Terms Of Stress Concentration Research Articles

Peridynamic Model for Tensile Elongation and Fracture Simulations of Polymethyl Methacrylate Notched Specimens

This paper presents the peridynamic (PD) numerical model for simulating a tensile test until total fracture for a brittle polymeric material namely polymethyl methacrylate (PMMA). U-notched and V-notched specimens were used to investigate the effect of the notches on the elongation and fracture of PMMA. The tensile elongation of PMMA exhibits nonlinearity with respect to the applied load, while the fracture occurs when the material stress has reached the ultimate tensile stress of the material. Similar elongation and fracture properties were applied on PD simulations. Two types of elongation equation are used namely brittle and ductile equations to form PD-brittle and PD-ductile models. The published experimental data of tensile fracture test on notched PMMA specimens are used as reference to validate the simulations of the PD models. The PD numerical force-extension curves have good quantitative similarity for V-notched specimen but adequate quantitative similarity for U-notched specimen. As for the quality of the fractured specimen shape, the PD simulations have good similarity for the V-notched specimen but adequate similarity for the U-notched specimen. The plot of the internal force distribution from the simulations of PD shows good qualitative similarity to the plot of the stress distribution from the published data of FEM in terms of stress concentration. From the PD results, it is observed that the PD-ductile model has better capability in producing accurate simulation of the notched specimens than the PD-brittle model.

Optimal design and biomechanical analysis of sandwich composite metal locking screws for far cortical locking constructs.

In the interests of more flexible and less stiff bridge constructs to stimulate bone healing, the technique of far cortical locking has been designed to improve locked plating constructs in terms of stress concentration, stress shielding, and inhibition of issues around fracture healing. However, far cortical locking screws currently lack objective designs and anti-fatigue designs. This study investigates an optimization algorithm to form a special locking screw composed of various metals, which can theoretically achieve the maintenance of the excellent mechanical properties of far cortical locking constructs in terms of fracture internal fixation, while maintaining the biomechanical safety and fatigue resistance of the structure. The numerical results of our study indicate that the maximum von Mises stress of the optimized construct is less than the allowable stress of the material under each working condition while still achieving sufficient parallel interfragmentary motion. Numerical analysis of high cycle fatigue indicates that the optimized construct increases the safety factor to five. A high cycle fatigue test and defect analysis indicates that the sandwich locking constructs have better fatigue resistance. We conclude that the sandwich locking construct theoretically maintains its biomechanical safety and fatigue resistance while also maintaining excellent mechanical properties for fracture internal fixation.

Peri-implant stress distribution assessment of various attachment systems for implant supported overdenture prosthesis by finite element analysis - A systematic review.

BackgroundVarious attachments like ball, bar-clip, magnetic attachments are used in implant supported overdentures. Finite Element Analysis (FEA) a newly innovated technology has been used in dental implantology to evaluate stress distribution patterns. There is little evidence available regarding the stress distribution in peri-implant region for implant supported overdentures. The purpose of the review was to generate scientific evidence on peri-implant stress distribution in FEA model with different types of attachments employed in implant supported overdentures.Materials and methodsSystematic review was conducted as per the Preferred Reporting Items for Systematic Reviews Guidelines and Meta-Analyses statement (PRISMA). A comprehensive search was undertaken by two reviewers from January 2020 to June 2020 with no year limits to published articles. Only in-vitro FEA studies were included. Following electronic databases were searched for published studies- PubMed, Web of Science. Characteristics of the studies tabulated and analysis of articles was done to compare different attachment systems.ResultsLocator attachments showed better stress distribution than ball attachment system in all the studies but one. Two studies showed results in favour of ball attachment compared to bar-clip attachment system when stress was evaluated distal to the implants. No significant difference in terms of stress concentration could be generated between ball versus magnetic/equator versus locator attachment system due to less number of studies and conflicting results.ConclusionVarious studies showed different results due to heterogenicity in selected attachment systems and study designs. Locator attachments showed favourable stress distribution around peri-implant bone than other attachments.

Numerical and Experimental Investigations of Fracture Behaviour of Welded Joints with Multiple Defects.

