Local Plastic Deformation Research Articles

Quantitative analysis of local plasticity accompanying hydrogen-related fracture in low-carbon martensitic steel

In this study, the local plasticity accompanying hydrogen-related fracture in a low-carbon martensitic steel was quantitatively investigated by correlative microscopy analysis combining digital image correlation technique and electron backscatter diffraction. The results revealed that strain localization occurred mainly in the bulk of martensite blocks and at prior austenite grain boundaries, both with and without the presence of hydrogen. Moreover, hydrogen enhanced strain localization during deformation and facilitated crack nucleation and propagation. The quantitative analysis showed that the hydrogen-related quasi-cleavage cracking was closely related to the local plastic deformation, and increasing hydrogen content decreased the local strain level required for quasi-cleavage cracking. It was found that strain localization also occurred around the intergranular cracks, suggesting that hydrogen-induced intergranular cracking was not a classical simple decohesion process and strain localization was involved in the sequence of hydrogen-induced intergranular cracking. These findings shed light on the understanding of the intrinsic nature of hydrogen embrittlement.

Crystal plasticity finite element modelling and mechanical deformation mechanism of nanolaminated graphene reinforced metal matrix composites

Metal matrix composites with certain hierarchical structures are potential to break out the conflicts between strength and toughness. Nano-carbon like graphene (Gn) reinforced metal matrix composites with nanolaminated structures have been largely developed to overcome this issue. However, the plastic deformation mechanism hiding behind the nanolaminated structures is still lacking. In this study, crystal plasticity finite element simulations and composite structure modeling are combined in order to uncover the mechanical deformation behavior of nanolaminated Gn/Al composites. Different lateral Gn sizes of 180, 360 and 720 nm are considered, while the hybrid lateral Gn sizes of 180 and 720 nm are applied as well. Good consistency between experimental and numerical results indicates the reliability of performed simulations. It is found that the strengthening and hardening behaviors of Gn/Al composites are dominated by Gn rather than Al matrix. The edges of Gn serving as a major dislocation nucleation source produce localization of plastic deformation, while the interiors of Gn activate more slip systems but reduce total shear strains. The edges and interiors of Gn determine the stability of plastic deformation of Gn/Al composites; large edge vs. interior area of Gn will lead to early necking. The rule of mixture fails in predicting the peak tensile strength and tensile strain of Gn/Al composites with hybrid lateral Gn sizes, because the microscopic plastic deformation can largely localize around 180 nm Gn in these hybrid Gn/Al composites. This work is enlightening to design and develop novel smart metal matrix composites.

Investigation of Passive Pile Groups’ Responses Induced by Combined Surcharge-Induced and Excavation-Induced Horizontal Soil Loading

Excessive lateral forces and the deformation of pile groups caused by adjacent surcharge loading and excavation can potentially lead to the failure or collapse of nearby pile foundations. This paper presents a simplified analytical method to investigate the lateral displacement of passive pile groups caused by combined surcharge-induced and excavation-induced horizontal soil loading and validates it by utilizing two published centrifuge tests and numerical modeling. Firstly, the passive load resulting from surcharge-induced horizontal soil loading is determined using the improved formula of Boussinesq and the theory of local plastic deformation. The additional horizontal force exerted on the pile axis subjected to excavation-induced horizontal soil loading is also taken into account using the Mindlin solution. Furthermore, the shielding effect between pile groups is also proposed to analyze the response of a laterally loaded pile group due to excavation unloading. Secondly, according to the Pasternak foundation model of soil–pile interaction, the equilibrium differential equations are solved by the finite difference method. Finally, parametric analyses are undertaken to evaluate the behavior of different constraints on the pile head and surcharge procedure. The results demonstrate that the capped-head pile groups can provide greater restraint compared to the free-head groups for the lateral displacement of pile groups. The front pile experiences greater forces than the rear one in passive pile groups as a result of the smaller soil pressure on the rear pile. Additionally, increasing the horizontal stiffness of the capped head with fully fixed ends can significantly decrease the horizontal deformation of the pile groups.

