Stress Concentration Sites Research Articles

Strain rate sensitivity in Zr-based metallic glass: Experiments and molecular dynamics study

This study presents results on the strain rate sensitivity (SRS) of Zr-based metallic glass over a wide range of specimen size (macro and micro) and strain rates (quasi-static and dynamic), and provides information on the correlation among shear band initiate, stress concentration and strain rate in metallic glass. Shear bands readily initiate from stress concentration sites, and metallic glasses are more sensitive to stress concentration at high strain rate. The inevitable stress concentration in bulk metallic glass could result in negative SRS. While its SRS could be enhanced by reducing the stress concentration extent. Even positive SRS could be obtained in micro-pillar metallic glass with little stress concentration. Molecular dynamics simulations show that metallic glass without defects (i.e. stress concentration) exhibits positive SRS, because the atomic mobility is not sufficient for the shear transformation zone operation to initiate a shear band at higher strain rate.

Local deformation and interfacial damage behavior of partially stabilized zirconia-reinforced metastable austenitic steel composites: Numerical simulation and validation

In this work, two simulation models were used to investigate the local deformation and damage behavior of partially stabilized zirconia-reinforced metastable austenitic steel composites and validated by experimental results. An in situ quasi-static tensile test on a miniature dog bone sample is performed under an electron microscope up to 6.4% true strain. A commercial digital image correlation program, VEDDAC, is used to process micrographs obtained at incremental strains to calculate the evolution of the local experimental strain distribution. The initial experimental micrograph was transformed into a geometry file for simulation model. In two different simulation models, the ceramic/matrix interface is assumed once to be perfect, and secondly, the interface is modeled using cohesive elements. The results showed that the stress concentration sites of the cohesive model continuously change along the interface because of the effect of debonding, while it maintains its position in the non-cohesive model. At 6.4 % global strain, the relative error of the strain between the DIC and the cohesive model is only 6 %. The numerical model with cohesive interface accurately predicts local stress and strain, as well as the position and evolution of damage accidents, and can therefore be reliably used by other researchers.

Directed energy deposition of γ/γ' Co-Al-W superalloys

In this study, Co-Al-W superalloys forming γ/γ' microstructures were fabricated by laser-directed energy deposition and investigated with respect to their susceptibility to hot-cracking during manufacturing. Two alloys of different nominal compositions (Co-12Al-8W, Co-7Al-8W (at.%)) were deposited to characterize their microstructures on multiple length scales. While the as-deposited Co-12Al-8W alloy was free of cracks, the Co-7Al-8W alloy exhibited numerous cracks, particularly along high-angle grain boundaries due to internal oxidation cracking. The former Al-rich alloy showed a continuous and dense-packed Al 2 O 3 scale on its surface, preventing internal oxidation. After homogenization heat-treatment at 1300°C for 24 h and subsequent aging at 900°C for 24 h, the Co-12Al-8W alloy revealed a homogeneous γ/γ' microstructure without hot-cracking. By contrast, the Co-7Al-8W alloy did not exhibit a passivation layer but only internal Al 2 O 3 particles due to the reduced Al content and insufficiently fast transport of Al to the oxidation front. These particles acted as stress concentration and crack initiation sites during directed energy deposition, thereby limiting the printability of the Co-7Al-8W alloy. The compressive creep results reveal a similar creep resistance of crack-free Co-12Al-8W alloy compared to conventionally processed ternary Co-Al-W-alloys.

