Search

The investigation on the behavior of a specimen under uniaxial tension and the process of microfracture attracts considerable interest with a view to understanding strength characterization of brittle materials. Little attention has been given to the detailed investigation of influence of heterogeneity of rock on the progressive failure leading to collapse in uniaxial tension. In this paper, a numerical code RFPA3D (Realistic Failure Process Analysis), newly developed based on a three-dimensional model, to simulate the fracture process and associated fractal characteristic of heterogeneous rock specimen subjected to direct uniaxial tension. Specimens with different heterogeneity are prepared to study tension failure. In a relatively homogeneous specimen, the macrocrack nucleates abruptly at a point in the specimen soon after reaching peak stress. In more heterogeneous specimens, microfractures are found to appear diffusely throughout the specimen, and the specimens show more ductile failure behavior and a higher residual strength. Development of fractal theory may provide more realistic representations of rock fracture. The fractal dimension of distributed AE is computed during the fracture process. For all specimens, the fractal dimension increases as the loading proceeds, and it reaches the peak value when macrocrack nucleates abruptly. It is also found that fractures scatter more diffusely in relatively heterogeneous specimens, and the fractal dimension has a smaller value. The homogenous rock specimens have flat and smooth rupture faces which are consistent with the fractal results.

Not Available

Not Available

In this work Ni36Al27Co37 ferromagnetic shape memory alloy was studied. Selective laser melting (SLM) was carried out at the modes providing the volumetric energy density (VED) of 70-200 J/mm3. It was shown that in order to obtain dense samples, VED should be at least 90 J/mm3 for this alloy. The maximum density was achieved at 150 J/mm3.With an increase in VED, the hardness nonlinearly decreases from 447 HV to 413 HV. The samples without visible defects showed a mean conventional tensile failure stress of 479 MPa and a residual deformation of about 0.04%, which confirms their brittleness when not heat-treated.Investigation into crystallographic structure of Ni36Al27Co37 SLM-samples showed that the laser power, scanning speed and hatch distance effects should not be considered separately but in complex. It was found that the most promising approach to obtain anisotropy in this alloy is a simultaneous decrease in the hatch distance and an increase in the scanning speed. In this case, the crystallites aspect ratio achieves values of up to 12 that makes the alloy prospective to show high magnetically induced strain.

This paper examines the stress distribution around a fiber break in alumina-fiber reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts the tensile strength using tensile failure simulations. In particular, we discuss the effect of the matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic-perfect plastic matrix and an elastic-plastic hardening matrix using finite element analysis. Second, the procedure for simulating fiber damage evolution in the Al2O3/Al composites is presented. The simulation incorporates the analytical solution for the axial fiber stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites, and discuss the effect of matrix hardening on the tensile strength of the Al2O3/Al composites. Coupled with size-scaling analysis, the simulated results express the size effect on the strength of the composites, which is seen in experimental results.

In this paper, the macroscopic mechanical behavior of tensile fracture of X70 pipeline steel is combined with the evolution of mesoscopic pores. The ABAQUS finite element software was used to analyze the variation of pore volume fraction in the tensile failure process of unilateral notched specimens with Gurson-Tvergaard-Needleman (GTN) mesoscopic damage model. Combined with the unilateral notched tensile test and the numerical simulation results, the damage mechanics model of pipeline steel based on void volume fraction is established. The results show that the volume fraction of pores is a mesomechanical parameter based on the damage of the micro-holes in the metallic plastic material, which is a bridge between the mesoscopic damage characteristics and the macro-mechanical parameters. Establishing the damage mechanics model of high strength pipeline steel with hole volume fraction as damage variable can truly reflect the macro-meso-mechanics behavior of X70 pipeline steel during unilateral notching.

This research investigated mechanical degradation of powder injection moulded SiCp-reinforced aluminium composite subjected to moderate temperature exposures. Aluminium composite of 20 vol.% SiCp reinforcement was produced by powder injection moulding and sintering at 680°C, followed by 500°C solution treatment plus 150°C for 6 hours artificial aging, and subsequent exposures at 100, 200 and 300°C for 10 and 100 hours. It was found that short-term exposure for 10 hours provided increasing hardness with increasing exposure temperature, while long-term exposure for 100 hours led to an opposite result. The maximum micro Vickers hardness was obtained at 182.2 Hv for Al-SiCp composite exposed at 300°C for 10 hours. Tensile strength was however found deleterious with increasing both exposure temperature and time. The maximum tensile strength was achieved at 191.2 MPa for Al-SiCp composite exposed at 100°C for 100 hours. The formations of AlN, Mg2Si and Al2Cu were observed in both age-hardened and as-exposed conditions. Furthermore, the highest temperature exposure at 300°C and extended exposure time at 100 hours resulted in the lowest hardness and tensile properties due possibly to the loss of coherency of precipitates. SiCp clusters were the main cause of the tensile failure.