Isothermal low-cycle fatigue and fatigue–creep behaviour of 2618 aluminium alloy

A series of strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on a 2618 aluminium alloy for various strain amplitudes at temperatures of 20 °C, 160 °C and 195 °C. LCF tests at elevated temperatures were performed for three different strain rates between 1×10−4/s and 1×10−2/s, while fatigue–creep tests were performed with either tensile or compressive strain dwell. At 20 °C, cyclic hardening was associated with a high dislocation density. In contrast, a localization of plastic deformation into slip bands and cyclic softening were observed at 195 °C. The investigation of the damage mechanisms revealed transgranular cracking as the predominant failure mode, significantly influenced by Al9FeNi intermetallic particles. Finally, fatigue–creep tests were found to be more damaging than equivalent LCF tests, particularly at 195 °C, where a negative strain rate sensitivity on fatigue lifetime was evident.

Evaluation of process parameter of friction stir processing of AA2024-T351 Alloy using RSM and GRA

A fundamental method for changing the characteristics of metals through localised severe plastic deformation is friction stir processing (FSP), which is both straightforward and efficient. Because of its high strength-to-mass ratio and resilience to fatigue, aluminium alloy 2024 is commonly utilised. The experiments were designed utilising the Box–Behnken method and the process parameters’ chosen range values. In this research, the impact of FSP process variables on the tensile, surface roughness, and hardness properties of AA2024 will be examined. These parameters include tool rotation speed, welding speed, and axial load. The base sample tensile strength, surface roughness and microhardness values were 325 MPa, 0.5125 μm, and 120 HV. The optimised sample’s maximum tensile strength, minimum surface roughness, and maximum microhardness values were 420 MPa, 0.4 μm, and 144 HV. The grey relational analysis (GRA) determines the ideal combination of process parameter values for the best values in all responses. XRD was used to assess residual stress, dislocation density and micro-strain. The optimal process parameters have been obtained at a rotational speed of 1000 rpm, traverse speed of 45 mm/min, and axial load of 10.25 kN.

Insights into building a digital twin of closed-cell aluminum foam during impact loading: Microstructural, experimental and finite element investigations

The mechanical behavior of metal foams under impact loading depends on multiple and complex parameters like impact velocity, strain-rate, local plastic deformation, oscillating and micro-inertial effects, etc. The prediction of the behavior of metal foams that are subject to impact loads is still challenging and engineering application of these materials typically requires time-consuming experimental tests. Numerical models based on the finite element method (FEM) can contribute to minimizing the experimentation effort. Realistic FEM models were built that account both for the macro- and micro-scopic characteristics of the porous material, explain the acting mechanisms that take place during impact, and study the yield properties as well as the energy absorption during the impact of closed-cell aluminum foams. The simulation results are compared with the ones derived from respective experimental uniaxial tests. Two different modeling approaches were applied thus creating two models. The first model relies on a cell-based method where the initial geometry of the foam was generated based on the Voronoi tessellation algorithm and the second one relies on the isotropic, strain-hardening, and continuum-based model developed by Deshpande-Fleck. The outcome of the investigation sheds light on the metal foam behavior under impact by explaining macro- and micro-structural phenomena that develop during impact.

Investigation of the Material Elasto-Plastic Response under Contact Indentation: The Effect of Indenter Material

When dealing with joints and bearings, high pressures localised at the contact interface lead to residual plastic strain. The present paper combines numerical simulations and experimental tests to investigate the role of the material constitutive law in the indentation process. Numerical indentation tests between similar materials showed a good agreement with the experiments when classical material laws recovered from tensile-compressive tests on bulk samples were accounted for. On the other hand, when simulating indentation between different materials in contact, the comparison between the numerical and experimental results highlighted the limits of using classical material laws. Bilinear material laws were then derived for different steel materials (ASP 2060 PM, 100Cr6, 440C, Marval X12, and Z15 CN17-03) in contact with a ceramic indenter, leading to the correct simulation of the residual indentation profiles (error less than 5%). The proposed approach to determine suitable material laws for indentation between dissimilar materials can be further applied when dealing with applications involving contacts undergoing local plastic deformation.

