Indentation Method for Determining the Macroscopic Fracture Energy of Brittle Bimaterial Interfaces

Based on the mechanics analysis of crack-interface interaction, a simple and direct micro-indentation technique has been developed to evaluate the fracture energy of a bimaterial interface. The technique, when applied to a pristine planar SiC-Si₃N₄ interface at various angles of attack, is shown to provide a reasonable estimate of the interfacial fracture resistance. The experimentally obtained fracture energy has been compared favorably with a proposed atomic model of bond breaking between SiC and Si₃N₄.     

Fracture Energy of Epoxy Interfaces with Layers of Different Silane Coupling Agents

We investigated the fracture energy, Gc, and the threshold fracture energy, Gth, in water at 80°C of interfaces between epoxy and layers of five silane coupling agents: (3-glycidoxypropyl)trimethoxysilane (GPS), (3-aminopropyl)trimethoxysilane (APS), [3-(phenylamino)propyl]trimethoxysilane (PAPS), (3-mercaptopropyl)trimethoxysilane (MPS), and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPS) on the native silicon oxide surface. While 10-nm-thick layers of these all had similar high Gc, APS had the best overall values of Gth, but all adhesion promoters, except for AEAPS, performed roughly similarly. The results from the XPS analysis of the silicon side of the fracture surfaces suggest that for all silane layers, the fracture occurs somewhere within that layer. When the silane coupling agents were added to the epoxy, the samples with the higher threshold fracture energies exhibited a thicker layer of the epoxy remaining on the native silicon oxide surface. Silane coupling agents that perform well as deposited layers do not necessarily perform well as additives to epoxies.     

Fracture Toughness of Ceramic Precracked Bend Bars

Fracture toughness was measured for four ceramic materials using precracked bend bar specimens. The effect of the precracking parameters, used for the bridge indentation method on fracture toughness values, was determined. Excellent agreement was obtained between fracture toughness values measured by this method and values obtained by other techniques.     

Effects of Substrate Orientation and Metal Film Thickness on the Intrinsic Strength, Intrinsic Fracture Energy, and Total Fracture Energy of Tantalum–Sapphire Interfaces

Parameters that characterize interface fracture are defined, and procedures to measure them are discussed. The interface strength σ_o is measured by using a novel laser spallation experiment, which uses a laser-induced stress wave to separate the interface. The intrinsic ( G _o) and total ( G _c) fracture energies are measured using a double cantilever beam experiment performed at ambient and cryogenic temperatures, respectively. These experiments are used to obtain relationships between G _c and G _o, and between σ_o and G _o, for interfaces between sputter-deposited polycrystalline Ta coatings and sapphire substrates. The intrinsic toughness and strength were modified by changing the orientation of the sapphire surface (basal and prismatic), while G _c was varied by changing the test temperature (ambient and cryogenic) and the thickness (1–3 μm) of the ductile Ta layer. Besides providing values that have interest in their own right, the work presented here provides a general framework for designing interfaces in bonded structures and serves as a basis to develop atomistic and continuum interface fracture models.     

Bimaterial Composites via Colloidal Rolling Techniques: III, Mechanical Properties

The mechanical properties of layered Ce-TZP/Al₂O₃ composites with laminate and cellular morphologies have been investigated. The strength and toughness increase as the layer thickness decreases, and the amount of transformation in the Ce-TZP layer increases discontinuously at the laminate/cellular transition. Very high strengths (1.1 GPa) and toughnesses (16 MPa·m^1/2) have been obtained in some cases. These results indicate that the progressive refinement of layer microstructure and the disruption of planar connectivity of phases are beneficial to the mechanical performance, because they provide more stress concentrators to trigger stress-assisted transformation for toughening functions. The composites of finer microstructure, with a layer thickness of lessthan equal to20 µm, have a homogeneous hardness of 11.5 GPa, which is a considerable improvement over that of Ce-TZP alone.     

