Fracture behavior of silicon nitride ceramics under combined compression-torsion stresses analyzed by multiaxial fracture statistics

Combined compression-torsion tests were performed on the thermal-treated and as-machined silicon nitride ceramics to investigate their fracture behavior under multiaxial stress states. The thermal-treated samples showed considerable high strength and low anisotropy to the grinding direction in flexure tests compared to the as-machined samples. Under combined compression and torsion stress states, the thermal-treated samples showed considerably higher tensile strength than that of as-machined samples at low compressive stress states and weakening with increasing compression stress. The as-machined samples showed little decrease in tensile strength with increasing compression stress and comparable tensile strength with the thermal-treated samples under a highly compressive stress state. The behavior of thermal-treated samples were well described by the statistical theory of multiaxial fracture for volume-distributed flaws combined with a mixed-mode fracture criterion with the shear sensitivity constant of 1.75 and 1.65 for Shetty’s criterion and the ellipsoidal criterion, respectively.     

Understanding and control of adhesive crack propagation in bonded joints between carbon fibre composite adherends II. Finite element analysis

Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The simulation work presented here complements the experimental work presented in Part I. The observed failure processes and their sequence are successfully described and modelled.     

Finite element analysis of fracture behavior in ceramics: Prediction of strength distribution using microstructural features

The estimation of the strength scatter caused by internal defects is necessary in analyzing a reliable design of advanced ceramic components. In this study, we proposed a finite element analysis methodology to predict the stochastic fracture behavior of ceramics based on the microstructural features obtained by the scanning electron microscopy and X-ray computed tomography. Here, the two- and three-dimensional distribution microstructural data are approximated by various probability density functions and are reflected in the dispersion of parameters of the damage model via a fracture mechanics model. We then applied the proposed method to alumina fine ceramics sintered at three different temperatures, and performed the three-point bending test. Furthermore, the numerically created Weibull distributions were compared with those obtained experimentally. Our analysis results confirm that the proposed method can reasonably predict the strength scatter in ceramics.     

Roughness of fracture surfaces and compressive strength of hydrated cement pastes

A new type of roughness number Rn_o is formulated as a possible analytical tool for surface studies using confocal microscopy. The formulation accommodates fractal dimension and both of the boundary length scales limiting the fractal region of the fracture surface under investigation. Besides the number Rn_o other roughness characteristics are discussed and their effectiveness in the field of cementitious materials is tested. 3D-surface profile SP and surface roughness SR parameters designed for surface 3D-analysis were calculated for fracture surfaces of hydrated Portland cement pastes with different values of water-to-cement ratio. The surface profile SP parameters monotonically increased with water-to-cement ratio and monotonically decreased with compressive strength. A short discussion of possible reasons for such a behavior is presented.     

Mathematical modeling of the rheological behavior of thermoplastic slurry in the molding process of beryllium ceramics

The article presents the results of calculations of a mathematical model of the flow and heat exchange of a thermoplastic beryllium oxide slurry in a circular channel of a foundry installation. Taking into account the peculiarities of coagulation structure formation and the flow mechanism with boundary conditions, the article presents calculations of the speed of the viscoelastic flow of the slurry on the basis of two rheological models of Shvedov-Bingham and Herschel-Bulkley. The three-parameter equation is used to check the consistency of the experimental data of the non-isothermal slurry flow in comparison with the Shvedov-Bingham model. According to the results of calculations of various hydrodynamic conditions and thermal parameters, the fields of velocities and temperatures were obtained to determine the optimal modes of casting beryllium oxide slurry. The heat transfer parameters were determined in accordance with the physical properties of the cavity wall. The positions of the crystallization front along the circular channel are also shown depending on the thermal parameters. The results of the calculated data on the Shvedov-Bingham and Bulkley-Herschel rheological models showed sufficient adequacy with the experimental data.     

Effect of air voids on salt scaling and internal freezing

By combining calorimetric measurements with dilatometry, it has been possible to calculate the contributions of thermal expansion, pore pressure, and crystallization pressure of ice to the strain observed in a mortar during freezing/thawing cycles. Air-entrained mortars contract upon freezing, while non-air-entrained mortars expand. The expansion of the latter is attributed primarily to hydraulic pressure, owing to the rapid growth of ice, which nucleates at low temperatures in laboratory samples. Poromechanical calculations account quantitatively for the contraction of samples with air entrainment, assuming that ice crystals form in the air voids. As originally proposed by Powers and Helmuth, those crystals create suction in the pore liquid that offsets the crystallization pressure of ice in the mesopores of the paste, resulting in a net contraction. Ice in the matrix also contributes significantly to the increase in the thermal expansion coefficient of the mortar.The magnitude of the contraction in air-entrained mortar is shown to account for a reduction of salt scaling damage. According to the glue-spall theory, the damage results from cracking of the ice on the surface of concrete, when the thermal expansion mismatch stress exceeds the strength of the ice. The contraction of the mortar caused by air entrainment offsets the thermal expansion mismatch sufficiently to prevent cracking.Based on observations of the nucleation temperature of ice in laboratory samples of various sizes, it is estimated that there is one site capable of nucleating ice at − 1 °C in a cube of mortar roughly 34 cm on an edge (or, one per square meter in a slab 3 cm thick). This suggests that ice nucleates in the field at high temperatures, compared to what is typically seen in the laboratory, and propagates slowly through the pores as the temperature drops. This mode of growth may lead to fatigue damage over many cycles, owing to local stresses from crystallization pressure, where the contribution of hydraulic pressure is insignificant.     

