Maximum Shear Stress Plane Research Articles

In Situ Study on Fracture Behavior of Thick‐Section Friction Stir‐Welded AA 5083 Alloy

Friction stir welding (FSW) produces a gradient microstructure with distinct metallurgical zones. The present study focuses on characterizing the fracture behavior during uniaxial loading across thick‐section FSW joint in nonheat‐treatable 5083 aluminum alloy. The hardness variation is linked to microstructural features across the weld zones through correlative microscopy. Notably, the heat‐affected zone (HAZ) exhibits a lower hardness of 75 HV, while the thermomechanically affected zone (TMAZ) and stir zones (SZ) display a hardness of around 79HV. A marginal difference in hardness in HAZ prompts strain localization in HAZ on the advancing side of the weld, resulting in offset in fracture from the SZ, which is captured through in situ monitoring of the tensile test. Further, the postdeformation analysis reveals crack propagation along the maximum shear stress plane due to the coalescence of microvoids resulting from the fracturing and debonding of second‐phase particles. Overall, this study provides valuable insight for optimizing the FSW process of AA 5083 alloy for thick‐section applications.

Crystal plasticity model of BCC metals from large-scale MD simulations

Accurate crystal plasticity models that faithfully capture the behavior of single crystals under a wide range of loading conditions, such as loading direction, strain rate, and temperature, are still lacking. Here we introduce a novel approach in which a crystal plasticity (CP) model is informed directly from and calibrated to large-scale quantum-accurate MD simulations in which single crystal BCC Ta serves as a testbed material. By analyzing our large set of MD simulations several key insights are obtained leading us to modify constitutive assumptions in order to address deficiencies of existing CP models. Importantly, we observe that the standard notion of fixed slip systems – pairs of slip directions and slip planes – is inadequate for describing high-rate plasticity in BCC tantalum at room temperature. Instead, pencil glide defined as dislocation motion in the maximum resolved shear stress planes (MRSSP) of each Burgers vector is fully consistent with our MD simulation data while providing significant simplifications of the constitutive relations. Our resulting new CP model closely matches the behavior of single crystals observed in high-rate MD simulations while being fully consistent with lower rate experimental results.

Crack Initiation Mechanism of AISI 4340 Steel for High‐Cycle Torsional Fatigue Loading

The nucleation of microstructural fatigue cracks in AISI 4340 steel is investigated. The pre‐electropolished specimen is employed for the observation of the very early stage of fatigue crack initiation correlated with the microstructure. It is found that there are precipitate‐free zones (PFZs) around the lath and packet boundary, and the strength of PFZ is lower than that of the lath due to the lack of second‐phase strengthening. Therefore, the fatigue crack would first initiate within the PFZ. Besides, the initiated crack plane usually is the maximum shear stress plane. However, the MnS inclusions would cause stress concentration so that the lath boundary (LB) deviating from the maximum shear stress plane could also have sufficient driving force to crack. In contrast, when the LB is vertical to the cracked MnS inclusion, it would prevent the crack from propagating into the matrix because of the anisotropy of lath martensite. The effects of the stress concentration caused by the MnS inclusion on the maximum deviation angle and the S‐N curve are discussed.

A Multidirectional Cyclic Direct Simple Shear Device for Characterizing Dynamic Soil Behavior

AbstractA newly constructed multidirectional cyclic direct simple shear (mcDSS) device with unique capabilities is introduced. This mcDSS apparatus, called the Illinois mcDSS (or I-mcDSS) device, for the first time brings together the following capabilities: (1) servo-hydraulic control that can apply stress- or strain-based monotonic, cyclic (e.g., sinusoidal, saw tooth, and square), and high-frequency broadband loads at realistic earthquake loading rates, improving over previous devices with pneumatic control; (2) unidirectional and bidirectional loading; (3) bender elements to measure (S-wave) velocity; (4) a cell for applying consolidation stresses different from at-rest values as well as back-pressure saturation; and (5) a multidirectional load cell on top of the specimen to minimize the effect of compliance and component friction on load measurements. The I-mcDSS device tests cylindrical specimens confined using either a wire-reinforced membrane or stacked rings. Experiments conducted on a uniformly graded Ottawa sand are presented to illustrate each key I-mcDSS feature. Test repeatability is demonstrated for monotonic and bidirectional cyclic tests. Drained or constant volume, K0-consolidated, strain-controlled monotonic, unidirectional cyclic, and bidirectional (circular, figure-8, and broadband) cyclic tests on dry specimens yielded shear stress–shear strain relations, peak effective-stress friction angles (ϕ′peak-DSS), and volumetric strains, or excess pore water pressures, consistent with the literature. However, depending on load cell and displacement transducer locations, device compliance was observed to affect shear stress–shear strain response at shear strains less than 1 %. When compared with triaxial compression (TC) peak effective-stress friction angles (ϕ′peak-TC), ϕ′peak-DSS was about 5° smaller than ϕ′peak-TC if the horizontal plane is considered the failure plane, whereas ϕ′peak-DSS ≈ ϕ′peak-TC if the horizontal plane is considered the plane of maximum shear stress. Lastly, measured S-wave velocities at varying confinements are consistent with published correlations.

