Principal Tensile Stress Research Articles

On modeling fracture of ferritic steels due to hydrogen embrittlement

Experimental studies in the literature show that the fracture of ferritic steels in the presence of hydrogen occurs primarily perpendicular to the direction of the maximum principal tensile stress, is transgranular in nature, and has fracture surfaces which show “quasi-cleavage” features. Such features are cleavage-like but not along any known cleavage plane or grain boundaries. While the precise micromechanisms which lead to a transition from regular plastic deformation of the steel to “quasi-brittle” fracture are at present unclear, in this paper we have formulated a continuum theory for the diffusion of hydrogen coupled with the elastic-viscoplastic response of metals, together with a simple model which leads to quasi-brittle fracture in the presence of hydrogen. We assume that when an internal variable φ of the theory reaches a critical value φcr, then the inelastic deformation transitions from (a) a volume-preserving plastic stretching which occurs in the direction of the stress deviator — as in standard Mises-type plasticity, to (b) a different form of inelastic stretching which occurs in the direction of the maximum principal stress, and which is dilational in nature. We call this latter form of inelastic deformation craze inelasticity. We view the strain produced due to crazing as a “quasi-brittle” form of inelastic deformation. We allow only a small amount of inelastic craze-strain before the process of craze-breakdown and fracture sets in. To model craze-breakdown we introduce a damage variable d∈[0,1]; if d=0 at a point then that point is intact, while if d=1 at some point then that point is fractured. With the aim of “regularizing” the strain-softening behavior during craze-breakdown, and to avoid mesh-dependency related issues during finite element simulations, we develop a damage theory which depends not only on d but also its gradient ∇d, which is considered to be a measure of the spatial inhomogeneity of the damage during the craze-breakdown process. We have numerically implemented our coupled diffusion-deformation-damage theory in a finite element program, and we present representative numerical examples which show the ability of the simulation capability to qualitatively replicate the process of fracture due to hydrogen embrittlement in some technically relevant geometries.

Crevasses as Indicators of Surge Dynamics in the Bering Bagley Glacier System, Alaska: Numerical Experiments and Comparison to Image Data Analysis

One of the largest sources of uncertainty in sea level rise prediction is glacial acceleration, of which the surge phenomenon is the least understood type. The surge of the Bering Bagley Glacier System (BBGS), Alaska, in 2011–2013 has provided a rare opportunity to study the surge phenomenon in a large and complex glacier system. A surge results in widespread crevassing throughout the glacier system complicating many traditional techniques used to study glacier dynamics. In this paper, we utilize crevassing as a means to investigate the recent BBGS surge through numerical modeling and geostatistical data analysis. Following the principles of structural glaciology, image‐based crevasse characterizations are obtained through geostatistical methods applied to Landsat‐7 data, supplemented by airborne field observations. On the modeling side, a 3‐D full‐Stokes finite element model of the BBGS is developed and applied to investigate ice dynamics and surface structures during the recent surge. A von Mises criterion is adopted to simulate crevassing at the glacier surface, oriented along the axes of maximum principal tensile stress. To facilitate evaluation of model‐ and data‐derived crevasse characteristics, three different comparison methods are introduced. General agreement in the model‐data comparisons indicates that the model has the ability to represent the BBGS system during peak acceleration. The crevasse‐based approach is also employed to optimize the basal sliding parameter and the von Mises stress threshold in the model. Results further indicate that bed topography is an important constraint in modeling the surge process.

Effects of Crack Surface Diffusion on Central Crack Evolution in Active Particle of Li-Ion Batteries

The active particle fracture is one of the major reasons for capacity fading in lithium ion batteries (LIBs). In this study, a model that couples Li-ion diffusion, stress evolution, and extended finite element method (X-FEM) is proposed to investigate the effects of central crack surface diffusion (CSD) in active particle. The different initial central crack length is adopted to simulate the crack evolution. The simulation results show the fracturing process occurs according to the following stages: no crack growth, stable crack growth, and unstable crack growth. When the influence of CSD is considered, the maximum principal tensile stress near the crack tip decreases and fracture of active particle becomes more difficult.

