New Development of the J-Based Fracture Testing Technique for Ceramic-Matrix Composites

The J -based fracture testing technique is newly extended to experimentally determine the tension-softening (σ–δ) relations in ceramic-matrix composites. The J -based technique originally proposed for concrete has been well established for quasi-brittle materials where the fracture process is primarily dominated by the formation of a fracture process zone and the contribution of the crack tip toughness is negligibly small. In this study, the J -based technique is further developed to cover a more general case, i.e., a material in which the crack tip stress singularity coexists with the fracture process zone. This is the case, in particular, for modern fiber-reinforced ceramic composites and coarse-grained ceramics. The newly derived testing technique has been applied to foam glass composites reinforced with SIC and stainless steel short fibers. The validity of the deduced tension-softening relations has been examined by microscopy observations and by comparing with other conventional testing methods: the fracture energy method and the R- curve approach. It is suggested that the J -based fracture testing technique can provide reasonable tension-softening relationships and fracture parameters in modern ceramics and ceramic-matrix composites.     

The plane-strain essential work of fracture as a measure of the fracture toughness of ductile polymers

We have extended the essential work of fracture technique to allow for the determination of the plane-strain essential work of fracture. The new technique is to measure the specific work of fracture as a function of ligament length in deeply double edge notched samples. This type of data is then experimentally corrected to remove the plastic work of fracture and leave only the essential work of fracture as a function of ligament length. By extrapolating the essential work of fracture to zero-ligament length, we claim to be measuring the plane-strain essential work of fracture. This new technique was applied to two rubber toughened nylons and to a series of polyethylenes. The plane-strain essential work of fracture was found to be independent of thickness. Where comparison can be made to J-integral testing, the plane-strain essential work of fracture was similar to the critical J-integral, J_Ic.     

The effect of processing conditions on the fracture behavior of syndiotactic polystyrene films

A fracture behavior map for syndiotactic polystyrene (sPS) film as a function of annealing temperature and the extension ratio has been developed. The fracture initiation and propagation processes of the film were characterized by a specialized video-based real-time thin-film fracture technique. An unusual layered complex deformation mechanism has been observed for the films fabricated at high extension ratios. This mechanism enhances the resistance to crack growth in sPS films. © 1996 John Wiley & Sons, Inc.     

Temperature Dependence of Hertzian Indentation Fracture Surface Energy of ThO2

The fracture surface energy, γ, of ThO₂ was measured between room temperature and 400°C using the Hertzian indentation technique with cold or preheated steel and alumina indentors. The value of γdecreased strongly with temperature with a suggested saturation at 350°C. This may indicate the change from an unrelaxed fracture surface to a relaxed surface at temperatures where oxygen becomes mobile. The result obtained at 400°C is γ=1.4 J m⁻².     

Fatigue fracture behavior of chlor-alkali membranes

The fatigue fracture behavior of chlor-alkali monolayer membranes is investigated under a condition similar to the membrane electrolysis cell service environment, i.e., at 86°C in brine solution. The linear elastic fracture mechanics (LEFM) approach, i.e., da/dN vs. AK, is implemented in this study. It is found that the fatigue crack propagation behavior of the chlor-alkali membranes can be successfully characterized via the LEFM technique. The fatigue fracture surfaces of the membranes are also investigated using stereo-optical microscopy and are found to show typical fatigue striation patterns. Detailed experimental procedures for preparing and testing of the membranes are presented.     

Fracture behaviours of in situ silica nanoparticle-filled epoxy at different temperatures

Fracture behaviours of nanosilica filled bisphenol-F epoxy resin were systematically investigated at ambient and higher temperatures (23 °C and 80 °C). Formed by a special sol-gel technique, the silica nanoparticles dispersed almost homogenously in the epoxy resin up to 15 vol.%. Stiffness, strength and toughness of epoxy are improved simultaneously. Moreover, enhancement on fracture toughness was much remarkable than that of stiffness. The fracture surfaces taken from different test conditions were observed for exploring the fracture mechanisms. A strong particle-matrix adhesion was found by fractography analysis. The radius of the local plastic deformation zone calculated by Irwin model was relative to the increment in fracture energy at both test temperatures. This result suggested that the local plastic deformation likely played a key role in toughening of epoxy.     

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.     

