Variation Of Fracture Strength Research Articles

Crystallized fraction and crystal size effects on the strength and toughness of lithium disilicate glass-ceramics

This work investigated the variation in fracture strength and toughness of stoichiometric lithium disilicate (LS2) glass-ceramics as a function of crystal size (d) and crystallized volume fraction (f), with three average crystal sizes (8, 13 and 34 µm) and a wide range of crystallized fractions (0–100 %). The fracture strength and toughness increased with increasing the crystallized volume fraction. For constant crystallized fraction, KIC increased with crystal size, indicating an R-curve behavior. The mean free path between the crystals limits the maximum size of the critical defect and is the crucial feature controlling fracture strength. Finally, we verified that the contribution to the toughness of R-curve mechanisms in this glass-ceramic is proportional to (f.d)1/2, which agrees with R-curve models for ceramics.

Sinterability and mechanical properties of glass-ceramics in the system SiO2-Al2O3-MgO/ZnO

The sintering, crystallization and mechanical behavior of three glass compositions in the SiO2-Al2O3-MgO/ZnO system were investigated. Partial substitution of MgO by ZnO causes a decrease in Tg, a slight increase in the crystallization temperature and improves sinterability. Cordierite and enstatite were crystallized in bulk glasses heat treated at 1000 °C. Addition of ZnO resulted in the crystallization of cordierite, willemite and gahnite for the heat treated bulk glasses and cristobalite, enstatite, gahnite and willemite for the sintered glass powders. The first phase to crystallize is the β-quartz solid solution at 850−900 °C for all compositions. The glass and glass-ceramic with ZnO had the lowest hardness and elastic modulus. There was no variation of fracture strength with composition. The glass-ceramic with ZnO had a similar wear resistance in comparison to the composition with higher MgO content.

Numerical modeling of microcrack behavior in encapsulation-based self-healing concrete under uniaxial tension

We investigated microcrack behavior in encapsulation-based self-healing concrete subjected to uniaxial tension by using finite element analysis. 3D circular capsule with particular shell thickness embedded in the mortar matrix samples was modeled. To represent potential cracks, zero thickness cohesive elements with bi-linear traction-separation law were pre-inserted into the initially generated meshes. Effects of fracture strength variation among the mortar matrix, the capsule, and the interface between them on crack nucleation, initiation, and propagation were investigated. The results showed that the mismatch among fracture strengths of the capsule, the mortar matrix, and the interface of them has a significant influence on crack nucleation, initiation, and propagation. Using similar fracture strength between capsule and mortar matrix, together with high fracture strength of their interface, will initiate a crack from the mortar matrix and then propagate directly into the capsule. This condition is the most favorable case in the capsule-based self-healing concrete since a capsule contained with a healing agent will likely fracture. Thus, the self-healing process in the concrete can be achieved effectively. In addition, the interface with lower fracture strength than the mortar matrix and the capsule strengths will initiate a crack from the interface and then leave the capsule intact. Hence, the self-healing mechanism could not be achieved. These results will become some valuable assets for the experimentalists to assist in their experimental works.

Mechanical reliability of short carbon fiber reinforced ABS produced via vibration assisted fused deposition modeling

PurposeMechanical reliability (variations in mechanical properties) of fused deposition modeled (FDMed) short-fiber-reinforced composites are unknown, which limits wider and safer use of these composites. Accordingly, this paper aims to investigate the mechanical reliability of FDMed model material short-carbon-fiber-reinforced acrylonitrile butadiene styrene (SCFR-ABS). A new vibration-assisted FDM (VA-FDM) process was used to reduce porosity.Design/methodology/approachTensile tests were performed on FDMed SCFR-ABS produced with and without vibrations. Weibull analysis was performed to quantify the variation in fracture strength, tensile strength, strain at break and strain at tensile strength.FindingsIntroduction of vibrations to the extrusion head during FDM decreased the inter-bead porosity in SCFR-ABS and thus improved elastic modulus, toughness, fracture strength, tensile strength and strain at break. Weibull modulus of fracture strength increased from 25 to 57 with vibrations.Practical implicationsThe reported Weibull analysis offers a practical design guideline to predict failure rates at specific service stresses.Originality/valueA detailed Weibull analysis of the variations in the mechanical properties of FDMed SCFR-ABS was performed for the first time. A new vibration-assisted FDM process was reported to reduce inter-bead porosity in FDMed composites.