Proper orthogonal decomposition assisted eigendeformation-based mathematical homogenization method for modeling cracks in 3D polycrystalline microstructures

In this manuscript, a reduced order model (ROM) is proposed for full three-dimensional (3D) polycrystalline microstructures with tortuous short cracks. The model reformulates eigendeformation-based homogenization method (EHM) assisted with the proper orthogonal decomposition (POD) method to account for local plastic deformation and crack separation fields. A constrained optimization problem is formulated with reduced order objective functions and un-reduced constraints to solve for the separation. To make the optimization process more feasible, an integrated form of the constraint is introduced, enabling the use of only one constraint regardless of the crack morphology. The model is verified against direct numerical simulations (DNS) with crystal plasticity as the constitutive relation. The use of priori knowledge in building crack separation field basis functions as well as sampling strategies in the absence of prior knowledge are discussed. The performance and accuracy of the ROM are assessed under various loading conditions and different crack configurations.

Mapping microstructure to shock-induced temperature fields using deep learning

The response of materials to shock loading is important to planetary science, aerospace engineering, and energetic materials. Thermally activated processes, including chemical reactions and phase transitions, are significantly accelerated by energy localization into hotspots. These result from the interaction of the shockwave with the materials’ microstructure and are governed by complex, coupled processes, including the collapse of porosity, interfacial friction, and localized plastic deformation. These mechanisms are not fully understood and the lack of models limits our ability to predict shock to detonation transition from chemistry and microstructure alone. We demonstrate that deep learning can be used to predict the resulting shock-induced temperature fields in composite materials obtained from large-scale molecular dynamics simulations with the initial microstructure as the only input. The accuracy of the Microstructure-Informed Shock-induced Temperature net (MISTnet) model is higher than the current state of the art and its evaluation requires a fraction of the computation cost.

Local Plastic Deformation and Quality of Cu-Cu Joints Obtained by Ultrasonic Welding

Joints of copper sheets with a thickness of 0.8 mm were produced by ultrasonic welding. To assess the quality of the joints, tensile lap-shear strength, area fraction of bonding, distributions of normal strains in the cross sections of welded samples, linear weld density at a magnification of ×1000, and the microstructure and microhardness of welded samples were analyzed. It was proved that the arrangement of microbonds and length of gaps in joint zones significantly depended on the local normal strains of welded samples caused by the penetration of tool ridges under the clamping pressure. Joint regions with a linear weld density of more than 70% were observed if the local compression strains of the sample exceeded 15%. The appearance of local tensile strains was accompanied by a drop in the linear weld density of the joints in some regions, down to 5%. The distribution of normal strains depends on the mutual positions of the ridges of the welding tip and anvil. It is concluded that in order to improve the quality of joints obtained by ultrasonic welding and reduce the scatter of their strength values, welding tools should provide sufficiently high normal compression strains in the weld spot area.

Unraveling the effect of sulfide-oxide complex inclusions on the localized corrosion mechanism for carbon steel

The localized corrosion induced by (Ca, Mn)S-(Al, Mg)O complex inclusions was investigated. It revealed that MgO·Al2O3 served as cores and MnS·CaS as shells in the carbon steel. MnS and CaS acted as an anodic phase in the galvanic corrosion formed with steel matrix, while MgO and Al2O3 had no galvanic couple effect with MnS and CaS. MnS was the most preferentially dissolved and triggered localized corrosion, which expanded in the order of CaS-Fe-MgO·Al2O3. The driving force for sulfides and steel matrix dissolution was the effect of galvanic couple, and high local plastic deformation caused by oxides, respectively.

