Papers by Keyword: Creep

Abstract: To investigate creep rupture properties and microstructural changes due to creep of the Mod.9Cr-1Mo steel with high initial hardness, creep rupture tests were carried out at 600°C and 650°C. The hardness of the gauge and grip portions of the ruptured specimens were also measured. This steel exhibited higher creep rupture strength than the conventional material at the two temperatures, but its creep rupture strength showed a larger decreasing tendency at 650°C. For the specimens ruptured at 600°C, an increasing tendency in lath width and a decreasing one in hardness were confirmed in gauge portions, but they were not observed in grip portions. However, for the specimens ruptured at 650°C, the hardness of both gauge and grip portions tended to decrease with the time to rupture, and the recovery of the lath structure and the coarsening of M₂₃C₆ carbides were particularly noticeable in the gauge portions.

Abstract: Polymethylmetharylate (PMMA) has been widely used for aircraft canopies and transparent structural components, and processed into various parts through vacuum forming. In this study, the effects of forming speed and deformation characteristics on thickness uniformity during high-temperature vacuum forming of PMMA were analyzed. First, creep tests and high-temperature tensile tests were conducted at the specimen level to quantitatively distinguish between creep deformation and plastic deformation. Creep tests were performed under constant temperature and load conditions, and strain was measured through Digital Image Correlation. For plastic deformation analysis, tensile tests at room temperature and elevated temperatures were carried out to compare yield strength and elongation changes. To analyze thickness uniformity during the forming process, rectangular-shaped parts were fabricated using vacuum forming under various conditions where temperature and forming speed are key variables. After forming, thickness uniformity and surface transparency of the products were measured. Additionally, internal structural changes according to forming speed and temperature conditions were analyzed, and a comprehensive evaluation of material stability was performed.

Abstract: Power generation components that operating under extreme conditions are susceptible to creep deformation. Such components rely on comprehensive creep data to ensure its integrity, smooth plant operation, and avoid fatal accident due components catastrophic failure. The small punch creep (SPC) test has emerged as a promising alternative to traditional uniaxial creep testing (UCT), offering advantages in terms of the small amount of material required for test sample. This study aims to develop a cost-effective SPC test rig integrated with an existing UCT machine and investigate its reliability in predicting the creep properties. Comparative analysis establishes a robust correlation factor (Ψ=2.5) between SPC and UCT data for Grade 91 steel at 600°C, enabling accurate estimation of creep rupture life across a broad stress spectrum. Fractographic investigations reveal the transition from ductile to brittle fracture as load levels decrease. The successful of SPC rig development and validation not only expands the creep testing toolset but also enables more efficient material characterization, optimized component design, and improved life prediction methodologies.

Abstract: Creep and shrinkage of concrete are important parameters for verification of ultimate and serviceability limit states. The prediction models which can be found in design codes, are applicable for ordinary concrete types. Unusual concrete types, like e.g., white concrete can be used, but their properties should be investigated by experimental testing. The paper is focused on measurement of shrinkage and creep of white and grey concrete of the same strength class. The experiments showed that both, creep and shrinkage of white concrete are higher than those of ordinary grey concrete. The measured shrinkage strains were compared with predicted shrinkage strains using various prediction models.

Abstract: Super304H is austenitic steel used predominantly for boiler and turbine components in thermal power plants due to the high strength, excellent creep and oxidation resistance in high-temperature steam environments. Microstructural degradation is inevitable upon long-term high-temperature exposure. Therefore, understanding the processes of degradation is critical for the determination of residual lifetime of the parts. In this study, the hardness, tensile strength and creep strength of a Super304H steel that was in service as a reheater for ~8 years at about 600°C was investigated with corresponding microstructural analysis. Grain growth is evident, but microhardness and tensile strength did not decrease, due to the precipitation of nanosized NbX and Cu-rich particles in grain interior. However, the creep rupture life of the aged S304H steel is ~90% lower at 650°C than that of the virgin S304H, due to the coarsening of σ phase and formation of M₂₃C₆ on the grain boundary. Our measurements and analysis of the specimens in this study indicates the remaining service life longer than those in the NIMS database.

