Maximum Principal Stress Criterion Research Articles

Rate-dependent augmented finite element method for fast arbitrary crack growth

Experimental observations indicate that fast crack growth often exhibits a rate dependency, which is caused by the viscosity of the material and the dissipation of energy resulting from the branching of microcracks. To address this phenomenon in simulation modeling, this study introduces a rate-dependent augmented finite element method designed for analyzing fast arbitrary crack growth. The method utilizes the maximum principal stress criterion to simulate arbitrary crack growth and predict the path of crack growth. The elements where crack growth is expected are divided into subelements, and they are re-assembled by incorporating rate-dependent cohesive elements between them to construct augmented finite elements. The cohesive elements are based on a rate-dependent trapezoid traction–separation relationship, where the critical traction and fracture toughness are modeled as functions of the separation rate and crack growth speed. The augmented finite elements only contain external nodal degrees of freedom (DoFs), while internal nodal DoFs are condensed. They can be utilized as conventional elements. A numerical algorithm based on the proposed method was developed in this article, and it was implemented in ABAQUS via user element subroutine. The effectiveness of this method and algorithm is validated through an example of dynamic crack growth in a double cantilever beam. The results demonstrate agreement with experimental and computational findings reported in the literature. Additionally, the method was applied to other structures, yielding reasonable outcomes. Analysis of these examples confirms the method’s utility in predicting fast arbitrary crack growth under dynamic load conditions.

Predicting Stress Intensity Factor for Aluminum 6062 T6 Material in L-Shaped Lower Control Arm (LCA) Design Using Extended Finite Element Analysis.

The aim of this study is to solve a practical problem encountered in the automotive industry, especially the failure of a cracked lower control arm made of al 6062 T6 material during static and crash physical tests, and to characterize the behavior of cracked parts made of aluminum materials using the fracture mechanics parameters. As a first step, we carried out a numerical study and simulation using Abaqus/CAE 2020 software and the finite element method to determine the stress concentration and load limit capacity for different car weight cases. The von Mises stress variation shows crack initiation and propagation to be in the area of the lower control arm's attachment to the vehicle platform, where stress is concentrated. These numerical results are consistent with the experimental test results found by automotive manufacturers. Also, we find that the mechanical load that can support this part is below 4900 N for good performance. In the second step, we use the results of the first section to simulate the failure of a lower control arm with a crack defect. This paper investigates the stress intensity factor KI in mode I for different lengths (L) and depths (a) of the crack in the lower control arm using the extended finite element method (XFEM) under Abaqus/CAE. For crack failure initiation and progression, we relied on the traction separation law, specifically the maximum principal stress (MAXPS) criterion. The KI factor was evaluated for the materials steel and Al 6062 T6. The results obtained from the variation of the KI coefficient as a function of crack depth (a) and the thickness (t) show that the crack remains stable even when a depth ratio (a/t = 0.8) is reached for the steel material. However, the crack in the Aluminum 6062 T6 material becomes unstable at depth (a/t = 0.6), with a high risk of total failure of the lower control arm.

Scratch-induced surface formation mechanism in C/SiC composites

Due to the extremely complex composition of C/SiC composites, their material removal mechanisms are significantly different from those of conventional single-phase ceramic materials. Therefore, the grinding removal mechanism of C/SiC composites must be investigated so that the components based on these composites can be machined efficiently and with low damage. In this paper, nano-scratch experiments with various cutting depths are carried out along different fiber orientations to simulate the material removal process in grinding and theoretically clarify the damage failure mechanism based on different cutting thicknesses. According to the assessment of surface and subsurface morphology of the various constituents in the scratch grooves regarding different fiber orientations, the multiple brittle breakage modes and surface integrity of carbon fiber are revealed. Green’s function is applied to establish a half-space anisotropic analytic elastic stress field considering the microstructural characteristics of material, and the crack nucleation and propagation principles during micro brittle fracture regime for nano-scratch experiment are elucidated based on the fracture mechanics and the maximum principal stress criterion. The formation mechanism of machined surface morphology in macro brittle removal process is investigated under the crack deflection effect at the weak interface and based on the elastic foundation beam theory. The related findings can provide a reasonable reference framework for optimizing the grinding technology, material and tool design regarding C/SiC composites.