Classification of ceramics and glass (edge chipping and fracture toughness)

The standard classification of advanced ceramics is based on their strength, although in many cases the performance of products made of such materials is controlled by their deformation behavior and fracture resistance. In this article, ceramics and glass are classified according to their edge chipping resistance (EF-method). Such a classification is based on the idea of a baseline (direct proportionality between edge chipping resistance and fracture toughness of ceramics that are similar to the model material of linear elastic fracture mechanics). Use was made of various elastic and inelastic, oxide and non-oxide ordinary ceramics, composite ceramics capable and incapable of retarding cracks and intended for engineering and biomedical applications. Attention is also given to silicate glass.     

Mechanical behavior of ceramic-metal joint under quasi-static and dynamic four point bending: Microstructures,damage and mechanisms

This paper studies the mechanical behavior of Alumina ceramic-Kovar joint under quasi-static and dynamic four-point bending (FPB). The joint is fabricated by molybdenum-manganese (Mo-Mn) metallization method with extra additions. The bend strength of the joint is improved by glass phase migration. Electronic universal testing machine and modified split Hopkinson pressure bar (SHPB) are employed to realize the loading process. The microstructure of the joint is investigated by scanning electron microscope (SEM) and the chemical composition is determined by energy dispersive spectrometer (EDS). Digital image correlation (DIC) technique is used to determine the displacement contours and ultra-high speed camera is used to monitor the deformation and crack evolution around the joint. It is found that the specimen will slip due to the different Young's modulus of base material. The dynamic bend strength of the joint is lower than the quasi-static bend strength. The failure mechanism of the ceramic-metal joint is mainly intergranular failure for the dynamic bend but mixed transgranular/intergranular failure for the quasi-static case. The crack starts from the inherent voids inside the ceramic and then expands along the metallization band between solder and ceramic.     

Mechanical and fracture properties of cement-based bi-materials after thermal cycling

This study investigates the effect of thermal cycles on the fracture properties of the cement-based bi-materials. Sixty eight cubes were exposed to a varied number of 24-hour thermal cycles ranging from 0 to 90 and subsequently were tested in a wedge splitting configuration. The mechanical and fracture properties of normal strength and high strength concretes are substantially improved after 30 thermal cycles, but less so after 90 thermal cycles both in isolation and when bonded to an ultra high-performance fibre-reinforced cement-based composite.     

The corrosion pattern of reinforcement and its influence on serviceability of reinforced concrete members in chloride environment

This paper deals with two corroded reinforcement concrete beams, which have been stored under sustained load in a chloride environment for 14 and 23 years respectively. The evolution of corrosion pattern of reinforcement and its influence on serviceability are studied. In chloride-induced corrosion process, corrosion cracking affects significantly the corrosion pattern. During the corrosion cracking initiation period, only local pitting corrosion occurs. At early stage of cracking propagation, localized pitting corrosion is still predominant as cracks widths are very small and cracks are not interconnected, but a general corrosion slowly develops as the cracks widen. At late cracking stage, interconnected cracking with wide width develops along large parts of the beam leading to a general corrosion pattern. Macrocells and microcells concepts are used for the interpretation of the results.Mechanical experiments and corrosion simulation tests are performed to clarify the influence of this corrosion pattern evolution on the serviceability of the beams (deflection increase). Experimental results show that, when the corrosion is localized (early cracking stage), the steel–concrete bond loss is the main factor affecting the beams serviceability. The local cross-section loss resulting from pitting attack does not significantly influence the deflection of the beam. When corrosion is generalized (late cracking stage), as the steel–concrete bond is already lost, the generalized steel cross-section reduction becomes the main factor affecting the beams serviceability. But, at this stage, the deflection increase is slower due to the low general corrosion rate.     