Investigation of the effect of BC layer surface roughness and TC layer porosity on stress values in plasma sprayed coatings based on SEM images

This study investigates the effect of BC surface roughness and TC porosity on the stress values in atmospheric plasma sprayed (APS) thermal barrier coatings using the finite element method based on SEM images. Thermal aging treatment was performed at 1100°C for 12 hours on TBC samples with different BC surface roughness. Grinding and grit blasting treatments were performed on the BC surface to achieve different roughnesses. It was observed that increasing the BC surface roughness increases the contact surface with oxygen and Al diffusion paths towards the BC/TC interface, increasing the TGO thickness. The finite element results show that in the initial state without the TGO layer, after the heating stage, the stress is tensile in the peak areas of the interface and compressive in the valley areas, and with the TGO growth, the stress in the peak becomes compressive and in the valley becomes tensile. By increasing the BC surface roughness from Ra=1.3μm to Ra=3.7μm, the maximum tensile stress in the valley region of the TGO/TC interface reached from 34.84 MPa to 91.70 MPa. As a result, increasing the surface roughness increases the stress values in TBCs. The TC porosity shifts the stress concentration sites to the boundary of pores and reduces the stress at the TGO/TC interface. For this reason, cracks have initiated from the discontinuities within the TC layer in the coating with the highest BC surface roughness. • Effect of BC surface roughness on the TGO thickness was experimentally investigated. • FE simulation was performed to calculate the stress distribution. • Effects of BC layer surface roughness and TC layer porosity on stress values were numerically investigated.

Effect of rotary swaging on microstructure evolution and adiabatic shear sensitivity of 90W–7Ni–3Fe alloy under dynamic loading

Tungsten heavy alloys (WHAs) are widely used in the preparation of kinetic penetrators. However, the penetrating performance of alloys, measured by the “self-sharpening effect”, is often limited by the low sensitivity of the adiabatic shear band (ASB). To address this issue, the 90W–7Ni–3Fe rods with deformation degrees of 20% and 40% were prepared by rotary swaging (RS) at high strain rates (4000 s−1 and 6000 s−1, respectively). The deformation and ASB behavior of RS-treated alloys were compared to those of undeformed samples. According to the results, RS pretreatment significantly increased the dynamic penetration strength and ASB sensitivity of the alloys. Besides, the characterization of the recrystallized microstructure within ASB and the calculation of the adiabatic temperature rise indicated that microstructure softening could be the primary cause for the initiation of ASB, which was consistent with the rotational dynamic recrystallization (RDR) mechanism. Moreover, the higher strain energy of the 40% RS deformation sample with high densities of dislocations and twins promoted the subgrain rotation and RDR initiation, providing a recrystallization prerequisite for the propagation and proliferation of ASB. The recrystallized twins formed after the completion of the RDR in the 40% RS deformation sample in order to adapt to the reloading impeded the movement of dislocations, which became a stress concentration site promoting microdamage more likely to nucleate and expand, making the ASB more prone to fracture. Therefore, this study is of great significance for the development of self-sharpening WHA kinetic penetrators.

Microstructure characteristics and their effects on fatigue behaviors of the bond zone between Al‐12Si/ABOw composites and Al‐12Si alloys

AbstractIn order to research the fatigue properties of the bond zone between aluminum borate whiskers (ABOw)‐reinforced composites and matrix Al‐12Si alloys, specially designed specimens consisted of Al‐12Si/ABOw composites and Al‐12Si alloys (named C/A specimen for short) were used in fatigue tests at different temperatures. The microstructure evolution of the bond zone of C/A specimen was studied through electronic microscope analysis. The results showed that the aluminum borate whiskers could cause segregation of alloying elements in the Al matrix and formation of numerous microscopic pores. Moreover, whiskers that were perpendicular to the load tended to cause interface between whiskers and Al matrix or Si particles cracking and thus lead to deterioration of fatigue performance. With the change of temperature from 25°C to 350°C, the toughness of Al matrix increased and the strength decreased, which also made the most serious stress concentration site of C/A specimen varied at different temperatures.

Effect of S addition on mechanical and machinability properties in austenitic Fe–Mn–Al–C lightweight steels

In this study, the machinability of austenitic LWS and the effect of MnS formation on the mechanical and machinability properties were investigated. For this purpose, S was added over a wide range from 0.0048 to 0.0713 wt% to control MnS inclusions. As the S content increased, the number and area fraction of the MnS inclusions increased linearly, whereas the inclusion size first rapidly increased, and then its slope decreased. The tensile properties hardly changed regardless of the S content, whereas the Charpy impact energy deteriorated linearly with increasing S content. Furthermore, the machinability of LWS was considerably improved by the formation of MnS inclusions, which play a role in the site of mechanical stress concentration and thermal cavity and pore generation during machining.