Research on wing crack propagation of closed crack under uniaxial compression based on peridynamics

To investigate the influence of the interaction between closed crack surfaces on crack propagation, a nonlocal friction contact model is proposed for numerical simulation of wing crack propagation under uniaxial compression. The closed cracks with different inclination angles are defined by mathematical models, and cracks are generated by breaking off bonds at specific positions. In terms of numerical model validation, the comparison with FEM results indicates the correctness of the friction contact numerical model. Subsequently, the maximum principal stress criterion was applied to determine the breakage of the bond. The simulation results showed that under uniaxial compression, the closed crack tips undergo tensile failure, and the wing cracks will gradually expand towards the direction of the maximum principal stress. Both the crack inclination angle α and friction coefficient fc have an impact on crack initiation and propagation, increasing α or fc will reduce the initiation angle θ of crack propagation. At the same inclination angle α, the larger the friction coefficient fc, the shorter the crack propagation length, while the smaller the inclination angle α, the more significant the influence of friction coefficient fc on the initiation angle θ and crack path.

Steady-State Crack Growth in Nanostructured Quasi-Brittle Materials Governed by Second Gradient Elastodynamics

The elastodynamic stress field near a crack tip propagating at a constant speed in isotropic quasi-brittle material was investigated, taking into account the strain gradient and inertia gradient effects. An asymptotic solution for a steady-state Mode-I crack was developed within the simplified strain gradient elasticity by using a representation of the general solution in terms of Lamé potentials in the moving framework. It was shown that the derived solution predicts the nonsingular stress state and smooth opening profile for the growing cracks that can be related to the presence of the fracture process zone in the micro-/nanostructured quasi-brittle materials. Note that similar asymptotic solutions have been derived previously only for Mode-III cracks (under antiplane shear loading). Thus, the aim of this study is to show the possibility of analytical assessments on the elastodynamic crack tip fields for in-plane loading within gradient theories. By using the derived solution, we also performed analysis of the angular distribution of stresses and tractions for the moderate speed of cracks. It was shown that the usage of the maximum principal stress criterion within second gradient elastodynamics allows us to describe a directional stability of Mode-I crack growth and an increase in the dynamic fracture toughness with the crack propagation speed that were observed in the experiments with quasi-brittle materials. Therefore, the possibility of the effective application of regularized solutions of strain gradient elasticity for the refined analysis of dynamic fracture processes in the quasi-brittle materials with phenomenological assessments on the cohesive zone effects is shown.

Residual life prediction of compressor impeller with microcrack damage

Taking compressor impeller with pressure ratio less than 3 as research object, a prediction method of residual life of compressor impeller was proposed. Considering the action of “inertial” load centrifugal force, finite element method (FEM) technology and sub-model technology were used to comprehensively analyze the mechanical quantities, such as blade stress, and the hot spot stress location of compressor impeller was determined. Crack orientation was determined according to maximum principal stress criterion. The linear elastic fracture mechanics (LEFM) based on FEM and Forman-Newman-de Koning (FNK) model were used to investigate the whole process of crack growth, including stable growth and instability growth stages. The crack growth law and residual life of impeller at different rotate speeds were given. The results show that with the increase of rotate speed, the stress intensity factor (SIF) of initial crack front increases in a quadratic power law, the critical length decreases linearly, and the residual life decreases nonlinearly. The crack grows along the height of blade and then fractures after a deflection of about 90 degrees to the outer edge of blade. The obvious deflection of crack is the sign of crack transition from stable growth stage to instability growth stage. The life of crack in the instability growth stage can be neglected compared with that in the stable growth stage. The analysis method proposed in this article can provide a basis for the damage tolerance evaluation of compressor impeller, and can also provide a reference for maintenance work.

