Showing papers on "Fracture toughness published in 2016"

Metal Additive Manufacturing: A Review of Mechanical Properties

Abstract: This article reviews published data on the mechanical properties of additively manufactured metallic materials. The additive manufacturing techniques utilized to generate samples covered in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, EBF3). Although only a limited number of metallic alloy systems are currently available for additive manufacturing (e.g., Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg), the bulk of the published mechanical properties information has been generated on Ti-6Al-4V. However, summary tables for published mechanical properties and/or key figures are included for each of the alloys listed above, grouped by the additive technique used to generate the data. Published values for mechanical properties obtained from hardness, tension/compression, fracture toughness, fatigue crack growth, and high cycle fatigue are included for as-built, heat-treated, and/or HIP conditions, when available. The effects of test...

Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy

Abstract: High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33–43 MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of –0.63 independent of orientation. Planar ½ {111} dislocations dissociate into Shockley partials whose separations range from ~3.5–4.5 nm near the screw orientation to ~5–8 nm near the edge, yielding a stacking fault energy of 30 ± 5 mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20–50 at.%, and atomic size misfit of ~4%.

Metal Additive Manufacturing: A Review of Mechanical Properties (Postprint)

Abstract: : This article reviews published data on the mechanical properties of additively manufactured metallic materials. The additive manufacturing techniques utilized to generate samples covered in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, EBF3). Although only a limited number of metallic alloy systems are currently available for additive manufacturing (e.g., Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg), the bulk of the published mechanical properties information has been generated on Ti-6Al-4V. However, summary tables for published mechanical properties and/or key figures are included for each of the alloys listed above, grouped by the additive technique used to generate the data. Published values for mechanical properties obtained from hardness, tension/compression, fracture toughness, fatigue crack growth, and high cycle fatigue are included for as-built, heat-treated, and/or HIP conditions, when available. The effects of test orientation/build direction on properties, when available, are also provided, along with discussion of the potential source(s) (e.g., texture, microstructure changes, defects) of anisotropy in properties. Recommendations for additional work are also provided.

Enhancement of fracture toughness, mechanical and thermal properties of rubber/epoxy composites by incorporation of graphene nanoplatelets

Abstract: Carboxyl terminated butadiene acrylonitrile (CTBN) was added to epoxy resins to improve the fracture toughness, and then two different lateral dimensions of graphene nanoplatelets (GnPs), nominally

Fracture toughness anisotropy in shale

Abstract: The use of hydraulic fracturing to recover shale-gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in-situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations; Divider, Short-Transverse and Arrester, using a modified Short-Rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam1/2 where the crack plane is normal to the bedding, and values as low as 0.21MPam1/2 where the crack plane is parallel to the bedding. For cracks propagating non-parallel to bedding, we observe a tendency for deviation towards the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions.

A review of Finite Fracture Mechanics: crack initiation at singular and non-singular stress raisers

Abstract: Crack initiation in brittle materials is not covered by classical fracture mechanics that deals only with the growth of pre-existing cracks. In order to overcome this deficiency, the Finite Fracture Mechanics concept assumes the instantaneous formation of cracks of finite size at initiation. Within this framework, a coupled criterion was proposed at the beginning of the 2000’s requiring two necessary conditions to be fulfilled simultaneously. The first one compares the tensile stress to the tensile strength, while the other uses an energy balance and the material toughness. The present analysis is restricted to the 2D case, and, through a wide list of references, it is shown that this criterion gives predictions in agreement with experiments in various cases of stress concentration, which can be classified in two categories: the singularities, i.e. indefinitely growing stresses at a point, and the non-singular stress raisers. It is applied to different materials and structures: notched specimens, laminates, adhesive joints or embedded inclusions. Of course, a lot of work remains to do in these domains but also in domains that are almost not explored such as fatigue loadings and dynamic loadings as well as a sound 3D extension. Some ideas in these directions are issued before concluding that FFM and the coupled criterion have filled a gap in fracture mechanics.

