Behavior Of Metallic Materials Research Articles

Strain-rate and size dependence of gradient lamellar nickel investigated by in-situ micropillar compression

Strain rate plays a nonnegligible role in the plastic deformation behavior of metallic materials with heterogeneous nanostructure, while little research focuses on the topic. In-situ compression tests at various strain rates were conducted on the micropillars located at different depths from the gradient lamellar Ni. These tests intended to reveal the strain-rate dependence of micropillars with different lamellar thicknesses and illustrated the competitive relationship between strain rate and intrinsic grain size on the plastic deformation behavior of metallic materials. In addition, due to the strain rate sensitivity related to not only intrinsic size but also external size, single Ni micropillars with different diameters were also compressed at various strain rates in order to clarify the competitive relationship between intrinsic size and external size on the strain rate sensitivity. It is found that lamellar thickness rather than strain rate has more effect on plastic deformation, and external size exhibits a more obvious impact on the strain rate sensitivity. The plastic deformation behaviors of different micropillars are mainly explained by transitions in the effect of lamellar grain boundaries on the dislocation motion at various strain rates and over different lamellar thicknesses, resulting in the change of dislocation multiplication and annihilation rate.

Modelling the effects of void swelling and spatial heterogeneity on the fracture of metallic materials by crystal plasticity

To theoretically analyse how void swelling and spatial heterogeneity impact the fracture behaviour of metallic materials, a coupled porous crystal plasticity framework has been developed. The proposed framework is then applied to analyse the fracture behaviour of metallic materials under different swelling conditions and types of spatial heterogeneity. It is revealed that at low porosity, the spatial heterogeneity of voids will have limited influence on the fracture behaviour of the material. However, at high porosity, owing to void swelling, a concentrated distribution of voids near grain boundaries can lead to a strong deformation localization, which can be seen as an indicator of possible quasi-brittle fracture. It is further discovered that the inner pressure of the voids tends to reduce the yield stress of the material and promote localized plastic deformation. As a result, such pressure will enhance the impact of concentrated distribution of voids, which can lead to further embrittlement of the material. The present study reveals the key role of void swelling and spatial heterogeneity in regulating the fracture behaviour of metallic materials, a step towards a better understanding over the failure mechanisms of metallic materials under extreme conditions.

Revealing and predicting the mechanical and strain hardening behaviors of Cu-Al alloys based on microscopic mechanism

In order to reveal and predict the strain hardening behaviors of metallic materials, mechanical properties of Cu-Al alloys processed by rolling were investigated by tensile tests. The microstructure evolution of Cu-Al alloys processed by rolling were characterized. The true stress-strain curves were fitted by an exponential strain-hardening (ESH) model. Based on the microscopic mechanism of dislocation growth and annihilation, the true stress strain curves were predicted through macroscopic mechanical properties. The strengthening effects on Cu-Al alloys of different methods were analyzed quantitively by hardening exponent z and saturation strength σs which were derived from the ESH model. The fundamental assumptions and physical meaning of parameters have been validated by investigating strain hardening behaviors of Cu-Al alloys. The limited applicable materials and future research of ESH model are also proposed for the further development. This study could provide guidance to understand work-hardening behavior and predict tensile performance of Cu-Al alloys.

Experimental Characterization and Phase-Field Damage Modeling of Ductile Fracture in AISI 316L

(1) Modeling and characterization of ductile fracture in metals is still a challenging task in the field of computational mechanics. Experimental testing offers specific responses in the form of crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM) implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model (PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed, with a threshold to propose a new relationship between critical fracture energy and critical total strain value. In addition, the threshold value of potential internal energy—which controls damage evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test. The PFDM model is validated against the experimental results obtained in the fracture toughness characterization test, with the simulation results being within 8% of the experimental measurements. (4) The novel implementation offers the possibility for better control of the ductile behavior of metallic materials and damage initiation, evolution, and propagation.

