Additional Stress Concentration Research Articles

Criterion for void nucleation by particle debonding in metals obtained from molecular dynamics simulations

The critical conditions for void nucleation by particle debonding at a θ-particle in an aluminum matrix are identified through a comprehensive set of molecular dynamics simulations spanning a range of stress triaxialities and Lode parameter values. Specifically, it is determined that nucleation occurs when the local normal stress at the matrix–particle interface reaches a critical value of 8.14 GPa. When plasticity occurs in the matrix prior to nucleation, an additional plastic stress concentration is observed as a result of the plastic strain build up around the particle. Our results indicate that this plastic stress concentration factor increases roughly linearly with equivalent plastic strain, independent of triaxiality and Lode parameter. Comparing our results with the literature, we observe a much stronger influence of plastic strain than is predicted by continuum plasticity theory. We argue that this difference derives from the fact that nucleation is fundamentally driven by local stress hot spots resulting from the heterogeneity of plastic strain, while continuum theory is based on homogenized plastic strain fields. These results provide guidance for development of ductile fracture engineering models, while giving a warning regarding the use of continuum theory to predict damage initiation phenomena in metals.

Fatigue Studies of Welded Stainless Steel 304L: Effect of Manufacturability and Weld Type, Choice of Fatigue Model, and Comparison to Pristine Material

Abstract This study investigates fatigue life and fracture in welded stainless steel 304L (SS 304L) resulting from load-controlled fatigue. This was done for specimens from two welding manufacturers. Mechanical testing was carried out using a MTS test frame in uniaxial tension. Specimens fabricated in 3 weld configurations (butt, lap, butt-with-backer) were cycled at stress ratio R = 0.1, frequency f = 5 Hz, and experienced varying mean tensile stress. A comparison of the fatigue life of welded and unwelded (or pristine) specimens determined that the fatigue life of welded specimens is considerably shorter. The microstructure in the parent material (PM), heat affected (HAZ) and fusion zones (FZ) was analyzed using SEM, and results suggest that, for a limited number of specimens (backer welds), fracture in the (FZ) was primarily the result of Zener-Stroh cracks. Most welded specimens fractured at the weld-HAZ interface (weld toe), and we postulate that stress concentrations primarily due to abrupt geometrical change at the weld interface led to crack initiation there. Additional stress concentrations at welding induced defects (resulting from high temperature) such as second phase particles, fatigue induced defects, and pre-existing impurities are responsible for crack initiation, crack growth, and eventual fracture. Comparison of unwelded and welded SN curve data using numerical curve fitting methods reveal that the fatigue life of pristine SS 304L follows the expected sigmoid shape found in literature (specifically the Chandran fatigue model), while that of welded SS 304L deviates considerably in low cycle fatigue (LCF) and follows the Basquin power law equation.

Fatigue reliability assessment of load‐carrying cruciform welded joints with undercuts and misalignments

AbstractThe additional local stress concentration caused by undercut and misaligned defects could lead to lower fatigue strength of welded joints. The paper aims to investigate the effects of undercuts and misalignments on fatigue performance and reliability of Load‐carrying Cruciform Welded Joint (LCWJ) quantitatively by probabilistic statistics theory and fracture mechanics theory. Firstly, the geometrical characteristics of undercut and misalignment are summarized from the literature and our experiments. Subsequently, the probabilistic distributions of material fracture properties, the configurations of geometries, and flaws were evaluated under specific loading conditions. The fatigue strengths are further estimated by the probabilistic reliability theory by combining the actual fatigue data. The results demonstrate a clear disparity of fatigue crack growth threshold and resistances for the tested data of base metal and weldment. The level of defects determines the interval of fatigue life, and the larger the dispersion accompanying earlier fatigue failure occurs.