Effects of indenter angle on micro‐scale fracture toughness measurement by pillar splitting

We present improvements to a recently developed pillar splitting technique that can be used to characterize the fracture toughness of materials at the micrometer scale. Micro‐pillars with different aspect ratios were milled from bulk Si (100) and TiN and CrN thin films, and pillar splitting tests were carried out using four different triangular pyramidal indenters with centerline‐to‐face angles varying from 35.3° to 65.3°. Cohesive zone finite element modeling (CZ‐FEM) was used to evaluate the effect of different material parameters and indenter geometries on the splitting behavior. Pillar splitting experiments revealed a linear relationship between the splitting load and the indenter angle, while CZ‐FEM simulations provided the dimensionless coefficients needed to estimate the fracture toughness from the splitting load. The results provide novel insights into the fracture toughness of materials at small‐scales using the pillar spitting technique and provide a simple and reliable way to measure fracture toughness over a broad range of material properties.     

Theory for the Load Relaxation Technique in Fracture Studies Using Four-Point-Bend Chevron-Notched Specimens

The load-relaxation technique is reviewed using the energy approach for four-point-bend chevron-notched samples. The influence of the sample and bending geometry and the stiffness of the testing system on the strain-energy release rate is considered. Proper experimental design allows the study of fracture initiation, propagation, and arrest, Special attention is paid to the effect of overload at fracture initiation on crack propagation. It is concluded that the chevron–notched sample offers great flexibility for gaining information on the fracture properties of ceramic materials.     

Coarse-grain CeO2 doped ZrO2 ceramic prepared by spark plasma sintering

In the present work, coarse grain cerium stabilized zirconia bulk ceramic was prepared by spark plasma sintering technique. The relatively high temperature of 2000 °C used for sintering led to enormous grain growth up to approximately 100 μm. Sintering at high temperatures and in the vacuum caused oxygen depletion and thus transformation from tetragonal to cubic phase during the sintering process. The tetragonal phase was recovered by annealing at 1400 °C in air. This led to a change in fracture behavior. Mostly transgranular fracture of the cubic phase was changed to intergranular fracture after recovering the tetragonal phase. On the intergranular fracture surface, twinning-like structure and structures similar to antiphase domain were observed.Mechanical properties represented by indentation hardness of prepared samples were evaluated.     

Relationship between fracture toughness characteristics and morphology of sintered Al2O3 ceramics

The influence of sintering temperature and soaking time on fracture toughness of Al₂O₃ ceramics has been investigated. The samples were prepared by solid state sintering at 1500, 1600 and 1700 °C for different soaking time periods. The fracture toughness of the sintered samples was determined by inducing cracks using Vickers indentation technique. Microstructural investigations on fracture surfaces obtained by three point bend test mode were made and correlated with fracture toughness. Crack deflection in the samples sintered at 1500 and 1600 °C for which ranges of fracture toughness are 5.2–5.4 and 5.0–5.6 MPa m^1/2 respectively, are found. The samples sintered at 1700 °C have lower fracture toughness ranging between 4.6 and 5.0 MPa m^1/2. These samples have larger grains and transgranular fracture mode is predominant. The crack deflection has further been revealed by SEM and AFM observations on fracture surface and fracture surface roughness respectively.     

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.     

Visualization of Impact Damage in Ceramics Using the Edge-On Impact Technique

Projectile impact generates severe fragmentation in ceramics which propagates at high velocities and precedes the penetration of the projectile. The high-speed photographic technique of the Edge-On Impact (EOI) has been developed at the Ernst-Mach-Institute (EMI) in order to visualize dynamic fracture in brittle materials. In a typical EOI test the projectile hits one edge of a specimen and fracture propagation is observed during the first 20 us after impact by means of a Cranz-Schardin highspeed camera. EOI tests allow a characterization of different ceramics by the macroscopic fracture patterns, single crack velocities, and crack front velocities (damage velocities). The phenomenology of damage propagation in several ceramics and a ceramic-metal composite is discussed. The EOI technique is useful for the evaluation of damage models for brittle materials because it enables a direct comparison of model predictions to experimental data obtained during the impact process.     

The effect of water on the fracture surface energy of fiber-reinforced composite materials

The effect of absorbed water on the fracture surface energy of both glass and graphite fiber unidirectional epoxy composites has been studied. The fracture energy parallel to the fiber direction was measured by the double torsion technique. Water was absorbed at 70°C while the sample was subject to an external tensile stress both parallel and perpendicular to the fiber direction. No degradation of fracture surface energy was measured for specimens stressed and immersed in water at 70°C for up to 1000 h. The fracture surface energy for the glass composite was greater than for the graphite composite, which was attributed to fiber pull-out in the case of the glass composite.     

