Meso-structural study of concrete fracture using interface elements. I: numerical model and tensile behavior

A recently developed FE-based mesostructural model for the mechanical behavior of heterogeneous quasi-brittle materials is used systematically to analyze concrete specimens in 2D. The numerical model is based on the use of zero-thickness interface elements equipped with a normal-shear traction-separation constitutive law representing non-linear fracture, which may be considered a mixed-mode generalization of Hillerborg’s “Fictitious Crack Model.” Specimens with 4 × 4 and 6 × 6 arrays of aggregates are discretized into finite elements. Interface elements are inserted along the main lines in the mesh, representing potential crack lines. The calculations presented in this paper consist of uniaxial tension loading, and the continuum elements themselves are assumed to behave as linear elastic. In this way, the influence of various aspects of the heterogeneous geometry and interface parameters on the overall specimen response has been investigated. These aspects are aggregate volume fraction, type of arrangement and geometry, interface layout, and values of the crack model parameters chosen for both the aggregate-aggregate and matrix-aggregate interfaces. The results show a good qualitative agreement with experimental observations and illustrate the capabilities of the model. In the companion second part of the paper, the model is used to represent other loading states such as uniaxial compression, Brazilian test, or biaxial loading.     

The influence of nanoadhesives on the tensile properties and Mode-I fracture toughness of bonded joints

Nanoadhesives of epoxy resin are synthesized and evaluated. They are organically modified by multiwalled carbon nanotubes (MWCNT) (1% by weight) as reinforcement. Tensile tests are conducted on multiple identical unnotched and notched specimens to evaluate the overloading and fracture behavior of the nanoadhesives and are compared with the neat epoxy resin. In comparison with the neat epoxy, it is found that the 1% MWCNT reinforcement increased the ultimate and residual strength by about 29% and 56%, respectively. In comparison with the neat resin, there is a 265% increase in the fracture toughness of the MWCNT adhesive. Fracture surface analysis revealed the various mechanisms by which the MWCNT adhesives acquire their superior strength and toughness in comparison with the neat resin.     

Meso-structural study of concrete fracture using interface elements. II: compression,biaxial and Brazilian test

In the previous companion paper, a recently proposed meso-mechanical model using fracture-based zero-thickness interfaces was used to analyze 2D concrete specimens subject to uniaxial tension, to compare and discuss the results with respect to well-known experimental behavior, and to study the influence of composition and parameters on the overall behavior. In this second part paper, the model is used to simulate additional situations including uniaxial compression for conventional and high-strength concrete, Brazilian test including effects of platen size, and biaxial tests. The results show that the simulations are equally realistic in this wider range of material behavior, always within the limitation imposed by its two dimensional character.     

Cd-O共掺杂ZnTe第一性原理计算

采用基于密度泛函理论的第一性原理平面波超软赝势方法计算了未掺杂,Cd、O单掺杂及Cd-O共掺杂ZnTe的几何结构、能带结构、态密度分布、光吸收谱和介电常数等性质。结果表明:掺杂后的ZnTe晶格常数发生变化,其中Cd掺杂的ZnTe晶格失配最大;三种掺杂均使ZnTe禁带宽度减小,并引入杂质能级,其中O掺杂和Cd-O共掺杂的ZnTe的禁带宽度变化较为明显,同时掺杂后ZnTe吸收带边出现不同程度的红移。     

Pre-cracking technique for fracture mechanics experiments along interface between thin film and substrate

Utilizing the difference in interface strength due to fabrication process, a technique for producing a sharp pre-crack between a thin film and a substrate is proposed. A cracked specimen for examining fracture toughness of interface between a sputtered copper (Cu) thin film and silicon (Si) is made by the method. A vacuum-evaporated Cu thin film, which has poor adhesion to Si, is inserted between the sputtered Cu thin film and the Si substrate as a release layer. The release layer debonds from the Si substrate at very low load, and the sharp pre-crack is successfully introduced along the interface. Using the pre-cracked specimen, the interface fracture toughness test is conducted and the critical J-integral, J_C, is evaluated as about 1 J/m² for the sputtered Cu/Si interface.     