Current standards related to welded joint defects (EN ISO 5817) only consider individual cases (i.e., single defect in a welded joint). The question remains about the behaviour of a welded joint in the simultaneous presence of several different types of defects, so-called multiple defects, which is the topic of this research. The main focus is on defects most commonly encountered in practice, such as linear misalignments, undercuts, incomplete root penetration, and excess weld metal. The welding procedure used in this case was metal active gas welding, a common technique when it comes to welding low-alloy low-carbon steels, including those used for pressure equipment. Different combinations of these defects were deliberately made in welded plates and tested in a standard way on a tensile machine, along with numerical simulations using the finite element method (FEM), based on real geometries. The goal was to predict the behaviour in terms of stress concentrations caused by geometry and affected by multiple defects and material heterogeneity. Numerical and experimental results were in good agreement, but only after some modifications of numerical models. The obtained stress values in the models ranged from noticeably lower than the yield stress of the used materials to slightly higher than it, suggesting that some defect combinations resulted in plastic strain, whereas other models remained in the elastic area. The stress–strain diagram obtained for the first group (misalignment, undercut, and excess root penetration) shows significantly less plasticity. Its yield stress is very close to its ultimate tensile strength, which in turn is noticeably lower compared with the other three groups. This suggests that welded joints with misalignment and incomplete root penetration are indeed the weakest of the four groups either due to the combination of the present defects or perhaps because of an additional unseen internal defect. From the other three diagrams, it can be concluded that the test specimens show very similar behaviour with nearly identical ultimate tensile strengths and considerable plasticity. The diagrams shows the most prominent yielding, with an easily distinguishable difference between the elastic and plastic regions. The diagrams are the most similar, having the same strain of around 9% and with a less obvious yield stress limit.

Inverse homogenization using isogeometric shape optimization

Finding periodic microstructures with optimal elastic properties is usually tackled by a highly resolved, regular finite element model and solid isotropic material penalization. This procedure not only has many advantages, but also requires a comparably high computational effort and challenges in representing stresses accurately. Therefore, an isogeometric shape optimization approach is applied to the inverse homogenization problem and combined with a reconstruction procedure for nearly optimal rank-3 laminates, which provides an efficient solution strategy with more accurate stress modelling. This allows to investigate the sensitivity of optimized microstructures in terms of stress concentrations.

Experimental Study forthe Effect of Additional Weld on Fatigue Strength inOut-of-Plane Gusset Welded Joints

Grinding ofweld toesis generally used toimprovethe fatigue strength of fillet weld joints. In this study,a technique employingadditional weldswas applied toout-of-plane gusset welded jointsand the relationship between the shape of the additional weld and fatigue strength was investigated. The shape of the additional weld was controlled by changing the aiming position. When there was more distance between the first weld toe and the aiming position of the additional weld, the flank angle and toe radius tended to be smaller and larger, respectively. In the nominal stress range of 150 MPa, fatigue life was extended to 2.7 times that of weld jointswithout additional welds. Although the toe radius of the additional weld was smaller than that created bygrinding, the resultant fatigue strength of the additional weld joint was almost equivalent to that of the grinding case. This can beexplained in terms of stress concentration, since the flank angle and toe radius in the additional weld toe canbe improved without reducing the throat thickness.

Improvement in mechanical and structural properties of poly(ethylene terephthalate) nanohybrid

The wide range of applicability of poly(ethylene terephthalate) (PET) in various fields such as packaging, automobile parts, sports and textile due to its transparent nature, light weight and mechanical properties creates strong case to enhance its mechanical properties. By embedding nanofillers and thereby improving the mechanical properties, this spectrum can further be widened. The effect of addition of nanofiller in terms of stress concentration is a crucial phenomenon to understand the mechanical property enhancement. The nanohybrids of poly(ethylene terephthalate) (PET) and organically modified nanoclay have been prepared through solution casting route to enhance the properties significantly without any trade-off of deteriorating other properties. The dispersion of nanoclays in the polymer matrix has been observed through TEM images. Good level of dispersion has been achieved in nanohybrids due to specific interactions between nanoclay and PET matrix explored through XRD and FTIR spectroscopic measurements. Thermal stability of the nanohybrids has been tested through TGA and DSC analyses. The mechanical properties have been tested and found to be enhanced in the presence of nanoclay with increasing filler concentrations. The modulus has been increased for nanohybrids up to 93%, while the optimum value of modulus and toughness has been observed at 4 wt% filler percentage. The improvement in mechanical properties has been predicted using different models for randomly dispersed filler and has been found to be fitted in acceptable range. The hardness of the nanohybrids has been tested using Vickers hardness test and is found to be increasing continuously with filler loading. The structural formations upon uniaxial stretching have been contemplated through two-dimensional small-angle X-ray scattering as well as through wide-angle XRD. The nanohybrids have shown short-range ordering on uniaxial stretching in comparison with pristine PET. Blob size has been found out using Debye–Bueche model and is found to be increased upon stretching the samples. The polymer nanohybrids have been prepared for the mechanical property improvement without imparting high brittleness for practical applications. The behavior of nanoclay on stretching the polymer nanohybrids has also been studied in detail.