Behaviour of recycled aggregate concrete beam-column connections in presence of PET fibers at the joint region

In this paper the behavior of reinforced concrete (RC) beam-column connections under cyclic loading was analyzed. The specimens, manufactured in a reduced-scale were made of (a) recycled aggregate concrete (RAC) by replacing 30% of natural coarse aggregate (NCA) with recycled coarse aggregate (RCA) and (b) RAC incorporating Polyethylene terephthalate (PET) fiber i.e., PET fiber-reinforced concrete (PFRC) at the joint region. PET fiber (aspect ratio=25) of 0.5% by weight of concrete used in the PFRC mix was obtained by hand cutting of post-consumer PET bottles. A reference specimen was also prepared using 100% of NCA and subjected to similar loading sequence. Comparing the results the structural behavior under cyclic loading of RAC specimens are quite similar to the reference specimens. Damage tolerance, load resisting capacity, stiffness degradation, ductility, and energy dissipation of the RAC specimens enhanced due to addition of PET fibers at the joint region. PFRC specimens also presented a lower damage indices and higher principal tensile stresses as compared to the RAC specimens. The results obtained gave experimental evidence on the feasibility of RAC for structural use. Using PET fibers as a discrete reinforcement is recommended for improving the seismic performance of RAC specimens.

The effect of adhesive failure and defects on the stress distribution in all-ceramic crowns

ObjectivesTo explore the effect of adhesive failure and defects between the crown and cement on the stress distribution within all-ceramic crowns and the corresponding risk of failure. MethodsAn IPS e.max crown of lithium disilicate produced by CAD/CAM for a first mandibular molar was modeled using finite element analysis based on X-ray micro-CT scanned images. Predefined debonding states and interfacial defects between the crown and cement were simulated using the model. The first principal stress distribution of the crown and cement was analyzed under a vertical occlusal load of 600 N. A concept of failure risk was proposed to evaluate the crown. ResultsStress concentrations in the crown were identified on the occlusal surface surrounding the region of loading, beneath the area of loading and at the margin of the interior surface. Stress concentrations in the cement were also evident at the boundary of the debonded areas. The lower surface of the crown is safe to sustain the 600 N vertical load, but the top surface of the cement would undergo cohesive failure. According to the evaluation of failure risk of the crown, the conditions of highest risk corresponded to the conditions with highest percentage of cement damage. The risk of failure is not only associated with debonding between the crown and cement, but also associated with its distribution. ConclusionsDebonding related defects and cementing defects are more deleterious to the interfacial stress than debonding itself. The axial wall plays a critical role in maintaining the principal tensile stress of the crown at an acceptable level.

Shear resistance of steel fiber-reinforced concrete beams without conventional shear reinforcement on the basis of the critical shear band concept

Recently, a concept to estimate the shear capacity of slender reinforced concrete beams without stirrups by examining the principal tensile stress within the shear span was introduced. Considering the existence of both the concrete normal stress and the shear stress in the tension zone, the calculated principal tensile stress is in excess of the concrete tensile strength in a narrow band in this concept. In this band, damage in form of micro-cracking is assumed. The critical shear crack is formed by coalescence of micro-cracks when the damaged band reaches a critical size. By limiting the tensile cracking in the shear band, steel fibers can postpone the critical shear crack development and, thus, enhance the shear resistance in this concept. This paper presents an extension of this method to estimate the shear capacity of slender steel fiber-reinforced concrete beams without conventional shear reinforcement. The proposed method is shown to properly estimate the shear resistance of a large set of available test data, being able to account for most influencing parameters.

