Fracture characteristics of concrete at early ages

The purpose of this study is to experimentally investigate, at early ages, the fracture characteristics of concrete such as critical crack tip opening displacement, critical stress intensity factor, fracture energy, and bilinear softening curve based on the concepts of the effective-elastic crack model and the cohesive crack model.A wedge-splitting test for Mode I was performed on cubical specimens with an initial notch at the edge. By taking various strengths and ages, load-crack mouth opening displacement (CMOD) curves were obtained and these curves were evaluated by linear elastic fracture mechanics and finite element analysis.The results from the test and analysis indicate that critical crack tip opening displacement decreases and critical stress intensity factor and fracture energy increase with concrete ages from Day 1 to Day 28. By numerical analysis, four parameters of bilinear softening curves from Day 1 to Day 28 were obtained. In addition, it was observed that the parameters f_t and f₁ increase and the parameters w₁ and w_c decrease with increasing age. The obtained fracture parameters and bilinear softening curves at early ages may be used as a fracture criterion and an input data for finite element analysis of concrete at early ages.     

Mechanical behavior and thermal shock resistance of MgO-C refractories: Influence of graphite content

The mechanical and thermo-mechanical properties of MgO-C refractories are of major importance in the industrial applications, and highly depend on the optimization of their microstructural design. In the present work, the influence of flaky graphite content on mechanical behavior and thermal shock resistance of such refractories was investigated with the aid of the wedge splitting test, fractal and microscopic fractographic analysis. The results showed that the increase of graphite content in the specimens led to an enhanced non-linear fracture behavior, a reduced nominal notch tensile strength (σ_NT), and a higher specific fracture energy (G_f), characteristic length (l_ch) and thermal shock resistance parameter (R_st). The fractal analysis of the crack propagation path of the specimens after the wedge splitting test indicated that increasing graphite content in the refractories can enhance their irregularity of the crack propagation path during fracture. Also, it was suggested from microscopic fractographic analysis that the improvement of thermal shock resistance of MgO-C refractories was positively correlated with the increase of interface crack propagation.     

Notched Fracture Behavior of an Oxide/Oxide Ceramic-Matrix Composite

The fracture behavior of an oxide/oxide ceramic-matrix composite, alumina/alumina-silica (Nextel610/AS), was investigated at 23° and 950°C using a single edge notched specimen geometry with clamped ends. Crack growth and damage progression were monitored during the tests using optical microscopy, ultrasonic C-scans, and crack mouth opening displacement. The net section strength of Nextel610/AS was less than the unnotched ultimate tensile strength. The failure mode was nonbrittle with considerable nonlinear deformation prior to and after the peak load at 23° and 950°C. The effect of temperature on the notched strength was significant. Net section failure stress decreased 50% when temperature was increased from 23° to 950°C. Observations of damage progression indicated that the reduction in notch strength with temperature was associated with self-similar crack growth at 950°C. Ultrasonic C-scans were found to be an effective method of monitoring damage progression. Ultrasonic attenuation ahead of the notch tip was correlated with surface matrix cracks and exposed fiber lengths on the fracture surface.     

基于ESPI技术的混凝土单轴拉伸断裂特性研究

为了研究混凝土的拉伸断裂特性,对带有预制切口的C80高强混凝土棱柱体进行单轴拉伸试验,采用电子散斑干涉(ESPI)技术测量棱柱体表面的场位移等信息,分析加载过程得到混凝土的断裂参数以及断裂特性等。为进行对比分析,本次试验还采用线性位移计和夹式位移计测量试件的变形程度。对比结果表明ESPI技术的测量结果与夹式位移计结果吻合较好,证实了ESPI技术用于测量混凝土表面位移场的精确性和可行性。分析得到:初裂点应力约为峰值应力的82%;峰值下裂缝口张开位移(CMOD)为11 μm,结合公式得到混凝土在单轴拉伸下的断裂韧度约为0.41 MPa·m^1/2,断裂能约为24.71 N/m;利用两种拟合公式对试验应力-应变关系进行拟合,得到了较好的拟合结果。最后,通过不同加载步骤下ESPI技术测得的位移云图分析裂缝口张开位移随试件宽度的变化规律,得到了单轴拉伸下的裂纹扩展规律。     

