Isothermal Loading Research Articles

Fracture studies on cruciform bend specimens of pressure vessel steel subjected to thermo-mechanical loading

The safety of a nuclear power plant during incidents, such as a Loss of Coolant Accident (LOCA), heavily relies on conducting a comprehensive structural integrity assessment of both the Reactor Pressure Vessel (RPV) and its components, specifically to withstand Pressurized Thermal Shock (PTS). PTS is characterized by a combination of steep temperature gradient, resulting from the injected emergency coolant during a LOCA, and internal pressure within the RPV. Majority of the reported fracture assessment studies on RPV steel, whether experimental or numerical investigations, have predominantly focused on standard uniaxial specimens at iso-thermal loading conditions. To better simulate the thermal shock scenario in RPVs, the present work aims to assess the impact of a biaxial stress field on both fracture parameters (crack mouth opening displacement and J-integral) and plastic collapse load. This assessment is conducted through experimental and numerical investigations, both with and without prior transient thermal load. Fracture experiments are performed on cruciform bend specimens, featuring two different biaxiality ratios (1:1 and 2:1). Moreover, numerical studies on cruciform specimens are conducted using finite element analysis to validate and corroborate the observations derived from the fracture experiments.

High temperature fatigue behavior of coated and uncoated nickel-based single crystal superalloy DD6: Microstructures evolution, damage mechanisms and lifetime prediction

Thermal barrier coatings (TBCs) are widely used to further extend the lifetime of turbine blades by protecting the blades from high temperature corrosion and oxidation. However, the mechanical behavior of turbine blades is obviously affected by the TBCs. In this study, microstructures evolution, damage mechanisms and life prediction of coated and uncoated nickel-based single crystal (NBSX) superalloy DD6 under isothermal fatigue load were investigated at 980 °C. The effects of TBCs on fatigue failure behavior and lifetime of DD6 were addressed. The results showed that the fatigue lifetime reduced with the increase of load. The effect of TBCs on the fatigue lifetime was related to the stress amplitude, as the effect was beneficial at high stress but almost negligible at low stress. Fracture morphologies showed that the cracks more likely initiated and propagated from basal defects for both coated and uncoated DD6, and the microstructure evolution also showed stress amplitude dependence. The crack density of uncoated DD6 increased first and then decreased with the increase of stress amplitude. However, the TBCs reduced the number of cracks that penetrate into the DD6 substrate, and the stress amplitude exerted a significant effect on crack propagation paths. In addition, the rafting behaviors of the DD6 substrate of coated and uncoated samples was compared, and results showed that TBCs could reduce the rafting degree of DD6. Finally, the fatigue lifetime of coated samples was predicted based on the modified Basquin model, and the prediction results fitted well with the experimental results.

Deformation and microregion fracture mechanisms in type 316L welded joint under isothermal and thermo-mechanical fatigue loadings

Isothermal (IF) and thermo-mechanical fatigue (TMF) tests were conducted on 316L welded joints to investigate the effects of phase angle (in-phase (IP), clockwise-diamond (CD), and out-of-phase (OP)) and mechanical strain amplitude (±0.4 %, ±0.5 %, ±0.6 %) on deformation and microregion fracture mechanisms. Results reveal that increasing strain amplitude induces a higher softening rate, reduced fatigue life, and more pronounced dynamic strain aging (DSA). Nevertheless, there is a complicated discrepancy in the cyclic deformation between various phase angles. At high strain amplitudes, local embrittlement along the austenite/δ-ferrite interface induces cracking in the weld metal (WM) under both IF and TMF loadings, with TMF life increasing with phase angle. At a lower strain amplitude of 0.4 %, however, fracture location migration and different TMF life were observed. Microstructural analysis reveals that during TMF a lower strain amplitude of 0.4 %, the microstructure in both base metal (BM) and WM evolves from simple planar structures to low-energy substructures, characterized by an increase in kernel average misorientation, geometrically necessary dislocations, and low-angle grain boundaries, together with a reduction in high-angle grain boundaries and twin boundaries. In the WM, increasing phase angle tends to reduce dislocation density and reproduces planar dislocation structures due to enhanced DSA, thereby lowering cellular structure maturity. Conversely, in the BM, dislocation proliferation and interaction intensify, enhancing cellular structure maturity and de-twinning effect, which reduces BM fracture resistance and causes fracture migration from the WM to the BM. Moreover, active DSA under CD loading improves deformation resistance in both microregions, prolonging fatigue life and contributing to the observed different TMF life.