A theoretical and experimental study of deformation mechanism dictated by disclination-dislocation coupling in Mg alloys at different temperatures

Dislocations and disclinations are fundamental topological defects within crystals, which determine the mechanical properties of metals and alloys. Despite their important roles in multiple physical mechanisms, e.g., dynamic recovery and grain boundary mediated plasticity, the intrinsic coupling and correlation between disclinations and dislocations, and their impacts on the deformation behavior of metallic materials still remain obscure, partially due to the lack of a theoretical tool to capture the rotational nature of disclinations. By using a Lie-algebra-based theoretical framework, we obtain a general equation to quantify the intrinsic coupling of disclinations and dislocations. Through quasi in-situ electron backscatter diffraction characterizations and disclination/dislocation density analyses in Mg alloys, the generation, coevolution and reactions of disclinations and dislocations during dynamic recovery and superplastic deformation have been quantitatively analyzed. It has been demonstrated that the obtained governing equation can capture multiple physical processes associated with mechanical deformation of metals, e.g., grain rotation and grain boundary migration, at both room temperature and high temperature. By establishing the disclination-dislocation coupling equation within a Lie algebra description, our work provides new insights for exploring the coevolution and reaction of disclinations/dislocations, with profound implications for elucidating the microstructure-property relationship and underlying deformation mechanisms in metallic materials.

Influence of an Engineered Notch on the Electromagnetic Radiation Performance of NiTi Shape Memory Alloy.

This work explores the influence of a pre-engineered notch on the electromagnetic radiation (EMR) parameters in NiTi shape memory alloy (SMA) during tensile tests. The test data showed that the EMR signal fluctuated between oscillatory and exponential, signifying that the specimen's viscosity damping coefficient changes during strain hardening. The EMR parameters, maximum EMR amplitude, and average EMR energy release rate remained constant initially but rose sharply with the plastic zone radius with progressive loading. It was postulated that new Frank-Read sources permit dislocation multiplication and increase the number of edge dislocations participating in EMR emissions, leading to a rise in the value of EMR parameters. The study of the correlation between EMR emission parameters and the plastic zone radius before the crack tip is a vital crack growth monitoring tool. An analysis of the interrelationship of the EMR energy release rate at fracture with the elastic strain energy release rate would help develop an innovative approach to assess fracture toughness, a critical parameter for the design and safety of metals. The microstructural analysis of tensile fractures and the interrelation between deformation behaviours concerning the EMR parameters offers a novel and real-time approach to improve the extant understanding of the behaviour of metallic materials.

Analytical modelling and electrochemical impedance spectroscopy (EIS) to evaluate influence of corrosion product on solution resistance

Electrochemical impedance spectroscopy (EIS) is a technique used to evaluate the electrochemical behavior of metallic materials in different environments. In this study, a mathematical model has been developed to analyse the relationship between solution resistance and concentration conductive-corrosion products (Fe2O3) of metallic corroded materials. This model has been designed as a part of an experimental series to use EIS as a tool for mapping the spatial distribution of corrosion by-product from bridge, in order to evaluate the impact of conductive-corrosion on the properties of the solution. The influence of Fe2O3 on the solution resistance at varying concentrations, has been modelled. Repetitive electrochemical tests were conducted to investigate the relationship between the impedance and concentration in three different concentrations of corrosion by-product. Nyquist and Bode's graphs have been used to quantitatively analyse the EIS data. The implementation of the proposed mathematical model can quantify the solution resistace based on the mass of presented particles, and provide significant efficiency and methodological advancement over EIS technique. The experimental outcomes show a clear link between solution resistance and iron oxide concentration within the solution which is consistent with the model's finding.