Temperature dependence of crack initiation fracture toughness of various nanoparticles filled polyamide 66

In the present study, the crack initiation fracture toughness of various nanoparticles filled polyamide 66 was investigated in a broad temperature range (23-120 °C) by using an essential work of fracture (EWF) approach. Four types of spherical nanoparticles, i.e. two types of TiO₂ (21 nm, with/without surface modification), SiO₂ (13 nm) and Al₂O₃ (13 nm), were selected with a constant volume content of 1% in nanocomposites, which were compounded using a twin-screw-extruder. The addition of nanoparticles led to an enhanced specific EWF item at most test temperatures at the cost of the reduction of the non-EWF item. The value of the specific EWF was also estimated by a crack opening displacement method. Associated with SEM fractograph analysis, it was clear that two basic factors, i.e. crack tip blunting and net section stress, finally determined the EWF value. With the addition of nanoparticles, the item of crack tip blunting was increased at most temperature range, which may be incidental with the formation of numerous dimples and sub-dimples induced by nanoparticles; while the item of net section stress was correlated with the particle distribution, especially at room temperature, which was notably decreased in case of poor nanoparticle distribution.     

Effect of nanocarbon sources on microstructure and mechanical properties of MgO–C refractories

A study of microstructural evolution, mechanical and thermo-mechanical properties of MgO–C refractories, based on graphite oxide nanosheets (GONs), carbon nanotubes (CNTs) and carbon black (CB), was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and thermal shock tests. Meanwhile, these results were compared to the conventional MgO–C refractory containing 10 wt% flaky graphite prepared under the same conditions. The results showed that higher cold modulus of rupture was obtained for the composition containing GONs, and the composition containing CNTs exhibited larger displacement after coking at 1000 °C and 1400 °C. Also, the addition of nanocarbons led to an improvement of the thermal shock resistance; in particular, both compositions containing CNTs and CB had higher residual strength ratio, approaching the thermal shock resistance of the reference composition containing 10 wt% flaky graphite, as it was associated with the presence of nanocarbons and in-situ formation of ceramic phases in the matrix.     

Toughness improvement of epoxy networks by nanophase‐separating antiplasticizers

Two epoxy‐amine networks were toughened by using a nanophase separating antiplasticizer at various contents ranging from 0 to 20 mol % with respect to the epoxide. These model networks were chosen to differ markedly by their glass transition temperature in the absence of additive, namely 180°C and 114°C. Net stress was measured as a function of crack opening displacement on single edge notch bending specimens. Analysis of the experimental data yielded both stiffness and fracture toughness. In addition, fracture surfaces were observed by scanning electron microscopy. Effect on both stiffness and toughness of network characteristics (nature of the hardener, amount of additive) were carefully examined. Conditions were found for which fracture energy at crack initiation substantially increases without any detrimental effect on stiffness. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fracture energy evaluation of refractories in wedge splitting tests from notch opening displacements

The work of fracture of refractories is commonly calculated from crack mouth opening displacements (CMODs) in wedge splitting tests (WSTs). This paper proposes a methodology for estimating the fracture energy from notch opening displacement (NOD) measurements, which is useful for setups where CMOD is not accessible. NODs and CMODs are calculated for both faces of two WSTs experiments on a castable refractory via digital image correlation (DIC) and finite element simulations. A quadratic function fits well the non-linear CMOD vs. NOD behavior in the crack initiation regime, while an affine trend describes the propagation regime. Although the nonlinearity associated with crack initiation is more complex, the crack propagation energy can easily be estimated from NOD data when CMODs cannot be measured.     