Numerical computation of central crack growth in an active particle of electrodes influenced by multiple factors

Mechanical degradation, especially fractures in active particles in an electrode, is a major reason why the capacity of lithium-ion batteries fades. This paper proposes a model that couples Li-ion diffusion, stress evolution, and damage mechanics to simulate the growth of central cracks in cathode particles $$(\hbox {LiMn}_{2}\hbox {O}_{4})$$ by an extended finite element method by considering the influence of multiple factors. The simulation shows that particles are likely to crack at a high discharge rate, when the particle radius is large, or when the initial central crack is longer. It also shows that the maximum principal tensile stress decreases and cracking becomes more difficult when the influence of crack surface diffusion is considered. The fracturing process occurs according to the following stages: no crack growth, stable crack growth, and unstable crack growth. Changing the charge/discharge strategy before unstable crack growth sets in is beneficial to prevent further capacity fading during electrochemical cycling.

Analysis on stress state of box-girder web under prestressing effect

In order to study the effect of prestressed box girder webs stresss state, determining the stress distribution within a web, research and analysis of vertical prestressed box girder, curved beam prestressed sensitivity under the web. Establishing the finite element model of the box girder web vertical prestressing effect is analyzed, results show that the principal tensile stress of the web is sensitive to the vertical prestress, applying the vertical prestress can effectively reduce the principal tensile stress of the web; with the decrease of the effective vertical prestress, the neutral axis above the principal compressive stress decreases rapidly, while below the neutral axis decreases relatively slow; Under the same vertical preloading stress level, the roots of cross section of the compressive stress of web reserves than L / 4 section of the web. Calculation and analysis of curved beam under bending point, different bending angles and bending radius of principal stress effect on the web, Results show that the set of curved beam web when the curved beam bending stress concentration easily, appear the main tensile stress; Increase the bending radius can effectively reduce beam cross-section of web principal tensile stress, along with the rising of the next corner, principal tensile stress peak value increases gradually, thus setting bending beam, should try to reduce the bending angle.

Shock Response Analysis of a Large LNG Storage Tank Under Blast Loads

Ensuring the safe operation of large LNG storage tank projects under blast loads is very important. First, using LS-DYNA software, numerical simulations of air blast wave propagation are studied with the ALE algorithm and fluid-solid coupling theory. A new corrected formula for the peak overpressure is proposed based on the numerical results, and its accuracy is verified by comparing the numerical results with the empirical formula and test results. Then, a finite element model of a large LNG storage tank is built, assuming that the explosion source is located 10 m away from both the ground and the tank wall. This model is used to analyse the overpressure and dynamic response of the outer concrete tank and to predict the TNT equivalent quantity that the tank can withstand. The results show that the tank wall facing the burst loads is in a more dangerous state and that the principal tensile stress increases approximately linearly with increasing TNT equivalent quantity. When the TNT equivalent quantity reaches 2.5 t, concrete element failure occurs on the centre facing the explosions, and the outer tank structure is damaged. The conclusions can provide a reference for the blast-resistant design of large LNG storage tanks.

Damage evolution and ductile fracture prediction during tube spinning of titanium alloy

In order to analyze and control surface crack during tube spinning of titanium alloy, six ductile fracture criteria (DFCs) were incorporated into the finite element software (ABAQUS) to simulate the damage evolution in the tube spinning process under various thinning rates. For improving the prediction accuracy of damage evolution during tube spinning, the valid damage strain was proposed to eliminate or diminish the strain fluctuation caused by the ALE adaptive meshing technique. Through analyzing the influence of the equivalent plastic strain, stress state and the principal stresses on the damage accumulation, the mechanisms of surface cracks were revealed. The results show that only the McClintock model predicted that the crack in tube spinning of Ti–15–3 alloy could be eliminated under moderate thinning rates, while lower or higher thinning rates may induce cracking on the tube surface, which was consistent with the experimental results. For the outer layer, as the thinning rate increased, the increasing equivalent plastic strain and tensile principal stresses contributed to the damage accumulation for all the DFCs studied in present research. For the inner layer under moderate thinning rates, the compressive axial and tangential stresses slowed down the damage accumulation, leading to the occurrence of the safe zone of thinning rate (∼20–30%) without surface crack during tube spinning of Ti–15–3 alloy.

