Stress Concentration Zone Research Articles

Fatigue fracture failure investigation of splitter in engine intermediary casing

The splitter fairing welded by sheet metal of stainless steel was installed in the aircraft engine intermediate case. Under operation load, cracks initiated and propagated along the welded seam in the splitter fairing, which seriously threatens the safe operation of the engine. In this work the cracks in the splitter fairing were detected, and the morphological characteristics of the crack surface were analyzed in detail using a scanning electron microscope (SEM). The cracks in the splitter fairing were initiated from the edge of the weld seam, which is the assembly slot for the splitter. The microstructure analysis of the crack initiation zone confirmed that the microstructure defects such as false welding and oxidation are the direct causes of the fatigue failure of the splitter. Mechanical analysis of splitter fairing under random vibration using the finite element method was performed to confirm that the crack initiated at the stress concentration zone during random vibration load.

АНАЛІЗ КОНСТРУКТИВНО-ТЕХНОЛОГІЧНИХ ОСОБЛИВОСТЕЙ ЗАБЕЗПЕЧЕННЯ РЕСУРСУ СЕРЕДНЬОЇ ЧАСТИНИ ФЮЗЕЛЯЖУ В ЗОНІ ВИРІЗУ ПІД ДВЕРІ ЛІТАКІВ ТРАНСПОРТНОЇ КАТЕГОРІЇ

This article is devoted to the analysis of the design and technological features of ensuring the resource of cutouts in the middle part of the pressurized fuselage of transport category aircraft. Aircraft fuselages, due to their functional features, have a large number of different cuts and irregularities. During flight, the fuselage structure perceives loads in the form of transverse and longitudinal forces, bending and torsional moments, which are caused by the general bending of the structure due to the action of aerodynamic, inertial and dynamic loads, as well as overpressure, which is formed in the middle of the pressurized part of the fuselage during operation of the air conditioning system.The main cutouts can be: cutouts for windows, doors, luggage, maintenance and emergency hatches, cutouts for the cargo hatch. Particular attention must be paid to the cutouts that are in the sealed area of the fuselage. The presence of a cutout in the structure of the fuselage shell leads to a weakening of the cross section of the fuselage in the cutout area. As an object of study, a cutout was chosen, which is located in the middle part of the fuselage under the door (1922x1012 mm). The presence of such a cutout in the fuselage shell creates a sufficiently high concentration of stresses, namely in the area of the cutout corner rounding radii. There, in addition to the concentration due to the corners of the cutout, which are rounded, there is also a concentration around the holes for fasteners, which are located near the rounding radii. It is in this zone that there is the highest probability of the formation and development of fatigue cracks.To achieve the desired durability indicators in the zone of significant stress concentration, a number of design and technological solutions are used. The use of constructive and technological solutions provides the requirements of static strength and durability in the areas of cutouts in the pressurized fuselage and puts forward the problem of obtaining a structure with a minimum mass

Multi-Criteria Evaluation of the Failure of CFRP Laminates for Frames in the Automotive Industry.

Methods to predict the fracture of thin carbon fibre-reinforced polymers (CFRPs) under load are of great interest in the automotive industry. The manufacturing of composites involves a high risk of defect occurrence, and the identification of those that lead to failure increases the functional reliability and decreases costs. The performance of CFRPs can be significantly reduced in assembled structures containing stress concentrators. This paper presents a hybrid experimental-numerical method based on the Tsai-Hill criterion for behavior of thin CFRPs at complex loadings that can emphasize the threshold of stress by tracing the σ-τ envelope. Modified butterfly samples were made for shearing, traction, or shearing-with-traction tests in the weakened section by changing the angle of force application α. ANSYS simulations were used to determine the zones of maximum stress concentration. For thin CFRP samples tested with stacking sequences [0]8 and [(45/0)2]s, the main mechanical characteristics have been determined using a Dynamic Mechanical Analyzer (DMA) and ultrasound tests. A modified Arcan device (AD) was used to generate data in a biaxial stress state, leading to the characterization of the material as a whole. The generated failure envelope allows for the prediction of failure for other combinations of normal and shear stress, depending on the thickness of the laminations, the stacking order, the pretension of the fasteners, and the method used to produce the laminations. The experimental data using AD and the application of the Tsai-Hill criterion serve to the increase the safety of CFRP components.