Future Parabolic Trough Collector Absorber Coating Development and Service Lifetime Estimation

This work presents a study on the optical and mechanical degradation of parabolic trough collector absorber coatings produced through the spray coating application technique of in-house developed paint. The main aim of this investigation is to prepare, cure, load, and analyze the absorber coating on the substrate under conditions that mimic the on-field thermal properties. This research incorporates predicted isothermal and cyclic loads for parabolic trough systems as stresses. Biweekly inspections of loaded, identical samples monitored the degradation process. We further used the cascade of data from optical, oxide-thickening, crack length, and pull-off force measurements in mathematical modelling to predict the service life of the parabolic trough collector. The results collected and used in modelling suggested that cyclic load in combination with iso-thermal load is responsible for coating fatigue, influencing the solar absorber optical values and resulting in lower energy transformation efficiency. Finally, easy-to-apply coatings made out of spinel-structured black pigment and durable binder could serve as a low-cost absorber coating replacement for a new generation of parabolic trough collectors, making it possible to harvest solar energy to provide medium-temperature heat to decarbonize future food, tobacco, and paint production industrial processes.

Prediction of crack growth in polycrystalline XH73M nickel-based alloy at thermo-mechanical and isothermal fatigue loading

Prediction of crack growth in polycrystalline XH73M nickel-based alloy at thermo-mechanical and isothermal fatigue loading

A modified constitutive model for whole-life thermal-mechanical fatigue incorporating dynamic strain aging in 316LN stainless steel

A comprehensive analysis of experimental data is presented for 316LN stainless steel subjected to isothermal and thermal-mechanical fatigue loading conditions within the temperature range of 350–600 °C. The study aims to provide a thorough understanding of the cyclic behavior through meticulous data integration, supplementation, and the use of stress decomposition methods combined with a classical damage evolution definition. The modeling approach employed in this study is based on the classical AF-OW-Kang model, with the incorporation of the Arrhenius term in the plastic flow rate equation. Furthermore, to consider the effect of dynamic strain aging at varying temperatures, temperature-dependent terms are introduced into the equations that govern the evolution of isotropic stress and backstress, resulting in enhanced accuracy in describing cyclic hardening behavior. Additionally, a modified damage evolution equation is utilized, along with equations for isotropic stress and backstress evolution, to address cyclic softening. Simulation results confirm the effectiveness of the modified model in capturing the cyclic hardening/softening behavior of 316LN stainless steel throughout the whole-life time, under both isothermal and thermal-mechanical fatigue loading conditions.

Characterization of the temperature-dependent superelastic and elastocaloric effects of a NiTi tube under compression at 293–330 K

Comprehensive characterizations of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMA) in the operation temperature region are highly desirable for using them in elastocaloric coolers with a large temperature lift. In this article, we report the superelastic and elastocaloric effects of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm and a wall thickness of 1 mm between 293 and 330 K. The NiTi tube sample was subjected to a training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that temperature dependencies existed for both superelastic and elastocaloric effects of the NiTi tube, and stress–strain curves differed much between isothermal and adiabatic loading conditions. The largest temperature rise and temperature drop measured at 293 K under an applied strain of 3.66% and a strain rate of 0.1 s−1 during loading and unloading were 21 and 11 K, respectively. The loading conditions (loading function and holding time) also impacted the superelastic effect of the NiTi tube. We identified two major reasons for the irreversibility of the adiabatic temperature change: the hysteresis heat dissipation and the temporary residual strain after unloading, and they affected the cooling performance of the elastocaloric cooler in different ways. We investigated the dependencies of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results are beneficial to the modeling of elastocaloric coolers with large temperature lifts.