Room-temperature plastic deformation of single crystals of α-manganese - hard and brittle metallic element

The deformation behavior of single crystals of α-manganese has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. When the specimen size is reduced to the micrometer-range, single crystals of α-manganese are found to plastically deform by dislocation motion at room temperature for the first time, accompanied by very high yield stresses of the range of 4–6 GPa. Slip along [111] and [001] are identified to operate for compression axis orientations near [001] and near [011] and [1¯11], respectively. Any low-indexed planes cannot be designated as the slip plane for both slip along [111] and [001], because of the significantly wavy nature of slip lines caused by the occurrence of frequent cross-slip. Slip along [111] tends to prefer the slip plane of {112} rather than {110}. Slip along [001], on the other hand, tends to occur on the maximum resolved shear stress plane. The 1/2[111] dislocation carrying slip along [111] moves as a perfect dislocation without dissociating into partials and does not have any preferred orientation. The [001] dislocation carrying slip along [001] also moves as a perfect dislocation without dissociating into partials. Although the Peierls stress for the motion of these dislocations must be very high, there seems no deep Peierls valleys along particular directions, unlike the screw direction for the 1/2[111] dislocation in body-centered cubic metals.

Multiscale analysis of interior cracking behavior of Ni-based superalloy fabricated by selective laser melting under very-high-cycle-fatigue at high-temperature

The fatigue test at high-temperature of selective laser melting Ni-based superalloy was performed to reveal the mechanism of interior cracking behavior by multiple analysis methods in the very-high-cycle-fatigue regime. Due to the inhibition effect of oxide layer, the interior multi-defects assisted facetted cracking becomes a significant failure mode. Large grain deformation mainly occurs on the fracture surface, and slip systems with high Schmid factor are activated by high-temperature. The precipitates in the matrix hinder the dislocation movement, resulting in dislocation accumulation and stress concentration increase, which makes it easier for microcracks initiation. The initiated microcracks with crack deflection caused by high angle grain boundaries propagate transgranularly to form a nearly circular region and undergo fast unstable growth, eventually leading to fracture. • The microcracks propagate transgranularly along the maximum shear stress plane. • {111}<112> slip system with high Schmid factor is activated by high-temperature. • Interior multi-defects assisted facetted cracking is the predominant failure mode.

Multiaxial fatigue of Inconel 718 produced by Selective Laser Melting at room and high temperature

This work investigates the fatigue and cyclic plasticity of Inconel 718 produced by Selective Laser Melting (SLM) at room and elevated temperature. Strain-controlled axial, torsional and proportional tests with Rɛ=0 were carried out at 20 °C and 450 °C using thin-walled specimens, resulting in fatigue lives ranging from 103 to 105 cycles. SLM Inconel 718 exhibits a columnar microstructure along the build direction, and the dominant defect related to the manufacturing process was gas pores. Fatigue crack initiation occurred a carbides and/or metallic inclusions for all tests and, therefore, intrinsic defects related to the manufacturing process did not induce premature initiation of fatigue cracks. The orientation of macroscopic fatigue cracks was consistent with the plane of maximum shear stress for all loading conditions. The fatigue crack propagation mechanism of SLM Inconel 718, on the other hand, depends on the loading path and temperature. Both the equivalent strain amplitude and the Fatemi–Socie criterion could correlate fatigue life of all loading conditions at 20 °C and 450 °C within factor-of-two boundaries. As a general trend, the fatigue endurance of SLM Inconel 718 is inferior to its forged counterpart, especially at 450 °C. The cyclic plasticity behaviour of SLM and forged Inconel 718 is similar at both 20 °C and 450 °C and can be simulated by the Chaboche model.