Numerical study of response behaviors of natural gas hydrate reservoir around wellbore induced by water jet slotting

The trial production of natural gas hydrate reservoirs remains poor. Reasonable reservoir reconstruction, which can improve formation permeability, is an important approach to increasing the efficiency and enhancing production. In this work, water jet slotting is proposed to reconstruct an natural gas hydrate reservoir near a wellbore. The spatial slots formed by water jet slotting not only directly constitute high-permeability channels, but also generate disturbances to the surrounding in-situ sediment. Water jet slotting disturbances to nearby sediment was investigated using a three dimensional flow-structure coupling model to evaluate the proposed reconstruction method. The reservoir at the SH2 site in the Shenhu area of the South China Sea was used as the reference. A horizontal slotting arrangement along the vertical well was adopted. The results demonstrate that water jet slotting can change the primary stress state of the sediment around the wellbore, and generate a dominant stress relaxation zone and small stress concentration zone. Within the stress relaxation zone, the in-situ compressive stress was remarkably reduced or even transformed into tensile stress, accompanied by sediment displacement and volumetric expansion strain. This is conducive to loosening the sediment around the wellbore and improving the permeability characteristics. In addition, the influence of the water jet slotting parameters including slot radius, spacing, and number on disturbances to the nearby sediment was studied. Reservoir responses to water jet slotting under balanced and unbalanced bottom-hole pressures were compared and analyzed. This study provides a reference for natural gas hydrate reservoir reconstruction using water jet slotting. Cited as: Huang, M., Su, D., Zhao, Z., Wu, L., Fang B., Ning, F. Numerical study of response behaviors of natural gas hydrate reservoir around wellbore induced by water jet slotting. Advances in Geo-Energy Research, 2023, 7(2): 75-89. https://doi.org/10.46690/ager.2023.02.02

Mechanical performance of composite flat specimens and pressure vessels produced by carbon/epoxy towpreg dry winding

The main purpose of this study is to evaluate the effects of certain processing parameters on the mechanical performance of carbon/epoxy towpreg wound composite structures. For this purpose, composite sample productions and their evaluations were conducted in two steps. In the first step, dry winding of carbon/epoxy towpregs was used to produce flat composite plates. Their evaluation was performed by rheological analysis, interlaminar shear tests, and unidirectional tensile tests. In the second step, towpreg dry winding was used to produce composite pressure vessel samples. Their performance was evaluated by observing the effects of various winding process parameters on the safety of the vessels via hydrostatic burst pressure tests. Compared to the traditional wet filament winding, the main difficulty observed was maintaining the “straight towpreg path” necessary for efficient winding operations. This problem was prevented by applying higher tension forces during dry winding. Evaluation of the hydrostatic burst tests in terms of burst pressure, hoop strain and safe failure mode revealed that the optimum pressure vessel performance could be obtained in the vessel samples with “helical-hoop-helical winding layer sequence.” On the other hand, use of “complex helical pattern” resulted in no advantages at all, due to basically higher number of undulation zones acting as stress concentration zones.