Investigations on Material and Mechanical Properties, Air‐Permeation Behavior and Filtration Performance of Mullite‐Bonded Porous SiC Ceramics

A theoretical relation between processing parameters and porosity (29–56%) of mullite‐bonded porous SiC ceramics was derived and validated with experimental data. Porosity‐dependent variation of fracture strength (9–34 MPa) and elastic modulus (7–28 GPa) was explained by the minimum solid area model. At room temperature, the Darcian, k1 (1.2 × 10−13–1.6 × 10−12 m2) and the non‐Darcian, k2 (4.6 × 10−9–2.7 × 10−7 m) permeability coefficients showed linear variation with porosity. Tests conducted up to 650°C indicated an increase in k1 with temperature and a reverse trend for k2. Airborne NaCl nanoparticle filtration tests showed good performance of SiC ceramics with fractional collection efficiency of >99% at 46–56% porosity levels.

On the residual stress and fracture strength of crystalline silicon wafers

This letter reports on residual stress measurement in thin crystalline silicon wafers with a full-field near-infrared polariscope. Residual stress is analyzed in combination with observed surface defects, and the results are related to measured fracture strength variation in the wafers. Measurements indicate that there is a sawing process-related residual stress in the as-cut wafers, and that etch-removal of ∼5 μm from the wafer surface eliminates a damage layer that can significantly reduce the residual stress in the wafer, and therefore increases the observed fracture strength. There is a corresponding 2 to 3 μm reduction in the observed characteristic defect size after etching. Fracture strength anisotropy observed in the wafers is related to defect orientation (scratching grooves and microcracks) caused by the sawing process.

Comparative Analysis of Fracture Strength of Slurry and Diamond Wire Sawn Multicrystalline Silicon Solar Wafers

AbstractThis paper presents an analysis of fracture strength of multicrystalline silicon (mc‐Si) solar wafers produced by slurry and diamond wire sawing. The wafers were bent in two orthogonal orientations relative to the saw marks. The fracture strength of slurry sawn wafers increases gradually from wire entry to wire exit whereas the strength variation in the wire feed direction is small. The fracture strength of diamond wire sawn wafers is bi‐directional, with a higher strength if bent perpendicular to the saw marks and a lower strength if bent parallel to the saw marks. The fracture strength variation is related to the microcracks generated in the vicinity of grit‐induced surface damage.

Microstructure and mechanical properties of nanocrystalline WC-particle-reinforced Ti-based composites with nano/ultrafine-grained intermetallic matrix from spark plasma sintering and crystallization of amorphous phase

Abstract High-strength Ti66Nb13Cu8Ni6.8Al6.2 composites with intermetallic matrix reinforced by nanocrystalline WC particles were fabricated via spark plasma sintering and crystallization of amorphous phase. Microstructure analysis indicates that the bulk composites contain isolated nanocrystalline WC particles surrounded by crystallized intermetallic matrix, including ductile bcc β-Ti regions surrounded by a mixed-phase region of (Cu, Ni)-Ti2, TiC, and remaining amorphous phase. With increasing sintering temperature the crystallized intermetallic matrix transforms from nanocrystalline to an ultrafine-grained structure. The composite with ultrafine-grained intermetallic matrix reinforced by 4.5 vol.% WC exhibits high yield and fracture strengths of 2194 and 2277 MPa, respectively. The variation of fracture strength for the fabricated composites can be explained based on the volume fraction of the crystallized ductile β-Ti phase and the scale of the crystallized phase regions.

Temperature and strain-rate dependent fracture strength of graphene

We investigate the variation in fracture strength of graphene with temperature, strain rate, and crack length using molecular dynamics (MD) simulations, kinetic analysis of fracture with a nonlinear elastic relation, and the quantized fracture mechanics theory. Young’s modulus does not vary significantly with temperature until about 1200 K, beyond which the material becomes softer. Temperature plays a more important role in determining the fracture strength of graphene. Our studies suggest that graphene can be a strong material even, when subjected to variations in temperature, strain rate, and cracks.

