Subcritical crack growth in porcelains, glass-ceramics, and glass-infiltrated alumina composite for dental restorations

The objective was to compare fracture toughness (K_Ic), stress corrosion susceptibility coefficient (n), and stress intensity factor threshold for crack propagation (K_I0) of two porcelains [VM7/Vita (V) and d.Sign/Ivoclar (D)], two glass-ceramics [Empress/Ivolcar (E1) and Empress2/Ivlocar (E2)] and a glass-infiltrated alumina composite [In-Ceram Alumina/Vita (IC)]. Disks were constructed according to each manufacturer’s processing method, and polished before induction of cracks by a Vickers indenter. Crack lengths were measured under optical microscopy at times between 0.1 and 100 h. Specimens were stored in artificial saliva at 37°C during the whole experiment. K_Ic and n were determined using indentation fracture method. K_I0 was determined by plotting log crack velocity versus log K_I. Microstructure characterization was carried out under SEM, EDS, X-ray diffraction and X-ray fluorescence. IC and E2 presented higher K_Ic and K_I0 compared to E1, V, and D. IC presented the highest n value, followed by E2, D, E1, and V in a decreasing order. V and D presented similar K_Ic, but porcelain V showed higher K_I0 and lower n compared to D. Microstructure features (volume fraction, size, aspect ratio of crystalline phases and chemical composition of glassy matrix) determined K_Ic. The increase of K_Ic value favored the increases of n and K_I0.     

Effect of Strain Rate and Preloading on the Fracture Toughness of ZrO2-Based Ceramics

We study the variation of the fracture toughness K_Ic ofZrO₂ - Y₂ O₃ ceramics (density 98%) as a function of the testing machine crosshead speed (0.005–50 mm/min) and preloading at K_I < K_c. The fracture toughness is shown to be practically constant in the speed range from 0.05 to 5 mm/min. At a loading rate of 50 mm/min, the quantity K_Ic substantially decreases (by a factor of more than two), whereas at a rate of 0.005 mm/min it slightly increases. Preloading leads to a 1.5-fold increase in K_Ic. Variation of the fracture toughness is associated with structural transformations.     

Kinetics of isothermal phase transformations in a dental porcelain

The dissolution of leucite (KAlSi₂O₈) and the precipitation of sanidine (Na _x K_1–x AlSi₃O₈; 0.1<x<0.3), occurring during isothermal heat treatments of a dental porcelain for porcelain-fused-to-metal restorations, was studied. The identification of phases was performed by X-ray diffraction, optical and scanning electron microscopy. An isothermal time-temperature-transformation (TTT) diagram, from 800 to 1000°C, and for periods up to 1440 min, is proposed. No metastable cubic leucite was retained by air-quenching in any sample. No increase of leucite volume fraction was observed.     

Fracture surface formation of ceramics in the precritical crack growth range

A model is suggested for forming ceramic fracture geometry based on ideas about the failure process as a number mutally independent events of intra- and intergranular tensile failure, and the fraction of areas of intergranular cleavage inclined at an angle of less than to the average plane of a macrocrack is determined as the probability of a random plane intersecting an elementary cell of the material at an less than to one of the crystallographic planes along which cleavage is possible at a given load. It is established that fracture surface geometry changes with an increase in stress intensity factor from a maximum developed with an SIF less than K_Ic for the plane of simplest cleavage for a single crystal of a given material to an almost plane geometry with K_Ic for polycrystalline material. The fracture toughness for A1₂O₃ ceramics is estimated by calculation from values of K_Ic for crystallographic planes of simplest cleavage for sapphire.Translated from Problemy Prochnosti, No. 1, pp. 37–43, January, 1990.     

Processing and mechanical properties of biocompatible Al2O3 platelet-reinforced TiO2

The mechanical behaviour of Al₂O₃ platelet-reinforced TiO₂ bioceramics produced by hot-pressing has been investigated. The variation of the elastic constants, fracture strength and fracture toughness with the volume fraction of platelet content was studied. The addition of platelets did not affect the critical flaw size of the composites. This fact, and the good matrix/platelet interfacial bond resulted in a simultaneous increase of the fracture strength and toughness. The mechanical properties increased from K_Ic=2.4 MPa m^1/2 and ₀=215 MPa for pure TiO₂ to K_Ic=3.3 MPa m^1/2 and ₀=265 MPa for a 30 vol% platelet-containing composite. The indentation technique demonstrated the anisotropic behaviour of the fracture toughness in the composites due to platelet orientation during hot-pressing. Load transfer was identified as the main reinforcing mechanism and the toughening effect could be assessed by a load transfer-based model equation. Fracture surface analysis showed mainly intercrystalline fracture for the TiO₀ matrix, whilst with the composites, fracture became more transcrystalline with increasing platelet content.     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Influence of dissolution rate on crack growth and fatigue of Na2O-Al2O3-B2O3 SiO2 glasses