Моделювання впливу кільцевого включення на концентрацію напружень навколо видовженого еліптичного отвору у сферичній оболонці

Shell structures are widely used in various branches of technology and industry due to a combination of a high strength and a relatively light weight. In the majority of cases, actual structures have openings for manufacturing or design reasons, thus leading to a sharp increase in local stresses and, as a result, to a decrease in the strength and reliability of the structure as a whole. That is why reducing stress concentration in thin-walled structural elements is an important and topical problem in deformable body mechanics. This paper presents the results of a computer simulation and finite-element analysis of the stress and strain field of a thin-walled spherical shell with an elongated elliptical opening and an annular inclusion that surrounds the opening at a certain distance therefrom. The effect of the geometrical and mechanical parameters of the inclusion and its distance from the opening contour on the concentration of the stress and strain field parameters of the shell is studied. The stress and strain intensity distribution in the local stress concentration zones is obtained. It is shown that a rigid annular inclusion located at a certain distance from an opening allows one to reduce the stress concentration factor by nearly 27 percent with a proportional decrease in strain intensity in the vicinity of the opening. The elliptical opening elongation degree greatly affects the concentration of the stress and strain field parameters. If an opening is reinforced with a rigid annular inclusion immediately along its contour, the stress intensity in its vicinity increases, while the strain intensity decreases. The numerical calculations conducted show that surrounding an opening with a rigid annular inclusion located remotely therefrom reduces both the stress and the strain intensity in the vicinity of the opening. If an opening is reinforced immediately along its contour, a decrease in the maximum strain intensity is somewhat greater in comparison with the case where the rigid annual inclusion surrounding the opening is located at some distance therefrom. The use of specially selected and located reinforcements of elongated elliptical openings in spherical shells allows one to control the stress and strain intensity distribution and magnitude in the zones of local concentration of their stress and strain field parameters.

Fretting Characteristics of Rubber X-Ring Exposed to High-Pressure Gaseous Hydrogen

The rubber ring is an essential component of high-pressure hydrogen storage systems. However, the fretting damage can lead to the seal failure of the rubber ring, which may cause hydrogen leakage. Rubber X-ring has been proven to own excellent static sealing performance, while its fretting characteristics under high-pressure hydrogen remain unclear. In this study, a numerical model is developed to explore the fretting characteristics of the X-ring combined seal, in which the effect of hydrogen swelling is well considered. The stress distribution of the fretting seal and the effects of fretting amplitude, friction coefficient, hydrogen pressure, and pre-compression ratio on the fretting behavior of the X-ring are investigated. Moreover, the similarities and differences in the fretting performance of X-ring and O-ring under high-pressure hydrogen are discussed. It is shown that the evolution of the stress concentration zone inside the X-ring is closely linked to the cover’s drag direction and the wedge-ring’s blocking action. The X-ring enters the fretting cycle earlier with increased fretting amplitude, whereas the fretting period remains unchanged. Furthermore, increasing the hydrogen pressure and pre-compression ratio could enhance the fretting sealing performance. The friction coefficient and hydrogen pressure increase the possibility of failure due to cracks or elasticity. The fretting characteristics of the X-ring are superior to those of the O-ring, denoting that X-ring may be an alternative to the O-ring in the fretting seal structure. This work is of theoretical importance for the quantitative evaluation and reliable prevention of fretting damage to rubber rings and improving the safety of high-pressure hydrogen storage systems.

Research Progress on Stress–Fracture–Seepage Characteristics for Hazard Prevention in Mine Goafs: A Review

Large-scale coal mining has created many goaf areas, which have become one of the most frequent sources of mine hazards. Investigations on the stress–fracture–seepage characteristics around goafs could help with identifying and controlling goaf-area hazards. Scholars have conducted theoretical analyses, similar simulation experiments, numerical simulations, and field measurements to analyze the multifield coupling development of mining stopes, including the stress variations, fracture advancement, and permeability-change characteristics. In the longwall-mining process, a stress-relief zone is formed above the goaf area, while a stress-concentration zone is formed in the adjacent coal seams. Mining-stope fracture goes through a process of stress-relief expansion, stress-recovery closure, and end-fracture expansion. The permeability of coal rock in mining stopes rises in all directions with the increase in the fracturing ratio. Further studies could concentrate on the distribution characteristics of the abutment pressure around the goaf area. A permeability model based on the coupling of the coal stress, damage, gas adsorption, and desorption is expected to be established to improve the accuracy of the permeability prediction and seepage analysis at the boundary of the goaf area. Relevant studies could provide better theoretical guidance for preventing hazards, such as gas-related incidents and coal spontaneous combustion in the goaf, as well as for the stability control of the goaf boundary.