Multiaxial high-cycle fatigue failure and life prediction based on critical plane method

Multiaxial high-cycle fatigue failure criterion and life prediction is important for structural components under complex low stress. A new multiaxial high-cycle fatigue damage parameter based on the plane of maximum shear stress is proposed, which includes the maximum shear stress amplitude, normal stress amplitude and maximum equivalent normal stress. The maximum equivalent normal stress is defined in the form of SWT stress function considering the influence of mean stress. The feasibility of the proposed failure criterion and life prediction model is verified by test data of different materials. The predicted results show that the life prediction error of the proposed fatigue model is basically within three times by comparing with McDiarmid, Matake, Crossland and Papadopoulos fatigue models.

Constitutive modeling of principal stress rotation associated with sand under simple shear loading

AbstractAn elasto‐plastic constitutive model is proposed to account for the effect of principal stress rotation that commonly occurs and significantly influences soil behavior. The model is based on a concept of adding plastic strain increments obtained from two mobilized planes: a plane of maximum shear stress which swings as the principal stress rotates, and a horizontal plane which is spatially fixed. Models based on the plane of maximum shear stress alone are particularly sensitive to horizontal effective stress ratioK0(=σ′x/σ′y), whereas the proposed model gives a similar skeleton behavior for soils at the same density and mean effective stress, regardless of the value ofK0, as observed in laboratory tests. The model is firstly calibrated by data from drained and undrained monotonic and cyclic Direct Simple Shear (DSS) tests onK0‐consolidated specimens of Nakdong River sand in loose and dense states. The capability of the proposed model in modeling of the conventional liquefaction effects: initial shear stress conditionα(=τ′xy/σ′y) withKαcorrections, and initial confining stress conditionσ´ywithKσcorrections, is also emphasized in this paper.

Sweep-tracing algorithm: in silico slip crystallography and tension-compression asymmetry in BCC metals

Direct Molecular Dynamics (MD) simulations are being increasingly employed to model dislocation-mediated crystal plasticity with atomic resolution. Thanks to the dislocation extraction algorithm (DXA), dislocation lines can be now accurately detected and positioned in space and their Burgers vector unambiguously identified in silico, while the simulation is being performed. However, DXA extracts static snapshots of dislocation configurations that by themselves present no information on dislocation motion. Referred to as a sweep-tracing algorithm (STA), here we introduce a practical computational method to observe dislocation motion and to accurately quantify its important characteristics such as preferential slip planes (slip crystallography). STA reconnects pairs of successive snapshots extracted by DXA and computes elementary slip facets thus precisely tracing the motion of dislocation segments from one snapshot to the next. As a testbed for our new method, we apply STA to the analysis of dislocation motion in large-scale MD simulations of single crystal plasticity in BCC metals. We observe that, when the crystal is subjected to uniaxial deformation along its [001] axis, dislocation slip predominantly occurs on the {112} maximum resolved shear stress plane under tension, while in compression slip is non-crystallographic (pencil) resulting in asymmetric mechanical response. The marked contrast in the observed slip crystallography is attributed to the twinning/anti-twinning asymmetry of shears in the {112} planes relatively favoring dislocation motion in the twinning sense while hindering dislocations from moving in the anti-twinning directions.

Pseudo-ductility under tensile and compressive loading in ±45º laminates

This work focuses on analyzing the uniaxial response of carbon fiber reinforced polymer matrix laminates with the configurations [&plusmn;45]2S under tension and [&plusmn;45]4S under compression. These present non-linear stress-strain evolutions that allow them to withstand large deformations before losing their load-carrying capacity. Both responses are characterized by a first linear stage, followed by a plateau in which the strain grows without increasing the stress level, and finally by a re-stiffening phase. But there are quantitative differences that lead to different failure patterns, which are explained with the help of the state of stress in the plane of maximum shear stress. For a better understanding of the process, tensile load-unload-reload tests are performed to verify if the energy recovered in each cycle could be related to the loss of the apparent stiffness. During the first two stages of the mechanical response, the laminate suffers a progressive damage with a reduction in stiffness related to the dissipated energy, but this pattern is not repeated in the last stage of strain hardening. Based on the experience of other authors, it is assumed that the re-stiffening stage follows a different pattern due to possible microstructural changes in the matrix during the plateau. These are promoted by a narrowing process under tensile loads and a local widening of the test zone under compression. The dimensional changes perpendicular to the load direction are observed thanks to the strain fields obtained by Digital Image Correlation.

