Hoop Stress Level Research Articles

Biomechanics of the medial meniscus in the osteoarthritic knee joint.

BackgroundIncreased mechanical loading and pathological response of joint tissue to the abnormal mechanical stress can cause degradation of cartilage characteristic of knee osteoarthritis (OA). Despite osteoarthritis is risk factor for the development of meniscal lesions the mechanism of degenerative meniscal lesions is still unclear. Therefore, the aim of the study is to investigate the influence of medial compartment knee OA on the stress state and deformation of the medial meniscus.MethodsThe finite element method was used to simulate the stance phase of the gait cycle. An intact knee model was prepared based on magnetic resonance scans of the left knee joint of a healthy volunteer. Degenerative changes in the medial knee OA model were simulated by nonuniform reduction in articular cartilage thickness in specific areas and by a decrease in the material parameters of cartilage and menisci. Two additional models were created to separately evaluate the effect of alterations in articular cartilage geometry and material parameters of the soft tissues on the results. A nonlinear dynamic analysis was performed for standardized knee loads applied to the tibia bone.ResultsThe maximum von Mises stress of 26.8 MPa was observed in the posterior part of the medial meniscus body in the OA model. The maximal hoop stress for the first peak of total force was 83% greater in the posterior horn and only 11% greater in the anterior horn of the medial meniscus in the OA model than in the intact model. The reduction in cartilage thickness caused an increase of 57% in medial translation of the medial meniscus body. A decrease in the compressive modulus of menisci resulted in a 2.5-fold greater reduction in the meniscal body width compared to the intact model.ConclusionsHigher hoop stress levels on the inner edge of the posterior part of the medial meniscus in the OA model than in the intact model are associated with a greater medial translation of the meniscus body and a greater reduction in its width. The considerable increase in hoop stresses shows that medial knee OA may contribute to the initiation of meniscal radial tears.

Parametric Study on Effect of Friction and Overbanding in Screening Current Stress of LBC Magnet

Magnetic stress is of particular concern for ultra high field magnets, especially, peak hoop stress of those magnets may be locally concentrated by the screening current stress (SCS). For instance, the Little Big Coil (LBC), which was designed, constructed, and operated by a collborate team led by NHMFL, generated the world record magnetic field of 45.5 T, but the conductor in LBC experienced plastic deformation due to excessive SCS. In this paper, to further analyze details of the mechanical stress behavior of LBC magnet, the effect of friction coefficient and overband radial build on stress alleviation is investigated by the parametric study. First, total current density with screening current is calculated by using the H-formulation together with domain homogenization method. Later, the entire magnet structure is modeled by the 2D finite element method to represent preload and axial interactions between each adjacent single pancake (SP), and 3 turns are assumed to be one engineering turn with frictionless radial turn-to-turn contact. While, friction acts between the coil and spacer in a direction that limits the radial displacement. Finally, we discuss the effects of friction coefficient and overband thickness by sweeping those parameters. Our parametric study implies that both friction and overband radial build can reduce peak hoop stress levels of LBC, however, effect of friction is dominant when overbanding is thicker than 0.5 mm.

Radiological aspects and behavior of spent fuel considering long-term storage

For spent nuclear fuel management in Germany, the concept of dry interim storage in dual purpose casks before direct disposal is applied. Current operation licenses for storage facilities have been granted for a storage time of 40 years. Due to the current delay in site selection, an extension of the storage time seems inevitable. In consideration of this issue, GRS performed burnup calculations, thermal and mechanical analyses as well as particle transport and shielding calculations for UO2 and MOX fuels stored in a cask to investigate long-term behavior of the spent fuel related parameters and the radiological consequences. It is shown that at the beginning of the dry storage period, cladding hoop stress levels sufficient to cause hydride reorientation could be present in fuel rods with a burnup higher than 55 GWd/tHM. The long-term behavior of the cladding temperatures indicates the possibility of reaching the ductile-to-brittle transition temperature during extended storage scenarios. Surface dose rates are 3 times higher when a cask is partially loaded with 4 MOX fuel assemblies. Due to radioactive decay, long-term storage will have a positive impact on the radiological environment around the cask.