Microstructure- and damage-nucleation-based crystal plasticity finite element modeling for the nucleation of multi-type voids during plastic deformation of Al alloys

During deformation of aluminum (Al) alloys, there are three kinds of voids being formed by distinct mechanisms, i.e., matrix-cracking voids (MCVs), particle-cracking voids (PCVs), and interface-debonding voids (IDVs), due to mesoscale heterogeneous deformation. The three kinds of void nucleation have different effects on the subsequent void growth, void coalescence, and ductile fracture of Al alloys. To determine the complex nucleation behavior of the three kinds of voids under mesoscale heterogeneous deformation, a microstructure- and damage-nucleation-based crystal plasticity finite element (MDNCPFE) model considering different nucleation mechanisms was established. To realize the MDN-CPFE modeling, the deformation and void nucleation of the matrix, particle, and their interface in Al alloys were described by a non-local dislocation density CP constitutive model and a local stored energy density criterion, an elastic constitutive model and a maximum principal stress criterion, and a cohesive zone model and fracture energy criterion, respectively. The developed MDN-CPFE model was validated by the nucleation locations and quantitative variations of the three kinds of voids in an in-situ tensile test. Using the developed model, the nucleation characteristics of the three kinds of voids during the small deformation of Al alloys were analyzed. It is found that PCVs are likely to nucleate within particles that are located at the deformation bands and with greater shape irregularity; IDVs are prone to nucleate at the interfaces around smaller particles in the deformation bands; MCVs tend to nucleate within grains with a higher Schmid factor and aspect ratio. The void area fractions (VAFs) of the three kinds of voids all evolve with an incubation stage and then followed by an exponentially increasing stage during deformation. The duration of the incubation stage for the three kinds of voids, designated by the nucleation strain, presents the following sequence: PCV < IDV < MCV; while, their VAFs and quantities present the opposite sequence. Additionally, the effects of the particle volume fraction and grain size on the nucleation strain, total VAF, and proportion of VAFs of the three kinds of voids were unraveled.

Development and Constitutive Model of Fluid–Solid Coupling Similar Materials

The Cretaceous Zhidan group (K1zh) pore fissure-confined water aquifer in Yingpanhao Coal Mine, Ordos City, China, has loose stratum structure, high porosity, strong permeability and water conductivity. In order to explore the fluid–solid coupling similar material and its constitutive model suitable for the aquifer, a kind of fluid–solid coupling similar material with low strength, strong permeability and no disintegration in water was developed by using 5~20 mm stone as aggregate and P.O32.5 Portland cement as binder. The controllable range of uniaxial compressive strength is 0.394~0.528 MPa, and the controllable range of elastic modulus is 342.22~400.24 MPa. The stress–strain curve and elastic modulus of similar materials are analyzed. It is found that the elastic modulus of similar materials with different water–cement ratios conforms to the linear law, the elastic modulus of similar materials with the same water–cement ratio after soaking treatment and without soaking treatment also conforms to the linear law. Based on the material failure obeying the maximum principal stress criterion and Weibull distribution, combined with the elastic modulus fitting formula, a constitutive model suitable for the fluid–solid coupling similar material was established, and the parameters of the constitutive model were determined by differential method. By comparing the theoretical stress–strain curve with the experimental curve, it is found that the constitutive model can better describe and characterize the fluid–solid coupling similar materials with different water–cement ratios and before and after soaking.

