Multi-step Loading Research Articles

Bending properties of GCr15 steel guide rail under the multi-step loading

To analyze the bending properties of GCr15 steel guide rail based on the elastic-plastic theory, the novel bending loading method consisting of multi-step loading and corresponding unloading was applied in three specimens with different cross section shape and different heat treatment condition. According to the experimental results, using numerical calculation software program and the numerical simulation with finite element analysis (FEA), the relationships among the maximal load and displacement on cross section shape with each step bend loading, the loading stroke with the heat treatment condition, and the loading stroke with cross section shape were gained, and also those curves were discussed qualitatively. Finally, the contrast results between the numerical simulation and experiment were carried out to study the influence about the multi-step loading on specimen. It is put forward that enlightenment for the straightening stroke in the precision linear guide rail manufacture process.

Study on the Development of Load Shedding Scheme for Improving Voltage Stability of Seoul Metropolitan Area using Synchro-phasor Data

Recent technology advancement related to computer & communication and measuring devices allows system operators to adopt more intelligent monitoring and control systems to their power systems in order to prevent massive system blackout. Among them, wide-area monitoring and control(WAMAC) system based on synchro-phasor technology has been widely applied to power systems for their own purposes. In this paper, the study on the development of load shedding scheme to improve voltage stability in KEPCO system is introduced. Summary of WAMAC technology being developed and applied in the world through extensive literature survey is proposed. And methodology to develop voltage stability index and multi-step load shedding scheme based on synchro-phasor data is also presented.

Novel imprint lithography process used in fabrication of micro/nanostructures in organic photovoltaic devices

A novel multistep loading and demolding process in nanoimprint lithography (NIL) with a soft mold is developed to fabricate 3-D micro- or nanoscale cathode structures in polymer photovoltaic (PPV) devices. Experiments show that this new NIL process, called distortion reduction by pressure releasing (DRPR) and two-step curing method for the demolding process, can reduce and avoid the distortions of the imprint mold and wafer stage, and through the two-step curing method, the transformation of resist from liquid to solid state can be controlled, which is helpful to decrease the demolding force and avoid some defects caused by demolding. With this new NIL process, the main replicating error caused by distortions and blind demolding can be limited effectively, and the micro- or nanoscale cathode structures in PPV devices can be fabricated with high fidelity to the imprint mold, which can improve the power conversion efficiency of PPV.

Strategy to Improve Computation Precision in Inverse Analysis Method of Sheet Metal Stamping

The traditional inverse analysis method (TIAM) of sheet metal stamping gives bad estimation of stress distribution mainly due to the loss of history of deformation. Elements near the die entrance radius are subjected to bending and unbending during forming operation. Moreover,stress accumulation can usually be found in die entrance zone. A fast and reliable searching scheme is used to find elements passing by die entrance radius during the deformation process. The followed procedure has been used to modify stress distributions. An efficient stress-strain relation for multi-step loading is introduced to update stress distributions. This constitutive equations can avoid the complexity of incremental finite element method (FEM) based stress/strain updating algorithms,while can consider the effects of die entrance radius on deformation history. Deep drawing of the U-bending test proposed in Numisheet'93 demonstrates that these constitutive equations can significantly enhance the prediction of stress distribution by comparison with incremental FEM based software ABAQUS/Standard.

Rate Dependence and Short-Term Creep Behavior of a Thermoset Polymer at Elevated Temperature

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at 288°C. The effect of loading rate on monotonic stress-strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. The effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods on both loading and unloading paths. The strain-time response was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 288°C. The model was employed to predict the response of the material under monotonic loading/unloading and multistep load histories.

A novel loading and demoulding process control in UV nanoimprint lithography

In UV nanoimprint lithography (NIL) with elastic mould, a novel multi-step loading and demoulding process, called distortion reduction by pressure releasing (DRPR) and two-step curing method for demoulding, is developed. This novel imprint process is continuous, the pressure releasing method, used to optimize the loading process, can reduce the distortions of imprint mould and wafer stage, while obtain better cavity filling and thin and uniform residual layer; through two-step curing method instead of traditional simple demoulding, the curing degree of resist can be controlled, which is helpful to decrease the demoulding force and avoid residual layer pulled-up while ensure replicated protrusions not collapse. It is a novel and robust process with high fidelity of pattern replication in micro/nano structures fabrication, and the replication error caused by distortions and “blind” demoulding can be reduced effectively.

