Cross Crack Research Articles

Mode-I Plane Elasticity Problem of Two Asymmetrical Edge Cracks Emanating from an Elliptical Hole in Two-Dimensional Decagonal Quasicrystals

We consider the plane elasticity problem of two asymmetrical edge cracks emanating from an elliptical hole in two-dimensional decagonal quasicrystals (QCs) under remotely uniform tensile stress. A complex variation method of two-dimensional QCs is developed to solve the plane elasticity problem of two-dimensional decagonal QCs containing complex defects. The analytical solutions for the stress field and the stress intensity factors near the crack tip are expressed by using a conformal mapping technique and complex potential theory. Some special cases of the results are also obtained, such as the T-type crack, cross crack, and Griffith crack. The effects of geometrical parameters of crack configuration on the stress intensity factors are presented graphically.

Penetration behavior of reactive projectiles impacting TC4 plates

The reactive projectile presents a tremendous potential to induce the combined damage effect of kinetic energy and chemical energy to the plate. A series of ballistic impact experiments and numerical simulation studies were carried out to study the penetration behavior of reactive projectile against TC4 plate. The experimental results showed that with the impact velocity of the reactive projectile increasing from 537m/s to 682m/s, the damage mode of the plate gradually changes from bulge, cross crack to plug. According to the experimental results, the ballistic limit velocity of reactive projectile perpendicular penetrating 5mm TC4 is 648.33m/s. The numerical simulation results showed that the reactive projectile reaches GPa high pressure within a few microseconds when it impacted with the plate. During reactive projectile penetrating TC4 plate, the chemical energy release significantly improves the projectile penetrating ability, and the contribution of chemical energy to projectile penetrating can reach 55.07%.

An improved SPH method for simulating crack propagation and coalescence in rocks with pre-existing cracks

An improved smoothed particle hydrodynamics (SPH) method is proposed to simulate the failure process of rock samples with pre-existing cracks under compression loads. By improving the kernel function of SPH, the brittle fracture characteristics of rock materials are captured. In the improved SPH, a prefabricated node segment (PNS) method is used to generate arbitrarily complex open or closed cracks, avoiding the kernel truncation caused by digging out particles. In order to improve numerical stability, the tension instability control (TIC) technique is embedded in the momentum equation. Moreover, the improved SPH adopts the coupled dynamic buffer solid boundary treatment algorithm, which reduces the configuration of boundary virtual particles, makes the application of boundary conditions more efficient. The accuracy of the improved SPH method and the feasibility of TIC technique are verified by a benchmark example. Then, the effect of the center dislocation spacing of the pre-existing cross crack on the failure mode and compressive strength of rock samples is studied. In addition, the effects of confining pressure on the coalescence type and penetration failure mode of cracks in a rock sample with two pre-existing cracks are also investigated. The numerical results agree well with the experimental results, which verifies the accuracy and validity of the improved SPH method.

Investigation into a comprehensive forming limit curve considering multi-parameters effects for semi-cured FMLs through uniform pressure loading test

Recently, hydroforming has paid dividends in composite forming due to its uniform pressure distribution, which helps in reducing the delamination defects. Considering the need to develop an appropriate criterion for evaluating the formability of FMLs, inspired by sheet metal forming and testing, the uniform pressure loading test was adopted to characterize the formability of semi-cured FMLs by forming limit curve ( FLC). In this paper, both theoretical analysis models considering and ignoring the tangential cohesive effect were established, and the stress expression of the critical zone was given. The results show that the tangential cohesive stress demonstrates a non-negligible influence on the FLC. A new form of comprehensive FLC was given by defining global equivalent strain and global strain ratio and analyzed by the finite element method. In addition, the numerical simulation model and the experimental parameters were verified. A simplified Hashin criterion was proposed and developed into the numerical simulation as a subroutine based on theoretical analysis. Parameters such as blank holder pressure, pressure rate, and temperature were considered both in experiments and simulation. Only fiber stretching damage was distinguished in the cross, single, and arc crack but no wrinkling. As predicted theoretically, a lower forming limit is expected when FMLs are subjected to greater tangential cohesive stress. Finally, with the help of numerical simulation, the influence of different boundary conditions on the comprehensive FLC was studied. Instead of a simple evaluation of FMLs, this comprehensive FLC could be used directly to optimize hydroforming processes in the engineering field.