Investigating the Microstructural and Mechanical Properties of Novel Ternary Reinforced AA7075 Hybrid Metal Matrix Composite.

This study investigates the comparison of the microstructural and mechanical properties of a novel ternary reinforced AA7075 hybrid metal matrix composite. Four samples, including AA7075 (base alloy), AA7075-5wt %SiC (MMC), AA7075-5wt %SiC-3wt %RHA (s-HMMC), and AA7075-5wt %SiC-3wt %RHA-1wt %CES (n-HMMC) were developed using the stir casting liquid metallurgy route, followed by the heat treatment. The experimental densities corresponded with the theoretical values, confirming the successful fabrication of the samples. A minimum density of 2714 kg/m3 was recorded for the n-HMMC. In addition, the highest porosity of 3.11% was found for n-HMMC. Furthermore, an increase of 24.4% in ultimate tensile strength and 32.8% in hardness of the n-HMMC was recorded compared to the base alloy. However, its ductility and impact strength was compromised with the lower values of 5.98% and 1.5 J, respectively. This was confirmed by microstructural analysis, which reveals that n-HMMC has mixing issues and forms agglomerates in the matrix, which served as the potential sites of stress concentration leading to low impact strength and ductility. Nevertheless, the hybrid composites showed superior mechanical properties over the MMC and its base alloy.

Evaluation of load-independent microhardness values in Plateau regions of Vanadium substituted Bi-2212 ceramics

This study reveals extensively effect of homovalent V/Bi partial replacement in Bi2.0−xVxSr2.0Ca1.1Cu2.0Oy ceramic matrix (0.00 ≤ x ≤ 0.30) on the key mechanical design performance parameters and load-independent Vickers microhardness parameters in plateau limit region by means of experimental microhardness tests and semi-empiric approaching models. It is found that the vanadium substitution level of x = 0.01 is observed to be optimum amount in the Bi-2212 crystal lattice for refinement of fundamental mechanical properties due to the enhancement in stabilization of durable tetragonal phase, surface residual compressive stress and elastic recovery mechanism. Conversely, from the replacement level of x = 0.01 onwards, the lattice strain field and stress concentration sites enhance significantly depending on the increase of microscopic structural problems, interaction problems between adjacent layers and crack-initiating flaws in Bi-2212 ceramic system. Correspondingly, stress-induced phase transformation begins to play predominant role, and excess vanadium substituted ceramic materials are easily broken at relatively smaller test load. Moreover, the models indicate that every ceramic compound shows standard indentation size effect (ISE) feature due to predominant behavior of elastic recovery in crystal structure. Hence, presence of optimum vanadium ions strengthens typical ISE characteristic behavior. Furthermore, among semi-empirical models the indentation-induced cracking (IIC) model exhibits the highest performance to inspect real microhardness values of Bi2.0−xVxSr2.0Ca1.1Cu2.0Oy ceramic compounds in the plateau limit region.

Precipitation Evolution in the Austenitic Heat-Resistant Steel HR3C upon Creep at 700 °C and 750 °C.

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be 57.1 MPa and 37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.

Grain Size and Heterophase Effects on Mechanical Properties of Mg-Cu Nanoglasses

Tailoring heterogeneities in amorphous alloys is a promising strategy for promoting the strength-ductility synergy. Here, molecular dynamics simulations are performed to investigate the effects of grains size and heterogeneous chemical composition on the mechanical properties of Mg-Cu nanoglasses (NGs). The reduced grain size in single-phase NGs improves the plasticity but at the expense of strength. In addition, the mechanical properties of dual-phase NGs composed of two chemical compositions depend critically upon the fraction of softer phase. In particular, the plasticity is improved for the low fraction of the softer phase, but is deteriorated for the high fraction of the softer phase, which is in striking contrast to the observations of the plasticity improvement reported in the traditional nanostructured metals/alloys. This is because that heterogeneities at the glass-glass interfaces intentionally introduce more stress concentration sites which are easier to accelerate the shear band formation. For an appropriate fraction of heterogeneous composition, a balance among strength and plasticity can be realized, which is useful for the design of novel NGs with high strength and superior ductility.

