Load Sequence Effects Research Articles

Effect of dynamic loads on the compressibility behavior of peat soil reinforced by bamboo grids

The compression behavior of peat was investigated by a program of static and dynamic load testing. The comprehensive model tests were conducted in the laboratory to evaluate the effect of loading sequences on the behavior of peat soil. The result test indicated that compression behavior of peat soil was significantly influenced by loading sequences and reinforcement of bamboo grids. The compression behavior after the dynamic load appears smaller than before the dynamic load. Reinforcement of bamboo grids can reduce the compressibility of peat soil, especially for one layer and two layers of bamboo grid, while for three layers of bamboo grid resulted in similar settlement for two layers of bamboo grid on the pressure above 9 kPa.

Cyclic hardening/softening behavior of 316L stainless steel at elevated temperature including strain-rate and strain-range dependence: Experimental and damage-coupled constitutive modeling

In this study, the cyclic mechanical characters of 316L stainless steel at elevated temperature are extensively investigated by the experimental and cyclic constitutive models. The experiments include the monotonic tensile tests with different loading rates and the low cycle fatigue tests considering the effect of strain amplitudes, strain rates and loading sequences. The evolution of cyclic stress amplitudes, hysteresis loops and elastic modulus under various loading conditions are comprehensively analyzed. The experimental results show that the 316L steel at elevated temperature performs a typical three-stage cyclic mechanical response, i.e., initial hardening, subsequent saturation and final accelerated softening. The cyclic softening in both stiffness and flow stress is mainly caused by the nucleation of micro-voids or micro-cracks, and the subsequent coalesce and propagation. Furthermore, although the nearly rate-independent mechanical behavior is observed at monotonic tensile and first several fatigue cycles due to the DSA effect, the cyclic hardening/softening behavior shows a significant strain-rate and loading history dependence. Finally, inspired by the experimental observations and analyses, a damage-coupled cyclic elastic-viscoplastic constitutive model involving strain-range, strain-rate and loading history dependence is proposed to predict the complex cyclic behaviors of the material at elevated temperature. A hardening factor is incorporated into the Chaboche kinematic hardening equations to model the kinematic-induced hardening behavior. And the plastic strain memory surface and the maximum plastic strain rate are introduced to model the strain-range, strain-rate and loading history dependence of cyclic behavior. The proposed model is proved to effectively describe the complex evolution of not only cyclic stress amplitude but also hysteresis loops for the 316L steel at elevated temperature.

A hysteresis energy dissipation based model for multiple loading damage in continuous fiber-reinforced ceramic-matrix composites

In this paper, a hysteresis energy dissipation based damage model for fiber-reinforced ceramic-matrix composites (CMCs) subjected to multiple loading stress levels is developed. Considering the combination effects of multiple loading sequences and multiple fatigue damage mechanisms, i.e., matrix cracking, fiber/matrix interface debonding and interface wear, the evolution of the fiber/matrix interface debonding and sliding, fatigue hysteresis loops, fatigue hysteresis dissipated energy and fatigue hysteresis modulus changing with increasing applied cycles are analyzed. The effects of fiber volume fraction, matrix crack spacing, fatigue peak stress and fatigue stress range on the damage development inside of CMCs are discussed. The difference of the fiber/matrix interface shear stress existed between the interface wear region and new interface debonded region affects the fiber/matrix interface debonded length and loading carrying ability of intact and broken fibers. The damage evolution for C/SiC and SiC/SiC composites subjected to multiple fatigue loading sequences are predicted using the hysteresis energy dissipation damage model. Under multiple loading stress, the fatigue hysteresis dissipated energy increases when high peak stress and stress range increases due to the increase of fiber/matrix interface sliding range. However, when the low peak stress increases, the evolution of fatigue hysteresis dissipated energy depends on the interface debonding and sliding state.

The effects of tribological factors and load sequence on surface pitting and cracks in bearing steel

This paper presents an investigation of the influence of various tribological parameters on surface initiated damage through Rolling Sliding Tests (RSTs) using bearing steel specimens. The RSTs were conducted on a benchtop twin-disc machine, consisting of a tribometer and a rolling contact fatigue testing system. The parameters investigated were contact pressure, slipping ratio, rotational speed, lubricant viscosity and load sequence, with each of them varying between two values. The first step was an investigation of the Coefficient of Traction (COT) under different testing conditions, followed by a set of RSTs to investigate surface damage initiation. It was found that the COT increased significantly under certain conditions of opposite rotational direction. The RST results showed that cracks and spalls on the surface were severer when higher slip ratio, higher contact pressure and higher rotational speed were applied first than that when lower levels of these parameters were applied first.