Toughening mechanisms of Al2O3-matrix composites with SiC whiskers

Whisker reinforced ceramic tool is currently used in the field of high-speed machining of difficult-to-machine materials due to its excellent high temperature mechanical properties. However, the toughening contribution values of crack deflection, whisker bridging and whisker pull-out effect in the whisker toughening model still depend on experimental measurement, which are time-consuming and laborious. In this paper, the scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS) are used to observe the number of main toughening models on the whole crack under different loads (98 N, 196 N and 294 N). The weights of crack deflection, whisker bridging and whisker pull-out effect toughening mechanisms are calculated by analytic hierarchy process (AHP), entropy method and synthetic method. The model parameters are optimized. According to the relation between the areas of whisker cross section and crack cross section, the spacing size between the nearest neighbor whisker centers of crack deflection model is calculated. The variation law of the toughening contribution value of whisker bridging and whisker pull-out effect with the change of angles between crack propagation path and whisker axis (ACW) is calculated by simulation. The ACWs with the highest contribution of whisker toughening mechanism to fracture toughness are obtained. Finally, the toughening contribution values of the three toughening models are determined. It provides a reference for the prediction of fracture toughness of whisker toughened ceramic tools.

Experimental study on uniaxial compression after creep characteristics of similar soft rock containing cross cracks

In order to study the creep phenomenon and failure law of rock materials containing cracks, uniaxial compression test and uniaxial compression test after creep are carried out on similar soft rock samples containing different angles of cross cracks, and the differences of rock mechanical properties after two tests are compared and analyzed. Results show that creep effect increases the compactness of the internal structure of the sample, the elastic modulus improve, enhance the ability of resistance to elastic deformation, the peak strength increases, and 15° cross crack sample is more seriously damaged, 30°, 45° and 60° cross crack samples generate deeper and longer crack propagation and broken degree is lower, 75° and cross sample produce fewer cracks.

Analysis of the Fatigue Failure of Rail Steel

The correlation between the mechanical properties of a material in microvolumes and the material under a load has been studied. A fractographic analysis is performed for the surface of fatigue failure of a railway rail with an internal cross crack. The correlation of the fractographic features with the material structure is discussed. A sample with an internal cross crack in the rail head had been withdrawn from service after severe cyclic loading in a turnout switch. For the metallographic study, the sample was subjected to a three-point bending. Three regions with different characters of the material fracture have been revealed at the surface of the fracture.

Evaluating the Stress and Vibration Conditions of the Rotor of Steam Turbine with the Defect

Consideration was given to the vibrations of the two-bearing weighty rotor of the steam turbine with the acquired defect in the form of cross crack caused by the working fluid temperature field. Rotor vibration equations were derived during the three-D simulation using the method of finite elements taking thus into consideration the temperature field in the form of additional volume forces. The equations are supplemented by a special mass matrix whose introduction allows us to take into consideration a change in centrifugal forces in a linear statement caused by the offsets and retain their radial directionality. The alternating two-dimensional temperature field is defined by the solution of the nonstationary problem of heat conductivity. The stressed state, the coefficients of stress intensity along the crack front and the crack edge contact conditions were defined for different rotor rotation phases. The stress intensity coefficient values calculated using asymptotic formulas for the rotor with the 50 % cross-section crack depth are much lower than critical and present no danger for the quasistatic state. However, these values can increase significantly in the case of the resonance state. The rotor vibration state was studied for different rotation frequencies in the presence and absence of temperature field. A maximum effect of the influence of nonstationary temperature field in the rotor with the cross crack on its vibrations is manifested at a maximum radial temperature difference. The temperature field reduces considerably maximum inflection values and peak-to-peak values. The superresonance 2/1 that occurred in the rotor in the absence of temperature field disappears and the vibrations approach a monoharmonic type and it is explained by the constancy of the contact area of crack edges during the rotor rotation.

