Asymmetric Loading Conditions Research Articles

Effects of Cyclic and Monotonic Deformations on Nonlinear Ultrasonic Response of Austenitic Stainless Steel: A Comparative Study

The effect of plastic deformations on the nonlinear ultrasonic response in austenite stainless steel was investigated under the tensile, asymmetric cyclic, and symmetric cyclic loadings. Nonlinear ultrasonic wave measurement was performed on the interrupted specimens. Results show that cyclic and monotonic plastic deformations lead to the significantly different acoustic nonlinear response. The increase of dislocation density and martensite transformation causes the increase of acoustic nonlinearity. By contrast, the well-developed cell structures decrease the acoustic nonlinear response. Under the asymmetric cyclic loading condition, the lightly decrease of acoustic nonlinearity is caused by the development of cell structures, while the slight increase of acoustic nonlinearity should be attributed to the increase of martensite transformation. Comparatively, the severe increase of acoustic nonlinearity during the first stage under symmetric cyclic loading is ascribed to the fast generation of dislocation structures and martensite transformation.

Effect of micro-cavities on different plastic zones at the fatigue crack tip of a compact tension specimen

Knowledge of cyclic plastic deformation response at the fatigue crack tip is crucial to understand the nature of cyclic damage acting on the crack tip. Interaction between crack and defect is equally important to understand the cyclic damage progression at crack tip; however isotropic hardening model was adopted in all existing literatures. Isotropic hardening model is suitable to model only the monotonic plastic zone and unable to model the cyclic/reverse plastic zone. Kinematic hardening model is suitable to model the key cyclic plastic deformation responses like Bauschinger effect, ratcheting, and mean stress relaxation. A non-linear kinematic hardening (Chaboche) model is used in this present investigation to represent the material’s cyclic stress–strain response accurately.Effect of micro-cavity positions (angle and distance from crack tip) and sizes on plastic zones near a crack tip is investigated in this study by two dimensional plane strain finite element model of a compact tension specimen. It is observed that the size and shape of the monotonic and cyclic plastic zones are affected by position and size of the micro-cavity. During asymmetric loading condition, the ratcheting strain accumulation direction is also affected by the position of micro-cavity.

Lava heating and loading of ice sheets on early Mars: Predictions for meltwater generation, groundwater recharge, and resulting landforms

Recent modeling studies of the early Mars climate predict a predominantly cold climate, characterized by the formation of regional ice sheets across the highland areas of Mars. Formation of the predicted “icy highlands” ice sheets is coincident with a peak in the volcanic flux of Mars involving the emplacement of the Late Noachian – Early Hesperian ridged plains unit. We explore the relationship between the predicted early Mars “icy highlands” ice sheets, and the extensive early flood volcanism to gain insight into the surface conditions prevalent during the Late Noachian to Early Hesperian transition period. Using Hesperia Planum as a type area, we develop an ice sheet lava heating and loading model. We quantitatively assess the thermal and melting processes involved in the lava heating and loading process following the chronological sequence of lava emplacement. We test a broad range of parameters to thoroughly constrain the lava heating and loading process and outline predictions for the formation of resulting geological features. We apply the theoretical model to a study area within the Hesperia Planum region and assess the observed geology against predictions derived from the ice sheet lava heating and loading model. Due to the highly cratered nature of the Noachian highlands terrain onto which the volcanic plains were emplaced, we predict highly asymmetrical lava loading conditions. Crater interiors are predicted to accumulate greater thicknesses of lava over more rapid timescales, while in the intercrater plains, lava accumulation occurs over longer timescales and does not reach great thicknesses. We find that top-down melting due to conductive heat transfer from supraglacial lava flows is generally limited when the emplaced lava flows are less than ∼10m thick, but is very significant at lava flow thicknesses of ∼100m or greater. We find that bottom-up cryosphere and ice sheet melting is most likely to occur within crater interiors where lavas accumulate to a sufficient thickness to raise the ice-melting isotherm to the base of the superposed lavas. In these locations, if lava accumulation occurs rapidly, bottom-up melting of the ice sheet can continue, or begin, after lava accumulation has completed in a process we term “deferred melting”. Subsurface mass loss through melting of the buried ice sheets is predicted to cause substantial subsidence in the superposed lavas, leading to the formation of associated collapse features including fracture systems, depressions, surface faulting and folding, wrinkle-ridge formation, and chaos terrain. In addition, if meltwater generated from the lava heating and loading process becomes trapped at the lava flow margins due to the presence of impermeable confining units, large highly pressurized episodic flooding events could occur. Examination of the study area reveals geological features which are generally consistent with those predicted to form as a result of the ice sheet lava heating and loading process, suggesting the presence of surface snow and ice during the Late Noachian to Early Hesperian period.

