Driving Force Calculation Research Articles

Internal oxidation behavior and its effect on the surface crack formation in a Fe-Mn-Al-C high Mn steel

The correlation between internal oxidation and surface crack was investigated in high Mn steel. Time- and temperature-dependent internal oxidation behaviors are examined in the temperature ranges of 1000– 1250 °C, revealing faster and deeper oxidation along grain boundaries than in grain. Oxides at the internal oxidation layer are confirmed to be MnO and MnAl2O4. The grain boundary penetration depths of the internal oxide increase from 13 to 34 μm as the temperature increases. The tensile crack on the surface of the specimen was evaluated after 5 % straining with 10−3/s strain. Temperature-dependent crack formation probability (LAC/LT) increases also from 0.4 to 1.6 % as the temperature increases. The evolution of the internal oxidation layer during the hot compression test was investigated to clarify its role in surface crack formation. The transition of the internal oxide layer to the external oxide layer was observed. Exfoliation of the external oxidation layer was also confirmed by further compression. Driving force calculation on the internal and external oxides reveals that the abundant oxygen supply originating from the cracking of oxide and thickness reduction of the internal oxide layer transforms the internal oxide layer to scale during the compression. High-resolution analysis of the grain boundary crack reveals the cracking of grain boundary MnAl2O4. The comparison between the grain boundary oxides’ penetration depth and the crack formation behavior clarifies the grain boundary internal oxide’s crucial role in the surface crack formation in a high Mn steel. This study provides vital insights into the surface crack formation mechanism in a Fe-Mn-Al-C-based high Mn steel during hot-rolling processes, highlighting the significance of grain boundary internal oxidation in production.

Modeling cable-pulley deployment systems of transformable rod structures

The aim of this article is to develop a simplified method for modeling cable-pulley deployment systems of rod structures based on the calculation of cable tensions and nodal driving forces with account for friction and other features of the system. Methods of theoretical mechanics, multibody dynamics, numerical integration of differential equations, and computer modeling were used during the research. The task of developing a simplified approach to modeling cable-pulley deployment systems for rod structures is considered. It is proposed to determine nodal driving forces by calculating cable tensions with account for friction and other features of the cable-pulley system, cables, and pulleys. To develop a model of cable-pulley deployment system, a rod system was chosen as the research object, which represents two sections of the transformable support truss of a reflector. Each section consists of diagonal and horizontal rods with tubular cross-sections. The sections are interconnected by hinge units. The structure is deployed using an upper and a lower cable, which pass through pulleys and are tensioned by an electric motor. The deploying forces are implemented by transferring the cable tension forces to the structure due to static friction and pressure between the cables and the pulleys. For further implementation of the model in an open-source software package, some simplifications were made due to the complexity of the design. A simplified method was developed for nodal driving force calculation in simulating rod structure deployment with the help of cables. The tensions, elongations, slacks, and neutral length of the cables and the forces transmitted from the cables to the pulleys were calculated as a function of time. Using them, the deployment of a rod structure was simulated for a constant cable speed. The results make it possible to control the rod system deployment time and rate depending on the characteristics and tension forces of the cables. The proposed approach is implemented using open-source software, and it provides modeling flexibility and reduces the model development and run time.

Phase-field simulation of misfit dislocations in two-phase electrode particles: Driving force calculation and stability analysis

Dislocations in lithium-ion battery materials significantly influence the performance of the battery. A chemo-mechanical phase-field model combined with a non-singular continuum theory of dislocations is developed to simulate misfit dislocations at the phase boundary of two-phase LiFePO4 particles. The configurational mechanics of dislocations in the mechanically coupled phase transformation problem is proposed and then used to formulate a dislocation formation criterion. The model is numerically implemented using the finite element method to compute driving forces on misfit dislocations. The driving force is further used to study the stability of the misfit dislocation to predict the critical size of dislocation-free two-phase electrode particles. The orthotropic and non-homogeneous elastic constants in two phases, anisotropic misfit strain, different aspect ratios of the particle, and different positions of the phase boundary in equiaxed particles are considered. The large misfit strain at the phase boundary is found to cause a redistribution of the concentration and thus a relaxation of the stress field at the interface. Stress relaxation is crucial to the driving force on the dislocation and ignoring the stress relaxation leads to an underestimation of the critical particle size. The critical particle size decreases with increasing aspect ratios and shows a saturation after aspect ratios above 2. An optimum aspect ratio of the dislocation-free electrode particle is around 1.5 which features the highest specific surface area.