Improving fracture toughness of dental nanocomposites by interface engineering and micromechanics

The fracture toughness of dental nanocomposites fabricated by various methods of mixing, silanization, and loadings of nanoparticles had been characterized using fatigue-precracked compact-tension specimens. The fracture mechanisms near the crack tip were characterized using atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The near-tip fracture processes in the nanocomposties were identified to involve several sequences of fracture events, including: (1) particle bridging, (2) debonding at the poles of particle/matrix interface, and (3) crack deflection around the particles. Analytical and finite-element methods were utilized to model the observed sequences of fracture events to identify the source of fracture toughness in the dental nanocomposites. Theoretical results indicated that silanization and nanoparticle loadings improved the fracture toughness of dental nanocomposites by a factor of 2-3 through a combination of enhanced interface toughness by silanization, crack deflection, as well as crack bridging. A further increase in the fracture toughness of the nanocomposites can be achieved by increasing the fracture toughness of the matrix, nanofilled particles, or the interface.     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

Electronic structure and transport properties of Co(Si1−xMx) (M = Al, P): First-principles study

In this study, by using the full-potential linear augmented plane wave (FLAPW) method based on the density functional theory (DFT), the lattice parameter of CoSi was calculated theoretically and the calculations of the electronic structures of CoSi and CoSi_1−xM_x (M = Al, P and x = 0.03125, 0.125) were performed. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semi-metal and the density of states (DOS) is very small at the Fermi level. M-doping can tune the Fermi level and the hole pockets and the electron ones, which is very valuable to modulate the transport properties. Based on the calculated electronic structures and our experimental results on CoSi [C.C. Li, W.L. Ren, L.T. Zhang, K. Ito, J.S. Wu, J. Appl. Phys. 98 (2005) 063706], the intrinsic relations between electronic structures and transport properties of CoSi and CoSi_1−xAl_x are discussed in detail. The transport properties along main crystallographic directions of binary CoSi and CoSi_1−xAl_x are experimentally examined. The experimental results show that the electrical resistivity of CoSi-based compounds is anisotropic, while the Seebeck coefficient is almost isotropic. The calculated band structures of CoSi_1−xAl_x can theoretically interpret the anisotropy of the electrical transport properties.     

SiCp／Al复合材料的微观结构与界面

对用压力铸造法制造的碳化硅颗粒增强铝合金（ＳｉＣｐ／Ａｌ）复合材料的微观结构和界面进行了研究。结果表明：碳化硅颗粒在复合材料中均匀分布，复合材料的基体中有较高的位错密度，碳化硅颗粒中有少量的层错。研究还发现ＳｉＣｐ／Ａｌ复合材料中界面结合良好，没有反应物生成，并且在界面处没有发现孔隙存在。在复合材料拉伸断口上没有发现裸露的碳化硅颗粒，说明在复合材料拉伸破坏时ＳｉＣｐ－Ａｌ界面没有开裂，反映了压铸ＳｉＣｐ／Ａｌ复合材料中良好的界面结合。     

