Plastic Region Research Articles

Efficiency of an Improved Grouted Corrugated Duct (GCD) Connection Design for Precast Concrete Bridge Pier: Numerical and Parametric Study

In this study, finite element analysis (FEA) has been conducted for an improved grouted corrugated duct (GCD) connection design with a reserved recess in bridge footing. This study aims to understand the damage progression mechanism and to evaluate the contribution of each component in the improved GCD connection design. Numerical model based on the experimental results are first created, validated and calibrated. It is found that the confining effect (support and friction force) provided by recess sidewall keeps the connection in good integrity. It also prevents early deformation and early development of transverse cracks along the connection interface, which further avoids the damage concentration at connection joint, transfers the plastic hinge region. Parametric study is then carried out by considering different recess depths, cushion thicknesses, recess diameters, and mortar strengths. The effect of recess details on mechanical behavior is thus studied. Recess depth can be designed as 6–20% of the column section size to ensure a higher upper limit of overall strength and ductility, and it also influences the stress distribution area of the joint local. The stiffness and strength of recess control the local damage, while has limited impact on the overall performance. In addition, preliminary suggestions on the GCD design of recess depth, thickness of mortar cushion, recess diameter, the strength of mortar are proposed.

Plastic loading induced high strength-ductility for creep aged Al–Cu–Li alloys

Large-curved or high-ribbed large-size integral panels are often subjected to plastic deformation during the loading stage of creep age forming (CAF). Although this plastic loading helps to improve the creep deformation, whether both good strength and ductility of the creep-aged alloy under plastic loading can be guaranteed remains unclear. Thus, under the applied stress ranging from elastic (230/250/270 MPa) to plastic regions (320/340 MPa) at 180 °C, mechanical properties and microstructural evolution of creep-aged 2195-T4 Al–Cu–Li alloys were systematically investigated. Surprisingly, high yield strength up to 620 MPa and ductility above 9.4% are obtained for peak-aged alloys under plastic loading, increasing by 9.0%∼21.5% and 6.7%∼30.1% over elastic loading and stress-free aging, respectively. High strength is attributed to the combined effect of high density of dislocations, densely distributed fine T1 precipitates in the matrix, and narrowed precipitate free zones (PFZ) at grain boundaries. Meanwhile, it was observed that shearing of T1 plates happens only as single shearing step, and numerous single-shearing events occur at different locations, accompanied by elastic and/or plastic distortion and progressively segmented rotation (accumulated angle up to 20°) of T1 precipitate, which induce high ductility during plastic deformation. The above good mechanical properties were much less sensitive to aging time for plastic-loading creep-aged alloys relative to stress-free or elastic-loading counterparts, which helps to widen the CAF process window. This paper offers a new strategy of tailoring shape and properties during CAF, which would break the limitation of CAF currently only for the small-curved components.

Mechanical Properties of a Bone-like Bioceramic-Epoxy-Based Composite Material with Nanocellulose Fibers.

Several composite materials are being investigated as reinforcement fillers for surgery simulations. This study presents an artificial composite material with properties similar to those of the human bone, which may be used in surgery simulations. Moreover, considering the potential toxicity of debris generated during sawing, a safe epoxy-based composite material was synthesized using cellulose nanocrystals (CNCs) and bioceramics (i.e., hydroxyapatite, Yttria stabilized zirconia oxide, Zirconia oxide), which were used to mimic the stiffness of human bone. To examine the change in mechanical properties according to the composition, 1, 3, and 5 wt% of CNCs were mixed with 5 wt% of the bioceramics. When CNCs were added at 1 wt%, there was a confirmed change in the non-linear stiffness and ductility. The CNC-added specimen fractured when forming a nano-network around the local CNCs during curing. In contrast, the specimen without CNCs was more densely structured, and combined to form a network of all specimens such that a plastic region could exist. Thus, this study successfully manufactured a material that could mimic longitudinal and transverse characteristics similar to those of real human bone, as well as exhibit mechanical properties such as strength and stiffness. Bioceramics are harmless to the human body, and can be used by controlling the added quantity of CNCs. We expect that this material will be suitable for use in surgery simulations.

