Experimental study on asphalt mixture recovery

Asphalt materials used in road pavements are exposed to repeated heavy traffic loading under changing climates. These phenomena make pavements prone to fatigue deterioration as a consequence of the formation of micro-cracks, which can coalesce into a network, ultimately leading to macro cracking and structural collapse. Susceptibility of asphalt mixtures to fatigue is usually evaluated through cyclic laboratory testing, where asphalt specimens are subjected to sinusoidal loading cycles. As the number of cycles increases, a significant loss in material stiffness occurs. However, if loading is interrupted by introducing a rest period between two continuous loading phases, an important change in material behavior is observed. This is associated with a substantial stiffness recovery, which in turn triggers the material’s fatigue life. In this study, the phenomenon of stiffness recovery during rest periods is investigated. Cyclic uniaxial tension–compression loading tests are conducted in stress-control mode and rest periods of different durations are considered. Dissipated energy is analyzed and used to assess the material’s capability for recovery and a new recovery index is proposed. It is found that the newly developed index can successfully assess the recovery properties of asphalt mixture.     

The effects of load sequencing on the fatigue life of 2024-T3 aluminum alloy

Current fatigue life analysis of metallic rotorcraft dynamic components are based on a linear cumulative damage concept known as Miner's rule. This type of analysis assumes that there is no load sequence effect that occurs during the fatigue loading history. Past studies have shown that linear cumulative damage analysis of fatigue tests has produced life predictions that have been conservative as well as unconservative. Some of this uncertainty has been attributed to the fact that load sequence effects do exist in most fatigue load spectra. As a first phase the study reported herein was done to evaluate the load sequencing effects that could exist in commercial fixed-wing fatigue load spectra. To evaluate these effects a typical commercial wing spectra was reordered using a scheme that had previously been shown in fatigue block loading to produce the shortest fatigue lives. This scheme starts with the smallest load range in a load sequence and proceeds in ascending order until the largest load range is reached. Tests on open hole test specimens made of 2024-T3 aluminum alloy were conducted on the normal sequence of loads as well as the reordered scheme called lo–hi. Test results showed no significant differences between the fatigue lives of the normal load sequence and the reordered load sequence. A computer program called FASTRAN which calculates total fatigue life using only crack growth data was shown to predict the fatigue life of the spectrum tests with acceptable accuracy.     

Influence of rest periods on the fatigue strength of structural steel

The effect of rest periods on fatigue life was investigated. Small size specimen and constant load amplitudes were used with a new developed fatigue testing machine in alternating bending. Rest periods of 1.5 min were introduced after each half cycle. The cycling rate was 0.25 cpm. Comparative tests on specimen tested without rest periods showed a substantial increase in fatigue life due to rest periods.     

小丝径65Mn金属橡胶疲劳性能研究

研究了小丝径65 Mn金属橡胶的疲劳性能,结果表明:随着65 Mn金属橡胶密度的增加,小位移段刚度增加,静态、动态损耗因子减小,疲劳寿命下降。     

Modeling and assessment of self-healing and thixotropy properties for modified binders

The fatigue endurance limit of binders is the results of more phenomena (e.g. viscoelasticity, damage, healing, thixotropy, steric hardening) that interact simultaneously making the mechanisms behind the fatigue behavior not properly understood.Currently, there is no consolidated analytical approach inclusive of such phenomena to characterize fatigue performance.This research proposes a criterion to determine the fatigue resistance of binders subjected to monotonous cyclic loading with multiple rest periods. The main rheological properties are measured during each stage using a Dynamic Shear Rheometer and the modeling of their evolution is proposed. The experimental program includes different binders in order to investigate the effects of SBS modification levels and aged binder contents on self-healing potential and fatigue behavior.The proposed criterion enables to identify fundamental contributions leading to a comprehensive fatigue endurance limit. This approach allows different binders to be distinguished taking into account their self-healing capacity and can help to establish a better correlation with in-service performance of mixtures.Moreover, a comparison with a previous analytical approach based on the same kind of test (time sweep) with only one rest period is proposed in order to evaluate the effectiveness and reliability of the proposed criterion. Results show that multiple rest periods are needed in order to fully understand the self-healing and fatigue behavior of bituminous binders and to quantify the contributions given by thixotropy.     

Fatigue life prediction of nanoparticle/fibrous polymeric composites based on the micromechanical and normalized stiffness degradation approaches

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.     