Finite element analysis of failure mechanisms in HDPE/CaCo3 particulate composite

Abstract

A finite element analysis of crack propagation in an HDPE/CaCo₃ composite was carried out using a combination of the extended finite element method (XFEM) and the cohesive zone method (CZM). A unit cell of an entire composite consisting of one particle was chosen as the study zone. The interphase was assumed as a cohesive surface between the matrix and the particle. Variable parameters were the interface adhesion, position of initial crack, volume fraction, and size of the particle. The results showed that, the energy release rate increases when increasing the particle size. Increasing the volume fraction from 5 to 10% has positive effects in decreasing the strain energy release rate; however, the effects of 10 and 15% of volume fraction on the energy release rate are almost the same. Increasing the values of interfacial adhesion strength increases the strength of composite.     

Wettability assessment of plasma-treated PBO fibers based on thermogravimetric analysis

Changes in the surface wettability of poly(p-phenylene benzobisoxazole) (PBO) fibers were investigated by thermogravimetric analysis (TGA) following an air dielectric barrier discharge (DBD) plasma treatment. The results were then supplemented and confirmed by scanning electron microscopy (SEM), dynamic contact angle analysis (DCAA), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. After exposure to the DBD plasma at a pre-determined power level, TGA analysis showed that the residual rates retained by the PBO composites decreased, which meant an increase in the amount of resin coating the PBO fibers in the composites. Observations by SEM confirmed that there was more resin adhering to the treated PBO fibers and the wetting behavior of resin on the fibers was greatly improved. Meanwhile, DCAA for the treated fibers showed a significant enhancement in fiber surface free energy. XPS and AFM were performed in order to reveal any variations in fiber surface activity and surface morphology resulting from the surface treatment. The resulting data showed that increases in oxygen-containing polar groups and surface roughness on the plasma-treated PBO fibers contributed to the above improved wetting behavior. With comprehensive analyses, it was concluded that TGA could be used as a supporting method assessing the surface wettability of PBO fibers before and after air DBD plasma treatment.     

Measurement methods of carbonation profiles in concrete: Thermogravimetry, chemical analysis and gammadensimetry

This paper deals with two experimental methods to determine carbonation profiles in concrete. Gammadensimetry is a non-destructive test method able to measure the total penetrated CO₂ and to monitor the carbonation process during laboratory accelerated tests. The second method is thermogravimetric analysis (TGA) supplemented with chemical analysis (CA): as TGA is performed on a small mortar sample not representative of the whole tested concrete, CA is needed to proportion the sample cement content, the sand content and to correct the TGA results becoming thus representative of the concrete mix. Consequently, TGA-CA gives accurate quantitative profiles in carbonated cementitious materials. Results are reported for an ordinary Portland cement paste, and three concrete mixes, containing siliceous or calcareous aggregates. The CO₂ mass loss due to carbonation occurs from 530 to 950 °C, which overlaps the temperature range of the calcareous aggregate dissociation. To solve the problem, the origin of CaCO₃ is carefully analyzed. Calcium carbonate ensuing from C-S-H carbonation dissociates in a lower temperature range than the more stable one ensuing from portlandite carbonation and from limestone, which enables C-S-H carbonation to be distinguished from calcareous aggregates. Therefore, TGA-CA allows the CaCO₃ ensuing from C-S-H carbonation to be measured and to calculate the portlandite degraded by carbonation. Thus, the total calcium carbonates profiles can be deduced even when calcareous aggregates is present in the concrete mix.     

The scratch test for strength and fracture toughness determination of oil well cements cured at high temperature and pressure

Recent advances in scratch test analysis provide new ways to relate measured scratch test properties not only to strength properties but fracture properties of materials as well. Herein, we present an application of such tools to oil well cements cured at high temperatures and pressures. We find a concurrent increase of strength and toughness of different oil well cement baseline formulations which we relate to the water-to-binder ratio for a series of cementitious materials prepared with cement and silica flour. The scratch test thus emerges as a self-consistent technique for both cohesive–frictional strength and fracture properties that is highly reproducible, almost non-destructive, and not more sophisticated than classical compression tests, which makes this ‘old’ test highly attractive for performance-based field applications.     

Finite element analysis of fracture statistics of ceramics: Effects of grain size and pore size distributions

A novel numerical simulation method based on finite element analysis (FEA), which can evaluate the fracture probability caused by the characteristics of flaw distribution, is considered an effective tool to facilitate and increase the use of ceramics in components and members. In this study, we propose an FEA methodology to predict the scatter of ceramic strength. Specifically, the data on the microstructure distribution (i.e., relative density, size and aspect ratio of pore, and grain size) are taken as the input values and reflected onto the parameters of a continuum damage model via a fracture mechanical model based on the circumferential circular crack emanating from an oval spherical pore. In addition, we numerically create a Weibull distribution based on multiple FEA results of a three‐point bending test. Its validity is confirmed by a quantitative comparison with the actual test results. The results suggest that the proposed FEA methodology can be applied to the analysis of the fracture probability of ceramics.     

Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis

The purpose of this study was to evaluate the effects of coping designs on the stress distributions in posterior zirconia crowns by non-linear three-dimensional finite element analysis. Three-dimensional finite element models of a mandibular right first molar with layers of veneering porcelain, zirconia coping, cement, and abutment tooth were designed by computer software (HyperWorks 10.0). Ten zirconia crowns with different designs were produced according to various shoulder positions and heights. The shoulders (1-mm width) exhibited incremental height increases of 1 mm, 2 mm, and 3 mm on the buccal, lingual, and proximal sides, respectively. An axial compressive dynamic load simulating the progressive load was applied until a stainless steel ball model (7 mm in diameter) deepened the veneer surface to 0.7 mm in depth. Loads were placed on the inner inclines of the mesiobuccal, distobuccal, and mesiolingual cusps. Residual maximum principal stresses (MPSs) at the veneer and coping under progressive loading were determined for each zirconia crown. Reinforcements with the shoulders on the buccal, lingual, and proximal axial walls resulted in lower MPSs in the veneering porcelain but higher MPSs in the zirconia coping. As the shoulder height increased, the tensile stresses decreased, while the compressive stresses increased in the veneering porcelains. It can be concluded that the shoulder height and position in the zirconia coping will affect the MPSs of the crown. Our findings conclusively reveal the critical role of the shoulder design of the coping in preventing veneer fracture on posterior zirconia restorations by reducing tensile stresses in veneering porcelain.     

ASR expansion behavior of recycled glass fine aggregates in concrete

The alkali-silica-reaction (ASR) expanding behavior of different types of glass, all derived from cullet with different chemical composition, has been investigated. The glass reactivity was determined in different alkaline solutions based on sodium and/or calcium hydroxide to simulate concrete environment. The expansion of mortar containing different amounts of the investigated glass as fine aggregate has been carried out in different conditions: data collected underline a different response of glass towards the alkaline environment. Soda-lime glass shows negligible expansion, lead-silicate glass always generates expanding trends while boro-silicate glass has different behaviors depending on its colour. An attempt to link the behavior to the solubility and chemical reactivity of the glass is proposed.     

Effect of high temperature cycling on both crack formation in ceramics and delamination of copper layers in silicon nitride active metal brazing substrates

Crack formation in Si₃N₄ active metal brazing (AMB) ceramic substrates and delamination of copper layers on the AMB substrates subjected to temperature cycling from −40 to 250 °C were investigated to evaluate the reliability of these substrates under harsh environments. Acoustic scanning microscopy (ASM) observation of the Si₃N₄ substrates with 0.30 mm thick Cu layers revealed crack formation beneath the corner of the copper plate after 100 cycles, whereas no cracks were detected on the Si₃N₄ substrate with a 0.15 mm thick Cu layer, even after 1000 cycles. The residual bending strength of the Si₃N₄ substrates with 0.30 mm thick Cu layers was 78% of the as-received substrate after 10 thermal cycles, and gradually decreased with an increase in the number of thermal cycles until ca. 65% of the initial strength after 1000 cycles. The Si₃N₄ substrates with 0.15 mm thick Cu layers exhibited a gentler degradation of residual strength than those with 0.30 mm thick Cu layers. In contrast, the residual bending strength of AlN-AMB substrates with 0.15 mm or 0.30 mm thick Cu layers were reduced by 50% within only 10 thermal cycles. The depth of cracks developed during the thermal cycles was measured from the fractured surface of the Si₃N₄-AMB and AlN-AMB substrates. The crack-growth rate in the Si₃N₄-AMB substrates was much slower than that in the AlN-AMB substrates, which could account for the different degradation behavior of the residual bending strength.     

Interaction between corrosion crack width and steel loss in RC beams corroded under load

This paper presents results and discussions on an experimental study conducted to relate the rate of widening of corrosion cracks with the pattern of corrosion cracks as well as the level of steel corrosion for RC beams (153 × 254 × 3000 mm) that were corroded whilst subjected to varying levels of sustained loads. Steel corrosion was limited to the tensile reinforcement and to a length of 700 mm at the centre of the beams. The rate of widening of corrosion cracks as well as strains on uncracked faces of RC beams was constantly monitored during the corrosion process, along the corrosion region and along other potential cracking faces of beams using a demec gauge. The distribution of the gravimetric mass loss of steel along the corrosion region was measured at the end of the corrosion process. The results obtained showed that: the rate of widening of each corrosion crack is dependent on the overall pattern of the cracks whilst the rate of corrosion is independent of the pattern of corrosion cracks. A mass loss of steel of 1% was found to induce a corrosion crack width of about 0.04 mm.