Review of Fatigue Assessment Approaches for Welded Marine Joints and Structures

Welded joints are widely used in many sectors and represent the main joining technique also in the marine industry. The welded joints are sites of high stress concentrations and are subject to severe conditions for the marine environment. The design of marine welded joints has to consider the effects from wave loads, ship motions and loading/unloading operations and corrosion effects. The aim of this scientific work is to discuss about the state of the art of the standards and the approaches for predicting the fatigue life of welded joints used for the marine industry. Several approaches are examined in order to provide an overview and highlight the advantages and limitations of each method. Furthermore, recent advances in welding of dissimilar metals and autonomous welding are considered.

Role of active slip systems induced with holmium impurity in Bi-2212 ceramics on mechanical design performance and morphological properties

Effect of Ho/Bi partial replacement in Bi2.1-xHoxSr2.0Ca1.1Cu2.0Oy (Bi-2212) superconductors on the fundamental structural, morphological and mechanical performance properties are investigated by Scanning Electron Microscopy (SEM) and Vickers hardness (Hv) measurement techniques. Crystallinity quality and surface morphology including the microcrystal coalescence orientations, grain alignment distributions, microscopic structural problems, microvoids, internal defects, uniform surface view, porosity and particle growth distribution are visually examined with the aid of SEM. Basic mechanical performance and characteristic features of Bi/Ho substituted Bi-2212 superconducting ceramics (0.00 ≤ x ≤ 0.10) are also determined with Vickers tests conducted at various loads intervals 0.245–2.940 N. Experimental findings show that the characteristic features enhance seriously in case of x = 0.01 due to refinement of crystallinity quality and slip systems. Thus, the optimum Ho concentration presents the highest mechanical fracture strength to the load applied as a result of better uniform surface appearance and grain orientations, well-connected flaky layers, larger particle size distribution and denser structure, confirmed by the SEM investigations. Namely, much more load is required to accelerate the dislocation movement and crack propagation to the terminal velocity for critical size growth. The fracture predominantly takes place in the transcrystalline regions and hence the propagations are easily controlled with the optimum Ho dopant ions. On the other hand, the increase in the Ho ions in Bi-2212 structure induces the crack-initiating defects for new stress concentration sites. In conclusion, the permanent and non-recoverable deformations appear at even lower indentation test loads. All samples present indentation size effect feature depending on the dominant character of elastic recovery mechanism. Further, original hardness parameters are semi-empirically analyzed in the plateau limit regions using mechanical modelling approaches for the first time. Based on the analyses, Hays–Kendall model exhibits the closest results to the experimental findings.

Comparative Evaluation of the Effect of Core Type and Antirotational Post on Stress Distribution in an Endodontically Treated Maxillary First Molar: FEA.

Background This study aims to analyze and compare the stress distribution in an endodontically treated maxillary first molar restored with various post and core systems and assess the benefit of the presence of an antirotational post and the effect of its length using finite element analysis. Materials and Methods Five 3D models of maxillary first molar restored with variable designs of post and core were constructed using CT scanning and information obtained from textbook and FEM software. Variables were types of core, presence or absence of an antirotational post, and length of the antirotational post. A load of 480 (N) vertically and a load of 240 (N) with 45° to the occlusal plane were applied. Results were analyzed using 3D von Mises criteria. Results The results showed that the most homogeneous stress distribution pattern along with dentin and the post-core system was observed in the model with one main post in the palatal canal and in the model with a two-piece core and one main post in the palatal canal and shorter antirotational post in the distal-buccal canal. However, models with one-piece core and antirotational post (either the same length or shorter than the main post) in the distal-buccal canal had the most stress concentration sites. Conclusion In this study, it was found that the presence of an antirotational post, its length, and type of core affected the distribution of stress along the dentin and post-core system. This study indicated that if the antirotational post is considered in the post-core design, it is better to use a two-piece core type due to more homogeneous stress distribution along the dentin and post-core system.