Fracture toughness, macroscopic crack growth and dynamic fatigue of glasses of the system 15Na₂O-4Al₂O₃-xB₂O₃-(81-x)SiO₂ are studied. The fracture toughness as a function of B₂O₃ content correlates to the dependence of elastic modulus which has a maximum between 20 and 30 mol%. The shape of the crack growth curve changes characteristically. Region 1 of the curves normalized toK _Ic is shifted to higher crack growth velocities (smallerK _I/K _Ic values, respectively) for increasing B203 content. The rise of velocity correlates approximately to the dissolution rate of the glasses in water. The determination of the slope in region 1 is problematic, particularly for poorly resistant glasses. The slopen of the crack growth velocity fitted by the power law decreases above 20 mol% B₂O₃ in correspondence with the results of the dynamic fatigue (23 >n > 7). A clear fatigue limit at one fifth of the inert strength occurs in glasses with B₂O₃ content above 20 mol%.     

Effect of heat treatment,deoxidation procedure,and low temperatures on the crack resistance of full-profile rails

Studies of the crack resistance of nonthermally strengthened (NT), bulk hardened (BH), and surface hardened type R65 rails of quality groups I and II are presented. Results are summarized for tests on 616 full-profile rail samples from 57 different batches at 20, ?20, and ?60°C. It is shown that deoxidation of rail steel with master alloys which do not contain aluminum (rails of group I) does not lead to an increase in rail crack resistance. As a result of bulk hardening for rails the values of K_Ic increase a little, whereas with surface hardening the crack resistance is 30–40% of that for nonthermally hardened rails. A reduction in test temperature from 20 to ?60°C causes a reduction in K_Ic for both nonthermally and bulk hardened rails by about 35%. The difference between NT- and BH-rails remains small. This fact points to the special importance of the problem of rail cold resistance.     

Surface and catalytic properties of CuO/MgO system doped with K2O and Cr2O3

The effects of doping of CuO/MgO system with K₂O or Cr₂O₃ (1–6 wt.%) on its surface and catalytic properties were investigated. The analytical techniques employed were XRD, nitrogen adsorption at −196 °C and decomposition of H₂O₂ at 30–50 °C. The results revealed that K₂O-treatment of the system investigated, followed by calcinations at 400 °C resulted in the conversion of some of the surface copper oxide into potassium coprate K₃Cu₅O₄ phase whose diffraction lines disappeared upon heating at 500 °C. Chromium oxide-doping did not lead to the formation of any copper oxide-Cr₂O₃ compound(s) in the treated sarples calcined at 400 and 500 °C. Also, K₂O-doping conducted at 400 °C increased the degree of crystallinity and particle size of MgO phase and exerted opposite effects in the crystallinity and particle size of CuO phase. The doping process either with K₂O or Cr₂O₃ effected a measurable progressive decrease (42–53%) in the specific surface area of the investigated system subjected to heat treatment at 400 and 500 °C. However, the opposite effect was observed upon doping with 1 and 2 wt.% Cr₂O₃ followed by heating at 500 °C. The catalytic activity of the investigated system decreased by doping with K₂O. The addition of 6 wt.% effected a decrease of 78% and 68% in the value of the reaction rate constant per unit surface area for the catalytic reaction carried out at 30 °C (k_{30 °C}⁻) over the doped catalysts calcined at 400 and 500 °C, respectively. Cr₂O₃-doping on the other hand, much increased the catalytic activity of the treated samples and an increase of 106% and 80% in the value of k_{30 °C} was observed upon doping with 6 wt.% Cr₂O₃ for the solids calcined at 400 and 500 °C, respectively. The results were discussed in terms of creation of new in pairs and conversion of some active sites (surface CuO) into less active site (K₃Cu₅O₄).     

Investigation of the thermodynamic criterion of crack resistance of steels

The article shows the validity of the previously proposed criterion K_Ic = K_Ic ^* + _0.2iK_Ic/_0.2 (K_Ic is reduced crack resistance, _0.2i is internal component of yield strength _0.2) for a wide range of body-centered cubic metals such as iron—carbon alloys (cast iron; low-, medium-, and high-strength carbon and alloy steels). It examines the relationship between this criterion and energy and force concepts of the fracture micromechanism. Existence of a common temperature dependence of the effective yield strength component in ferrite—pearlite carbon and alloy steels in the annealed, normalized, and heat-treated states is established. It is shown that the fraction of effective stress In the total yield strength, i.e., _0.2 ^*/_0.2 controls crack resistance K_Ic over a wide range of temperatures and deformation rates. For impact strength KCV, linear dependences of KCV — KCV/_0.2 and KCV-KCV/HB are observed In the zone of transition temperatures and cold brittleness threshold. A correlation equation connecting KCV and K_Ic over the indicated range is obtained. An applied software package has been worked out for computer-aided prediction of crack resistance and impact strength.Translated from Problemy Prochnosti, No. 8, pp. 14–22, August, 1993.     

