Tensile Creep Tests Research Articles

Structural behavior of an ablated reactor pressure vessel wall with external cooling

In a severe accident scenario of a nuclear power plant involving core meltdown and relocation to the lower head of the reactor pressure vessel (RPV), the vessel may undergo serious deformation and even failure due to extreme thermo-mechanical loads from the relocated core melt. Proper material models and detailed structural analysis are paramount in predicting the timing and mode of possible vessel failure.This paper presents a strain hardening creep model with optimal parameters to simulate the material behavior of the reactor steel 16MND5 under extreme thermo-mechanical loads. First, validations against two experiments, a tensile-creep test and the EU-REVISA RUPTHER #14 test, show that the proposed model is best overall compared to three previous models. Next, the creep model is implemented for the thermo-mechanical analysis of an ablated RPV under a severe accident scenario with external vessel cooling as a mitigation strategy. The effect of internal pressures from 3 to 50 bars is investigated with the assumption that the corners of the ablated part of the vessel have sharp corners. In this case, we found that the vessel fails above 40 bars. However, if we model the corners with varying smoothness or fillet sizes, we found significant delay in failure time and an increase in failure internal pressure.

Highly improved creep resistance of hot-extruded Mg-0.9Mn-0.5Ce alloy by conventional forging

The insufficient creep resistance seriously restricts the practical application of Mg alloys, which needs to be preferentially resolved. In this work, the conventional forging was adopted to highly improve the creep resistance of an extruded Mg-0.9Mn-0.5Ce (wt%) alloy, and the corresponding mechanism on the improvement of creep resistance was carefully analyzed. The forging was conducted at 623 K, 673 K and 723 K with the strain rate of 1.0 × 10−3 s−1, 1.0 × 10−2 s−1 and 1.0 × 10−1 s−1 to a true strain of ε = 0.4 along the extrusion direction (ED), and the tensile creep tests were conducted at 423 K along the ED. It was shown that a mixed microstructure with both coarse and fine grains was obtained, and the basal poles tilted from the transverse direction (TD) to the ED after forging. Microstructure characterization revealed that forging barely showed influence on the active dislocation slip systems even the texture obviously changed. They were basal slip and pyramidal <c + a > slip in both as-extruded and as-forged samples, and the highly improved creep resistance was mainly derived from two aspects. On the one hand, the strong dislocation entanglements and obvious dislocations cells introduced by forging and the tough twin boundaries consolidated by precipitates effectively impeded the continuous movements of dislocation and highlighted the creep resistance. On the other hand, the dynamically precipitated Mg12Ce precipitates on dislocation lines and the obvious segregation of Ce elements at grain boundaries decreased the mobility of dislocations and grain boundaries, which also contributed to the improvement of creep resistance.

Microstructural Evolution during Accelerated Tensile Creep Test of ZK60/SiCp Composite after KoBo Extrusion.

In the current study, the creep properties of magnesium alloy reinforced with SiC particles were investigated. For this purpose, ZK60/SiCp composite was produced by the stir casting method following the KoBo extrusion and precipitation hardening processes. The creep tests were performed at 150 °C under 10–110 MPa. The results showed that the stress exponent (n) and the average true activation energy (Q) was changed at high stresses, was found with increasing stress, the creep mechanism changing from grain boundary sliding to dislocation climb. The results of microstructure characterization after the creep test showed that at low stresses, the dynamic recrystallization resulting from twinning induced the GBS mechanism. However, at high stresses, with increasing diffusion rates, conditions are provided for dynamic precipitation and the dislocation climb of the dominant creep mechanism. Examination of the fracture surfaces and the surrounding areas showed that the cavity nucleation in the ternary boundary and surrounding precipitation was the main cause of damage. The evaluation of the samples texture after creep showed that the unreinforced alloy showed a moderately strong fiber texture along the angle of ϕ1 = 0–90°, which was tilted about Φ = 10°. A new strong texture component was observed at (90°, 5°, 0°) for the composite sample, which crept due to minor splitting of the basal pole by ~5° toward RD.

