Cyclic Thermomechanical Loading Research Articles

Effect of Design and Surface Treatments on Fracture Resistance of Zirconia Inlay-Retained Fixed Dental Prosthesis After Thermo-Mechanical Cycling: An in vitro study

Objective: This study evaluated the effect of preparation designs and surface treatment on the fracture resistance of zirconia inlay retained fixed dental prosthesis (IR-FDPs). Material and Methods: Twenty secondary premolar and twenty secondary molar were used in this study. Twenty zirconia IR-FDPs were constructed, divided into two main groups, ten specimens each, according to abutment preparation (box & tube shape), each main group was subdivided into two sub groups according to surface treatment (Air abrasion & MDP primer surface treatment). The IR-FDPs were cemented using dual cure resin cement, then the specimens were subjected to thermo-mechanical cyclic loading (TMCL). The fracture resistance was measured. Results: The results showed that there was no statistically significant difference between air abrasion and MDP for both designs (box & tube shape).The effect of design,showed that there was statistically significant difference for both surface treatments between tube and box design where box design showed higher fracture resistance than tube design for both surface treatments. Conclusion: Inlay retained fixed dental prosthesis (IR-FDPs) with box shape provide more fracture resistance than with tube shape. The air abrasion surface treatment shows similar fracture as the MDP primer surface treatment.

Microstructural analysis of failure progression for coated carbide tools during high-speed milling of Ti-6Al-4V

In this study, the microstructural failure progression of coated carbide tools was analyzed during milling Ti-6Al-4V under dry conditions. The microstructure and compositional characterization of worn tool inserts were examined using SEM and EDS analysis. It was found that the coated carbide cutting tools undergo several stages during interrupted cutting Ti-6Al-4V alloy: coating delamination, cobalt diffusion, attrition wear, cracking and flaking. Firstly, coating delamination firstly occurred around the cutting edge and then extended to the rake face and flank face as cutting process proceeds. The diffusion of the binder phase cobalt (Co) was observed after coating delamination and cutting speed significantly promoted the process of cobalt diffusion. Then, attrition wear occurred around the cutting edge as the WC particles were pulled out and removed from the tool substrate. In addition, Microcracks initiated and propagated from the voids introduced by cobalt diffusion due to cyclic thermomechanical loadings. Brittle fractures were found to be the main failure modes of coated carbide tools at high cutting speeds.

Notch stress-strain estimation method based on pseudo stress correction under multiaxial thermo-mechanical cyclic loading

In this paper, a notch stress correction method based on the structural yield surface is proposed, in which the actual stress increment at the notch root can be obtained by multiplying two quantities, that is, the pseudo stress increment and the ratio of the slopes of the material and structural stress-strain curves. In the proposed method, the influence of temperature on the notch stress correction can be taken into account, and no iteration is required during the calculation process. Moreover, based on the proposed notch correction method and a unified viscoplastic constitutive model at high temperature condition, a notch stress-strain estimation method is developed under multiaxial thermo-mechanical cyclic loading. In order to assess the prediction accuracy of the proposed method, the calculated results are correlated to those from thermal-structural non-linear finite element analysis under various proportional and non-proportional mechanical loadings at non-isothermal high temperature condition. The comparison between the calculated and analyzed results shows that the proposed method can precisely estimate the notch stress and strain under multiaxial thermo-mechanical cyclic loading.

Numerical schemes based on the stress compensation method framework for creep rupture assessment

Evaluation of creep rupture limit and prediction of creep rupture life are two significant issues for high-temperature devices under the action of cyclic thermo-mechanical loadings. In this paper, the shakedown solution procedure proposed recently by the authors, so-called stress compensation method (SCM), is extended for creep rupture assessment via an extended shakedown theory including creep. Two distinct numerical schemes based on the SCM framework are presented, where Scheme 1 is utilised for evaluation of creep rupture limit and Scheme 2 is utilised for prediction of creep rupture life. Instead of using the detailed creep constitutive equations, the present methods just need to know several material parameters including the creep rupture data. A holed plate is provided as a typical example to validate the reliability of two numerical schemes. Detailed cycle-by-cycle analyses are carried out to illustrate the good accuracy of these calculated creep rupture limits and reveal the failure mechanisms of the structure under different combinations of loads. A numerical study on a pipe junction is also conducted to show the engineering application of the methods. As a result, the two numerical schemes are proved to be effective and reliable for solving practical industrial problems.