On Stress Concentration on Edges of Defect Holes in Plates and Critical Issues on Filling-in Repairs

Repair is now increasingly used for damaged structures due to its distinguished advantages in prolonging the service life of structures, substantial saving in materials, and preservation of related crucial resources. This work presents a novel filling-in repair procedure for localized damages in panel structures, and conducts a series of theory and numerical studies to examine the repair effects in terms of stress concentration. It aims providing some fundamental understandings important in the design of repairing procedures for effective filling-in repairs to mitigate stress concentrations. First, the damaged zone (or cracks) is “cut-off” by simply drilling a hole through the panel to remove possible stress singularity. The hole is then filled-in with a circular patch of slightly smaller diameter made of similar material of the panel using adhesives. Stress concentration is then investigated to reveal the possible ranges of the stress concentration factors (SCF), through both analytical and numerical means using finite element (FE) models. The repair effects are then examined for repaired plates with filled-in holes in terms of SCF reduction, comparing with plates with open-holes. Both aluminum and composite plates are studied, and the filling-in repairs are carried out using epoxy adhesives. It is found that the SCF increases with the adhesive thickness, decreases with hole radius and Young’s modulus of adhesives. Parametric studies are performed to produce two-stage curve-fitted formulae for SCF predictions from given hole radius, physical properties and thickness of the adhesive layer, which are useful for the design and assessment of the repairs, leading to a prolonged life of services of structures.

Failure behaviour of the adhesive layer and angle ply composite adherends in single lap joints: A numerical study

In the last decades, composite materials have been increasingly used in advance engineering fields such as aerospace and automobile industries due to their advantages in terms of stress concentrations, structural weight and manufacturing. Joining composite parts in such applications has been increasingly achieved by adhesives compared to welded, riveted or bolted connections. Mechanical testings of adhesively bonded composite joints are often time consuming and expensive due to a number of parameters involved and their analytical modellings are complicated. Therefore, numerical analysis is increasingly used in recent years to overcome these limitations. In this work three-dimensional FE modelling of the single lap joints (SLJs) of angle ply composite adherends subjected to tensile loading was developed using ABAQUS/Explicit. Cohesive zone model was adopted for the modelling of damage in the adhesive layer. The continuum damage mechanic model implemented via a user defined VUMAT subroutine was used to model the intraply damage in composite adherends. The model accounted for the damage initiation, the damage evolution, and the nonlinear shear behaviour of the material. Experiments of the SLJs of [±10]5s, [±20]5s, [±45]5s and [±55]5s adherends under tensile loading were performed. The adherends were manufactured from glass reinforced polymer matrix and the adhesive layer was AF163-2K. For verification of the developed model, numerical results were compared with those of the experiments mentioned above. The SLJs of [±10]5s and [±20]5s adherends failed due to failure in the adhesive layer and those of [±45]5s and [±55]5s failed mainly due to failure in the adherends. The peel and shear deformation modes in the adhesive layer were invoked in different ways for different SLJ configurations. The effects of adhesive layer thickness and the overlap length on the response of the SLJ were investigated in detail.

Numerical analysis of optimum treatment parameters by high frequency mechanical impact

In the study, finite element analyses were carried out systematically to investigate the optimum treatment parameters of high frequency mechanical impact (HFMI) on the basis of notch effect reduction and residual stress, which are generally considered to be the dominating factors of fatigue strength improvement of welded joints by HFMI treatment. Taking strain rate into account, the combined isotropic-kinematic hardening model was used to describe the elastic-plastic behavior of the HFMI-treated welded joint of S355 steel. The notch stress concentration factors (SCFs) and residual stresses under different impact depths and diameters of indenter were determined, respectively. Moreover, their synthetic effects on stress distributions of HFMI-treated joints under static load were investigated. It is found that the stress distributions of HFMI-treated joints under static load are affected significantly by impacted groove depth, independent of indenter diameter. The HFMI treatment can achieve the optimal status when the original weld toe is just removed with a groove depth ranging from 0.1 to 0.2 mm. At final, the optimized diameters of 3 to 4 mm are recommended for the indenter in terms of stress concentration and avoidance of crack.