Load sequence effects and variable amplitude fatigue of superelastic NiTi

Fatigue behavior of superelastic NiTi alloys under variable amplitude loading is experimentally studied and two energy-based fatigue models were used to analyze the fatigue behavior. Cyclic tests were conducted in strain-controlled condition. Load sequence effects are studied by performing fatigue testing under load paths consisting of two steps of constant amplitude loading. Both low-high (LH) and high-low (HL) load paths were considered in experimentations. Variable amplitude load paths were also generated at a constant strain rate using random values, which resulted in reversals of different strain amplitudes and mean strains. Analysis of the fatigue results was carried out using two energy-based fatigue models that have been shown to appropriately account for the effects of mean strain and stress. The energy-based total-fatigue-toughness parameter, ∑Wt, together with the Rainflow cycle counting method provided satisfactory fatigue life predictions for this alloy. Based on this combination of the fatigue parameter and cycle counting technique, the predicted fatigue lives fall within prediction bands of 2.5 for all the different variable amplitude loading paths.

A New Cumulative Fatigue Damage Rule Based on Dynamic Residual S-N Curve and Material Memory Concept

This paper introduces a new phenomenological cumulative damage rule to predict damage and fatigue life under variable amplitude loading. The rule combines a residual S-N curve approach and a material memory concept to describe the damage accumulation behavior. The residual S-N curve slope is regarded as a variable with respect to the loading history. The change in slope is then used as a damage measure and quantified by a material memory degeneration parameter. This model improves the traditional linear damage rule by taking the load-level dependence and loading sequence effect into account, which still preserves its superiority. A series of non-uniform fatigue loading protocols are used to demonstrate the effectiveness of the proposed model. The prediction results using the proposed model are more accurate than those using three popular damage models. Moreover, several common characteristics and fundamental properties of the chosen fatigue models are extracted and discussed.

Ratcheting effect of reinforced graphite sheet with stainless steel insert (RGSWSSI) under cyclic compression at elevated temperature

AbstractRatcheting and creep of reinforced graphite sheet with stainless steel insert (RGSWSSI) are tested under cyclic stress‐controlled compression by a self‐designed clamp from 500°C to 600°C. The effects of insert type, temperature, stress amplitude, stress rate, creep, and loading sequence are considered. Results present that ratcheting deformations for RGSWSSI with 316‐L stainless steel tanged and bonded insert approach to each other and show little rate dependence, while they slightly increase with the increment of stress amplitude and temperature. Moreover, obvious ratcheting effect takes place under cyclic pulsating loads with the peak stress of 32 MPa at 500°C or higher. The accumulated deformations of RGSWSSI under small stress amplitude only increase during about the first 25 cycles and then always turn to shakedown. It can also be estimated by the corresponding static creep strain in practical engineering with good accuracy at high temperature. This work provides important data and understanding of RGSWSSI under harsh fluctuating loads.

Vibration Fatigue Damage Accumulation of Ti–6Al–4V under Constant and Sequenced Variable Loading Conditions

This study aims to investigate the effect of loading factors on damage accumulation under variable amplitude vibration (VAV). Vibration fatigue experiments are conducted under both constant amplitude vibration (CAV) and VAV loading cases. The effects of loading sequence, loading amplitude, stress difference, and cyclic ratio on damage accumulation are analyzed. It is found the damage accumulation rate is strongly affected by the loading sequence: the fatigue lives can be ranked in descending order as the one-way low-high loading, the constant loading, and the one-way high-low loading. The effect of stress difference on damage accumulation is not significant, while the damage accumulation varies a lot according to the cyclic ratio of the two-level loading blocks and the fatigue life could be extended by increasing the lower loading cycles. Comparing with linear and double linear damage rules, models based on nonlinear damage rules have apparent advantages in predicting accuracy in VAV conditions, in which the nonlinear continuous damage model has the best compromise between availability and precision.

