Indentation Crack Length Research Articles

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.

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.

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.

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.

Tunneling radial cracks in layered structures from contact loading

Glass/epoxy laminates glued onto a compliant substrate are indented with a hard ball. The damage is characterized by a set of transverse cracks which pop out from the subsurface of the glass layers due to flexure and propagate stably in the radial direction with load in a bell-shape front under a diminishing stress field. Compliant interlayers, even extremely thin ones, are effective in inhibiting crossover fracture. This leads to crack tunneling and crack multiplication in the hard layers, which enhances energy dissipation and reduces the spread of damage relative to the basic bilayer configuration. The experiments show that the fracture in a given layer is well approximated by a power-law relation of the form c 3/2 K C/ P = δ, where P, c, and K C are the indentation load, crack length and fracture toughness, in that order, and δ an implicit function of the layer position and material and geometric variables, derived with the aid of available tunnel crack solutions. The model specimen studied provides a useful insight into the fracture behavior of natural, biological and synthetic layered structures from concentrated loading. The analysis shows that the crack arrest capability of a thin interlayer increases in proportion to the modulus misfit ratio between the layer and interlayer, and that the spread of radial cracks in a laminate of given thickness reduces in proportion to n 1/3, where n is the number layers in the laminate.

Strength and grinding residual stresses of Y-TZP with duplex microstructures

The influence of grinding surface stresses on the strength and apparent fracture toughness of Y-TZP doped with 2.8% molar Yttria has been analysed. It is shown that by means of heat treatments of 2.8Y-TZP at 1650 °C for different times, a duplex cubic-tetragonal microstructure can be produced, which is more transformable and tougher than the starting 2.8Y-TZP. The effect of grinding is studied in samples heat-treated under different conditions and it is shown that compression residual stresses are induced in a surface layer. These are estimated by X-ray diffraction and by measuring the length of indentation cracks in ground and in annealed specimens. The effect of residual stresses on strength and fracture toughness is discussed in terms of a simple model based on a uniform compression residual stress in a surface layer.

Influence of indenter angle on cracking in Si and Ge during nanoindentation

The influence of indenter angle on the nanoindentation cracking behavior of single crystal Si and Ge was systematically explored through nanoindentation experiments with a series of triangular pyramidal indenters with different centerline-to-face angles in the range 35.3–85.0°. The relationships between indentation load, crack length and indentation size and their dependence on indenter angle were carefully examined and compared with previous indentation cracking studies. The results are discussed in terms of ways to estimate fracture toughness and indentation cracking threshold loads more precisely through nanoindentation.

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown α-Mo, Mo 3Si, and Mo 5SiB 2 phases. In addition, particles of SiO 2 formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while α-Al 2O 3 formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of α-Mo and Mo 3Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by α-Mo, crack deflection and branching. Flexural strength is related to volume fraction of the α-Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.

Marginal toughness of bonded dental ceramics evaluated by determination of the crack length

The objective of this study was to evaluate the toughness of dental ceramics near a bonded interface using indentation microfracture. The ceramics used in the present study were two leucite-reinforced ceramics (IPS Empress and Finesse), one lithia-based ceramic (IPS Empress 2) and one mica-based ceramic (OCC). The ceramic discs were processed according to the manufacturers instructions and were bonded to resinous discs using a resin composite cement. The bonded specimens were sectioned perpendicularly, and micro-Vickers indentations were imprinted at several distances from the bonded interface in the polished ceramic sections. The lengths between the two ends of the radial cracks along the bonded interface were measured. Crack lengthening was recognized when the distance was within 70 microm. In each ceramic, the indentation crack length at 40 microm from the interface was significantly longer than the length at the inner point of the ceramic material (p<0.05). The order of the mean indentation crack lengths at 40 microm was OCC>Finesse>IPS Empress 2>IPS Empress, while the order at 490 microm was Finesse>OCC>IPS Empress>IPS Empress 2. There was a difference between the orders at 40 microm and at 490 microm. The behaviour of the indentation crack near the interface differs from that which is distant from the interface. Therefore, it is impossible to estimate the marginal toughness of the ceramic based on the indentation microfracture at the inner point of the ceramic.