Analyzing Support Stability of Deep Shaft Based on Plastic Softening and Dilatancy of Hard Rock Mass

To explore the stability analyses and control methods for surrounding rocks in deep hard rock shafts, this paper is based on field engineering geological surveys and laboratory rock mechanics tests and relies on the main shaft being constructed in the Shaling Gold Mine of China as the engineering background. The quality of the rock mass is evaluated by the Q system, rock mass rating (RMR) and geological strength index (GSI). The mechanical parameters of the surrounding rock mass of the shaft are calculated by using the generalized Hoek–Brown failure criterion, and the main support system is determined based on the rock mass classification system. Based on the finite element method, a two-dimensional plane strain model is constructed to analyze and evaluate the deformation and plastic region range of surrounding rocks for different support systems. On this basis, considering the dilatancy and plastic softening characteristics of hard rock masses, an analytical solution of the stress, strain and plastic region radius of hard rock around shafts in homogeneous media is proposed. Finally, the plastic region of the surrounding rock is measured by the P-wave velocity test method. The results show that after considering the dilatancy and plastic softening characteristics of the rock mass, the numerical simulation, theoretical analytical solution and measured results are basically consistent, and the proposed support system can effectively ensure the stability of the shaft.

Fatigue crack growth behavior of 42CrMo high-strength steel with tempered sorbite/bainite microstructures: Roles of grain and constituent in microstructures

Microstructural differences through the thickness have always been an attention focus for large-scale steel structures, since they could significantly influence fatigue crack propagation (FCP) characteristics. Here, the effects of microstructure in FCP behaviors of 42CrMo steel were analyzed with emphasis on two different tempered sorbite/bainite (TS/B) microstructures: (i) fine TS/B structures (M42); (ii) coarse TS/B structures (M52). The results show that the FCP rate is relatively slower for M52 than for M42 in the regime through the later stage of near-threshold (I-L) to the early stage of Paris-regime (Ⅱ-E) with low ΔK, but is fairly similar for the two cases in the other ΔK regimes. Under relatively low ΔK, the FCP path of M52 is more tortuous with numerous micro-cracks in B and larger-sized crack deflections at the carbides and boundaries compared to M42, especially in the I-L where fatigue micro-cracks can deflect at the single coarse carbide, thereby resulting in roughness-induced crack closure and crack retardation behavior. Theoretical analysis shows that FCP behaviors are closely related to the cyclic plastic region size (Δrp) at the crack-tip. In the early stage of near-threshold region, Δrp is restricted within a grain-size scale for both steels, thus leading to a similar FCP rate. However, from the I-L to the Ⅱ-E, coarse B/ferrite-grains in M52 are always larger than the Δrp of fatigue crack-tip, and thereby lead to numerous micro-cracks formation and crack deflections at the carbides and B boundaries, inducing crack closure. In the later stage of Paris region, B/ferrite-grains in the two cases are much smaller than Δrp, so that FCP behaviors depend on the microstructure group effects, or overall strength and fracture toughness of materials, which cause the close FCP rate.

Effect of Overburden Depth and Stress Anisotropy on a Ground Reaction Caused by Advancing Excavation of a Circular Tunnel

The assumption of the Convergence–Confinement Method (CCM) is the analysis of the interaction behavior of the support and ground of a deep circular tunnel under an isotropic stress field. Aiming to improve this method, this paper proposes a discussion on the influence of the overburden depth and stress anisotropy. To consider the influence of the overburden effect, the ground reaction in different depths due to tunnel advancing excavation is investigated. Under anisotropic stress conditions, the analytical solutions of the stress/displacement in the plastic and elastic regions of this ground reaction can also be suitable for theoretical analysis in a consistent manner. The key factor in this study is the use of confinement loss, which can not only describe the simulation of tunnel advancing effects but also become a superimposed value of the incremental procedure. In addition, the calculation spreadsheets can be used to estimate and implement the theoretical analytical solutions into executable computational solutions. To check the validity of the analytical solution, finite element analysis is used to examine the distribution of stress/displacement around the tunnel, especially the distribution along the overburden pressure line in the circular tunnel cross-section. Comparing the analytical solution calculated by the incremental procedure with the result of the numerical analysis shows a consistent trend.

