Internal Stress State Research Articles

Characterization of phonon thermal transport of Ti3C2T x MXene thin film

Two-dimensional MXene materials with high electrotonic conductivity, good chemical stability, and unique laminar structure show great potential in the field of electrochemistry. In contrast to the widely concerned electrical properties, studies on the thermal properties of MXene materials are very limited. This paper presents a comprehensive analysis of the thermal properties of Ti3C2T x MXene thin film. Thermal diffusivity and thermal conductivity of Ti3C2T x films are characterized by the transient electro-thermal technique. The experimental results show a 16% enhancement in thermal conductivity when the temperature is increased from 307 K to 352 K. The phonon transport contributes substantially to thermal conductivity compared with electron transport. Molecular dynamic simulation is employed to further investigate the role of phonon thermal transport of Ti3C2 layer. It is found that the combined effect of specific heat capacity, stacking structure and internal stress states is responsible for the thermal transport performance of Ti3C2T x MXene thin film.

Optimization of the photoelastic conoscopic fringe pattern processing for the stress measurement on uniaxial PWO crystal cut parallel to the a-c plane and with variable thickness.

This paper presents a case study in which conoscopic, non-invasive, photoelastic techniques and the associated model are applied for the first time to investigate the internal stress state of the PbWO4 (PWO) uniaxial crystal, cut normal to the crystallographic a-c plane and with non-uniform thickness. As a matter of facts indeed, in the mass production of crystals it is rather probable to have specimen of non-constant thickness. In addition, the crystal cut, normal to the a-c plane, lead to a complex fringes structure and modeling. This can affect the fringe patterns interpretation, and in turn the interpretation of the stress state within the sample. In this paper we analyze the dependence of some model parameters on the thickness, and the relative interpretation of the fringe pattern in order to obtain a reliable measure of the stress for observations normal to the a-c plane. This procedure can speed up the crystal analysis whatever the thickness of the crystal is.

Chemical vapor deposited alumina hard coatings: Residual stress states decisively determined by the physical material properties

Machining tools made from cemented carbides coated with protective hard layers are indispensable in machining technology and are examined down to the smallest detail for reasons of patent law. As established in thin film analysis practice, a biaxial stress model is in general used for residual stress evaluation. But important information gets lost as a result of this simplification and the potential for product improvement is wasted. If someone goes into more detail the appearance changes and enables a deeper and more fundamental understanding. The X-ray diffraction study presented demonstrates that the residual stress state in the top α-Al2O3-layer is more complex than the usual model assumption of a biaxial stress nature suggests. It can again be shown that the internal stress state arising during the chemical vapor deposition process, is strongly influenced by the thermal expansion behavior of the material under investigation. But the presented results reveal that the impact is much higher than discussed in previous studies. The internal stress field is triaxial, anisotropic and strongly direction-dependent, presumably due to the crystal anisotropy in combination with growth textures of [0 0 1] type. Nevertheless, with respect to direction-dependent grain interaction the sample elasticity is not affected, and the sample state proves to be quasi-isotropic.

Induced internal stresses and their relation to FLASH sintering of KNN ceramics

Direct link between FLASH sintering, structure, microstructure and higher residual compressive internal stress state compared to conventional sintering.

Bone fragility: conceptual framework, therapeutic implications, and COVID-19-related issues.

Bone fragility is the susceptibility to fracture even for common loads because of structural, architectural, or material alterations of bone tissue that result in poor bone strength. In osteoporosis, quantitative and qualitative changes in density, geometry, and micro-architecture modify the internal stress state predisposing to fragility fractures. Bone fragility substantially depends on the structural behavior related to the size and shape of the bone characterized by different responses in the load–deformation curve and on the material behavior that reflects the intrinsic material properties of the bone itself, such as yield and fatigue. From a clinical perspective, the measurement of bone density by DXA remains the gold standard for defining the risk of fragility fracture in all population groups. However, non-quantitative parameters, such as macro-architecture, geometry, tissue material properties, and microcracks accumulation can modify the bone’s mechanical strength. This review provides an overview of the role of different contributors to bone fragility and how these factors might be influenced by the use of anti-osteoporotic drugs and by the COVID-19 pandemic.

The Influence of Different Lay-Up Parameters on the Fatigue Response of Carbon/Epoxy Laminates under Internal Multiaxial Stress States.