Examining the pathways for deformation band formation at the mesoscale

At the macroscopic scale, strain accumulation is known to form parallel to the plane of maximum shear stress. In this work, plastic strain accumulation is tracked across four mesoscale areas of interests. Each representing a different configuration of tension-torsion loading. The local deviations in the deformation pathways were investigated relative to the plane of maximum shear stress. The deviations in the deformation pathways were observed to be a function of the local plastic strain magnitude and the microstructural features. From these results, a representative volume element was created based on the macroscopic and micromechanical deformations as well as the spatial statistics of the deformation bands.

Molecular dynamics simulations of screw dislocation mobility in bcc Nb

The screw dislocation mobility in bcc Nb has been studied by molecular dynamics (MD) simulations at different strain rates and temperatures using an embedded-atom method (EAM) potential. Static properties of the screw dislocation, as determined with the EAM potential, are in agreement with previous density-functional-theory calculations. The elementary slip plane of the screw dislocation remains (110) for all studied strain rates (in the range 6.3 × 107–6.3 × 109 s−1) and temperatures (5 to 550 K). However, the consecutive cross-slip on different symmetry-equivalent (110) planes leads to an effective glide on (112) planes. It is demonstrated that the screw dislocation trajectories, velocities and waviness of the screw dislocation depend on the crystallographic indices, (110) or (112), of the maximum resolved shear stress plane. The waiting time for the start of the screw dislocation motion increases exponentially with decreasing strain rate, substantiating the necessity to apply in future accelerated MD techniques in order to compare with macroscopic stress-strain experiments.

Influence of uniaxial and biaxial pre-straining on the high cycle fatigue performance of DP590 steel

Present work aims to assess tensile and high cycle fatigue (HCF) performance of dual-phase (DP590) steel subjected to uniaxial and equi-biaxial pre-strain. Pre-strain increases material’s strength, reduces ductility and simultaneously enhances HCF performance, quantified by increased material’s tensile yield strength 12% & 21%, ultimate tensile strength 6% & 7%, endurance limit 11% & 18%, after 10% uniaxial and equi-biaxial pre-strain. The rotation of one maximum shear stress plane during uniaxial re-loading after equi-biaxial pre-strain results in increased fatigue life. Finite element analysis suggests an alteration in the location of deformation concentration zone during uniaxial fatigue cycling of equi-biaxial pre-strain material.

Persistent slip observed in TiZrNbHfTa: A body-centered high-entropy cubic alloy

Slip properties of TiZrNbHfTa were evaluated to obtain ψ–χ relationships, where χ is defined as the angle between the maximum shear stress plane and (1¯01). ψ is defined as the angle between the apparent slip plane and (1¯01). Bending tests were conducted with micrometer-sized cantilevers fabricated from a single grain with a focused ion beam. Slip bands appear to be straight on a micrometer length-scale, as for Fe–Si alloys, which suggests a persistent slip. However, the ψ–χ relationship indicated that ψ is nearly the same as χ, suggesting that there is no persistent slip. Detailed observations with an atomic force microscope confirm that cross-slipping is so frequent at a sub-micrometer length-scale that the slip bands appear to be straight and the maximum shear stress plane is observed on a micrometer length-scale. Here, TiZrNbHfTa shows abnormally frequent cross-slipping, which is a novel characteristic of high-entropy body-centered cubic alloys.

Influence of uniaxial and biaxial pre-straining on the low cycle fatigue performance of DP590 steel

The present work aims to examine the low cycle fatigue (LCF) behavior of dual-phase (DP590) steel subjected to different pre-straining (uniaxial and equi-biaxial) paths. The LCF tests reveal all specimens showed continuous cyclic softening throughout the fatigue life. However, the rate of cyclic softening is higher for the equi-biaxial pre-strain specimen. The rotation of one maximum shear stress plane during fatigue cycling after equi-biaxial pre-straining introduces non-proportional loading condition, thereby significantly reducing the fatigue life. The larger lattice rotation and in-grain misorientation due to the non-proportional loading is notable for the equi-biaxial pre-strain specimen.

