Overload Ratio Research Articles

Probabilistic modelling of fatigue damage accumulation in steel bridge decks under stochastic and dynamic traffic load

A realistic traffic simulation is a premise of the fatigue damage estimation. This study presents a novel computational framework for probabilistic modelling of fatigue damage accumulation in steel bridge decks under stochastic traffic load. The stochastic traffic load is simulated based on bridge weigh-in-motion measurements, and the dynamic effects are solved in a vehicle-bridge coupled vibration system. A response surface method is utilised to replace the finite-element analysis. Probability models of the fatigue stress ranges are finally used for fatigue reliability evaluation of an orthotropic steel bridge deck. Numerical results show that the stress spectrum under the actual traffic loads follows a bimodal distribution. Both the fatigue reliability index and the fatigue life decrease rapidly under a continuous traffic growth. A strict threshold overload ratio reduces the decreasing range of the reliability index caused by traffic growth.

Evaluating sustainability of truck weight regulations: A system dynamics view

Purpose: Targeting the problem of overload trucking in Highway Transportation of iron ore from Caofeidian to Tangshan (HTCT), this paper aims to assess long-term effects of alternative Truck Weight Regulation (TWR) policies on sustainability of HTCT. Design/methodology/approach: A system dynamics model was established for policy evaluation. The model, composed of six interrelating modules, is able to simulate policies effects on trucking issues such as freight flow, truck traffic flow, pavement performance, highway transport capacity and trucking time, and further on the Cumulative Economic Cost (CEC) including transport cost and time cost of freight owners and the Cumulative Social Cost (CSC) including pavement maintenance cost, green house gas emission cost, air pollutants emission cost and traffic accidents cost, so the effects of TWR policies on sustainability of HTCT could be evaluated. Findings: According to different values of overload ratio which a TWR policy allows, alternative TWR policies are classified into three types, which are The Rigid Policy (TRP), The Moderate Policy (TMP) and The Tolerant Policy (TTP). Results show that the best policy for sustainability of HTCT depends on the importance of CSC which is expected by the local government. To be specific, (1) if CSC is considered much less important than CEC, the local government should continue implementing the current TTP with the maximum overload ratio; (2) if CSC is considered much more important than CEC, then TRP is recommended; and (3) if CSC is considered slightly more important than CES, TMP with overload ratio of 80% is the best. Practical implications: Conclusions of this paper may help the local government design appropriate TWR policies to achieve sustainability of HTCT. Originality/value: To the best of our knowledge, this is the first effort to evaluate TWR policies on sustainability of regional freight transportation based on system dynamics modeling.

Overload effect revisited − Investigation by use of configurational forces

The configurational force concept enables the derivation of the incremental plasticity J-integral Jep, which is, in contrast to the conventional J-integral, physically appropriate to characterize the crack driving force in cyclically loaded elastic–plastic materials with growing cracks. In this paper we apply Jep, combined with an analysis of the configurational force distribution, for the investigation of fatigue crack growth retardation after a single tensile overload. The motivation for this investigation is that the main reason for the overload effect, i.e. crack flank contact behind or residual stresses around the growing crack tip, is today still an open question in fatigue. Numerical case studies are performed for two-dimensional Compact Tension specimens with long cracks that grow under cyclic Mode I loading and plane strain conditions. Variables of the numerical case studies are the overload ratio and the load ratio during constant cyclic loading. The influence of crack flank contact is examined by a comparison of two different simulations: The first simulation assumes frictionless contact between the upper and lower crack flank; in the second, fictive case, it is assumed that crack flank overlap is possible. The results show that all features of the overload effect even occur, if crack flank contact is not possible. Finally, the ability of the effective stress intensity range ΔKeff to characterize the overload effect is also discussed.