Fatigue crack growth predictions based on damage accumulation ahead of the crack tip calculated by strip-yield procedures

Elber assumed a long time ago that ΔKeff is the driving force for fatigue crack growth (FCG), and his hypothesis is the basis for strip-yield models widely used to predict residual lives of cracked components. However, this hypothesis cannot explain many load sequence effects observed in practice. Hence, it is at least worth to verify if FCG models based on ΔKeff are indeed intrinsically better than concurrent models based on other principles. To do so, the same mechanics is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation caused by the cyclic strain history ahead of the crack tip, an idea does not need or use the ΔKeff hypothesis. To compare both approaches fairly, FCG rates are estimated by damage accumulation using the cyclic strain ranges induced by plastic displacements calculated by the very same procedures used by strip-yield models, assuming there are strain limits associated both with FCG thresholds and with material toughness. Despite based on apparently conflicting principles, both models can reproduce quite well FCG curves, a somewhat surprising result. Besides confirming that data fitting cannot be used to prove any model superiority, this result indicates that the ΔKeff hypothesis is not a necessary requirement to explain the FCG behavior.

Au/C catalysts promoted with Ni for glycerol electrooxidation in alkaline media

Au/C catalysts promoted by Ni at different Au:Ni atomic ratios (3:1, 2:1, and 1:1) were prepared for glycerol electrooxidation in alkaline media by a polyvinyl alcohol protection method. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, and inductively coupled plasma atomic emission spectroscopy. According to the cyclic voltammogram, Ni enhanced the catalytic activity of gold for glycerol electrooxidation in the alkaline media, although the average particle size of the AuxNiy/C catalysts was slightly larger than that of the Au/C with small cluster formation. The optimum atomic ratio of Au:Ni was found to be 2:1, since the Au2Ni1/C provided the maximum mass current density at approximately 18% higher than did the Au/C. The stability of the Au2Ni1/C also improved, as evidenced by chronoamperometry. These improvements are attributed to the ability of Ni to enhance the OH-adsorbed species on the Au surface, thereby inducing a lower onset potential and helping to remove intermediates from the active site. To reduce cluster formation, the effects of metal sol preparation and metal loading sequence on the catalysts’ performance were further studied at this atomic ratio (2:1). Catalysts with preloading of Ni (Au2/Ni1/C) were found to provide the highest maximum mass current density with comparable decay rates among all the catalysts tested.

Investigation of fatigue crack growth under loading sequence effects using in-situ SEM testing

Fatigue damage is one of the most important failure mechanisms in engineering components. The excited structures are usually subjected to spike loads in the fatigue weakness area during their service lives. The nonlinear loading sequence effects due to overloads are significant in the crack propagation process. In this paper, an in-situ scanning electron microscope (SEM) testing is performed to analyse the mechanisms of nonlinear fatigue crack growth affected by the load sequence. The crack tip behaviours under constant amplitude loading cycles superimposed by tensile overload were observed. The SEM experiment results reveal that the overload effects include the transient weakened area (shear bands and micro-cracks) and the relatively long-term retardation. Additionally, the observation loading sequence influence region is larger than the theoretical value. According to these SEM testing analyses, the Willenborg is modified considering the nonlinear loading sequence effects. In this approach, the damage zone concept is introduced to account for the instantaneous acceleration. Moreover, the loading sequence effect area is defined as the whole plastic zone due to overload rather than part of it. The proposed algorithm is validated by experiment data of 350WT steel and Al 2024-T351 specimens under constant loading with overloads. Good agreements are observed.

Elastic plastic approximation procedure for notched bodies subjected to thermal transient loadings