Phasing effects on thermo-mechanical fatigue damage investigated via crystal plasticity modeling

Nickel (Ni)-based superalloys have high strength at elevated temperatures, making them ideal candidates for aerospace components exposed to thermo-mechanical loads. Thermo-mechanical fatigue's (TMF) influence on material failure is highly dependent on the temperature-load phase profile as a consequence of path-dependent thermo-mechanical plasticity. In order to understand the direct influence phasing effects have on TMF performance, a microstructure-based fatigue model has been developed to connect damage mechanisms to crack initiation events. In this work, in-phase (IP) TMF, out-of-phase (OP) TMF, and iso-thermal (ISO) loading profiles are investigated with a temperature-dependent, dislocation density-based crystal plasticity model to pinpoint a relationship between microstructural damage and TMF phasing effects. A crystal plasticity modeling framework with capabilities to isolate phasing (IP, OP, and ISO) effects is presented to locate fatigue damage in a set of statistically equivalent microstructures (SEMs) with an energy-based damage indicative parameter. Local stress, accumulated damage, intragranular misorientiation, grain interactions, and slip alignment activity at the damaged regions are studied under the various temperature-load phase profiles to connect microstructural material damage to TMF performance. Based on the local microstructural material damage, it is postulated the material's stiffness at maximum load influences OP TMF performance, whereas the hardening/plasticity at maximum load influences IP TMF. The results from the presented framework is consistent with experimental evidence that at high mechanical strain amplitudes IP TMF has a shorter life as a consequence of hardening/plasticity influencing path-dependent thermo-mechanical plasticity, whereas at low strain amplitudes OP TMF has a shorter life due to stiffness influencing path-dependent thermo-mechanical plasticity.

Research on welding simulation for vacuum chamber test specimens

The vacuum chamber is an important component of nuclear fusion devices, and the deformation control of the welding process is relatively strict. In this paper, the isothermal multi-level cyclic loading tests have been conducted to explore a nonlinear mixed hardening mode, which can be used as the material input for the welding simulation. Then, welding simulation has been performed for test specimens of the vacuum chamber, and the deformation of test specimens has been obtained under different welding paths. After the first welding simulation is completed, the clamp and limiting plates have been designed based on the deformation of the test specimens. The welding simulation has been performed again with the clamp and limiting plates, in order to evaluate the rationality of the welding process and the design for the clamp and limiting plates. Finally, according to the determined welding process and design for the clamp and limiting plates, welding tests have been conducted, and the welding deformation has been measured. It is found that the difference between the measurement values of the tests and the simulation results is small, which implies that the welding simulation is reasonable.

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

Validated assessment of power plant components with flaws and defects operating under long-term creep and isothermal creep-fatigue loading

An extensive database of frequently used heat-resistant steels 1 %Cr, 9–10 %Cr and 12 %Cr steels at application-typical temperatures was evaluated using the Nikbin-Smith-Webster model. Influencing factors, such as the initial plasticization and the specimen thickness, were investigated in the evaluation of the fracture mechanics material characteristic values. A thickness influencing factor fB for crack initiation is proposed, which improves the crack initiation estimation using the NSW model and reduces its over-conservatism notably. It could be shown that the crack initiation and especially the crack growth under creep loading can be described with sufficient accuracy by the modification of the NSW model. The limitation in the application of the C*-parameter could also be shown and explained by the Two-Criteria-Diagram (2CD). It should be applied to crack geometries with predominant crack tip damage. The 2CD is shown as preferred method for a more precise, material-related understanding and assessment of the crack initiation process.Based on the recalculation of fracture mechanics tests on CT und DENT specimens, the analytical approaches for the description of crack initiation and growth under creep and isothermal creep-fatigue loading could be validated in the long-term range by comparison with the experimental results up to 100,000 h.Finally, the transferability of these approaches from CT und DENT specimens to large components was investigated by calculating cracked components under practical creep and isothermal creep fatigue loading using these methods. It has been shown that the load level and the duration of holding time are decisive for the crack initiation and the crack growth under isothermal creep-fatigue loading. In the case of high stress and shorter holding times, the damage is predominantly influenced by fatigue. With lower stress and longer holding times, creep plays a more important role.