Crack stop holes in steel bridge decks: Drilling method and effects

Force analysis using a compact tension model, as recommended by ASTM, was carried out on a crack stop hole. The stress before, and after, drilling the hole was compared in terms of stress concentration and stress gradient. The optimum drilling location and diameter were studied through analysis of different locations and diameters. By analyzing the effects of flank holes and an additional hole, drilling advice was proposed and fatigue testing of the cracks in a steel bridge deck with a crack stop hole was conducted. The results show that the stress at the crack tip with a crack stop hole decreased, and the major principal stress around the hole was distributed accordingly. The optimum position of the crack stop hole centre was where the centre of the crack stop hole was situated behind the crack and the hole edge coincided with the crack tip. Therefore, hole diameters larger than 8 mm, or those weakening the section by 10%, were suggested as the best diameters. In terms of multi-hole crack stopping, a flank hole was not recommended. The optimum horizontal position of flank holes was at a distance of 1/4 of a single hole diameter from, and in front of, the single hole. Besides, the experiment showed that crack stop hole could only prevent cracks from growing and had no influence on crack growth rate.

Minimizing Stress Concentrations in the Femoral Heads of Hip Joint Prostheses: Effect of Borehole Shapes

This paper presents a stress analysis of the ceramic femoral heads of hip joint prostheses with different borehole shapes to evaluate their mechanical reliability in terms of stress concentration. Under the ideal loading conditions used for ceramic rupture tests specified by the ISO 7206-5 standard, a finite element (FE) modeling is performed to determine the tensile and hoop stress distributions in the ceramic femoral heads. Two borehole shapes that are currently used in the manufacturing industry for hip joint prostheses, namely the flat bottom and keyhole, were first studied. Two new borehole shapes, dome arc and dome ellipse, were then introduced by the authors in the paper to minimize the stress concentration. It was found that while the currently used borehole shapes lead to high tensile notch stresses at their critical corners causing possible fracture failure of ceramic heads, the authors’ borehole designs can improve the mechanical reliability significantly. In addition, the effects of taper-bore contact length and their interface friction are investigated and discussed.

Interfacial shear stress in FRP-plated RC beams under symmetric loads

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre-reinforced polymer (FRP) plates to their soffits. An important failure mode of such plated beams is debonding of the FRP plates from the concrete due to high level interfacial stresses near the plate ends. A closed-form rigorous solution for the interfacial stresses in simply supported beams bonded with thin plates and subjected to arbitrary loads has been found, in which a non-uniform stress distribution in the adhesive layer was taken into account. This paper uses the rigorous solution to investigate the impact of symmetric loading configurations on the interfacial shear stress distributions, and concludes that the bending moments on the cross sections at the plate ends play a significant role in terms of stress concentration, while the shear forces on the same cross-section contribute little to the concentration. On the basis of this observation, this paper proposes a simplified approximate solution to the shear stress along the interface between concrete and adhesive layer. Compared with the rigorous and other approximate solutions, the simplified solution exhibits sufficient accuracy in terms of stress distribution and stress concentration localized near the plate ends. Due to its compact feature, the simplified solution is more suitable for engineering applications using a portable calculator and to be adopted in the codes of practices.

Stress concentration due to shear lag in continuous box girders

Quite a few researches on shear lag effect in box girder have been reported in the past, and many of them employed the finite element method. The past researchers, however, do not seem to have paid much attention to the influence of the finite element mesh on the shear lag, although the shear lag effect in terms of stress concentration can be quite sensitive to the mesh employed in the finite element analysis. In addition, most of the researchers on the shear lag have focused on simply supported girders and cantilever girders, while continuous girders have been dealt with in very few researches. The present study investigates the shear lag effect in a continuous box girder by using the three-dimensional finite element method. The whole girder is modeled by shell elements, and an extensive parametric study with respect to the geometry of a box girder is carried out. The influence of finite element mesh on the shear lag is carefully treated by the multimesh extrapolation method. Based on the numerical results thus obtained, empirical formulas are proposed to compute stress concentration factors that include the shear lag effect.

Transversely isotropic viscoplasticity model for a directionally solidified Ni-base superalloy

A transversely isotropic continuum viscoplasticity model has been formulated to capture the fatigue and creep responses of a directionally solidified (DS) polycrystalline Ni-base superalloy used mainly in turbine blades. This model has been implemented as an ABAQUS User MATerial (UMAT) subroutine using a semi-implicit integration scheme. Isothermal uniaxial fatigue data from tests conducted with and without hold times and creep data are used to characterize the stress–strain response at temperatures ranging from 427 °C to 1038 °C. The scheme leads to reduction of the associated computational costs when compared to a crystal viscoplasticity model that explicitly considers 3-D grain structure. The macroscopic elastoviscoplastic model is shown to simulate the homogenized deformation response of the polycrystalline DS alloy for various isothermal histories. The predictive capability of this model is verified using both in-phase and out-of-phase TMF data, and is compared to the results of analysis of a single crystal in terms of stress concentration and stress distribution for a model problem of a plate with a central hole.