Pressure Pipe Damage: Numerical Estimation of Point Load Effect II

It has been shown that the service lifetime of buried pipes can be reduced especially in relation to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated there and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper, the effect of a hard indenter, material properties, and the hoop stress level on the stress intensity factor of the axial inner crack are evaluated. A simple relation for estimation of the stress intensity factor value without FEM simulations is proposed.

Superconducting and Mechanical Properties of Impregnated REBCO Pancake Coils Under Large Hoop Stress

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the BJR relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at Iop = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.

Pressure Pipe Damage: Numerical Estimation of Point Load Effect

The most relevant loading conditions for real polymer pipe systems are not only internal pressure, but also loading caused by sand embedding including bending or different kinds of point loads. It has been shown that service lifetime of buried pipes can be reduced especially due to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated here and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper the effect of the hard indenter, Poissons ratio, hoop stress level and pipe wall thickness on the crack shape was estimated using numerical simulations of the creep crack propagation based on finite element method. Relation between crack length and crack width was found and expressed by simple relationship. A deeper understanding of the point load effect in order to prevent unexpected failure of the pipelines is of paramount importance for pipeline design.

Radiation creep and phase instability of low-activation austenitic steel 12 Cr-20 Mn-W under neutron irradiation in the FFTF fast reactor

Results of a radiation creep experiment conducted in the FFTF fast reactor using the MOTA-2B temperature control facility maintained at ±5°C are presented. Helium-pressurized creep tubes were constructed from a reduced activation austenitic Fe-12 Cr-20 Mn-W steel and irradiated in contact with the sodium coolant. Five hoop stress levels were employed ranging from 0 to 140 MPa. The neutron fluence reached varied from 5.3 to 5.8 × 1022 n/cm2 (E > 0.11 MeV) at target temperatures of 428, 519, and 599°C. The total strains were found to be strongly dependent on irradiation temperature, but not to be completely linear with stress level. The measured stress-free stains are thought to arise from both phase instability and void swelling.

Health assessment of 22 years service-exposed radiant tube from an oil refinery

Radiant tube conforming to ASTM A-200 T5 grade of steel and service exposed for 20 years from an oil refinery was investigated for health check and safety aspect. Microstructural study of the service exposed tube revealed a ferritic–martensitic structure with significant spheroidization of second phase particles. It appears that there is total disintegration of martensitic lath boundaries in some areas. The martensitic lath grains seemed to have broadened and lath boundaries are decorated with coarse and rounded carbides. As a result, with carbides the material has softened. Analysis of hot tensile and accelerated stress rupture data revealed that the residual life at 575 °C and an operating hoop stress level of (28.11 MPa) gave a minimum balance of life of approximately four and a half years for the service-exposed radiant tube. This is provided there is no evidence of localised damage in the form of surface cracks, cavitations or dents. A further life assessment of the service-exposed radiant tube is recommended after an additional 2–3 years of service life an economical and safety reasons viewpoint. During the next shut down of the plant, NDT (non-destructive) tests viz. dimensional (thickness and diameter)measurement, hardness measurement and in situ metallography may be carried out to assess the condition of the materials for their future serviceability.

A model analysis of static stress in the vestibular membranes

BackgroundThe scheme of the core vestibular membranes, consisting of serially connected utricle, ampulla and semicircular canal, first appeared hundreds of millions of years ago in primitive fish and has remained largely unchanged during the subsequent course of evolution. The labyrinths of higher organisms build on this core structure, with the addition of the phylogenetically newer membrane structures, namely, saccule, lagena and cochlea. An analysis of static stress in these core vestibular membranes may contribute to a better understanding of the role of stress in the evolution of derivative membrane structures over the long term as well as the short-term membrane distortions seen in Meniere's disease.MethodsA model of these core vestibular membranes is proposed in order to analyze the distribution of stress in the walls of the component chambers. The model uses basic geometrical elements of hollow cylinders and spheres to emulate the actual structures. These model elements lend themselves to a mathematical analysis of static stress in their membranes.ResultsHoop stress, akin to the stress in hoops used to reinforce barrel walls, is found to be the predominant stress in the model membranes. The level of hoop stress depends not only on pressure but as well on a geometric stress factor that incorporates membrane shape, thickness and curvature. This result implies that hoop stress may be unevenly distributed in the membranes of the several vestibular chambers due to variations in these dimensional parameters. These results provide a theoretical framework for appraising hoop stress levels in any vestibular labyrinth whose dimensions are known.ConclusionStatic hoop stress disparities are likely to exist in the vestibular membranes given their complex physical configurations. Such stress disparities may contribute to the development of membrane pathologies as seen in Meniere's Disease. They may also factor in the evolutionary development of other derivative membrane structures such as the saccule, the lagena, and the cochlea found in higher animals.