Toughening of a Cordierite Glass-Ceramic by Compressive Surface Layers

An indentation fracture mechanics analysis is developed to characterize the toughening effects of a compressive surface layer in brittle materials. The analysis is used to describe the enhanced toughness of cordierite glass–ceramic laminate composites, in which thermal-expansion mismatch effects induced uniform stress in the exterior layers of the symmetric exterior:interior:exterior structures. Interpretation of indentation crack length and inert strength tests via the analysis shows that cracks can be viewed as experiencing discrete regions of decreasing stabilization on propagation from small cracks and complete containment within the compressive layer to large cracks and partial extension into the compensating tensile interior. The observations are described using a stress-intensity factor for circular cracks in linear stress fields that includes different base and surface values for extended cracks. Deconvolution of inert strength data for the model cordierite system studied suggested an increase in toughness from 1.4 MPa·m1/2 for the base material to a peak of about 5 MPa·m1/2 for a 1:18:1 composite structure, with attendant increases in strength and flaw tolerance.

Fracture toughness from submicron derived indentation cracks

Indentation tests with loads between 0.5 and 10 mN were performed on fused quartz, (0001) oriented sapphire and (001) oriented barium titanate. The resulting submicron cracks were used to determine the fracture toughness KIC of the tested samples. The indentation crack length method was applicable, but a c/a dependency of the constant of proportionality was found. In addition, a very effective and simple approach—using the extra penetration of the indenter, due to the formation of cracks, so called pop-in—was used to determine KIC.

Crack opening profiles of indentation cracks in normal and anomalous glasses

A comparative Vickers indentation study is made of two glasses, soda-lime and borosilicate. Indentations in the two glasses reveal substantially shorter radial cracks in the borosilicate, even though toughness values measured by conventional (double-cantilever beam) methods are similar in the two glasses. Here, indentation toughness is measured in two ways: by optical measurement of the indentation crack lengths (ICL) and by atomic force microscopy (AFM) measurement of crack-opening displacements (COD) in the near-crack-tip regions. The ICL measurements indicate artificially high values for the borosilicate relative to the soda-lime, consistent with previously documented indentation results. The COD measurements indicate similar values for the two glasses, in line with expectations from the independent determinations. In the case of soda-lime glass, the COD and ICL values are mutually consistent. In the case of borosilicate, the COD and ICL values differ widely, indicating “anomalous” indentation behavior, typical of glasses with open, network-former structures. It is concluded that the COD route provides more reliable evaluations of intrinsic toughness, albeit at some expense in experimental simplicity. Residual elastic–plastic contact stresses responsible for driving the radial cracks, deconvoluted from COD measurements over the entire radial crack lengths, are shown to be significantly smaller in the borosilicate relative to soda-lime, indicative of a compaction rather than volume-conserving contact deformation mode.

A numerical fracture analysis of indentation into thin hard films on soft substrates

In this paper, finite element simulations of spherical indentation of a thin hard film deposited on a soft substrate are carried out. The primary objective of this work is to understand the mechanics of fracture of the film due to formation of cylindrical or circumferential cracks extending inwards from the film surface. Also, the role of plastic yielding in the substrate on the above mechanics is studied. To this end, the plastic zone development in the substrate and its influence on the load versus indentation depth characteristics and the stress distribution in the film are first examined. Next, the energy release rate J associated with cylindrical cracks is computed. The variation of J with indentation depth and crack length is investigated. The results show that for cracks located near the indenter axis and at small indentation depth, J decreases over a range of crack lengths, which implies stability of crack growth. This regime vanishes as the location of the crack from the axis increases, particularly for a substrate with low yield strength. Finally, a method for combining experimental load versus indentation depth data with simulation results in order to obtain the fracture energy of the film is proposed.

Indentation Crack Length Anisotropy in Silicon Nitride with Aligned Reinforcing Grains

Vickers indentation was performed on surfaces of silicon nitride with an aligned microstructure in order to study the interaction between cracks and the microstructure. Although there was not much evidence of crack bridging, the transverse radial cracks were very short, resulting in high fracture toughness values. The longitudinal radial cracks tended to propagate along the grain boundary of the reinforcements and were much longer than the transverse cracks. As the sintering temperature increased, the lateral cracks on the casting surface led to spalling and consumed more energy for the crack formation, making the longitudinal cracks shorter. On the surface normal to the alignment direction, there was no spalling and the indentation cracks became longer as the sintering temperature increased.