Enhancement in fatigue life of NiTi shape memory alloy thermal actuator wire

The effect of intermittent overload cycles (OLCs) on the fatigue life of NiTi wires has been investigated. The results showed that the OLCs were effective in enhancing the fatigue life of the NiTi wires undergoing thermo-mechanical cycling (TMC). Three distinct regions were identified in the No. of cycles to failure vs. over load ratio (OLR) plot. Region I (OLR ≤ 1.5) corresponded to threshold OLR, at and below which the OLCs did not have any effect on the fatigue behaviour of the NiTi wires. In region II (1.5 < OLR < 2.5) the fatigue life was found to increase monotonically with the increase in OLR. In region III (OLR ≥ 2.5), the increase in fatigue life was quite substantial. The actuator wire with OLCs of OLR 2.5 showed fatigue life of 3 times of that of the wire with OLR of 2.25 and 9 times of that without OLCs. Study revealed that the effect of OLCs towards enhancement in fatigue life was cumulative, and its withdrawal at any stage during the TMC resulted in a life lower than that of the wire subjected to OLCs till fracture. The presence of stabilized martensite phase in the microstructure was found beneficial towards enhancement in fatigue life and its restoration by healing treatment had a detrimental effect on the life of the actuator wires.

Delamination and fatigue crack growth behavior in Fiber Metal Laminates (Glare) under single overloads

Delamination extension and fatigue crack growth behaviors under single overloads were investigated for GLARE 2-2/1-0.3 with fiber direction of 00/00. The results indicate that the stress intensity factor at the crack tip in metal layer while overload applied, Ktip,ol is a key controlling variable which influences fatigue crack growth and delamination behaviors. When Ktip,ol becomes bigger and exceeds a critical value, an obvious kink in the delamination shape is observed nearby the location of overload applied. Crack growth rate after application of overload could not return to its original level even the crack grows beyond the overload plastic zone. The reduction magnitude of the crack growth rate becomes bigger with the overload ratio (intrinsically Ktip,ol) increasing. These new results for the crack growth behavior have never been reported before, which can be well explained by the delamination extension behavior observed after overload applied.

Combined Effects of Different Overload Ratios and Nitriding Process on Fatigue Crack Growth of AISI 4140 Steel

The combined effects of different overload ratios (OLRs) and nitriding process on fatigue crack growth (FCG) were analyzed. Steel compact tension specimens made of AISI 4140 steel were subjected to the plasma nitrification process using a gas mixture of 50% N2 and 50% H2 at 500 °C for 2 h. Structural properties of the samples were studied by scanning electron microscopy, optical microscopy, microhardness testing and digital imaging. Plasma nitriding slowed down FCG, but the crack growth retardation caused by the overload was reduced when the OLR was 2. 0 and 2. 5. Nitriding did not take effect when the OLR was 1.5 and 3.0.

Stability Analysis of Single-Pier Variable Cross-Section Box-Girder Continuous Bridge

On the basis of Umansky box-girder torsion theory, the software of ANSYS is applied to simulate related engineering model. In considering a variety of live load conditions, the lateral stability of multi-span single-pier variable cross-section box-girder under torsion eccentric load is analyzed. Analysis results reveal that under different working conditions, the factors affecting the transverse stability of the bridge and the proportion of the value on each, such as gravity, the span ratio and the distance between support are different; When the overload ratio is 2 times larger, the bridge stability safety coefficient decreases along the curve and changes rapidly. In the future design of this kind of bridge, the overall lateral stability should be paid much more attention. Some methods and suggestions about how to prevent the emergence of security and stability problems are put forward in the end.

A new method for cyclic crack-tip plastic zone size determination under cyclic tensile load

This paper presents a new method to determine the cyclic crack-tip plastic zone size (CTPZS) in the center cracked plate subject to cyclic tensile load. The maximum crack opening displacement (MCOD) and the variable quantity of MCOD (VMCOD) are used to estimate cyclic CTPZS in this method. Based on the series of calculation results by finite element analysis, an explicit expression of cyclic CTPZS versus MCOD and VMCOD is fitted by the least square method. The expression enables to eliminate the influences of crack geometry, plate sizes, elastic modulus, yield stress, plane state, stress ratio, overload ratio and load sequence. Therefore, the presented method is suitable for center cracked finite plate models with different material under cyclic tensile load.