Influence of a graphite shell on the thermal and electromagnetic characteristics of FeNi nanoparticles

Graphite-coated FeNi alloy nanoparticles have been prepared by a modified arc-discharge method in an alcohol atmosphere and have been characterized by means of X-ray diffraction, energy dispersive spectroscopy, transmission electron microscopy, Raman spectroscopy, thermal gravimetric analysis and scanning differential thermal analysis. The results show that the nanoparticles have a core/shell structure, with FeNi alloy as core and graphite layers as shell. Compared with FeNi nanoparticles with an oxide shell, the graphite shell restricts the growth of the FeNi nanoparticles, which leads to lower saturation magnetization and higher natural-resonance frequency. Due to the enhancement of the thermal stability by the graphite shell and its oxidation protection, the graphite-coated FeNi nanoparticles are stable in air below 240 °C. The electromagnetic characteristics of the graphite-coated FeNi nanoparticles have been studied in the 2-18 GHz range. The graphite shell dramatically improves the magnetic/dielectric loss and the attenuation constant in the 9-18 GHz range through the enhancement of the electrical resistivity and the protection of the FeNi cores, leading to enhanced microwave-absorption properties in this range.     

Mechanical properties of phenolphthalein polyether ketone: Yield stress,young's modulus,and fracture toughness

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using phenolphthalein polyether ketone (PEK-C). Yield stress, Young's modulus, fracture toughness, and crack opening displacement data were obtained for various conditions. In general, both yield stress and Young's modulus increase with decreasing temperature. However, the relationships between fracture toughness, loading rate, and temperature are very complex. This behavior is due to the simultaneous intersection of viscoelasticity and localized plastic deformation. The increased yield stress is the main factor contributing to the reduction in fracture toughness and crack opening displacement. The relationship between fracture toughness and yield stress are discussed. © 1995 John Wiley & Sons, Inc.     

Critical stress intensity factor of epoxy mortar

Fracture behavior of epoxy mortar was investigated in Mode I fracture using single edge notched beams with varying notch depth and beam thickness. The beams were loaded in both 3-point and 4-point bending. Influence of polymer content and temperature on the fracture behavior of epoxy mortar was studied using uniform Ottawa 20–30 sand. The polymer content was varied between 10 percent and 18 percent of the total weight of the composite. The temperature was varied between 22°C and 120°C. The flexural strength of the polymer mortar increases with increase in polymer content while the flexural modulus goes through a maximum. The critical stress intensity factor (K_IC) was determined by several methods including compliance method (based on crack mouth opening displacement) and finite element analysis. The K_IC for epoxy mortar increases with increase in polymer content and epoxy mortar strength but decreases with increase in temperature. The critical stress intensity factor of epoxy mortar is represented in terms of polymer content and polymer strength or stiffness. Numerical tests based on random sampling and stratified sampling procedures were performed to substantiate the experimentally observed fracture toughness values of epoxy mortar.     

Fracture behavior and analysis of fiber reinforced concrete beams

Fiber reinforced concrete beams with varying notch depths and different volume fractions of steel fibers were tested. The results were analyzed to examine the applicability of various fracture mechanics approaches including: critical stress intensity factor, J-integral, critical crack opening displacement, compliance technique for determining the slow crack growth and R-curve analysis. Attempts were made to identify a fracture parameter which is independent of test-specimen geometry and which can correctly predict the effects of fiber addition. R-curve method appears to be promising.     

High-Temperature Bridging in a Vitreous Bonded Alumina

The load supported by crack wake bridges was studied in glassy alumina at 1240°C using short double cantilever beam specimens with a rear notch wedge. Direct observation of fracture surfaces revealed the sizes and shapes of the high-temperature bridges. The bridged area was stochastic in nature but there was a clear trend toward decreased area bridged with increasing crack opening displacement consistent with existing models. Fracture surface observations on specimens without a rear notch were compared to bridge distributions in wedged specimens. Because of the complex bridge morphology the load supported by the bridges cannot be modeled as viscous uniaxial columns.     

Thermal‐Shock Fracture and Damage Resistance Improved by Whisker Reinforcement in Alumina Matrix Composite

Fracture resistance of SiC‐whiskers‐reinforced Al₂O₃‐matrix composite under thermal shock was examined. Equibiaxial tensile thermal stress in the composite was significantly reduced before fracture, because the whiskers made percolation paths that increase heat flux and thereby reduced the temperature gradient. The thermal‐shock fracture resistance (R′) of the composite is thus much higher than that of monolithic Al₂O₃. Thermal‐shock damage resistance (R″″) was estimated from the thermal‐shock stress when a surface crack propagates. R″″ of the composite is also much higher than that of monolithic Al₂O₃ owing to an increment of work‐of‐fracture due to crack‐face bridging of the whiskers.     