Multi-step loading of titania on mesoporous silica: Influence of the morphology and the porosity on the catalytic degradation of aqueous pollutants and VOCs

Titania nanoparticles have been deposited on inert porous silica supports with high specific surface area. These materials have potential applications in paint and textile industry as the titania particles selectively deposited on the inner surface of the silica supports act as a photocatalyst. The inert external surface is necessary to avoid photodegradation of the textile material or the paint components. The photocatalytic activity of the catalysts has been evaluated with two catalytic setups. One setup in aqueous phase, for the degradation of dyes such as rhodamine-6G, is commonly used. The second setup is a continuous flow gaseous phase setup which was used for the mineralization of ethanol as a representative volatile organic compound (VOC). The influence of the porosity and the morphology of the silica supports on the photocatalytic activity are discussed.

Critical voltages and blocking stresses in nematic gels

We present a model of the dynamics of director rotation in nematic gels under combined electro-mechanical loading. Focusing on a model specimen, we describe the critical voltages that must be exceeded to achieve director reorientation, and the blocking stresses that prevent alignment of the nematic director with the applied electric field. The corresponding phase diagram shows that the dynamic thresholds defined above are different from those predicted on the sole basis of energetics. Multistep loading programs are used to explore the energy landscape of our model specimen, showing the existence of multiple local minima under the same voltage and applied stress. This leads us to conclude that hysteresis should be expected in the electro-mechanical response of nematic gels.

Multistep Loading of Titania Nanoparticles in the Mesopores of SBA-15 for Enhanced Photocatalytic Activity

A multistep deposition of anatase nanoparticles was employed to incorporate high amounts of titania into the mesopores of SBA-15. Anatase nanoparticles were synthesized and deposited following the Acid Catalyzed Sol Gel method. With this method, the size of the anatase nanoparticles can be controlled and therefore, the titania loading into the mesopores of SBA-15 can be controlled. Through multistep deposition of anatase nanoparticles, a further increase of titania loading into the mesoporous channels can be obtained. For the degradation of Rhodamine-6G, the samples synthesized by multistep deposition showed an enhanced photocatalytic activity.

Fluid pressure driven fibril reinforcement in creep and relaxation tests of articular cartilage

Biological tissues exhibit diverse mechanical behaviors because of complex material properties. As has been shown for ligaments and intervertebral discs, mathematical models often appear to well predict load responses individually by adjusting model parameters, but likely fail to describe several different load responses simultaneously using the same model parameters. In the present study, we attempted to describe and explain both creep and relaxation responses of articular cartilage using a fibril-reinforced model, which has been successfully used to account for the load response of the relaxation tests of articular cartilage. Experiments were performed on bovine articular cartilage disks ( n = 8) using multi-step loading protocols, involving both creep and relaxation in each protocol. The experimental results indicated that mechanical changes, such as fiber recruitment in collagen network during stretch, recovered fully upon unloading. Creep loading did not affect relaxation properties, and vice versa. Relaxation proceeded much faster than creep, because of different fluid pressure profiles. The load sharing among the proteoglycan matrix, collagen network and fluid pressurization was predicted to differ for the creep and relaxation testing. The experimentally observed strong creep and relaxation responses in unconfined compression could not be predicted if either fibril reinforcement or fluid pressurization were neglected. It was essential to consider the interplay between nonlinear fibril reinforcement and fluid pressurization for the transient response (this interplay may be best termed as fluid pressure driven fibril reinforcement). Fibril reinforcement played a relatively insignificant role in the compressive load response at equilibrium, in agreement with previous findings for cartilage stress relaxation testing.

Failure behaviour of EUROFER 97 in the low-cycle fatigue region under multi-step loading

For verification of high temperature creep–fatigue interaction rules for EUROFER 97, different LCF multi-step experiments (2- and 3-step tests) were performed at 450 and 550 °C. The load conditions were varied by changing the total strain amplitude during an experiment. In 2-step tests the load was changed after a given number of cycle from a high level to a low level or vice versa and afterwards the experiments were continued until fracture (high–low, or low–high loading). In 3-step tests, the load was changed in the first and second steps after given numbers of cycles. In the third step, it was switched back to the load applied in the first step and the experiments were continued until fracture (high–low–high, or low–high–low loading). As expected, the results show a non-linear damage accumulation. In 2-step experiments, it was found that beginning the tests with a low total strain amplitude resulted in longer normalised total lifetimes than in the case of high total strain amplitudes at the beginning. In 3-step tests, one can detect a similar behaviour. But here, additionally it is also observed that the number of cycles in the first step influences the lifetime. A comparison of the results with the standard design curve shows that there is more than enough space available to be on the safe side for calculations of constructions.