Assessing seismic response of a 2D roller-compacted concrete dam under variable reservoir lengths

Finite element method (FEM) is effectively used for evaluating roller-compacted concrete (RCC) dams, especially in high seismicity zones. The aim of this study is to investigate the effect of various reservoir lengths on the seismic response of a selected RCC dam under strong ground motion effects. Six different reservoir lengths, the lengths varies from h/2 to 10h (h, the height of dam), are used within finite element models. The hydrodynamic pressure of the reservoir water is modelled with the 2D fluid finite elements based on the Lagrangian approach. The horizontal and vertical components of the 1989 Mw 6.9 Loma Prieta earthquake are utilized in numerical analyses. The non-linear time-history analyses of those six models are investigated by using Drucker-Prager material model. According to the numerical analysis, it is obvious that the seismic behavior of the RCC dams is considerably dependent on the reservoir length. By increasing the length, we have obtained higher displacement values, which exist until the reservoir length reaches the 3h; at increased lengths, the values remain stable. This result proposes that 3h reservoir length is adequate to assess the seismic response of RCC dams. The principal tensile stresses are relatively lower in non-linear analysis compared to linear analyses. However, the principal compression stresses are close to each other in linear and non-linear analyses. The results imply that the non-linear response is influential on the total seismic response of a dam, which cannot be neglected in numerical analysis.

Stress analysis of irradiated human tooth enamel using finite element methods

The objectives of this project were to use finite element methods to determine how changes in the elastic modulus due to oral cancer therapeutic radiation alter the distribution of mechanical stresses in teeth and to determine if observed failures in irradiated teeth correlate with changes in mechanical stresses. A thin slice section finite element (FE) model was constructed from micro CT sections of a molar tooth using MIMICS and 3-Matic software. This model divides the tooth into three enamel regions, the dentin-enamel junction (DEJ) and dentin. The enamel elastic modulus was determined in each region using nano indentation for three experimental groups namely – control (non-radiated), in vitro irradiated (simulated radiotherapy following tooth extraction) and in vivo irradiated (extracted subsequent to oral cancer patient radiotherapy) teeth. Physiological loads were applied to the tooth models at the buccal and lingual cusp regions for all three groups (control, in vitro and in vivo). The principal tensile stress and the maximum shear stress were used to compare the results from different groups since it has been observed in previous studies that delamination of enamel from the underlying dentin was one of the major reasons for the failure of teeth following therapeutic radiation. From the FE data, we observed an increase in the principal tensile stress within the inner enamel region of in vivo irradiated teeth (9.97 ± 1.32 MPa) as compared to control/non-irradiated teeth (8.44 ± 1.57 MPa). Our model predicts that failure occurs at the inner enamel/DEJ interface due to extremely high tensile and maximum shear stresses in in vivo irradiated teeth which could be a cause of enamel delamination due to radiotherapy.

Multi-objective optimization of an arch dam shape under static loads using an evolutionary game method

ABSTRACTThis article attempts to evaluate the safety and economic performance of an arch dam under the action of static loads. The geometric description of a crown cantilever section and the horizontal arch ring is presented. A three-objective optimization model of arch dam shape is established based on the arch dam volume, maximum principal tensile stress and total strain energy. The evolutionary game method is then applied to obtain the optimal solution. In the evolutionary game technique, a novel and more efficient exploration method of the game players’ strategy space, named the ‘sorting partition method under the threshold limit’, is presented, with the game profit functions constructed according to both competitive and cooperative behaviour. By way of example, three optimization goals have all shown improvements over the initial solutions. In particular, the evolutionary game method has potentially faster convergence. This demonstrates the preliminary proof of principle of the evolutionary game method.

