Mechanical behavior and numerical estimation of fracture resistance of a SCS6 fiber reinforced reaction bonded S13N4 continuous fiber ceramic composite

Continuous fiber ceramic composites (CFCCs) have advantages over monolithic ceramics: Silicon Nitride composites are not well used for application because of their low fracture toughness and fracture strength, but CFCCs exhibit increased toughness for damage tolerance, and relatively high stiffness in spite of low specific weight. Thus it is important to characterize the fracture resistance and properties of new CFCCs materials. Tensile and flexural tests were carried out for mechanical properties and the fracture resistance behavior of a SCS6 fiber reinforced Si₃N₄ matrix CFCC was evaluated. The results indicated that CFCC composite exhibit a rising R curve behavior in flexural test. The fracture toughness was about 4.8 MPa m^1/2 , which resulted in a higher value of the fracture toughness because of fiber bridging. Mechanical properties as like the elastic modulus, proportional limit and the ultimate strength in a flexural test are greater than those in a tensile test. Also a numerical modeling of failure process was accomplished for a flexural test. This numerical results provided a good simulation of the cumulative fracture process of the fiber and matrix in CFCCs.     

Manufacturing and mechanical characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/β-TCP/TiO<Subscript>2</Subscript> biocomposite as a potential bone substitute

This paper focuses on the mechanical characterization of a bioceramic based on commercial alumina (Al₂O₃) mixed with synthesized tricalcium phosphate (β-TCP) and commercial titania powder (TiO₂). The effect of β-TCP and TiO₂ addition on the mechanical performance was investigated. After a sintering process at 1600 °C for 1 h, various mechanical properties of the samples have been studied, such as compressive strength, flexural strength, tensile strength, elastic modulus, and fracture toughness. The measurements of the elastic modulus (E) and the tensile strength (σ _t ) were conducted using the modified Brazilian test while the compressive strength (σ _c ) was determined through a compression test. Also, semi-circular bending (SCB) specimens were used to evaluate the flexural strength (σ _f ) and the opening mode fracture toughness (K _IC). From the main results, it was found that the best mechanical performance is obtained with the addition of 10 wt.% TCP and 5 wt.% TiO₂. Alumina/10 wt.% tricalcium phosphate/5 wt.% titania composites displayed the highest values of mechanical properties and a good combination of compressive strength (σ _c ?≈?352 MPa), flexural strength (σ _f ?≈?98 MPa), tensile strength (σ _t ?≈?86.65 MPa), and fracture toughness (K _IC?≈?13 MPa m^1/2).     

A modified model to determine limiting values of coating toughness by nanoindentation

Abstract

The bound model to determine coating toughness limits by nanoindentation tests was originally proposed by Toonder et al. However, the assumptions in this method conflict with each other. It provides lower and upper toughness limits for load control tests; however, it is unable to give the lower toughness limit for displacement control tests. In this paper, a modified model is presented by more comprehensive analysis of the unloading curves at the points where the crack starts and stops respectively. This modified method equally provides the lower and upper limits of fracture dissipated energy for both load control and displacement control. The model developed here has been successfully applied to fullerene like CN_x coatings deposited on various substrates such as Si, SiC and Al₂O₃. It is also applicable to other coated systems provided that the through thickness fracture induced excursion is observed in the load–displacement curve.     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

Wear and fracture toughness of partially stabilized zirconia ceramics under dry friction against steel

The wear of two ceramic materials containing partially stabilized zirconia is studied under unlubricated friction against steel. The first material, with ZrO₂ and 3 mol % Y₂O₃, was obtained by cold isostatic pressing and sintering. The second material, comprising ZrO₂ and 4 mol % Y₂O₃, was fabricated by additional hot isostatic pressing. The samples of both materials were fabricated with high and low values of fracture toughness. The samples with high fracture toughness are found to wear more intensively. This fact can be explained by surface micro-cracking during braking as a result of phase transformations.     

Mechanical properties and scratch test studies of new ultra-hard AlMgB14 modified by TiB2

Nanoscale composites of AlMgB₁₄ with AlN, TiC or TiB₂ were prepared by mechanical alloying in a vibratory mill, followed by compression molding at 1400 °C. Determination of microhardness and fracture toughness indicated that TiB₂ was an effective addition for increasing both quantities. The optimum percent TiB₂ addition corresponding to the highest hardness and fracture toughness was 60–70 wt%. The abrasion resistance of AlMgB₁₄ composites with varying amounts of TiB₂ has been studied using single-point diamond scratch tests with loads ranging from 20 to 70 N in 10 N increments. The scratch width, as measured by stylus profilometer, increased almost linearly with the applied load and decreased with increasing TiB₂ proportion up to 70 wt%. Furthermore, macroscopic abrasion resistance increased with both mean hardness and fracture toughness. Energy dispersive spectroscopy (EDS) revealed the presence of Al₂MgO₄ (spinel) and FeB₄O₇ in AlMgB₁₄. Cracks were observed on the surface of AlMgB₁₄−70 wt% TiB₂ when scratched under 70 N load, but there was no cracking in the absence of TiB₂.     