Statistical Validation of a New Tensile Creep Test for Asphalt Mastics

Abstract Currently, the increasing traffic level and climatic change are exacerbating the premature distress of pavement infrastructures. In order to assess this problem, the proposed research activity aims to statistically evaluate the parameters of a new mastic tensile creep test, which is based on a uniaxial tensile configuration. This allows the material deformation to be kept in the small-strain domain and the mechanical behavior to be considered as linear viscoelastic (LVE). Particularly, the main tasks of the study herein were the statistical analysis of the proper testing temperature and load. Moreover, the storage period of the samples was also analyzed, comparing four storage durations (1, 2, 4, and 12 h). The proposed analysis was performed using three fillers (one pure limestone and two limestone-based fillers) and two asphalt binders, neat and 3.5 % Styrene-butadiene-styrene (SBS) modified, respectively. The validation of the testing protocol also involved the analysis of the operator sensitivity using the one-way ANOVA statistical test. The results showed that the testing protocol is not operator-sensitive and allows the LVE behavior to be kept in the small-strain domain. The results also indicated that the m-value seems to interpret the accumulation of the permanent deformation for the mastic materials, which may be used as the property of materials, well.

Tensile creep behavior of HfNbTaTiZr refractory high entropy alloy at elevated temperatures

Tensile creep, which is one of the most important deformation modes for high temperature applications, is rarely reported for refractory high entropy alloys (RHEAs). In the present study, the optical floating zone (OFZ) technique was used to fabricate HfNbTaTiZr with grain size larger than 1 mm on average; tensile creep tests under vacuum at 1100-1250°C and stepwise loading of 5–30 MPa were conducted. The stress exponents and creep activation energies were determined to be 2.5–2.8 and 273 ± 15 kJ mol–1, respectively. The stress exponents determined have suggested solute drag creep behavior, and deformation was governed by a/2<111> type dislocations. To elucidate the effect of alloying constituents on solute drag creep, intrinsic diffusion coefficients of all elements were determined by simulation, and theoretical minimum creep strain rates were compared with those of experimental values. Analysis suggests that creep rate of HfNbTaTiZr appears to be controlled by Ta, which possesses the lowest intrinsic diffusivity and contributes the most to drag dislocations. To our knowledge, this work is the first to report tensile creep deformation mechanism of HfNbTaTiZr, especially up to 1250°C.

Regularities of the creep deformation and failure of 15Kh2NMFA-A steel within the temperature range of 900–1200 °C

This paper presents and summarizes main results of the research to define the high-temperature creep parameters and strength characteristics of the unirradiated steel 15Kh2NMFA-А tested in the temperature range of 900–1200 °C. This ferritic-pearlitic steel is one of the main types of steel used for the production of the VVER RPVs of the Russian design. The results of research of the strength properties and creep parameters for 15Kh2NMFA-А steel are meant to support, first and utmost, the estimation of the integrity and predicting the behavior of the VVER pressure vessel during the SA, when it may deform due to high-temperature creep and the overpressure in the RPV. One of the main goals of the investigation was to determine such strength properties and creep parameters of the investigated steel as, for instance, steady-state creep rate, creep tensile strength under the condition of isothermal creep at different temperatures and test stress. The creep tensile tests were conducted in vacuum and in the protective argon medium. A series of creep tests was conducted in air. The analysis and comparison of the test results demonstrated significant impact of the surficial oxidation process on the nature of their damage and the time to failure of the test samples. The absence of a protective medium during testing, as observed in the case of the test in air, causes decarburization and the damage of the surficial layer of the steel samples, so that the specimen fractures prematurely. Based on the statistic estimation the (B) and (n) parameters for the secondary creep rate equations were defined for the researched steel within the temperature range of 900–1200 °C. The agreement between the values of the steady-state creep rate of 15Kh2NMFA-A steel from the conducted creep tensile tests and similar calculation values from the obtained parameters (B) and (n) for the temperature range from 900 to 1200 °C was satisfactory. The paper also presents the definition of the LMP parametric dependence coefficients, completed on the basis of the creep test results for 15Kh2NMFA-А steel with a time span up to 100 h in the given temperature range. The comparative analysis of the defined LMP dependence demonstrated good agreement of the predicted values for the failure time with similar values defined during the experiment.The research of the steel specimens’ microstructure following the tests demonstrated a difference in the mechanisms of damage accumulation and fracture at the temperatures over 1000 °C in comparison to similar mechanisms at lower temperatures. It is important to apply more precise research methods to understand this phenomenon to a better extent; such research may take place later.

Modeling blended cement concrete tensile creep for standard ring test application

AbstractThe tensile creep of concrete with supplementary cementitious materials (SCMs) such as fly ash (FA) and ground granulated blast furnace slag (GGBFS) is investigated in this paper. A total of 21 series of tensile creep tests using dog‐bone specimens under uniaxial tensile loading have been carried out. The cement replacement rates considered were 30% fly ash, 40% and 60% GGBFS. The characteristic compressive strength of concrete was ranging from 25 to 100 MPa. The tests were conducted from the age of 2 days until 28 days. It is observed that the tensile creep of fly ash concretes was slightly lower than that of the reference mixtures without SCM. For GGBFS concrete, the higher the GGBFS content, the higher the tensile creep. Existing creep models, originally developed for creep in compression, could not predict the experimental tensile creep results. Thus, a new tensile creep model was proposed including creep prediction for concrete with fly ash and GGBFS. The new model was calibrated only for controlled environmental conditions (23°C and 50% RH) and has been validated by analyzing the development of concrete tensile stress in the restrained ring test.