变刚度橡胶球铰的刚度特性与疲劳寿命分析

进行变刚度橡胶球铰的承载特性研究,有限元数值分析与实验结果均表明其刚度曲线表现为明显的非线性特性。正常载荷工况刚度值小、平稳,极限载荷刚度曲线出现拐点,刚度值显著增大。改变橡胶球铰的止挡高度,会直接影响刚度曲线的拐点位置。可据载荷工况,通过调整止挡高度改变刚度特性。据橡胶超弹特性,用有限元分析数据计算疲劳载荷工况下球铰危险点的等效应力范围,结合S-N曲线对橡胶球铰的疲劳寿命分析预测,并通过台架疲劳实验验证。结果显示橡胶球铰经150万次疲劳试验后未失效,与寿命预测值基本吻合。     

Fatigue on the flight deck: The consequences of sleep loss and the benefits of napping

The detrimental effects of fatigue in aviation are well established, as evidenced by both the number of fatigue-related mishaps and numerous studies which have found that most pilots experience a deterioration in cognitive performance as well as increased stress during the course of a flight. Further, due to the nature of the average pilot's work schedule, with frequent changes in duty schedule, early morning starts, and extended duty periods, fatigue may be impossible to avoid. Thus, it is critical that fatigue countermeasures be available which can help to combat the often overwhelming effects of sleep loss or sleep disruption. While stimulants such as caffeine are typically effective at maintaining alertness and performance, such countermeasures do nothing to address the actual source of fatigue – insufficient sleep. Consequently, strategic naps are considered an efficacious means of maintaining performance while also reducing the individual's sleep debt. These types of naps have been advocated for pilots in particular, as opportunities to sleep either in the designated rest facilities or on the flight deck may be beneficial in reducing both the performance and alertness impairments associated with fatigue, as well as the subjective feelings of sleepiness. Evidence suggests that strategic naps can reduce subjective feelings of fatigue and improve performance and alertness. Despite some contraindications to implementing strategic naps while on duty, such as sleep inertia experienced upon awakening, both researchers and pilots agree that the benefits associated with these naps far outweigh the potential risks. This article is a literature review detailing both the health and safety concerns of fatigue among commercial pilots as well as benefits and risks associated with strategic napping to alleviate this fatigue.     

Fatigue behaviour of composite adhesive lap joints

This paper is concerned with a fatigue study of composite adhesive lap joints produced from a bi-directional woven E-glass fibres and polypropylene. The adhesive used was a Bostik 7452 (Rubber & Plastics Grade) ethyl cyanoacrylate type. The effects of layer orientation, lap joint length and water immersion on the fatigue behaviour were studied. The specimens were immersed in water during periods until ninety days for controlled temperatures of 20, 40 and 70 °C. The results are presented in the form of curves of stress amplitude versus number of cycles to failure and also in the form of number of cycles to failure against time to exposure in water for fixed stress amplitudes. The fatigue damage and failure mechanisms were analysed and discussed. The joint shows that creep deformation within the temperature range of this study was probably the mainly cause of the dynamic stiffness reduction observed.     

Finite Element Analysis of Laser Cut Edge Beam Section for High Stress Intensity Structural Assessment

This research has analyzed the stability of laser cut-edges in the location of high stress intensity of a structural beam. The cut-edge characteristic properties formed during laser-cutting processing have been over prolonged periods determined to have beneficial effects on fatigue life. During this study two high strength steel grades S355MC and DP600 have been examined. Optimal fatigue lives were attained by minimizing the laser cut-edge surface damage, maintaining the formation of shallow striations and by controlling the near edge microstructural deformations during the cutting process. This was validated using a bespoke component in which was tested under four-point loading in which there is a area of stress concentration forming a localized plastic zone. The importance of this region is critical due to the fact that it is the area that influences the fatigue life of the structure. Predicting the lives to crack initiation was determined through FE analysis which is based on the use of E–N cut-edge fatigue data.     

The relationship of creep,recovery and fatigue in polycarbonate

The quantitative relationship between creep and recovery which had been previously developed by Mindel and Brown [1] has been applied to interrupted creep tests. Single and multiple interruptions (fatigue) were investigated. In general it was observed that interruptions decreased the time to failure. The experiments were conducted in compression in the range of high stresses. Failure was caused by excessive deformation or accelerated creep which is produced by a softening mechanism that is common to all linear polymers. The temperature changes associated with the creep and recovery parts of the cyclic loading were measured. The temperature rise during loading exceeds the decrease during unloading so that there is a net rise in temperature if the creep and recovery intervals are equal. However, the temperature change is not the primary cause for the decrease in time-to-failure for cyclic loading as compared to the failure time during steady stress creep. It has been concluded that fatigue failure under compressive deformation is related fundamentally to the constant stress creep curve.     

Gradient‐enhanced damage modelling of high‐cycle fatigue

Continuum damage mechanics can be used to model the initiation and growth of fatigue cracks. However, finite element analyses using standard fatigue damage formulations exhibit an extreme sensitivity to the spatial discretisation of the problem. The mesh sensitivity is caused by the fact that the underlying continuum model predicts instantaneous, perfectly brittle crack growth as soon as a crack has been initiated. The growth of damage localizes in a vanishing volume during this instantaneous growth. This localization is not so much due to loss of ellipticity of the problem, but is caused by the fact that the damage rate is singular at the crack tip. The damage rate singularity can be removed by the introduction of higher‐order deformation gradients in the constitutive modelling. As a result, crack growth at a finite rate and with a positive amount of energy dissipation is predicted. Finite element analyses converge to this solution and are thus no longer pathologically dependent on the spatial discretization. Copyright © 2000 John Wiley & Sons, Ltd.     