Microtensile Bond Strength Evaluation of Composite Resin to Discolored Dentin After Amalgam Removal

BackgroundTo obtain prolonged clinical success with composite restorations, better bonding of resin to the tooth substrate is crucial.AimThe study was aimed to evaluate the microtensile bond strength (µTBS) of bulk-fill composite resin restorations when bonded to a cavity previously restored with amalgam, comparing with that of freshly prepared dentin.Materials and methodMesio-occlusal cavity preparations were done on 80 extracted human mandibular molars with a buccolingual width of 4 mm and a 1.5 mm axial depth by placing the gingival seat 0.5 mm coronal to the cementoenamel junction (CEJ) and were restored with fine-grain amalgam alloy. After thermocycling, the amalgam restorations were removed. Disto-occlusal cavities with similar dimensions of mesial cavities were prepared, and both the proximal surfaces were filled with bulk-fill composite using either etch-and-rinse or self-etch adhesives. Following thermomechanical cyclic loading, all the teeth were sectioned for µTBS testing. Bond strength data expressed in megapascals (MPa) were subjected to statistical analyses using analysis of variance (ANOVA) and Tukey’s multiple post-hoc tests.ResultsThe total-etch adhesive exhibited statistically higher bond strength values to both dentin substrates compared to self-etch adhesives (p<0.05). Failure mode analysis reported more of adhesive failures.ConclusionThe µTBS of bulk-fill composite resin restorations bonded to a cavity previously restored with amalgam was significantly lower than that of freshly prepared dentin. Total-etch adhesives bond strength was higher than self-etch adhesives to both the substrates tested.

Inelastic design for type 316L stainless steel at ambient and 600 °C under uniaxial loading

This study aims to determine the safe elastoplastic shakedown limit of type 316L stainless steel at room temperature and at an elevated temperature of 600°C. Components made of type 316L stainless steel are commonly subjected to combined thermomechanical loads in various engineering applications. Shakedown based designs can be used to expand the feasible design space under combined thermomechanical cyclic loading conditions compared to conventional yield-limited designs. Extensive experimental and numerical effort has been made to model ratchetting and cyclic hardening/softening of stainless steels under uniaxial cyclic loading but a very limited number of studies have focused on determining the shakedown limit. In this work, shakedown limits are determined experimentally and numerically. A numerical model that combines time-dependent creep deformation and time-independent hardening is used to develop Bree load-interaction diagrams. It is found that shakedown occurs at room temperature at stress levels up to two times the linear elastic limit of the material and is limited by the time-dependent creep deformation. At 600oC, an enhanced shakedown behavior is observed that expands the feasible design space up to 4 times the linear elastic limit.

Anisotropic continuum damage analysis of thin-walled pressure vessels under cyclic thermo-mechanical loading

The present study intends to analyze damage in thin-walled steel cylinders undergoing constant internal pressure and thermal cycles through use of anisotropic continuum damage mechanics (CDM) model coupled with nonlinear kinematic hardening rule of Chaboche. Materials damage in each direction was defined based on plastic strain and its direction. Stress and strain distribution over wall-thickness was described based on the CDM model and the return mapping algorithm was employed based on the consistency condition. Plastic zone expansion across the wall thickness of cylinders was noticeably affected with change in internal pressure and temperature gradients. Expansion of plastic zone over wall-thickness at inner and outer surfaces and their boundaries demarking elastic and plastic regions was attributed to the magnitude of damage induced over thermo-mechanical cycles on the thin-walled samples tested at various pressure stresses.

Evaluation of Marginal Adaptation and Wear Resistance of Nanohybrid and Alkasite Restorative Resins

Introduction: Polymerisation shrinkage stresses developed during curing of adhesive resin cements may cause debonding at the margins of cavity leading to microgap formation with secondary caries and consequently restorative failure. Thus, a restoration should have good marginal integrity and wear resistance to obtain high success with clinical longevity. Aim: To compare the influence of thermo-mechanical stresses on marginal quality and wear of class II Mesio-Occluso-Distal (MOD) restorations filled with Solare Sculpt or Cention N. Materials and Methods: An in vitro study was conducted at GITAM Dental College and Hospital, Visakhapatnam on 80 human extracted mandibular molars by preparing mesio-occlusal-distal class two cavities that restored either with Solare Sculpt or Cention N resin composites. Baseline evaluations were recorded for marginal gaps (in μm) through Scanning Electron Microscope (SEM), and the weights of the samples were recorded by an electronic balancing instrument to measure the amount of wear (in grams). Following thermo-mechanical cyclic loading, all the restored teeth were evaluated again to record the same parameters. The statistical analysis was done using Statistical Package for the Social Sciences (SPSS) programme for windows version 22.0 (IBM, NY) software. An independent t-test for intergroup comparison and a dependent t-test for intragroup comparisons were done to analyse the tested parameters with the significance level established at (p≤0.05). Results: In intergroup comparison, no difference in marginal adaptation was observed before thermo-mechanical loading between two materials with a p-value of 0.3625, but after therm-omechanical loading Cention N exhibited significantly superior marginal adaptation with a p-value of 0.0374. Both the materials have shown a significant difference in the marginal adaptation after thermo-mechanical loading in the intragroup comparison with Cention N (0.0002) and Solare sculpt (0.0001) p-values. The wear rate was not different statistically between the materials with a p-value of 0.7144 before thermo-mechanical loading and 0.2285 after thermo-mechanical loading. Conclusion: Marginal adaptation of Cention N was superior to Solare Sculpt, whereas both the materials exhibited a similar wear rate.