Research on the Deformation Mechanism of Mining Roadway Stratiform Surrounding Rock with Nonuniform Stress Field

Aiming at the failure problems of mining roadway stratiform surrounding rock influenced by asymmetrical high stress, firstly, the rock instability conditions of different structural plane angles were summarized based on the mechanical model of rock joints and instability criterion. Secondly, stresses, plastic zone, and displacement of surrounding rock with different structural plane angles were calculated and analyzed with the theoretical calculation method or numerical simulation method, respectively. The distribution characteristics of plastic zone of surrounding rock under different influence factors are further studied, and the results show that the lateral pressure coefficient can change the size and shape of plastic zone at the same time. The damage mechanism was analyzed through the above study. Research studies show that the shear and tension failure area of plastic zone were directly determined by the maximum shear stress and the minimum principal stress. The effect of structure planes on stress has two sides, one to destroy the continuity and the other to strengthen discreteness and laddering nature. When the angle between maximum shear stress and structure plane increases, the extended mode and distribution pattern of plastic zone change substantially as well. The #15 haulage roadway is taken as engineering background, and the above research’s results are verified by the comprehensive analysis. The research results can enrich the butterfly plastic zone theory and provide the basis for roadway supporting design.

Effect of age on the failure properties of human meniscus: High-speed strain mapping of tissue tears

The knee meniscus is a soft fibrous tissue with a high incidence of injury in older populations. The objective of this study was to determine the effect of age on the failure behavior of human knee meniscus when applying uniaxial tensile loads parallel or perpendicular to the primary circumferential fiber orientation. Two age groups were tested: under 40 and over 65 years old. We paired high-speed video with digital image correlation to quantify for the first time the planar strains occurring in the tear region at precise time points, including at ultimate tensile stress, when the tissue begins losing load-bearing capacity. On average, older meniscus specimens loaded parallel to the fiber axis had approximately one-third less ultimate tensile strain and absorbed 60% less energy to failure within the tear region than younger specimens (p < 0.05). Older specimens also had significantly reduced strength and material toughness when loaded perpendicular to the fibers (p < 0.05). These age-related changes indicate a loss of collagen fiber extensibility and weakening of the non-fibrous matrix with age. In addition, we found that when loaded perpendicular to the circumferential fibers, tears propagated near the planes of maximum tensile stress and strain. Whereas when loaded parallel to the circumferential fibers, tears propagated oblique to the loading axis, closer to the planes of maximum shear stress and strain. Our experimental results can assist the selection of valid failure criteria for meniscus, and provide insight into the effect of age on the failure mechanisms of soft fibrous tissue.

Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model

The practical constitutive model UBCSAND2, which combines two-mobilized planes—a maximum shear stress plane and a horizontal plane within a framework of classical plasticity approach—is used to incorporate shear-induced effects in both loading and unloading as well as principal stress rotation effects. UBCSAND2 was calibrated by capturing cyclic direct simple shear (CDSS) test results of Pohang sand, which was collected from liquefied paddy fields due to the 2017 Pohang earthquake (Mw = 5.4) in South Korea. The model procedure focuses on simple shear condition because it best simulates field conditions under earthquake loading. The calibrated UBCSAND2 model is then used to assess the liquefaction-induced damages that occurred at the quay wall and backfill layer in Youngil-man port near the epicenter of the Pohang earthquake. The numerical results show that liquefaction mostly occurred in silty sand layers, in which the excess pore pressure ratio reached almost one. The estimated displacements of the quay wall and the predicted settlement of reclaimed area obtained from the analysis were in good agreement with those obtained from field measurements.

Plastic deformation of a Zr-based bulk metallic glass fabricated by selective laser melting

Fully amorphous Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass (BMG) samples with a relative density exceeding 98% were fabricated via selective laser melting (SLM). High fracture stresses of around 1700 MPa and a reproducible plastic strain of about 0.5% were obtained for cylindrical SLM samples. The analysis of the observed serrations during compressive loading implies that the shear-band dynamics in the additively manufactured samples distinctly differ from those of the as-cast glass. This phenomenon appears to originate from the presence of uniformly dispersed spherical pores as well as from the more pronounced heterogeneity of the glass itself as revealed by instrumented indentation. Despite these heterogeneities, the shear bands are straight and form in the plane of maximum shear stress. Additive manufacturing, hence, might not only allow for producing large BMG samples with complex geometries but also for manipulating their deformation behaviour through tailoring porosity and structural heterogeneity.