The inherent anisotropy of composites complicates their damage response. The influence of multiaxiality, particularly in carbon-based composites, is not thoroughly understood due to obstacles related to damage monitoring during loading. In this study, the response of different carbon/epoxy laminates under fatigue is examined through dedicated in situ microscopic observations. By varying the orientation of off-axis layers, the impact of multiaxiality on the mechanical and damage response is evaluated. Furthermore, balanced and unbalanced laminates are compared, considering the limited information for the latter. The influence of the number of off-axis layers is finally assessed leading to important conclusions about optimal fatigue response. The fatigue response is evaluated in all cases considering both the mechanical properties and the damage characteristics. Significant conclusions are drawn, especially for the benefits of unbalanced laminates and the impact of shear stresses, allowing for the utilization of the obtained data as important input for the establishment of reliable fatigue damage models.

Understanding the rigid-block equilibrium method by way of mathematical programming

This paper discusses and extends some main features of the rigid-block equilibrium (RBE) method. RBE is a numerical approach that frames the equilibrium problem of rigid-block assemblies as an optimisation problem to compute possible internal and equilibrated singular stress states. The contact between blocks is considered having a finite friction capacity and the unilateral behaviour is modelled through a penalty formulation. In particular, the penalty formulation widens the standard admissible solution space of compressive-only forces by allowing for tensile forces appearing on potentially unstable regions. The RBE objective function minimises the interface forces whereas the constraints are linear functions enforcing the static equilibrium of the whole assembly. In this paper, along with the original quadratic objective function, the authors propose a linear function to illustrate and explore the role played by both the nodal forces and the interface resultants. Moreover, the authors show how RBE can be used to explore different admissible internal stress states – for example, due to increasing, static, horizontal actions.

Influence of deformation degree of austenitic steels welded joints on structural state and internal stresses felds in weld line zone

Nowadays initial assessment of welding quality is performed by testing equipment with increased loads (high pressure) at technical devices of hazardous production facilities. Test requirements are regulated by standardized documents of the Federal Service for Environmental, Technological and Nuclear Oversight of Russia (Rostekhnadzor). Recently, along with traditional tests, a “stress test” was used – the essence of which is to load pipeline section to the yield point, followed by leak test. However, in scientifc publications there is practically no information about physical processes occurring in the base metal and in welded joints during such tests. In addition, effect of preload (deformation) on the parameters of substructure and internal stresses feld in welded joints of austenitic steels and, consequently, on the further trouble­free operation of the tested equipment was not evaluated. The paper analyzes changes in structural state and values of internal stresses in the samples of austenitic steel under the action of high loads. It substantiates the use of modulated current welding with automatic control of heat input process in molten weld pool. The admissible limits values of plastic deformation are argued when testing technical devices with high pressure for this type of steel. In order to reduce the risk of damage to austenitic steels welded joints of technical devices of hazardous industrial facilities, performed by pulsed welding with small­drop transfer, and to exclude formation of microdefects in them, high pressure tests (stress test) can be performed under loads that create deformations in metal, not exceeding 5 %. For joints welded by manual arc welding, deformations should be less than 5 %. Welded joints made by pulsed welding with large­drop transfer (with and without defects) are not recommended to be tested with high pressure.

Study on fault sealing mechanism based on rock physics simulation experiment

At present, the development of fault oil and gas reservoirs has become an important way for to increase oil and gas productivity in China. However, due to the complex internal stress state of rock faults, the phenomenon of oil and gas escape is prone to occur during the mining process, so it is of great significance to study the sealing mechanism of the rock fault. For this reason, based on the theory of fault sealing, rock physical simulation experiments of fault sealing were carried out, and the mechanism of fault sealing and its main controlling factors were deeply studied. The results show that the rock fault dip and the argillaceous content of the fault filling are the main controlling factors for fault sealing; The essence of fault lateral sealing and vertical sealing is the sealing of the displacement pressure difference; As the dip angle of the fault increases, the lateral sealing of the fault becomes better, while the vertical sealing becomes worse; with the increase of the argillaceous content of the fault filling, the lateral and vertical sealability of the fault becomes better.