Full Resolution Structural Finite Element Analysis of the CEA/Iseult 11.75T MRI Coil

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems requires a highly homogeneous 11.75T magnet with a 900 mm warm bore. We describe the principle of calculation of the main coil deformations in order to perform field homogeneity loss assessment and mechanical dimensioning. This coil is wounded as double pancakes laterally wetted by HeII. As preliminary calculation runs led to high hoop stress levels, and due to the composite nature of this winding pack, a detailed finite element model of the coil structure is needed to obtain accurate computation of the displacement and stresses. Also, this finite element model serves as reference solution to validate a homogenized model. Then, a mixed full resolution/homogenized model is constructed in order to reduce the mesh size. A good correlation between this mixed model and full resolution one is observed. These three kinds of models will be exploited during the development phase of the magnet, to conduct mechanical design and estimation of field homogeneity losses.

Creep behavior of reduced activation ferritic/martensitic steels irradiated at 573 and 773 K up to 5 dpa

The irradiation creep behavior of F82H and several variants of JLF-1 steel has been measured at 573 and 773 K up to 5 dpa using helium-pressurized creep tubes irradiated in HFIR. These tubes were pressurized with helium to hoop stress levels of 0–400 MPa at the irradiation temperature. The results for F82H and JLF-1 with a 400 MPa hoop stress showed small creep strains (<0.25%) after irradiation at 573 K. The irradiation creep strain at 573 K in these steels is linearly dependent on the applied stress at stress levels below 250 MPa. However, at higher hoop stress levels, the creep strain becomes nonlinear. At 773 K, the irradiation creep strain of F82H is linearly dependent on the applied stress level below 100 MPa. At higher stress levels, the creep strain increased strongly. The creep compliance coefficients for F82H and JLF-1 are consistent with the values obtained for other steels. These data contribute to the materials database for ITER test blanket design work.

HFIR 照射された低放射化フェライト鋼の照射下クリープ挙動

The irradiation creep behavior of F82H and several JLF-1 steels has been measured at 573 and 773 K up to 5 dpa using pressurized creep tubes irradiated in HFIR. These tubes were pressurized with high-purity helium to hoop stress levels of 0 to 400 MPa at the irradiation temperature. The results for F82H and JLF-1 with hoop stress of 400 MPa showed small creep strains (<0.25%) after irradiation at 573 K. The irradiation creep strain at 573 K in these steels is linearly dependent on the applied stress at stress levels below 250 MPa. However, at higher hoop stress levels, the creep strain is nonlinear. At 773 K, the irradiation creep strain of F82H is linearly dependent on the applied hoop stress level below 100 MPa. At higher stress levels, the creep strain increased strongly. The creep compliance co-efficient for F82H and JLF-1 is consistent with the values obtained for other steels. These data contribute to the materials database for ITER test blanket design work.

Health Assessment of Radiant Heater Tube

A radiant heater tube in an oil refinery made of a particular grade of austenitic steel and service exposed for 61,000 hours was investigated for health assessment and safety. Experimentally determined yield strength and ultimate tensile strength as well as estimated 10,000 hours - 100,000 hours rupture strength as obtained from experimental data in the temperature range of 773 to 873K exhibit a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization etc. Analysis of hot tensile and accelerated stress rupture data revealed that the service exposed radiant heater tube could remain in service for a length of more than ten years at the operating hoop stress level and maximum allowable skin temperature (158.78 MPa / 865 K), provided no localised damage in the form of cracks or dents had developed. It is recommended that a similar health check of these tubes should be carried out after 50,000 hours of service exposure at the desired operating conditions.

