Local Stress Concentration Research Articles

Influence of Loading Modes on Stress Concentration Locations in Tubular T-Joints

Influence of Loading Modes on Stress Concentration Locations in Tubular T-Joints

Experimental investigation of the geometry of geocell on the performance of flexible pavement under repeated loading

To evaluate the benefit of geocells of different geometrical configurations for pavement application, full-scale instrumented model tests were performed on pavement sections reinforced with geocells of different geometrical configurations subjected to monotonic and repeated loading. The responses studied were stress distribution in different pavement layers, induced strains in geocell walls, and settlement characteristics. The reinforced sections exhibited a significant reduction in rut depth as well as localized stress concentration compared to the unreinforced section. The reduction in rut depth was found to be influenced by the geocell height as well as weld spacing. The geocell reinforcement was found to distribute the stresses in the subgrade and subbase layers more efficiently, thus reducing the stress concentration in these layers. The strain measurements were found to be higher at the bottom of the geocell walls indicating a higher confinement effect on a lower part of the geocell. In the field, mostly geocells of 356 mm weld spacing and 150 mm height (SW356-H150) are used. However, this study suggests that a geocell of 330 mm weld spacing and 100 mm height (SW330-H100) having approximately 30% lower cost compared to SW356-H150 is as effective in reducing the rut depth and localized vertical stress distribution.

Cyclic behaviour modelling of additively manufactured Ti-6Al-4V lattice structures

The present work is concerned with the numerical modelling of the cyclic behaviour of Ti-6Al-4V lattice structures. In the study, diamond structures of titanium alloy produced by the additive laser powder bed fusion (LPBF) method with different degrees of relative density were used. Realistic geometric models of the studied mesostructures were generated using computed microtomography, taking into account the imperfections of the material resulting from the manufacturing process. The numerical calculations also took into account the actual material hardening curve in the elastic-plastic strain range. One of the achievements of this work is the numerical modelling of cyclic loading of realistic mesostructures with their imperfections. The areas of the mesostructures most susceptible to fatigue cracking have been identified and analysed. True hysteresis loops and values of local stress and strain amplitude were determined at the locations of highest stress concentration in cyclically loaded diamond structures. The main achievement of the present work was the modelling of the macroscopic fatigue life of the investigated mesostructures based on the true values of stress and strain at the locations most exposed to fatigue cracking. For this purpose, a stress criterion for fatigue cracking of Ti-6Al-4V lattice structures fabricated by the additive LPBF method was proposed.

Comprehensive effect of simulated centrifugal force on characteristics of different extent notch-type foreign object damages

To systematically reveal effects of the centrifugal force on macro and micro characteristics and residual stress distributions of notch-type foreign object damages (FODs) on the leading edge of rotating titanium alloy fan/compressor blades, four series of notch-type damages with different depths were obtained under the same impact condition but four kinds of preloading conditions by using a light gas gun equipped with a 4-axial adjustable fixture. Macro and micro characteristics, stress concentrations and predicted numerical residual stress distributions of the notch-type damage impacted under the different preloading conditions were observed, calculated and compared. Finally, a qualitative analysis of fatigue crack initiation locations of FOD specimens was conducted by the comprehensive comparison of locations of stress concentrations, maximum residual stresses, micro damages such as micro cracks.

Characterization of Ti-6Al-4V Bar for Aerospace Fastener Pin Axial Forging

Ti-6Al-4V warm forged fasteners are a critical part of the aerospace industry, as they are used in vast quantities for mechanical joining of components for the fuselage, wing-skin and aero-engine. These components are produced in vast quantities at rapid production rates through multi-blow axial forging However the rate that they are manufactured means that manufacturers rely upon periodic part conformance testing to understand if the part is within tolerance or if any undesirable manufacturing defects such as cracks or underfilling are present. Thus, a right-first-time manufacturing approach is essential to minimize non-conformant scrap. An analysis of the Ti-6Al-4V supplied raw material for axial forging, in a variety of different bar diameter sizes and from different industrial suppliers, was conducted. This was to attempt to understand whether material property variation or operator variation was the root cause for some material behaving differently during the manufacture route. Experimental testing was performed through microstructure characterization and mechanical testing methods. The volume fraction of the β-phase was noted to be marginally higher in material with good forgeability. The hardness of the inner core of the bar appears to be a critical material property for the Ti-6Al4V bar, with an overly hard bar-core hindering forgeability of the bar. This is believed to be due to the hotter central region malleability being key for forgeability. Micro-void porosity was also noted which could lead to stress concentration locations, or crack initiation, and as such is a deleterious property for forgeability. The experienced forgeability of the Ti-6Al-4V bars have been demonstrated to be sensitive to rather small variation in measured microstructure and mechanical property. It is believed that cumulative impacts of small differences, 1% variation in α-phase volume fraction, small variations in elongation to failure, 1% variation in elastic modulus and microhardness profile variation at the center of the bar of less than 10 HV0.3, can combine to significantly impact the forgeability of Ti-6Al-4V bar.