Creep behavior of different elliptical film cooling holes in complex temperature fields

This paper investigates the creep damage of film cooling holes with varying ellipticity within complex temperature fields. The temperature field, derived through the conjugate heat transfer method, serves as a boundary condition for crystal plastic finite element simulations. We discovered that film hole patterns with an ellipticity (λ) of ≤0.5 exhibit superior cooling effectiveness, particularly at a blowing ratio (B) of 1.0. For λ = 0.5, these holes demonstrate exceptional cooling effectiveness and mechanical properties. Reducing the ellipticity of film holes induces a shift in the high-stress region around the hole, subsequently diminishing stress concentration on the acute side of the hole. Moreover, the temperature difference between the edges on both sides of the hole perimeter and the upstream zone on the high-temperature surface for hole patterns with λ ≤ 0.5 introduces additional stress concentrations, with a significant temperature difference causing creep damage in this region.

A numerical study on stress mitigation in through-thickness tailored bi-adhesive single-lap joints

In bi-adhesive single-lap joints (SLJs), longitudinal grading of adhesive compliance leads to peak stress concentration to the bi-adhesive interface (due to mismatch in properties). Due to this additional stress concentration at the bi-adhesive interface, the failure initiation threshold is lowered. In this paper, the stress distribution in the compliance-tailored bi-adhesive joints is investigated numerically. Peak stress at the bi-adhesive interface is diffused by geometrically grading the bi-adhesive interface in the longitudinal and transverse (through the thickness) directions. Linear and non-linear (material and geometric) finite element (FE) analyses were performed for two different set of bi-adhesive bondlines and joint configurations. FE results were benchmarked with published experimental and numerical data. Peak peel stress and peak shear stress are decreased by 29–70% and 8%, respectively, when the bi-adhesive interface is graded geometrically in the transverse and longitudinal directions as opposed to longitudinal tailoring alone. Moreover, the bi-adhesive interface’s stress concentration is reduced significantly. This leads to a more homogeneous stress distribution throughout the bond-layer. A thorough investigation reveals that through-thickness compliance tailoring of the bi-adhesive in SLJ can be used to increase tolerance to high load levels, achieve uniform stress distribution along the bond-layer, and reach the optimum joint stiffness.HighlightsA new approach for stress mitigation involving through the thickness tailoring of adhesive is introducedProposed approach is validated for two sets of bi-adhesive systems and two joint configurationsFinite element results are validated with published experimental dataLoad carrying capacity of joints is enhanced by diffusing peak stresses away from the bi-adhesive interface

Stress amplification effect and fatigue strength evaluation of marine thin plate welded structure considering welding deformation: Theoretical and experimental analysis

Due to the low bending stiffness of thin plate welded joints, welding deformations such as welding dislocation, welding angle deformation and initial welding deformation are easy to occur in the welding process. The existence of these deformations will cause additional stress concentration effect and reduce the fatigue performance of thin-plate welded structures. Therefore, the influence of various welding deformations should be considered when evaluating the fatigue strength of the welded joints of thin plates. Combined with the test data, the calculation formula of stress amplification factor considering various welding deformations proposed by this paper is compared with the calculation formula in the existing literature and finite element calculation results. Moreover, the analytical formula is applied to the fatigue assessment of 5059 aluminum alloy welded joints. According to the evaluation results, it is found that the structural stress method and equivalent structural stress method based on the proposed bending stress amplification coefficient formula have good evaluation results for aluminum alloy thin plate specimens with welding deformations. Fatigue strength class curves suitable for thin plate aluminum alloy welded joints corresponding to different fatigue evaluation methods are further suggested.

Mixed-mode load bearing estimation of the cracked additively manufactured materials using stress/strain-based models

PurposeThe main aim of this study is to investigate the mixed-mode I/II failure and the cracking manner of three-dimensional (3D)-printed components made by the fused deposition modeling technique in an experimental and theoretical manner.Design/methodology/approachAcrylonitrile butadiene styrene (ABS) material and a modified printing method (that increases the adhesion and integrity between the layers and strands) are used for manufacturing the semicircular bending (SCB) test samples. In addition to precracking, the effect of additional stress concentration on the stress field is studied by introducing three small holes to the SCB fracture samples. The critical mixed-mode I/II failure loads obtained from the experiments are predicted using different stress/strain-based fracture theories, including maximum tangential stress (MTS), maximum tangential strain (MTSN), generalized form of MTS and MTSN and combination of them with equivalent material concept (EMC). The effects of plastic deformation, as well as the structural stress concentration, are considered for a more realistic prediction of mixed-mode fracture load.FindingsThe stress-based criteria are more suitable than the strain-based theories. Among the investigated fracture models, the EMC–generalized maximum tangential stress theory provided the best agreement with the experimental results obtained from 3D-printed SCB tests.Originality/valueThe influences of stress risers and applicability of different failure theories in cracked layered 3D-printed parts are studied on the fracture behavior of tested specimens under mixed-mode I/II.