Dynamic tensile deformation behavior of AISI 316L stainless steel fabricated by laser-beam directed energy deposition

The dynamic deformation behavior of metallic materials is the key to crash-safe design in many applications, e.g., in the automotive sector. However, in the field of additive manufacturing, dynamic deformation properties have only been scarcely studied. Based on the intrinsic interrelationships of process parameters, post-process heat treatments and the resulting microstructure, the present study seeks to provide a holistic overview of the dynamic tensile deformation properties of AISI 316L stainless steel, fabricated by laser-beam directed energy deposition. Utilizing in situ digital image correlation, the local deformation behavior at varying strain rates from 100 s−1 to 1000 s−1 was unveiled. In combination with pre- and post-mortem microstructure analysis using electron backscatter diffraction and scanning transmission electron microscopy, the microstructural effects on the active deformation mechanisms were studied. The results show that twinning contributes to the overall deformation in the heat-treated condition owing to the weaker crystallographic texture. On the contrary, twinning is less prominent in the non-heat-treated condition due to its pronounced <001> texture. As a result of the differences in terms of active deformation mechanisms, superior fracture elongations are observed in heat-treated condition, which are rationalized by delayed necking due to a more homogeneous strain distribution along the gauge section. Independently from the sample condition, fractography analysis revealed a ductile mode of failure being characterized by equiaxed dimples on the fracture surfaces. Consequently, it can be concluded that AISI 316L stainless steel is excellently suited for dynamic tensile loading, although heat-treatment-depended differences have to be taken into account.

Coupling effect of strain gradient strengthening and thermal softening on the microscale plastic behavior of metallic materials

Size effects and thermal effects together determine the microscale plastic response of metallic structures in high-integrated microelectromechanical systems (MEMS) at different temperatures. To investigate the microscale plastic behavior of metallic materials at different temperatures, a thermo-mechanical coupled microscale plasticity model is developed. In the mechanical part of this model, the framework of conventional plasticity theory is maintained, and the plastic strain gradient is introduced as an internal variable to increase the tangential hardening modulus without the introduction of higher-order stress and higher-order boundary conditions. The influence of temperature on the mechanical parameters (e.g. the yield strength) is calibrated by experiment results. In the thermal part of this model, the heat generation in the plastic deformation stage is calculated by the difference between the plastic work and the hardening stored energy influenced by the plastic strain gradient. Due to the strong nonlinearity of the coupled equations, a finite element solution algorithm that combines the radial return method and the staggered solution scheme is proposed. The effectiveness of this model and its solution algorithm is verified by comparisons with the experiment results and a numerical benchmark example. Finally, taking the tensile behavior of a plate with a hole in its center as an example, the coupling effect of strain gradient strengthening and thermal softening on the microscale plastic behavior of metallic materials is investigated. The results show that the microscale plastic behavior of metallic materials at high temperatures depends on the competition between thermal softening and strain gradient strengthening. Our study provides a theoretical basis and a reliable simulation method for the design of MEMS at different temperatures.

Ductile fracture prediction of HPDC aluminum alloy based on a shear-modified GTN damage model

In this paper, we investigate how the shear-modified Gurson-Tvergaard-Needleman (GTN) model can be used to reveal the effect of manufacturing-process-induced porosity on the scatter of ductile fracture properties of a high-pressure die-casting aluminum alloy. The GTN model exhibits great advantages in predicting the variation of failure strain/displacement by altering the initial void volume fraction (f0), compared with uncoupled ductile damage models. For a specific metallic material, one unique set of model parameters is usually determined from experiments, which leads to the fact that only a deterministic failure strain/displacement can be obtained under a fixed porosity. To overcome this shortcoming of the shear-modified GTN model, a novel parameter calibration scheme is proposed and validated in the present study. Following the physical background of the GTN model, the twelve material parameters of the shear-modified GTN model have been determined based on a combination of macroscopic mechanical tests covering a wide range of stress states and micromechanics-based unit cell simulations. Different from the existing calibration strategies, the simulation results of unit cells embedded with a spherical void have revealed a mutual dependence between the f0 and critical void volume fraction (fc). By conducting interrupted shear tests, the evolution of secondary nucleated void volume fraction was identified according to the statistical results of the fractured brittle silicon particles. The remaining parameters have been further iteratively determined. In the end, the effectiveness of the proposed parameter determination approach is confirmed based on the accurate prediction of stochastic ductile failure properties (both the global failure displacements and the local failure patterns) in different fracture tests. Given the proposed parameters identification strategy, the GTN material models have been enriched to predict the ductile failure behavior of metallic materials, which possess relatively high porosities and more pronounced scattering in failure properties.