Numerical Investigation on Precipitation Hardening of Mg-Gd Alloys.

The second-phase particles in magnesium alloys could affect the mechanical properties of the material significantly. In this work, 3D finite element models with explicit incorporation of second-phase particles are established. The simulations are calibrated with the experimental results of the Mg-1Gd alloy. The influences of factors, such as the particle distribution, size, and orientation of cylindrical particles, on precipitation hardening are investigated in detail. Three interface conditions between particles and the matrix-perfect bonding and high- and low-strength bonding-are studied at the same time. The interface conditions are shown to exert a stronger influence on precipitation hardening compared to the factors of particle distribution and size. In contrast, the influence of the orientation of cylindrical particles at grain boundaries outweighs the effect of interface property. When second-phase particles are relatively large and all located at grain boundaries, the hardening effect can be improved, and the magnesium alloy shows relatively high flow stress. However, the high hardening effect from the second-phase particles could result in high local stress concentration and possible early failure or low ductility of Mg alloys.

On the Thermally Induced Interfacial Behavior of Thin Two-Dimensional Hexagonal Quasicrystal Films with an Adhesive Layer

The mechanical response of a quasicrystal thin film is strongly affected by an adhesive layer along the interface. In this paper, a theoretical model is proposed to study a thin two-dimensional hexagonal quasicrystal film attached to a half-plane substrate with an adhesive layer, which undergoes a thermally induced deformation. A perfect non-slipping contact condition is assumed at the interface by adopting the membrane assumption. An analytical solution to the problem is obtained by constructing governing integral–differential equations for both single and multiple films in terms of interfacial shear stresses that are reduced to a linear algebraic system via the series expansion of Chebyshev polynomials. The solution is compared to that without adhesive layers, and the effects of the aspect ratio of films, material mismatch, and the adhesive layer, as well as the interaction between films, are discussed in detail. It is found that the adhesive layer can soften the localized stress concentration. This study is instructive to the accurate safety assessment and functional design of a quasicrystal film system.

Grain boundary engineering using self-organized high-index twins produced by twin-twin reactions in β-Ti alloys: A phase field study

Introducing coherent twin boundaries is an effective way to develop submicron/nano-sized grains with improved thermal stability in polycrystals, which exhibit enhanced mechanical properties of metals, such as high strength, high hardness and high fracture toughness. However, crystalline defects, such as dislocations and disclinations, usually arise at twin boundary junctions, which lead to local stress concentration and property degradation. Here we show that a large number of high-index coherent twin boundaries (e.g., Σ11 and Σ19a) can be produced through a well-controlled deformation twinning process. By using metastable β-Ti alloys as an example and analyzing the broken symmetry associated with deformation twinning, we demonstrate that the abnormal high-index twins result from intercorrelated twinning pathways and twin-twin reactions. The formation mechanism of self-organized multi-twin structures has been analyzed systematically through phase field modeling and simulations. It has been found that, under well designed thermo-mechanical condition, regularly distributed high-index twin boundaries and geometrically-compatible twin boundary junctions (i.e., dislocation/disclination-free) can be produced. The stress/strain condition required to obtain self-organized twins has been quantitatively determined, which provides a theoretical guideline for the development of self-organized submicron/nano-twin metals for unprecedented properties.

Atomic-scale understanding of twin intersection rotation and ε-martensite transformation in a high Mn twinning-induced plasticity steel

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions.

