentation measurements. Additions of alumina to the zirconia layers were effective in reducing both sources of residual stress and allowed crack formation during processing to be avoided. Residual stresses were also shown to improve mechanical performance.     

Cracking of Laminates Subjected to Biaxial Tensile Stresses

During the processing of laminar ceramic, biaxial residual stresses can arise due to differential thermal contraction between unlike layers. A tensile stress can cause preexisting flaws to extend across the layer and into the adjacent layers and then tunnel until they meet either another crack or a free surface. A previous analysis has shown that for a given residual stress there is a critical layer thickness, below which no tunnel cracks will exist, regardless of initial flaw size. Here, the previous analysis was modified to take into account the crack extension into adjacent layers. To determine the validity of the analysis, laminates composed of alternating layers of zirconia and alumina/zirconia were fabricated by a sequential centrifugation technique. The composition of the alumina/zirconia layer was varied to change the biaxial, tensile stresses in the zirconia layer. Observations were then made to determine the critical layer thickness for tunnel cracks and their extension into the adjacent layers. These observations were compared to the theoretical predictions.     

Toughening of Layered Ceramic Composites with Residual Surface Compression

Effects of macroscopic residual stresses on fracture toughness of multilayered ceramic laminates were studied analytically and experimentally. Stress intensities for edge cracks in three-layer, single-edge-notch-bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a function of the crack length by the method of weight function. The selected weight function and the method of calculation were validated by calculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress-intensity equation recommended in ASTM Standard E-399. The stress-intensity calculations for the three-layer laminates with the macroscopic residual stresses were used to define an apparent fracture toughness. The theoretical predictions of the apparent fracture toughness were verified by experiments on three-layer SENB specimens of polycrystalline alumina with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer. A residual compression of ∼400 MPa developed in the outer layers by the constrained transformation of the unstabilized zirconia from the tetragonal to the monoclinic phase enhanced the apparent fracture toughness to values of 30 MPa^.m^1/2 in a system where the intrinsic fracture toughness was only 5 to 7 MPa^.m^1/2.     

Surface Properties of Ceramic Laminates Fabricated by Die Pressing

The simple die-pressing technique has been used to fabricate thick-layer zirconia/alumina ceramic laminates in the forms of film/substrate and multilayer systems. A "vibration sieve" method is used to achieve uniform layer thickness. In order to suppress surface crack initiation and propagation, the laminates are designed to retain residual compressive stresses in the surface layers. As a result, the surface fracture/fatigue resistance has been significantly improved, and the microhardness of the surface layer has also been increased to a certain degree.     

Characterisation of mechanical and fracture behaviour of Al2O3/ZrO2/BaTiO3 laminate by indentation

The paper describes the preparation of laminate piezo-ceramic composite consisting of Al₂O₃, ZrO₂ and BaTiO₃ layers and proves the idea of residual stresses utilization for crack deflection and handling with the brittleness of BaTiO₃. The laminate was prepared by alternate electrophoretic deposition. Although the laminate was sintered at 1300 °C and consisted of layers having a density between 57 % (ZrO₂) and 73 % (BaTiO₃), the hardness and elastic modulus of layers corresponded to those of free sintered monolithic ceramics at a comparable level of porosity. The crack deflection at the interface between individual layers was observed having the same effect and magnitude as deflection observed in the case of fully dense Al₂O₃/ZrO₂ laminates. An interlayer developed on the interface between Al₂O₃ and BaTiO₃ had no negative impact on crack propagation.     

Residual stresses in polymers. II. Their effect on mechanical behavior

The distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above T_g to below T_g temperatures. Simultaneous to buildup of residual stresses, profiles of density and tensile properties were observed. The profiles were obtained using the layer removal technique, which was found not to affect the measured properties. Quenching of the material results in a steep density gradient in the surface layers. Correspondingly, the tensile modulus increases significantly from the surface to the inner layers and so are also the ultimate tensile properties. This behavior could be accounted for neither by the conventional packing volume approach nor by superposition of internal and external stresses. However, observations of the fracture surfaces are very supportive and indicate that the fracture initiation sites are influenced by the residual stresses. Hence, the mechanical behavior is strongly affected by both density and residual stresses profile. Density is the controlling factor in determining the elastic properties whereas residual stresses determine the ultimate strength and fracture mechanism.     