Integration of optical fibre sensors by material extrusion 3-D printing – The effect of bottom interlayer thickness

Thermoplastic composites are becoming the materials of choice for lightweight structures. Distributed optical fibre sensors can be valuable for structural health monitoring of thermoplastic composites, improving safety against damage and extending the operational life of composite components. However, the practical integration of optical fibres into thermoplastic composites remains still to be solved. To this end, a novel sensor integration approach by 3-D printing is proposed, and mechanical testing is conducted to test its performance. Optical fibres were placed inside thermoplastic embedding elements at controlled interlayer thicknesses (0 to 1.6 mm) from the substrate surface. Self-sensing experiments by optical fibres identified a uniform level of residual strains at −1600 µε from the 3-D printing attachment process. Furthermore, tensile creep testing (up to ca. 1% strain) revealed that distributed strain transfer coefficients for optical fibre sensors remain constant with time. A simple calculation method that accounts for the shear lag effect between the component and optical fibre strains was derived from previous analytical work. This method enables the empirical prediction of strain transfer coefficients for optical fibre surface attachments with arbitrary embedding interlayer thickness.

An initial study on the impression creep behaviour of stir-cast GNP reinforced AA6061 composite (AMMC)

Impression creep testing technique is used to screen out creep behaviour of materials as it is relatively faster method, requires small volume of the specimen, and is relatively material non-invasive technique when compared to conventional uniaxial tensile creep test method. This technique also possesses the benefit of maintaining constant stress throughout the experiment because a flat cylindrical indenter is used to apply load on the specimen. Owing to commercial unavailability of the materials under development such as metal matrix composites (MMC), the impression creep testing approach for such materials becomes a best method. In present work, creep behaviour of stir-cast graphene nanoplatelets (GNP) reinforced AA6061 composite (AMMC) is studied by using impression creep tests. The AMMC was prepared by adding GNPs in AA6061 through stir casting process by melting the alloy up to 1073 K. The impression creep experimentation included stress levels of 140 MPa, 170 MPa, and 200 MPa at a temperature of 573 K, 603 K and 633 K. The tests were conducted at constant stress by varying the temperature and vice-versa in vacuum. Two distinct regions i.e., primary and secondary/steady-state were observed and minimum creep strain rate was obtained in steady-state region. Tertiary region was absent, since the load/stress applied was compressive. During the experiment, depth of indentation of 0.03 mm to 0.14 mm was produced in the specimen. The microstructure of the composite has been analyzed using optical microscope and scanning electron microscope (SEM) to correlate with impression creep behaviour of the studied materials.

Use of Tensile Creep Test (TCT) for evaluation of low temperature performance of bituminous mixtures used for bridge pavement

Use of Tensile Creep Test (TCT) for evaluation of low temperature performance of bituminous mixtures used for bridge pavement

Class I type creep behavior of coarse-grained Al0.5CoCrFeMnNi high entropy alloy

The high-temperature deformation behavior of homogenized cast Al0.5CoCrFeMnNi with a coarse-grained face centered cubic (FCC) phase and a small amount of body centered cubic phase-dispersed within the FCC matrix grains was investigated using tensile creep and elongation tests at three temperatures of 1023, 1098 and 1173 K in a wide strain rate range between 10−8 and 10−1 s−1. As the strain rate increased, the strain rate regime associated with n = 3 (where n is the stress exponent) and n = 5 and power law breakdown (n > 7) appeared in sequence. As the temperature increased, the upper limit of the strain rate for n = 3 increased. The inverse primary creep behavior occurred and subgrain formation was not observed at the low strain rates and high temperatures associated with n = 3, strongly suggesting that solute drag creep is the rate-controlling deformation mechanism. Furthermore, the deformation mechanism of Al0.5CoCrFeMnNi was very similar to that of binary Al–Mg alloys known as typical class I solid solution alloys. The concept of breakaway stress from solute atmosphere could explain the transition of deformation mechanism from solute drag creep to dislocation climb creep at high strain rates.