Stress Balance in Synthetic Serpentinized Peridotites Deformed at Subduction Zone Pressures

AbstractWeak serpentine minerals affect the mechanical behavior of serpentinized peridotites at depth, and may play a significant role in deformation localization within subduction zones, at local or regional scale. Mixtures of olivine with 5, 10, 20 and 50 vol. % fraction of antigorite, proxies for serpentinized peridotites, were deformed in axial shortening geometry under high pressures (ca. 2–5 GPa) and moderate temperatures (ca. 350°C), with in situ stress and strain measurements using synchrotron X‐rays. We evaluate the average partitioning of stresses at the grains scale within each phase (mineral) of the aggregate and compare with pure olivine aggregates in the same conditions. The in situ stress balance is different between low antigorite contents up to 10 vol. %, and higher contents above 20 vol. %. Microstructure and stress levels suggest the deformation mechanisms under these experimental conditions are akin to (semi)brittle and frictional processes. Unlike when close to dehydration temperatures, hardening of the aggregate is observed at low serpentine fractions, due to an increase in local stress concentrations. Below and above the 10–20 vol. % threshold, the stress state in the aggregate corresponds to friction laws already measured for pure olivine aggregates and pure antigorite aggregates respectively. As expected, the behavior of the two‐phase aggregate does not evolve as calculated from simple iso‐stress or iso‐strain bounds, and calls for more advanced physical models of two‐phase mixtures.

Finite element study on post-screw removal stress in toy poodle radius with different plate designs and screw arrangements.

The study employs finite element analysis to investigate stress distribution in the radius of toy poodles after screw removal. The examination focuses on the biomechanical implications of varied screw hole configurations using 1.5 and 2.0-mm locking compression plates (LCPs) with notched head T-Plates. To provide a noninvasive approach to analyzing the immediate consequences of screw removal from the radius bone in toy poodles. Specifically, it explores the impact of varied plate designs and screw arrangements on stress distribution within the forelimb bones. The study constructs a three-dimensional bone model of the toy poodle's forelimb based on computed tomography (CT) images. Simulations were designed to replicate jumping and landing from a 40 cm height, comparing stress distribution in the radius post-screw removal. The analysis reveals significant variations in stress distribution patterns between the two LCPs. The radius implanted with the 2.0-mm LCP displays a uniform stress distribution, contrasting with the 1.5-mm plates. Localized stress concentration is observed around the screw holes, while trabecular bone regions near the screw holes exhibit lower stress levels. The study highlights the plate designs and screw configurations that affect bone stress in toy poodle forelimbs post-screw removal. The findings provide valuable insights for veterinarians, aiding informed decisions in veterinary orthopedic practices.

Parameters Influencing the Fatigue Behavior of Ti6Al4V

The fatigue strength of components is affected by loading and environmental conditions, their geometry, the material and the manufacturing process adopted for their production. As a consequence of local stress concentrations, fatigue cracks can propagate to failure from the defects created by the manufacturing processes or introduced during the service life, as occurs in impact-damage. Ti6Al4V titanium alloy is commonly used in the aeronautical sector and other engineering applications due to its high specific strength. The study of the fatigue behavior of Ti6Al4V is therefore critical for designing reliable and durable structures. The work reports a synthesis of the results present in the literature for the fatigue behavior of Ti6Al4V in inert and aggressive environments, in the presence of a deposited coating, in the case of impact-damage, when components are produced by additive manufacturing and in the absence of solution treatment and over-aging.

Defect-induced fracture topologies in Al2O3 ceramic-graphene nanocomposites

Models of ceramic-graphene nanocomposites are used to study how the manufacturing process-dependent arrangement of reduced graphene oxide (rGO) inclusions governs nano-crack network development. The work builds upon recent studies of such composites where a novel combinatorial approach was used to investigate the effect of rGO arrangements on electrical conductivity and porosity. This approach considers explicitly the discrete structure of the composite and represents it as a collection of entities of different dimensions - grains, grain boundaries, triple junctions, and quadruple points. Here, the combinatorial approach is developed further by considering the effects of rGO agglomerations, stress concentrators and adhesion energies on intergranular cracking. The results show that the fracture networks can be effectively controlled by the local ordering of rGO inclusions to allow for a concurrent increase in the strength and conductivity of the ceramic composites. It is shown that the ratio of local stress concentrators related to rGO inclusions and cracks is the most significant factor affecting the nano-crack network topology. The local spatial arrangement of rGO inclusions becomes an effective tool for controlling nano-crack network topology only when this ratio approaches one. It is anticipated that these results will inform future design of toughness-enhanced composites.

