Geometric Stress Concentration Research Articles

On Defect Evolution in EBM Additively Manufactured Ti-6Al-4V via In Situ Investigations.

This study concerned the in situ investigation of the defect evolution and fracture mechanism of additively manufactured (AM) Ti-6Al-4V under uniaxial tensile tests. In order to achieve this, microstructure characterization was initially carried out in order to identify the defects within the matrix of the candidate material. In situ testing was then performed, focusing on the spherical defect to observe its evolution under tensile loading. It was found that, before the fracture stage, the geometric evolution of the spherical defect towards an ellipse shape was dominated by the load in the tensile direction. In addition, the slip band density was found to be aggravated near the spherical defect due to the geometric discontinuity-induced stress concentration. During the fracture process, the defect geometry evolved as an irregular shape, which was mainly attributed to the micro-void-induced localized multi-axial stress state. The fracture analysis indicated that defects play a key role in crack initiation, leading to the fracture of LPBF materials.

Highly detailed structural integrity assessment of the reactor pressure vessel nozzle of Atucha-I during a pressurized thermal shock event

In pressurized water reactor (PWR) nuclear plants, reactor pressure vessel (RPV) nozzles are critical due to geometric stress concentrations that worsen with large temperature gradients. In the context of Atucha-I Long Term Operation (LTO) assessment, this work presents a high-fidelity thermo-mechanical simulation approach to verify the stress levels on the main nozzles of the vessel under a pressurized thermal shock event. Nucleoeléctrica Argentina S.A. (NA-SA) has provided the boundary conditions, including the thermal plume that results from computational fluid dynamics (CFD) simulations of the transient event. Then, using a one-way and non-conformal projection procedure, CFD data is coupled to a highly-detailed 3D finite element model (FEM), including the RPV, closure head, nuts and bolts, and moderator tank, all interconnected through non-linear contact condition that accounts for friction. Finally, a 3D FEM submodel assuming an elastoplastic material behavior with non-linear isotropic hardening is used to verify the structural integrity of the nozzle. Results indicate that the plastic strain and stresses on the nozzles are low enough to ensure the correct function of the reactor during the LTO period.

Analysis of the uniaxial fatigue behaviour of 42CrMo4 Q&T steel specimens extracted from the big end of a marine engine connecting rod using the heat dissipation approach

Analysis of the uniaxial fatigue behaviour of 42CrMo4 Q&T steel specimens extracted from the big end of a marine engine connecting rod using the heat dissipation approach

Specimen design for gigacycle fatigue testing of structural steel welded joints

Structures and components across a range of industries exhibit service lives above 107 cycles, but there is limited fatigue data in this domain. Conducting fatigue tests in the gigacycle regime is only feasible by using ultrasonic fatigue testing machines in which specimens are typically excited at 20 kHz. This research focuses on the design of a novel specimen which captures the geometric stress concentration at the weld toe, as an alternative to typically used cylindrical ‘hourglass’ shaped specimens. The gas-shielded flux-core arc welding method was used to manufacture butt joints of 080A15 structural steel. Specimens were tested in fully reversed axial loading at room temperature using a Shimadzu USF-2000A ultrasonic fatigue testing machine. Specimens failed in the range of 104-107 stress cycles with all fatigue cracks initiating at the weld toe. One specimen was classed as a run-out after being subjected to 3 × 109 stress cycles without failure. The fatigue data obtained was assessed using the effective notch stress method and showed similar fatigue performance to the IIW guidelines.

Fatigue performance of fiber laser fusion cut edges on thick plates

Fatigue performance of cut edges is an important factor in structural design, especially when geometrical stress concentration factors (SCFs) are present. High power laser cutting systems have enabled efficient and fast fusion cutting with nitrogen as an assistant gas. The fusion cutting induces recast layer, which features crack-like defects close to cut edge. In this study, plates with thickness 16 ≤ t ≤ 30 mm have been fatigue-tested. A 10 kW fiber laser cutting system was used to cut plates with nitrogen as an assistant gas. High strength steel (HSS) S690 specimens included unnotched coupon specimens and welded T-profile with longitudinal gusset in post-weld treated conditions, where laser cut edge was found critical. Fatigue performance of S690 cut edges were found close to 170 MPa m = 3 when SCFs of welded specimens were included on analysis. Local properties at the cut edge including hardness measurements and scanning electron microscope (SEM) fractography. Fractography obtained local quality use with theory of critical distances -based fatigue strength assessment was demonstrated.