Rockburst mechanism and the law of energy accumulation and release in mining roadway: a case study

The rockburst dynamic disasters in the process of deep coal mining become more and more serious. Taking the rockburst occurred in the 23130 working face of Yuejin Coal Mine as the engineering background, we study the characteristics of mining stress field around roadway, the plastic failure morphological characteristics of surrounding rock and the accumulation/release law of elastic energy before and after burst. An analysis model quantitatively describing the physical process of rockburst in the mining roadway is established, and the calculation method of dynamic release of elastic energy in the physical process of rockburst is educed. The mechanism of rockburst in mining roadway is revealed. The results show that an “L-shaped” stress concentration zone is formed within 100 m of the 23130 working face, and the principal stress ratio of the surrounding rock of the transportation roadway is 2.59–4.26. The change of the direction of the maximum principal stress has a significant effect on the burst appearance characteristics. The failure strength of different sections of the mining roadway is characterized by the elastic energy release value. With the increase of the working face distance, the elastic energy released by burst failure and the expansion variation of failure boundary radius show a nonlinear variation law that tends to decrease steadily after sharp fluctuation. The closer to the working face, the higher the burst risk. At a distance of 10 m from the working surface, the maximum principal stress reaches its maximum value. The butterfly-shaped failure system generated by the surrounding rock of the roadway has energy self-sustainability, and the elastic energy released by the sudden expansion of the butterfly leaf is enough to cause a burst damage of 1.9 magnitude. This work could provide theoretical support for the prediction and prevention of rockburst.

Optimum utilization of on-demand manufacturing and laser polishing in existence of supply disruption risk

3D printing has moved from being a rapid prototyping tool to an additive manufacturing method within the last decade. Additive manufacturing can satisfy the need in dire situations where spare parts distribution is an issue but access to a 3D printer is much more likely and rapid than access to original parts. Managing inventories of spare parts can be tackled with more ease thanks to the reduced part types with additive manufacturing. While quality (in terms of reliability) of additively manufactured spare parts in terms of mechanical properties seem to be lower than original parts (particularly due to the inherent staircase appearance and the corresponding stress concentration zones that can lead to premature fatigue failure), use of post-processing subtractive techniques to correct such surface irregularities are found to improve reliability. While each process adds another layer of complexity to the cost minimization problem, demand uncertainty and risk of supply disruption represent the modern global problems faced recently. The problem tackled in this study is the joint optimization of the supply reliability considering the effect of laser polishing parameters and the demand uncertainty. In this problem, a condition of random breakdowns of identical products is considered. Also, the original supplier of machine components is subject to exogenous disruptions, such as strikes, raw material scarcity, or the COVID-19 pandemic. As a result, the optimum control policy with the right cost parameters was shown via numerical experiments originated from mathematical analyses. This optimality can be critical in managing the system in the best possible way, particularly during times of unforeseen circumstances such as pandemics.

Study on the Bending Effect and Rock Burst Mechanism of Middle Rock Pillars in Extremely Thick Subvertical Coal Seams