Microstructure dependent hardness and fracture behavior in liquid-phase-sintered Al 2O 3

The liquid-phase-sintered Al 2O 3 (LPS) derived from commercial powders of different particle size, e.g. coarse (70–100 μm), medium (3.6–7.0 μm) and reactive (<2.0 μm) showed a substantial differences in Vickers indentation fracture behavior depending upon their grain size distribution and thermal expansion coefficient mismatch between the matrix grain and the intergranular phases. A high true hardness and a low indentation crack length was observed in the case of the LPS with reactive powder which was attributed due to enhanced dissolution of Al 2O 3 into the glassy phase. A high flexural strength was achieved with the LPS of medium powder. A high K ic-short always resulted either due to (i) the MgO/(CaO+BaO+KNaO) ratio of nearly 1 in the chemical composition of LPS, or (ii) higher modulus of elasticity to hardness ratio, or (iii) reinforcement of coarse grains (>12 μm) in the fine-grained (∼2 μm) microstructure. The crack path was predominantly intergranular at lower MgO/(CaO+BaO+KNaO) ratio (<1.0) for the indentation load in between 9.81 and 49.03 N, whereas it was transgranular at a higher ratio (∼1.6). A low K ic-short was observed due to precipitation of anorthite phase in the LPS with a high MgO/(CaO+BaO+KNaO) ratio. Finally the sintered density of 91–94 wt% LPS materials comprising of all powders produced a linear relationship with both the hardness and the modulus of elasticity.

Residual Stress Evaluation in Brittle Coatings Using Indentation Technique Combined with In-situ Bending.

The indentation crack length approach was adopted and further elaborated to evaluate residual stress and toughness of the brittle coatings:two kinds of glass coatings on steel. The influence of the residual stress on indentation cracking was examined in as-received coating condition and by in-situ superimposing a counteracting tensile stress. For purpose of providing reference toughness values stress-free pieces of separated coating material have also been examined. Thus results of the two complementary sets of experiments were assumed to prove self-consistently toughness and residual stress data of the coating. In particular, the in-situ bending of specimen in combination with the indentation test allowed us to vary deliberately the residual stress situation in glass coating. Thus experiments which utilized the combination of bending test and micro-indentation were introduced as a method to provide unambiguous information about residual compressive stress. Toughness and residual compressive stress of glass coatings used in this study were 0.46∼0.50 MPa·m1/2 and 94∼111 MPa, respectively. Furthermore, a thermoelastic calculation of the residual compressive stress was performed and it is found that the value of residual compressive stress at coating surface of specimen was 90∼102 MPa.

Processing and properties of MoSi2–SiC and MoSi2–Al2O3

Molybdenum disilicide (MoSi 2 ) is a candidate material for high temperature structural applications due to its high melting point and excellent oxidation resistance. A major impediment to implementing this material is its low temperature fracture toughness. Earlier studies examined polycrystalline MoSi 2 for the role of grain size and SiO 2 impurity distribution on the fracture response via indentation. The goal of this study is to apply this indentation analysis to MoSi 2 containing additions of either Al 2 O 3 or SiC. These additions modify the microstructure and result in changes to the indentation cracking behavior. Indentation crack responses and crack lengths are recorded and the fracture toughness values are determined. An explanation is presented for the formation of a tortuous crack path that increases the fracture toughness for the sample with the SiC addition.

Mechanical characterization of Al 2O 3/ZrO 2 hybrid laminates

Significant residual stresses due to the thermal expansion mismatch were found in the Al 2O 3/Ce–ZrO 2 laminated composites investigated in this study. The residual stresses were measured by evaluating the difference in indentation crack length, with and without the residual stresses. The obtained residual stress values are in good agreement with the calculated residual stresses from the measured coefficients of thermal expansion and elastic constants. The laminated structure was found to give rise to an increase in indentation strength compared to monolithic alumina. This apparent increase in fracture toughness can be explained as the effect of the compressive residual stress in the outer Al 2O 3 layer and can be controlled by changing the relative layer thickness. For the size scale studied here, the zirconia-based layers did not appear to play a major role in the final failure process.

Indentation Response of Molybdenum Disilicide

The influence of microstructure on the indentation cracking behavior of molybdenum disilicide (MoSi2) has been examined. The indentation response of samples produced by various methods has been measured to examine the elastic/plastic nature, hardness, and fracture toughness. Fracture toughness comparisons were made by measuring indentation crack lengths, observing the elastic/plastic indentation response, and quantifying the differences in the indentation cracking behavior. Further information was gained by monitoring the acoustic activity during indentation for selected specimens. It has been observed that the fine grain size and the dispersion of the silica phase promote microcracking and crack deflection.

Strength vs Stressing Rate and Indentation Crack Growth Behavior of Indium(III) Phosphide

Susceptibility of InP to environmentally enhanced crack growth behavior was investigated in water and in 50% relative humidity by strength vs stressing rate and indentation crack length measurements. Statistical analysis indicated that, at the 95% confidence level, no environmentally enhanced fracture was detected for InP in liquid or gaseous water.