On the determination of the stress-free temperature for alumina–zirconia multilayer structures

Internal residual stresses can enhance the fracture resistance and mechanical reliability of layered ceramics. The magnitude of the stresses depends on the elastic and thermal properties of the layers and the typically assumed reference (stress-free) temperature, below which internal stresses develop. A novel combined experimental and numerical simulation approach has been developed to determine the reference temperature and experimentally proved in alumina–zirconia ceramic laminates. Dilatometric data of monolithic phases are input for the numerical simulation and experimental data on the laminate properties are used for the stress-free temperature determination. In contrast to typical assumptions, reference temperature very near the sintering temperature (i.e. approx. T_ref≈1470 °C) was found, which should be considered for the estimation of internal (residual) stresses in alumina/zirconia-based layered ceramics.     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Comparison of Analytical, Numerical, and Experimental Methods in Deriving Fracture Toughness Properties of Adhesives Using Bonded Double Lap Joint Specimens

Stress and fracture analysis of bonded double lap joint (DLJ) specimens have been investigated in this paper. Numerical and analytical methods have been used to obtain shear- and peel-stress distributions in the DLJ. The generalized analytical solution for the peel stress was calculated for various forms of the DLJ geometry and, by using crack closure integral (CCI) and by means of the J-integral approach, the analytical strain energy-release rate, G, was calculated. Experimental fracture tests have also been conducted to validate the results. The specimens were made of steel substrates bonded by an adhesive and loaded under tension. Specimens with cracks on both sides and at either end of the DLJ interface were tested to compare the fracture behavior for the two crack positions where tensile and compressive peel stresses exist. Tests confirmed that the substrates essentially behave elastically. Therefore, a linear elastic solution for the bonded region of the DLJ was developed. The fracture energy parameter, G, calculated from the elastic experimental compliance for different crack lengths, was compared with numerical and analytical calculations using the experimental fracture loads. The stresses from analytical analysis were also compared with those from the finite element results. The strain energy-release rate for fracture, G_f, for the adhesive has been shown to have no R-curve resistance, was relatively independent of crack length, and compared well with those obtained from numerical and analytical solutions. However, it was found that fracture energy for the crack starter in the position where the peel stress was tensile was about 20% lower than where the crack was positioned at the side, where the peel stress was found to be compressive.     

Crack propagation and residual stress in laminated Si3N4/BN composite structures

The level of residual stress and crack propagation in a new generation of laminates, based on silicon nitride (Si₃N₄) layer and a mixture of boron nitride (BN) and alumina (Al₂O₃) interlayer, was presented. The structure consists of alternated concentric rings of Si₃N₄ separated by the weak BN interlayer possessing no planes of easy crack propagation and fracture resistance much larger than that of any classical planar laminates. The results on direction of crack propagation and residual stress in relation to inter-layer composition, the number of layers, and their thickness are investigated and reported. The effect of residual stress on crack propagation was studied by using Vicksrs intentation. The highest compressive residual stress of ∼170 MPa was found in samples with five layers possessing an average layer thickness of ∼310 × 10⁻⁶ m.     

Mullite/Alumina Particulate Composites by Infiltration Processing: IV, Residual Stress Profiles

Residual stress profiles through mullite/alumina bodies prepared by an infiltration process were determined. In one approach, residual stress profiles were predicted by obtaining concentration profiles and treating the bodies as laminates. In a second approach, a strain gage technique in which part of the sample was ground away while recording the resulting strain was used. The results showed that residual compressive stresses existed in the surface region of all composite samples. The strain gage approach indicated values of surface compression in the range of 176 to 291 MPa, which were higher than those predicted using the laminate approach. The results were compared with changes in the mechanical properties and found to be consistent with the increases in the strength and indentation fracture toughness which resulted when mullite was added to alumina by the infiltration technique.     

Fracture Behavior of Layered Alumina Microstructural Composites with Highly Textured Layers

A new class of layered microstructural composites that combines equiaxed and textured alumina layers was fabricated. Template loading was used to change the texture fraction and porosity in the textured layers. Due to the thermal expansion anisotropy of the textured layers, residual compressive stresses as high as 100 MPa were achieved during cooling from the sintering step. Fracture experiments showed that the interface between the basal planes of highly oriented alumina grains in the textured layers changes from a “strong interface” to a “weaker interface” as the porosity changes from 1% to 5%. Composites with 5% porous textured layers show both crack bifurcation and crack deflection in the textured layers. Crack deflection is attributed to the anisotropic fracture energy of the oriented microstructures and crack bifurcation is ascribed to the compressive stresses that arise from the thermal expansion mismatch between adjacent layers.     

Evaluation of the fracture behaviors of multilayered propylene–ethylene copolymer/polypropylene homopolymer composites with the essential work of fracture

As one of the most appropriate techniques for evaluating the fracture behavior, the essential work of fracture (EWF) was introduced to investigate the fracture toughness of multilayered composites. Propylene–ethylene copolymer (CPP)/polypropylene homopolymer (HPP) alternating multilayered composites with 2–128 layers were prepared though multilayered coextrusion. Polarized optical microscopy photographs revealed that the CPP and HPP layers aligned alternately vertical to the interfaces and continuously parallel to the extrusion direction. The dichroic Fourier transform infrared spectroscopy results showed that the coextrusion sheet had a preferential orientation parallel to the melt flow direction (MD); this caused crack propagation along the blunted MD and the necking ligament section. After heat treatment, the orientation parallel to the MD could been largely eliminated, and the crack propagated in a stable manner. The specific essential work of fracture (w_e) of the multilayered composite was higher than that of the blend; this indicated a higher resistance of crack propagation. The number of layers had little effect on the toughness of the multilayered composites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40574.     