Numerical and experimental study on the optimization of overload parameters for the increase of fatigue life

Fatigue life and crack growth retardation due to periodic tensile overloads, which are superposed on constant amplitude cycles, have been investigated in the present study. In the numerical analyses, the Walker equation with the generalized Willenborg model has been used. The analyses and experiments have been performed on C(T) specimens made of 7075 aluminium. The periodic overloads have been induced at two different periodicities and the effect of overload ratio and overload periodicity on the fatigue life has been studied. The results of the experiments and analyses reveal that the normalized fatigue life versus overload ratio curve has a maximum point, which indicates that there are greater retardation effects in the intermediate overload ratio values. Also, the numerical and experimental results indicate that at the occurrence ratios (the ratio of the number of overload cycles to the number of constant-amplitude cycles between overloads) of 1/50 and 1/500, the maximum interaction occurs at the overload ratios of 1.5 and 2.0, respectively.

Prediction of Fatigue Crack Growth of Aged Hardening Al-Alloys under Variable Amplitude Loading

In this investigation, variable amplitude loading effect was studied on aged hardening Al-alloys in series 2000 and 7000. Generalised Willenborg model was used in order to show loading interaction effects (overload effects). Variable amplitude loading under different form of spectrum has affected highly the fatigue life and fatigue crack growth rates. Fatigue lives were increased and fatigue crack growth rates (FCGRs) were decreased in increasing of overload ratio in single overload case. In application of overload band, the fatigue lives and FCGRs were affected by band overload and R-ratio of them when level in FCGRs was increased.

Influence of an overload applied within compressive base-line loading on crack propagation retardation in AM60B magnesium alloy

Application of a tensile overload within an otherwise constant amplitude cyclic loading scenario is known to retard the resulting crack propagation in metals, thus resulting in longer fatigue life. The magnitude of retardation and crack length increment that would be affected by such an overload would depend on the overload ratio and the nature of baseline loading. However, the influence of an overload within a baseline loading with negative stress ratio on crack propagation has not been extensively investigated. Consequently, the influence of overload (with various overload ratios), applied within various baseline loadings consist of both positive and negative stress ratios, is considered in our experimental investigation. It is shown that the retardation response of AM60B magnesium alloy varies when the stress ratio of the baseline loading changes. Wheeler model's parameters are evaluated separately for positive and negative stress ratios. For positive stress ratio, the affected zone was modified based on the baseline loading's stress ratios.The experimental results reveals that retardation trend in crack propagation would be affected significantly by negative stress ratios; therefore, Wheeler's model would have to be modified to account for this important issue. Moreover, the influence of loading sequence (i.e., when the overload is followed by a compressive underload) on retardation of crack propagation is also investigated.Finally, surface roughness of the fractured specimens is carefully examined by a profilometer and its variation with respect to stress ratio is reported in this paper.

Instrumented indentation measurements of residual stresses around a crack tip under single tensile overloads

Abstract In this study, the residual-stress fields around crack tip of M(T) specimens under three single tensile overload ratios ( R ol ) during fatigue crack growth of aluminum alloy 2524-T3 were investigated using the instrumented indentation tests (IIT). The results showed that both the magnitude and region of the compressive residual stress around the crack tip increase with the increase of R ol . Finite element method was also used to simulate the residual stress distributions near the crack tip at the same loading conditions as experiments, which confirmed that the estimation of the crack-tip residual stress using Suresh et al.'s model based on IIT was feasible. The profiles of the crack tip under constant-amplitude loading and three overload ratios were examined using optical microscopy. It was found that the geometry of crack tip changes significantly from the sharp crack to the blunt crack with increasing R ol after applying a tensile overload. The probable mechanisms resulting in the change of the crack-growth behavior due to tensile overloads were also discussed. It was suggested that the combined effects of the residual stress, crack tip profile and the crack tip hardening may be responsible for the observed overload retardation effect of the present M(T) specimens during fatigue crack growth.