The spall fracture is a shock wave induced dynamic fracture phenomenon, it’s difficult to capture the features of the spall fracture when traditional finite element method based on continuum mechanics is applied. In this paper, a new and flexible meshless method, material point method, is used to study the spall fracture of metal material in the case of hypervelocity impact. Firstly, a computational process is given in which Johnson-cook plasticity model, Mie-Grüneisen equation of state and several failure models including hydrodynamic tensile failure model, effective plastic strain model and Johnson-cook failure model are considered. Then a 3D simulation of spall fracture of an armco iron target under impact loads by a 2024-T351aluminum projectile is carried out. At last, the numerical results show that the material point method can accurately capture important features of spall fracture such as the arrival times, magnitudes and shapes of both the compressive waves and tensile reflections in the spall region, and it’s proven that material point method is suitable to simulate the spall fracture in engineering applications.

Tensile failure of metals often occurs through void nucleation, growth and coalescence. This work is concerned with the study of plastic nanovoid cavitation in face-centeredcubic (FCC) crystals at finite temperature. In particular, the Quasicontinuum (QC) method,suitably extended to finite temperatures, is taken as the basis for the analysis. We specificallyfocus on nanovoids in copper single crystals deforming in uniaxial and triaxial tension. Thecomplex structure of dislocations around the nanovoid and the evolution of stress, deformationand temperature of the sample is described in the present work.

External bonding of fiber reinforced polymer (FRP) plates or sheets, because of their advantages, such as high strength to weight ratio and good resistance to corrosion, has become a popular technique for the strengthening and upgrading of structurally inadequate or damaged reinforced concrete (RC) structures. Interface debonding failure is one of the most common failure modes of the FRP strengthened RC structures. In this paper, the damaged concrete constitutive model is established and the effects of crack on the interfacial stresses of RC beam strengthened with CFRP are investigated. Longitudinal stress in the CFRP, shear stress in the adhesive layer and the first principal stress in the concrete at the crack tips of the retrofitted RC beams with cracks at different locations are analyzed. The results show that when cracks locate at the loading position, the longitudinal stress in the CFRP is the largest and the tensile failure of the CFRP is the most likely occurred.

Composite damage modelling with cohesive elements has initially been limited to the analysis of interface damage or delamination. However, their use is also being extended to the analysis of inplane tensile failure arising from matrix or fibre fracture. These interface elements are typically placed at locations where failure is likely to occur, which infers a certain a priori knowledge of the crack propagation path(s). In the case of a crack jump for example, the location of the jump is usually not obvious, and the simulation would require the placement of cohesive elements at all element faces. A better option, presented here, is to determine the potential location of cohesive elements and insert them during the analysis. The aim of this work is to enable the determination of the crack path, as part of the solution process. A subroutine has been developed and implemented in the commercial finite element package ABAQUS/Standard[1] in order to automatically insert cohesive elements within a pristine model, on the basis of the analysis of the current stress field. Results for the prediction of delamination are presented in this paper.

The dynamic compressive behavior of Al2O3 (10% vol.) / TiB2 ceramic composite had been tested by using a split Hopkinson pressure bar in this paper. The results show that the main failure modes of the ceramic composite include crushed failure and split fracture along the loading direction. The former is the typical compressive failure of brittle materials. The later is tensile failure along the flaws produced during the composite manufacturing. The numerical simulation was also used to study the effect of the diameter/length ratio of the samples on the experimental results. The effect of the deformation in the bars’ ends, which contacted with the samples, was also studied in the numerical models.

The Brazilian splitting tests have been commonly and widely used as a standardized test method on disc or cylinder specimens to measure the indirect tensile strength of rocks in mining engineering and other rock engineering. In this paper, a novel numerical code, 3D Rock Failure Process Analysis code, was applied to implement the splitting tensile failure tests on rock discs. The influences of the heterogeneity on stress distribution in rock are also discussed and the splitting failure patterns of specimens subjected to Brazilian tests are simulated. The simulated splitting results of rock discs were found quite realistic, which indicate that the rock failure analysis method is applicable and practical for the study of rock disc splitting failure.

When a tunnel or an underground structure is excavated in rock mass, rock disturbed or damaged zone (EDZ) is formed around the excavation due to the stress concentration resulting from stress redistribution. Recent studies on the rock EDZ revealed it’s important to structural stability around underground opening. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by AE tests. The results of the experiments showed that tensile failure was the major microscopic failure mechanism of rock in excavation damaged and disturbed zone. The expression of the damage magnitudes in each AE source leads to accurate prediction of macroscopic failure mechanisms. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.