Experimental assessment of the functional fatigue in biocompatible Ti67Zr19Nb11.5Sn2.5 shape memory alloy in the vicinity of drilled holes

Shape memory alloys (SMA) exhibit desirable and unique functionalities that are suitable for a plethora of applications in the automotive, aerospace, and biomedical industries. The TiNbZrSn SMA has been specifically developed for biomedical applications owing to its enhanced biocompatibility compared to the widely available and heavily investigated NiTi-based SMA. In this work, the effects of mechanical rolling, heat treatment, and grain size on the functional fatigue properties of TiNbZrSn are investigated under cyclic loading conditions. Superelasticity was observed for samples rolled to reduction levels exceeding 90 %. However, the recovery magnitudes improved significantly following 96.6 % rolling due to the optimization of the grain size. The reduction in grain size, from about 400 μm in the homogenized conditions to 50 μm for the 96.6 % rolled state, increases slip resistance which consequently promotes superelasticity. The functional superelastic properties were also evaluated for samples with geometric stress concentration (drilled hole). Using full-field measurement techniques, the applied, recoverable, irreversible strains, and their evolution under cyclic loading conditions were carefully measured. The use of virtual extensometers located in multiple locations around the geometric stress concentration provides means to not only evaluate the functional degradation of local superelastic properties, but also the transition to structural damage through the initiation and propagation of fatigue cracks.

Effect of combined (radial-axial-angular direction) misalignment on sliding wear of spur gears: A comprehensive study

This study explores accelerated gear failures due to misalignment-induced geometric stress concentration. A mathematical model is derived to assess misalignment effects on sliding wear of driver gears using MATLAB. The model's accuracy is validated against literature and experiments. A parametric study investigates the impact of design parameters on wear in combined misaligned gear pairs. Findings show that increasing radial misalignment raises pressure and sliding wear in the single-tooth contact zone, while axial misalignment decreases the contact zone, resulting in increased wear throughout the mesh cycle. Angular misalignment reduces the contact zone, leading to non-linear wear distribution along the tooth flank. Combined misalignment has the most severe effects, causing higher contact pressure, non-linear wear distribution, and increased wear in transition zones.

SELECTION OF A WELDING ELECTRODE TO PREVENT CONTACT CORROSION OF A STEEL 1.0402 WELD JOINT

Welded structures operate, as a rule, under conditions of simultaneous exposure to aggressive media and workloads. Thus, when protecting the metal of welded joints from corrosion fatigue, a number of specific problems arise due to the heterogeneity of their structure, the presence of multilayer welds, physical and geometric stress concentrators, and the staged nature of the change in the fine structure of the metal during fatigue loading. The result is an accelerated destruction of the metal in the junction zones of various structural forms, which are unequally adapted to the action of fatigue stresses. This fully applies to such critical structures of the oil and gas industry as pipelines, tanks, gas holders, etc. The purpose of the research was to establish the features of corrosion of welded joints made by various brands of electrodes by analyzing the change in the value of the electrode potential of the metal, which reflects the level of free energy of the surface layers in dynamics. Laboratory samples of carbon steel 1.0402 were subjected to manual arc welding (OZL-6, LB-52, MP-3 electrodes) and semi-automatic welding (EMK 6D polished wire and E71T1 rutile flux-cored welding wire with rapidly crystallizing slag). Further, their microstructural analysis was carried out and the hardness of various sections of welded joints was determined. It has been established that samples welded by semi-automatic welding are less susceptible to corrosion damage, since the distribution of electrode potentials in the zones of the welded joint indicates their lower structural inhomogeneity. In this case, general uniform corrosion takes place. In addition, the use of EMK 6D and E71T1 welding wires ensures better mechanical properties of welded joints.