Rock bursts occur in nearly vertical coal seam mines at shallow to moderate burial depths, which endangers safe mining. To study the rock burst mechanisms of nearly vertical and extremely thick coal seams, the characteristics of rock bursts were studied via on-site investigation, and a field test of in situ stress was carried out. The mechanical behavior of rock pillars in the middle of the B1+2 and B3+6 coal seams was analyzed using theoretical and numerical simulation methods. The results show that the horizontal maximum principal stress orientation and the nearly vertical coal seam strike were both 82°. The bending of the rock pillars occurred due to the horizontal unbalanced force, and a large amount of bending energy was accumulated within 50 m above the mining level. Rock pillars were bent toward the B1+2 mining goaf and exerted a reverse bending and squeezing effect on the B3+6 coal seam below the mining levels. In addition to the inclination and compression of the B3+6 coal seam roof, stress concentration zones formed in the B3+6 coal seam, where a large amount of elastic energy had accumulated in the coal-rock mass. Consequently, both the rock pillars and the B3+6 coal body at the mining level are in an unstable state undue to mining disturbance. Rock burst energy theory and numerical calculation results showed that in the stress concentration zones of the B3+6 coal seam, the energy density of the coal mass reached or exceeded its critical value before rock burst occurred, and rock bursts were prone to occur under mining disturbances. The in situ microseismic results showed that high-energy microseismic events were mainly concentrated in middle rock pillars around the mining levels and the coal mass in high-stress concentration zones.

Propagation Characteristics of Magnetic Tomography Method Detection Signals of Oil and Gas Pipelines Based on Boundary Conditions.

The magnetic tomography method (MTM) is a non-contact external inspection method for detecting metal magnetic memory signals. It has great potential for application in long-distance oil pipeline and subsea pipeline inspection. However, the spatial distribution characteristics and propagation laws of magnetic signals are not yet clear, which makes the MTM passive detection. In this study, a three-dimensional mathematical model of the magnetic field distribution of the stress concentration zone outside the pipe was established based on the boundary conditions. For the two cases in which the stress concentration zone was located at the top and bottom of the inner wall of the pipe, the model was solved by finite element analysis. The variation law of the magnetic signal outside the pipe was analyzed, and experiments were designed to verify the model. The results show that the shape of the magnetic memory signal remained unchanged after passing through the pipe wall. As the magnetic permeability of the pipe medium is much larger than that of air, the magnetic memory signal is significantly attenuated after penetrating the pipe wall. As the detection height increases, the magnetic induction outside the tube decays exponentially. The results also prove that the magnetic tomography method can detect the stress concentration zone at any position of the pipeline, and the detection accuracy is higher when it is located at the top of the pipeline.

Characterising the stress ratio effect for fatigue crack propagation parameters of SAE 1045 steel based on magnetic flux leakage

The aim of this study was to determine the relationship between fatigue crack length, a, the number of cycles, N, the magnetic flux gradient intensity, dH/dx and the stress ratio, R. Fatigue crack growth tests were performed using a constant tensile load amplitude on SAE 1045 steel in the form of single-edge cracks. Stress ratio values of 0, 0.1, 0.2, 0.3 and 0.4 were used to study the characteristics of the magnetic flux gradient. A crack opening displacement device was used to detect the parameters of the stress intensity factor range, ΔK, fatigue crack growth rate, da/dN, and N. Meanwhile, a metal magnetic memory sensor device was used to detect the H value at the stress concentration zone when a 1 mm-increase in the crack occurred. The experimental results showed that as the fatigue cycle and crack length increased for each R, dH/dx also increased exponentially. The governing equations were derived based on the magnetic flux gradient for the fatigue crack length equation and the fatigue cycle equation, with the stress ratio. Based on a statistical analysis of the magnetic flux leakage signals, it was found that all the data were within a range based on a 90 % confidence level. Thus, the magnetic flux leakage parameters could be used to construct fatigue crack growth behaviour models for ferromagnetic materials and potentially as an alternative method for predicting the fatigue life.

Major controlling factors and hydrocarbon accumulation models of large-scale lithologic reservoirs in shallow strata around the Bozhong sag, Bohai Bay Basin, China

Based on the practice of oil and gas exploration and the analysis of shallow lithologic reservoirs, combined with the allocation relationship and enrichment law of oil and gas accumulation factors, main controlling factors and models of hydrocarbon accumulation of large lithologic reservoirs in shallow strata around the Bozhong sag are summarized, and favorable exploration areas are proposed. The coupling of the four factors of “ridge-fault-sand-zone” is crucial for the hydrocarbon enrichment in the shallow lithologic reservoirs. The convergence intensity of deep convergence ridges is the basis for shallow oil and gas enrichment, the activity intensity of large fault cutting ridges and the thickness of cap rocks control the vertical migration ability of oil and gas, the coupling degree of large sand bodies and fault cutting ridges control large-scale oil and gas filling, the fault sealing ability of structural stress concentration zones affects the enrichment degree of lithologic oil and gas reservoirs. Three enrichment models including uplift convergence type, steep slope sand convergence type and depression uplift convergence type are established through the case study of lithologic reservoirs in shallow strata around the Bozhong sag.