Strength Behavior of Polycrystalline Alumina Subjected to Thermal Shock

Theoretical predictions of crack propagation behavior in brittle solids under conditions of thermal shock were verified by water quenching of cylindrical polycrystalline alumina rods followed by strength testing. The calculated quenching temperature difference (Δ T _O) required to initiate thermal-stress fracture agreed fairly well with experiment. When fracture was initiated, strength decreased catastrophically, in agreement with theory. An expression for the strength remaining after thermal stress fracture was derived in terms of the pertinent physical parameters. Values of surface fracture energy similar to those reported in the literature agreed with experiment. Strength after thermal shock was predicted to be inversely proportional to the 1/4 power of the rod diameter; this prediction was supported by experimental data for two rod sizes. Over a range of quenching temperature differences Δ T Δ T ₀ strength remained constant, in agreement with the theoretical expectation that the newly formed cracks were subcritical. Only at the highest quenching temperature differences could further decreases in strength be observed; the quantitative changes, however, were masked by nonlinear deformation (evidenced by permanent crack opening). It was concluded that, although thermal shock behavior of brittle ceramics can be approximated fairly well, reliable quantitative estimates require considerably more information about strength and surface fracture energies as a function of environment, stress distribution, strain rate, and temperature and specimen size effects.     

On the calibration of cohesive parameters for refractories from notch opening displacements in wedge splitting tests

Cohesive elements are commonly used to describe crack propagation in heterogeneous materials with toughening mechanisms. This work aims to provide a guideline on how these fracture parameters can be calibrated using notch opening displacements (NODs) measured via digital image correlation and force data from wedge splitting tests (WSTs). Weighted finite element model updating was applied to calibrate material and boundary condition parameters in the same framework. The influence of each parameter on force and NOD data are given together with uncertainties for the calibrated parameters. Numerical results were in very good agreement in terms of splitting force, NOD, displacement and gray level residual fields. It is shown that images obtained during WSTs focusing on the crack path (i.e., hiding the loading region) can be used to drive numerical simulations and obtain cohesive parameters.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

Fracture toughness of particle filled fiber reinforced polyester composites

Fracture behavior of polyester composite systems, polyester mortar and glass fiber reinforced polyester mortar, was investigated in mode I fracture using single edge notched beams with varying notch depth. The beams were loaded in four-point bending. Influence of polymer content on the flexural and fracture behavior of polyester composites at room temperature was studied using a uniform Ottawa 20–30 sand. The polymer content was varied between 10 and 18% of the total weight of the composite. The flexural strength of the polyester mortar systems increase with increase in polymer content while the flexural modulus goes through a maximum. The critical stress intensity factor (K_IC) for the optimum polyester mortar (14%) was determined by two methods including a method based on crack mouth opening displacement. The K_IC for polyester mortar is linearly related to the flexural strength. Polyester mortar (18%) reinforced with 4% glass fibers was also investigated, and crack growth resistance curve (K_R) was developed with crack extension (Δa). A model has been proposed to represent the fracture toughness with change in crack length, K_R - Δa relationship, of fiber reinforced polyester composite.     

Toughening of isotactic polypropylene with CaCO3 particles

The mechanisms of deformation and fracture of isotactic polypropylene filled with CaCO₃ particles were studied. Three types of particles with average diameters of 0.07, 0.7, and 3.5 μm were used at filler volume fraction from 0.05 to 0.30. The experiments included slow tensile tests, notched Izod impact tests with varying notch depths, and fracture resistance tests using double-cantilever-beam sample configurations. In slow tension, addition of fillers increased the modulus and decreased the yield stress independently of filler type. The strain at break increased with initial incorporation of fillers but decreased at higher loadings. The 0.7 μm diameter particles improved Izod impact energy up to four times that of the unfilled matrix. The other particles had either adverse or no effect on the impact toughness. The toughening mechanisms at work were plastic deformation of interparticle ligaments following particle-matrix debonding with additional contribution coming from crack deflection toughening. The failure of the 0.07 and 3.5 μm diameter particles to toughen the matrix was attributed to poor dispersion.