Fracture Behavior of Directionally Solidified Y3Al5O12/Al2O3 Eutectic Fiber

The strength and fracture of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth technique. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The tensile strength and Weibull's modulus of the eutectic fibers were determined in the as-fabricated state and after extended thermal exposure at 1460°C in air. Fractographic analysis was used to identify and classify the strength-limiting mechanisms. The fracture toughness and crack growth behavior were characterized by an indentation technique. A fracture mechanics analysis was also used to establish the relationships between surface flaw size, tensile strength, and fracture toughness of the fiber.     

Reinforcement of precursor-derived Si–C–N ceramics with carbon nanotubes

Nanocomposites consisting of precursor-derived Si–C–N ceramics incorporated with carbon nanotubes (CNTs) were successfully prepared by casting of a mixture of CNTs and a liquid precursor polymer followed by cross-linking and thermolysis. The effect of CNTs on the fracture toughness of these nanocomposites was investigated by a thermal loading technique. The results reveal a dependence of the fracture toughness on the type of the CNTs. One type shows a significant increase of the fracture toughness at CNT contents of only 1–2 mass%, whereas the other one exhibits no effect. The microstructural effects of CNTs observed at the fracture surfaces of the nanocomposites by scanning electron microscope (SEM) and transmission electron microscope (TEM) can be correlated with the observed fracture toughness behavior.     

Microstructure and properties of directionally solidified LaB6 (100)-ZrB2 eutectic composite prepared by the optical zone melting method

Directionally solidified (DS) LaB₆ (001)-ZrB₂ eutectic composite is successfully prepared by optical zone melting technique. The effect of the solidification rate on the microstructure and properties is systemically investigated.With the increase in the solidification rate from 20 to 100 mm/h, the eutectic rods present a homogeneous eutectic microstructure. The relationship between the average eutectic spacing and the solidification rate is established. The fracture toughness shows an obvious anisotropy. Crack deflection and crack bridging play important roles in improving the fracture toughness of the material. As the solidification rate is increased from 20 to 300 mm/h, the fracture toughness, bending strength and current density firstly increase and then decrease. The highest fracture toughness of 5.16 MPa.m^1/2, bending strength of 809.04 MPa and current density of 36.24 A/cm² at 1873 K belong to the DS LaB₆ (001) - ZrB₂ eutectic composite obtained at V = 100 mm/h.     

Determination of the fracture energy in polymeric films by in situ photoelasticimetry on double edge notch specimen

The fracture toughness is key parameters to select polymeric films. The essential work of fracture (EWF) is a phenomenological but efficient way to characterize this resistance to fracture. One can gain valuable information on the resistance to perforation and propagation of flaws. A new technique was developed to better understanding the EWF experiments. A tensile test combined to photoelasticimetry allows following in situ the geometry and amount of plastic deformation on double edge notched specimen. The EWF parameters are determined when the plastic deformation appears constant, so when the fracture energy W_f only contributes to rupture filament. This new methodology requires just a single sample, whereas at least five specimens are required for general method. It will help characterize expensive polymeric films or reveal the heterogeneous behavior, for instance after polymer ageing. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42854.     

High-Speed fracture mechanics by photography of polypropylene copolymers

The use of photography applied to stroboscopy in analyzing the processes inherent to the starting and propagation of cracks in materials is a technique which has proved to be of great interest, especially since it enables one to check and “directly” study the evolution of such phenomena. Using fracture mechanics criteria this technique has been applied to the study of the impact behavior of some polypropylene copolymers at different rubber contents obtained either by blending or by synthesis. This technique makes it possible to determine numerous parameters of fracture mechanics, including C.O.D. (crack opening displacement), C.O.A. (crack opening angle), JIc, the plastic work parameter, determined from the resistance curve R, tearing modulus, and crack propagation velocity. Furthermore, under high strain rate conditions, the value taken on by the coefficient “λ” relating the J-integral to C.O.D. (1, 2) were checked for those materials using the equation J = λ. σy · C.O.D., Hayes and Turner (3) and Boyle (4). From analysis of the materials it was possible to note that the synergetic effect of the EP (ethylenepropylene) rubber increased, especially when present at percentages of more than 10 percent. Annealing the materials, on the other hand, produced an increase in fracture toughness for those products having a low rubber content; however it did not have any effect on those with an elevated rubber content (26 percent).     

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).