Components of power plants are often subjected to thermo-mechanical loading conditions. Thermal loadings alone are strain-controlled loadings, inducing locally high mechanical strains and stresses, which may result in low cycle fatigue issues. Furthermore, these cycles are mixed with numerous cycles of lower stress and strain ranges. Regarding applicable design codes such as ASME, French RCC-M or German KTA, fatigue evaluation of such components can be based on the simplified elastic plastic fatigue analysis as the standard option and alternatively on elastic plastic finite element analysis. With regard to processing of long load-time histories (e.g. within an online or offline fatigue monitoring approach), elastic plastic finite element analyses are too time-consuming and not feasible. In contrast, the simplified elastic plastic fatigue analysis is a comparatively fast method, but may yield overly conservative results (and in some rare cases underestimate elastic plastic strain ranges). This may lead to unsatisfactory results by neglecting important influences (cyclic plastic deformation behavior, load sequence and mean stress). In order to consider effects of load sequence and mean stress in fatigue evaluation, it is necessary to calculate the local stress-strain paths over the entire load-time history, using the elastic plastic deformation behavior of the material. The application of commonly used notch approximation procedures (e.g. Neuber’s rule, equivalent strain energy density method) fail under thermo-mechanical loading conditions by overestimating the local stresses and strains.As a general application e.g. for the purpose of long-term fatigue monitoring, measured or calculated temperature-time sequences have to be transferred to fatigue relevant stress and strain time sequences at critical locations. In order to support this task, a fast approximation procedure will be developed in order to overcome the shortcomings of plasticity estimation as an essential part of the fatigue analysis.

Fatigue Lives of Power Plant Structures Due to Load Sequence Effects Originating from Fluctuating Production of Renewable Energy

The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudo-random sequence derived from measurements in power plants. The developed non-linear fracture mechanics based model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.

Full-Scale Testing of Deep Wide-Flange Steel Columns under Multiaxis Cyclic Loading: Loading Sequence, Boundary Effects, and Lateral Stability Bracing Force Demands

This paper discusses the findings from 10 full-scale steel column tests subjected to multiaxis cyclic loading. The columns use deep wide-flange cross sections typically seen in steel moment-resisting frames designed in seismic regions. The effects of boundary conditions, loading sequence, local web, and member slenderness ratios on the column hysteretic behavior are investigated. The test data underscore the influence of boundary conditions on the damage progression of steel columns. Local buckling followed by out-of-plane deformations near the plastified column base are the dominant failure modes in fixed base columns with a realistic flexible top end. Twisting may occur only at drifts larger than 3% even when the member slenderness is fairly large. The test data suggest that bidirectional loading amplifies the out-of-plane deformations but does not significantly affect the overall column performance. The loading sequence strongly affects the column’s plastic deformation capacity but only at story drifts larger than 2%. Above this drift amplitude, column axial shortening grows exponentially and becomes a controlling failure mode. Measurements of the lateral stability bracing force demands at the column top exceed the lateral brace design force specified in North American standards.

Effect of Rhenium Loading Sequence on Selectivity of Ag–Cs Catalyst for Ethylene Epoxidation

Rhenium is an important promotor for silver catalyst, and very little work has been focused on the effect of rhenium loading sequence on the selectivity of ethylene oxide. Re was introduced to the Ag–Cs catalyst supported on α-Al2O3 in three different ways: co-impregnation (Ag–Cs–Re), step-impregnation with Re loaded first (Re/Ag–Cs), and step-impregnation with Re loaded last (Ag–Cs/Re). The selectivity of Re/Ag–Cs and Ag–Cs–Re are about 20% higher than that of Ag–Cs/Re, which indicates that the performance of the catalyst is influenced by the synergistic effect of Re and Cs. Rhenium oxides with the valence of + 4 to + 7 are distributed around the Ag particles. The binding energy of Re is increased by about 0.2 eV by the addition of Cs in the Ag–Cs–Re catalyst, and the presence of Re with high valence around Ag will cause the electrons to shift toward the Re atom, facilitating the formation of weakly adsorbed oxygen and improving the selectivity of the Ag–Cs catalyst. On the Ag–Cs/Re catalyst, Re is mostly loaded on the external Ag surface, and the binding energies of Ag 3d5/2 and Cs 3d5/2 increase by 0.2 and 0.4 eV, respectively, from the Ag–Cs/Re catalyst to the Ag–Cs–Re and Re/Ag–Cs catalysts. The higher binding energy of Ag leads to the stronger oxygen adsorption, which contributes to the complete oxidation reaction, thereby resulting in lower selectivity.

A model to quantify fatigue crack growth by cyclic damage accumulation calculated by strip-yield procedures

Elber's hypothesis that DeltaKeff can be assumed as the driving force for fatigue crack growth (FCG) is the basis for strip-yield models widely used to predict fatigue lives under variable amplitude loads, although it does not explain all load sequence effects observed in practice. To verify if these models are indeed intrinsically better, the mechanics of a typical strip-yield model is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation induced by the cyclic strain history ahead of the crack tip, which does not need or use DeltaKeff ideas. The main purpose here is to predict FCG using the cyclic strains induced by the plastic displacements calculated by strip-yield procedures, assuming there are strain limits associated both the with the FCG threshold and with the material toughness. Despite based on conflicting principles, both models can reproduce quite well FCG data, a somewhat surprising result that deserves to be carefully analyzed.