On the Effect of Testing Frequency on High and Very High Cycle Fatigue Behavior of AA2024-T3, Ti-6Al-4V, and Inconel 718

An increase of testing frequency to reduce the testing time could affect the fatigue behavior of metallic materials. The current paper investigates the effect of testing frequency on the high and very high cycle fatigue behavior of AA2024, Ti-6Al-4V, and Inconel 718 alloys tested at 20, 90, and 1000 Hz. To quantify the effect of testing frequency a two-parameter Weibull distribution and Basquin’s equation were utilized as well as the threshold stress intensity factor range was determined. Fatigue crack initiation sites were evaluated on the fracture surface with respect to different testing frequencies. The results showed that changes in the testing frequency significantly influence the mechanical response of the Ti-6Al-4V alloy, while no substantial impact on the fatigue properties of AA2024 and Inconel 718 alloys was identified. The findings of the study contribute to a more complete knowledge of the frequency sensitivity of the alloys and provide valuable insights for the design and evaluation of materials used in high-frequency applications.

A damage-based uniaxial fatigue life prediction method for metallic materials

Determining the fatigue behaviour of metallic materials using standardised testing methods is costly and time-consuming. Therefore, several methods have been proposed to shorten the testing time and improve the fatigue optimisation of materials and components. This work presents a new fatigue testing method based on fatigue damage monitoring that allows determining the fatigue resistance in a short time and with few specimens. The presented method, named as the stiffness method, monitors the inelastic strains as an indicator of fatigue damage evolution. Strain measurements were carried out by digital image correlation techniques and showed to effectively follow damage evolution during fatigue tests. Results are convincing and more evident to obtain and discuss than other monitoring techniques, like temperature dissipation. In addition, the method overcomes the main limitations of the existing fast testing methods by avoiding the utilisation of complex apparatus, like infrared cameras or acoustic emission sensors. The approach has been validated in ten different metallic materials, as titanium and aluminium alloys, carbon steels, and stainless steels. The estimated fatigue limit was compared with values obtained following standardised tests, showing excellent agreement. Results allow pointing out the stiffness method as an efficient and effective tool for rapidly determining the fatigue behaviour of metallic materials.

Inverse characterization of a material model using an ensemble-based four-dimensional variational method

The identification accuracy of material model parameters is essential for accurately predicting the deformation behavior of metallic materials (e.g., metal forming) using a finite element (FE) simulation. Numerous researchers have studied inverse material model characterization, where parameters are determined by minimizing a cost function that quantifies the difference between experimental data and mechanical test simulation results. However, sensitivity analysis in the optimization process hinders the extensibility of inverse methods due to issues like computational cost and complex numerical implementation. In this study, we developed a novel inverse methodology for material model characterization that improves extensibility by applying an ensemble-based four-dimensional variational method (En4DVar), which has the potential to address the challenges associated with conventional FE-based inverse material model characterization. The developed method was verified through numerical experiments in which En4DVar was applied to an elastoplastic FE simulation of the deformation of an aluminum alloy during a uniaxial tensile test, including diffuse necking. We investigated the estimation accuracy of the strain-hardening parameters in Swift’s hardening law and evaluated the simulation results under various conditions through numerical experiments. We focused on the effect of time and location to incorporate synthetic experimental data into the simulation to examine the quantities of synthetic experimental data required for parameter estimation. The results of the numerical experiments showed that En4DVar is a powerful approach for estimating the parameters and characterizing the deformation behavior of a material. Moreover, it was shown that accurate estimation results can be obtained even using synthetic experimental data with a relatively low temporal resolution or a small field of view. The proposed method's ease of extensibility using En4DVar expands the range of problems solvable in the field of material model characterization.

