Indentation Cracks Research Articles

Material Fracture Character of Marble Processing by Single Diamond Tools

Material fracture character of marble processing by single diamond tools was studied by single diamond indentation experiment and fast milling experiment. The surface morphology and inner cracks were achieved based on double lights transmission of stereomicroscope, marble semitransparent character and SEM technology. Results showed Indentation cracks break crystal or extend along cleavage plane from three directions. The marble indentation fracture progress could be divided into three phases: plastic deformation, fracture deformation and block broken. Material broken types were diverse for different compositions in marble.

FIB Tomographic Analysis of Subsurface Indentation Crack Interactions with Pores in Alumina

This article describes a study involving three‐dimensional FIB tomographic characterization of Vickers microindentation sites in an alumina material containing pores. The study was carried out using a dual beam FIB instrument for loads in the range 50–200 g. The results obtained show that this technique can be used to obtain 3‐D distributions of indentation‐generated cracks. Although, compared to a dense material, presence of pores in alumina reduced the ability to generate and/or propagate radial cracks, the significant probability of crack‐pore interactions indicates that pores assist in the generation/propagation lateral cracks. The subsurface images show that pores act as sites for crack initiation and/or assist in crack propagation. In addition, a clearly reduced ability to generate and/or propagate radial cracks has been noted. However, considering that FIB tomography is destructive and invasive, microstructural changes seem to have occurred during the process and consequently it is not possible to identify all the generated cracks. Moreover, propagation of some of the cracks also occurred due to material removal in FIB milling.

Fracture toughness of a directionally solidified Al–Nb–Ni ternary eutectic

Fracture toughness ( K IC), an important property of brittle materials, can be determined by indentation cracking tests. This paper reports on an investigation of the fracture toughness of a directionally solidified Al 3Nb–Nb 2Al–AlNbNi ternary eutectic, based on the Vickers indentation test applied to longitudinal and transverse sections of its microstructure. The measurements were taken using indentation loads varying from 2.45 to 24.5 N. Correlations between the resulting crack parameters and indentation load were evaluated using Palmqvist and half-penny cracks models. In the range of indentation loads studied, the results suggested that the Palmqvist model provided a better fit to the experimental data. Fracture toughness was calculated using equations developed for Palmqvist crack mode. The indentation fracture toughness values for longitudinal and transverse sections are in the range of 2.82–3.05 MPa m 1/2 and 2.98–3.59 MPa m 1/2, respectively. It was found that the addition of Ni and incorporation of a third phase to the Al 3Nb–Nb 2Al eutectic improved fracture toughness of this in situ composite material.

Adhesive and cohesive properties of nanostructured Zr0 2 coatings by the original Vickers Indentation Cracking technique

In order to characterize the adhesion of nanostructured plasma-sprayed Y 2O 3–ZrO 2 coatings presenting some geometrical specificities, it is here suggested to apply the Vickers Indentation Cracking (VIC) technique, which is capable of initiating a crack at the interface by performing the indentation test close to the interface within the substrate. This method renders it possible to overcome the difficulties concerning the characterization of the adhesion of nanostructured Y 2O 3–ZrO 2 (YSZ) coatings. It was suggested to calculate an adhesive stress parameter by employing two physical parameters, i.e., the critical load to initiate the crack and the indentation distance measured between the interface and the indent center. In addition, for the case of brittle coatings, the crack was found to deviate from the interface toward the coating. Under such conditions and on the basis of the critical point (given by the two parameters: load and indentation distance), a stress parameter representative of the cohesive properties of the coating was defined according to the crack deviation conditions.

Determination of fracture toughness and energy dissipation of SiO2-poly(methyl metacrylate) hybrid films by nanoindentation

In this paper we have applied different methods based on nanoindentation techniques to measure the toughness of SiO2-poly methyl-methacrylate hybrid films on organic acrylic substrates. The hybrid films were deposited by the Sol–Gel method from precursor solutions containing tetraethyl-orthosilicate, methylmethacrylate and 3-trimethoxysilyl propyl methacrylate (TMSPM) as the coupling agent. The influence of TMSPM content in the hybrid precursor solution on the fracture behavior of the hybrid films was studied. The classical indentation crack length method was applied from nanoindentation tests to determine the stress intensity factor by direct measurement of crack length from atomic force microscopy images. A second method, based on the pop-in analysis allowed the separation of crack formation from film delamination from multiple pop-ins. Finally, a third method based on energy methods is also reported and discussed. The amount of TMSPM in the precursor solution showed a strong influence on the toughness of the hybrid films.