Interlaminar fracture toughness of CFRP laminates with silk fibers interleave

Interlaminar fracture toughness of CFRP laminates with silk fibers interleave was evaluated in this paper. Silk fibers were obtained from silkworm cocoon. Long silk fibers were wound around a bobbin and cut into specimen size. Resin films were bonded on both sides of a sheet of silk fibers. Silk fibers with resin films were put between [0₁₂] and [0₁₂] and cured by following the curing process. Evaluation of mode I and mode II interlaminar fracture toughness was accomplished by DCB and ENF test, respectively. Mode I interlaminar fracture toughness of CFRP laminates with silk fibers interleave was 59% higher than that of CFRP. Mode II interlaminar fracture toughness of CFRP laminates with silk fibers interleave was 44% higher than that of CFRP. It seems that CFRP laminates with silk fibers interleave will be useful to structures which need high interlaminar fracture toughness.     

Mode I fracture toughness analysis of a single-layer grapheme sheet

To predict the fracture toughness of a single-layer graphene sheet (SLGS), analytical formulations were devised for the hexagonal honeycomb lattice using a linkage equivalent discrete frame structure. Broken bonds were identified by a sharp increase in the position of the atoms. As crack propagation progressed, the crack tip position and crack path were updated from broken bonds in the molecular dynamics (MD) model. At each step in the simulation, the atomic model was centered on the crack tip to adaptively follow its path. A new formula was derived analytically from the deformation and bending mechanism of solid-state carbon-carbon bonds so as to describe the mode I fracture of SLGS. The fracture toughness of single-layer graphene is governed by a competition between bond breaking and bond rotation at a crack tip. K-field based displacements were applied on the boundary of the micromechanical model, and FEM results were obtained and compared with theoretical findings. The critical stress intensity factor for a graphene sheet was found to be K _IC = 2.63 ~ 3.2MPa \(\sqrt m \) for the case of a zigzag crack.     

Effect of temperature on the fracture toughness of A516 Gr70 steel

Fracture toughness JIC and KIC tests were performed on A516 Gr70 carbon steel plate at the temperature ranging from −160°C to 600°C, and test results were analyzed according to ASTM E 813 and ASTM E 399. Unloading compliance J-integral tests were performed on ITCT specimens. The relation between the J_IC value and the test temperature was obtained. It was concluded that the temperature ranging from −15°C to 600°C is the upper shelf region of ductile-brittle transition temperature, and in this temperature range, fracture toughness J_IC values decreased with increasing temperature. The ductile brittle transition temperature of the material may be around −30°C. In the region near −30°C, the tendency of J_IC to decrease with decreasing temperature was significant.     

Enhancement of master curve method for inhomogeneous material

Although the well-known master curve method has been widely used to estimate the ductile to brittle transition temperature and to prevent subsequent brittle fracture of reactor pressure vessels, it has a limitation to determine reference temperatures on heat affected zone and weld metals. The present paper is to propose an enhanced master curve method. Prior to this, several schemes to provide the master curve of inhomogeneous materials such as bimodal master curve estimation, randomly inhomogeneous master curve estimation and single point estimation were reviewed to confirm their applicability. As a result, the single point estimation scheme was chosen as the basic algorithm to calculate the reference temperature and modified by adopting three T _0sp parameters concept to enhance the accuracy. The proposed method can be used to inhomogeneous materials for more accurate reference temperature calculation of reactor pressure vessels even with dispersed fracture toughness test data.     

Widerstand gegen abrasiven verschleiss und dynamische bruchzähigkeit weisser vanadingusseisen

The wear behaviour under abrasive sliding loading, the structure and dynamic fracture toughness of white cast iron containing (4 – 6)% V are considered in this paper. In wear systems containing hard abrasive particles (silicon carbide), the dynamic fracture toughness and wear resistance increase with increasing austenite content in the structure. In systems with less hard abrasive particles (e.g. flint, garnet) these alloys exhibit a combination of high fracture toughness and high wear resistance. The vanadium-alloyed white cast irons with a predominantly austenitic matrix show more favourable values with respect to both dynamic fracture toughness and wear resistance in comparison with a simultaneously tested chromium white cast iron.     