Thermal effects on one-way cyclic behaviour of clay-structure interface

In energy geostructures, which exploit the heat in soil using earth contact elements, the interface is subjected to cyclic thermo-mechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay-clay and clay-structure interface at different temperatures (22 and 60 °C). Different cyclic and average stress ratios (CSR and ASR) were applied to the kaolin clay-structure interface under 300 kPa of normal stress. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. In cyclic clay-structure tests, decreasing the cyclic stress ratio, increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60 °C for the different cyclic stress ratios tested.

Multiscale synchrotron scattering studies of the temperature-dependent changes in the structure and deformation response of a thermoplastic polyurethane elastomer

The distinct molecular architecture and thermomechanical properties of polyurethane block copolymers make them suitable for applications ranging from textile fibers to temperature sensors. In the present study, differential scanning calorimetry (DSC) analysis and macroscopic stress relaxation measurements are used to identify the key internal processes occurring in the temperature ranges between −10 °C and 0 °C and between 60 °C and 70 °C. The underlying physical phenomena are elucidated by the small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of synchrotron beams, allowing the exploration of the structure-property relationships as a function of temperature. In situ multiscale deformation analysis under uniaxial cyclic thermomechanical loading reveals a significant anomaly in the strain evolution at the nanoscale (assessed via SAXS) in the range between −10 °C and 0 °C owing to the ‘melting’ of the soft matrix. Furthermore, WAXS measurement of crystal strain within the hard regions reveals significant compressive residual strains arising from unloading at ∼60 °C, which are associated with the dynamic shape memory effect in polyurethane at these temperatures.

Investigating the Effects of Cyclic Thermo-Mechanical Loading on Cyclic Plastic Behavior of a Ninety-Degree Back-to-Back Pipe Bend System

Abstract Pipe bends are generally employed for routing piping systems by connecting to straight pipes but back-to-back pipe bends are often necessary for confined space applications. In order to achieve safe operation under complex loading, it requires a thorough pipeline integrity assessment to be commenced. This paper investigates the effects of cyclic thermo-mechanical loading on cyclic plastic behavior of a 90-deg back-to-back pipe bend system, including temperature-dependent yield stress effects. Structural response interaction boundaries are determined for various different combinations of cyclic and steady loading. Constructed structural responses are verified by full cyclic incremental, step-by-step, finite element analysis. The numerical studies provide a comprehensive description of the cyclic plastic behavior of the pipe bends, and semi-empirical equations for predicting the elastic shakedown limit boundary are developed to aid pipeline designers in the effective assessment of the integrity of the pipe bends without a requirement for complex finite element analysis.

Evolution of principal stress of a turbine rotor under cyclic thermo-mechanical loading

The target of the present study is to analyze the local variation of stress and strain state in a steam turbine rotor under cyclic service-type thermo-mechanical creep-fatigue loading conditions. Toward this end, a Chaboche model based material constitutive model is applied to simulate the multi-axial stress-strain behavior in the rotor. Comparison of von Mises stress, stress tensors and principal stresses is carried out to demonstrate the importance of the stress state which is very useful to the optimization of the components and operation, and then principal stress and strain during a whole cyclic loading condition is analyzed to illustrate the variation of tension and compression states. Both the temperature and rotating speed related centrifugal force influence the evolution of the principal stresses and strain, but the influential extent has a large difference at different locations; furthermore, the discrepancy of cyclic softening/hardening behavior and ratcheting behavior due to creep at different locations is also significant. This outcome can be used to carry out the optimization or damage assessment in the rotor.