Influence of dislocations on domain walls in perovskite ferroelectrics: Phase-field simulation and driving force calculation

Dislocations in ferroelectrics cause domain wall pinning and nucleation of ferroelectric domains, which significantly influence the electromechanical properties of ferroelectrics. To have an in-depth understanding of the role of dislocations in ferroelectrics, this work presents a finite element analysis of the driving force on domain walls and their interaction with dislocations. The phase-field model coupled with the non-singular continuum dislocation theory and the generalized configurational force theory are derived for dislocated ferroelectrics. In particular, by making use of the driving force on domain walls and dislocations, we propose a new approach to evaluate the critical electric field for domain walls to break through dislocations. It is shown to be computationally much more efficient than the brute force approach of scanning all electric fields. The influences of dislocations on the 180° and 90° domain walls are analyzed numerically. The dislocation-induced stress field causes fluctuation of the polarization, which results in multiple equilibrium positions of the domain wall. Increased polarization was observed on the tensile side of the dislocation, which is responsible for the strong pinning strength of the dislocation. The needle domain nucleated from the dislocation core also causes the pinning of domain walls, and the interaction between the domain wall and the needle domain results in nonlinear dependency of the critical electric field on the Burgers vector.

Reconciling the Driving Force and the Barrier to Charge Separation in Donor–Nonfullerene Acceptor Films

We investigate the dependence of charge yields on the driving force for photoinduced electron transfer in a series of all-small-molecule, semiconducting films made from indacenodithiophene nonfullerene acceptors (IDT NFAs). In contrast to reports of efficient, barrierless charge separation at near zero driving force for NFA-containing organic photovoltaics, we find that barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential, ≥300 meV, based on time-resolved microwave conductivity and PL quenching measurements of 5 mmolal sensitized and 700 mmolal blended films comprising IDT NFAs paired with three different donors. This discrepancy with recent literature is caused by a difference in the way that we calculate driving force. Far from being semantic, the driving force calculation is crucial because its value controls the mechanisms needed to explain experimental observations. We provide a candid assessment of the uncertainties for our methods and popular ones used in the literature and emphasize the importance of standardizing methods across this field.

Specific of the Recrystallization Driving Force Calculation on the early Stages of Thermomechanical Treatment of Aluminum Alloys

The article investigates the effect of the strain rate on the driving force of recrystallization during hot working of the as-cast structure. For the study, we applied previously obtained experimental data of recrystallization kinetics during this stage of thermomechanical treatment. In addition, hot laboratory rolling, followed by saltpeter bath soaking, were performed in order to obtain supplemental data on grain structure size and orientations. Grain structure size was examined by optical microscopy, and its orientation was examined by X-ray texture analysis. The studies demonstrated, that overestimated recrystallization driving force not only results in erroneous kinetics estimation, but also gives excessive number of recrystallization centers and undersized grain structure. Besides, unaccounted effect of recrystallization driving force on grain size leads to distorted predictions of texture composition. In order to avoid this, it was recommended to apply an special exponential accumulated strain dependent coefficient.

Selective Laser Sintering-Based 4D Printing of Magnetism-Responsive Grippers.