Insights into the effects of tensile and compressive loadings on microstructure dependent fracture of trabecular bone

The micro-scale finite element models used in the past to understand yielding failure of trabecular bone have not addressed the microcrack formation and its effect on microstructure dependent fracture. An understanding of microcrack based failure mechanisms can be important to develop insights into response of trabecular bone to external loading before final failure. With this goal, we analyze tensile and compressive fracture failure at two different ages in two trabecular bone micrographs obtained from an ovine femur using a recently developed cohesive finite element method (CFEM) framework. The results and analyses indicate that examined trabecular microstructures are optimally designed for resisting compressive loading. Under tensile loading, initial damage in a microstructure is localized in a single random trabecula. Final microstructure failure occurs immediately after the failure of the trabecula. However, under compressive loading, failure of the first trabecula does not precede immediate complete failure of microstructure. Under compression the propagation fracture toughness (characterized by change in energy release rate as a function of crack density) increases with increase in crack density. However, under tension the propagation fracture toughness decreases with increasing crack density. The fracture mechanism remains unaffected by age variation. Effect of tissue property random variation on the variation in fracture strength diminishes under tension and increases under compression with increase in the age. Overall, results indicate that structural arrangement of the trabecular bone (besides the hierarchical chemical composition) can be an important contributor to its unique fracture resistance properties.

Evaluation of fracture properties of silicon by combining resonance frequency and microtensile methods

Fracture properties of single- and polycrystalline silicon films were evaluated by the resonant frequency and microtensile methods. Fracture strength was measured using a piezoelectric-driven stress–strain measurement system. The variation of fracture strength with etching and doping was analyzed by surface and doping condition. A new surface micromachining technique including a two-step sacrificial layer removal process was proposed to combine the resonant frequency and microtensile methods in order to evaluate the fracture toughness of silicon thin films in strict adherence to fracture mechanics. A pre-crack was generated in an electrostatic-driven test specimen and a load was applied by piezoelectric-driven microtensile equipment. Before the microtensile test, a second sacrificial-layer removal was performed. The fatigue pre-crack was successfully introduced and the fracture toughness was derived on the basis of fracture mechanics. The fracture toughness of the pre-cracked specimen was lower than that of the notched specimen and was independent of specimen geometry. The effects of the notch-tip radius and doping condition were determined.

Effects of Residual Stress on Fracture Strength of Si<SUB>3</SUB>N<SUB>4</SUB>/Stainless Steel Joints With a Cu-Interlayer

The variation in fracture strength of a brazed Si3N4/Cu/steel joint was compared with the change in residual stress as a function of the Cu-interlayer thickness that was used. The higher residual stress and the lower measured fracture strength for the joint, using a 0.1 mm thick Cu-interlayer, were ascribed to the entire dissolution of the Cu-interlayer into the brazing alloy. The finite element analysis of residual stress, which considered the microstructure at the interface region, could explain the fracture behavior for the brazed joints, which is dependent on the thickness of the Cu-interlayer.

Structural reliability of alumina-, feldspar-, leucite-, mica- and zirconia-based ceramics