Development of cleavage fracture toughness locus considering constraint effects

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter andq-parameter as well as theK andj-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length(a/W=0.11) is remarkable whilst the effect of temperature from - 20°C to-60°C is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter orq-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering con-straint effects.     

Residual stresses in TiN, DLC and MoS2 coated surfaces with regard to their tribological fracture behaviour

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS₂ coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 μm TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS₂ and DLC coatings, being K_C = 4-11, about 2, and 1-2 MPa m^1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface.     

Experimental investigation on the essential work of mixed-mode fracture of PC/ABS alloy

The mixed-mode fracture of polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) alloy is experimentally investigated in this work. The essential work of fracture method is employed to measure the mixed-mode fracture toughness of PC/ABS alloy. Results show that the essential fracture work w _e increases significantly with the decrease in loading angles. Crack initiation angles are measured on the fractured specimens. The deviation of crack growth direction from the initial crack plane also increases with the decrease in loading angles, which corresponds to the increase of shear components. Microfracture processes of PC/ABS alloy with different loading angles are examined in situ by a scanning electron microscope. Crazing structures are clearly seen around the crack tip. The appearance, growth, and coalescence of crazes can be observed in the microfracture processes, and the macrocrack propagation direction is finally determined by the coalesced direction of crazes.     

Effect of addition of hydrophobic nano silica on viscoelastic properties and scratch resistance of an acrylic/melamine automotive clearcoat

Scratches are a consequence of tribological events encountered by automotive coatings during their service lives. A series of analytical techniques such as laboratory carwash simulator, nano-indentation, tensile, DMTA, SEM, etc. were used in order to compare the effects of addition of various contents of a nano silica on changes in the viscoelastic properties and scratch morphologies of the resultant clearcoats. The results illustrate, that incorporating increased amounts of hydrophobic nano silica particles into an automotive clearcoat decreased its T_g whilst increasing its toughness. Increasing nano silica content also gave progressive transitions from fracture type scratches to plastic type scratches.     

Toughness and processes of material removal

When material is removed from a body, processes of separation must be involved, whatever the form the removed material appears in (small particles, continuous ribbons or large chunks), whatever the method of removal and irrespective of whether removal is controlled or uncontrolled. Separation in brittle materials is customarily modelled in terms of elastic fracture mechanics (including friction where appropriate) to predict forces, energy, dimensions and shapes of separated pieces, etc., the separation work being the fracture toughness R where K_C² = ER. Removal of material in ductile solids is customarily modelled in terms of plastic flow and friction, with the mechanisms of separation either being ignored or considered as consuming negligible work. At the other end of the spectrum, slicing of floppy materials such as soft foodstuffs is all about separation work and little else.The history of the assumption of negligible separation work when cutting ductile solids is re-examined in the light of elastoplastic fracture mechanics (Shaw, Orowan, Irwin, etc.). Given that even for separation by cleavage most surfaces have severely deformed sub-surface boundary layers as a result of their manner of formation (void growth and coalescence in commercial metals and so on), it is questioned whether the chemical surface free energy is the correct property to employ to assess the magnitude of separation work. Experimental evidence, algebraic and FEM modelling all show that material removal processes are branches of fracture mechanics and that a material's fracture toughness is just as important as its ‘strength’ or hardness in saying how easy or difficult it will be to remove material by cutting; during wear and erosion; in ballistic perforation, etc.; and in what form the separated material will appear. Simply put, R makes ‘ductility’ quantitative.The well-known scale effect in fracture mechanics (energy stored or dissipated depending upon volume, but energy required for fracture depending on area) is revealed throughout removal processes. For example, it explains why glass may be machined at the micrometer scale but shatters at larger scales; why, in comminution, there is a limit below which powders cannot be ground; why, in cutting a given material, transformations from continuous chips to ‘knocking lumps out’ are easily produced by altering tool geometry and depth of cut. From this sort of thing it becomes clear that fracture is part and parcel of all material removal processes. What is called a scale effect in cutting, where the specific cutting pressure or unit power (and derived yield strengths) increase at small depths of cut for no apparent reason, is not a scale effect and is expected when cutting is modelled to include toughness as well as plasticity and friction.Why great progress has been made in understanding the mechanics of chip formation in metalcutting without regard to separation work is discussed: the boundary layers forming the top of the machined surface and the bottom of the chip are thin (relatively so thin that they are missed in FEM modelling unless very fine meshes are employed), that the chip formation plastic flow field is essentially uncoupled from the work of separation.Examples are given from a wide range of separation processes that demonstrate that the toughness-to-strength (R/τ_y) ratio of a material is important and, when combined with the depth of cut t to make the non-dimensional parameter (R/τ_yt), or ER/τ_y²t to include elastic behaviour, will determine how a material will behave in removal processes. Applications to abrasion (Krushschov and Babichev), erosion (Finnie), and engraving are highlighted.     