Damage characterization of belt conveyor made of the 330Nb alloy after service in a carburizing atmosphere in a continuous heat treatment furnace

In this paper, the failure of the belt conveyor made of 330 Nb heat-resistant steel, working in a furnace for continuous heat treatment in a carburizing and oxidizing atmosphere is investigated. On the basis of microscopic tests and X-ray phase analysis, changes in the microstructure in the cross-section of the conveyor wire after 731, 1642 and 2138 cycles carburizing and oxidizing in the temperature range 30–850 °C in 160 min were determined. Moreover, tensile tests were carried out to investigate the effect of the microstructure on the mechanical properties of heat-resistant steel. The failure was due to a cross crack of wires of the conveyor belt mesh. The results show that the on the surface of the alloy, Cr2O3 chromium oxides and FeCr2O4, Mn1.5Cr1.5O4 oxides are formed. On the border of the alloy and chromium oxides, the airtight SiO2 layer is created. As a result of the actions of external forces, transverse and longitudinal cracks are formed in the oxide layer. These cracks are the ways of carbon and oxygen diffusion. In addition, in the oxidation and carburization process, degradation of the SiO2 layer is promoted by the presence of Na, which is applied in washing agents used for cleaning products before placing them on the conveyor. Microstructure of the unused wire of the conveyor belt consisted of G and H phases and NbC in austenitic matrix. Under the operating conditions of the conveyor belt, phase H dissolves in the matrix with the simultaneous increase of the G phase, and also the G phase is formed around the NbC carbides. High temperature carburizing leads to the precipitation of massive carbides M23C6 with simultaneous significant depletion of chromium into an austenitic matrix to about 13%at. Quantitative microstructural analysis showed that the there was an increase in the size and participation, of hard phases M23C6, G, NbC in the alloy microstructure. The volume fraction of precipitation increased from 3 to 24%vol. The resulting structure is characterized by low strength. The ductility of the alloy has decreased significantly.

Failure Behavior for Rocklike Material with Cross Crack under Biaxial Compression

AbstractThis paper presents an experimental study in which molded gypsum specimens with different crack geometries (T-shaped and X-shaped cross crack) were tested in biaxial compression. Crack prop...

The Anti-Plane Shear Problem of Two Symmetric Cracks Originating from an Elliptical Hole in 1D Hexagonal Piezoelectric QCs

Using the complex variable function method and the technique of conformal mapping, the fracture mechanics of two symmetric collinear cracks originating from an elliptical hole in a one-dimensional (1D) hexagonal piezoelectric quasicrystals (QCs) are investigated under anti-plane shear loading and electric loading. The crack is assumed to be either electrical impermeable or permeable. The exact solutions in closed-form of the stress intensity factors (SIFs) of the phonon field and the phason field, and the electric displacement intensity factors (EDIFs) are obtained. In the limiting cases, the new results such as Griffith crack, a circular hole with equal two edge cracks and cross crack can be obtained from the present solutions. In the absence of the phason field, the obtainable results in this paper match with the classical results.

Non-Destructive Testing of Rods Using a Potential Component of a Magnetic Field

An essentially new method for non-destructive testing of elastic electrically conductive rods using non-vortex electromagnetic induction is proved theoretically. An experimental technique for defining a location of a cross crack is offered.

Fracture properties of a cylindrical superconductor with a central cross crack

In this work, the central cross crack problem is investigated for a long cylindrical high-temperature superconductor under electromagnetic forces. The distributions of both the current density and the magnetic flux density in the cylinder are obtained analytically in either the Bean or the Kim critical state model for both the zero-field cooling and the field cooling magnetization processes. Based on the finite element method, lots of the stress intensity factors at the crack tips for decreasing magnetic fields are numerically calculated. Numerical results obtained show that the field cooling activation process has more significant influence on the stress intensity factors than the zero-field cooling activation process, and that for every activation process, as the applied field decreases, the superconducting cylinder is most dangerous when the currents in the crack region are just be influenced. The safety of the superconducting cylinder with a cross crack depends on not only the magnetic process but also the crack length. Additionally, the introduced dimensionless parameter p in the Kim model has different effects on the stress intensity factors obtained from the most dangerous field and from the trapped field. The present study should be helpful to the design and application of high-temperature superconductors.

A Crack Segmentation Approach Using the Combination of Gray Thresholds and Fractal Feature

A segmentation method of combining gray-level threshold and fractal feature for crack images is proposed, and the fractal law for the perimeter and area of the target is introduced as the constraint condition for the image segmentation of crack. At first, Otsu algorithm is used for the initial segmentation of the crack image, and then the edge of crack is optimized in accordance with fractal law. At last, boundary of crack is determined, and the final result of the image segmentation is obtained. This method makes full use of the fractal geometry law and image information, to effectively solve the problems such as crack contour detection, regional connection and cross crack identification. Several typical examples are analyzed, and the results show that this method has a good segmentation effect on crack images, and it can also be used to identify the other images which have fractal feature.