ELECTRIC, MAGNETIC AND MECHANICAL COUPLING EFFECTS ON FERROELECTRIC PROPERTIES AND SURFACE POTENTIAL OF BiFeO3 THIN FILM

BiFeO 3 (BFO) is usually considered as a multiferroic material, which shows both ferroelectric and antiferromagnetic properties at room temperature. It is an important material having potential applications in various spintronics devices. Many studies have been done to understand the ferroelectric and magnetic responses of BFO; therefore, this work is focused on characterizing the coupling effects of electric, magnetic and mechanical field on the ferroelectric properties and surface potential of the BFO thin films. A polycrystalline BFO film (~200 nm) is deposited on the Si substrate by the Pulse Laser Deposition techniques. The Vertical Piezoresponse Force Microscopy (PFM) is used to study the ferroelectric domain structure and the hysteresis loop of as-deposited BFO film as well as the effects of the mechanical stress and magnetic field on those properties. Kelvin Probe Force Microscopy (KPFM) is used to measure the surface potential of the samples. To apply the mechanical stress on the film, micro-indentations were made on the surface of the film with two different loads (1.96 N, 2.94 N). Then the ferroelectric domain structure, strain amplitude, domain switching (hysteresis response) behavior and surface potential are measured at the locations near the side of the indentation cavity and cracks. This way, the coupling effects of electric, magnetic and stress field on BFO film are studied at nanoscale.

Crack-Tip Toughness from Vickers Crack-Tip Opening Displacements for Materials with Strongly Rising R-Curves

Accurate measurements of the intrinsic crack-tip toughness, KI0, are essential to understanding the strength and toughness of bridging ceramics. Furthermore, even the most carefully made R-curve measurements cannot accurately assess this initial toughness point. In this manuscript, both rigorous and approximate methods are proposed for determining KI0 from crack-tip opening displacement (COD) measurements on Vickers indent cracks for materials with steeply rising R-curves. Such approaches are attractive because of the relative ease of producing indentation cracks and analyzing the CODs; further, the latter method is of particular interest because it saves considerable computational effort. Both the advantages and limitations of the proposed methods are discussed. Finally, based on applying the above methods to three Si3N4 ceramics, it was concluded that there is a common crack-tip toughness of KI0≈2.2±0.3 MPa·√m.

Quantitative optical fluorescence microprobe measurements of stresses around indentations in Al 2O 3 and Al 2O 3/SiC nanocomposites: The influence of depth resolution and specimen translucency

Residual stresses around 1 kg Vickers indentations in Al 2O 3 and Al 2O 3/SiC nanocomposites were measured using high-resolution Cr 3+ fluorescence microscopy. Experiments and modelling showed that the use of non-confocal microscopes can lead to significant underestimation of the surface stress in Al 2O 3 because of the sampling of subsurface regions where the stresses are lower. The nanocomposites were less sensitive to the depth resolution of the microscope because their strong absorption limited the depth from which fluorescent radiation was collected. The use of confocal microscope settings allowed accurate measurements to be made and the indentation stresses were found to be very similar in Al 2O 3 and the Al 2O 3/SiC nanocomposites. The stresses measured were significantly different from the predictions of the Yoffe model for indentation stresses. This was because of indentation cracking, which is not accounted for in the model. Cracking was also considered to be important in determining the plastic zone size in ceramics, which is much smaller relative to the indentation size than in metals.

Strength Evaluation of Alumina Spray Coating (2nd Report, Proposal for Judgment of Crack Interference and Correction Methods of Crack- Shape)

The alumina spray coating is a material containing many pores (cracks) and there is a crack-interference in the alumina spray coating, therefore, the crack-interference leads to a decrease in the fracture strength. There are some studies that have used the elastic analysis for two or more cracks. Moreover, there are some standards for the combining of the cracks. However, in ceramics including alumina, a study of quantifying the internal spread of the crack when interfering and presuming a static fracture strength with judgment of the crack-interference after the stable crack propagation is not known. In this study, after the alumina spray coating single layer was taken out, a Vickers' indentation crack was introduced, a 4-point bending test was performed and a R-curve was acquired. As a result, the fracture strength characteristic of the alumina spray coating single layer was clarified. Three judgments of the crack-interference with the R-curve were then proposed. Moreover, a correction method of the crack-shape when the cracks interfere was proposed. The validities of the proposals were verified with bending tests after two Vickers' indentation cracks were introduced.