Impact of High-Pressure Gaseous Hydrogen on the Fatigue Behaviour of Austenitic Steel A-286 under Asymmetric Loading Conditions

Ultrasonic techniques are an established means for carrying out fatigue tests at very highnumbers of cycles. These techniques are based on the formation of a standing ultrasonic wave inthe specimen and usually use frequencies around 20 kHz. Although such systems allow testing to avery high number of cycles in a relatively short time, the use of a standing wave for creating thestrains restricts them to symmetric push-pull mode. This limitation can be overcome by coupling an ultrasonic test device to a universal test rig. In this work a different approach is presented that is particularly well suited for studying environmental effects. The load train with the specimen is enclosed in a pressure vessel. An acoustic horn divides this pressure vessel into two separate chambers. Applying a pressure difference between the two chambers then leads to a static stress in the specimenon which the oscillating stress from ultrasonic excitation is superposed. The addition of both stresses allows testing at varying R ratio. The deteriorating effect of high-pressure gaseous hydrogen on the steel A-286 is investigated as function of oscillating and static stresses at room temperature. SEM analysis of the fracture surface is presented.

Design and Analytical Analysis of a Novel DBA Algorithm with Dual-Polling Tables in EPON

Ethernet passive optical network is a point-to-multipoint structure, and an effective media access protocol should be designed for collision avoidance and bandwidth allocation. Most previous researches have aimed to solve the problem using dynamic bandwidth allocation (DBA) algorithms with polling-based schemes. Unfortunately, idle channel time among the polling cycles cannot be overcome efficiently. In this paper, a dual-polling DBA (DP-DBA) algorithm which consists of an interpolling mechanism and an intrapolling mechanism is proposed. The interpolling mechanism helps eliminate the idle time problem while the intrapolling part aggregates the unused bandwidth of the light-loaded users and those due to nondefragmentation of the Ethernet frames of the general users. We also evaluate the performance of the DP-DBA algorithm by a comparison with the interleaved polling with adaptive cycle time (IPACT), the dynamic bandwidth allocation with a modified grant table generation algorithm (DBA2), the Double-Phase Polling (DPP), and the adaptive DBA algorithm with sorting report messages (Sort-DBA) schemes with simulation results. For greater realism with regard to general applications, we also simulate the asymmetric traffic loads at the optical network units (ONUs).The results show that DP-DBA outperforms the other DBA schemes under asymmetric load conditions.

Prediction of fracture healing under axial loading, shear loading and bending is possible using distortional and dilatational strains as determining mechanical stimuli

Numerical models of secondary fracture healing are based on mechanoregulatory algorithms that use distortional strain alone or in combination with either dilatational strain or fluid velocity as determining stimuli for tissue differentiation and development. Comparison of these algorithms has previously suggested that healing processes under torsional rotational loading can only be properly simulated by considering fluid velocity and deviatoric strain as the regulatory stimuli. We hypothesize that sufficient calibration on uncertain input parameters will enhance our existing model, which uses distortional and dilatational strains as determining stimuli, to properly simulate fracture healing under various loading conditions including also torsional rotation. Therefore, we minimized the difference between numerically simulated and experimentally measured courses of interfragmentary movements of two axial compressive cases and two shear load cases (torsional and translational) by varying several input parameter values within their predefined bounds. The calibrated model was then qualitatively evaluated on the ability to predict physiological changes of spatial and temporal tissue distributions, based on respective in vivo data. Finally, we corroborated the model on five additional axial compressive and one asymmetrical bending load case. We conclude that our model, using distortional and dilatational strains as determining stimuli, is able to simulate fracture-healing processes not only under axial compression and torsional rotation but also under translational shear and asymmetrical bending loading conditions.

Stability analysis of single-layer elliptical paraboloid latticed shells with semi-rigid joints

Based on the bending stiffness of socket joints obtained through experiments, the finite element models of single-layer elliptical paraboloid latticed shells with the semi-rigid socket joints are established. Parametric analysis of the latticed shells is carried out using ANSYS. The parameters considered are bending stiffness, ratio of rise to span, rotation-stiffness, ball size, asymmetric load distribution, tube section, support conditions and initial imperfections. The effects of the different parameters on the critical loads of the shells are thereby identified. Both geometric and material nonlinearity are considered during the analysis. Finally, formulae are derived for the calculation of critical loads of the shells under symmetric and asymmetric loading conditions. This will be useful for the design and application of single-layer latticed shells with semi-rigid joints.