Effect of isothermal temperature and load on wear of nanobainitic cast steel

The wear behaviour of a nanobainitic cast steel: 0.76C-2.08Si-0.74Mn-0.79Cr (wt. %), isothermally treated at different temperatures, 250 and 300°C; have been investigated by pin on disk test in dry conditions. The tests were carried out at different loads: 2, 7, 12 and 15 N. The results show that the wear rate is higher when the isothermal transformation temperature and load increases. Also, at low loads adhesion, oxidation and abrasion are the main wearing mechanisms, while at high loads contact fatigue becomes dominant. The results are related to the microstructural differences produced by the change in the isothermal temperature of the heat treatment.

Phase-field description of fracture in NiTi single crystals

A phase-field model for thermomechanically-induced fracture in NiTi at the single crystal level, i.e., fracture under loading paths that may take advantage of either of the functional properties of NiTi–superelasticity or shape memory effect–, is presented, formulated within the kinematically linear regime. The model accounts for reversible phase transformation from austenite to martensite habit plane variants and plastic deformation in the austenite phase. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interfaces and is accounted for by introducing an interaction term in the free energy derived based on the Mori–Tanaka and Kröner micromechanical assumptions and the hypothesis of martensite instantaneous growth within austenite. Based on experimental observations suggesting that NiTi fractures in a stress-controlled manner, damage is assumed to be driven by the elastic energy, i.e., phase transformation and plastic deformation are assumed to contribute in crack formation and growth indirectly through stress redistribution. The model is restricted to quasistatic mechanical loading (no latent heat effects), thermal loading sufficiently slow with respect to the time rate of heat transfer by conduction (no thermal gradients), and a temperature range below Md, which is the temperature above which the austenite phase is stable, i.e., stress-induced martensitic transformation is suppressed. The numerical implementation of the model is based on an efficient scheme of viscous regularization in both phase transformation and plastic deformation, an explicit numerical integration via a tangent modulus method, and a staggered scheme for the coupling of the unknown fields. The model is shown able to capture transformation-induced toughening, i.e., stable crack advance attributed to the shielding effect of inelastic deformation left in the wake of the growing crack under nominal isothermal loading, actuation-induced fracture under a constant bias load, and crystallographic dependence on crack pattern.

The Effect of the Origin and Inner Structure of Limestones on the Burning Process and the Formation of Lime

The burning process of limestones is an important process in the modern industries, which can be described in two parts, CaCO3 decarbonation due to the thermal stress and formation of CaO crystalline structure. It was already observed that the different composition and structure of a raw material influence the transformation process and has affect on the chemical and mechanical properties on the formed lime. This study is focused on the characterization of the raw material (porosity, chemical composition, geological age and origin) and its effect on the burning process and the formation of CaO and its properties. The microstructure of studied material burnt at different times of isothermal load was observed by SEM and the reactivity test was measured and analyzed. The limestone with a more porous inner system was burnt faster and is inclinable to overburn at longer thermal load.