Objectives: The objective of this study was to test the hypothesis that industrially manufactured ceramic materials, such as Cerec Mark II and Zirconia-TZP, have a smaller range of fracture strength variation and therefore greater structural reliability than laboratory-processed dental ceramic materials. Methods: Thirty bar specimens per material were prepared and tested. The four-point bend test was used to determine the flexure strength of all ceramic materials. The fracture stress values were analyzed by Weibull analysis to determine the Weibull modulus values ( m) and the 1 and 5% probabilities of failure. Results: The mean strength and standard deviation values for these ceramics are as follows: (MPa±SD) were: Cerec Mark II, 86.3±4.3; Dicor, 70.3±12.2; In-Ceram Alumina, 429.3±87.2; IPS Empress, 83.9±11.3; Vitadur Alpha Core, 131.0±9.5; Vitadur Alpha Dentin, 60.7±6.8; Vita VMK 68, 82.7±10.0; and Zirconia-TZP, 913.0±50.2. There was no statistically significant difference among the flexure strength of Cerec Mark II, Dicor, IPS Empress, Vitadur Alpha Dentin, and Vita VMK 68 ceramics ( p>0.05). The highest Weibull moduli were associated with Cerec Mark II and Zirconia-TZP ceramics (23.6 and 18.4). Dicor glass-ceramic and In-Ceram Alumina had the lowest m values (5.5 and 5.7), whereas intermediate values were observed for IPS-Empress, Vita VMK 68, Vitadur Alpha Dentin and Vitadur Alpha Core ceramics (8.6, 8.9, 10.0 and 13.0, respectively). Conclusions: Except for In-Ceram Alumina, Vitadur Alpha and Zirconia-TZP core ceramics, most of the investigated ceramic materials fabricated under the condition of a dental laboratory were not stronger or more structurally reliable than Vita VMK 68 veneering porcelain. Only Cerec Mark II and Zirconia-TZP specimens, which were prepared from an industrially optimized ceramic material, exhibited m values greater than 18. Hence, we conclude that industrially prepared ceramics are more structurally reliable materials for dental applications although CAD–CAM procedures may induce surface and subsurface flaws that may adversely affect this property.

The dependence of surface damage induced by electrical-discharge machining on the fracture strength of Al 2O 3-Cr 3C 2 composites

The feasibility of electrical-discharge machining (EDM) conducted on the Al 2O 3-Cr 3C 2 composites is explored for various contents and particle sizes of Cr 3C 2 in composites. The characteristics of surfaces after EDM were examined by scanning electron microscopy. The surface roughness after EDM, the material removal rate and the variation of fracture strength were studied in terms of EDM processing variables, e.g., pulse current, pulse duration time, and electrical polarity. Mechanisms for removal of Al 2O 3-Cr 3C 2 composites by EDM are also proposed.

On the Mixture Laws of Strength and Variation in FRP

The fiber reinforced plastics (FRP) are now available for increased use in structural materials. FRP are, however, generally characterized by a considerable scatter in mechanical properties. Understanding the sources and nature of these variabilities is the key to better utilization of these composite materials. This paper deals with the two typical types of glass fiber reinforced composites (GRP) which are reinforced by plain-woven glass cloth and chopped strand glass mat. The main results obtained in this work are summarized as follows;(1) The mixture law of strength is applied to the modeled laminate which consists of three parts; the first part is the unidirectional layer aligned parallel to the load direction, the second is the layer which does not contribute to tensile strength and the third is the matrix layer. The mixture laws of the average and the variation of tensile strength are developed under an assumption that the fracture strengths of the first and third layers and the fiber volume content are random variables.(2) The average and the coefficient of variation of fracture strength in the first and third layers can be obtained by use of the bundle theory and the weakest link theory. The coefficient of variation of fiber volume content is given by the combination of the coefficient of variation of volume of reinforced material and that of thickness of laminate.(3) The calculated results of the average of fracture strength are found to agree well with the experimental whether each of the random variables are independent or not. But the coefficient of variation of strength is dependent on the correlation of each variables.

The deformation and fracture behaviour of glass-filled ABS

The tensile and fracture characteristics of pigmented ABS containing 30 wt % rubber reinforced with 30 wt % glass fibre have been examined over a range of strain-rates extending from approximately 10−4 to 10−1 sec−1 within the temperature range 293 to 353 K. The glass fibre-reinforced composite had significantly increased fracture strength compared with the base polymer but possessed decreased ductility. The marked yield point which is characteristic of the ABS base polymer was absent from the reinforced material. Two different regions were found to exist on the fracture surfaces of composite specimens. One region possessed the characteristics of a weak interfacial bond while the other showed evidence of strong, interfacial bonding. In both regions extensive fibre pull-out was observed. The variation in fracture strength and morphology with strain-rate and temperature of testing is explained in terms of the properties of an interfacial region adjacent to the fibres which possesses viscoelastic properties different from those of the bulk polymer. The effect of adiabatic heating at the crack tip is also taken into account in the high temperature—high strain rate regime.