Comparison of Analytical,Numerical, and Experimental Methods in Deriving Fracture Toughness Properties of Adhesives Using Bonded Double Lap Joint Specimens

Stress and fracture analysis of bonded double lap joint (DLJ) specimens have been investigated in this paper. Numerical and analytical methods have been used to obtain shear- and peel-stress distributions in the DLJ. The generalized analytical solution for the peel stress was calculated for various forms of the DLJ geometry and, by using crack closure integral (CCI) and by means of the J-integral approach, the analytical strain energy-release rate, G, was calculated. Experimental fracture tests have also been conducted to validate the results. The specimens were made of steel substrates bonded by an adhesive and loaded under tension. Specimens with cracks on both sides and at either end of the DLJ interface were tested to compare the fracture behavior for the two crack positions where tensile and compressive peel stresses exist. Tests confirmed that the substrates essentially behave elastically. Therefore, a linear elastic solution for the bonded region of the DLJ was developed. The fracture energy parameter, G, calculated from the elastic experimental compliance for different crack lengths, was compared with numerical and analytical calculations using the experimental fracture loads. The stresses from analytical analysis were also compared with those from the finite element results. The strain energy-release rate for fracture, G _f , for the adhesive has been shown to have no R-curve resistance, was relatively independent of crack length, and compared well with those obtained from numerical and analytical solutions. However, it was found that fracture energy for the crack starter in the position where the peel stress was tensile was about 20% lower than where the crack was positioned at the side, where the peel stress was found to be compressive.     

Preparation and fracture behavior of bionic layered SiCp/Al composites by tape casting and pressure infiltration

In this study, the bioinspired laminated composites with alternating soft Al layers and hard SiCp/Al were fabricated through the tape casting followed by pressure infiltration. In-situ bending and digital image correlation technology (DIC) analysis were carried out on the laminated composites. The results showed that the uniform layers of SiCp/Al and Al were obtained with the thickness of 30 μm and 10 μm, respectively. The interfaces between layers had an intimated combination. The bending deformation process of the laminated composites could be divided into three stages, i.e., crack initiation, crack stable diffusion and crack propagation instability. During deformation, the laminated structure changed the state of strain and strain distribution, further restricted the development of the crack, and the whole materials presented a stepped fracture. This study provides support for preparation and fracture process analysis of biomimetic layered composites prepared by tape casting.     

Toughening of laminated ZrB2-SiC ceramics with residual surface compression

Laminated ZrB₂-SiC ceramics with residual surface compression were prepared by stacking layers with different SiC contents. The maximum apparent fracture toughness of these laminated ZrB₂-SiC ceramics was 10.4 MPam^1/2, which was much higher than that of monolithic ZrB₂-SiC ceramics. The theoretical predictions showed that the apparent fracture toughness was strongly dependent on the position of the notch tip, which was confirmed by the SENB tests. Moreover, laminated ceramics showed a higher fracture load when the notch tip located in the compressive layer, whereas showed a lower fracture load as the notch tip within the tensile layer. The toughening effect of residual compressive stresses was verified by the appearance of crack deflection and pop-in event. The influence of geometrical parameters on the apparent fracture toughness and residual stresses was analyzed. The results of theoretical calculation indicated that the highest residual compressive stress did not correspond to the highest apparent fracture toughness.     

Ceramic Composites with Three-Dimensional Architectures Designed to Produce a Threshold Strength—II. Mechanical Observations

Finite element modeling and linear elastic fracture mechanics are used to model the residual stresses and failure stress of ceramic composites consisting of polyhedral alumina cores surrounded by thin alumina/mullite layers in residual compression. This type of composite architecture is expected to exhibit isotropic threshold strength behavior, in which the strength of the composite for a particular assumed flaw will be constant and independent of the orientation of tensile loading. The results of the modeling indicate that the strengths of such architectures will be higher than those of laminates of similar architectural dimensions that were previously found to exhibit threshold strength behavior for a particular flaw type. Flexural testing of the polyhedral architectures reveals that failure is dominated by processing defects found at junctions between the polyhedra. Fractography revealed the interaction of these defects with the residual stresses in the compressive layers that separate the polyhedra.     

Edge Chipping Resistance of Alumina/Zirconia Laminates

The edge chipping test was used to measure the fracture resistance of alumina/alumina‐zirconia laminated structures. Tailored, symmetrical laminated structures were prepared with a variety of layer thickness. The laminates had a significantly greater edge chipping resistance. Laminates with thin layers were just as effective in impeding edge chips as laminates with thick layers.     

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.