Creep analysis of a main steam pipe system

ABSTRACT The present work is performed on the main steam pipe system in Heleneholmsverket, a CHP in Sweden and consists of the following parts (i) numerical analysis of the in-service creep behaviour of the pipe system, (ii) creep testing of new and service exposed materials from welded components, (iii) characterisation of the creep damage distribution in creep tested welds of the actual piping. The entire system has been modelled for creep evaluation to make it possible to compare the simulated creep stress and strain distributions in selected welds with observed amounts of creep cavitation, which can be correlated to the accumulated creep strain. Creep data for the analyses were produced by creep testing of service exposed base and weld metals from a pipe weld and a T-piece branch weld from the system. In addition, the creep tested welds were studied metallographically to map the creep damage and make it possible to compare the damage development with the resulting creep stress and strain distributions in the weld. In the previous project also a T-piece branch weld was investigated in a similar way and those results were used for verification of the re-analyses in present project with the updated system model. The following results were achieved: The model of the entire steam pipe system was created in Abaqus and the strain distributions were verified in comparison to a corresponding elastic Caepipe model. The Norton creep law was used for the simulations. In addition, also primary creep was analysed. The effects of primary creep on the long-term creep behaviour was significant and the results shows the importance of including primary creep into the model. There is no effect of starts and stops on the stress and strain distributions in the system during creep. The system analysis results showed enhanced strains up to 2.1% at one bend and 0.5–1.0% in some parts of the system. Although replica testing had not been conducted directly at the bend the high strains indirectly agreed with the observations of small creep cracks had been observed in replica testing of in a weld at one of the ends of the actual bend. Furthermore, several components in the system have been exchanged due to creep crack formation. Moderate levels creep damage was observed in the pipe weld. The analysis of this pipe weld gave somewhat lower creep strains than expected. The stress and strain distributions matched with the maximum principal stress criterion but not with the von Mises stress that Abaqus uses for creep analyses by default. The analysis of the branch weld matched well with observed creep damage distributions whereas the maximum strain level of 0.4% appears to be rather low in comparison to the quite extensive creep damage. However, local constraint and multiaxiality in welds lead to significantly lower creep ductility compared to uniaxial creep and contribute to a reasonable agreement between the strain and the damage levels. The creep tests of service exposed material resulted in relatively high Norton creep law exponents and no shift to lower values at the lowest tested stresses. It is hardly possible to perform tests at even lower stresses and therefore the simulations at service conditions resulted in unreasonably low creep strains.

Substrate geometry effect on the strength of repaired plates: Combined XFEM and CZM approach

Aluminum alloys are commonly used in aeronautical applications because of their specific strength and improved corrosion resistance. These structures, during their service, are exposed to various loading cycles, eventually leading to failure at the loci of geometric discontinuities. Repairing by metal or composite patch bonding is widespread to extend the structures’ life by limiting stress concentrations and delaying crack initiation. This work consists of a numerical study, validated by experimental test data, to assess the effect of a central circular notch in an aluminum plate, either reinforced or not by an adhesively bonded composite patch, on the global tensile response of the structure. The constitutive law of the aluminum and adhesive is assumed nonlinear and follows Von Mises equivalent stress flow theory with a hardening variable in incremental form. Damage initiation in the aluminum alloy is modeled by the XFEM (eXtended Finite Element Method), using the maximum principal stress criterion (MAXPS) for damage initiation prediction. Damage evolution is based on the energy approach. The adhesive layer was modeled by CZM (Cohesive Zone Model). A good agreement was found between the experimental results of the tensile curves of the repaired and unrepaired plates with those resulting from numerical modeling. Once the numerical model was validated, several parameters werenumerically studied, namely the shape of the composite patch, the size of the notch, the nature of the adhesive and repair mode by single and double patch, to reduce maximum stress of the damaged area and provide maximum repair efficiency.

Strength of asymmetric tapered composite foam core sandwich structures subjected to thermomechanical loading

An experimental test program and companion analytical study were designed and conducted to gain understanding of the tensile failure mechanisms in asymmetrical tapered sandwich core structures at cold temperatures. Representative test coupons were subjected to a tensile load in an environmental chamber to induce foam tensile failure in the core. Finite-element modeling with a proposed maximum principal stress criterion was used to predict the failure loads of the tapered test coupon design at the test temperatures. The predicted failure loads were in good agreement with test results. An important finding is that the cure stresses in the foam are significant and should not be ignored. Additionally, the analysis correctly predicted the failure initiation location, which was verified using high speed photography during the tensile test. The study identified the critical failure regions of asymmetrical tapered sandwich core designs and the failure load dependence on the temperature gradient in the structure.

Numerical method to characterize probabilistic energy distribution of drop weight tear test at Ductile-Brittle transition temperatures

This paper proposes a numerical method to characterize probabilistic energy distributions of drop weight tear tests (DWTTs) at ductile–brittle transition temperatures. The method employs finite element ductile–brittle fracture simulation using the stress-modified fracture strain model for ductile tearing and the maximum principal stress criterion for cleavage fracture. The probability of energy scatters is expressed using the two-parameter Weibull distribution in terms of the maximum principal stress. The maximum principal stress is then obtained as a function of the absorbed energy using FE simulation. The characterized energy scatters are compared with API X80 DWTT data at transition temperatures, showing good agreement. The proposed method can be used to effectively predict DWTT data at transition temperatures.

