Elasto-plastic Creep Research Articles

Coupling Characteristics of Creep Fracture of Rock Foundation on Wind Turbine under Wind-Induced Vibration

In view of the nonlinear mechanical characteristics of rock foundation creep fracture and its influence on the stability of wind turbine under wind load, based on the influence of wind load, this paper proposed the elastoplastic creep fracture and rock foundation bearing capacity on wind turbine. Considering the superstructure concrete with rock foundation and its boundary conditions, the wind load standard value was obtained and wind turbine system composition was constructed. The two grades of freedom system of viscous damping vibration model was proposed. Furthermore, the frequency characteristic equations and the relations of the first- and second-order natural frequencies were obtained. Considering plastic yield theory on power hard rock base material, the analytical expressions of principal stress distribution, plastic zone, and plastic state of I-II composite crack were obtained and used for coupling Mohr–Coulomb plastic yield condition and creep fracture characteristics. Furthermore, the nonlinear creep model equation and accelerated creep fracture time were also obtained, to be used for the modified Kelvin nonlinear accelerated creep model. Combined with the calculation examples, it is verified that the accelerated creep displacement and crack propagation of rock foundation are obvious, taking full account of the wind bracing and creep characteristics of rock foundation. Final, the failure mode of rock foundation is of compressive shear, local shear, and bending-shear; so, it is necessary to reinforce the interface of the rock foundation in a timely manner.

A stress triaxiality based modified Liu–Murakami creep damage model for creep crack growth life prediction in different specimens

Liu–Murakami creep damage model is improved to predict the creep life of various cracked specimens. The modified creep damage law is implemented in the framework of extended finite element method (XFEM) for performing elasto-plastic creep crack growth simulations. Experiments show that the crack tip constraints vary from component to component which leads to variation in crack growth rates. A stress triaxiality function is introduced in the modified Liu–Murakami damage model to address the variation in crack growth rates. Moreover, a new definition of stress triaxiality (ratio of a linear combination of maximum principal stress and hydrostatic stress to von Mises stress) is proposed based on Leckie and Hayhurst failure criterion. The new definition of stress triaxiality is a key parameter in the prediction of time to failure. The modified Liu–Murakami creep damage model is used for the creep crack growth (CCG) simulations of several specimens under different loading conditions. Parametric studies are also performed to study the influence of various parameters on the CCG. Moreover, a combined framework of continuum damage mechanics and XFEM is used to predict the CCG life of a turbine blade. This work establishes that the modified Liu–Murakami creep damage law accurately predicts the creep life of cracked components under different constraint conditions.

Creep crack simulations using continuum damage mechanics and extended finite element method

In the present work, elasto-plastic creep crack growth simulations are performed using continuum damage mechanics and extended finite element method. Liu–Murakami creep damage model and explicit time integration scheme are used to evaluate the creep strain and damage variable for various materials at different temperatures. Compact tension and C-shaped tension specimens are selected for the simulation of crack growth analysis. For damage evaluation, both local and nonlocal approaches are employed. The accuracy of the extended finite element method solutions is checked by comparing with experimental results and finite element solutions. These results show that the extended finite element method requires a much coarser mesh to effectively model crack propagation. It is also shown that mesh independent results can be achieved by using nonlocal implementation.

Changes in the Structure and Properties of Single-Crystal Refractory Alloys Under Long-Term Operation

The structure and properties of carbon-containing single-crystal alloy ZhS32 and carbonless single-crystal alloys ZhS36, VZhM4 and VZhM5U are studied after 500 – 3000-h aging at 850 – 1100°C. The resistance to elastoplastic strain, creep and fatigue and the characteristics of long-term strength of the alloys are determined. It is shown that the expected values of long-term strength and creep resistance of the single-crystal alloys should be corrected with allowance for the changes in their initial structure after long-term operation.

On the load-carrying capacity of structural members

Various approaches to determining the load-carrying capacity of beams and shells under elastoplastic deformation and creep deformation conditions are analyzed.

Residual Strain of OFC Using Synchrotron Radiation

Residual strain in oxygen-free copper, one of the materials used in high-heat-load components at SPring-8 front end, was investigated. A strain scanning method with oscillation was employed for strain measurement. The results were compared with those obtained using an elasto-plastic creep FEM analysis.

Effect of creep constitutive equation on simulated stress mitigation behavior of alloy steel pipe during post-weld heat treatment