Remaining Life Assessment of Service Exposed Reheater and Superheater Tubes in a Boiler of a Thermal Power Plant

This paper presents the high temperature tensile and the stress rupture properties of 150,000 hours service-exposed superheater and reheater tubes made of 2.25Cr-1 Mo steels in a 120 MW boiler of a thermal power plant. These were used to estimate the remaining life for safety. Experimentally determined yield strength and ultimate tensile strength as well as estimated 10,000 hours - 100000 hours rupture strength as obtained from experimental data in the temperature range of 793 to 853K exhibit a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization etc. In general, analysis of tensile and stress rupture data reveal that the service exposed superheater and reheater tubes can remain in service for a length of more than ten years at the operating hoop stress level 40 MPa / 813K, provided no localised damage in the form of cracks or dents has been developed. It is recommended that a similar health check should be carried out after 50,000 hours of service exposure at 813K.

Remnant life assessment of service-exposed pendent superheater tubes

Remnant life assessment and life extension has become an integral part of plant maintenance activities, Cr–Mo steels are widely used in thermal power plants at a temperature of about 550°C. This paper deals with the high temperature tensile and creep rupture properties of 1Cr–0.5Mo steels for pendent superheater tubes in a boiler of a thermal power plant after five years service exposure for 43,000 h. Based on stress vs Larson Miller Parameter plot and at the operating hoop stress level, the pendent superheater tubes in general are found to be in a good state of health and can continue to remain in service for a length of 10 years, provided the temperature of the tube is brought down below 500°C. This observation may not be true for those selected tubes where already expansion in the diameter has taken place. Such tubes should be identified and replaced. In the case that it is not possible to bring down the temperature below 500°C, it may be desirable to use a superior grade of material like 2.25Cr–1Mo or 9Cr–1Mo steel. A thicker tube of the same grade may also be considered provided the overall load of the panel does not exceed the limit. It is, however, recommended that a similar health check be carried out after five years.

Mechanical Property Evaluation and Remaining Life Assessment Of Service-Exposed Steam Pipe of Boilers in a Thermal Power Plant

This paper is aimed at investigating the residual life of more than eleven years service - exposed main steam pipes of various boilers in a thermal power plant. The remaining life assessment for safety were made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 823 K is concerned, it is possible to obtain a life of greater than 100000 hours, both at the allowable as well as operating hoop stress levels of the service - exposed pipes, provided no localised damage in the form of cracks or dents has been developed. It is recommended that a health check should be carried out after 50,000 hours of service exposure at 823 K.

Mechanical Property Evaluation and Remaining Life Assessment of Service Exposed Steam Pipe of Boilers in a Thermal Power Plant

This paper is aimed at investigating the residual life of more than eleven years service - exposed main steam pipes of various boilers in a thermal power plant. The remaining life assessment for safety were made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 823 K is concerned, it is possible to obtain a life of greater than 100000 hours, both at the allowable as well as operating hoop stress levels of the service - exposed pipes, provided no localised damage in the form of cracks or dents has been developed. It is recommended that a health check should be carried out after 50,000 hours of service exposure at 823 K.

Evaluation of mechanical properties and assessment of residual life of a service-exposed water wall tube

This paper is aimed at investigating the residual life, spanning 32 years, of a service-exposed water wall tube from one of the steam generating boilers of a thermal power station. The remaining life prediction and safety assessments were made using destructive accelerated stress rupture and tensile tests at different temperatures, as well as some non-destructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service-exposed water wall tube. So far as the remaining life at 425°C is concerned, it is possible to obtain a life greater than 100,000 h at the hoop stress level of the service-exposed tube, providing that no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure.

Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant

This paper deals with residual life prediction methodology for more than 12.5 years service exposed main steam pipes of various boilers in a thermal power plant. Health assessment was made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 550°C is concerned, it is possible to obtain a life of greater than 100,000 h at the hoop stress level of the service exposed pipes, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure at 550°C.

Prediction of remaining life of a FCCU reactor plate

Evaluation of mechanical properties is a powerful tool for investigating and assessing the performance of service exposed materials. The trends recently observed in service exposed (32 years) plain carbon steels used as a reactor vessel material in a petrochemical plant have been highlighted in this paper. Health assessment of both base and welded joint was made using impact tests at room temperature, destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. From the stress rupture tests it could be concluded that a life of more than 10 years at the hoop stress level of the material can be obtained, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a health check should be carried out after 5 years of service exposure at 485°C.