A fully coupled thermo-viscoelastic-viscoplastic-damage framework to study the cyclic variability of the Taylor-Quinney coefficient for semi-crystalline polymers

In the present work, a rigorous and consistent thermo-viscoelastic-viscoplastic ductile damage model is proposed to address the variability of the Taylor-Quinney coefficient (the storage to anelastic energy rate ratio) during the anelastic deformation of thermoplastic polymers. More specifically, the constitutive model developed is mainly dedicated to the description of recoverable viscoelastic effects occurring on different time scales, irreversible strains observed above the stress threshold and energy responses arising from the anelastic deformation process (storage of energy, dissipation of energy, thermomechanical coupling, etc.). One of the aims of this paper is to evaluate the partitioning between stored and heat energy without assuming any Taylor-Quinney values. The proposed approach is incorporated into the framework of Thermodynamics of Irreversible Processes and Generalized Standard Materials formalism to offer the thermodynamic consistency of all the constitutive equations. The numerical algorithm for the proposed model is implemented in the well-known finite element code Abaqus via the user material subroutine UMAT using an implicit formulation of the constitutive equations coupled with a radial return mapping algorithm. The model is then calibrated and validated across monotonic tensile and cyclic tensile-tensile tests by comparing predicted and experimentally observed energy responses. This comparison shows a good level of accordance between experimental findings and model predictions in terms of stress-strain responses for both monotonic and cyclic thermomechanical loading conditions. The model can also numerically capture the cyclic kinetics of the storage and dissipation energies. The capabilities of the fully-coupled model have been demonstrated through simulating the thermo-mechanical response of a complex 3D structure. The numerical analysis establishes the model’s capability to accurately render the spatio-temporal patterns of the Taylor-Quinney coefficient and the self-heating induced part of the ductile damage.

A novel graphene oxide/trans-1,4-polyisoprene (GO/TPI) shape memory polymer nanocomposite and its multifunctional properties

In this work, we have synthesized a novel graphene oxide/trans-1,4-polyisoprene (GO/TPI) shape memory polymer (SMP) nanocomposite by adding GO (0.0–1.5 phr) to bulk TPI polymer to enhance its mechanical properties. We have studied and examined its mechanical, thermal and thermomechanical properties through systematic tests from microscale to macroscale. The shape memory properties and the effect of the GO content on the thermal and mechanical properties of the manufactured SMP nanocomposites were studied by differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis, thermal conductivity, and static tensile and mechanical as well as thermomechanical cyclic loading tests. The experimental results indicate that the SMP nanocomposite with 0.9 phr GO has superior thermal and mechanical properties compared to all other cases, and good shape memory was obtained in the GO/TPI nanocomposites.

Finite element simulation of thermomechanical training on functional stability of shape memory alloy wave spring actuator

Pre-service thermomechanical training is of great significance to achieve functional stability for shape memory alloy device. This article presents a finite element simulation of the training behavior of a shape memory alloy wave spring actuator using a thermomechanically coupled and finite-strain shape memory alloy model (Wang et al., 2017a). The model is implemented into ABAQUS/Explicit by means of a user-defined material subroutine VUMAT. The introduction of a finite-Hencky-strain return-mapping integration scheme substantially improves the numerical efficiency and stability. Model predictions are validated against the experimental data. The good agreement between both demonstrates the capabilities of the model of well describing the training behavior of shape memory alloy when subjected to large cyclic thermomechanical loading. Simulation results illustrate several primary thermomechanical characteristics during training process, such as the expansion of the phase transformation zone, the accumulation of the residual deformation, and the concentration of the internal stress. The present finite element approach provides a powerful tool in design and optimization of shape memory alloy wave spring actuator, especially to improve the geometric precision and to enhance the two-way shape memory effect.

Ductile to brittle transition in ITER specification tungsten assessed by combined fracture toughness and bending tests analysis

The evaluation of the fracture toughness of tungsten is required for the design of plasma-facing components in order to ensure safe and durable operation in ITER reactor, being under construction in France. During operation, plasma facing materials will be exposed to cyclic thermo-mechanical loads combined with high energy neutron flux, which, in general, reduce the fracture toughness. Characterization of the degradation of the mechanical properties after exposure to the neutron flux involves time consuming and expensive procedures due to nuclear activation and special handling. Subsequently, development of sample miniaturization and protocols to reduce the volume of material under inspection is critical to speed up the progress in R&D. In this work, we propose a combined approach for the reconstruction of the fracture toughness – temperature curve, which is applied in the ductile to brittle transition temperature range. The approach consists of two steps: (i) application of the three point bending tests using miniaturized samples to reveal the transition temperature range on the basis of flexural strain data; (ii) execution of standardized fracture toughness tests at the upper temperature of the transition regime. The results allow the determination of the fracture toughness as a function of temperature with a reasonable accuracy. The validity of the approach has been demonstrated on two commercial tungsten grades produced according to ITER specification and tested in the as-fabricated state. The conclusions are supported by microstructural analysis performed on both standardized and miniaturized samples.