OS2208 押込み法による高強度鋼の水素ぜい化感受性評価法の提案(OS22-3 水素と強度,OS-22 水素環境の材料強度特性への影響)

This study investigates hydrogen embrittlement cracking (HEC) upon indentation in a high-strength steel. When an indentation test is applied to the high-strength steel, i.e., steel that has absorbed hydrogen, radial cracks appear around the impression, whereas as-received steel with no hydrogen absorption does not produce any cracks. An experimental/computational investigation is carried out to elucidate the mechanism of such indentation cracks caused by hydrogen embrittlement. The crack propagation is computed based on cohesive zone model in finite element method in order to investigate stress intensity factor at the crack. It is found that crack initiates at loading, and then it propagates under unloading. The value of stress intensity factor at indentation crack tip agrees well with the threshold value of crack propagation of HEC (K_<ISCC>). This indicates that the length of the indentation HE crack can roughly estimate the value of K_<ISCC>.

Cracks formed by Vickers indentation adjacent to the interface in bonded dental ceramics with various marginal angles

The objective of this study was to assess the influence of the ceramic marginal angle on the length and nature of indentation cracks in ceramics near the ceramic/resin bonded interface. Disks of a leucite-reinforced ceramic or a diopside-based glass-ceramic bonded to a resin composite were sectioned so that the ceramic marginal angles were 45, 60, and 90°. Vickers indentations were placed in the ceramic at various distances from the bonded interface. The lengths of the indentation cracks running near parallel to the interface were measured and the orientation of crack propagation was characterized. The crack length and orientation were significantly affected by the distance from the interface and by the marginal angle, respectively. The crack length extended as the distance from the interface was shortened. Smaller marginal angles resulted in more oblique cracks. The toughness of the ceramic affected the indentation crack length, shape, and direction.

Finite Element Analysis of Indentation Cracks for Brittle Materials under Static Loading

The indentation method is able for use in determining the fracture properties of materials. In the present work, a source code programme for two-dimensional finite element method has been applied to simulate cracking behaviour during indentation on brittle specimens under static loading conditions. The study also aims to predict the crack propagation trajectories under Linear Elastic Fracture Mechanics (LEFM). The source code is written in FORTRAN language. The FE mesh is generated using an advancing front method, where the generation of the background mesh and the construction of singular elements are also added to this developed programme to realise the fracture analysis. In evaluating the accuracy of the crack path predictions, the results are compared and validated with the sets of experimental data of relevant published research work. Upon comparison, it is proven that this developed source code programme is capable of demonstrating the crack indentation in terms of predicting the crack trajectories as well as the evaluation of the stress intensity factors.

Fracture Behavior of Silicon Carbide Measured by Different Test Methods

Single edge notched beam (SENB) and indentation methods (IM) were used to test the fracture toughness (KIC) of SiC ceramic in present work. Both of testing results were compared. The applicative computational equation of indentation crack was determined through studying indentation morphology formed by IM. The results showed that the indentation crack in SiC could be Babcock crack, and the KIC value calculated by Niihara (P) indentation equation approximated to real value mostly with minimum relative deviation.

Fracture Toughness of CNTs/AlN Ceramics Tested by Indentation

CNTs/AlN ceramics were fabricated by hot-pressing sintering process. The fracture toughness was measured by indentation method. The morphologies of indentation cracks were analyzed by SEM. The results show that the facture toughness of AlN was slightly improved because appreciate toughening mechanisms such as CNTs pull-out, crack bridging and deflect operate in CNTs/AlN. In addition, the facture toughness of CNTs/AlN increased with increasing CNTs content up to 3wt%. Then, the fracture toughness decrease when the CNTs content is 4wt%.

Effect of Humidity on Delayed Propagation of Indentation Crack under Sustained Electric Field in Ferroelectric Ceramics

The effect of humidity on delayed propagation of indentation crack in lead zirconate titanate ferroelectric ceramics under sustained electric field was investigated. The results indicated that delayed propagation of indentation crack could occur in humid air at 98%RH without electric field but it did not in air at RH  30%. The threshold electric field and incubation time for delayed propagation of indentation crack increased but the increment of propagation length decreased with decreasing the humidity. When the electric field is large of 0.14 times of the coercive field, the delayed propagation could also occur in vacuum.