Effects of loading frequency on fatigue behavior of metallic materials—A literature review

AbstractThe topic of loading frequency effect on fatigue behavior is in close relation to high‐cycle fatigue (HCF) and very‐HCF (VHCF) research. The conventional loading frequency in fatigue tests is normally between 10 and 100 Hz. In order to accelerate testing process, researchers have developed ultrasonic vibration device with a very high frequency, for example, 20 kHz, to perform VHCF tests. Thus, a question has always been asked: Does loading frequency affect the fatigue behavior of tested material? It is obvious that the answer is yes because the remarkable difference in strain rate with respect to different loading frequencies may cause the change of fatigue mechanism and the related fatigue performance. The subsequent question is how is the effect of loading frequency on fatigue behavior? This paper attempts to address such questions by two cases of experimental investigations. Moreover, typical results in literature are quoted for the further discussion of this issue.

Strain rate effect and dynamic constitutive model of 7A04-T6 high-strength aluminium alloy

In recent decades, various aluminium alloys have been developed and employed in construction engineering. They are light, corrosion-resisting and recyclable, making them attractive to engineers and researchers. Among these aluminium alloys, 7A04 high-strength aluminium alloy, which is an Al-Zn-Mg-Cu alloy, has now gained focus for its excellent strength-to-weight ratio, hardness, formability, etc. To date, the investigations into its dynamic mechanical properties are still limited. In this context, the mechanical behaviour of 7A04-T6 aluminium alloy at high strain rates was studied in this work. A split Hopkinson pressure bar test system was employed to conduct the dynamic experiments. 6061-T6 aluminium alloy was also included in the tests for reference. Then, comparisons were made between the tested 7A04-T6 and 6061-T6 aluminium alloys and among the studied 7A04-T6 aluminium alloy and steels with approximate yield strengths (Q420 and Q550 structural steels) and similar strength-density ratios (4340 M and Aermet 100 steels), to assess the strain rate sensitivity of 7A04-T6 aluminium alloy. Based on the Johnson-Cook model and the Cowper-Symonds model, which are widely used models that describe the dynamic constitutive behaviour of metallic materials, a modified dynamic constitutive model was developed for 7A04-T6 aluminium alloy, in which the influence of deformation-induced temperature rise on the dynamic mechanical properties is well considered.

Corrosion fatigue crack growth in stainless steels: A peridynamic study

Fatigue crack growth (FCG) behaviors are accelerated by corrosive environments, resulting in higher corrosion FCG (CFCG) rates and more rapid structural failure. Current local-theory-based numerical methods do not effectively capture the CFCG behaviors of metallic materials. Recently, peridynamic (PD) models, which are nonlocal-theory-based, have demonstrated good descriptive ability for discontinuous problems, making it promising to simulate CFCG via PD methods. In this paper, a peridynamics-based CFCG simulation framework is proposed and applied for the CFCG simulation of the stainless steel, Type-634. This framework provides explanations of corrosion-induced faster crack growth from the view of degradations of material properties. And the simulated results agree well with the experimental testing results, demonstrating the effectiveness of the proposed model. Moreover, the effects of corrosion-related degradation function parameters and loading frequencies on CFCG results are explored, and the results demonstrate the generalizability of the simulation method. With further development and embedding more accurate physical models, this simulation framework will effectively assist real CFCG tests.