ProCrackPlast: a finite element tool to simulate 3D fatigue crack growth under large plastic deformations

Many structural components and devices in combustion and automotive engineering undergo highly intensive cyclic thermal and mechanical loading during their operation, which leads to low cycle (LCF) or thermomechanical (TMF) fatigue crack growth. This behavior is often characterized by large scale plastic deformations and creep around the crack, so that concepts of linear-elastic fracture mechanics fail. The finite element software ProCrackPlast has been developed at TU Bergakademie Freiberg for the automated simulation of fatigue crack growth in arbitrarily loaded three-dimensional components with large scale plastic deformations, in particular under cyclic thermomechanical loading. ProCrackPlast consists of a bundle of Python routines, which manage finite element pre-processing, crack analysis, and post-processing in combination with the commercial software Abaqus . ProCrackPlast is based on a crack growth procedure which adaptively updates the crack size in finite increments. Crack growth is controlled by the cyclic crack tip opening displacement varDelta CTOD, which is considered as the appropriate fracture-mechanical parameter in case of large scale yielding. The three-dimensional varDelta CTOD concept and its effective numerical calculation by means of special crack-tip elements are introduced at first. Next, the program structure, the underlying numerical algorithms and calculation schemes of ProCrackPlast are outlined in detail, which capture the plastic deformation history along with the moving crack. In all simulations, a viscoplastic cyclic material law is used within a large strain setting. The numerical performance of this software is studied for a single edge notch tension (SENT) specimen under isothermal cyclic loading and compared to common finite element techniques for fatigue crack simulation. The capability of this software is featured in two application examples showing crack growth under mixed-mode LCF and TMF in a typical austenite cast steel, Ni-Resist. In combination with a crack growth law identified in terms of varDelta CTOD for a specific material, the tool ProCrackPlast is able to predict the crack evolution in a 3D component for a given thermomechanical loading scenario.

Can Martensitic Phase Transformation Measured by Magnetic Methods be an Indicator of Fatigue Damage in Austenitic Steel at Elevated Temperature and Thermo-Mechanical Loading?

The phenomenon of phase transformation from paramagnetic γ austenite to ferromagnetic α' martensite in metastable austenitic stainless steels under fatigue loading is well investigated, especially at ambient temperature. Generally, it can be said that at ambient temperature, the amount of martensite is proportional to the accumulated plastic strain and, therefore, can be an early indicator of fatigue damage. The problem arises at elevated temperatures when the thermal energy can severely or even completely inhibit the phase transformation. In this case, the amount of α'-martensite can no longer be a good indicator of fatigue damage. The question is, where is the limit the deformation-induced martensite can still be used as a fatigue damage indicator? To answer this question, several fatigue tests have been performed on AISI 347 (X6CrNiNb18-10), a metastable austenitic stainless steel often used in the piping of European nuclear powerplants. Tests included isothermal fatigue tests as well as thermo-mechanical fatigue (TMF) loading in the range of 25 to 320 °C. Deformation-induced martensite was measured in situ by means of a uniaxial magnetic balance, from which the rate of phase transformation was obtained. Fatigue specimens were then cut to investigate the distribution of martensite in the gauge length by means of magnetic force imaging. The results showed that isothermal fatigue loading results in a steady decrease in phase transformation rate with temperature. Initial martensite nucleation was observed to be different at elevated temperatures and occurred only at the surface, where sufficient shear deformation was present. TMF tests showed higher than isothermal rates of phase transformation. Peak temperatures above 300 °C were nevertheless enough to severely reduce the formation of α' martensite, even when cooled back to ambient temperature in the other half of the TMF cycle.

Experimental and numerical study of TGO-induced stresses of plasma-sprayed thermal barrier coating

Experimental and numerical studies were conducted in order to investigate the stress changes of ZrO2-8 wt% Y2O3 (YSZ) thermal barrier coatings (TBC) under isothermal loading. After conducting the isothermal oxidation test at a temperature of 1070 °C, the Thermally Grown Oxide (TGO) growth process and the life of samples were evaluated using scanning electron microscopy (SEM). The X-ray diffraction (XRD) approach was selected for the measurement of surface residual stress and its change in terms of TGO growth. Furthermore, a finite element model based on a real TGO profile, extracted by SEM, was executed, where TGO-induced stress was measured at the TGO/Top Coat (TC) interface direction. Employing the finite element model, following the selection of the appropriate creep strength based on experimental results, the stress evolution and distribution were assessed during time exposure at maximum temperature. It was shown that the lifetime of samples was about 240 h under isothermal loading of 1070℃. By increasing the time exposure, stresses increased and resulted in the formation of a delamination line above peak regions.