A closed-form analytical variational approach to thermally induced thin-film delamination

Thin polymeric films applied on stiff substrates are prone to delamination upon thermal loading. Different thermal expansion coefficients and different elastic properties can cause localized stress concentrations in the interface between applied layer and substrate. The present work proposes a variational approach to compute residual stresses and strains within the film. Using finite element calculations, it is shown that the presented closed-form analytical solution provides accurate interfacial stresses. Employing a critical distances failure criterion, these stresses are used to predict thermally induced crack nucleation. The proposed model is validated using experimental data of thermally loaded epoxy–glass specimens. It is in good agreement with the test data and provides predictions for thin-film delamination.

Enhancing strength-ductility synergy in an extruded Al-Cu-Li-Mg-Ag alloy via homogeneous GP zones and dislocation configuration

Balancing strength and ductility proves challenging in high-strength wrought Al-Cu-Li alloys, particularly in extruded rods. The abundance of subgrain structures hinders dislocation movement, inevitably sacrificing the ductility. Herein, an effective strategy for enhancing the strength-ductility synergy in an extruded Al-3.7Cu-1Li-0.40Mg-0.75Ag alloy was reported by developing a novel combinatorial pretreatment (CPT), including pre-ageing after solution treatment, followed by pre-stretching. Subsequently, the CPT alloys undergo artificial ageing at 150 °C. Desirable comprehensive mechanical properties (YS = 668 MPa, UTS = 700 MPa, and EL = 12.5 %) were obtained through pre-ageing at 110 °C before 4 % pre-stretching. Notably, the elongation showed a remarkable improvement of 60.3 % compared to the non-CPT sample, without sacrificing the yield strength. Microstructure analysis indicates that the interactions between the GP zones and dislocations facilitated energy dissipation during pre-stretching, effectively mitigating local stress concentration. Consequently, promoted ductility was realized through a more uniform dislocation distribution. Furthermore, the dense GP zones formed during pre-ageing served as the heterogeneous nucleation sites for precipitates, increasing the volume fraction of θ′ phases. This adjustment in dislocation motion further contributed to ductility improvements. Our approach offers new insights into developing high-performance extruded Al-Cu-Li alloys with both high strength and desirable ductility.

Investigation of tensile deformation behavior of pure titanium at 20 K

The cryogenic tensile properties and deformation behaviors of pure titanium at liquid hydrogen temperature (20 K) were investigated. Compared with 298 K, high ultimate tensile strength (UTS) of 978 MPa and the excellent elongation-to-fracture (El) of 48 % were achieved. A large number of {101‾2}<1‾011 > and {112‾2}<112‾3‾ > deformation twins released local stress concentration and lead to high work hardening rate, which endow pure titanium with excellent strength and ductility. In addition, the inhibition of multiple necking of pure titanium at 20 K is due to a large number of deformation twins released the stress concentration and produced a strong dynamic hardening effect, which relieved the local plastic deformation. And the serrated stress-strain curve is caused by intermittent activation of twining and dislocation slip. Finally, the twins are thinner at 20 K are attributed to the more obvious stress concentration. The cryogenic deformation mechanism and fracture behavior are explained from two aspects of dislocation accumulation and thermal physics.

Insight into double twinning behavior of Mg–Gd rolled alloy at the nano scale: Role of micro-texture entropy

101¯1}− {101¯2} double twinning is a classical twinning mode with diverse variants in magnesium and its alloys. However, its variant selection criterion and its influence on the mechanical behaviors are still in debate. In this study, we proposed a view of micro-texture entropy to elucidate the double twinning behavior during large strain rolling of Mg–1.5Gd (wt%) alloy. Combined with the high spatial resolution scanning electron microscopy-transmission Kikuchi diffraction (SEM-TKD) technique, the detailed microstructure and micro-texture of the double twins in early stage of evolution were detected, and new insight into the variant selection and evolution of double twins was obtained. The observations suggest that the nucleation events of different double twin variants are influenced by their micro-texture entropy, the variants with lower micro-texture entropy are preferred. While the growth stage of double twins is affected both by their micro-texture entropy and the geometric factors between the primary and secondary twin systems; the former will influence their growth potential and the later can affect their growth space. A novel {101¯1}− {101¯3} double twin was also triggered to release the high local stress concentration during large strain rolling process.