Fracture toughness of hydrogels: measurement and interpretation

Abstract: The fracture mechanics of hydrogels, especially those with significantly enhanced toughness, has attracted extensive research interests. In this article we discuss the experimental measurement and theoretical interpretation of the fracture toughness for soft hydrogels. We first review the definition of fracture toughness for elastic materials, and the commonly used experimental configurations to measure it. In reality most gels are inelastic. For gels that are rate insensitive, we discuss how to interpret the fracture toughness associated with two distinct scenarios: crack initiation and steady-state crack propagation. A formulation to estimate energy dissipation during steady-state crack propagation is developed, and connections to previous models in the literature are made. For gels with rate-dependent behaviors, we review the physical mechanisms responsible for the rate-dependence, and outline the difficulties to rigorously define the fracture toughness for both crack initiation and propagation. We conclude by discussing a few fundamental questions on the fracture of tough gels that are yet to be answered.

Fracture toughness and tensile strength of 316L stainless steel cellular lattice structures manufactured using the selective laser melting technique

Abstract: Selective Laser Melting (SLM) process is a metallic additive manufacturing technique that directly manufactures strong, lightweight and complex three dimensional parts in a layer-by-layer to scan and melt the metal powder for aerospace applications. However, there are still certain evaluation criteria such as fracture toughness and tensility of cellular structure made by SLM which were not reported before. This study presents new and novel methods in additive manufacturing and evaluates the local failure mechanism of 316L cellular lattice structures made by SLM under uniaxial tension and three point pending load. The effect of different build directions of the 316L lattice structure on the fracture toughness properties are compared to the Ashby and Gibson models. Also, the effect of different build directions on tensile properties of 316L cellular structures has been investigated. Microcomputer tomography (CT) reveals that the cellular structure parts with different build directions were manufactured free of defect by the SLM. The density of the lattice structure samples was found at 1.35 g/ cm 3 for both vertical and horizontal building directions while the relative density of solid struts is 96.25%. The tensile and fracture toughness properties in vertical building direction samples are higher than those samples that were built in horizontal building direction. There was no big difference between the Ashby and Gibson micromechanical model to predict fracture toughness and Single Edge Notch Bend (SENB) test results from 0.2 to 0.5 MPa m 0 . 5 .

Influence of B4C particle reinforcement on mechanical and machining properties of Al6061/B4C composites

Abstract: This study investigates the mechanical and machinability properties of the aluminum 6061 reinforced with boron carbide (B4C). Four aluminum 6061 composite specimens reinforced with 5 wt%, 10 wt%, 15 wt%, and 20 wt% B4C were fabricated using a powder metallurgy and hot-extrusion method. The composite samples were investigated to elucidate the influence of different weight fractions of B4C reinforcement content on the hardness, fracture toughness, tensile strength, transverse rupture strength (TRS) and milling properties of the resulting composites. The milling tests were performed based on the Taguchi mixed-orthogonal-array for experiments, L16 (44 × 21), to determine the effect of B4C content on surface quality and energy consumption for different cutting parameters under dry- and compressed-air cooling and using an uncoated carbide insert. The results reveal that the B4C particles are uniformly distributed in the matrix and that the fracture toughness decreases and the hardness increases as the weight fraction of the reinforcement increases. The highest tensile and transverse rupture strength are for Al6061/5 wt% B4C and Al6061 reinforced with 10 wt% B4C composite material has the best fracture toughness from among the specimens measured. At higher milling speed and lower cutting feed and under dry machining conditions, an excellent surface quality is obtained after milling all composites materials and the surface finish improves with increasing B4C content in the matrix. The power consumption and surface roughness increases when cooling with compressed air.

Mixed mode fracture toughness testing of PMMA with different three-point bend type specimens

Abstract: Mixed mode fracture behavior of PMMA, was studied experimentally and theoretically using four different test configurations and with different crack types. Although all the test samples were subjected to three-point bend loading but completely different mixed mode fracture behaviors were observed for the tested samples. For each test configuration, especial mixed mode fracture toughness curves were obtained for fracture initiation direction and fracture toughness in the whole ranges of mixed mode I/II loading conditions. The level of crack tip constraint in the tested samples (which is related to the stress intensity factors ( K ) and the T -stress) was used as the main affecting parameter for predicting the mixed mode fracture results based on a two parameter ( K-T ) fracture theory. It was observed that the loss of crack tip constraint in the tested samples increases the mixed mode fracture resistance and decreases the fracture initiation direction and vice versa.