Components fabricated by four-dimensional (4D) printing hold the potential for applications in soft robotics because of their characteristics of responding to external stimuli. Grippers, being the common structures used in robotics, were fabricated by the selective laser sintering (SLS)-based 4D printing of magnetism-responsive materials and tested for remote-controllable deformation in an external magnetic field. A composite material consisting of magnetic Nd2Fe14B powder and thermoplastic polyurethane powder was selected as the raw material for the SLS; the magnetic particle acquired permanent magnetism by magnetization after the SLS process. Microscopic characterization showed the homogeneous dispersion of magnetic particles inside the polymer matrix. The magnetic induction intensity distribution was systematically investigated by both experiments and numerical simulations. The reliability of the numerical model proposed was justified by the excellent consistency between them. The deformation of the grippers could be regulated by tuning the magnetic particle content and the distance from the external magnet; the deformation mechanism is investigated numerically. The magnetic driving force and the corresponding horizontal displacement are calculated, thus having high accuracy compared with the existing research that obtained the deformation amount by only visual inspection. Mechanical properties of the SLS-fabricated magnetic polymer composite specimens were also studied because of their close relationship with the deformation behaviors. These findings provide guidance for future research on controllable deformation and driving force calculation for 4D printing.

Crack Driving Force Calculation for a Railway Wheel Considering Thermoplastic Properties of the Wheel Material

Crack Driving Force Calculation for a Railway Wheel Considering Thermoplastic Properties of the Wheel Material

On the Role of Chromium in Dynamic Transformation of Austenite

The effect of Chromium (Cr) on the dynamic transformation (DT) of austenite to ferrite at temperatures up to 430 °C above Ae3 was studied in a medium-carbon low-alloy steel. Hot compression tests were performed using Gleeble 3800® thermomechanical simulator followed by microstructural examinations using electron microscopy (FESEM-EBSD). Driving force calculation using austenite flow stress and ferrite yield stress on an inverse absolute temperature graph indicated that Cr increases the driving force for the transformation of austenite to ferrite; however, when the influence of stress and thermodynamic analysis are taken into account, it was observed that Cr increases the barrier energy and therefore, emerges as a barrier to the transformation. An analysis, based on lattice and pipe diffusion theories is presented that quantifies the role of stress on the diffusivity of Cr and is compared with other the main alloying elements such as C, Si and Mn and its impact, positive or negative, on the DT barrier energy. Finally, a comparison is made on the differential effects of temperature and stress on the initiation of DT in medium-carbon low-alloy steels.

A Combined Experimental and Modelling Approach to Elastic–Plastic Crack Driving Force Calculation in the Presence of Residual Stresses

Since all residual stress measurement methods have inherent limitations, it is normally impractical to completely characterise a three-dimensional residual stress field by experimental means. This lack of complete information makes it difficult to incorporate measured residual stress data into the analysis of elastic–plastic fracture without resorting to simplified methods such as the Failure Assessment Diagram (FAD) approach. We propose a technique in which the complete residual stress field is reconstructed from measurements and used in finite element analysis of the fracture process. Residual elastic strains and stresses in three-point bend fracture specimens were measured using neutron diffraction and an iterative method was used to generate a self-consistent estimate of the complete residual stress field. This enabled calculation of the J contour integral for a specimen acted on by both residual stress and an externally-applied load, allowing the interaction between residual and applied stress to be observed in detail.

Dynamics Verification Experiment of the Stewart Parallel Manipulator

As the basis of dynamic analysis and driving force calculation, dynamic models and dynamic parameters are important issues in mechanical design and control. In this paper, a dynamics verification experiment, which covers both dynamic models and dynamic parameters as a whole, is carried out on the typical Stewart parallel manipulator. First, the complete dynamic model of the Stewart manipulator is derived, considering the force sensors. The Newton-Euler method with clear physical meaning is adopted to facilitate understanding and parameter definitions. The dynamic parameters are deduced based on the established three-dimensional virtual prototype and adjusted with actual measurements. The recorded trajectory, instead of the theory trajectory, is adopted to calculate the theoretical limb forces. The practical limb forces are measured using pull pressure sensors. Finally, the dynamic model and identified parameters are verified by comparing the limb forces obtained using the above two approaches. Experiment results show that theoretical and practical limb forces coincide well, with a small maximum RMS (root mean square) error of 1.516N and forces ranging from 10N to 40N. Additionally, the established dynamics verification algorithm, which involves dynamic modelling, a parameter identification approach and a data analysis method, are generic and practical, and can be flexibly applied to the dynamic analysis of other parallel manipulators.