The multi-walled carbon nanotubes (MWNTs) and carbon fibers (CFs) were added to the phenolic resin to fabricate MWNTs/phenolic, MWNTs/CFs/phenolic nanocomposites and CFs/phenolic composites by hot press method. The differential scanning calorimetry (DSC) test was performed for the above-mentioned three kinds of composites. The valley points on the slope of endothermic responses correspond to the glass transition temperatures of the composites. The MWNTs/phenolic nanocomposites had the lowest glass transition temperature among the three kinds of composites discussed, which indicated a better thermal conductivity property of MWNTs. Phenolic-based composites reinforced by different weight percentages of MWNTs and CFs were also investigated. The tensile failure morphologies of nanocomposite specimens were examined using a scanning electron microscope to evaluate the possible effects on the glass transition temperature of nanocomposites..

Tensile failure behavior of ceramic matrix composites (CMCs) was characterized with nondestructive evaluation (NDE) techniques. Prior to the mechanical testing, infrared (IR) thermography was employed to obtain thermal diffusivity maps for CMC specimens. IR thermography also was used for quantitative analyses of the progressive damage and in-situ monitoring of the damage during tensile tests, while ultrasonic (UT) C-scans were used to present defect distributions of the composites. The thermal diffusivity map showed good consistency with ultrasonic C-scan results of CMC specimens. In this investigation, qualitative relationship between UT signatures and thermal diffusivity has been introduced, and the temperature changes of CMC specimens during tensile test have been measured. Moreover, the correlation between NDE results and fracture behavior of CMCs has been presented to understand tension fracture behavior of CMCs.

Concrete is taken as three-phase composites consisting of aggregate, interface and mortar matrix in the paper. The tensile strength and elastic modulus of each phase material are assumed to obey Weibull distribution. A few random aggregate and random parameter models are generated by Monte-Carlo method. Tensile failure of micro element is described by tension damage model, in which the stress will be reduced linearly after the stress reaches the maximum tensile strength. The results show that more cracks of lower homogenous specimens are observed than those of higher homogenous specimens. The phenomenon of discontinuous cracking is observed in the former, while strain localization and brittle behavior is observed in the latter. The ultimate load obtained in the numerical simulation agrees well with that of the test.

Two different types of multi-walled carbon nanotube (MWNT), the dispersed and the network MWNTs, were used to reinforce the phenolic resin. The MWNTs/phenolic nanocomposites were tested by a dynamic mechanical analyzer (DMA) to characterize their dynamic mechanical properties. The results showed that increasing the MWNT content can increase the storage modulus, the loss modulus and the glassy transition temperature of the MWNTs/phenolic nanocomposites. A subambient loss transition is seen in the nanocomposites with network MWNTs which results in a better impact resistance property in the nanocomposites. The glassy transition temperature of the nanocomposites with network MWNTs is higher than that of nanocomposites with dispersed MWNTs. The MWNT additive in phenolic resin can be used to improve the dynamic mechanical properties of the MWNTs/phenolic nanocomposites. The tensile failure morphologies of MWNTs/phenolic nanocomposites were also examined using field emission scanning electron microscope (FESEM) to explain the difference between the two types of nanocomposites.

A series of formulas about two-parameter parabolic Mohr strength criterion(2-PP Mohr criterion) are derived. Based on the results of uniaxial tension and uniaxial compression tests, the parameters involved in the criterion can be easily determined, then the criterion in terms of the major principal stress and the minor principal stress is derived, and the damage pattern is also discussed. At last, the formulas about the rupture angle and the friction angle are presented, and their relationship is also given. 2-PP Mohr criterion can describe not only shear but also tensile failure. In this criterion the ratio of the uniaxial compression strength and the uniaxial tension strength is not confined as in Griffith criterion. The formula about the rupture angle provides steady theoretical foundation for determining the direction of crack faces and damage patterns in the computation of macro crack propagation. In fact, Griffith criterion is only a special case of the two-parameter parabolic Mohr strength criterion proposed in this present paper.

In the present paper, axisymmetric smooth and notched specimens of aluminium alloy LY12CZ were monotonic tension tested to study the ductile fracture behavior and evaluate the suitability for the critical void growth ratio, VGC of the material, from the micro and macro points of view. It's found that the material would change fracture morphology from tensile to shear failure in the various stress states; the ductile fracture parameter VGC is sensitive to the fracture morphology which is applicable for the case of tensile failure while not suitable for the case of the shear failure, i.e. the case of smooth tensile specimens.

After removal of the resin from PAN-carbon fibers, the fibers were coated with C or SiC by chemical vapor deposition (CVD) and the effects of these CVD coatings on the mechanical and chemical properties of carbon fibers were investigated. DTG analysis was conducted at different temperatures (993K, 1053K and 1113K) to establish the oxidation reaction models of the C/C or C/SiC coated fibers. The results showed that the CVD coatings enhance the oxidation resistance of the carbon fibers. The surface morphology of the CVD C or SiC fibers was investigated by SEM. The tensile strength of the fibers was found to decrease rapidly after CVD, while the Young’s modulus remained almost constant. These changes can be explained by the interface performance of the carbon fibers and the mechanism of CVD C or SiC deposition. The difference in dilatability between the CVD C or SiC layers and the carbon fibers was the main reason for the tensile failure.