Loading history effect on fatigue crack growth of extruded AZ31B magnesium alloy

An experimental investigation was conducted on the fatigue crack growth (FCG) behavior of an extruded AZ31B magnesium (Mg) alloy in ambient air. Compact tension (CT) specimens oriented in three different directions with respect to the extrusion direction were employed in the study. The influences of overloading and two-step high-low sequence loading on FCG were investigated in detail. Single tensile overloads with three overload ratios (1.5, 1.75, and 2.0) were applied during otherwise constant amplitude loading. A single overload retarded the crack growth rate in all the three specimen orientations. The crack growth rate decreased immediately to a minimum value right after the application of a tensile overload and increased quickly to a stable value expected at constant amplitude loading. No visible torn fracture was observed at the crack tip right after overloading but the crack tip was clearly blunted. Results from the two-step high-low sequence loading reveal that FCG retardation occurred at the beginning of the lower amplitude step when the maximum load was lowered in the second loading step, which is similar to the case of a single overload. Walker’s model can correlate well the crack growth experiments with different R-ratios. Wheeler’s model can reasonably predict the influence of overload and high-low loading sequence on the FCG in the AZ31B Mg alloy.

A material sensitive modified wheeler model for predicting the retardation in fatigue response of AM60B due to an overload

Application of an overload within an otherwise constant-amplitude loading scenario causes retardation in crack propagation. Several models have been proposed for predicting retardation in crack propagation due to an overload cycle. Among them, the widely used Wheeler model, assumes the “affected zone dimension” to be a function of the current and overloaded plastic zone radii. When one considers the actual shape of the plastic zone, however, one realizes that the affected zone dimension does not agree with that assumed by Wheeler.In this paper, the influence of a single overload (but by considering three different overload ratios) on the fatigue crack growth retardation of center-cracked AM60B magnesium alloy plates is experimentally investigated. The retardation effect on crack growth due to an applied overload within a random-amplitude loading scenario, using various “clipping levels”, is also investigated. The sensitivity of this material to overload is compared with the response of some other materials.The actual radius of the plastic zone is evaluated for various stress intensity factors, using the finite element method. The results indicate that depending on the material, the affected zone would be sometimes larger or smaller than that produced by Wheeler’s model. Subsequently, a new parameter, hereafter referred to as the “sensitivity parameter” (β), is introduced that enables one to evaluate the affected zone dimension more accurately. It is shown that the proposed modified model is more effective than the original one in predicting the retardation response of the alloy. The integrity of the modified model is also investigated by evaluating the retardation in some other materials.

Effect of tensile overloads on fatigue crack growth of high strength steel wires

Fatigue of the tensile armor wires is the main failure mode of flexible risers. Techniques to increase the life of these components are required to improve the processes safety on oil exploration. This work evaluates the crack growth retardation of high strength steel wires used in flexible pipelines. Fracture toughness tests were performed to establish the level of stress intensity factor wherein the wires present significant plastic deformation at the crack tip. The effect of tensile overload on fatigue behavior was assessed by fatigue crack growth testing under constant ΔK control and different overload ratios with two different load ratios. The outcomes show that the application of controlled overloads provides crack retardation and increases the fatigue life of the wires more than 31%. This behavior is also evident at stress ratio of 0.5, in spite of the crack closure effect being minimized by increasing the applied mean stress.

Three-Dimensional Finite Element Analysis of Dynamic Response of the Stress in Pavement Structure under Overload

In recent years, the phenomenon of vehicle overload in highway transportation is more and more serious. The three-dimensional dynamic FEM was adopted and a three-dimensional simulative model for asphalt pavement with subgrade based on cross-anisotropy under moving load was established by the large-scale finite element analysis software ABAQUS. The response of the stress in the pavement structure was got by using ABAQUS/Standard code. During deceleration, The simulation analysis results under the rated load were compared with those under the different overload ratio. The results show that, under moving load, the surface course of pavement is mainly in three dimension compressive stress state. The horizontal load caused by deceleration has a remarkable effect on the longitudinal stress and the horizontal shearing stress. The stress of the pavement structure under overload is much greater than that caused by the rated load. And the greater the overload ratio, the greater the deviation between them.

