Theoretical Fracture Strength Research Articles

Microstructure evolution and defect characteristics of multilayer Fe-Cr alloy coatings fabricated by laser melting deposition

To clarify the microstructure evolution, defect characteristics and suppression methods for multilayer cladding by laser melting deposition (LMD), experimental research on multilayer LMD of Fe-Cr alloy with different laser energy densities was carried out. The phase composition and the evolution rules of the phase during the melting-solidification process were analyzed. The crystal growth trend, dendritic transition and dendrite spacing of multilayer Fe-Cr coatings were investigated. The formation mechanism of crack, porosity and surface spheroidization defects were revealed, the effects of laser energy densities on the defects were analyzed, and the defect suppression methods were proposed and verified. The results showed that the primary and secondary remelting zones of multilayer Fe-Cr coatings were fine equiaxed dendrites and irregular cellular structures, respectively. When the laser energy density increased from 22.74 J/mm2 to 55.70 J/mm2, the dendrite spacing increased gradually, and the porosity and spheroidization defects showed a trend of improvement and then deterioration. The cracks were considered cold cracks with transgranular characteristics. Meanwhile, when the residual stress exceeded the theoretical fracture strength of 1.6 GPa, the multilayer cladding experienced brittle fracture and formed cracks. Selecting an appropriate laser energy density and preheating treatment can effectively improve the forming quality and inhibit the formation of defects.

Deformation and fracture of crystalline tungsten and fabrication of composite STM probes

Fracturing microscale constrictions in metallic wires, such as tungsten, platinum, or platinum-iridium, is a common fabrication method used to produce atomically sharp tips for scanning tunneling microscopy (STM), field-emission microscopy and field ion microscopy. Typically, a commercial polycrystalline drawn wire is locally thinned and then fractured by means of a dislocation slip inside the constriction. We examine a special case where a dislocation-free microscale constriction is created and fractured in a single crystal tungsten rod with a long side parallel to the [100] direction. In the absence of dislocations, vacancies become the main defects in the constriction which breaks under the tensile stress of approximately 10 GPa, which is close to the theoretical fracture strength for an ideal monocrystalline tungsten. We propose that the vacancies are removed early in the tensile test by means of deformation annealing, creating a defect-free tungsten constriction which cleaves along the W(100) plane. This approach enables fabrication of new composite STM probes which demonstrate excellent stability, atomic resolution and magnetic contrast that cannot be attained using conventional methods.

Mechanical Properties of α-SiC and Correlation to SiC/Si Interface at Nanoscale from Reaction Bonded SiC/Si Composites (RBSC)

Reaction bonded SiC/Si (RBSC) composites composed of α-SiC, β-SiC and crystalline-Si phases manufactured at high temperature are widely used in different applications due to their outstanding performances in extreme service conditions. Although the macroscopic mechanical properties of these materials have been extensively explored, there are questions remaining unanswered such as the local material behavior compared to the SiC/Si interface, mechanistic responses in nanoscale and the micro- to nanoscale mechanical properties of major individual components after experiencing the reaction bonding process. In this study, nanoscale specimens were prepared by utilizing Ga focused ion beam (FIB) and an in-situ tensile testing platform was established with a testing stage accommodated inside a field emission scanning electron microscope (FE-SEM). Maximum tensile strength, elastic modulus and Weibull modulus of the nanoscale α-SiC specimens were measured to be 22.9 GPa, 321 GPa and 4.1, respectively. The maximum failure strength was found to be as high as 80% of the theoretical fracture strength. The fracture was found to originate at the side of the specimen surface and appeared to propagate in a brittle manner. The overlap of tensile strength ranges of α-SiC and SiC/Si interface suggests the consistency with an observation of mixed fracture modes in RBSC.

Fracture strength of the proximal femur injected with a calcium sulfate/hydroxyapatite bone substitute

BackgroundAvailable interventions for preventing fragility hip fractures show limited efficacy. Injection of a biomaterial as bone substitute could increase the fracture strength of the hip. This study aimed to show the feasibility of injecting a calcium sulfate/hydroxyapatite based biomaterial in the femoral neck and to calculate the consequent change in strength using the finite element method. MethodsFive patients were injected with 10 ml calcium sulfate/hydroxyapatite in their femoral neck. Quantitative CT scans were taken before and after injection. Five additional patients with fragility hip fractures were also scanned and the images from the non-fractured contralateral sides were used. Finite element models were created for all proximal femora with and without injection and the models were tested under stance and sideways fall loading until fracture. The change in fracture strength caused by the injection was calculated. Additionally, perturbations in volume, location, and stiffness of the injected material were created to investigate their contribution to the fracture strength increase. FindingsThe 10 ml injection succeeded in all patients. Baseline simulations showed theoretical fracture strength increases of 0–9%. Volume increase, change in location and increase in stiffness of the material led to increases in fracture strength of 1–27%, −8-26% and 0–17%, respectively. Altering the location of the injection to a more lateral position and increasing the stiffness of the material led to increases in fracture strength of up to 42%. InterpretationThis study shows that an injection of calcium sulfate/hydroxyapatite is feasible and can theoretically increase the hip's fracture strength.