ディープホールドリリング法による溶接部残留応力の高精度測定

This paper presents a method to improve accuracy of measuring through–thickness residual stress for welds through deep hole drilling (DHD) technique. The focus of this study is the effect of dimensions of the hollow cylinder, which is manufactured by drilling and trepanning processes, on residual stress measurement at the welds. First, stress concentration–induced plastic region around a reference (drilled) hole was theoretically quantified according to value of existing stress. The plastic region around the hole is not negligible for the welds at where high tensile residual stress exists. In order to reduce the influence of the plastic region, it is considered effective to set a large ratio of trepanning diameter to drilling diameter. Another noteworthy thing is that weld residual stress is present in a relatively narrow region along a weld line. Then, DHD technique was applied to measure through–thickness residual stress at the welds of melt–run welded–plate specimens under conditions of various drilling and trepanning diameters. These DHD residual stress measurement results were compared with those measured by X–ray diffraction and contour methods and calculated by weld thermal elastic–plastic analysis. As the results, it was shown that through–thickness residual stresses measured by DHD technique were obviously influenced by dimensions of the hollow cylinder. Measurement accuracy becomes higher with increasing the ratio of trepanning diameter to drilling diameter. Meanwhile, trepanning diameter should not be larger than width of generation area of high tensile residual stress to be measured at the welds. It was concluded that a method for determining appropriate dimensions of the hollow cylinder to be drilled and trepanned was successfully proposed to accurately assess through–thickness residual stress at the welds through DHD technique.

Electrolytic Co-Deposition Mechanisms, Texture Layers, and Residual Stresses in Nanocomposite Coatings Processes: A Review

The composite coating has gained wider attention due to its property to protect materials used in energy, bridges, offshore platforms, underground pipelines, and the aviation industry from corrosion and deterioration. In this work, a literature review was conducted about the processes of nanocomposite coating, the mechanisms of electrolytic co-deposition, the texture of layers, and the residual stresses. An important aspect, residual stress, was emphasized, which represents the persistent stress after removing the external force affecting a metal in the plastic region. Because it cannot be measured directly and may be determined by measuring strain and indirect methods, the sources and methods for measuring residual stresses (XRD, SEM, TEM, EDS) were described in the last section to provide a comprehensive overview. Based on the thorough analysis of the published literature, it was concluded that nanoparticles could be electrodeposited with Ni on an Al substrate using a direct current and Ni sulfamate as an electrolytic solution, and Nickel will not reside on the oxide layer covering Al, so chemical changes are needed to prepare the Al surface. In addition, texture changes with the thickness of the coated layer must be investigated.

The application of standard nonlinear solid material models in modelling the tensile behaviour of the supraspinatus tendon

Tendons transmit forces from muscles to bones through joints. Typically, tendons and muscles work together to innovate a motion. In addition, tendons are often subjected to much higher stresses than the muscles that they serve in any given action. As a result, tendons are susceptible to injuries that may lead to a permanent dysfunction in joint mobility due to the fact that the scar tissue that forms after healing often does not have the same mechanical properties of the original tissue. It is therefore very important to understand the mechanical response of tendons. In this paper the performances are examined of two viscoelastic standard nonlinear models in modelling the elastic and plastic behaviour of the tendon in the light of a well-known hyperelastic Yeoh model. The use of the Yeoh model is more for validating the performances of the viscoelastic models within the elastic region than for comparison purposes. The Yeoh model was selected based on its superior performance in modelling the elastic phase of soft tissue, as reported in previous studies, combined with its simplicity. The results show that the two standard nonlinear solid models perform extremely well both in fitting accuracies and in correlating stress results. The most promising result is the fact that the two standard nonlinear models can model tendon behaviour in the nonlinear plastic region. It is also noted that the two standard nonlinear models are physically insightful since their optimisation parameters can easily be interpreted in terms of tendon elasticity and viscoelastic parameters.