Effect of free surfaces on localized plastic deformation in single-crystal nickel containing helium bubbles and radiation-induced self-interstitial atom clusters

This paper presents molecular dynamics (MD) simulations of plastic deformation in single-crystal Ni models containing internal distributions of helium (He) bubbles and self-interstitial atom (SIA) clusters corresponding to experimental characterization of ex-service Inconel X-750 components. We simulate single crystals with and without free surfaces at room temperature and 588K, the latter temperature corresponding to the Inconel X-750 service conditions. Both models are deformed under multiple slip to a maximum deviatoric strain of ∼10 %. A dislocation slip band forms in the model without free surfaces, but not in the model with free surfaces. However, the latter exhibits nano-twin nucleation from surfaces. Helium (He) bubbles that overlap with a slip band or a nano-twinned region deform more severely than other bubbles in the model. Nevertheless, the bubbles do not link up and do not form configurations that resemble incipient cracks.

Study on corrosion perforation behavior of copper nickel alloy pipe during service in marine environment

A CuNi90/10 copper nickel alloy seawater pipeline experienced premature perforation and leakage during service in the marine environment. The reasons of perforation and leakage of CuNi90/10 copper nickel alloy pipe in seawater pipelines were studied through macroscopic morphology, chemical composition, metallographic structure, microscopic morphology, and energy spectrum analysis, the influencing factors and key points for anti-corrosion and leakage prevention were analyzed. The results show that the chemical composition, hardness, and grain size of the CuNi90/10 copper nickel alloy pipe with perforation leakage meet the standard requirements, and the composition, microstructure distribution in the perforation leakage and nearby areas are uniform. CuNi90/10 copper nickel alloy pipe undergoes erosion-corrosion and perforation near the straight pipe section (approximately 10 mm from the weld toe) on the inner arch side of the elbow. The high flow rate of the seawater on the inner arch side of the elbow generates significant shear stress on the alloy pipe wall; solid particles such as sediment in the seawater impact the alloy pipe wall, causing damage to the passivation film and local plastic deformation; and due to the protrusion of the weld joint near the elbow, the local micro-turbulence of the seawater is generated; Under the combined action of erosion and corrosion, premature perforation and leakage of the copper nickel alloy pipe are caused. In order to avoid premature perforation leakage of the copper nickel alloy during service in the marine environment, safety technical operating procedures should be strictly followed during use, such as controlling fluid flow rate, maintaining seawater flow in the early stages of the service to form a protective film, and reducing the content of solid particles (such as sediment) in the fluid, etc.

Lateral Deformation Response of an Adjacent Passive Pile under the Combined Action of Surcharge Loading and Foundation Excavation

With the increasing development of civil engineering in large cities, more and more excavations and surcharge loadings are being constructed or planned adjacent to existing building piles in crowded urban areas. Previous study on pile deformation has primarily focused on surcharge loading or foundation excavation and given little concern to the combined action of surcharge loading and foundation excavation. The article develops a two-stage process to assess the lateral displacement of nearby pile foundations induced by the combined action of surcharge loading and excavation. Firstly, the local plastic deformation theory and Boussinesq solution are used to accurately predict the passive loading of adjacent pile foundations caused by surcharge loading; Mindlin formulas are adopted to predict the passive pile’s additional lateral stress applied by excavation. Secondly, Pasternak models are adopted and the finite difference method is used to establish the deflection differential formula of the single passive pile. Last but not least, a parametric study is conducted to investigate the influence of the loading dimensions, loading magnitudes, and three-dimensional excavation dimensions. The findings of the calculations reveal that the loading magnitudes have a more significant impact on the lateral displacement of the pile compared to the loading dimensions. Therefore, a concentrated surcharge loading should be avoided. Additionally, the excavation depth has a greater influence on the lateral displacement of the pile compared to the excavation area. In order to mitigate this situation, a step excavation should be implemented for each layer of soil, with the soil excavated away from the pile foundation first.