Prestressed Ceramics Lateral Confined by Aluminum Alloy

This article presents the results for the prestress of ceramics lateral confined by aluminum alloy. Prestressed ceramics composite is produced by casting molten aluminum alloy around a hot ceramic tile and then cooling it down with blowing air. Different technical parameters of the casting are investigated. Al2O3/ZrO2(4Y) composite ceramics alumina tiles prepared by combustion synthesis under high gravity are used in this study to investigate the effect of confinement and prestress. The samples of ceramics are lateral confined by aluminum alloy, which lead to monaxial compressive stress in the tile and tensile stress in the metal. Thin and thick target configurations are used to understand the effect on pre-stressing performance, which simply and approximately allows high monaxial compressive pre-stresses. The residual stress in the metal should be lower than the yield strength of the metal, and this requirement can be approximately realized by Vickers indentation crack length.

Mechanical Properties and Plastic Behaviour Mechanism Induced by Nanoindentation Technique of YSZ and GDC Materials used as Electrolytes in Fuel Cells Devices

In this study, mechanical characterization at nanometric scale was carried out for different ceramic materials (Yttrium–Stabilized Zirconium, YSZ, and Gadolinium–doped Cerium, GDC) both widely employed as electrolytes in Solid Oxide Fuel Cells (SOFCs). The applied load (P)–displacement (h) curves, and the energy data (total, plastic, and elastic energy) were analysed to evaluate the minimum work necessary to produce plastic deformation. The different mechanical properties were calculated by the Oliver and Pharr approach, and the work of indentation method. As a result, it was determined that the hardness for YSZ is higher than for GDC materials, and the Young’s modulus for both studied materials is similar. Finally, fracture behaviour of each of the materials during indentation (chipping, cracks, and other mechanisms) was also discussed in detail from the residual imprints observed with Atomic Force Microscopy(AFM).

Micro- and macro-indentation behaviour of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−d perovskite

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−d (BSCF) is a candidate material for the application as oxygen separation membrane. However, the requisite mechanical reliability needs to be warranted. Indentation tests on dense BSCF yielded data for hardness, stiffness and fracture toughness up to a temperature of 340 °C. Complementary to this, the fracture toughness was also evaluated up to 800 °C based on an indentation-strength method. Up to 200 °C, the values of all characteristic mechanical parameters decreased. At high temperatures they increased. The morphology of the indentation cracks depended on the applied indentation load. This was taken into account while selecting suitable expressions for calculating indentation toughness. The temperature dependence of the normalised fracture toughness as determined by indentation technique and indentation-strength method matched quite well. They revealed a good agreement with the temperature dependence of previously reported normalised fracture stresses. In addition to this, the effect of annealing on the mechanical properties of the material was also studied.

Adhesion of YSZ suspension plasma-sprayed coating on smooth and thin substrates

The design concept of the gas-cooled fast reactor which is a 4th generation nuclear reactor, requires protective coatings able to operate at 850 °C and protect the underlying structure in case of sudden increase of the functional temperature up to 1250 °C and depressurization from 0.70 MPa to atmospheric pressure. The parts to be covered are made of 1-mm thick materials resistant to heat and erosion and exhibiting high mechanical properties at high temperatures, such as the Haynes® 230 nickel-based alloy. In this study, the use of suspension plasma spraying to manufacture zirconia coatings is explored. The spraying conditions were optimized for the elaboration of coatings on stainless steel AISI 304L substrates and then adapted for Haynes 230 substrates. A special attention was paid to coating adhesion that was investigated by using a Vickers indentation cracking method.

Contraction stresses in dental composites adjacent to and at the bonded interface as measured by crack analysis

The objective of this study was to calculate stresses produced by polymerization contraction in regions surrounding a dental resin composite restoration. Initial cracks were made with a Vickers indenter at various distances from the edge of a cylindrical hole in a soda-lime glass disk. Indentation crack lengths were measured parallel to tangents to the hole edge. Resin composites (three brands) were placed in the hole and polymerized (two light irradiation protocols) at equal radiation exposures. The crack lengths were re-measured at 2 and 10 min after irradiation. Radial tensile stresses due to polymerization contraction at the location of the cracks ( σ crack) were calculated from the incremental crack lengths and the fracture toughness K c of the glass. Contraction stresses at the composite–glass bonded interface ( σ interface) were calculated from σ crack on the basis of the simple mechanics of an internally pressurized thick-walled cylinder. The greater the distance or the shorter the time following polymerization, the smaller was σ crack. Distance, material, irradiation protocol and time significantly affected σ crack. Two-step irradiation resulted in a significant reduction in the magnitude of σ interface for all resin composites. The contraction stress in soda-lime glass propagated indentation cracks at various distances from the cavity, enabling calculation of the contraction stresses.