Abstract: A single pocket cage is the SKF product, which is used in Large Size Bearings for wind industry. The function of a bearing cage is to hold, guide and separate rolling elements, and differently from the conventional cage, the current one consists of segments, which eases the bearing assembly and reduces its weight. The long life challenge (25 years!) requires considering fatigue, and since the single pocket cage is made of PEEK polymer, it is also susceptible to creep (in near room temperature), which enhances fatigue damage. The current work proposes the numerical model capturing non-linear viscoelasticity of PEEK. The mechanical behavior of this material is identified in uniaxial tension test and is modeled in Finite Elements (FE) by means of the Parallel Rheological Framework (this numerical tool has been recently implemented in the commercial software ABAQUS). The current FE model enables to apply cyclic loading, simulating the material response of cage when it operates in running bearing. By applying sub-modeling technique only a small domain is modeled which improves the computational time efficiency. The sub-model domain corresponds to the cage region, where the stress is high resulting to the material yielding, fatigue/creep degradation (due to inelastic cyclic deformation) and initiation of fatigue crack. The FE results were combined with the test data, in attempt to relate the numerically predicted damage to the cage life. The development of irreversible deformation during cyclic loading, shakedown analysis and the stress volume effect, are the main focuses of the current work.

Abstract: Two types of carbon fiber-reinforced epoxy composite laminates are chosen for long-term tensile creep tests under different temperatures and load levels. Their time-dependent and non-monotonic deformations indicate clearly both temperature effect and physical aging effect. To characterize these viscoelastic behavior, two phenomenological constitutive models and one physical model are developed. The linear viscoelastic model based on the Boltzmann superposition principle is able to describe reasonably the deformations at relatively lower stress levels and temperatures. The nonlinear viscoelastic model of Schapery’s single-integral form, together with a usage of effective time theory, could describe nicely all the effects of temperature, stress, and physical aging. The physical model based on Ngai’s coupling mechanism concept is further combined with the framework of Schapery’s nonlinear viscoelastic theory, which may provide certain physical understanding about the effect of aging behavior on long-term creep deformation of the laminated composites. Numerical modelling by finite element method are implemented, and comparisons between the experimental and simulation results are demonstrated.

Abstract: Conventional finite element (FE) modelling, which employed structured mesh, is unable to simulate local damage evolution at microstructure level. This paper aims to investigate the creep rupture and damage behaviour of Grade 92 steel under a creep environment using microstructural-type FE mesh. The idealised microstructures of the material were generated based on the Voronoi tessellation technique. Three notched bar specimens with different notch acuities were modelled in Abaqus v6.13 software and a ductility exhaustion based damage model was employed to estimate the damage state. The influence of the notch constraint on the ductility is accounted for in the simulation. It is found that the results obtained from the proposed technique are in good agreement with the experimental data. All the prediction points fall within the scatter band of +/- factor of 2. The damage was predicted to initiate at a distance offset from the notch tip. As the acuity increases, the damage initiation site shifts further away from the notch.

Abstract: Effect of rare earth (RE) on creep rupture behavior of 316LN austenitic stainless steel (316LN steel) was investigated after crept at 700°C under the stress in the range from 125MPa to 200MPa, by the optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show RE addition in 316LN steel increased the creep rupture ductility at high stress, but reduced the creep rupture ductility at low stress. Under 200MPa, RE addition increased the creep rupture strain of 316LN steel from 0.558 to 0.787 but the creep rupture strain after crept under 150MPa was decreased from 0.875 to 0.566. The fracture mode of 316LN steel was also apparently impacted by the RE addition. The typical ductile fracture feature of homogeneous dense dimples was obviously observed in NRE steel after crept rupture under all stresses. While in 32RE steel, small amount of intergranular fracture fractographs under low stress appeared instead of partial dimples under high stress. Moreover, it is noted that RE addition in 316LN steel promotes to precipitate a great number of fine Laves particles within grains. These Laves particles strengthening the matrix resulted in the strain concentration on grain boundaries, which might sensitively induce crack initiation on grain boundaries during long-term creep under the low stress.

Abstract: A Group of three specimens from 1312 steel were subjected to a range of elevated temperatures (700, 800, 900) Celsius to demonstrate the hazard of fire on steel columns mechanical proprieties. The results show a dangerous increase in creep strain buckling after 60 minutes from the beginning of the test and after 67 minutes failed to occur. Mechanical tests for normal and elevated temperatures were made to compare the fire hazards, the results depict reduction by 29%, 46%, 55% for young modulus of elasticity for elevated steel specimens (700, 800, 900) °C. for yield strength the value decreased by (128.4, 169.6, 189) Mpa for specimens (700, 800, 900) °C respectively. The ultimate stress reduction by (64, 78, 83) % from the normal value. whenever higher temperatures go up the lower the ultimate strain falls dawn by (50, 60, 66) % to the original ultimate strain value 0.5. the 0.2 strain % is decreased from 0.0029 to 0.0027 for 700 °C and increased to 0.011, 0.015 for (800, 900) °C respectively.