Numerical shape, thickness and stacking sequence optimisation and experimental study of hybrid composite plates under in-plane shear loading

Shape, thickness and stacking sequence optimisation of a damage tolerant hybrid (GFRP-CFRP) composite laminate is performed using the commercial Optistruct solver. The results of the optimisation study are compared to both a benchmark non-damage tolerant CFRP laminate (without protective surface GFRP plies known as type 1 laminate) and a damage tolerant traditionally optimised hybrid CFRP-GFRP laminate (having X shape CFRP plies known as type 2 laminate), designed and tested in a previous study. The optimised laminate is manufactured using three different manufacturing techniques. The experimental buckling and post-buckling performance of the manufactured laminates are investigated. The optimised hybrid laminate is approximately 8% heavier than the type 1 but 17% lighter than type 2, but with the benefit of protective surface GFRP plies in favour of a damage tolerant design as shown in a previous study. Both numerical and experimental buckling and post-buckling performance studies show that the optimised laminates demonstrate higher pre-buckling stiffness compared to the type 1 design. However, the experimental buckling and failure/collapse loads, unlike the numerically predicted loads, are 24.31% and 26.70% lower, respectively. This is due to the significant number of ply drop-offs in the hybrid laminate design, and hence geometric imperfections and stress concentration effects at these locations leading to early buckling and failure in the post-buckling region.

The role of hardening and roughening during the incubation period in water droplet impingement erosion of Ti–6Al–4V

The present work investigates the role of surface hardening and roughening on the damage accumulation during the incubation period in water droplet erosion (WDE). Water droplet erosion tests were carried out on Ti–6Al–4V alloy at an impact velocity of 250 m/s. The evolution of hardness and surface roughness during the incubation period were evaluated using microhardness tester and Confocal Laser Scanning Microscope (CLSM). Scanning Electron Microscopy (SEM) was conducted on the impact area at several intervals during the incubation period. Finite Element (FE) simulations were performed to obtain the impact stresses on both smooth and rough surfaces. It was found that the solid surface plastically deforms by the action of the initial impingements, and as a result, hardness and surface roughness increase progressively until the end of the incubation period. The cyclic increase in hardness results in gradual improvement in the erosion strength, making the surface more resistant to the subsequent droplet impacts. However, the finite element results revealed that the dynamic surface roughening process results in higher impact stresses for the same impact pressure due to geometrical stress concentration. It is concluded that the increase in impact stresses resulting from surface roughening negates the improvement in the erosion resistance due to hardening. Hence, cyclic hardening and roughening are crucial parameters in the damage accumulation process during the incubation period.

Modeling and simulation of belt bucket elevator head shaft for safe life operation

This research paper presents a step by step conceptual design and life prediction approach for the design, modeling and simulation of head shaft of a belt bucket elevator, to be used for conveying grains to a height of 33.5 m and at the rate of 200 tons/h. output. For this elevator system, the force and torque acting on the head shaft as well as the bending moment were calculated. Furthermore, the diameter of each cross section of the shaft was determined taking into consideration the geometric and fatigue stress concentration factors, due to shoulders which contribute significantly to most fatigue failures of shafts. The stress induced on the shaft by the force and the factor of safety for each cross section of the shaft was calculated using the DE-Goodman criterion. The model of the shaft was created from the calculated diameters and subjected to static and fatigue analysis using SolidWorks FEA. The results were validated by comparing the values from the FEA and the calculated values for stress and factor of safety of the critical section of the shaft, which showed an equivalent value. The FEA gave a fatigue load factor greater than one, which signifies that the shaft will not go into failure mode within the infinite life cycle of the shaft. The value of the fatigue strength obtained from FEA was higher than the value for the maximum von misses stress of the shaft, this result shows that the head shaft will sustain the loading stresses over a finite life prediction. This research is significant because the stress induced forces on the head shaft from each component of the elevator system were properly identified and analyzed so as to obtain precise results for life prediction.

Effect of Pore Defects on Uniaxial Mechanical Properties of Bulk Hexagonal Hydroxyapatite: A Molecular Dynamics Study.

Hydroxyapatite (HAP) is a calcium apatite bioceramic used in various naturally-derived and synthetic forms for bone repair and regeneration. While useful for the regrowth of osseus tissue, the poor load-bearing capacity of this material relative to other biomaterials is worsened by the propensity for pore formation during the synthetic processing of scaffolds, blocks, and granules. Here we use molecular dynamics (MD) simulations to improve the current understanding of the defect-altered uniaxial mechanical response in hexagonal HAP single crystals relative to defect-free structures. The inclusion of a central spherical pore within a repeated lattice was found to reduce both the failure stress and failure strain in uniaxial tension and compression, with up to a 30% reduction in maximum stress at the point of failure compared to a perfect crystalline structure observed when a 30 Å diameter pore was included. The Z axis ([0 0 0 1] crystalline direction) was found to be the least susceptible to pore defects in tension but the most sensitive to pore inclusion in compression. The deformation mechanisms are discussed to explain the observed mechanical responses, for which charge imbalances and geometric stress concentration factor effects caused by pore inclusion play a significant role.