The Effect of Relative Density on the Mechanical Properties of Hot-Pressed Cubic Li 7 la 3 Zr 2 O 12

Abstract: The effect of relative density on the hardness and fracture toughness of Al-substituted cubic garnet Li6.19Al0.27La3Zr2O12 (LLZO) was investigated. Polycrystalline LLZO was made using solid-state synthesis and hot-pressing. The relative density was controlled by varying the densification time at fixed temperature (1050°C) and pressure (62 MPa). After hot-pressing, the average grain size varied from approximately 2.7–3.7 μm for the 85% and 98% relative density samples, respectively. Examination of fracture surfaces revealed a transition from inter- to intragranular fracture as the relative density increased. The Vickers hardness increased with relative density up to 96%, above which the hardness was constant. At 98% relative density, the Vickers hardness was equal to the hardness measured by nanoindentation 9.1 GPa, which is estimated as the single-crystal hardness value. An inverse correlation between relative density and fracture toughness was observed. The fracture toughness increased linearly from 0.97 to 2.37 MPa√m for the 98% and 85% relative density samples, respectively. It is suggested that crack deflection along grain boundaries can explain the increase in fracture toughness with decreasing relative density. It was also observed that the total ionic conductivity increased from 0.0094 to 0.34 mS/cm for the 85%–98% relative density samples, respectively. The results of this study suggest that the microstructure of LLZO must be optimized to maximize mechanical integrity and ionic conductivity.

Matrix design of strain hardening fiber reinforced engineered geopolymer composite

Abstract: The feasibility of developing a fiber reinforced engineered geopolymer composite (EGC) exhibiting strain hardening behavior under uni-axial tension has been recently demonstrated. The effect of different alkaline activators on the matrix and composite behavior of such EGC has also been evaluated to enhance its compressive and tensile strengths with relatively low concentration activator combinations. The focus of this study, as a follow up investigation, is to evaluate the quantitative influence of geopolymer matrix properties on the strain hardening behavior of the recently developed fly ash-based EGC with the aim of selecting the appropriate type of geopolymer matrix to manufacture the strain hardening EGC with enhanced elastic modulus while maintaining the tensile ductility behavior of the composite. The effects of water to geopolymer solids ratio, sand size and sand content, as the most significant matrix-related parameters, on the matrix properties including workability, compressive strength, elastic modulus, fracture toughness and crack tip toughness, and the uni-axial tensile performance of the composite were evaluated. Experimental results revealed that lowering the water to geopolymer solids ratio and the addition of sand enhanced the elastic modulus of the geopolymer matrix and composite in all cases. However, the excessive use of fine sand and the use of coarse sand adversely affected the strain hardening behavior of the developed EGC due to the increase of the matrix fracture toughness and the first-crack strength of the composite. Only geopolymer matrices with suitable fracture toughness, as defined by the micromechanics design model, maintained the desirable tensile ductility of the developed fly ash-based EGC.

Atomically sharp crack tips in monolayer MoS2 and their enhanced toughness by vacancy defects.

Abstract: We combine in situ transmission electron microscopy and large-scale molecular dynamics simulations to investigate brittle fracture in 2D monolayer MoS2, revealing that cracks propagate with a tip of atomic sharpness through the preferential direction with least energy release. We find that sparse vacancy defects cause crack deflections, while increasing defect density shifts the fracture mechanism from brittle to ductile by the migration of vacancies in the strain fields into networks. The fracture toughness of defective MoS2 is found to exceed that of graphene due to interactions between the atomically sharp crack tips and vacancy clusters during propagation. These results show that monolayer 2D materials are ideal for revealing fundamental aspects of fracture mechanics not previously possible with thicker materials, similar to studies of dislocation behavior in 2D materials.