Late Systolic Myocardial Loading Is Associated With Left Atrial Dysfunction in Hypertension.

Late systolic load has been shown to cause diastolic dysfunction in animal models. Although the systolic loading sequence of the ventricular myocardium likely affects its coupling with the left atrium (LA), this issue has not been investigated in humans. We aimed to assess the relationship between the myocardial loading sequence and LA function in human hypertension. We studied 260 subjects with hypertension and 19 normotensive age- and sex-matched controls. Time-resolved central pressure and left ventricular geometry were measured with carotid tonometry and cardiac magnetic resonance imaging, respectively, for computation of time-resolved ejection-phase myocardial wall stress (MWS). The ratio of late/early ejection-phase MWS time integrals was computed as an index of late systolic myocardial load. Atrial mechanics were measured with cine-steady-state free-precession magnetic resonance imaging using feature-tracking algorithms. Compared with normotensive controls, hypertensive participants demonstrated increased late/early ejection-phase MWS and reduced LA function. Greater levels of late/early ejection-phase MWS were associated with reduced LA conduit, reservoir, and booster pump LA function. In models that included early and late ejection-phase MWS as independent correlates of LA function, late systolic MWS was associated with lower, whereas early systolic MWS was associated with greater LA function, indicating an effect of the relative loading sequence (late versus early MWS) on LA function. These relationships persisted after adjustment for multiple potential confounders. A myocardial loading sequence characterized by prominent late systolic MWS was independently associated with atrial dysfunction. In the context of available experimental data, our findings support the deleterious effects of late systolic loading on ventricular-atrial coupling.

Fatigue modeling for a thermoplastic polymer under mean strain and variable amplitude loadings

The applicability of several fatigue damage models for polyether ether ketone (PEEK) under mean strain and variable amplitude multi-block loadings is evaluated. The models utilized in this study are assessed against experimental data for PEEK under various cyclic loading conditions, including (1) constant amplitude loading with non-zero mean strains, (2) two-block loading with zero and non-zero mean strains, and (3) three- and four-block loading with zero mean strain. Several fatigue models, including a strain-based, a strain-stress-based, and an energy-based, are employed to correct for the effect of mean strain and stress on fatigue behavior of PEEK. Among these models, the energy-based approach, which considers the deformation response of the material throughout its entire life, better correlates the PEEK experimental data in the presence of mean strain. For specimens under block loading, three different damage accumulation models are employed to evaluate their applicability for PEEK. These include the Linear Damage Rule, as well as the non-linear Damage Curve Approach and Hashin-Rotem model. Additionally, a Direct Cumulative Damage (DCD) approach using an energy-based parameter is also employed to account for the load history and sequence effect on PEEK fatigue behavior. The DCD method is found to provide acceptable fatigue life predictions for PEEK under multi-block loading with various strain ratios and frequencies.

Influence of mechanical properties on load sequence effect and fatigue life of aluminium alloy

Influence of mechanical properties on load sequence effect and fatigue life of aluminium alloy

A new fatigue damage accumulation model considering loading history and loading sequence based on damage equivalence

In this study, a new nonlinear fatigue damage accumulation model is proposed to consider the effects of loading history and loading sequence under multi-level stress loading based on the Miner–Palmgren rule and S-N curve. By using damage equivalence, the new model is simplified and another form of the model is given. This model improves the application of the traditional Miner–Palmgren rule, by considering not only the loading sequence effect but also the loading history effect. The methods for calculating the degree of safety of specimens and cumulative damage of low-amplitude loads are also presented. Applicability of the new model is validated by predicting the fatigue life of 16Mn and 45 steel specimens under two-level stress loading. Further validation is carried out for the case of 41Cr4 and Aluminum alloys 6082 T6 under multi-level stress loading, and the strengthening and damaging effect of low-amplitude loads is considered. Comparing with the Miner–Palmgren rule and some new models, this new model gives more accurate and reliable prediction.