The Finite Element Analysis of CLAC Beam

In this paper, a comprehensive numerical investigation is presented on the behaviour of ceramsite light aggregate concrete(CLAC) beam by the non-linear finite element method. The investigation includes the entire process of the beam from loading to failure. Numerical analysis results show that the flexural behavior of CLAC beam is obtained by numerical simulation,including load-deflection curve, cross section stress and crack development ect. Numerical simulation may replace the actual experiment for study on flexural behavior of CLAC beam.

Exact analytic solutions for an elliptic hole with asymmetric collinear cracks in a one-dimensional hexagonal quasi-crystal

Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric collinear cracks in a one-dimensional hexagonal quasi-crystal is solved, and the exact analytic solutions of the stress intensity factors (SIFs) for mode III problem are obtained. Under the limiting conditions, the present results reduce to the Griffith crack and many new results obtained as well, such as the circular hole with asymmetric collinear cracks, the elliptic hole with a straight crack, the mode T crack, the cross crack and so on. As far as the phonon field is concerned, these results, which play an important role in many practical and theoretical applications, are shown to be in good agreement with the classical results.

Analytic solutions to problem of elliptic hole with two straight cracks in one-dimensional hexagonal quasicrystals

By means of the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with two straight cracks in one-dimensional hexagonal quasicrystals is investigated. The solution of the stress intensity factor (SIF) for mode III problem has been found. Under the condition of limitation, both the known results and the SIF solution at the crack tip of a circular hole with two straight cracks and cross crack in one-dimensional hexagonal quasicrystals can be obtained.

An investigation on the erosion resistance of rare earths nitrocarburized 40 Cr steel

The effect of rare earths (RE) on erosion resistance of nitrocarburized 40 Cr steel is investigated. Results indicate that a significant improvement occurs in erosion resistance of nitrocarburized 40 Cr steel by introducing RE in nitrocarburizing processes comparing conventional nitrocarburizing processes. Measurement of mechanical properties exhibits that both hardness and toughness of nitrocarburized 40 Cr steel increase in the case using RE nitrocarburizing processes comparing the conventional. Metallurgical microscopy reveals that there is an improvement in the nitrocarburizing layer microstructure attributed to an introduction of RE in nitrocarburizing, which results in an improvement in erosion resistance and mechanical properties. Surface morphology observation of tested samples reveals that predominantly furrow-like peeling from plastic deformation are observed in RE nitrocarburizied 40 Cr steel, while the furrow-like peeling with initial cross crack and large grinding peelings takes place in conventionally nitrocarburized samples.

Response Prediction of RC Beams Externally Bonded with Steel-Reinforced Polymers

This paper deals with an innovative technique for strengthening reinforced concrete (RC) structures using steel-reinforced polymer (SRP) materials. The results of an experimental campaign using RC beams strengthened in flexure with carbon fiber-reinforced polymer or SRP laminates are summarized, and the experimental outcomes are compared to the predictions provided by analytical models and code formulations in terms of flexural strength, curvature of the cross section, deflections, and crack widths. Under ultimate conditions, the ACI 440.2R-02 approach provided conservative flexural strength, and a modified expression for the bond coefficient km was proposed. Under serviceability conditions, good agreement was obtained between experimental results and a theoretical model developed by the writers. Comparisons of code models in terms of both crack width and deflections highlighted the need for a calibration of code formulas to account for effects due to externally bonded reinforcement.

On The Possibilities Of Verifying The CrossCracking Mechanism In The Con-cast Steel Slab

This contribution (which follows the previous articles [1,2]) tries to verify the hypothesis on the initiation and propagation of the cross crack by means of behaviour in tensile tests with the con-cast slab of the same basic chemical composition in the range of temperature moving from 25°C to 1450°C. The tensile tests were made on 18 test bars at suitably chosen temperature differences and gradually led to the tension ruptures of the bars under exactly defined conditions. Simultaneously, the roughness of the resultant rupture surface depending upon the temperature was studied in detail. Thus relations between the roughness of the rupture surface, tension force, work to rupture and contraction with the test bars at the given range of temperature were determined. On the basis of the comparison of the rupture surfaces of cross cracks found in the study [2] and the rupture surfaces obtained at the given interval of test temperatures in the course of this study, properties of material of the slab affected by the cross crack and its most probable behaviour during solidification and cooling were then evaluated by analogy. In this way it was possible to specify and verify the mechanism of cross crack initiation and propagation and to assess the temperature interval, within which the initiation was the most probable.