Step-wise R phase transformation rendering high-stability two-way shape memory effect of a NiTiFe-Nb nanowire composite

In this work, we designed a NiTiFe-Nb nanowire composite to achieve a high-stability and small-hysteresis R phase two-way shape memory effect (TWSME). The R-phase TWSME was achieved in the composite in an annealed state without thermomechanical training due to the inherent internal stresses associated with the Nb nanowires, as demonstrated by in situ synchrotron high-energy X-ray diffraction. Transmission electron microscopic analysis also revealed some details of the R-phase transformation process, which render an explanation of the high cyclic stability of the R-phase TWSME. The R-phase transformation in this NiTiFe-Nb nanowire composite was found to proceed over several stages, including the formation of precursor nanodomains of partial lattice distortion for the R phase, the formation of R phase particles of coordinated orientations, the coalescence of the R phase particles into selected plate variants of preferential orientations for TWSME, and the continued evolution of the R phase crystallographic structure for further TWSME with zero hysteresis. Such step-wise process of the R-phase transformation divides the total TWSME strain into segments of smaller magnitudes in the macroscopic behavior, and more importantly reduces the discrete lattice distortion mismatch at the transformation interface on the microscopic scale. This drastically reduces the chances of generating dislocations during the transformation process, thus rendering the high cyclic stability of the R-phase TWSME. Such intricate behavior may be closely related to the unique NiTiFe matrix – Nb nanowire microstructure and its unique inherent internal stress condition.

Crystallization and its kinetics of soft magnetic (Fe1−xNix)79B12P5Si3C1 glassy alloy ribbons

A glassy phase is formed over the whole composition range for melt-spun (Fe1−xNix)79B12P5Si3C1 with x = 0; 0.2; 0.4; 0.5; 0.6 alloy ribbons and these glassy ribbons exhibit good bending plasticity. The Ni-containing glassy alloys show glass transition, followed by a supercooled liquid (SCL) region and then multistage crystallization for the alloys with x = 0.5 and 0.6 (hereafter 0.5Ni and 0.6Ni alloys). The 0.5Ni glassy alloy exhibits the largest SCL region of 43 K and good soft magnetic properties even in as-spun state, i.e., low coercive force of 4.8 A/m, effective permeability of 7400 at 1 kHz and saturation magnetization of 0.80 T. The bending plasticity remains unchanged in the wide annealing temperature range below Tg. The crystallization proceeds in the processes of glassy (G) → bcc(α)Fe(Si) + Fe3B for the 0–0.2 Ni alloys, G → fcc(γ)(Fe,Ni) + Fe3(B,C) + Fe3Ni3(B,C) for the 0.4Ni alloy and G → G’ + γ(Fe,Ni) → γ(Fe,Ni) + Ni5P2 + Fe4P + Fe3Ni3(B,C) for the 0.5Ni and 0.6Ni alloys. Analyses of crystallization reaction in isochronal and isothermal conditions were performed via Kissinger and JMAK models and the activation energy for crystallization is much larger for the 0.5Ni glassy alloy. The appearance of the largest SCL region as well as the highest activation energy for the 0.5Ni alloy is due to the necessity of the simultaneous decomposition to the four crystalline phases. The better magnetic softness for the 0.5Ni alloy is presumably due to the development of medium-range ordered atomic configurations which enable the appearances of the largest SCL region as well as the lower internal stress state. The combination of useful properties for the 0.5Ni glassy alloy is promising for useful soft magnetic materials with mass production ability.

Comprehensive computational analysis of the crimping procedure of PLLA BVS: effects of material viscous-plastic and temperature dependent behavior

Recently, researchers focused their attention on the use of polymeric bioresorbable vascular scaffolds (BVSs) as alternative to permanent metallic drug-eluting stents (DESs) for the treatment of atherosclerotic coronary arteries. Due to the different mechanical properties, polymeric stents, if compared to DESs, are characterized by larger strut size and specific design. It implies that during the crimping phase, BVSs undergo higher deformation and the packing of the struts, making this process potentially critical for the onset of damage. In this work, a computational study on the crimping procedure of a PLLA stent, inspired by the Absorb GT1 (Abbott Vascular) design, is performed, with the aim of evaluating how different strategies (loading steps, velocities and temperatures) can influence the results in terms of damage risk and final crimped diameter. For these simulations, an elastic-viscous-plastic model was adopted, based on experimental results, obtained from tensile testing of PLLA specimens loaded according to ad hoc experimental protocols. Furthermore, the results of these simulations were compared with those obtained by neglecting strain rate and temperature dependence in the material model (as often done in the literature), showing how this lead to significant differences in the prediction of the crimped diameter and internal stress state.