Ratchetting strain prediction

A time-efficient method for predicting ratchetting strain is proposed. The ratchetting strain at any cycle is determined by finding the ratchetting rate at only a few cycles. This determination is done by first defining the trajectory of the origin of stress in the deviatoric stress space and then incorporating this moving origin into a cyclic plasticity model. It is shown that at the beginning of the loading, the starting point of this trajectory coincides with the initial stress origin and approaches the mean stress, displaying a power-law relationship with the number of loading cycles. The method of obtaining this trajectory from a standard uniaxial asymmetric cyclic loading is presented. Ratchetting rates are calculated with the help of this trajectory and through the use of a constitutive cyclic plasticity model which incorporates deviatoric stresses and back stresses that are measured with respect to this moving frame. The proposed model is used to predict the ratchetting strain of two types of steels under single- and multi-step loadings. Results obtained agree well with the available experimental measurements.

A Parametric Study on Crushability of Open-Cell Structural Polymeric Foams

A five-parameter nonlinear phenomenological constitutive model, capable of capturing the entire stress-strain response of crushable polymeric foams, is presented. The functional form of each parameter in the model was determined using uniaxial-strain compression experiments on cylindrical foam specimens of different initial bulk densities (or porosities). Two-step and multistep loading procedures were employed during the uniaxial-strain compression experiments to investigate crushability and residual crushability of foams and to generate additional data at different porosity levels that were not originally available. Thus, specimens with certain initial low densities were repeatedly used to obtain stress-strain curves at increasingly high densities. Based on the experimental data, each parameter of the model was expressed in terms of initial bulk density (or porosity). Such relationships were shown to be useful in generating ‘crushability-maps’ that could be utilized to evaluate the residual crushability and/or for selection of a suitable density of foam for a given application.

Crack-tip residual stresses and crack propagation in cyclically-loaded specimens under different loading modes

Abstract Residual stress distributions near crack tips in a fine-grained low-alloyed steel (German Grade S690QL1) were measured by X-ray diffraction techniques after single and multistep loading sequences, using mode-I, mode-II, or mixed-mode loading parameters. The consequences of overload cycles applied under different loading modes on the residual stress distributions and crack propagation rates were analyzed. Characteristic examples are presented demonstrating the influences of loading modes, loading sequences as well as of amount of loading on the resulting crack tip residual stress fields. Crack retardation effects on mode-I crack propagation after overloads were observed, which were most pronounced for mode-I overloads and negligible for mode-II overloads.

Running-in effect on the load-carrying capacity of a water-lubricated SiC thrust bearing

It is known the friction of self-mated SiC in water strongly depends on the roughness of their contact surfaces, and a proper running-in process is the way to obtain low friction by smoothing the contact surfaces of SiC with tribochemical wear. In this paper, the running-in process of surface-contacted SiC (thrust-bearing-type contact) in water is studied experimentally. It is found the maximum running-in load has a large influence on the load-carrying capacity, which is measured as the critical load for the transition from hydrodynamic to mixed lubrication in this research. A multi-step loading running-in method is proposed to increase the load-carrying capacity of SiC thrust bearings working in water. Finally, the running-in process of a laser textured SiC surface is studied, the effect of the pore area ratio on the roughness of the run-in surface is reported, and the mechanism of the effect of micropores is discussed.

Extensive validation of a thermodynamically consistent, nonlinear viscoelastic model for glassy polymers

The nonlinear thermoviscoelastic formalism presented in the preceding paper is validated with four amorphous polymer systems. Validation is performed over a broad range of relaxation phenomena in the glass transition region, including the temperature and rate-dependence of the stress–strain behavior through yield, volume and enthalpy relaxation, and stress relaxation during multi-step loading histories. The objective is to obtain quantitative agreement between the constitutive theory and all experimental results using one set of model parameters for each material system. The nonlinear viscoelastic formalism is shown to predict the wide range of behavior observed experimentally, indicating that the formalism does capture the essential physics of glassy polymers. Moreover, the material parameters required in the constitutive formalism can be readily obtained from independent experiments and are relatively insensitive to how these parameters are determined experimentally from the various characterization techniques.