Predicting fracture and fatigue crack growth properties using tensile properties

The safe-life assessment of components requires information such as the plane stress (K_c), plane strain (K_Ic), part-through fracture toughness (K_Ie), and the fatigue crack growth rate properties. A proposed parametric/theoretical approach, based on an extended Griffith theory is used to derive fracture toughness properties and generate fatigue crack growth rate data for a range of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial stress-strain material properties data to derive material fracture toughness values. However since the methodology is in part based on an empirical relationship a wide ranging validation with actual data is required. This paper uses steel, aluminum and titanium based alloys from a pedigree database to quantify material properties sensitivity to the predictions for K_Ic and K_c and the subsequent estimation of ΔK_th threshold and the Paris constants, C and n values. A sensitivity analysis using experimental scatter bounds show the range of da/dN predictions can be achieved. It is found K_Ic/ΔK_th ratios designated as α has a range of 5-25 irrespective of tensile ductility, ε_f, and is insensitive to it. The value of ΔK_th for all the alloys considered was found to be proportional to the final elongation, ε_f, and an empirical relationship describing ΔK_th as a function of ε_f was established. Furthermore it is suggested that, with the knowledge of appropriate tensile properties and the estimated range of K_Ic/ΔK_th ratios for the different alloys applying this method could be an appropriate tool that can be used to conservatively predict fracture and fatigue in similar alloy categories. Thus helping to reduce costs and optimize the number of experimental tests needed for alloy characterizations.     

Deformation mechanisms in tensile fracture of Al foils containing Si precipitates

Recently, Kiritani et al. proposed a new mechanism of plastic deformation without involving dislocations in tensile fracture of metal foils. The paper reports transmission electron microscopy (TEM) study of tensile fracture of Al containing hard precipitates (Si) that are considered to act as obstacles to dislocation motion. In sawtooth-shaped thin-foils formed at the fracture tip (‘sawtooth portion’), tensile strain was as high as 10³, but only a few dislocations were pinned to precipitates. Instead, voids were formed at precipitate/matrix interface, elongated in the direction of tension, and broke up into several smaller voids, due to stress concentration around hard precipitates. The thicker area of the specimen (‘base portion’), where tensile strain was 30, did not contain voids but showed a dislocation cell structure. In tensile fracture of pre-thinned specimen, voids were formed in the sawtooth portion, despite the tensile strain also being 30. These results suggest that the sawtooth portion is formed by a new mechanism that does not involve dislocations.     

A study to evaluate and understand the response of aluminum alloy 2026 subjected to tensile deformation

The strain concentration factors were determined for aluminum alloy 2026 in the T3511 temper using multi-hole structural coupon specimens. Samples of the alloy were evaluated for both the 6.25 mm (0.25 in.) thick and 10 mm (0.4 in.) thick specimens and having widths of 50 mm (2 in.) and 100 mm (4 in.), respectively. For the case of the specimens that were 50 mm in width the mechanical tests were conducted for both the open hole and filled hole conditions and the corresponding strain concentration value was determined. Threaded fasteners having collars were used for the case of the filled hole specimens. The fasteners posses a shank diameter that was slightly larger than the nominal hole size in order to provide for some interference. The strain concentration values were evaluated at both the failure strain (ε_f) and the strain at maximum load (ε_max). The average strain concentration value was then used to predict the results for the stack-up tests.     

Cohesive zone modeling of interface fracture in elastic bi-materials

Some basic issues regarding the cohesive zone modeling of interface fracture between two dissimilar elastic materials are studied. The dependence of the cohesive energy density on the phase angle is first discussed under small scale cohesive zone conditions. It is then shown that in general the stress singularities in tension and shear cannot be simultaneously removed at the cohesive zone tip if a single cohesive zone length is adopted for both tensile and shear fracture modes. Finally, the energy dissipation at the tip of a prescribed cohesive zone is examined using a bilinear cohesive zone model under the uncoupled tension/shear conditions.     

Mechanical properties and microstructure of Al/Al laminated structure produced via ultrasonic consolidation process

Mechanical properties of a 2024 aluminium alloy laminated structure produced by the ultrasonic consolidation were investigated. In comparison with the monolithic aluminium alloy, the existence of laminated structure gave different fatigue and fracture mechanisms that associated with the layer interfaces. The Al/Al laminated specimens had the lower tensile strength but much higher fracture toughness than the monolithic Al specimens due to the exit of interface delaminations around the crack tip. The fatigue life of the laminated specimens was comparable to that of the monolithic Al specimens, though the initiation and propagation of the crack in the laminated specimens depended strongly on the microstructure of each material. The interface between layers could arrest the fatigue crack and impede the further propagation.     