Post-weld heat treatment (PWHT) is usually conducted to reduce the residual stress and to improve the mechanical properties of the welded region. The thermal elasto-plastic creep analysis is used to estimate the mitigation of the residual stress in the welded region analytically. The stress mitigation is caused by effect of the creep relaxation behavior during PWHT. Thus, creep constitutive equation is used to estimate the residual stress mitigation. The material properties of Norton's law and the Norton-Bailey law for alloy steel pipe JIS STPA23 (equivalent to ASME SA335P11 seamless steel tubes) were experimentally investigated. Creep tests were conducted at 400, 500, 600, and 700 °C. Specimens were subjected to certain stresses, and creep strain was measured. The material properties of Norton's law and the Norton-Bailey law were calculated from the measured creep strain data. Five bead-on curved plate specimens were fabricated in order to verify the thermal elasto-plastic creep analysis done with these creep constitutive equations. The curved plate was made from a pipe cut in the hoop direction. The specimens were welded without filler metal by gas tungsten arc weld (GTAW) on both surfaces in the center of the curved plate. The residual stress in one specimen was measured without PWHT. The other specimens were measured under different PWHT conditions. The results of analysis with the two creep constitutive equations agreed well with the experimentally measured results. In addition, no differences in the creep constitutive equations were observed at high PWHT temperature. Accordingly, these analyses conducted with both types of creep constitutive equation were verified to be effective. The effect of creep constitutive equation on simulated stress mitigation behavior during PWHT has been clarified for analytically estimating residual stress in the welded region.

Prediction of fatigue life of high-heat-load components made of oxygen-free copper by comparing with Glidcop

Following a successful study on the prediction of fatigue life of high-heat-load components made of Glidcop, the thermal limitation of oxygen-free copper (OFC), which is used more commonly than Glidcop, has been studied. In addition to its general mechanical properties, the low-cycle-fatigue (LCF) and creep properties of OFC were investigated in detail and compared with those of Glidcop. The breaking mode of OFC, which was observed to be completely different from that of Glidcop in a fatigue fracture experiment, clarified the importance of considering the creep-fatigue interaction. An additional LCF test with compressive strain holding was conducted so that the creep-fatigue life diagram for out-of-phase thermal fatigue could be obtained on the basis of the strain-range partitioning method. The life predicted from elasto-plastic creep analysis agreed well with that determined from the void ratio estimated in the fatigue fracture experiment.

Material Modeling of Elasto-Plastic Creep Behavior for Gray Cast Iron Based on Damage Mechanics and Its Applications to Creep Fatigue Life Prediction

The elasto-plastic constitutive equation based on the concept of continuum damage mechanics is extended to cover the creep behavior for the gray cast iron as used in diesel engine parts. It employs the creep strain given by Perrin and Hayhurst and the creep damage given by Dunne and Hayhurst. Lots of material parameters in the constitutive equation are determined by using the uniaxial tensile/compressive, the creep, the relaxation, and the fatigue test results at 400°C. The identified model is used to predict the creep fatigue life with 600 sec hold-time at the maximum compressive strain. The validity of the proposed model is illustrated by comparing the calculated results with the experimental results. It is expected that the developed computational modeling can be used effectively to predict the creep fatigue life of gray cast iron machine parts.

Studies on Dynamic Character of Saturated Soft Clays through Cyclic Torsion Shear Tests

A large number of cyclic torsional shear tests on saturated soft clay under unconsolidated undrained condition have been carried out. The changing law between accumulative generalized shear strains vs. failure numbers has been analyzed and a cyclic shear strength curve has been received based on cyclic torsional shear tests. According to the test re- sults, the criterion of saturated soft clay is approved on plane. Further, the test results are in good accord with the pseudo-static elasto-plastic creep model, which based on the cyclic triaxial tests according to the theory of pseudo-static creep. Then, dynamic character of saturated soft clay is verified from test results. The reliable proof has been provided for understanding and using dynamic character of saturated soft clay with cyclic loads, at the same time the ground has been provided for using and popularizing pseudo-static elasto-plastic cyclic strength model and pseudo-static elasto-plastic cy- clic creep model.

Two-dimensional boundary element analysis of creep continuum damage problems with plastic effects

In this paper, a quadratic boundary element formulation for elasto-plastic creep damage problems in which strains and deformations are mainly creep dominant is presented. In order to cover both plastic and creep deformations, the initial strain formulation is employed. The Euler method with automatic time-step control scheme is implemented for time integrations. Creep rupture life is predicted using a continuum damage mechanics approach. The proposed formulation is applied to the uniaxial, perforated plate and rotating disk.

Creep-rupture lifetime simulation of unidirectional metal matrix composites with and without time-dependent fiber breakage

Abstract A numerical simulation for predicting the axial creep-rupture lifetime of continuous fiber-reinforced metal matrix composites is proposed, based on the finite element method. The simulation model is composed of line elements representing the fibers and four-node isoparametric plane elements representing the matrix. While the fibers behave as an elastic body at all times, the matrix behaves as an elasto-plastic body at the loading process and an elasto-plastic creep body at the creep process. It is further assumed in the simulation that the fibers are fractured not only in stress criterion but time-dependently with random nature. Simulation results were compared with the creep-rupture lifetime data of a boron-aluminum composite with 10% fiber volume fraction experimentally obtained. The simulated creep-rupture lifetimes agreed well with the averages of the experimental data. The proposed simulation is further carried out to predict a possibility of creep-rupture for the composite without time-dependent fiber breakage. It is finally concluded that the creep-rupture of a boron-aluminum composite is closely related with the shear stress relaxation occurring in the matrix as well as time-dependent fiber breakage.