Analysis of the hydraulic fracturing mechanism and fracture propagation law with a new extended finite element model for the silty hydrate reservoir in the South China Sea

The existing hydrate exploitation technology is characterized by low gas production in single wells and a small exploitation range, owing to the low porosity and low permeability of submarine hydrate reservoirs; therefore, commercial exploitation is difficult to achieve. There is an urgent need to improve exploitation efficiency by means of reservoir reconstruction. Hydraulic fracturing technology may be useful for low-permeability reservoirs such as silty hydrate reservoirs in the South China Sea (SCS). In this study, we established a new hydraulic fracturing model of a horizontal single well for gas-hydrate-bearing-sediments (GHBS) using the cohesive zone method (CZM) based on the extended finite element method (XFEM). To this end, we utilized field measured data of GHBS at site GMGS3-W19. The effect of fluid-solid coupling was considered, and the fracture propagation criterion was set as the maximum principal stress criterion to analyze the hydraulic fracture propagation law of hydrate reservoirs in the SCS. First, we illustrated the effectiveness of the CZM based on XFEM in simulating the fracture morphology of GHBS hydraulic fracturing. A Parametric study showed that the hydrate saturation, horizontal geo-stress coefficient, perforation angle, and injection rate play crucial roles in the fracture propagation path and characteristic parameters of GHBS hydraulic fractures. The fracture development pattern and propagation law of a silty hydrate reservoir within the SCS were theoretically obtained. The results of the present study partly provide theoretical support for hydrate exploitation engineering in the SCS.

ВПЛИВ КОНЦЕНТРАЦІЇ НАПРУЖЕНЬ НА ДЕФОРМУВАННЯ СТАЛІ 20 ПРИ ЦИКЛІЧНИХ НАВАНТАЖЕННЯХ

The effect of stress concentration on fatigue behavior of steel 20 is studied. Experimental study is carried out on tubular smooth and notched specimens under uniaxial cyclic loading and alternating torsion. Load and strain controlled fatigue tests were carried out at room temperature using servohydraulic machine with independent control of push-pull and torsion loads with frequency of 0.5-3 Hz. One basic geometry for two different types of specimens shown in Figure 1 with 1.1 mm wall thickness, 22 mm inside diameter, and 40 mm gauge length was used in this work. One type was a tubular smooth thin-walled specimens, the other type was the same thin-walled specimen with 3.4 mm circular through-thickness hole at the middle of gauge length. The 5% load drop for uniaxial strain control tests and 5% strain and rotation angle increment for uniaxial and torsion load control tests respectively, as compared to midlife stable cycle for smooth specimens were considered as a small crack initiation life. The criterion of maximum principal stress was used as a control parameter of the test program. Torque loading was accompanied by a significant decrease in durability compared to uniaxial loading due to conservative methodology. Although maximum principal stress criterion could correlate axial and torsion constant amplitude data of notched specimens with a factor of 4, it could not correlate axial with torsion data of smooth specimens. It is found that the prediction of fatigue life can be successfully performed according to the shear form of Fatemi-Soci criterion by taking into account the mechanism of destruction of samples. The shear strain-life curve was generated based on von Mises criterion. The FS parameter was associated with local stress-strain condition based on FE analysis results.

New approach to failure of pre-cracked brittle materials based on regularized solutions of strain gradient elasticity

In this paper, we use the feature of the strain gradient elasticity theory (SGET) related to the regularization of classical singularity problems. We suggest that the structural analysis of pre-cracked materials can be reduced to failure analysis within SGET by using the appropriate failure criteria formulated in terms of the Cauchy stresses. These stresses are work-conjugated to strains, and they have non-singular values in SGET solutions for the problems with cracks and sharp notches. We confirm our suggestion based on the full-field numerical simulations and provide examples of identification of the length scale parameters of SGET for the known experimental data with pre-cracked brittle and quasi-brittle materials. We show that the identified length scale parameters allow us to predict the failure loads for the experimental samples with different type of cracks by using the maximum principal Cauchy stress criterion. In the experiments with a mixed-mode fracture of the quasi-brittle material (PMMA with inclined cracks), we found that there may arise a transition between the failure determined by the maximum principal stress criterion and the criterion related to the second invariant of the Cauchy stress tensor that can be explained by the change of the stress state triaxiality.