Mode II Critical Stress Intensity Factor of Medium-Density Fiberboard Measured by Asymmetric Four-Point Bending Tests and Analyses of Kink Crack Formation

Using medium-density fiberboard specimens, asymmetric four-point bending tests were conducted to obtain the Mode II critical stress intensity factor for this in-plane system. Because the medium-density fiberboard is in-plane quasi-isotropic about its board plane, the crack propagates obliquely with respect to the initial crack direction under the asymmetric four-point loading condition. A finite element analysis is required to obtain the Mode II stress intensity factor. The analysis herein was conducted to take into account the kink crack formation. In addition, a three-point bend end-notched flexure test was also conducted, and the results obtained by the experiments and numerical calculations were compared. When the initial crack’s length-to-specimen’s depth ratio ranged from 0.85 to 0.95 and when the additional crack length was taken into account, the Mode II critical stress intensity factor KIIc was appropriately obtained by the asymmetric four-point bending test.

Instrumented End Notched Flexure – Crack propagation and process zone monitoring. Part I: Modelling and analysis

Analysis of the three-point bending, End Notched Flexure (ENF), test is presented for exploitation with a new experimental methodology using the backface strain monitoring technique. The model consists of two Timoshenko beams joined with a two parameter elastic foundation, to evaluate stress and strain fields in both adherends and the adhesive layer, together with the specimen compliance and energy release rate. This model is compared to a simplified case, from which a comprehensive experimental method is proposed for precise monitoring of crack propagation and measurement of interface shear compliance during the ENF experiment. In addition, simple formulae are proposed for evaluating mode II energy release rate, and possible experimental artefacts due to asymmetric loading conditions are evidenced.

Asymmetric thermo-elastic analysis of long cylindrical shells of functionally graded materials by differential quadrature method

Asymmetric thermo-elastic analysis of a long cylindrical shell made of functionally graded material under the effect of thermo-mechanical loadings is carried out. Material properties of the cylindrical shell are assumed to be graded in the radial direction according to a power law function, while the Poisson's ratio is assumed to be constant. Numerical solutions of displacement, stress, and temperature fields are obtained using the Fourier and polynomial differential quadrature methods. The results are compared with the reported analytical solutions which are found to be in very good agreement. The cylindrical shell is considered to be under the effect of both axisymmetric and asymmetric loading conditions. Effects of grading parameter, temperature difference, and the ratio of the outer to inner radii of the cylindrical shell on stresses, displacement, and temperature fields are presented.

A Discrete-Time Decoupled Current Control Scheme Applied to VSC-Based Three-Phase Three-Line Systems

Accurate current control of the voltage source converters (VSCs) is one of the key research subjects in modern power electronics. To achieve a preferable solution to current coupling effect in the VSC-based three-phase three-line system, a discrete-time decoupled current control strategy is proposed in the paper. With integration of the α-β transform and two independent current controllers, the proposed methodology can effectively implement decoupled control of the three-phase currents, which can thereby eliminate the impact from the neutral-point voltage especially under asymmetrical loading conditions. Control performance under digital realization was characterized with extensive tests on a shunt active power filter (SAPF) platform. Both the simulative and experimental results have demonstrated that the SAPF could function well and thereby verified the feasibility and effectiveness of the proposed current control methodology.

Failure analysis on collapsed towers of overhead electrical lines in the region Münsterland (Germany) 2005

End of November 2005 strong south-west wind and heavy snowfall were predominant in the region Münsterland, north-western part of Germany. This led to accretion of a considerable quantity of wet snow to overhead electrical lines in form of snow rolls on the conductors. Eighty-two transmission towers failed catastrophically, most of them by buckling, however some by brittle fracture. As a consequence nearly 250,000 people have been cut off from electrical power supply for several days with major media attention. This paper describes the forensic analysis in order to investigate the failure cause. Therefore extensive materials investigations, mechanical testing of original components and specimens thereof, estimations for the real wind and snow loads and their combinations, structural analyses as well as detailed evaluations on the basis of previous investigations, literature and regulations were conducted. It was revealed that some of the examined components were manufactured from Thomas steel which was partially in embrittled condition. The investigated towers fulfilled the design codes valid at the time of erection. However the present line loads of the wet snow rolls on the conductors exceeded by far the ones given in the design codes valid at that time. The load case leading to failure was reconstructed by the derived positions of loads mainly caused by unequal and asymmetric distribution of snow rolls on left and right electrical system. The loads and corresponding stresses acting on the structure before failure were estimated. By comparison with the fracture forces from mechanical testing of original members of the collapsed tower the component that primarily failed was localised. The primary fracture occurred on a diagonal member under tension made of Thomas steel which was weakened by embrittlement. The failure cause was a combination of heavy weather conditions (storm, approx. 0 °C and wet snowfall leading to heavy snow rolls on conductors), asymmetric loading conditions and the usage of Thomas steel which was partially embrittled. Finally, recommendations for avoiding future failures are given.