Elasto-plastic creep behaviour and load capacity of steel columns during fire

Elasto-plastic creep behaviour and load capacity of steel columns during fire

Plastic load capacity and stability of frames in fire

The behaviour of real construction materials in different predictable conditions is so complex that it is difficult to describe it precisely in terms of mathematical equations. The problem of the mathematical modelling of materials is, therefore, to be solved rationally by taking into consideration the material type, construction method, load value and external conditions (e.g. temperature). In ambient temperatures there is an interaction of elastic form and plastic form of steelwork failure. This interaction is described by the Merchant-Rankine formula. An analogous interaction should be expected at elevated temperatures. This paper puts forward an interaction formula for elasto-plastic creep thermal frames. It also presents an analytical method which may be used for fire safety designs in the building of steel frames.

Creep Effects on Creep Fatigue Crack Propagation of Center Cracked Plate and Attendant Acceleration/Retardation Phenomena.

A computational analysis addressing to the creep crack propagation problems is conducted. An incremental finite element procedure is developed and applied for investigating the creep effect on a fatigue crack propagation of a center cracked plate. The near crack parameter T, for which the preliminary study showed the validity of its use in the elastoplastic creep regime, is used in the prosent fatigue crack study. The phenomena of crack acceleration and retardation due to loading spectra are discussed in terms of T and the crack opening/closure load level. T as well as crack opening levels indicates the acceleration and retardation. The creep crack propagates faster than that under the non-creep cracks. The creep strain seems to play the role of suppressing acccleration and retardation effects in low to high, high to low and single overload cases.

Simplified analysis of cyclic elastoplastic creep responses in elevated temperature structures under cyclic loading

A simplified analysis of cyclic elastoplastic creep responses in structures subjected to cyclic mechanical loads with a holding time at high temperature is presented in this paper. The main feature of this method is to transform the complicated response analysis to another approximately equivalent problem—the elastoplastic analysis of a structure subjected to a cyclic loading with variant load amplitude, while the load amplitude variation in each loading branch is determined by the amount of stress relaxation during the holding time before the subsequent load reversal. Then two methods which are used in simulating cyclic elastoplastic responses and creep stress redistribution respectively as well as a load equivalence technique to deal with their interaction are combined to track the response evolution of cyclic elastoplastic creep at a critical location concerned in a structure.

Elastoplastic creep analysis of thermal stress and strain in aluminum interconnects of LSIs

AbstractTo study a thermally induced open‐failure mechanism of aluminum interconnects of LSIs, a three‐dimensional (3‐D) simulation using a finite element method was carried out. In this calculation, a creep strain increasing with time in addition to an elastoplastic strain was taken into account. Further, the effect of a manufacturing process on a stress and a strain induced in an aluminum layer was examined using a simulation for a subsequent stacking of oxide‐layer/aluminum film/protective layer on an Si chip.The condition for a 3‐D plasticity to occur was calculated in which case a strain was represented using the von Mises criteria. As a result, after the manufacturing process is completed, the stress is found to decrease significantly with time, while a plastic strain is kept constant. However, the creep strain responsible for the failure is found to increase. the temperature dependence of the creep strain also was studied. the three different results previously reported, i.e., the lifetime becomes shorter as the temperature rises, there is no temperature dependence, and the shortest lifetime is at about 150°C are all found to be possible. the calculation was applied to the multilayered interconnects and the creep strain is shown to decrease by more than a few tens percent in an aluminum layer.

建築構造用耐火鋼柱の耐火性能に関する実験的研究

This paper presents experimental results of a full-scale fire test with loading by Fire Resistant steel (FR steel) columns and their numerical analysis. Because of the elements it contains, FR steel excels, compared to conventional structural steels, in strength at elevated temperatures. Sprayed rockwool and an intumescent coating are used in providing FR steel's fire protection. Experimental results show that FR steel columns can bear a load corresponding to two-thirds of their room temperature yield point at 600℃, reduce the thickness of rockwool compared to conventional steel columns, and provide one hour of fire resistance with an intumescent coating. Also, a numerical analysis shows that it is possible to simulate fire, resistance time and deformation behavior by the numerical properties of FR steel and an elasto-plastic creep thermal deformation analysis.

The numerical variational principle of structural analysis for elastoplastic creep damage problems

The numerical variational principle of structural analysis for elastoplastic creep damage problems

Stress concentration in plates under tension with local thinnings in elastoplastic deformations and creep

The results are given of a calculation-experimental investigation of the stress concentration in the zone of two-sided and one-sided craters in the form of spherical segments. The elastoplastic problem was solved by the method of variable parameters of elasticity with use of the finite element method. The experiments were made on plates of AL2 alloy with the use of short-base strain gauges. The relationship was obtained of the stress concentration factors to the parameters of thinning and loading at characteristic points. The picture is given of the successive transition of the elements into the plastic state. The simulation of creep based on the method of isochronous curves made it possible to obtain curves of the change in stress concentration factors with time for the characteristic points applicable to the steels used in turbine production from the data of experimental investigation of aluminum elastoplastic models.