Unified viscoplasticity model for type 316 stainless steel subjected to isothermal and anisothermal-mechanical loading

316FR-type low carbon stainless steel is usually used for many advanced gas cooled reactor (AGR) power plants which are subjected to operating temperatures in the range of 500–650 °C under thermo-mechanical loading, which may lead to isothermal low cycle fatigue (LCF) and thermal mechanical fatigue (TMF) damage. The strain controlled thermo-mechanical fatigue test with and without holding time was carried out for 316FR-type stainless steel under the temperature cycling between 500 °C and 650 °C, and the strain ranges are Δε = ± 0.4%, ±0.8%, ±1.0% and ±1.2%, respectively. Within the framework of Abdel Karim and Ohno model, an improved unified viscoplastic constitutive model (UVM) is proposed to simulate the cyclic plastic behavior of 316FR stainless steel. The improved UVM includes strain rate sensitivity, temperature rate dependence, static recovery, average stress evolution and strain range dependence of cyclic hardening. And then based on the radial return mapping and backward Euler integration methods, the implicit stress integration algorithm and the new expressions of the consistent tangent modulus are derived respectively, the improved UVM is implemented by a User Material Subroutine into ABAQUS software again. The good correlation between simulation results and experimental data is obtained.

Experimental investigation of buckling stability of superelastic Ni-Ti tubes under cyclic compressive loading: Towards defining functionally stable tubes for elastocaloric cooling

Elastocaloric cooling technology recently attracted significant attention as an environmental friendly alternative to vapor-compression technology. It is based on the elastocaloric effect, which occurs in superelastic shape memory alloys (SMAs) during stress-induced martensitic transformation. To date, several proof-of-concept devices (mostly based on tensile loading) have been developed, but limited fatigue life was shown to be one of the major issues. Compressive loading improves the fatigue life of such devices significantly, but in return buckling of SMA structure might occur. To overcome this challenge, it is crucial to understand the buckling phenomena of SMA structures, especially for thin-walled tubes that seem to be ideal candidates for application in elastocaloric cooling devices. Here, we experimentally investigated the effects of the diameter-to-thickness ratio (Dout/t) and slenderness (λ) on buckling stability of Ni-Ti tubes that were subjected to cyclic compressive loading. In total, 161 superelastic Ni-Ti tubes with outer diameter (Dout) ranging from 2 mm to 3 mm, Dout/t ranging from 5 to 25 and gauge lengths (Lg) ranging from 6 to 20 mm were tested. The loading procedure consisted of 3 parts: (I) 1 isothermal full-transformation loading cycle, (II) 50 training cycles, and (III) 20 adiabatic cycles to simulate loading conditions in elastocaloric device. We constructed experimental phase diagrams of buckling modes in λ - Dout/t space for constant Dout and in λ - Dout space for constant Dout/t ratio. Marked areas of functionally stable tubes in these phase diagrams give the design guidance for future developments of durable and efficient elastocaloric devices and other applications, e.g. actuators and dampers.

Multiaxial thermo-mechanical fatigue damage mechanism of TC4 titanium alloy

In this paper, the experimental investigation was carried out on the TC4 titanium alloy to understand the cyclic deformation behavior and fatigue damage mechanism of materials under multiaxial thermo-mechanical loading. The results showed that the fast cracking mode of the material may be activated by the combined action of high temperature, tensile stress and shear stress, inducing a fast degradation in the axial mechanical properties of the material, which results in a drastic decrease in the failure life of the material. Moreover, it is also found that the non-proportional additional hardening and the tensile mean stress can increase the fatigue and oxidation damages of the material, and the material damage behavior is temperature dependent and time dependent. It should be pointed out that the damage mechanisms identified in this paper can reasonably explain the life law under uniaxial isothermal fatigue loading with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue loadings.