Application of Scanning Acoustic Microscopy for Detection of Dental Caries Lesion

Introduction: A scanning acoustic microscope (SAM) is an apparatus for imaging acoustic properties. This apparatus can non-invasively and rapidly evaluate the hardness of materials in the elastic region. This device shows great potential for the diagnosis of dental caries in the clinical setting. However, since the tissue elastic modulus measured using a SAM is a property of the elastic region and the Knoop hardness is a property of the plastic region, the hardness properties differ completely. Therefore, we investigated whether the acoustic impedance measured using a SAM is related to the Knoop hardness, which is used as the standard for removal of carious dentin. Method: Polished sections were prepared from 20 extracted carious wisdom teeth. The acoustic impedance and Knoop hardness were measured for each section. In addition to comparing carious and healthy dentin in SAM images, we evaluated the difference between the carious and healthy dentin in terms of the acoustic impedance and Knoop hardness. We also evaluated the correlation between the Knoop hardness and acoustic impedance. Results: The SAM images were visualized as two-dimensional color images based on the acoustic impedance values. The mean acoustic impedance of carious dentin was significantly lower than that of healthy dentin, showing a similar trend as Knoop hardness. A strong correlation was observed between the two. Discussion: The acoustic impedance values obtained through acoustic microscopy differed significantly between carious and sound dentin. Both types of dentins were visualized using two-dimensional color images. A strong correlation was observed between the acoustic impedance value, which indicates the hardness of the elastic region, and the Knoop hardness, which indicates the hardness of the plastic region. The results of the present study indicate that acoustic impedance accurately reflects the hardness of dentin.

Flexural behaviour of 3D-printed carbon fibre composites: Experimental and virtual tests - application to composite adaptive structure

Flexural properties of 3D-printed carbon fibre (CF) composites are investigated, experimentally and numerically. A series of 3-point bending experimental tests are conducted on CF composite specimens, and a series of 3-point bending virtual tests are simulated in LS-DYNA using a composite modelling approach, which is user defined integration points through the composite thickness. The flexural stress-strain results are compared, and they show good agreement within the elastic region. In the plastic region, the experimental flexural modulus reduces significantly, which remains constant for a certain range of flexural strain, before structural failure. A new hypothesis on the flexural properties of the 3D-printed CF composite is suggested, which is the specimen behaves as two stacks of individual beams, after yielding/delamination. The hypothesis is supported by another series of FE simulations on the composite, modelled as two stacked composite beams, and subjected to 3-point bending load. Then the capability of the composite 3D-printing to manufacture a structure with complex geometries and to manufacture several parts as a single structure, are exploited to fabricate a CF composite corrugated structure with a trailing edge section. The structure which represents an internal structure of a morphing aerofoil is actuated by a NiTi shape memory alloy (SMA) wire with a 1.6% recoverable strain. A trailing edge deflection of 6.0 mm is obtained, which is measured using an IMETRUM optical system. It is reasonably close to the predicted deflection of 7.3 mm shown in the FE simulation, using a newly developed UMAT for SMA-actuation, in an explicit LS-DYNA.

Cyclic response of precast concrete columns connected with cast-in-place UHPC in plastic hinge region

Ultra high performance concrete (UHPC) is a promising material for lap splice connection of steel bars in reinforced concrete (RC) precast structures given its high compressive strength, its high toughness, and its high bond strength with steel bars. This paper focuses on the cyclic response of bottom frame columns connected by UHPC in the plastic hinge region. The effect of lap length of longitudinal bars in UHPC (6d, 8d and 12d, d = diameter of longitudinal bar), axial load ratio and stirrup ratio of the UHPC connection region on the global response of the columns were presented and analyzed, and a cast-in-site RC column was prepared for comparison. The behavior of the UHPC connected columns was flexural dominated, but columns with 6d and 8d lap length failed due to the gradual bonding degradation between UHPC and longitudinal bars, and the column with 12d lap length failed due to UHPC crushing. It was demonstrated that a 8d lap length could achieve equivalent lateral bearing capacity and ductility to the control RC column, but the lap length should be greater than 8d when applying UHPC connection technology in seismic regions to ensure the energy dissipation. Column with lower axial load ratio (0.17) exhibited lower ductility because shear sliding occurred between the interface of UHPC and footing at the post-peak stage. The UHPC connected specimens exhibited higher resilience and better damage resistance in earthquake because the ratios of the residual displacements to the corresponding peak displacements were between 17 and 39%. Besides, a flexural capacity model for the columns was developed and the results predicted by the model agreed well with the experimental results.