Fatigue behavior and stress distribution of molars restored with MOD inlays with and without deep margin elevation.

This study evaluated the effect of deep margin elevation (DME) and restorative materials (leucite-reinforced glass-ceramics [C] vs. indirect resin composite [R]) on the fatigue behavior and stress distribution of maxillary molars with 2-mm deep proximal margins restored with MOD inlay. Fifty-two extracted human third molars were randomly assigned into four groups (n = 13): C; DME + C; R; and DME + R. Inlays were fabricated in CAD-CAM and bonded to all teeth. The fatigue behavior was assessed with the stepwise stress test (10,000 cycles/step; step = 50N; 20Hz; initial load = 200N). Fatigue failure loads and the number of cycles were analyzed with 2-way ANOVA and Tukey's test (p < 0.05) and Kaplan-Meier survival plots. The stress distribution was assessed with finite element analysis. The models were considered isotropic, linear, and homogeneous, and presented bonded contacts. A tripod axial load (400N) was applied to the occlusal surface. The stress distribution was analyzed with the maximum principal stress criterion. For fatigue, there was no difference for DME factor (p > 0.05). For the material factor, the load and number of cycles for failure were statistically higher in the R groups (p < 0.05). The finite element analysis showed that resin composite inlays concentrated more stress in the tooth structure, while ceramic inlays concentrated more stress in the restoration. Non-reparable failures were more frequent in the resin composite inlays groups. DME was not negative for fatigue and biomechanical behaviors. Resin composite inlays were more resistant to the fatigue test, although the failure mode was more aggressive. DME does not impair mechanical behavior. Resin composite inlays failed at higher loads but with a more aggressive failure mode.

Effect of occlusal anatomy of CAD/CAM feldspathic posterior crowns in the stress concentration and fracture load.

ObjectivesThis study evaluated the effect of restoration occlusal design on the maximum fracture load and stress distribution of a feldspathic ceramic crown.Materials and MethodsTwenty dentin analogues were used to simulate a full‐crown preparation. Next, 20 feldspathic crowns were milled according to the occlusal design parameter available in the CAD database (Young or Adult). The crowns were cemented with dual cure resin‐cement and loaded until fracture at 1 mm/min crosshead speed. Data were analyzed by using one‐way ANOVA and Tukey tests (p < 0.05). The same geometry and experimental setup was modeled and exported to the computer aided engineering software and tensile stress concentration was calculated using the finite element method with 300 N occlusal load simulation.ResultsThe occlusal anatomy significantly influenced the load‐to‐fracture (p < 0.05). Adult design showed higher mean values (1149 ± 201 N) than Young design (454 ± 77 N). The maximum principal stress criteria showed similar stress pattern for both designs, however, the highest stress concentration was calculated for Young design (91 MPa) in the occlusal surface.ConclusionsAn anatomy design with reduced cusp angulation and less evident occlusal sulcus can reduce the stress concentration and increase the fracture load for feldspathic CAD/CAM posterior crowns.

Fatigue survival of endodontically treated teeth restored with different fiber-reinforced composite resin post strategies versus universal 2-piece fiber post system: An in vitro study