一方向CFRP積層板の高温における非主軸ラチェット挙動とその現象論的モデル化

Off-axis ratcheting behavior of a unidirectional carbon/epoxy (CFRP) laminate at high temperature has been examined with a particular emphasis on its load-waveform and fiber-orientation dependence. Development of an engineering model for describing the off-axis ratcheting deformation in unidirectional composites has also been attempted. First, off-axis ratcheting tests are performed on coupon specimens with different fiber orientations under different asymmetric cyclic loading conditions. Off-axis creep tests at constant loads equal in magnitude to the maximum levels of the ratcheting tests are also carried out, and the creep test results are compared with the ratcheting test results. Experimental results show that the off-axis ratcheting strain accumulates more as the stress ratio, i.e. the mean stress level, of cyclic loading becomes larger, regardless of fiber orientation. It is also found that the off-axis ratcheting is transient over the range of testing and similar to the off-axis creep in the unidirectional CFRP laminate in its stress, time and fiber-orientation dependence. Then, a simplified ratcheting model for transversely isotropic media is formulated by means of the classical Bailey-Norton equation and a newly defined effective stress that can account for the effect of the maximum stress as well as the mean stress of cyclic loading. It is shown that the proposed engineering ratcheting model allows predicting the off-axis ratcheting behavior of the unidirectional CFRP laminate for different fiber orientations over a range of stress ratio.

Influence of asymmetric cyclic loading on substructure formation and ratcheting fatigue behaviour of AISI 304LN stainless steel

The aim of this report is to examine the nature of accumulation of ratcheting strain ( ɛ r) vis-à-vis in situ substructural and microstructural variations in 304LN stainless steel under varied asymmetric cyclic loading (ACL) conditions. For this purpose, a series of ACL experiments have been carried out on cylindrical tensile specimens under different combinations of mean stress ( σ m) and stress amplitude ( σ a). In one set of experiments, σ m has been varied between 50 and 150 MPa keeping σ a constant, while in a second set σ m has been kept constant and σ a is varied between 300 and 400 MPa. From the results it can be inferred that ɛ r increases from cycle to cycle for ACL tests and the phenomenon is considered to be governed by the associated mechanics and mechanisms of deformation. Fatigue life of a specimen can either increase or decrease depending upon the test condition, which induces different variations in dislocation density, sub-grain size or micro-damage formation.

Direct Estimation of Critical Load for Single-Layer Reticulated Domes with Semi-Rigid Joints

Numerical model of a bolt-ball joint system is established using software ANSYS and verified through experiments. Based on the bendingrotation curves of the bolt-ball joints obtained through numerical simulation, a finite element model of single-layer reticulated domes with bolt-ball joint system is established using nonlinear beam element with end spring elements in software ANSYS. The analyses of the domes are carried out with different parameters including bending stiffness, ratio of rise to span, rotation-stiffness, ball size, asymmetric load distribution, tube section, support condition and initial imperfection. Several key parameters having influence on the load carrying capacity of a semi-rigidly jointed single-layer reticulated dome are thus identified. Finally, formulas are established for direct estimation of critical loads of semi-rigidly jointed single-layer reticulated domes under symmetric and asymmetric loading conditions.

J0301-2-1 一方向CFRPの非主軸ラチェット挙動とサイクル波形依存性([J0301-2]工業材料の変形特性とそのモデル化(2))

Ratcheting behavior of a unidirectional carbon/epoxy (CFRP) laminate under off-axis cyclic loading conditions at high temperature has been examined with a particular emphasis on its load-waveform and fiber-orientation dependence. Development of an engineering model for describing the off-axis ratcheting in unidirectional composites has also been attempted. First, off-axis ratcheting tests are performed on coupon specimens with different fiber orientation angles of 30, 45 and 90°, respectively, under different asymmetric cyclic loading conditions with stress ratios of 0 and 0.5. Experimental results show that the off-axis ratcheting strain accumulates more as the stress ratio of cyclic loading, thus the mean stress level, becomes larger, regardless of the fiber orientation. It is also found that the off-axis ratcheting is similar to the off-axis creeping in the unidirectional CFRP laminate in its stress, time and fiber-orientation dependence. It is demonstrated that the engineering ratcheting model proposed in this study allows adequately predicting the off-axis ratcheting behavior of the unidirectional CFRP laminate for different stress ratios as well as for different fiber orientations.