Experimental Research on Fatigue Crack Growth Behavior of Diffusion-Bonded Titanium Alloy Laminates with Preset Unbonded Areas.

This paper aimed to study the fatigue crack growth behavior of diffusion-bonded titanium alloy laminates (DB-TAL) with preset unbonded areas using an experimental method to understand the toughening mechanisms of presetting unbonded areas in DB-TAL. For two series of specimens of DB-TAL with preset unbonded areas with an open hole, which have a pre-notch at the open hole edge, fatigue experiments under tension–tension cyclic loading were conducted. The fatigue crack growth process, the crack growth rate, and the stress intensity factor on the crack front were analyzed. The results showed that the preset unbonded area leads the crack away from the stress concentration zone and slows down the crack growth rate. Therefore, the preset unbonded area significantly improved the fracture property of DB-TAL.

Development and justification of universal designs for energy tests in flow paths of hydroelectric power plants

Introduction. The authors present one of methods for measuring water flows through the intake of a hydroelectric power plant. The new structure has a metal frame and a folding rotary row. The authors analyzed the advantages of the proposed, and mase strength and hydraulic analyses. Computational studies of the stress-strain state, made with account taken of the actual hydrodynamic pressure, allow choosing the optimal position of measurement points, designing a frame structure, and making highly accurate measurements of energy characteristics.
 Materials and methods. Top international publications, as well as archived materials, were analyzed to select the universal frame structure. The most promising directions were identified; the advantages and disadvantages of the proposed solutions were taken into account. Complex computational studies were performed using ANSYS Mechanical, a universal industrial software package, and ANSYS CFX, a specialized module for modeling flows of liquids and gases with account taken of turbulence.
 Results. The position of measurement points that ensure the least distortion of the flow and tilt angles of hydraulic turntables were determined during the hydraulic simulation. The flow loads were taken into account when the stress-strain state of the universal frame structure was calculated; optimal design solutions were selected to ensure the strength and reliability of metal elements. Stress concentration zones were identified for monitoring purposes during installation.
 Conclusions. Given the mathematical modeling data and experimental field studies, a universal frame structure for energy tests was substantiated. The new design ensures a measurement error of ±0.67 %, which corresponds to the leading world standards.

Detection of Uniaxial Fatigue Stress under Magnetic Flux Leakage Signals using Morlet Wavelet

This paper demonstrates the application of continuous wavelet transform technique for magnetic flux leakage signal generated during a uniaxial fatigue test. This is a consideration as the magnetic signal is weak and susceptible to being influenced by an external magnetic field. The magnetic flux leakage signal response of API steel grade X65 is determined using Metal Magnetic Memory under cyclic load conditions ranging from 50% to 85% of the UTS. To facilitate further signal analysis, the magnetic flux gradient, the dH(y)/dx signal were converted from a length base into time series in this study. Magnetic flux leakage readings indicated a maximum UTS load of 56.5 (A/m)/mm at 85%, where a higher load resulted in a higher reading and the signal contained Morlet wavelet coefficient energy of 1.02×106 µe2/Hz. As increasing percentages of UTS loads were applied, the signal analysis revealed an increasing linear trend in the dH(y)/dx and wavelet coefficient energy. The analysis revealed a strong correlation between the wavelet coefficient energy and the dH(y)/dx amplitude, as indicated by the coefficient of determination (R2) value of 0.8572. Hence, this technique can provide critical information about magnetic flux leakage signals that can be used to detect high stress concentration zones.