Fatigue Life Assessment of Metals under Multiaxial Asynchronous Loading by Means of the Refined Equivalent Deformation Criterion

As is well-known, non-proportional fatigue loading, such as asynchronous one, can have significant detrimental effects on the fatigue behavior of metallic materials by reducing the fatigue strength/fatigue limit and by leading to a fatigue damage accumulation increased with respect to that under proportional loading. In the present paper, the novel refined equivalent deformation (RED) criterion is applied for the first time to estimate the fatigue lifetime of materials, sensitive to non-proportionality, subjected to asynchronous loading under low-cycle fatigue regime. The present criterion is complete since it considers: (i) the strain path orientation, (ii) the degree of non-proportionality, and (iii) the changing of material cyclic properties under non-proportional loading. To evaluate its accuracy, this criterion is applied to examine two different metals (a 304 stainless steel and a 355 structural steel) whose experimental data under multiaxial asynchronous loading are available in the literature. More precisely, the parameters of the criterion are firstly determined by using experimental strain paths, and then the computed refined equivalent deformation amplitude is used to represent the experimental data with a satisfactory accuracy. Finally, a comparison with the results obtained through two other criteria available in the literature is performed, highlighting the good prediction of the present RED criterion.

The mechanism of abnormal corrosive behavior of metallic materials in the pseudocritical region of supercritical water

To study the corrosive behavior of metal materials in the pseudocritical region of supercritical fluid and explain the abnormal mechanism in corrosive within the pseudocritical region, a corrosive experimental platform for supercritical fluids was built. Three different candidate materials for Supercritical Water Reactor (i.e. Q235 low carbon steel, 316 L austenite stainless steel and 3Cr13 martensitic steel) were selected and reacted for 200 h in the pseudocritical region (375.6 ℃, 22.5 MPa) and gas-like region (405.6 ℃, 22.5 MPa) respectively. The results show that 316 L exhibits abnormal corrosive exacerbation in the pseudocritical region with a corrosive rate 1.67 times that of the gas-like region. Considering the special variation of physical properties in pseudocritical region, the enhanced kinetic effect is further analyzed. It is found that the migration of corrosive media such as oxygen to the material matrix is accelerated, which aggravates the corrosive products shedding and freeing. The present study can provide a basis for improving the anti-corrosive performance of materials under supercritical fluid environments and providing a safe operation of the Supercritical Water Reactor.

Interfacial-constraint-induced intragranular deformation inhomogeneity of Ti−Al layered composites

AbstractA multilayered Ti−Al composite was designed and fabricated by hot pressing and hot rolling of alternately stacked Ti and Al foils. In situ electron backscatter diffraction was utilized to characterize the orientation evolution and deformed substructure upon tension. A remarkably inhomogeneous deformation mode was observed within the interior of Ti grains, in a way that the closer to the grain boundary or Ti/Al interfaces, the harder the plastic flow. Such a phenomenon is attributed to the synergetic constraint of phase interface and grain boundary. The present study thus provides a new strategy to tailor the deformation behavior of metallic materials by sufficiently leveraging the interfacial constraint effect for performance optimization.

Nano Mechanical Characterization and Physical Modeling of Plastic Deformation Chapter 2: Strengthening Mechanisms by Various Lattice Defects

Mechanical behaviors of metallic materials in a small scale are characterized and physically modeled based on experimental measurements and computational simulation. An effect of in-solution carbon or silicon in Fe on a plasticity initiation was characterizes by nanoindentation pop-in phenomenon. A coherency at a ferrite-cementite interface significantly affects to a plasticity initiation leading to a different macroscopic yielding behavior. A stability of an austenite phase in TRIP steel was evaluated though the analysis of indentation-induced deformation behavior, demonstrating a significant constraint effect on stabilization by an adjacent harder phase. Heterogeneous microstructures with a bimodal grain size in fcc metals show an inhomogeneous mechanical behavior depending on a location of the fine microstructure. An intermittent plasticity in pure iron was analyzed through a statistical model showing a Gaussian function by thermally activated process in the early stage and a power-law one by an avalanche type phenomenon in the subsequent stage. An elementally step in a plastic deformation in a metallic glass was characterized through Molecular Dynamics simulation indicating that the plasticity inhomogeneity depends remarkably on the initially inhomogeneous atomistic structure.