Seismic behavior of concrete columns with ends transformed via perforated H-shaped steel and engineered cementitious composite

A novel structural detail is proposed to strengthen or shift the plastic hinge regions of concrete column-footing connections by using perforated H-shaped steel and engineered cementitious composites (ECC) in this study. Two transformed concrete columns and a normal concrete column were tested under a constant axial load and quasi-static reversed cyclic load to explore their seismically important features, including failure modes, hysteretic response, energy dissipation, stiffness degradation, and strain. Furthermore, a numerical study was also conducted to investigate the influences of critical parameters on seismic performance, and a formula for predicting the shear capacity of concrete columns is proposed. The results reveal that the seismic performance of concrete columns can be improved by embedding perforated H-shaped steel and using ECC at each end, and the plastic hinge can be relocated away from the column ends. The finite-element results agreed well with the experimental results. A high shear-span ratio adversely affects the seismic performance of the transformed concrete columns. An increase in the axial load ratios and H-steel insertion depth can enhance the load-carrying capacity of the models.

Investigating the mechanism of splitting failure in deep high sidewall cavern based on complex function and strain gradient

Owing to excavation-induced unloading, high sidewalls of underground powerhouse are often subjected to splitting failure, which produces many adverse effects to the construction of the cavern. To reveal the formation mechanism of splitting failure, we proposed a novel elastic–plastic damage softening model based on the strain gradient theory and the elastic–plastic damage theory. Using the ODE45 program in MATLAB, the numerical solution of the displacement and stress of circular cavern was solved based on the newly proposed elastoplastic damage model. We then adopted the complex function method and used the Schwarz-Christoffel integral formula to obtain the mapping function from the outer domain of high sidewall cavern to the outer domain of the unit circle. Finally, the elastic–plastic region and the displacement distribution of high sidewall cavern were obtained by mapping the obtained elastic–plastic solution of the circular cavern under the axisymmetric condition. The numerical analysis results were consistent with the previous geomechanical model test results. The results presented that the stress of high sidewall of an underground powerhouse exhibits an oscillating variation with alternating peaks and troughs, which is the mechanical mechanism of splitting failure.

Solution for the Dynamic Elastically Compressible Power-Law Strain Hardening Cylindrical Cavity-Expansion Problem

A closed form analytical solution for both the quasi-static or dynamic strain hardening cylindrical cavity-expansion (CCE) problems requires the plastic region to be incompressible. This is due to the fact that in both cases a hydrostatic stress state must be explicitly included in the strain hardening function, making an analytical solution intractable. Additionally, for the dynamic problem the elastic region must be compressible to avoid a logarithmic singularity at infinity. An alterative for the dynamic analytical formulation is to use the finite element method (FEM). With the FEM weak formulation, solutions for the radial stress at the cavity surface and the elastic–plastic interface interface velocity as functions of the cavity-expansion velocity can be obtained when both the strain hardening plastic and elastic regions are elastically compressible. In this study, a comparison is made using results from both analytical (strong formulations), and FE simulations for both strain hardening and perfectly plastic dynamic CCE problems. It is concluded that with increasing cavity-expansion velocities, the elastically compressible strain hardening FEM solution is less resistive than the corresponding closed form analytical solution with an incompressible plastic region. Additionally, it is shown that the elastic–plastic interface velocity asymptotes at the bulk wave speed for completely elastically compressible solutions. Furthermore, the completely elastically compressible solutions remain valid for cavity-expansion velocities beyond the critical cavity-expansion velocity associated with the closed-form analytical strain hardening solution.

Ultra-low cycle fatigue performance of grid structure with bolted spherical joints

This study aimed to investigate the effect of the specification of each grid structure component on the ultra-low cycle fatigue performance of the rod element of a grid structure with bolted spherical joints. Thus, an ultra-low cycle fatigue test of three identical composite steel tube and bolted sphere specimens was performed. The test was performed under three kinds of constant amplitude loading regimes for large axial cyclic plastic strain loads. Based on steel ductility damage and initial geometric defects, a finite element simulation of the test was then conducted using Abaqus/Explicit, which was calibrated according to the test results. In addition, parametric numerical simulations were performed to analyze the effects of different steel tube slenderness ratios, bolt diameters, and steel tube wall thicknesses on the seismic performances of the composite steel tube and bolted sphere specimens. The results indicated that the specimen dissipated energy mainly through material damage in the plastic hinge region in the middle of the steel tube. The steel tube slenderness ratio mainly affected the fatigue life and energy dissipation capacity of the specimen. Further, the ultimate tensile and compressive bearing capacity as well as deformation capacity of the specimen both increased with increasing steel tube wall thickness. Finally, the energy dissipation pattern of the specimen was most sensitive to the bolt diameter; thus, the energy dissipation capacity of the steel tube can be fully utilized only by selecting the bolt specification that matches the rod element.