First-principles study of carbon segregation in bcc iron symmetrical tilt grain boundaries

Segregation of light elements can profoundly affect the energies and cohesive properties of grain boundaries. First-principles calculations have been performed to determine the carbon solution energies and cohesive properties of three different grain boundaries in presence of carbon. It is demonstrated that the most stable segregation sites possess the greatest coordination number and maximum Fe–C nearest neighbor distance. Thereby a geometric criterion for predicting the segregation sites is suggested. Open grain boundary structures are shown to be more attractive to C atoms than the compact grain boundary structure, vacancies and dislocations, and C segregation at open grain boundaries decreases the grain boundary energy. The theoretical fracture strength of grain boundaries increases with C concentration and tend to similar values for certain areal concentrations irrespective of the grain boundary structures. This implies that the maximum fracture strength of a grain boundary depends on the maximum C areal concentration it can accommodate.

Reserch of Dislocation Theory in Micro Cutting

This paper analysis Griffith 's Theory and the metal theoretical fracture strength, thus introducing a line defect that existence in actual crystal — dislocation; And in-depth analysis the metal fracture dislocation mechanism in the micro cutting, and the formation and expand of the Fracture cracks , provides the theory basis for micro cutting

Energetics, structure, and long-range interaction of vacancy-type defects in carbon nanotubes: Atomistic simulations

The presence of vacancy clusters in carbon nanotubes has been assumed to explain the formation of carbon peapods and the difference between the experimentally measured and theoretical fracture strength of nanotubes. We use atomistic simulations at various levels of theory to study the characteristics of large vacancies formed by up to six missing atoms. We show that the formation of big ``holes'' on nanotube walls is energetically unfavorable as the vacancies tend to split into smaller defects due to the reconstruction of the nanotube atomic network. We also demonstrate that there is a weak but long-ranged interaction between the vacancies not only through strain fields but, surprisingly, also due to electronic effects, similar to those of adatoms on metal surfaces.

Molecular dynamics simulation of intrinsic and extrinsic mechanical properties of amorphous metals

Mechanical response of amorphous metal Ni 40Zr 60 under applied tensile loading is investigated using large-scale atomistic simulations. To obtain intrinsic properties, homogeneous samples with atomically smooth surface are used while samples with deliberately introduced surface notches of varying depths and root radii are used to test extrinsic effects. It is found that the notch-free samples show strength close to the theoretical fracture strength and extremely large ductility. Apparent strain hardening and strain rate sensitivity are also observed in these studies. It is argued that the free volume generation and localized shear displacement are responsible for the mechanical properties. Therefore, the presence of surface imperfections can greatly reduce the strength and ductility. The results suggest that to retain and improve the intrinsic mechanical properties of amorphous metals, surface treatment may be needed, which has been practiced in oxide glass industry for centuries but received little attention in metallic glass community.

Measurement of the Bending Strength of Vapor−Liquid−Solid Grown Silicon Nanowires

The fracture strength of silicon nanowires grown on a [111] silicon substrate by the vapor-liquid-solid process was measured. The nanowires, with diameters between 100 and 200 nm and a typical length of 2 microm, were subjected to bending tests using an atomic force microscopy setup inside a scanning electron microscope. The average strength calculated from the maximum nanowire deflection before fracture was around 12 GPa, which is 6% of the Young's modulus of silicon along the nanowire direction. This value is close to the theoretical fracture strength, which indicates that surface or volume defects, if present, play only a minor role in fracture initiation.

A model for the stability and creep of organic materials

A model is presented for the thermally assisted breaking of a number of bonds arranged in parallel and stressed by an individual soft spring each. Using a simplified potential for the bond it is shown that in equilibrium there are two definite regions of elastic behavior: one with all bonds intact, the other with a variable fraction of bonds broken, therefore with a tangent modulus steadily decreasing with applied stress. Criteria are given for the existence of these regions. Beyond these regions time-dependent creep to rupture is found, limited, in turn, by the theoretical fracture strength, the stress necessary for fracture without any thermal assistance, beyond which a bound state is impossible. The time-to-fracture for creep rupture is calculated and an example of the time evolution of the accelerating creep given. The results of the calculations are applied to experimental data on Wallaby tendons by Wang and Ker (J. Exp. Biol. 198 (1995) 831) and data estimated for the bond potential depth, the theoretical fracture strength and the number density of bonds involved as well as the elastic modulus of the ensemble. Values are derived under the assumption of one deformation mechanism being dominant—e.g., (sub-)fibril sliding or sliding of collagen molecules along one another—but the model cannot definitely distinguish between mechanisms.

A statistical theory of time-dependent fracture for brittle materials

Abstract By considering the size distribution of pre-existing flaws and their slow crack growth characteristics, a statistical fracture theory is developed for the time dependence of the strength of brittle materials such as cementitious matrices, glasses and ceramics. Theoretical fracture strength and lifetime predictions in pure-bend specimens subjected to constant stress rates, sustained and cyclic stresses are presented. Both surface flaws and volume flaws are considered in the analyses. The time-to-failure experimental results for a soda-lime glass and a polycrystalline alumina agree well with the statistical fracture theory predictions, but the conventional single-crack analysis is inadequate. A simple energy balance method for the prediction of creep under sustained stresses is also given.

The effect of surface structure on the brittle fracture strength of metals

T he theoretical fracture strength for brittle material (approximately 0.1 E) has been reached for certain glass fibres and some metal whiskers. However, metals with very high fracture strengths, such as hard-drawn steel springs or pure copper, deformed at liquid IIe temperatures break at about one tenth of this value. A possible explanation of some of the observed fracture stresses is presented which is based on the effect of superimposed stress concentration fields. It seems possible that small surface slip steps could give rise to stress concentrations. The corresponding stress concentration factor is of the order of 4 to 40 and the theoretical fracture strength is reduced by this factor. Only metals without slip steps can reach the theoretical fracture strength.