Reinforcement layout and sizing optimization of composite submarine sail structures

A topology optimization approach that makes use of nonlinear design variable-to-sizing relationship is presented. A finite element (FE) model is used to describe the loaded structure, but unlike the microstructure approach, the decision is whether an element in the continuum should have maximum or minimum cross-sectional dimension while its material density and moduli are held constant. This approach is applied to reinforcement layout optimization of a very large and geometrically complex Composite Advanced Sail (CAS) structure under an asymmetric wave slap loading condition. A high-complexity model in the form of multilayered shell and a low-complexity model in the form of stiffened shell are developed for the layout optimization of the CAS and solved for minimum strain energy. The effects of constraints such as buckling instability on optimal placement of internal stiffeners are also explored. Based on the results of the layout optimization, a new FE model of the CAS is developed and optimized for minimum weight. Depending upon the degree of variability in skin thickness, the results show a weight saving of up to 19% over the original model.

Numerical Simulation Techniques to Study the Structural Response of the Human Chest Following Median Sternotomy

The optimal closure technique of median sternotomy remains controversial. The objective of this study was to analyze the structural response of the separated sternum using computer-based numerical discretization techniques, such as finite element methods. Thoracic computer tomographic scans (2.5-mm slices) were segmented, analyzed by image processing techniques, and transferred into a three-dimensional finite element model. In a first approach a linear elastic material model was used; neglecting nonlinear and damage effects of the bones. The influence of muscles and tendons was disregarded. Nonlinear contact conditions were applied between the two sternal parts and between fixation wires and sternum. The structural response of this model was investigated under normal breathing and asymmetric leaning on one side of the chest. Displacement and stress response of the segmented sternum were compared regarding two different closure techniques (single loop, figure-of-eight). The obtained results revealed that for the normal breathing load case the single loop technique is capable of clamping the sternum sufficiently, assuming that the wires are prestressed. For asymmetric loading conditions, such as leaning on one side of the chest, the figure-of-eight loop can substantially reduce the relative longitudinal displacement between the two parts compared with the single loop. The application of numerical simulation techniques using complex computer models enabled the determination of structural behavior of the chest regarding the influence of different closure techniques. They allowed easy and fast modifications and therefore, in contrast to a real physical model, in-depth parameter studies.

Three‐dimensional cohesive modeling of dynamic mixed‐mode fracture

AbstractA cohesive formulation of fracture is taken as a basis for the simulation of processes of combined tension‐shear damage and mixed‐mode fracture in specimens subjected to dynamic loading. Our three‐dimensional finite‐element calculations account explicitly for crack nucleation, microcracking, the development of macroscopic cracks and inertia. In particular, a tension‐shear damage coupling arises as a direct consequence of slanted microcrack formation in the process zone. We validate the model against the three‐point‐bend concrete beam experiments of Guo et al. (International Journal of Solids and Structures 1995; 32(17/18):2951–2607), John (PhD Thesis, Northwestern University, 1988), and John and Shah (Journal of Structural Engineering 1990; 116(3):585–602) in which a pre‐crack is shifted from the central cross‐section, leading to asymmetric loading conditions and the development of a mixed‐mode process zone. The model accurately captures the experimentally observed fracture patterns and displacement fields, as well as crack paths and crack‐tip velocities, as a function of pre‐crack geometry and loading conditions. In particular, it correctly accounts for the competition between crack‐growth and nucleation mechanisms. Copyright © 2001 John Wiley & Sons, Ltd.

Characterization of deformation behavior for waspaloy under creep-fatigue interaction

Hysteresis loops of strain controlled LCF tests for Waspaloy were characterized within the temperature range of 350°C and 600°C. Materials deformation behaviors under symmetric and asymmetric loading conditions were assessed using Chaboche’s viscoplasticity model. The strain hardening and stress relaxation occurring in the loading cycles were estimated. Hardening plays an important role at an early stage of loading, but the stress relaxation becomes dominant at a later stage. The observed change in the slope of hysteresis loops between the first cycle and the second cycle and mean stress relaxation under asymmetric loading conditions may be explained by the evolution of the kinematic hardening variable which depends on the inelastic strain range. The effect of creep on the cyclic shake down is expected to be limited.