Towards best practice in numerical simulation of blind rivet nut installation

The present paper derives best practices to accurately simulate the installation process of a Blind Rivet Nut (BRN) using FEA. A BRN is a mechanical fastener used to equip plate material with a threaded part. The installation of a BRN inherently induces a high contact force which can have detrimental consequences when applied to non-metals such as polymer composite materials. The effects of the localized stress in the plate on the mechanical performance of the BRN can be studied with the aid of finite element simulations. To this end, the joining by forming process itself is accurately simulated using a computational efficient axisymmetric 2D model. The 2D model enables to predict the metal flow and internal state of stress after setting with sufficient accuracy. The latter is validated using a full 3D model and a multitude of experimental observations. It is shown that the large strain flow curve of the BRN material needs to be adequately identified. An industrially relevant calibration procedure is presented mitigating the experimental effort. In addition, material test selection in case of highly anisotropic BRN materials is discussed based on a thorough stress state analysis. Finally, the sensitivity of the BRN geometry to a change in the most relevant geometrical parameters is demonstrated. The presented experimental methods and numerical models provide fundamental insights in the forming mechanism including process-induced ductile damage while installing the BRN. The latter will foster the development of new BRN applications in multi-material mechanical design.

Discrete Element Simulation Analysis of Biaxial Mechanical Properties of Concrete with Large-Size Recycled Aggregate

Concrete made with large-size recycled aggregates is a new kind of recycled concrete, where the size of the recycled aggregate used is 25–80 mm, which is generally three times that of conventional aggregate. Thus, its composition and mechanical properties are different from that of conventional recycled concrete and can be applied in large-volume structures. In this study, recycled aggregate generated in two stages with randomly distributed gravels and mortar was used to replace the conventional recycled aggregate model, to observe the internal stress state and cracking of the large-size recycled aggregate. This paper also investigated the mechanical properties, such as the compressive strength, crack morphology, and stress–strain curve, of concrete with large-size recycled aggregates under different confining pressures and recycled aggregate incorporation ratios. Through this research, it was found that when compared with conventional concrete, under the confining pressure, the strength of large-size recycled aggregate concrete did not decrease significantly at the same stress state, moreover, the stiffness was increased. Confining pressure has a significant influence on the strength of large-size recycled aggregate cocrete.

Tension–tension fatigue behavior and residual strength evolution of SiC/(PyC/SiC)2/SiC minicomposites prepared by CVI+MI and CVI+PIP processes

The residual tensile strength (RTS) evolution of SiC/(PyC/SiC)2/SiC samples, prepared by chemical vapor impregnation (CVI) combined with either melting reaction sintering (MI) or polymer impregnation and pyrolysis (PIP), was investigated after 106 cycles as the pre-fatigue stress increased. The fatigue limits of the two tested specimen types, CVI + MI and CVI + PIP, were 760 and 660 MPa, respectively, corresponding to 95% and 75% of the tensile strength, respectively. Although the RTS of the two specimen types first increased and then decreased, it was interesting that after pre-fatigue, the maximum RTS of the CVI + PIP-prepared sample at 680 MPa, was 16 MPa higher than that of the CVI + MI-prepared sample at 590 MPa. The nanoindentation results indicate that the matrix prepared by CVI could protect the fiber from heavy damage in subsequent preparation, and the matrix modulus and hardness were higher in samples prepared by PIP than those prepared by MI. According to the microstructure observations and hysteresis characteristics, it was concluded that the differences mainly came from the internal stress state, matrix, and fiber properties.

Analysis of stress influence and plastic strain on magnetic properties during the forming process

The aim of this paper is to analyze the relation between magnetic and mechanical properties during and after forming processes. For this purpose, several tensile tests were carried out on sheet metal samples up to a defined plastic strain. The specimens were left in the clamping device in order to relieve the force in several steps until the specimen was completely relieved. As a consequence, the gradual relief leads to a reduction of internal stress states. During the forming process, the initial magnetic relative permeability and magnetic anisotropy of the sample were measured several times. Both properties are related to the mechanical states in the material through the effects of magnetic embrittlement and magneto-elasticity. The plastic strain of the specimens was determined by optical measurements and the stresses in the measurement range during the tensile test was determined with the help of a subsequent numerical simulation. This made it possible for the first time to measure the magnetic properties of samples with different plastic strain and different stress states. The evaluation shows that there is a strong correlation between permeability and plastic strain as well as anisotropy and stress. Based on these findings, it has been confirmed, that the determination of the plastic strain by a soft sensor is possible.