A Four-Point Bend Technique to Determine Dynamic Fracture Toughness of Ceramics

The procedure for determining quasi-static fracture toughness of ceramics has been standardized. To expand the loading rate into the dynamic region, the dynamic equilibrium over the entire specimen needs to be satisfied to interpret the crack tip loading state with the far-field loading conditions. Furthermore, to determine the loading-rate effects, the loading rate at the crack tip should be nearly constant during an experiment. A new four-point bending experimental technique, based on a split Hopkinson pressure bar, has been developed to determine the dynamic fracture toughness of ceramics at high rates under valid conditions, which is demonstrated through the determination of the dynamic fracture toughness as a function of loading rate for a silicon carbide (SiC–N).     

A Triangular Double Cantilever Beam Test for Measuring Adhesive or Cohesive Fracture Energy

A thin layer of adhesive bonded between two stiff elastic plates of uniform thickness and triangular in shape is recommended as a test specimen for measuring cohesive or adhesive strength. A similar test was employed many years ago by Mostovoy et al. ^1,2 but appears to have received little attention in the intervening period. Nevertheless, it has marked advantages in comparison with current ASTM tests in simplicity of construction and use. Examples are given using silicone rubber layers bonded between steel plates.     

A Triangular Double Cantilever Beam Test for Measuring Adhesive or Cohesive Fracture Energy

A thin layer of adhesive bonded between two stiff elastic plates of uniform thickness and triangular in shape is recommended as a test specimen for measuring cohesive or adhesive strength. A similar test was employed many years ago by Mostovoy et al.^1,2 but appears to have received little attention in the intervening period. Nevertheless, it has marked advantages in comparison with current ASTM tests in simplicity of construction and use. Examples are given using silicone rubber layers bonded between steel plates.     

Bimaterial Composites via Colloidal Rolling Techniques: I, Microstructure Evolution during Rolling

Laminate composites of Al₂O₃ and ZrO₂ fabricated by repeated rolling and folding are demonstrated using aqueous slurries of ceramic powders without any plasticizers or binders. Crack-free composites with a layer thickness of 4-60 µm have been obtained. A novel interface instability during the rolling of bimaterial slurries, which leads to the entrapment of the rheologically harder phase in the softer phase, also has been discovered. This discovery has been exploited to obtain layered materials, by matching the yield stress of the slurries, or cellular materials with the desired included phase, by mismatching the yield stress of the slurries. Adjustment of the yield stress can be realized by varying only the NH₄NO₃ salt concentration while keeping the pH and the solids loading (64 vol%) constant.     

Fracture Velocity and Fracture Energy of Glass in the Fatigue Range

A method is developed for determining crack velocities from the stress-time curve of fracture. Velocities of glass broken in air and in vacuum converge at a value between 1 and 10 mm. per second. This convergence is considered to be the upper limit of the fatigue range. Fracture energy has been computed in terms of strain energy release rates. For glass broken in air under low stresses this energy is about equal to the surface energy of the glass, but when in vacuum it is fifteen times greater. At the upper limit of the fatigue range it is thirty times greater, whereas at the terminal velocity of fracture it is of the order of fifty times greater. It is concluded that surface energy must constitute only a small part of the energy absorbed in the fracture process. This excess energy has a pronounced influence on the fracture process and on the measured strength of glass.     

Influence of Covalent Bonds on the Adhesion Energy at Elastomer-Glass Interfaces

The aim of this study was to analyze the effect of interfacial covalent bonds on the adhesive behavior of an elastomer, a crosslinked polydimethylsiloxane, and a glass substrate. These covalent bonds were created by applying to both materials an appropriate surface treatment by means of plasma polymerization. Adhesion measurements were carried out by analyzing the contact area between a rubber hemisphere and a flat rigid glass plate. The contact was forced under a given compressive loading for different times t_c, then the load was removed and the fracture propagation at the interface was recorded as a function of relaxation time t_r. Finally, adhesion energies were also determined by means of a probe test using a tensile testing machine.     

Re-evaluation of Adhesive Fracture Energy

A model to evaluate the Energy Release Rate (ERR) of adhesives using the Double Cantilever Beam (DCB) specimen is described. The model accounts for the adhesive bond thickness and its material properties. The analysis, considered as an improvement to the built-in cantilever beam model, treats the adherend as a finite beam which is partly free and partly supported by an elastic foundation and the adhesive bond as a thin strip under prescribed displacement. The results show significant effect of the adhesive parameters on the total ERR and that the built-in cantilever model underestimates the ERR. In general, the contribution of the adhesive bond to the ERR increases for softer adhesives, shorter cracks and thicker bonds.     