Development of Tensile Test Specimens for Fused Deposition Modeling

Standard test specimens are usually used to determine the tensile strengths of plastics. Their shape assumes that the material is homogeneous and only normal stress occur inside their nominal length. Unlike injection molding, which is a common technology to produce plastic parts, the structure of additively manufactured parts is not homogeneous. In areas with variable geometry, an additional internal stress concentration occurs, which often leads to failure of the test specimens outside their nominal length, which in turn compromises the tensile test. The stress concentration increases with the nozzle diameter used. This requires improvement of the test specimens shape. In the present work a new form of the test specimens is proposed, in accordance with the peculiarities of the Fused Deposition Modeling (FDM) process. A series of test specimens have been produced, using a 1mm and 2mm nozzle as well as one wall, no walls and milled specimens with newly developed geometry. All specimens were tensile tested and the results were shown and compared. It has been found that the proposed new shape ensures failure within the parallel length for successful testing, provided infill strands are correctly positioned.

Microstructure Evolution and Shear Strength of Tin-Indium-xCu/Cu Joints

The low melting temperature In-48Sn alloy is a promising candidate for flexible devices. However, the joint strength of the In-48Sn alloy on the Cu substrate was low due to the rapid diffusion of Cu into the In-rich alloy. In this study, the effect of the addition of xCu (x = 2.0 and 8.0 wt.%) on wettability, interfacial reaction, and mechanical strength of the In-Sn-xCu/Cu joint is analyzed. The results demonstrate that both the In-48Sn and In-Sn-xCu alloys exhibit good wettability on the Cu substrate and that the contact angle increases with an increase in the Cu content. Furthermore, fine grains are observed in the alloy matrix of the In-Sn-xCu/Cu joint and the interfacial intermetallic compound (IMC) comprising the Cu-rich Cu6(In,Sn)5 near the Cu substrate and the Cu-deficient Cu(In,Sn)2 near the solder side. The In-Sn-2.0Cu/Cu joint with fine microstructure and a small amount of IMC in the alloy matrix shows the highest average shear strength of 16.5 MPa. Although the In-Sn-8.0Cu/Cu joint also exhibits fine grains, the presence of large number of voids and rough interfacial IMC layer causes the formation of additional stress concentration points, thereby reducing the average shear strength of the joint.

Experimental Study on Physical Similar Model of Fault Activation Law Based on Distributed Optical Fiber Monitoring

The impact ground pressure in coal mining is closely related to the fault structure, and the fault activation pattern is different when the working face advances along the upper and lower plates of the fault, respectively. In this paper, the F16 positive fault in the southern part of Yima coalfield is used as a prototype to carry out the physical similar model test simulating the process of the working face advancing from the upper and lower plates of the fault, and PPP-BOTDA optical fiber sensing technique is used to study the overburden deformation law and fault activation law when the working face is located in the upper and lower plates of the fault, respectively. The study shows that the key stratum breakage is closely related to the fault movement, and the shear stress concentration range occurs within the key stratum. The additional shear stress concentration at the fault surface caused by the working face advancing in the lower plate is much larger than that at the upper plate, which is the reason for the serious fault destabilization phenomenon at the lower plate. The upper rock layer on the fault face is affected by the mining action of the working face before the lower one, and the working face is affected by the fault in a larger range when advances in the lower plate than that in the upper plate, and the risk of fault activation instability occurs earlier when the working face advances in the lower plate than that in the upper plate. The distributed optical fiber sensing technology is used to verify the basic conclusions that the impact of the working face advancing from the lower plate is much greater than that from the upper plate, which is more likely to cause fault activation. The preferential placement of the working face in the upper plate in the fault area will be beneficial to mine pressure control. The results of the study provide an experimental basis for the application of distributed optical fiber sensing technology to the study of fault activation law.