Structural Performance of Demountable Hybrid Floor Systems Under Monotonic and Cyclic Loading

AbstractThe building sector is actively researching and reviewing technical solutions for de‐construction, driven by the increasing importance of sustainability requirements, as outlined in the EU Commission's ‘Green Deal,‘ which aims to achieve net‐zero greenhouse gas emissions by 2050. Two key research areas in efficient structural design strategies are the performance of hybrid structures, which combine mechanical properties and architectural appearance of different materials, and the techniques and mechanical properties of connections between structural components that enable deconstruction and reuse. However, there is a scarcity of studies and methods focused on demountable and hybrid structural systems, limiting the understanding of their overall structural performance at a real‐structure floor level. In this paper, a finite element model is presented to conduct a comparative study between the traditional floor system with welded stud connections and the demountable floor system with coupler‐embedded bolted connectors. The numerical results showed that the serviceability loading capacity of the demountable floor system was 8% lower, and the difference in ultimate resistance was further enlarged. Reusing a demountable floor system, analysed in the case study, had little effect on the ultimate resistance, but it is difficult to exclude local plastic deformation after the first life cycles.

Investigation of the influence of fatigue alternating loading on residual stress in materials

This study investigates the effect of pulsating cyclic fatigue loading on the internal stress of silicon steel material. A comparative analysis is conducted between the loaded material and the original material before treatment. The findings indicate that pulsating cyclic fatigue loading leads to a decrease in the residual stress of the material and an increase in its elongation, albeit with a reduction in its strength. Additionally, fatigue alternating loading reduces the roughness of the material and enhances its grain size, resulting in narrower grain boundaries. Transmission electron microscope (TEM) analysis indicates that finite fatigue loading causes an increase in the number of dislocations in the material, leading to local plastic deformation and contributing to the reduction of residual stress in the material. Notably, the mechanism underlying the reduction of residual stress resulting from pulsating cyclic fatigue loading is akin to that of non-temperature-affected residual stress reduction treatment, suggesting a possible similarity in their fundamental nature.

Impact behavior of high-strength steel beam with circular web openings

A drop-weight impact testing machine was used to conduct an impact test on 9 specimens in order to examine the impact resistance of high-strength steel beams with circular web openings (HSSBCWO). The experimental results indicated that the dynamic response of HSSBCWO is significantly affected by the change of impact energy and opening height. A coefficient β is proposed to assess the severity of localized deformation in HSSBCWO, and the range of β values for the specimens of test are found to be approximately between 0.5 and 0.8. Within the range of impact energy designed for the experiment, HSSBCWO primarily absorbed energy through localized plastic deformation. A finite element model which can predict the dynamic response of HSSBCWO under impact load is established. The results show that the optimal ratios of opening height to beam height and spacing between openings to beam height for the HSSBCWO are determined to be 0.50 and 0.93, respectively. Furthermore, a damage evaluation system for HSSBCWO under impact load was established, utilizing the ratio of mid-span residual displacement to calculated beam span Dr / L0 as the damage assessment index, with impact energy, opening height, and spacing between opening as the main control parameters.

Effect of Temperature and Thermal Stress on Micropores and Cracks in Aluminum Alloy Film During Micro-Arc Oxidation

In this paper, COMSOL Multiphysics software was used to simulate the micro-arc oxidation of 7075 aluminum alloy, so as to study the influence of temperature and thermal stress on the micropores and cracks in the process of micro-arc oxidation. The results show that: The high temperature and high-pressure environment at the discharge channel leads to the eruption of molten oxide at the discharge channel and the formation of irregular micropores on the surface of the oxide film. The reason for the formation of cracks is that the thermal stress around the discharge channel and in the corner region of the film layer is much larger than the tensile strength of the Al2O3 film layer, in which case the material will have local plastic deformation or cracks. The larger the temperature gradient span near the discharge channel is, the larger the thermal stress will be, resulting in larger diameter of micropores on the surface of the film, more irregular shape, and even bigger defects due to the penetration of cracks. Because the crack region has a relatively low resistivity, it is easier to generate a dense spark discharge, so the crack region has a denser distribution of micropores.