First-principles study of the (001) and (110) surfaces of superhard ReB2

Structural relaxations, electronic properties, and surface energies of ReB₂ (001) and (110) surfaces with various terminations are investigated with a first-principles method. It is found that the surface interatomic spacings of ReB₂ (001) and (110) surfaces are different from those of the bulk structure. The vertical spacings between the first and second layers of the studied surfaces are contracted. The (001)-Re surface is likely to be stable without introducing a large relaxation. Among these surfaces, only the (110) surface has surface rumpling, and the Re atoms on its first layer are apt to move inward. After atomic relaxation, some covalent bonds formed by the outmost atoms of the relaxed surfaces are shorter than those of the bulk system, which indicates that the covalent B-B and Re-B bonds of the surface layer have been strengthened. An analysis of surface energies shows that after relaxation, the (001)-Re surface is more stable than other types of surfaces.     

Cohesive zone modelling of interface fracture near flaws in adhesive joints

A cohesive zone model is suggested for modelling of interface fracture near flaws in adhesive joints. A shear-loaded adhesive joint bonded with a planar circular bond region is modelled using both the cohesive zone model and a fracture mechanical model. Results from the models show good agreement of crack propagation on the location and shape of the crack front and on the initial joint strength. Subsequently, the cohesive zone model is used to model interface fracture through a planar adhesive layer containing a periodic array of elliptical flaws. The effects of flaw shape are investigated, as well as the significance of fracture process parameters. The results from simulations of fracture in a bond containing circular flaws show that localization of crack propagation in the vicinity of a flaw has significant effect on the joint strength and crack front shape. The localization effects are highly dependent on the fracture process zone width relative to the flaw dimensions. It is also seen that with increasing fracture process zone width, the strength variation with the flaw shape decreases, however, the strength is effected over a wider range of propagation.     

Enhancing both strength and toughness of carbon/carbon composites by heat-treated interface modification

A simple method to increase both strength and toughness of carbon/carbon (C/C) composites is presented. This method is based on the heat treatment of the pre-deposited thin carbon coating, leading to the formation of more orderly pyrolytic carbon (PyC) as a functional interlayer between fiber and matrix that could optimize the interfacial sliding strength in C/C composites. Effects of such a heat-treated PyC layers on the microstructure, tensile strength and fracture behavior of unidirectional C/C composites were investigated. Results showed that although the in-situ fiber strength was deteriorated after the introduction of interfacial layer, tensile strength of the specimen was greatly improved by 38.5% compared with pure C/C composites without any treatment. The interfacial sliding stress sharply decreased, which was interpreted from finite element analysis and verified by Raman spectra. Therefore, the fracture behavior was changed from brittle fracture to multiple-matrix cracking induced non-linear mechanical behavior. Finally, the ultimate strength can be predicted by different models according to the interfacial sliding stress. Our research would provide a meaningful way to improve both strength and toughness of C/C composites.     

Mixed mode fracture criterion of interface crack between dissimilar materials

This study presents an application of fracture mechanics to the interface crack between dissimilar materials. In this study, a concept of the stress intensity factors of an interface crack is discussed, and various types of specimens are tested experimentally for investigating the mixed mode fracture toughness criterion of an interface crack. The fracture toughness based on the stress intensity factors of an interface crack is decided by the fracture test and the boundary element analysis using the contour integral method. The mixed mode fracture toughness criterion is successfully characterized by the stress intensity factors of an interface crack.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

High temperature tensile and fatigue behaviour of the unidirectionally reinforced MMC Ti64/SiC

The material employed in the present work is a Ti6Al4V matrix composite unidirectionally reinforced with eight plies of SiC fibres. This paper studies the tensile behaviour and the fatigue fracture processes at the operation temperatures planned for these materials (550 and 600 °C). The fractographic analysis and microstructural observations of the tested samples at both temperatures are also presented. These are used to determine the mechanisms of nucleation and propagation of damage until fracture.