Patterns of Vertical Root Fracture: Factors Affecting Stress Distribution in the Root Canal

Previous studies have indicated that vertical root fracture tends to occur in a buccolingual direction, where dentin thickness is greatest. Factors potentially influencing the location and direction of root fracture include root canal shape, external root morphology, and dentin thickness. In this finite-element study, simulated root sections were varied systematically with respect to canal size and shape, external root morphology, and dentin thickness to determine their relative contribution to vertical root fracture. Similar models were constructed based on cross-sections of human tooth roots that had been fractured clinically or experimentally. Finite-element models demonstrated that canal curvature seems more important than external root morphology, in terms of stress concentration, and that reduced dentin thickness increases the magnitude but not the direction of maximum tensile stress. Models based on actual root fractures showed a strong similarity between tensile-stress distribution and fracture patterns.

Whole-field assessment of the effects of boundary conditions on the strain field in off-axis tensile testing of unidirectional composites

The objective of this paper is to present an experimental assessment of the effects of boundary conditions on the strain field in 10° off-axis tensile tests on unidirectional carbon/epoxy composites. It is shown experimentally by using a whole-field phase stepped grid technique that oblique tabbing of off-axis coupons results in a homogeneous strain field over the whole specimen, thus preventing premature failure near the tabs. A numerical sensitivity study reveals that the oblique angle is not very sensitive to the elastic moduli values. Therefore only estimates of the moduli are needed to derive the oblique angle. It is also shown by finite-element analysis that a 3° error on the oblique angle is acceptable in terms of stress concentrations near the tabs, but that a 7° error leads to significant stress concentrations for this material.

A critical reevaluation of stresses generated during vertical and lateral condensation of gutta‐percha in the root canal

The finite element method was used to calculate the stresses in a maxillary canine tooth produced during lateral and vertical condensation. Description of the biological domain to be analyzed, and conversion of the manual operations of the endodontist to realistic load representation are intricate problems which must be overcome before stresses are recovered and plotted. The stresses in dentin are in general of comparable magnitudes during lateral or vertical condensation, but these magnitudes generally remain much below those presented in a previous study. From these calculations it appears that root fractures reported by some investigators may be explained in terms of stress concentrations and local irregularities of the samples tested.

Effects of grain size and test temperature on ductility and fracture behavior of A B-doped Ni 3Al alloy

Effect of grain size on ductility and fracture behavior of boron-doped Ni 3Al (Ni-23A1-0.5Hf, at.%) was studied by tensile tests using a strain rate of 3.3 × 10 −3 S −1 at temperatures to 1000°C under a high vacuum of < 1 × 10 −4 Pa. At temperatures below 700°C, the alloy showed essentially ductile transgranular fracture with more than 30% elongation whereas it exhibited ductile grain-boundary fracture in the temperature range from 700 to 800°C. In both cases, the ductility was insensitive to grain size. On the other hand, at temperatures above 800°C, the ductility decreased from about 17 to 0% with increasing grain size. The corresponding fracture mode changed from grain-boundary fracture with dynamic recrystallization to brittle grain-boundary fracture. The ductile transgranular fracture at lower temperatures is explained by stress concentration at the intersection of slip bands. The grain-size dependence of ductility is interpreted in terms of stress concentration at the grain boundaries. Finally, it is suggested that the temperature dependence of ductility in this alloy might be related to the thermal behavior of boron segregated to the grain boundaries.

Variation of Fatigue Notch Factor With Lifetime, Stress Ratio and Temperature

An understanding of the influence of various loading parameters on the fatigue notch factor Kf is important to the designer, who often relies exclusively on the limited fatigue data available in handbooks. The significance and limitations of Kf values so obtained are discussed in the light of geometry, fatigue lifetime, stress ratio, and temperature. Ultimately fatigue notch factor data in the literature are very limited, and often there is no recourse but to make use of the theoretical stress concentration factor Kt. The results of this procedure are usually very conservative. Handbook fatigue failure data were reexamined in order to determine the variation of Kf for a representative range of materials, including aluminum, steel, titanium and nickel alloys. Results are presented in the form of Kf over a wide range of notch and loading conditions. The results are interpreted and generalized in terms of stress concentration, and material mechanical and microstructural response.