Cohesive interface simulations of indentation cracking as a fracture toughness measurement method for brittle materials

Cracks in brittle solids induced by pyramidal indenters are ideal for toughness evaluation since the indentation stress fields decay rapidly from the contact center and any cracks will be eventually arrested. Thus, if the applied energy release rate can be determined analytically, the material toughness can be deduced by measuring the crack length. However, such a driving force calculation is a nontrivial task because of the complex stress fields; only a number of limit cases can be solved, such as the long half-penny cracks (at least two times larger than the contact size) in the classic Lawn–Evans–Marshall (LEM) model. Important questions such as the evolution from short cracks to median/radial and then to half-penny cracks, the form of the scaling relationship that relates fracture toughness to material hardness and indenter angles, the threshold load for indentation cracking, etc., cannot easily be answered without a detailed knowledge of the co-evolution history of the stress fields and crack morphology. To this end, a finite element model of four-sided pyramidal indentation adopting cohesive interface elements is developed to study the effects of indenter geometry, load, cohesive interface parameters, and material properties on the initiation and propagation of the median/radial/half-penny crack systems. The validity and artifacts of the cohesive interface model are carefully examined, and the crack morphologies under various indentation and material parameters are systematically studied. Numerical predictions lead to a quantitative evaluation of the threshold load for indentation fracture, and an improved method for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, which compare favorably to a large set of experiments in the literature. It is also found that the toughness evaluation method is very sensitive to Poisson’s ratio – an observation that has previously received little attentions. An approximate analysis for short cracks is developed based on the fracture mechanics of annular cracks and the embedded-center-of-dilatation model for indentation-induced residual stress fields.

Determination of Remnant Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

Abstract: Material fracture toughness data are required to undertake fitness‐for‐service assessments of engineering components containing cracks. Calculations of crack driving force in the component are compared with material fracture toughness values to assess the likelihood of subsequent failure. Experimental measurements of fracture toughness are usually made on small specimens extracted from a larger ‘parent’ component following strict experimental guidelines, formulated to ensure measured toughness values in the fracture specimens are appropriate for use in the full‐size component. Implicit in this procedure is the assumption that the extracted fracture specimens contain no residual stresses, with any residual stresses in the full‐size component being accounted for in the crack driving force calculation. This paper considers a recent conjecture within the structural integrity community that the extracted fracture specimens may themselves contain a residual stress field which may influence measurements of fracture toughness. This could potentially lead to a degree of ‘double accounting’, i.e. the effect of residual stresses may be included in both the material toughness and the crack driving force. This, in turn, could lead to unnecessary conservatism in safety assessments. To explore this conjecture, the results of numerical modelling and neutron diffraction measurements of residual stresses in fracture specimens extracted from two different welded parent components are presented. One of the components is significantly larger than the extracted specimens, with the other being marginally larger than the extracted specimens. Results confirm the intuitive expectation that the residual stresses in specimens extracted from much larger components are negligible, whereas if the dimensions of the extracted specimens are comparable with the larger component then significant residual stresses may remain.

Thermodynamic description on the miscibility gap of the Mg-based solid solution in the Mg–Zn, Mg–Nd and Mg–Zn–Nd systems

With the consideration of the existence of the metastable miscibility gaps, the thermodynamic parameters of the Mg-based solid solution have been assessed for the Mg–Zn and Mg–Nd systems. The new semi-empirical equation proposed by Kaptay was used to describe the interaction parameters between components to prevent the appearance of the artificial miscibility gap at high temperature. The obtained thermodynamic descriptions can reproduce the metastable miscibility gaps and the spinodal decomposition curves in the Mg–Zn and Mg–Nd systems. The driving force calculation can explain well the precipitation of the second hcp_A3 phase (Guinier–Preston zones) in the primary Mg-based hcp_A3 matrix, as well as the decomposition sequence following a quench from a single-phase supersaturated solid solution to a two-phase regime. The miscibility gap of the Mg-based phase in the Mg–Zn–Nd ternary system was predicted on the basis of the descriptions of the Mg–Zn and Mg–Nd binary systems without consideration of the three-component interactions.