Multiphase modeling of the effect of external loading on the shrinkage of concrete and its consequences for modeling tensile creep

Multiphase modeling is a powerful framework for the simulation of the early-age and long-term behavior of concrete. Nevertheless, state-of-the-art multiphase models do not capture the experimentally observed instantaneous change in internal relative humidity upon external loading, which also manifests in the lack of predicting load-induced shrinkage. The approach presented in this publication is a step to overcome this deficiency. It is based on a poromechanical multiphase model for coupled creep and shrinkage of concrete, which is enhanced by a strong hygro-mechanical coupling, introduced via a load-dependent desorption isotherm. The enhanced model is evaluated for sealed and drying uniaxial compressive and tensile creep tests. Instantaneous changes in internal relative humidity upon loading are recovered by the enhanced model. In compression, the influence on the mechanical response is small. In contrast, the representation of tensile creep is significantly improved by considering load-induced shrinkage.

Effect of grain-boundary θ-Al2Cu precipitates on tensile and compressive creep properties of cast Al–Cu–Mn–Zr alloys

Tensile and compressive creep tests were performed at 300 °C on high-temperature Al–Cu–Mn–Zr (ACMZ) alloys with 6 wt% Cu (6Cu) and 9 wt% Cu (9Cu) to evaluate the effect on creep properties of micron-size θ-Al2Cu intergranular precipitates. For compressive creep, the increased volume fraction of θ-precipitates at grain boundaries (from ∼0.7% in 6Cu to ∼ 6% in 9Cu) does not affect deformation rates across the investigated stress range of 15–110 MPa, consistent with creep being controlled by submicron θ′-Al2Cu precipitates within grains, whose size and fractions are the same in both alloys. In contrast, for tensile creep, 9Cu creeps faster than 6Cu at stresses above 20 MPa, and this difference increases with the stress level. This discrepancy between tensile and compressive creep behavior is explained by cavitation during tensile creep, which is favored by higher volume fraction and larger size of intergranular θ precipitates in 9Cu. Conversely, larger precipitates impede cavity linkage resulting in improved creep ductility of 9Cu as compared to 6Cu at 300 °C.

Effects of particle reinforcement on creep behaviour of AlSi1MgCu

Abstract The creep resistance of an unreinforced and a particle-reinforced AlSi1MgCu alloy (AA6061) produced by liquid metallurgy was studied by means of conventional tensile creep tests at 300 °C. Dislocation-creep mechanism is attributed to both materials in the range of 15 – 50 MPa. The particlereinforced metal (PRM) exhibits a higher creep rate than the matrix alloy because of the particles acting as dislocation sources and the increase of the apparent diffusion coefficient due to a higher defect density. The apparently smaller creep exponent for the matrix is explained by the overaging of Mg2Si precipitates during longterm tests. Void formation leads to strain localisation in the unreinforced alloy, which is dispersed by the particles reducing the damage rate in the PRM. Subsequently, the time to rupture of the PRM surpasses that of the unreinforced matrix at creep stresses smaller than 10 –3 G (20 MPa).

The effect of aspect ratio on the mechanical behavior of Li metal in solid-state cells

The Li metal anode is necessary for solid-state batteries to dramatically exceed the energy density of state-of-the-art conventional Li-ion batteries. To enable the Li metal anode, the mechanical properties of Li metal under compression in a commercially relevant cell configuration must be determined. This work examines the compression creep behavior of Li metal coupled with a relevant model solid-state electrolyte (Li 6.5 La 3 Ta 0.5 Zr 1.5 O 12 ) and current collector (Ni) as a function of applied stress (1–10 MPa) and Li foil thickness (100–700 μm). At 1 MPa applied stress, the strain rate of all Li foil thicknesses under compression was almost four orders of magnitude lower than what is predicted from tensile creep tests. Steady-state creep behavior was observed for Li foil ≥350 μm at 5 MPa and ≥250 μm at 10 MPa. Conversely, sustained creep deformation was not observed for 100 and 150 μm Li foils at any applied stress. It is suggested that friction at the Li/LLZO and Li/Ni interfaces leads to the formation of hydrostatic stress that controls Li deformation at low aspect ratios. These results have implications for battery pack stack pressure requirements, particularly relating to void formation during stripping and Li metal filament penetration during plating. • Compressive creep measurements of Li metal foils in solid-state cell configuration. • Strain and strain rate decreased with decreasing Li foil thickness. • The strain rate of Li measured in compression was lower than predicted from tension. • Friction at Li interfaces prevents substantial creep deformation for thin Li foils.

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™‐iBN and Hi‐Nicalon™‐S fibers

AbstractIsothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in‐plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™‐iBN or Hi‐Nicalon™‐S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi‐Nicalon™‐S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures &gt;1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.