The variation of load diffusion and ballast bed deformation with loading condition: an experiment study

ABSTRACT Ballast bed deformation is significantly influenced by its internal stress state. However, investigations concerning ballast bed deformation focus mainly on the deformation behaviour under various external loading conditions or ballast states, where the influence of load diffusion variation has rarely been considered directly. In this work, a full-scale model test was established to obtain simultaneously the external deformation and the internal stress of ballast bed, with respect to various loading conditions (equivalent axle load 14–30 t, 3–6 Hz). The results show that both average stress and stress ratio on ballast increase with loading magnitude. Meanwhile, a larger proportion of vertical load concentrates within the sleeper projection area. Accordingly, the plastic (elastic) deformation is more (less) sensitive to the load variation at larger wheel–rail load. Besides, the dynamic stiffness increases gradually with loading amplitude while showing a small drop as loading frequency increases.

Mortar pore pressure prediction during the first hours of cement hydration

During cement hydration, solid, fluid, and total volumes vary as water is consumed by chemical reactions. Those volume variations lead to pore water pressure decrease during the first hours of hydration. The deformations induced by early hydration are quantified in order to understand their consequences on the internal stress state within the hardening material.This paper aims to link pore water pressure with autogenous and chemical shrinkage during the first hours of hydration in the bulk of early setting cement mortar. It is demonstrated that specific volume variations are compensated by an increase of air-entrained volume. The variation of the volume of air is used to predict the air pressure. Using the size of a maximum capillary pore diameter filled with water, the capillary pressure is estimated using the Young-Laplace equation. In order to validate the approach, the pore water pressure is measured and compared to the thermodynamic prediction and air content.

Testing the Applicability of 119Sn Mössbauer Spectroscopy for the Internal Stress Study in Ternary and Co-Doped Ni-Mn-Sn Metamagnetic Alloys

The influence of both the Co addition and the internal stress on the atomic level magnetism is comparatively studied in Ni50Mn37Sn13 and Ni45Mn38Sn13Co4 alloys by magnetic measurements and 119Sn Mössbauer spectroscopy. The results show that the saturation magnetization and the hyperfine field follow the same temperature trend. The internal stress state is investigated by subjecting the samples to milling and annealing treatments, and tracking the singlet component revealed by 119Sn Mössbauer spectroscopy. Contrary to what was expected, in the Co-doped Ni-Mn-Sn sample the singlet component can be resolved between the milled and annealed states in both martensite and austenite phases. Therefore, the results demonstrate the feasibility of tracking the singlet component upon the structural recovery in Co-doped Ni-Mn-Sn alloys in a much wider range than in ternary alloys. In addition, it is concluded that the transferred dipolar field at Sn from the neighbor magnetic atoms depends very strongly on the stress field and on the microstructural order surrounding Sn atoms. The observed sensitivity of Sn Mössbauer probe atoms to slight microstructural distortions make 119Sn a powerful technique for the characterization of the stress present in Sn containing metamagnetic shape memory alloys.

A finite strain thermomechanically-coupled constitutive model for phase transformation and (transformation-induced) plastic deformation in NiTi single crystals

A 3D finite-strain constitutive model for the deformation response of NiTi at the single crystal level is proposed. The model accounts for reversible phase transformation from austenite to martensite habit plane variants, plastic deformation in the austenite phase, and rate effects induced from latent heat. It is developed within the formalism of irreversible thermodynamics with internal state variables based on the Eulerian logarithmic strain and its corrotational objective rate. The inelastic deformation is defined as an average over a representative volume element as classical in the micromechanics-based modeling approach. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interface. It is accounted for by introducing an interaction term in the free energy, which is described through the Eshelby tensor by regarding the habit plane variants as ellipsoidal inclusions embedded in the austenite matrix, in order to accurately reflect the internal stress states that contribute to dislocation slipping. The numerical implementation of the model in an efficient scheme and its calibration are described in detail. The proposed model is validated by comparing simulations with available experimental data in single NiTi crystals. Numerical simulations of polycrystals are performed to obtain an insight into the interaction between phase transformation and plastic deformation induced by intergrannular constraints. The efficiency of the numerical implementation of the model is verified by simulations of indentation tests.