Stress-sensitive fatigue crack initiation mechanisms of coated titanium alloy

The presence of hard coatings can reduce the fatigue life of metal substrates to an unacceptable level, causing the catastrophic failure of load-bearing structures. Unfortunately, the present fatigue crack initiation mechanism is difficult to explain some existing experimental phenomena, and the preventive guidance theory remains absent. Here, by performing tension–tension axial fatigue experiments of TiN coated Ti-6Al-4V alloys, we found that the fatigue crack initiation mechanisms of the coated alloys were stress sensitive, showing two-fold characteristics in S–N curve, which was never reported in previous studies. As the applied stress higher than critical stress, coating fracture induced cleavage cracking of substrate is responsible for the fatigue crack initiation, leading to the fatigue crack source appear at the interface. As the applied stress lower than critical stress, slip step causes coating fracture, forming additional stress concentration on brittle α phase where has already been pressed by dislocations pile-up, thus accelerating fatigue crack initiation at the subsurface. Based on the proposed mechanisms, a ductile chromium interlayer was introduced to decrease coating crack velocity and absorb dislocations escaped from substrate, leading to the increase in the percentage reduction from 40% to 20%. The proposed mechanisms in this work will provide theoretical guidance for designing hard coatings to reduce detrimental effects on metal loading-structure's fatigue life to an acceptable level, even improving substrate's the fatigue properties.

Structural Finite Element Evaluation of Twisted Stacked-Tape Cable Under Cyclic Lorentz Loads

Structural finite element analysis (FEA) was performed on a straight 40-tape Twisted Stacked-Tape Cable (TSTC) surrounded with a solid copper core to investigate the stress state of the tape-stack under cyclic transverse compressive Lorentz loads up to the tenth cycle. Two worst-case stack orientations, 45 degree and 90 degree, were investigated. In addition, the stack was modeled as simply placed into the core or soldered to the core. Since annealing of the copper core can occur during machining; the stress state of the stack was studied as a function of yield strength of the copper. The results showed that if the copper was strong enough, the stress state of the stack remained unaltered and low up to 300 kN/m cyclic transverse compressive loading. However, when the copper was fully annealed, the plastic deformation accumulated from cyclic loading more significantly affected the stress distribution in the stack. If the tape-stack was simply placed into the core, the tapes can move and slide after each loading cycle. The averaged stress went down for the first few cycles, and then plateaued. However, when the stack was soldered to the core, the stack had less room to move, and therefore the stress state remained at higher values and was less affected by cyclic loading. Additionally, when producing the stack, the tapes at the edge may not align perfectly, and this would introduce additional stress concentration. The misalignment of the stack was studied, and the results showed that this effect caused higher stress in the stack. The numerical modeling presented provides a technique to simulate cyclic loading of TSTC cable. Besides, the information on the stress state of the tape-stack can be used to investigate the behavior of more recently developed cable designs and optimize future configurations.

Time-Dependent Mechanical Response of Ice Adhesion on Aluminum Substrates.

Ice adhesion on aerospace-relevant materials is both complex and not well understood. Measuring such adhesion and understanding the underlying physics involved require reliable testing techniques that can yield multifaceted data sets. The latter includes the surface morphology, that is, roughness, and its spatial correlation structure, resolving substrate-induced strain, and direct mechanical testing of adhesion. As part of a continuing investigation of ice adhesion on a relevant surface, we performed time-dependent stress ramps on aluminum surfaces. The temperature range explored, from -20 to -7 °C, allowed spontaneous icing and ice morphologies, namely, below or above -15 °C. Additionally, we characterized the spatial correlation surface roughness maps of the specimens. Our novel test protocol yields reproducible and high-precision results when compared with alternative methods reported throughout the literature. The stress-ramp test data using the proposed protocol show that the apparent average critical stress (proportional to the adhesion strength) depends on both stress-ramp rate and temperature. More specifically, the adhesion strength is higher for higher stress rates and increases with decreasing temperature. The stress-ramp test yields the full span of the time-dependent adhesive behavior of ice and particularly the upper bound. Additional stress-concentration analysis is needed to correct for this effect and thereby yield the critical stress rather than the average value produced by our procedure. The results in this work should aid to improve our understanding of ice adhesion mechanisms.