Behaviour of α-AI2O3 insulator surfaces under electron irradiation

Cathodoluminescence (CL), Auger electron spectroscopy (AES) and direct crack measurements were performed on-Al₂O₃ samples in order to relate chemical, electrical and mechanical effects induced by electron irradiation of the surface. Electrical discharges and visible luminescence were observed during the excitation of the samples with a 80 keV electron beam. Changes of the surface state and of the toughnessK _Ic were subsequently detected. The results suggest that charging of the sample is related to the presence of defects and corresponding trap levels in the energy gap. The concentration of defects (oxygen vacancies or associated F, F⁺ centres) may be enhanced, especially in the vicinity of the surface, by the electrical discharges induced by the electron irradiation. This leads to an increase of mechanical stresses in the brittle material: a striking example was the fracture of a corundum single crystal in the electron microscope, which cannot be explained by the direct heating effect of the primary electron beam. On the contrary, an advantageous situation for the mechanical properties of the material may be achieved when the defects have a blocking effect on the crack propagation; a subsequent increase of the toughnessK _Ic is then recorded.     

Bruchmechanische Charakterisierung des Vergütungsstahls 30 CrNiMo 8 nach verschiedenen Anlaßbehandlungen

Fracture Mechanics Characterisation of the Q & T Steel 30 CrNiMo 8 after Different Heat Treatments The investigations show that the fracture toughness K_Ic of the Q & T steel 30 CrNiMo 8 for large structural members is more sensitive to a change of the tempering treatment than the Charpy energy A_v. For tempering temperatures between 200 and 250°C the fracture toughness K_Ic shows maximum values with K_Ic ? 87 MN/m^3/2, R_m = 1800 MN/m² and R_p0,2 = 1400 MN/m² at room temperature. With these values this steel may be an inexpensive alternative to the maraging steels, for example to 18 Ni 300 (X 2 NiCoMo 18 9 5) with K_Ic = 76 MN/m^3/2 at R_m = 2100 MN/m² and R_p0.2 = 2000 MN/m² (aged 4 h 480°C). Elastic-plastic fracture toughness values K_Ji were determined at onset of stable crack growth with specimens that fail after more or less large plastic deformations because the thickness of specimen is too small or test temperature and tempering temperature, respectively are too high. These values are essentially independent of specimen size, if some minimum size conditions are fulfilled, and they are in agreement with the linear elastic fracture toughness K_Ic. But the test of more especially large linear elastic failing specimens may clarify the uncertainty whether this procedure delivers conservative results in all cases.     

Strength Assessment for Butt Joints of Steel 50 by the Fracture Toughness Criteria

We propose an experimental-theoretical engineering procedure for assessing the strength of butt-welded joints using the force (K_Ic) and strain (_c) criteria of the brittle and quasibrittle fracture mechanics, respectively. For this purpose, beam specimens with square and bevel welds and an initial edge crack or notch in the weld metal are tested under three-point bending. The parameter K_Imax controlling fracture of a bent beam with an inclined (Mode I + Mode II) crack is assessed taking into account the values of the stress intensity factors K_I and K_II, and the crack inclination angle ^*. We also studied the plastic zone at the crack tip and the crack propagation kinetics depending on the weld geometry and the V-notch tip radius for butt-welded joints. The data obtained allow one to rate such joints by their strength according to the fracture toughness criteria K_Ic and _c.     

Mode I and mixed mode fracture of polysilicon for MEMS

An experimental study was carried out to investigate the local and effective fracture behaviour of polycrystalline silicon for microelectromechanical systems (MEMS). The apparent mode I critical stress intensity factor was determined from MEMS‐scale tension specimens containing atomically sharp edge pre‐cracks, while local deformation fields were recorded near the crack tip, with high resolution by the in situ Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method previously developed by this group. The effective mode I critical stress intensity factor varied in the range 0.843–1.225 MPa√m. This distribution of values was attributed to local (in grain) cleavage anisotropy and to enhanced grain boundary toughening. The same sources resulted in very different local and macroscopic (apparent) stress intensity factors, which, combined with the small grain size of polysilicon (0.3 μm,) were the reason for subcritical crack growth that was evidenced experimentally by AFM topographic and AFM/DIC displacement measurements. The effect of local in‐grain anisotropy and granular inhomogeneity was stronger under mixed mode loading of edge cracks inclined at angles up to 55° with respect to the applied far‐field load. The K_I–K_II locus was characterized by scatter in the K_Ic values but on average it followed the curves calculated by the maximum tensile stress and the maximum energy release rate criteria calculated assuming isotropy.     