Simulation of deformation and growth during surfacing of aluminum bronze nanograins

In additive manufacturing, it is important to determine the technological and structural factors that control the crystallization process and specify the required structure of the product. The crystallization structure is affected by the parameters of the starting material and the crystallization conditions. Using the orientation of the grains of the substrate, which is formed after electron-beam surfacing of aluminum bronze, the modeling parameters were set by the molecular dynamics simulation. The process of deformation of three adjacent grains under constrained conditions and structural changes in the process of interaction with a melt drop and subsequent crystallization were considered. An analysis of grain deformation made it possible to reveal the role of geometric shear stress concentrators and to determine the significance of constrained conditions for deformation of polycrystal grains. It has been established that under the influence of a drop of melt, stacking faults are the least thermally stable, and twins are the most stable. The crystallographic orientation of the crystallizing grains coincides with the orientation of the substrate grains. During crystallization, columnar grains continue to grow, in which stacking faults and twins are formed.

Electrohydrodynamic Pulling Consolidated High-Efficiency 3D Printing to Architect Unusual Self-Polarized β-PVDF Arrays for Advanced Piezoelectric Sensing.

Piezoelectric pressure sensors are important for applications in robotics, artificial intelligence, communication devices, etc. The hyperboloid is theoretically predicted to be an unusual 3D structure that allows concerted piezoelectric enhancement owing to its synergistic effects of geometrical stress confinement and stress concentration, but has not been experimentally fulfilled due to a lack of efficient architecting techniques. In this work, a 3D hyperboloidal arrayed self-polarized PVDF piezoelectric energy harvester (PEH) is successfully fabricated by incorporating electrohydrodynamic (EHD) pulling technology into fused deposition modeling (FDM) 3D printing. This strategy not only simplifies the layer-by-layer constructing procedure for arrays, but simultaneously realizes a self-polarized and high β-phase (92%) PVDF PEH in a single electric-pulling step, saving posttreatment such as poling and removing excessive additives. Such a PEH delivers a significantly enhanced piezoelectric potential which is around 8 times that of a 2D flat film sensor. Moreover, this PEH featuring excellent linearity within a wide pressure regime, enables the sensing of human activities in a relatively large force range, which is otherwise difficult for traditional film sensors to differentiate. This work demonstrates a potential roadmap to advanced piezoelectric sensors exploiting unusual 3D structures enabled by the unique EHD pulling coupled 3D printing technique.

Comparison of foam models from regular and irregular arrays of Gibson-Ashby open-cells

The paper studies effective elastic properties of foam or cellular materials modeled by a set of Gibson-Ashby open cells with regular or irregular structures. Currently, there are many papers that present results of studying cellular materials using theoretical, numerical and experimental methods. However, these papers consider either regular lattices, or a single cell, or representative volume models based not on the Gibson-Ashby models. In this paper, in addition to the regular lattice, irregular structures were numerically studied. A mathematical formulation of the homogenization problem based on the energy equivalence of a foam-like material and on a homogeneous comparison medium is described. Formulations of six boundary value problems are presented. The solutions of these problems allow us to determine a complete set of effective stiffness modules for foams with different types of physical and geometric anisotropies. All stages of the numerical study were implemented in the ANSYS finite element package. Two algorithms for forming solid-state and finite-element models of irregular Gibson-Ashby lattices with small and large porosity are described in detail. As an example, numerical calculations are carried out for polycarbonate foams. The values of the effective elastic modules for regular and irregular lattices and for the Gibson-Ashby analytical model are compared. The results of numerical experiments showed that the Gibson-Ashby model describes the behavior of highly porous materials quite well (for porosity more than 75%), but this model gives a less satisfactory prediction in case of lower porosity. It is noted that for a large number of cells, regular and irregular lattices statistically give similar results for effective modules. However, for individual structures of irregular lattices, especially with strongly differing cells in individual directions, the effective moduli can have significantly different values, and the effective homogeneous medium can have pronounced anisotropic properties. These effects are due to geometric anisotropy and stress concentration in long connecting beams and at the joints of beams of various sizes in highly irregular Gibson-Ashby lattices. Examples of such lattices are given. We analyze the scatter of value for relative modules, which characterizes the anisotropy of such structures.