Cutting force and tool wear of single diamond-coated bead

The aim of this study is to optimize stone materials cutting by diamond wire cutter. Attention is focused on the study of the cutting process through sintered diamond-encrusted beads used to work granite. This study is aimed at understanding the interaction between the bead and the material; in detail, experimental equipment was designed to test individual diamond-coated beads, and it was installed on a numerical-control work center. This equipment made it possible to test an individual bead and, in particular, to determine its cutting power and its main force components on the basis of various process parameters, such as cutting velocity (V _t) and feed velocity (V _a). The test runs also made it possible to determine wear on each bead on the basis of process parameters. This is a necessary first step to be able, in the future, to optimize the tool and the cutting process.     

Microstructural studies and impact toughness of dissimilar weldments between AISI 316 L and AH36 steels by FCAW

In this study, AISI 316 L austenitic stainless steel and AH36 low-alloy ship building steel pair were joined with flux-cored arc welding method by using E309LT1-1/4 filler metal under four different shielding gas compositions containing CO₂ at different ratios. Microstructure, impact toughness of welded materials, and their microhardness distribution throughout joining were determined. In macro- and microstructure examinations, stereo optical microscope, scanning electron microscope (SEM), SEM/energy dispersive spectroscopy, and SEM/mapping analysis techniques were used. After notched impact toughness, fracture surfaces were examined using the scanning electron microscope. This study investigated effects of shielding gas composition on microstructure, impact toughness, and microhardness distribution of transition zone between AH36 steel and weld metal of joined material. It is observed that based on an increase in amount of CO₂ in shielding gas, impact toughness values of the weldment decreased. Microhardness values change throughout weld metal depended on shielding gas composition. Moreover, an increase in amount of CO₂ within shielding gas decreased δ-ferrite amount in weld metal. The increase in amount of CO₂ within shielding gas leads to expanded transition zone in interface between AH36 and weld metal and also affects notched impact toughness values negatively due to the inclusion amounts occurring in weld metal and hence caused it to decrease.     

Dry sliding wear behaviors of La2O3–WSi2–MoSi2 composite against alloy steel

A molybdenum disilicide (MoSi₂) matrix composite with the addition of WSi₂ and La₂O₃ (RWM) was fabricated as a wear resistant material by self-propagating high temperature synthesis (SHS) and hot pressing (HP). This composite was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The wear resistance of MoSi₂ against steel is significantly improved by the addition of both WSi₂ and La₂O₃, and it is attributed to the increase in hardness and toughness of the composite. It is found that the wear behavior of the RWM is sensitive to sliding speed, load and hardness of the counter-face material. When worn against a steel with a lower hardness (A), the wear rate of RWM increases with an increase of sliding speed, and increases initially and then decreases with an increase of load. The material removal mechanisms varied from ploughing wear at low load and speed to serious adhesive wear at high load and speed. When worn against a steel with a higher hardness (B), the wear resistance of the RWM improved and the material removal mechanism were brittle fracture wear at low speed and adhesive wear at high speed.     

Moving finite element analyses for fast crack propagation in sheet metal

The conventional fracture mechanics parameters K_IC and/or J_IC are used as fracture toughness criteria necessary for the start of crack propagation under plane strain conditions. These criteria are defined only for small-scale yielding or infinitesimal deformation, though actual fractures involve large plastic deformation. Hence, measurement of fracture resistance during crack propagation is difficult with the conventional parameters.Estimating the mechanical conditions around the propagating crack tip is very useful for reducing damage during accidental fracture. Therefore, establishing a criterion for crack propagation with large-scale yielding is very important for not only science fields but also some industrial fields. For fractures with large-scale yielding, micro- or mesoscale damage processes in the crack tip vicinity have to be considered.In this study, Gurson's constitutive model for void occurrence and growth was introduced into the finite element method to discuss failure behavior in the crack tip vicinity. Fast crack propagation behavior under high-speed deformation was simulated using the moving finite element method based on the Delaunay automatic triangulation. The excellent far-field integral path independence of the T* integral was verified for pure mode I fast crack propagation and non-straight crack propagation under mixed mode conditions. The void growth conditions near the crack propagation path were evaluated.