Statement of problemVarious strategies have been proposed to reduce the cement space of foundation restorations for endodontically treated teeth. However, they may add more operative steps, or the dentist must keep different sizes of drills and posts in stock. A 2-piece universal adjustable post system has been developed to overcome this problem, but whether the system has acceptable fatigue survival performance is unclear. PurposeThe purpose of this in vitro study was to evaluate the fatigue survival and stress distribution of endodontically treated teeth without a ferrule and restored with different glass fiber post strategies versus a recently introduced universal 2-piece fiber post system. Material and methodsBovine incisor roots were randomly assigned to 3 groups as per the post used (n=13): adapted glass fiber post with post space preparation of the same size, composite resin-custom glass fiber post (CTM), and universal 2-piece glass fiber-reinforced composite resin post (UNI). The posts were adhesively luted, the composite resin core was added, and a composite resin crown was produced with computer-aided design and computer-aided manufacturing (CAD-CAM), and then adhesively luted to each core. A fatigue test was performed with the stepwise stress method (10 000 cycles/step; 20 Hz; load=100 N to 750 N; step=50 N) until fracture, and the failure mode analyzed. The stress distribution was evaluated by finite element analysis with the maximum principal stress criteria by following the parameters of the in vitro test. The solids were considered homogeneous, linear, and isotropic, except for the glass fiber post (orthotropic), and a load of 450 N at 30 degrees was applied. The fatigue failure load and the number of cycles for failure were analyzed with Kaplan-Meier and Mantel-Cox (log rank test) (α=.05). The finite element analysis results were analyzed with colorimetric graphs. ResultsThe highest fatigue failure load and the number of cycles for failure were found in the UNI system, whereas the lowest results were found in the CTM group. All groups exhibited repairable failures. The finite element analysis showed the lowest stress in root dentin in the UNI system. The CTM system had the largest stress regions at the dentin and dentin-core interface. ConclusionsThe use of a 2-piece universal glass fiber post system resulted in more fatigue behavior compared with composite resin-custom glass fiber posts.

Comparison of Various Criteria Determining the Direction of Crack Propagation Using the UDMGINI User Procedure Implemented in Abaqus.

This paper describes a method of predicting the direction of crack propagation implemented by user subroutines in the Simulia-Abaqus FEA system with the use of the extended finite element method (X-FEM). This method is based on displacements and stresses according to Westergaard’s solution of Griffith’s crack problem. During the calculations, in each crack increment, the algorithm reads the stresses and displacements in the model around the crack tip, calculates the criterion values at the read points, reduces them to a unit distance from the crack tip, fits a polynomial to these points, and finds the minimum of the function closest to the last propagation angle. The algorithm also decides when the crack grows, depending on a chosen criterion. Four criteria have been implemented to predict the direction of failure propagation: the maximum principal stress criterion, the Ottosen–Podgórski criterion, the new criterion described here based on the minimum component values of the displacement vector, and the maximum circumferential tensile stress (MTS). These criteria were verified in two tests: the three-point bending test of the notched beam and the anchor pull-out test. For these tests, the criterion built into Simulia Abaqus does not correctly define the crack path, which causes the crack propagation direction to “rotate” when simulating the fracture. The criteria developed here, in most cases, determine the crack path and the maximum force very well compared to real laboratory tests.

Computer Aided Design Modelling and Finite Element Analysis of Premolar Proximal Cavities Restored with Resin Composites.

This study evaluated the stress distribution in five different class II cavities of premolar models restored with conventional or bulk-fill flowable composite by means of finite element analysis (FEA) under shrinkage and occlusal loading. An upper validated premolar model was imported in the software, and five class II cavities with different occlusal extensions and dimensions were prepared: horizontal cavity on the mesial surface (horizontal slot), mesio-occlusal cavity, mesial cavity (vertical slot), tunnel type cavity and direct access cavity. The models were restored with conventional or bulk-fill flowable resin composite. The tested materials were considered as homogeneous, linear, and isotropic. The Maximum Principal Stress criteria was chosen to evaluate the tensile stress results. The lowest shrinkage stress value was observed in the direct access cavity restored with bulk-fill flowable resin composite (36.12 MPa). The same cavity, restored with conventional composite showed a score of 36.14 MPa. The horizontal slot cavity with bulk-fill flowable showed a score of 46.71 MPa. The mesio-occlusal cavity with bulk-fill flowable had a score of 53.10 MPa, while with conventional composite this was 55.35 MPa. Higher shrinkage stress was found in the vertical slot cavity with conventional resin 56.14 MPa, followed by the same cavity with bulk-fill flowable 56.08 MPa. Results indicated that the use of bulk-fill flowable composite resin more significantly decreased the polymerization shrinkage stress magnitude. The larger the cavity and the volume of material necessary to restore the tooth, the greater the residual stress on enamel and dentin tissue.