Experimental study of the destruction in CFRP laminates at the macro-level by acoustic emission method

The author conducted experiments using the method of acoustic emission under uniaxial tension of specimens of CFRP laminates with different stacking, geometry and stress concentration. The influence of a mutual arrangement of the material destruction zone and the extensometer installation site on the nonlinear deformation is revealed. It is shown that the damage accumulation process is highly informative when determining the energy of AE pulses. The possibility of visualizing the formation and development of destruction zones in orthogonal directions, as well as the possibility of determining the moment of crack start from the apex of the notch during splitting, is established. The mechanism of alternating stress relaxation in two developed zones of destruction is revealed. The paper shows a high degree of correlation between mechanic and acoustic-emission events. Finally, the author suggests assessing the mechanical state of material with the account of obtained damages by the degree of its integrity (one to zero) depending on the load history, its individual geometric features and stress concentration.

Turning-induced surface integrity for a fillet radius in a 316L austenitic stainless steel

Turning is a machining process extensively applied to produce revolution parts. Durability of these parts are known to depend on the turning process signature that is often referred as surface integrity. The surface integrity generated in a fillet radius has been barely studied in the literature so far, despite the well-known geometrical stress concentration factor of such singularities. Therefore this paper deals with the investigation of machining-induced surface integrity when turning a fillet radius in a 316L austenitic stainless steel. Different characterization methods are used for that purpose - SEM, EBSD, nanoindentation and X-Ray diffraction. It points out that the turning-induced consequences are not homogeneous along the machined profile. Residual stresses are strongly affected and microstructure is highly modified over a depth of 80 μm that leads to a mechanical properties gradient. It is evidenced that the average uncut chip thickness is the main governing parameter regarding surface integrity. It is also reported that deformation twins appear in the affected zone. It highlights that turning-induced microstructure evolution at a given depth is rather a consequence of severe plastic deformation at high strain rate than dynamic recrystallization. • For the first time, turning-induced surface integrity in 316L is studied along a fillet radius. • Turning-induced affected depth depends on angular position in the fillet radius. • Affected depth is twice larger for longitudinal turning than for face turning. • Microstructure evolution is correlated with average uncut chip thickness parameter and cutting edge radius. • Severe plastic deformation at room temperature or/and high strain rate governs microstructural evolution (deformation twins).

Prediction of spot weld fatigue life using finite element approach

Resistant spot welding is predominantly used in automotive industry for joining sheet metal. Mostly automobile fatigue failure occurs in and around spot welding (80%). So, accurate prediction of the deformation, stresses and fatigue life of spot weld is necessary. In this work finite element model of spot weld and experimental study of coach peel test of spot welded low carbon steel sheets are presented. The property change in various zones of spot weld is found by nano-indentation test followed by inverse finite element modelling. Spot weld failure happens at the interface between nugget and heat affected zone due to the geometric stress concentration and property change that occurs during the welding process. Solid elements are successful in capturing the geometric stress concentration, but they increase the simulation cost and hence a model with combined shell and solid elements is used. For nugget solid elements are used and tore present all other parts of spot weld shell elements are used. Tensile and fatigue tests on coach peel specimen are done as per ASTM standardD903-98. The simulation results are found to be in close agreement with experimental results.

Notched fatigue of additive manufactured metals under axial and multiaxial loadings, Part I: Effects of surface roughness and HIP and comparisons with their wrought alloys

Additive manufacturing (AM) technology has grown significantly in recent years, as an alternative manufacturing technique to conventional fabrication methods. However, fatigue behavior of AM metals is still not well understood, limiting their applications for critical load carrying components. This is particularly true when component-like features such as geometrical stress concentrations such as notches exist, or complex service loading conditions such as shear or multiaxial stresses are present. In this work notched fatigue behavior of AM metals under axial, torsion, and combined axial-torsion loadings are investigated and contrasted with their wrought counterparts, using two common AM metals, Ti-6Al-4V and 17-4 PH stainless steel. Different post fabrication heat treatments and surface roughness conditions were included. Part I of this two-part publication presents the fatigue test results and comparative analyses. In Part II, both crack initiation (i.e. critical plane-based method) and crack growth (i.e. fracture mechanics-based method) approaches to life predictions are used for predictive analysis and comparisons with the experimental results presented in part I.

Effects of Geometric Stress Concentration in the Design of Thick Compound Cylinders- A Review

Effects of Geometric Stress Concentration in the Design of Thick Compound Cylinders- A Review