Nucleation from a cluster of inclusions, leading to void coalescense

A cell model analysis is used to study the nucleation and subsequent growth of voids from a non-uniform distribution of inclusions in a ductile material. Nucleation is modeled as either stress controlled or strain controlled. The special clusters considered consist of a number of uniformly spaced inclusions located along a plane perpendicular to the maximum principal tensile stress. A plane strain approximation is used, where the inclusions are parallel cylinders perpendicular to the plane. Clusters with different numbers of inclusions are compared with the nucleation and growth from a single inclusion, such that the total initial volume of the inclusions is the same for the clusters and the single inclusion. After nucleation, local void coalescence inside the clusters is accounted for, since this makes it possible to compare the rate of growth of the single larger void that results from coalescence in the different clusters. Nucleation parameters leading to rather early nucleation, or to later nucleation, are considered. Also, different transverse stresses on the unit cell are considered to see the influence of different levels of stress triaxiality, and results are shown for different levels of strain hardening in the material.

Finite Element Analysis of Screw-Tightening Torque Applied to Custom and Conventional Abutment

The aim of this study was to design an abutment with an esthetic emergence profile contour using CAD technology and compare the stress distribution within the structure between the custom abutment and conventional abutment according to the screw tightening torque using 3D finite element analysis (FEA). The maximum tensile principal stress was found in the endpoint of the screw head and the start point of the screw line with regard to the application of the tightening torque of the screw. A similar pattern was observed in all of the following screws: 10N·cm, 20N·cm, and 30N·cm. The tightening torque of the screw had a significant impact on the changes in the stress of the abutment and screw fixture. This study also found that the condition in which the screw load was applied showed a more realistic description of the behavior of a single fixed dental implant than the condition in which the screw load was not applied. This study examined the optimal tightening torque value of the screw for the denture used in this study at a location slightly higher than 20N·cm. The difference in the custom abutment and conventional abutment did not have a significant impact on the supporting bone with regard to the external load. In regard to the stress occurring in the screw, the custom abutment had a lower degree of stress than the conventional type. Therefore, a screw fracture would occur less frequently in a custom abutment than a conventional abutment.

Axisymmetric torsional fretting contact between a spherical punch and an FGPM coating

• The maximum tensile principal stress increases with the increase of the gradient index. • The maximum Mises stress occurs at the end of slip annulus where the severest state of stress happens. • The FGPM coating can be used to resist the fretting contact damage at smart devices. The axisymmetric torsional fretting contact between a rigid conducting spherical punch and a functionally graded piezoelectric material (FGPM) coating is studied in this paper. The exponential model is used to simulate the inhomogeneous electro-mechanical properties of the FGPMs coating. The conducting spherical punch with a constant surface electric potential is considered in the contact. A normal force and a cyclic torque are applied to the two contact bodies. The applied torque produces an outer annular slip area and an inner stick area. The torsion angle is produced within the inner stick area as a rigid body. With the aid of the Hankel integral transform technique, we can reduce the contact problem to the singular integral equations of the Cauchy type. Then the unknown electro-mechanical fields and stick/slip area can be obtained numerically. The effect of the gradient index on the surface electro-mechanical fields is discussed at loading and unloading phases. The Mises stress and principal stress at the contact surface are also discussed to predict the possible location of fretting damage and failure.

Structural Behavior of Recycled Aggregate Concrete Beam-Column Connection in Presence of Micro Concrete at Joint Region

This paper investigates the effect of addition of micro concrete on the behavior of RC exterior beam-column connections prepared with recycled coarse aggregate (RCA) under the reversed cyclic loading. The results on mechanical strength evaluation of concrete showed an optimum replacement of 30% of natural coarse aggregate (NCA) by RCA. To extend the application of RCA concrete to structural elements, four beam-column specimens with two each of 30% RCA and 100% RCA were cast and the joint region of one specimen of 30% RCA and 100% RCA were cast with a micro concrete. A reference specimen was also prepared using 100% NCA and then subjected to a similar cyclic load. It was observed that, RCA specimens resulted in a brittle mode of failure as compared to NCA specimen. However, the presence of a micro concrete improved the damage tolerance, load resisting capacity and hence the seismic parameters such as stiffness degradation, ductility and energy dissipation increased. Further, the addition of micro concrete in RCA specimens presented a lower damage indices and higher principal tensile stresses, which are comparable to the NCA specimen. Conclusively, strengthening of joint region using micro concrete is found to have a significant contribution in improving the seismic performance of RCA beam-column connections.