Fracture Surface Energy of Glass

Fracture surface energies of six glasses were measured using the double-cantilever cleavage technique. Values ranged from 3.5 to 5.3 J/m² depending on the chemical composition of the glass and the temperature of the test. The fracture surface energy increased with decreasing temperature and increasing Young's modulus; however, exceptions to this behavior were noted. The magnitude of the values obtained is discussed with respect to the theoretical strength of glass and possible irreversible effects at the crack tip such as stress corrosion and plastic deformation are considered.     

Bimaterial Composites via Colloidal Rolling Techniques: II, Sintering Behavior and Thermal Stresses

Shrinkage behavior and crack formation during firing have been investigated for Al₂O₃/Ce-TZP composites that have been fabricated by colloidal rolling and folding. These composites show improved sinterability and sinter isotropically after repeated rolling. Interface instability in rolling creates corrugated interfaces with large layer waviness; therefore, rolling can substantially alleviate the in-plane sintering constraints, which leads to improved sinterability. A loss of sintering anisotropy also is observed and is directly correlated to the microstructure instability, which is coincident with the laminate-cellular transition. Sintering cracks during heating and thermal cracks during cooling both are limited to the thick Ce-TZP layers in the composites. The critical layer thickness and the normalized crack spacing of the thermal cracks follow the predicted behavior of elasticity theory. Thus, crack-free, high-density Al₂O₃/Ce-TZP composites with either a laminate or cellular microstructure can be obtained, with a layer thickness of 4-60 µm, via pressureless sintering.     

Fracture Analysis of the Constrained Blister Test

The constrained blister test is analyzed in the framework of linear elastic fracture mechanics. Exact results for the energy release rate and the mode mixedness are obtained as functions of quantities which are natural to measure in the test. Effects of geometrical non-linearities and initial in-plane stresses are included. An analysis of the range of loads for which the test results in configurationally stable delamination is performed.     

On the Vickers Indentation Fracture Toughness Test

The Vickers indentation fracture toughness test, or VIF, is addressed by considering its origins and the numerous equations that have been applied along with the technique to estimate the fracture resistance, or the K _{I c} of ceramics. Initiation and propagation of cracks during the VIF test are described and contrasted with the pre-cracking and crack growth for internationally standardized fracture toughness tests. It is concluded that the VIF test technique is fundamentally different than standard fracture toughness tests. The VIF test has a complex three-dimensional crack system with substantial deformation residual stresses and damage around the cracks. The VIF test relates to an ill-defined crack arrest condition as opposed to the rapid crack propagation of the standardized fracture toughness tests.
Previously published fracture toughness results employing the VIF technique are reviewed. These reveal serious discrepancies in reported VIF fracture toughness values. Finally, recent fracture resistance measurements by the VIF technique for the Standard Reference Material SRM 2100 are presented. These are compared with standardized test results for the same material. It is concluded that the VIF technique is not reliable as a fracture toughness test for ceramics or for other brittle materials. What the VIF actually measures in terms of fracture resistance cannot be readily defined. It is recommended that the VIF technique no longer be acceptable for the fracture toughness testing of ceramic materials.     

Measurement of Fracture Surface Energy of SiC

The fracture surface energies of 5 polycrystalline silicon carbides were measured at room temperature using the double-cantilever-beam, notched-beam, and work-of-fracture methods. All methods yielded comparable fracture surface energies,     

Grain-Size Dependence of Fracture Energy in Ceramics

A simple model incorporating thermal elastic anisotropy stresses is used to calculate the microcrack zone size around cracks in Al₂O₃. It is found that the ratio of microcrack zone size to grain size is almost constant for notched beam tests, but increases with grain size for double cantilever beam data. It is suggested that notched-beam ratios of fracture toughness are related to crack initiation, whereas double cantilever beam values are related to propagation and reflect R-curve behavior of the material.     

On the Fracture Energy, Rehealing Velocity and Refracture Energy of Cast Epoxy Resin

The fracture energy of cast epoxy resin was measured by a novel method in the presence of various agents. The crack spontaneously rehealed under heating to above the heat distortion temperature to give a fresh joint of about the same fracture energy as the virgin material.