In-situ EBSD observation and simulation of free surface roughening and ductile failure in the ultra-thin ferritic stainless steel sheet

Surface roughening is one of the most fundamental issues which greatly affects the plastic deformation and ductile fracture behavior of ultra-thin sheet materials. The present study investigated the influence of free surface roughening on the micromechanical failure in the ultra-thin ferritic stainless steel (FSS) sheet for bipolar plate in proton exchange membrane (PEM) fuel cell under in-situ tensile deformation. The microstructural evolution during uniaxial tension was monitored using in-situ electron backscatter diffraction (EBSD) technique. Then the initial microstructure was directly mapped onto the finite element mesh of representative volume element (RVE) in the crystal plasticity finite element (CPFE) simulation. The CPFE model was incorporated with the microscopic ductile fracture criterion in terms of plastic energy dissipation in order to simulate the initiation of ductile fracture caused by the free surface roughening. The in-situ EBSD observations reveal that the microstructure of the ultra-thin FSS sheet exhibits heterogeneous plastic deformation and strain localization on the surface at the large deformation owing to the considerable free surface roughening. The CPFE simulations well reproduce the stress/strain heterogeneity and the local hotspots of the ductile fracture initiation. It reveals that the increase of surface roughness with the deformation results in the increased inhomogeneity of thickness which accelerates the strain localization, consequently the premature necking and failure in the ultra-thin steel sheet. Due to the free surface roughening which can facilitate strain localization, the initiation of ductile fracture is obviously observed in both the ridges and valleys in the free surface and its propagation along the pronounced localization band through the thickness, leading to material failure. The good correlation of free surface roughening to the onset of plastic instability and ductile fracture further advances the insight into the micromechanical failure during plastic deformation of the ultra-thin FSS sheet for bipolar plate in PEM fuel cell.

Patterns of localized deformation at pre-fracture stage in carbon steel – stainless steel bimetal

The work is devoted to the study of strain localization at macroscale level during parabolic mechanical hardening and pre-fracture under quasi-static loading of a carbon steel – stainless steel bimetal. The problem of estimating the scale of the phenomena that determine plasticity is decisive in the development of any theories of plastic deformation, in particular, dislocation theories. The main difficulty in constructing such theories is the reconciling the dislocation scales, characteristic for most deformation and mechanical hardening mechanisms, with macroscopic parameters of deformation processes. In the framework of the autowave model of localized plastic deformation, this problem can be reduced to the possibility of obtaining parameters from the results of macroscale observations of localized plastic flow development. During the experiments, it was confirmed that in a bimetal at any forming stage, a specific pattern of localization centers distribution is spontaneously generated - a pattern of localized plastic flow. The shape of such patterns is determined by the law of mechanical hardening acting in the material. It is shown that the observed localization patterns can be used as an informative feature in predicting the plasticity margin. In the process of uniaxial tension at the stage of parabolic mechanical hardening of the bimetal, the deformation mode is realized with the formation of several potential fracture centers. It was established that at the pre-fracture stage, during the time evolution of the wave pattern of deformation localization, the zone of active plastic deformation narrows, but the number of centers in it either remains the same with a decrease in the distance between them, or even increases. The result of this process is the formation of a macroscopic neck, and then fracture. At the pre-fracture stage, the collapse point indicates the place of future fracture and signals the need to stop the deformation process in order to avoid the fracture of the bimetallic material. Thus, the well-known manifestation of deformation macroscopic localization – formation of a neck – is preceded by complex phenomena of mutually coordinated motion of localized plasticity centers at the pre-fracture stage.