Dislocation precipitate bypass through elastically mismatched precipitates

The Orowan bypass mechanism for elastically homogeneous precipitates has been thoroughly studied. In engineering materials, alternate phased precipitates exhibit elastic moduli differing from the host matrix. To further our understanding of realistic dislocation precipitate interactions, we employ a three dimensional coupled discrete dislocation dynamics and finite element computational scheme to compute the Orowan bypass stress (τ Orowan) required for a dislocation to bypass a row of elastically stiff precipitates. Specifically, we examine the influence of elastic mismatch between precipitates and the host matrix on τ Orowan. Unique to our computational study, our simulations span a range of precipitate diameters (D), inner precipitate spacings (L), and an order of magnitude in precipitate-matrix elastic mismatch ratio. We partition observed increases in τ Orowan into dislocation image stress interactions and additional stress concentrations due to the interaction of the applied stress with the elastically stiff precipitates. Finally, we incorporate the dependence of τ Orowan on precipitate-matrix elastic mismatch into our derived model for τ Orowan by introducing an effective D which depends explicitly on the elastic mismatch. Both our simulations and analyses suggest that strengthening due to relative increases in precipitate stiffness is modest compared to strengthening with precipitate width.

Non-disruptive method to decrease stresses in the web of the crane beam

Ensuring the required resource of the bearing capacity of the crane beams is one of the most urgent tasks from the point of view of the rational design. The approach proposed in this article is based on the principle of redistribution of the functions of the elements involved in the operation of the crane structure. Weak resistance to rotation of the top flange and the adjoining web zone is a critical disadvantage of a typical crane structure. Additional reinforce elements are proposed to eliminate the consequences of the influence of this disadvantage. At the same time, these reinforce elements practically do not perceive either the vertical or horizontal components of the crane load, preventing only the rotation of the top flange on the length between the beam stiffeners. Another feature is that no additional stress concentration zones are created either in the flange or in the web from welding or drilling holes. The article provides a comparative analysis of the stress-strain state of the crane beams based on the results of numerical calculations carried out with the use of ABAQUS software package. It is shown that the stresses in the upper zone of the beam web, reinforced by the proposed method, are up to 20% lower than in a conventional I-beam. However, despite the simplicity of the solution, the positive results of numerical analysis and the supposed manufacturability of the approach, it is undoubtedly necessary to conduct a series of full-scale experiments on already existing objects.

Equal Load-Carrying Design of Lapped Joints of Al–Cu Dissimilar Materials

In order to avoid the adverse effects of additional moment and stress concentration of traditional lap joints, a new lap joint was put forward, according to the concept of “equal load-carrying”. Through static analysis and brazing characteristics consideration, the equal load-carrying design method of Al–Cu lap joint based on brazing method was established. Through three types of brazing, the relationship among two fracture modes, brazing process and static tension curve of lap joint, was analyzed. The results demonstrated that the selection of solder was required to simultaneously meet the requirements of brazability and mechanical properties. A certain relationship existed between the fracture mode of the lap joint and the static tensile curve, while the segments of the static tensile curve corresponded to the fracture paths of the two fracture modes. When the brazing holding time was quite short, the interface bonding was poor, while the bearing capacity of the joint was low; when the holding time was suitable, the bearing capacity of the joint reached the corresponding highest, while the fracture mode conformed to the equal load-carrying design; when the brazing holding time was quite long, the bearing capacity of the joint remained at a high level, but the fracture mode was the same as the holding time was quite short.