Microstructure-based finite element analysis of mesoscale strain localization and macroscopic mechanical properties of aluminum alloy 2024 laser weld joint

The microstructure and residual stress can be modified by laser welding, which significantly affect the strength and ductility of materials. In this work, a microstructure-based finite element model is established to investigate the mesoscale strain localization and the macroscopic mechanical properties of the 2024 aluminum alloy laser weld joint. The residual stress in different welding processes is predicted, and its effect on the elastic-plastic constitutive model of constituent phases is evaluated. The stress-strain curves of α-Al phase and eutectic phase are calculated using instrumented indentation method. Actual microstructure graphs of the joints are used as the representative volume elements in the model. The mesoscale strain distribution, macroscopic deformation response and the damage initiation are predicted quantitatively during uniaxial tensile deformation. The model is validated by comparing with experimental results, i.e., fusion profile, distortion, strain localization bands, tensile test curves, and good agreements are achieved. It is found that the strain and stress in the constituent phases of the fusion zone are extremely inhomogeneous. There are significant strain localization bands in the α-Al phase and high stress concentration zones in the eutectic phase. The dendritic arm spacing and eutectic phase size decrease, and the percent of eutectic increases during high power matching with high welding speed within certain limits. In this case, the bearing capacity of eutectic phase increases, and the strain inhomogeneity decreases. As a result, the strength (358.8 MPa) and ductility (6.21%) of the joint can be increased by the optimized laser power (5 kW) and welding speed (140 mm/s).

Numerical study on rock failure around a tunnel destressed by a conceptualized notched technique

A destressing method for reducing the strainburst risk in burst-prone grounds is suggested. In this method, the rock is destressed by cutting notches at the excavation boundary. First, the concept of the proposed method is described both analytically and numerically. Then, the method is applied to a tunneling problem. Several numerical models are built to study the destressing process and the failure mechanism around a circular tunnel. Results show that when the notch is added to the model, the rock at the tunnel wall is destressed, and the stress concentration zones shift to a farther distance away from the wall. Also, the analysis of the failure zone around the tunnel and the velocity of the failed elements show that the failure in the notched tunnel is less violent compared to that in the tunnel without the notch. Finally, a parametric study is conducted to investigate the influences of the notch dimensions on the stress distribution, deformation, and failures around the tunnel.

Surface roughness effect on multiaxial fatigue behavior of additively manufactured Ti6Al4V alloy

Additive manufacturing (AM) is a special technology that offers several advantages compared to the conventional methodologies. For instance, it allows to build components with complex geometry, difficult to make in a different way, build integrated parts deleting joining issues, etc. However, its application in engineering fields is still limited because the dependency between the mechanical properties of the obtained components and the manufacturing parameters is not in depth understood. Improve the performances of AM components, under static and dynamic loadings, is crucial to ensure their durability and reliability and in recent years it became a challenging research field. In this work, the effect of the surface roughness on the multiaxial fatigue resistance of Ti6Al4V thin-walled tubular samples, made by the Selective Laser Melting (SLM) process, was investigated. In particular, experiments under combined axial–torsional loadings were carried out on two batches of samples, made by different surface roughness (machined and as built samples). Maximum valley depth Rv was used as a representative parameter of the surface roughness as it geometrically represents a stress concentration zone where crack initiates and propagates. An effective strain intensity factor range, based on the modified Smith Watson and Topper (MSWT) model, that includes the roughness parameter Rv, is proposed and used for a better correlation of the fatigue data. To prove the reliability of the proposed model, the Socie and the Reddy & Fatemi effective strain-based intensity factor range were also used but a bigger scattering of results was observed. The MSWT model was also applied to predict the failure plane and the obtained results were compared with the predictions of the Fatemi-Socie model. Results revealed that, for SLM Ti6Al4V alloy components, the MSWT model is more accurate.