Crack initiation and slow growth in soda-lime glass from a self-healed crack

Soda-lime glass (SLG) is a widely used structural material with many advantages in terms of thermal, physical and mechanical properties besides esthetics, sustainability and environmental impact. As a highly brittle, low-toughness and high-stiffness material, understanding its failure behavior in general and fracture mechanics in particular is critical for structural applications. The propensity for hairline crack formation at stress concentration sites during service is particularly a critical issue. Hairline cracks in SLG can occasionally heal when the structure is unloaded, become optically invisible and go undetected, causing unexpected and catastrophic failure upon reloading. In this work, crack initiation and quasi-static crack growth from a self-healed crack in SLG plate is recreated and investigated under laboratory conditions. A wedge-splitting test (WST) geometry is adopted to carry out the study. A pre-cut notch is loaded using a wedge to generate a hairline crack and let to heal without any external stimulus. An opto-mechanical study of crack (re)initiation and growth is carried out while mapping the crack-tip stress gradients in the whole field using the transmission-mode Digital Gradient Sensing (DGS) method. An abrupt re-initiation and extension of the self-healed crack, subsequent initiation of the natural crack-tip followed by a slow crack growth are all captured by the far-field load–displacement response as well as the local crack-tip fields. The stress intensity factor (SIF) history for the entire event is extracted subsequently. The results indicate approx. 50% lower value of critical SIF at re-initiation of the self-healed crack relative to the natural/virgin crack. The crack growth resistance behavior shows a nominally constant energy release rate of 6.5 ± 0.5 J/m2 during slow crack growth. A companion finite element (FE) model that mimics optical measurements is also included. The simulations suggest that the contact stiffness of the self-healed crack is about 60% of the virgin material.

Splashed gold bump dependence of cleavage of InSb chip under cyclic liquid nitrogen shocking tests

Local cleavage of indium antimonide (InSb) chip always occurs in the manufacture of the InSb infrared focal plane detectors (IRFPAs), and this specific fracture phenomenon restricts the improvement of the yield of the InSb IRFPAs. After analysis, we think that the cleavage of InSb chip in the edge region of the InSb IRFPAs is related to the splashed gold bump existing in this region, and this failure phenomenon dominates in the low-cyclic liquid nitrogen shocking tests. In order to clarify the influence of the splashed gold bump on the cleavage of the InSb chip, we establish a structural model of the InSb IRFPAs containing the splashed gold bump, and analyze the influence of the splashed gold bump on the thermal stress distribution in the InSb chip. Besides, we preset the initial cracks with different lengths at the stress concentration sites to describe the dislocations in InSb wafers. Using the energy release rate as criterion, we obtain the relationship between the cleavage of the InSb chip and the dislocation line length in the presence of splashed gold bump. The main conclusions are drawn as follows. 1) The influence of the splashed gold bump on the cleavage of the InSb chip is localized, and two stress concentration sites are formed in the outermost part of the contact region between the splashed gold bump and the InSb chip. 2) The energy release rate surrounding the preset crack increases promptly with the preset crack length increasing. 3) Cleavage of the InSb chip caused by the splashed gold bump belongs to the type I fracture failure mode. In the cyclic liquid nitrogen shocking tests, the dislocation line gradually punches through the InSb chip under the driving of the concentrating stress, and forms the macro cleavage of the InSb chip.