Investigating the potential of FGMs through numerical minimization of thermal stresses

Functionally graded materials (FGMs) have the potential to reduce high stress concentrations near material interfaces. In the ITER divertor monoblock, this may be achieved by replacing the distinct W–Cu interface by an FGM, which gradually changes the coefficient of thermal expansion (CTE) from copper to tungsten. To assess the full potential of FGMs in improving the thermal and mechanical behavior, a first proof-of-principle study is performed in this paper to numerically optimize the tungsten volume fraction distribution of a W–Cu FGM based ITER monoblock for minimal thermal stresses using a simple thermal conduction model. We show that the optimizer can ensure that the temperature remains below the recrystallization temperature of tungsten and that it minimizes the chosen cost function, based on the fractional margin between the local temperature and melting temperature, in a computationally efficient way. Furthermore, the resulting design has a lower Von Mises stress measure at the original position of the W–Cu interface. However, a new material interface is introduced, which results in an additional undesired stress concentration. Future work should therefore aim at integrating a complete stress evaluation in the optimization method.

Long-term mechanical performance of geothermal diaphragm walls in stiff clay

Diaphragm wall equipped with ground heat exchangers is one type of thermo-active foundations, which harness the energy stored by the ground for heating and/or cooling buildings. Past investigations on geothermal diaphragm walls mainly focused on the thermal performance, but paid little attention on their mechanical response to geothermal energy operation. This paper conducts thermo-hydro-mechanical (THM) finite element analyses to investigate the long-term performance of geothermal diaphragm walls in stiff clay. The numerical analyses take account of both the station excavation process in short-term and long-term behaviour of the diaphragm wall. The long-term soil-structure interaction simulation includes three scenarios, examining the effects of ground consolidation, external thermal solicitations and seasonal geothermal operation, respectively. A comparison between the mechanical behaviour of the geothermal diaphragm wall and that of the same wall without geothermal activation indicates that geothermal operation may have an impact on structural serviceability issues (e.g. thermal-induced concrete cracks) although unlikely cause critical safety problems. In particularly, the ground settlement near the station is very sensitive to the stiffness degradation of the stiff clay during geothermal operation, while specific attention should be given to the structural performance at the connections between the wall and slabs due to thermo-induced additional stress concentration.

Considering the stress concentration of fiber surfaces in the prediction of the tensile strength of unidirectional carbon fiber-reinforced plastic composites

The tensile strengths of unidirectional carbon fiber-reinforced plastic (CFRP) were predicted by using a spring element model that considers the surface stress concentration on fibers caused by a fracture site in an adjacent fiber. The surface stress concentration on the fibers was experimentally evaluated by implementing multi-fiber fragmentation tests in conjunction with a spring element model simulation. Four types of epoxy materials were utilized to explore the effects of matrix polymer properties on the surface stress concentration of the fibers. The size scaling results, coupled with the results of the spring element model simulation, designed to take into account the surface stress concentration, were reasonably consistent with the experimental data on the tensile strengths of the unidirectional CFRP composites, regardless of the differences in the matrix mechanical properties. Possible mechanisms by which additional stress concentration is generated on an intact fiber surface were analyzed numerically using the finite element method.

Stress and failure analysis of the connecting rod of diesel engine

Abstract In this study the failure and stress analysis of the connecting rod of turbocharged diesel engine was performed. Visual examination of the fracture showed that beach marks, typical for the fatigue failure were observed. Additional observations of the crack initiation area indicated that the crack origins were not covered by products of corrosion or the material defects. In order to explain the reasons of connecting rod damage, an advanced stress analysis using the finite element method (FEM) was utilized. In order to solve the problem the geometrical models of connecting rod, piston and adjacent components were created. Next the complex loads resulting from both the pressure of gas in cylinder and the inertial forces were defined. The contact between all touching surfaces of considered components was defined. The results of nonlinear static analysis showed that during work of engine with the maximum power the high stress zones were located in crack origins. Obtained results indicated that the bolt tightening torque has a significant influence on maximum value of stress at crack origin. Based on results of performed investigations it was concluded that the main reason of the connecting rod's failure was a high stress levels in zones near the bolt hole caused by high pretension of bolts. The crack initiation process could also be accelerated by small radius of fillet on surface near the bolt hole. In consequence the notch effect caused additional stress concentration in the critical zone of the connecting rod. In last part of the work the recommendations for increase of the fatigue life of connecting rod were formulated.