Thermal fatigue analysis and structural optimization of sliding composite leaf spring

The thermal failure mechanism of sliding composite leaf springs (SCLSs) is examined in this research. Bench fatigue testing is used in the experimental investigation of the thermal failure temperature and mode. The findings demonstrate that after 20 min, when the temperature reaches the glass transition temperature, cracks start to form around the bolt hole of the composite body. The findings from bench experiments indicate that thermal fatigue failure of SCLS can be divided into three main forms: delamination failure of the resin-rich layer, the resin-fiber interface, and fibre pull-out failure. The utilization of finite element simulation has demonstrated that when subjected to thermal load, the leaf spring’s internal defects can lead to localized stress concentration. The structure of the spring end is optimized using the insulating panel scheme and the metal joint scheme. The thermal fatigue issue with SCLS is best addressed by the heat dissipation approach, according to bench tests.

Coupled Thermo-Hydro-Mechanical Processes in Fractured Rocks: Some Past Scientific Highlights and Future Research Directions

Coupled thermo-hydro-mechanical (THM) processes in fractured rocks have been a topic of intense scientific research for more than 30 years. The present paper takes a look into the past and highlights some scientific advances which are of an unusual “out-of-the-box” nature, and then looks forward and discusses possible directions of future research in this interesting field of study. Concerning future research directions, we see a trend from a focus on coupled THM processes in single fractures or a few interacting fractures, to the study of coupled THM behavior in complex fracture network systems where the fractures act collectively giving rise to local stress concentration points and points of large pressure gradients. Three examples of future research directions are presented. First is an effort towards identifying characterizing parameters of a fracture network that play a direct controlling role in major coupled THM phenomena (such as induced seismicity and flow channeling), rather than parameters of stochastic distributions of fractures in the network. The second example of research direction is accounting for the heterogeneity and hierarchy of fractures in a fault or fracture zone which has been associated with major THM events in a number of geo-energy projects. The third example is at the opposite end of the first; here it is recognized that in some cases, the coupled THM processes in fractured rocks may be controlled dominantly by only a few key bridges. Identification, characterization, and evaluation of these key bridges should be one of the important research directions in the coming days.

Comparison of fatigue test results of high-strength steel DED-Arc specimens with milled and unmilled surfaces

The surface characteristics of direct energy deposition (DED)-Arc specimens have been previously explored, revealing the dual effect of the surface topography on fatigue strength. This effect includes a reduction in the nominal load-bearing cross-section due to the waviness of the surface and the initiation of fatigue cracks at combined geometric and metallurgical notches from the surface. To complement these findings, this study focuses on the fatigue life and crack initiation of milled DED-Arc samples, e.g., after removal of the geometric notches. Fatigue tests on DED-Arc specimens validate the impact of surface topography on fatigue strength, emphasizing the significance of surface characteristics in determining structural integrity. The additional tests conducted on milled surfaces provide insights into the failure mechanisms specific to these samples. Fracture surface analysis, microstructure characterization, and hardness measurements are performed. For both surface conditions, cracks originate in the interlayer zone, where local hardness is reduced. In unmilled specimens, this is due to the stress concentration effect, and in milled samples, this is due to softened interlayer zones. The fatigue life of milled specimens is comparatively high, demonstrating the importance of optimizing surface characteristics for improved fatigue resistance.Highlights1. Both, unmilled and milled specimens, show crack origins in the interlayer areas. For unmilled specimens, this is because of the local stress concentration in the notch; for milled specimens, it was shown that soft zones between the layers act as the weakest link.2. The fatigue strength of unmilled test coupons is significantly lower than those of milled specimens.3. Unmilled samples exhibit multiple crack origins.