Modeling of partial transformation cycles of SMAs with a modified hardening function

A physical constitutive model is combined with a new function expression that describes the hardening behaviour of Shape Memory Alloys to enable the accurate and efficient prediction of partial transformations during cyclic thermo-mechanical loading. The reversal point memory and the associated memory wipe-out are successfully modeled for single partial transformation cycles by introducing a pair of interpolation functions to scale the martensitic volume fraction predicted by the original model. Isobaric, thermally induced, partial transformation cycle experiments are performed on Ni51Ti49 wt% wires. Predicted partial transformation response results using the proposed model modification are presented and compared with two other approaches and correlated with obtained experimental data for four different cases of partial transformation cycles. Very good agreement is obtained with measured partial cycles, without need of additional calibration, especially for partial transformation branches formed near the middle of the major hysteresis loop. The new constitutive equations are suitable for inclusion into finite element frameworks due to their simple implementation and high accuracy. A case study of a morphing strip actuated by a single SMA wire is considered to demonstrate the capabilities of the modified constitutive equations under combined thermal and mechanical loading conditions.

Cyclic heat-up and damage-relevant substrate plastification of single- and bilayer coated milling inserts evaluated numerically

Milling processes are characterized by interrupted cutting, resulting in cyclic thermo-mechanical loading conditions affecting the milling tool's service life. In the current paper, a numerical method is built to predict the transient temperature and stress fields inside coated milling inserts during a dry milling application. The investigated milling tools are hard coated WC-Co hard metal milling inserts, the cut workpiece material is 42CrMo4. The thermal shielding of the substrate by three different hard coating layers, each with a thickness of 7 μm is quantitatively evaluated numerically. The compared coatings are: (i) a TiAlN single layer, (ii) a TiCN/α-Al2O3 bilayer and (iii) a TiAlN/α-Al2O3 bilayer. The deformation behavior and thermal properties of the hard metal substrate and the hard coatings were considered as a function of temperature by experimentally parameterized material models. A remarkable new feature of the presented model is that the simulated dry milling process includes an unprecedented number of 100 load cycles. The synergetic combination of 2D and 3D finite element models gives insight into the cyclic thermo-mechanical tool load that causes stresses and inelastic strains in the substrate. The applied modeling approach considers that the heat flux between the workpiece and the milling tool is changing as the tool heats up during milling. During successive milling cycles, a decreasing heat flux into the tool is taken into account. A comparison of hard coatings with different inherent thermal properties showed a damage-relevant reduction in substrate plasticization with decreasing thermal conductivity of the coatings.

An experimental investigation on deformation and shape recovery behavior of Fe-28Mn-6Si-5Cr shape memory alloy under cyclic thermo-mechanical loading

An experimental investigation on deformation and shape recovery behavior of Fe-28Mn-6Si-5Cr shape memory alloy under cyclic thermo-mechanical loading

Method of calculating the heat and stress-deformed state of the cylinder head of a transport diesel with liquid cooling

In an internal combustion engine (ICE), stresses arise from the action of a variable gas force and from a variable temperature due to the combustion of fuel in the combustion chamber in the cylinder head and piston, while the thermal stresses are much higher than the stresses from the power load. Therefore, at the design stage of a new engine design, they are calculating the durability of heat-stressed parts, since they limit the reliability of a piston engine. The paper discusses the method of selecting source data when calculating the thermal and stress-strain state of the cylinder head of a diesel engine using the finite element method and using the Solid Works software package. The stages of creating and preparing a solid model of a cylinder head to create a finite element model are described using the example of the 8ChN12 / 13 diesel engine (KAMAZ 740.75-440). The main loads acting on the cylinder head during the assembly period are considered: efforts from pressing in valve seats and bushings, as well as from tightening the bolts for fastening the head to the crankcase. The shape of the elements for the finite element model of the cylinder head was chosen from the condition of reducing the estimated time. This condition was suited to the shape of a finite element in the form of a tetrahedron with four nodal points. These elements made it possible to reduce the computational grid in cross sections, for example, stress concentration. It is shown that high-frequency temperature fluctuations that occur as a result of the flow of the working process of a piston engine do not affect the thermal strength of the cylinder head, and high-frequency temperature fluctuations due to changes in the engine operating conditions affect. The changes in the basic characteristics of the aluminum alloy from which the cylinder head is made with temperature and cyclic thermomechanical loading of low frequency are given. It is shown that with increasing temperature, the endurance limit of the material decreases at low frequency loads.