Influence of Solution Heat Treatment on Microstructure and Tensile Properties of Gd-Treated Al-15% Mg2Si In-Situ Composites

ABSTRACT Microstructural alteration and tensile properties of Al-15% Mg2Si composite specimens was examined after addition of gadolinium (Gd) and conducting solution heat treatment. Various percentages of gadolinium (0.5, 1.0, 2.0 and 5.0 wt. % Gd) were added to the composite Al-15% Mg2Si composite. The specimens then solutionized at 500 °C for 4h followed by quenching. The results showed that regular morphology and small size of primary Mg2Si particles is achieved after addition of 1.0 wt.% Gd compared to untreated composite. Due to solutionizing effect, Mg2Si dissolution occurred which led to alter the morphology of primary Mg2Si particles to round shape. Tensile testing results revealed that enhancement in UTS and El% values owns to influence of both Gd addition and solution heat treatment on the Al-15% Mg2Si composite. The fracture surface of untreated composite depicted a cellular fracture, while the fracture surface of Gd treated and heat treated composite showed a ductile surface containing fine dimples, in which alteration of fracture mode is due to the role of Gd and heat treatment on microstructural modification, which results in reduction of potential sites for stress concentration and crack initiation areas.

CREEP RUPTURE LIFE OF PRE-STRAINED SUPERALLOY N07080

The creep of the pre-strained superalloy N07080 is described in this work. The pre-strain was achieved by warm rolling at 1050 oC.-The warm rolling was performed due to additional strengthening, i.e increasing of the superalloy hardness.-The pre-strain drastically reduces the creep rupture life of the superalloy compared to the creep rupture life of the standard heat treated superalloy.-The drastic reductionof the creep rupture life is result of rapid creep cavity nucleation on stress concentration sites along primary grain boundaries of the pre-strained superalloy.-Recrystallization eliminates potential sites for rapid cavity nucleation and prolongates the creep rupture life.

Highly improved mechanical and thermal properties of alkali silicate and graphene nanoplatelet composite adhesives

Inorganic adhesives have received increased attention due to their high temperature resistance, non-toxicity, and environment friendliness. However, poor adhesion strength of inorganic adhesives hinders their application. Further, graphene improves the mechanical and thermal properties of polymer composites. Herein, graphene nanoplatelets (GNP) with different specific surface areas were added into a mixed silicate (MS) containing different ratios of alkali silicates. The results revealed that lap shear strengths increased from 7.4 MPa with the unmodified MS to 9.42 MPa with the MS/GNP 750_10% composite. In addition, the GNP powder with the highest surface area (MS/GNP 750_10% composite) formed maximum interfaces with the silicate matrix, leading to effective load transfer and reduction in stress concentration sites. Hardness and modulus increased as the GNP surface area increased; unmodified MS had the highest values because GNP changed MS from brittle to ductile. In addition, at room temperature, thermal conductivity increased by 60% in the vertical direction and 173% in the horizontal direction in the MS/GNP 750_10% composite compared to the unmodified MS. Furthermore, at a higher temperature (100 °C), the thermal conductivity of the MS/GNP 750_10% composite increased by 273% in the vertical direction and 412% in the horizontal direction. • This study investigated the effect of the surface area of GNP powder on the properties of Alkali silicate based adhesives. • The lap shear strength increased from 7.4 MPa in the bare MS to 9.42 MPa in the MS/GNP 750_10%. • The thermal conductivity increased by 60% in the vertical and 173% in the horizontal direction in the MS/GNP 750_10%.

The significance of inclusion morphology and composition in governing hydrogen transportation and trapping in steels

In this paper, two steels with different morphology of inclusions are studied. Scanning electron microscopy, finite element calculations, and electrochemical hydrogen permeation tests were conducted. Furthermore, in-situ observations of hydrogen de-trapping from the surface of steels were achieved using a hydrogen charging device and optical microscope. Results show that stripe-shaped complex inclusions exhibit a higher level of residual stress, especially at the boundary of oxide components. The hydrogen molecule is also prone to emerge at the oxide composition and intensifies the risks of cracking. In contrast, the residual stress levels around spherical inclusions are comparatively lower due to discrete sites of stress concentration. Thus, the effect of dispersed tiny dioxide sphere inclusions can help relieve the local hydrogen pressure and promote the resistance against hydrogen-induced cracking.