Effect of Intermittent Overload Cycles on Thermomechanical Fatigue Life of NiTi Shape Memory Alloy Wire

Effect of intermittent overload cycles on fatigue behavior of NiTi shape memory alloy wire during thermomechanical cycling (TMC) has been evaluated. Results showed that fatigue life of NiTi is enhanced when the intermittent overload is above certain minimum level. An enhancement in fatigue life by ~50 pct is observed when the overload ratio is 2.0. Accumulation of plastic strain in the material under such TMC condition is found to be relatively high compared to that of TMC with no overload cycles.

Corrosion fatigue crack propagation of an aluminum alloy under periodic overloads

ABSTRACTPeriodic tensile overloads superposed on constant amplitude cycles were used on compact tension fracture mechanics specimens of an aluminum (Al–Cu–Mg) alloy. The delayed retardation of corrosion fatigue crack propagation in the low part of the Paris regime was investigated in a 3.5% NaCl environment. The aluminum alloy exhibits the intensive retardation effect of crack propagation under overload ratios from 1.3 to 2.0 and overload intervals from 102 to 104 cycles. The crack propagation under the periodic overloads can be treated as the base constant amplitude crack propagation, i.e. . The periodic overloads decrease the crack propagation resistance coefficient of Bcf, but have less effect on the threshold value of obtained by best fitting of this model.

Crack retardation mechanism due to overload in base material and laser welds of Al alloys

To determine the retardation mechanisms due to overload and to predict the subsequent evolution of crack growth rate, investigations are conducted on crack retardation caused by single tensile overloads in base material and laser-welded sheets of AA6056-T6 Al alloy. The effect of the overload ratio on the fatigue crack propagation behaviour of the C(T) 100 specimens was analysed by using experimental and Finite Element (FE) methods. The crack growth rate and fracture surface features were investigated for both base material and laser-welded sheets. The retardation due to overload is described in terms of the affected regions in front of the crack tip. The size and shape of the crack-tip plastic zone and the damage profile induced during the application of the overload in the base material are predicted by FE analysis in conjunction with a porous-metal plasticity model. The results show that the mechanisms of retardation in under-matched welds are substantially different from that of the homogenous base material. More significant crack retardation due to overload has been observed in the laser weld of AA6056-T6. Based on SEM observations of the fracture surfaces and the damage profiles predicted by the proposed FE model, the shape of the crack front formed during the overload application can be predicted. During the overload, the crack front extends into a new shape, which can be predicted by the ductile damage model; a higher load results in a more curved crack front. These outcomes are used to determine the dominant retardation mechanisms and the significance of retardation observed in each region ahead of the crack tip and finally to define the minimum crack growth rate after overload.

Experimental investigation of random loading sequence effect on fatigue crack growth

An experimental study is proposed to investigate the effect of random loading sequence effect on the fatigue crack growth behavior of Al 7075-T6. The testing matrix includes different overload cycle percentage, overload ratios, and deterministic and random loading sequences in the current investigation. Multiple specimen tests and statistical data analysis are performed to show the effect of random loading sequence on the median and scatter behavior of fatigue crack growth. The proposed experimental study suggests that extreme value distribution is a good approximation of fatigue life distribution. It is observed that the effect of uncertain loading is different under different loading spectrums. For high overload cycle percentage spectrums, the random loading sequence has no major impact on the probabilistic crack growth behavior compared to the deterministic loading sequence with identical load cycle distributions. For low overload cycle percentage spectrums, the random loading sequence has huge impact on the probabilistic crack growth behavior compared to the deterministic loading sequence with identical load cycle distributions, for both the median and the scatter of the fatigue crack length curves. Finally, all experimental observations are reported in table format in Appendix A for future numerical model development and validation for interested readers.