Numerical Investigation of Dynamic Rock Fracture Toughness Determination Using a Semi-Circular Bend Specimen in Split Hopkinson Pressure Bar Testing

Abstract: The International Society for Rock Mechanics (ISRM) has suggested a notched semi-circular bend technique in split Hopkinson pressure bar (SHPB) testing to determine the dynamic mode I fracture toughness of rock. Due to the transient nature of dynamic loading and limited experimental techniques, the dynamic fracture process associated with energy partitions remains far from being fully understood. In this study, the dynamic fracturing of the notched semi-circular bend rock specimen in SHPB testing is numerically simulated for the first time by the discrete element method (DEM) and evaluated in both microlevel and energy points of view. The results confirm the validity of this DEM model to reproduce the dynamic fracturing and the feasibility to simultaneously measure key dynamic rock fracture parameters, including initiation fracture toughness, fracture energy, and propagation fracture toughness. In particular, the force equilibrium of the specimen can be effectively achieved by virtue of a ramped incident pulse, and the fracture onset in the vicinity of the crack tip is found to synchronize with the peak force, both of which guarantee the quasistatic data reduction method employed to determine the dynamic fracture toughness. Moreover, the energy partition analysis indicates that simplifications, including friction energy neglect, can cause an overestimation of the propagation fracture toughness, especially under a higher loading rate.

Portland cement mortar nanocomposites at low carbon nanotube and carbon nanofiber content: A fracture mechanics experimental study

Abstract: A thorough fracture mechanics characterization of Portland cement mortars reinforced with multi wall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) took place. The critical values of stress intensity factor, K IC S ; strain energy release rate, G IC S ; crack tip opening displacement, CTODc; and critical crack length, ac of 3, 7, and 28 days Portland cement mortars, reinforced with well dispersed carbon nanotubes and carbon nanofibers were experimentally determined. Prismatic notched specimens of neat mortars and mortars reinforced with 0.1 wt.% CNFs, and 0.1 and 0.2 wt.% MWCNTs were subjected to a three point closed loop bending test, using the crack mouth opening displacement, CMOD, as the feedback signal. The fracture parameters of the nanoreinforced mortars were then determined using the two parameter fracture model. The excellent reinforcing and toughening efficiency of MWCNTs and CNFs is demonstrated by a significant improvement in the critical stress intensity factor/fracture toughness (128.6%), critical strain energy release rate (154.9%), and critical crack tip opening displacement (39%). These results allow us to conclude that the MWCNTs and CNFs beneficially alter the nanostructure of the mortar matrix, resulting to a significant enhancement of all fracture and mechanical properties and provide the material, with the ability of performing multiple structural functions.

Effect of Point and Line Defects on Mechanical and Thermal Properties of Graphene: A Review

Abstract: New materials with distinctive properties are arising and attracting the scientific community at regular intervals. Stiffness and strength are the important factors in determining stability and lifetime of any technological devices, but defects which are inevitable at the time of production can alter the structural properties of any engineering materials. Developing graphene with specific structural properties depends upon controlling these defects, either by removing or deliberately engineering atomic structure to gain or tailoring specific properties. In this article, a comprehensive review of defective graphene sheets with respect to its mechanical and thermal properties are presented and examined.

A study on the failure behavior and mechanical properties of unidirectional fiber reinforced thermosetting and thermoplastic composites

Abstract: Failure behavior and mechanical properties of unidirectional (UD) carbon fiber reinforced polyamide 6 (CF/PA6) and epoxy resin (CF/Epoxy) laminates were investigated through tensile tests in this study. The fracture modes of both CF/PA6 and CF/Epoxy were discussed based on the fiber orientation, interfacial properties, Mode II interlaminar fracture toughness and the brush width. Meanwhile, Global load sharing (GLS) model was employed to compare with the experimental mechanical properties and corresponding fracture mechanics model was employed to analyze the fracture behavior. The results showed that UD CF/PA6 laminates with weak interface but high Mode II interlaminalr fracture toughness mainly exhibited step-like fracture modes (77%) in interfacial fracture mode (Adhesive failure), while CF/Epoxy laminates with stronger interface but lower Mode II interlaminalr fracture toughness mostly showed splitting fracture mode (69%) in matrix fracture mode (Cohesive failure).