Topology Optimization of the Fiber-Reinforcement of No-Tension Masonry Walls

An innovative approach is proposed to define the optimal fiber-reinforcement of in-plane loaded masonry walls, modeled as linear elastic no-tension (NT) bodies. A topology optimization formulation is presented, which aims at distributing a prescribed amount of reinforcement over the wall, so as to minimize the overall elastic energy of the strengthened element. Perfect bonding is assumed at the wall-reinforcement interface. To account for the negligible tensile strength of brickwork, the material is replaced by an equivalent orthotropic material with negligible stiffness along the direction (s) undergoing tensile principal stress (es). Compressive principal stresses in the reinforcement are not allowed. A single constrained optimization problem allows both the equilibrium of the NT body to be enforced, and the optimal reinforcing layout to be spotted out, without any demanding incremental approach. Some preliminary numerical examples are shown to assess the capabilities of the proposed procedure and to identify the optimal reinforcement patterns for common types of masonry walls with openings.

A new methodology for developing flexure-hinged displacement amplifiers with micro-vibration suppression for a giant magnetostrictive micro drive system

Flexure-hinged displacement amplifiers play an important part in ultra-precision engineering due to their advantages of no friction, no backlash, compactness, high resolution and so on. In fact, the flexure-hinged displacement amplifier is initially proposed for addressing the travel shortcoming of piezoelectric or magnetostrictive actuators. However, some new challenges such as micro-vibration isolation and long service life, etc., arouse attentions especially when working in the harsh environment. Therefore, a new methodology for designing amplifier which could handle such issues is proposed and discussed in this paper. The focus of this study is on the design, theory analysis, and optimization of amplifiers, not only with the purpose of providing a larger displacement output, but also achieving micro-vibration isolation in desired frequency band. First, a generalized model for flexure hinges, the largest principal tensile stress and fatigue safety factor are formulated mathematically under combined tension and bending loads. Then the applied loads acting on each flexure hinge is calculated according to pseudo-rigid-body model. The equivalent dynamic model is established by Lagrange method. Whereafter, the procedure of the methodology is summarized and a different optimization strategy is implemented. Finally, a prototype is fabricated and tested. The experiments and simulation results confirm that the amplifier can achieve the desired amplification ratio and passive vibration isolation, which validate the effectiveness of this methodology.

Shear performance of bolted side-plated reinforced concrete beams

Although the bolted side-plating (BSP) technique is feasible to upgrade the shear capacity of reinforced concrete (RC) beams, related studies on its strengthening effect have yet to be sufficient. Therefore, an experimental study of seven specimens was conducted to investigate the shear performance of BSP beams. The failure mode was found shifting from shear–tension to shear–compression after strengthening. The decrease of bolt spacing, the increase of plate breadth and thickness improved the shear capacity, stiffness and ductility effectively. Although the additional stiffeners improved the stiffness considerably, it exhibited no obvious improvement in the shear capacity and caused a serious reduction in ductility. Due to the shear deformation of bolt shaft and the tensile principal stress in the steel plates that was perpendicular to the main diagonal crack, uneven relative longitudinal and transverse slips existed on the interface between the bolted steel plates and the RC beams. An analytical model was developed according to the force equilibrium and deformation compatibility of the beam segment in the shear span. A design procedure, which can be conducted conveniently using ordinary computing software or even a calculator by structural engineers in their strengthening design practise, was also provided to estimate the shear bearing capacity of BSP beams.