Pinning effect on the correlations of nonaffine displacement in metallic glasses

We mimic the structural heterogeneity of inclusions in nanoscale by pinning a portion of atoms in CuZr metallic glasses (MGs) on molecular dynamics simulations with shear deformation. Results show yielding of the MGs transits from a system-spanning shear band (SB) to a localized plastic flow with pinning, due to the constrained length scale for the organization of the plastic events imposed by pinning. Spatial correlation of the nonaffine displacement exhibits a quadrupolar symmetry that shrinks with pinning, giving two characteristic lengths in the maximum and minimum directions that are both constrained by the static length for the pinned atoms. The two characteristic lengths signify the anisotropic nature of the plastic regions, which diminishes with pinning. These results shed light on the microscopic mechanism of tailoring the mechanical properties by tuning the static structure in MGs.

A general stress solution in a plastic region near a traction-free boundary of arbitrary shape under plane-strain conditions

A general stress solution in a plastic region near a traction-free boundary of arbitrary shape under plane-strain conditions

Experimental study on the seismic performance of self-centering bridge piers incorporating ECC and superelastic SMA bars in the plastic hinge regions

The seismic performance of pier columns is important to the safety of bridge structures. To improve the deformation capacity and energy dissipation, reduce the residual deformation and plastic damage, and resume normal functions with no or minimal repair after an earthquake, an innovative pier column with self-centering capability based on shape memory alloy (SMA) and engineered cementitious composites (ECC) was proposed. In the plastic hinge region of the pier column, SMA bars and ECC were used to replace the ordinary longitudinal reinforcements and concrete. The superelasticity of SMA can be used to achieve the self-centering function of the pier column; the strain hardening characteristic of ECC can improve the energy dissipation capacity of the pier column and reduce damage. Five specimens were produced, i.e., ordinary reinforced concrete bridge pier (R/C-BP), reinforced ECC bridge pier (R/ECC-BP), steel strand reinforced concrete bridge pier (SS/C-BP), steel strand reinforced ECC bridge pier (SS/ECC-BP) and SMA bar ECC bridge pier (SMA/ECC-BP). After low cycle repeated loading tests, the seismic performances of different piers, such as failure mode, bearing capacity, ductility and energy dissipation capacity, were compared and analyzed. The test results show that SMA material can significantly improve the deformation ability and ductility of the structure and reduce the residual deformation of the structure; ECC material can effectively improve the ductility and energy dissipation capacity of the structure and reduce the speed of cracking. Compared with ordinary reinforced concrete specimens, SMA/ECC pier columns were less damaged with better ductility and self-centering effects and exhibited excellent seismic performance.

Molecular dynamics simulation on creep-ratcheting behavior of columnar nanocrystalline aluminum

Creep-ratcheting deformation behavior of columnar nanocrystalline (NC) Al varying in grain sizes have been studied using molecular dynamics simulations at three different temperatures (300 K, 467 K, and 653 K). The underlying deformation mechanisms of columnar NC Al for two stress ratios of ratcheting are also evaluated in this study. The highest strain accumulation and rapid hysteresis loop proliferation are attained at 653 K. The dislocation density is low at 653 K in contrast to the other two deformation temperatures. The perfect dislocations support the creep-deformation process followed by Shockley partial dislocations. It is observed that the cyclic hardening phenomenon at 300 K and the cyclic softening phenomenon at 653 K dominate during creep-ratcheting loading. The specimen fails earlier at higher temperatures owing to the change in the shape of hysteresis loops. The predominant grain boundary-based deformation mechanisms of columnar NC Al specimens are grain boundary (GB) migration, GB diffusion, GB widening, GB sliding, and GBs merging for all the deformation temperatures. The variation in the extent of deformation with respect to temperatures depends on strain accumulation in the plastic region under creep-ratcheting loading.