Fatigue life and cyclic deformation behaviour of quenched and tempered steel AISI 4140 at two-step stress- and total-strain-controlled push–pull loading

Abstract The behaviour of steels at multi-step cyclic loading was explored up to now almost exclusively in fatigue-life-oriented investigations. Thus, only few works exist dealing with the cyclic deformation behaviour at two- and multi-step loading. Therefore, the cyclic deformation behaviour at two-step experiments with a single amplitude change (2-block experiments) and with multiple changes between two blocks of certain length and different amplitudes (multi-block experiments) was investigated in this work at the technically important steel AISI 4140 (German grade 42CrMo4).

超長寿命疲労破壊におけるＯＤＡ（Ｏｐｔｉｃａｌｌｙ Ｄａｒｋ Ａｒｅａ）の形成に及ぼす諸因子

The mechanism of fatigue failure in the ultra-high cycle regime was studied using a bearing steel, JIS SUJ2 (SAE52100). In the case of tension-compression fatigue testing, the S-N curve for high strength steels consisted of a single straight line in contrast to the dual S-N curve observed in rotating bending. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behavior has been discussed by Murakami et al. In order to investigate further the influence of hydrogen trapped by various types of inclusions, four materials of different chemical compositions were prepared. Those specimens having a longer fatigue life had a particular morphology designated as ODA (Optically Dark Area) surrounding the inclusion at the fracture origin. In order to investigate the growth of ODA with cycling, fatigue tests involving multi-step loading were carried out, and the appearance of the ODA was compared with that of specimens tested under a constant amplitude loading. A specimen which endured N=2×108 under a stress was fractured by tensile test at -190°C. The fracture origin in this tensile test was at a subsurface, nonmetallic inclusion. If the fatigue test had not been interrupted, the potential fatigue life of the specimen was estimated from the size of the inclusion and the ODA using the √area parameter model. The model was also used to estimate the fatigue limit in the ultra-high cycle range. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed using fracture mechanics. The critical stress intensity factor range ΔKODA for the critical size of the ODA can be correlated with the threshold stress intensity factor range ΔKth expressed by the √area parameter model for small cracks. A fatigue design method for ultra-high fatigue life regime was proposed based upon the growth behavior of the ODA, the statistics of extremes of inclusions, and the √area parameter model.

Prediction of Fatigue Life of Rolled Bolt-Nut Joints under Three-Step Loading and Three-Level Multi-Step Loading

Based on the previously proposed damage rule and life prediction method for rolled bolt-nut joints under two-step loading, life prediction method for three-step loading and three-level multi-step loading was investigated. The effect of load histories and change of material properties with damage accumulation was taken into account in the prediction method. Required material properties were obtained from fatigue tests under constant stress amplitude and two-step loading. In order to investigate applicability of the life prediction method, axial load controlled fatigue tests of rolled bolt-nut joints were carried out. Experiments indicated that the predicted results were in good agreement with the experimental results.

Sealing behavior of the HTR-10 pressure vessel flanges

The flanges of 10 MW high temperature gas-cooled reactor (HTR-10) pressure vessel play an important role in sealing the primary coolant of Helium. They are bolt-connected with a metallic O-ring and a welded Ω-ring. An elastic–plastic nonlinear analysis was performed to evaluate the stress and deformation of the contact flanges with the finite element software of MSC MARC 2000. The multi-step loading process was employed to simulate the processes of pre-tightening and pressurizing of the HTR-10 pressure vessel. The structural effects of the flanges on the opening and the shifting of the HTR-10 pressure vessel flanges at the O-ring position were studied to determine the flange height and the head closure thickness. The good sealing performance of the O-ring and the Ω-ring was verified both numerically and experimentally. The finite element model analysis results compared well with the hydraulic test of the HTR-10 pressure vessel. The results show that the flanges can meet the strength requirement and that the O-ring and the Ω-ring can effectively seal the HTR-10 pressure vessel during both pre-tightening and pressurizing.