Effects of hot compression on the fracture toughness and tensile creep behaviors of a Mg-Gd-Y-Zn-Zr alloy

The effects of hot compression on the fracture toughness and tensile creep behaviors of a Mg-Gd-Y-Zn-Zr alloy were researched in detail. The hot compression was performed to a true strain of 0.5 along the extrusion direction at 703 K with a strain rate of 3 × 10-1 s-1. Two types of samples were used in this work: one was the annealed sample without hot compression (NHC sample), the other was the annealed sample with hot compression (HC sample). Uniaxial tensile tests were performed with a strain rate of 3 × 10-3 s-1 at room temperature and 523 K. Fracture toughness tests were performed with a strain rate of 3 × 10-3 s-1 at room temperature. Tensile creep tests were carried out at 523 K under various applied stresses ranging from 90 to 130 MPa. All tests were conducted along the extrusion direction. The results showed the improved mechanical properties were obtained in the HC sample at room temperature. At elevated temperature, the yield strength of the HC sample was improved, while the ultimate tensile strength and elongation were decreased. Compared with the NHC sample, the room temperature fracture toughness of the HC sample was apparently improved due to 14H LPSO phases and kink bands had excellent blocking effects to cracks growth and turned the crack propagation direction, which helped to release stress and absorb energy. However, the creep resistance of the HC sample was poorer than that of the NHC sample at 523 K. The creep mechanism of the NHC sample was basal slip, while the HC sample were dominated by pyramidal <c+a> slip and grain boundary migration. Besides, the dynamic precipitation of β′ phases occurs during hot compression. Although pyramidal <c+a> slip and β′ phases had positive effects on the creep resistance, they could not offset the detrimental impacts derived from grain boundary migration and cracks nucleation at kink bands. As a result, a decreased creep resistance was obtained in the HC sample.

Application of PSO Method to Identification of Creep Material Constants Using Indentation Load Relaxation Curves

In the design of high-density mounting boards, the strength reliability of solder joints should be evaluated by conducting finite element analyzes (FEAs) considering the distribution of creep properties in the joints. To conduct such FEAs, a testing method that can evaluate the creep properties of the solder joints in-situ must be developed. The indentation creep test is an effective method for the evaluation, however, there are some problems with the method such as the creep law obtained by this method does not coincide with that obtained by tensile creep tests. Therefore, we previously proposed a new indentation test including the indenter depth maintenance process in which creep properties are evaluated by using the load relaxation curve given by the maintenance process. However, we have not developed a systematic method to estimate material constants in creep law based on the test results yet. Then, in this study, we proposed a method using the particle swarm optimization (PSO) method to estimate creep material constants from the load relaxation curve due to maintaining indenter depth. To discuss the availability of the proposed method, elastic-plastic-creep FEAs were conducted as the numerical experiments of the indentation test including the indenter depth maintenance process for an Sn-Ag-Cu (SAC) solder.

Influence of High Pressure Sliding and Rotary Swaging on Creep Behavior of P92 Steel at 500 °C

High-pressure sliding (HPS) and rotary swaging (RS) at room temperature were used to form severely deformed microstructures in martensitic creep-resistant P92 steel. The deformed microstructures contained markedly different ratios of low- and high-angle grain boundaries (LAGBs/HAGBs). The application of the RS method, with an imposed equivalent strain of 1.4, led to the formation of a heterogeneous microstructure with a high number of LAGBs, while the HPS method, with an imposed equivalent strain of 7.8, led to the formation of a relatively homogeneous ultrafine-grained microstructure with a significant predominance of HAGBs. Microstructure analyses after creep testing showed that the microstructure of RS- and HPS-processed P92 steel is quite stable, but a slight coarsening of subgrains and grains during creep testing can be observed. Constant load tensile creep tests at 500 °C and initial stresses ranging from 300 to 900 MPa revealed that the specimens processed by HPS exhibited higher creep strength (slower minimum creep rate) and ductility compared to the coarse-grained and RS-processed P92 steel. However, the HPS-processed P92 steel also exhibited lower values of stress exponent n than the other investigated states of P92 steel. For this reason, the differences in minimum creep rates determined for different states decrease with decreasing values of applied stress, and at applied stresses lower than 500 MPa, the creep resistance of the RS-processed state is higher than the creep resistance of the HPS-processed state.

Short-Term Creep-Rupture Behaviour of AISI 310S Austenitic Stainless Steel Sheets Manufactured in Salem Steel Plant, a Special Steels Unit of Steel Authority of India Limited, Ministry of Steel, Government of India

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steadystate creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Creep-rupture properties are compared with that of AISI 600 ironbased superalloy to analyze quantitatively its behavior