Fracture parameters for sintered steels

Measurements of the plane strain fracture toughness K _Ic of sintered steels have frequently been invalid because the requirement that P _max/P _Q<1.1 (where P _max = maximum load and P _Q=load used to calculate K _Ic) has not been met. We show that the reason for the criterion not being met is that sintered steels have a considerable crack growth resistance K _R. Values obtained in the past for K _Ic probably have been over-estimates of the initiation value of the crack growth resistance K _i and under-estimates of the maximum crack growth resistance K . The important point is that the assessment of the toughness of sintered steels by a single parameter is not appropriate. Test methods to determine the crack growth resistance of sintered steels are discussed. Crack growth, which is difficult to detect by visual observation, can be determined by compliance techniques. Because of the porous nature of sintered steel, fatigue cracks are unnecessary at the tip of the notch and indeed are undesirable as they can easily cause errors in toughness measurements through inadvertent overloading. The thickness requirement for plane strain measurements can also be relaxed.     

Subcritical crack growth in partially stabilized ZrO2(MgO)

The room temperature slow crack growth resistance in air ( 50% relative humidity) and in water for large cracks in MgO-partially stabilized zircoba (PSZ) improves with increase in critical fracture toughness,K _lc. Ageing the as-fired PSZ at 1400° C for 8 h results in decreasing K_lc from 8.5 MPa m^1/2 to 6 M Pa M^1/2. The ageing treatment also promotes the growth of eutectoid decomposition products on grain boundaries that is accompanied by a decrease in the dependence (A the change in Region I crack velocity with a change in the applied stress intensity. Calculated times to failures are markedly decreased in the aged as compared to the as-fired PSZ ceramic.     

Synthesis,characterization and evaluation of resin-based carbon spheres modified by oxygen functional groups for gaseous elemental mercury capture

A novel resin-based spherical carbon material was successfully prepared by suspension polymerization of alkyl phenol and formaldehyde and steam activation in combination with surface modification by heat treatment with dry air for enhancing Hg⁰ adsorption. The analysis results demonstrate that the oxidation-modified activated carbon spheres possess better mercury removal performance than untreated sorbents, and the ACS-O300 obtained by oxidation modified at 300 °C is the optimal sorbent at the adsorption temperature range from 25 to 150 °C. The main reason is assigned to the increase of the oxygen functional groups of C=O and C(O)–O–C that play an important role as effective active sites for binding the Hg⁰, even though the C(O)–O–C predominates in mercury removal performance under higher adsorption temperature. The optimum O₂ concentration is 4 vol% at the O₂ concentration range from 0 to 8 vol%. SO₂ and NO are favorable to the mercury adsorption under 4 vol% O₂, while H₂O leads to the inhibition of the mercury adsorption. The TPD results suggest that a strong desorption peak at temperature around 235 °C and a weak peak at 324 °C should generate from mercury desorption of C?=?O and C(O)–O–C, correspondingly. Moreover, the XPS analysis results of the fresh and used sorbent indicates that the C=O and C(O)–O–C serve as strong oxidizer and facilitate electron transfer for converting Hg⁰ to Hg²⁺ in the chemisorption process. These results suggest that the obtained resin-based spherical porous carbon (ACS-O300) is promising for Hg⁰ capture.     

Analysis of brittle fracture under the combined stresses of tension and compression

Brittle fractures occurring under biaxial stress states were analysed based on the weakest link model using the mixed mode fracture criterion. Expressions for the mixed mode fracture criterion were chosen for application to the negativeK _l region, corresponding to the compressive stress for the crack. Calculations for biaxial strength with randomly oriented constant-length cracks from the mixed mode fracture criterion were made in the region ofK _I>0 because an unstable fracture seems to occur in this region. The results indicated that the tensile stress component in the combined tension and compression stress state remains constant when the compressive component is smaller than the critical value, which is given by [1 –(K _c/K _c)²]_t derived from the mixed mode fracture criterion, (K /K _c) + (K /K _c)² = 1. Considering the statistical effects, however, calculation of the biaxial strength is modified to result in: (1) lowering the biaxial tensile strength, and in (2) a smooth transition from the constant tensile strength region to the decreasing strength region under the combined tension and compression stress. This suggests that the highK _IIc/K_Ic ratio results in the increase in the compressive strength relative to the tensile strength.