A comparative study of failure features in aerospace grade unidirectional and bidirectional woven CFRP composite laminates under four‐point bend fatigue loads

Damage mechanisms in unidirectional (UD) and bi‐directional (BD) woven carbon fiber reinforced polymer (CFRP) laminates subjected to four point flexure, both in static and fatigue loadings, were studied. The damage progression in composites was monitored by observing the slopes of the load vs. deflection data that represent the stiffness of the given specimen geometry over a number of cycles. It was observed that the unidirectional composites exhibit gradual loss in stiffness whereas the bidirectional woven composites show a relatively quicker loss during stage II of fatigue damage progression. Both, the static and the fatigue failures in unidirectional carbon fiber reinforced polymer composites originates due to generation of cracks on compression face while in bidirectional woven composites the damage ensues from both the compression and the tensile faces. These observations are supported by a detailed fractographic analysis.     

Dynamic Viscoelasticity of Polyester/Rubber Composites under Cyclic Loading

This paper focuses on the influence of dynamic viscoelasticity and surface temperature on the fatigue mechanism and fatigue lifetime of polyester/rubber composites. Rubber composites show significant viscoelasticity during fatigue process. The variations of dynamic elastic modulus, mechanical loss angle, loss energy per cycle exhibit different trend in fatigue initial stage and final stage. Due to high viscoelasticity high heat generation occurs under cyclic loading, which leads to a high surface temperature. It is found that the variation of specimen surface temperature depends strongly on cycling frequency and stress amplitude. SEM (scanning electron microscopy) observation and static residual stiffness studies reveal that the surface temperature affects fracture morphology and fatigue lifetime of rubber composites strongly because of heat aging.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

Stiffness reduction due to matrix cracking in a (0/90)_s glass fibre reinforced plastic (GFRP) laminate has been studied under both quasi-static and fatigue loading. The stiffness reduction is shown to be directly proportional to the density of cracks which accumulate in the transverse ply. A model for the transverse ply crack growth during fatigue gives good agreement with the experimentally determined stiffness reduction curves.     

Investigations of stiffness and fatigue properties of asphalt mixtures

Three conventional asphalt concrete mixtures containing one aggregate of a given size distribution and bitumens of different stiffness obtained from one and the same source are characterised with regard to rheological properties using uniaxial testing. Both non-destructive complex modulus and fatigue testing at different temperatures are performed. The results obtained from the non-destructive testing indicate that the mixtures studied exhibit similar rheological properties (stiffness and phase angle) in the range investigated, if compared at different temperatures (similar rheological state). The results presented also suggest that fatigue deterioration to a great extent is determined by the rheological status of the mixture. Some aspects on the test procedure, such as adequate strain measurement and heating during destructive (fatigue) testing as well as fatigue life characterisation using the classical approach, are discussed.     

A fracture mechanics analysis of fatigue crack growth data for various materials

A large amount of previously published fatigue crack growth data obtained from 10 in. wide centre-cracked sheet specimens of various materials has been re-analysed in terms of the range of stress intensity factor (ΔK) and the results presented as master curves of crack growth rate against ΔK. In addition, data obtained previously from fatigue tests on edge-cracked plate specimens concerning the minimum cyclic stress that would just not cause a crack of a given length to grow have been similarly analysed.     

碳纤维增强树脂基复合材料预紧力单齿接头的静态及疲劳性能试验

通过试验研究了预紧力、齿长和载荷水平对碳纤维增强树脂基复合材料（CFRP）预紧力单齿接头（PTSTC）静态和疲劳性能的影响。试验结果表明:CFRP PTSTC的疲劳极限承载力可以达到静态极限承载力的80%~85%,与螺栓等传统连接方式相比,其疲劳性能具有一定优势;预紧力可以显著改善CFRP单齿接头的静态和疲劳性能;CFRP PTSTC的静态极限承载力随齿长增加而升高,但是在相同载荷水平下增加齿长不一定可以延长接头的疲劳寿命,尤其是在低载荷水平下齿长增加反而会缩短疲劳寿命;在加载初期,PTSTC的疲劳裂纹快速萌生,之后扩展缓慢,在接近破坏前的几次循环中又骤然增大,预紧力可以减缓疲劳损伤的累积速率;疲劳过程中PTSTC的刚度衰退不明显,在前95%疲劳寿命阶段仅下降1%~4%,预紧力也可以减缓刚度的衰退速率。所得研究成果可为复合材料接头抗疲劳设计提供参考依据。      

Fatigue in soda-lime silica glass: influence of surface treatment

The influence of surface treatment on fatigue of soda-lime glass has not been previously explained. An explanation based on different distributions of crack tip radii for different treatments is proposed. Fatigue is considered to occur as a result of crack tip sharpening. Freshly abraded glass has crack tips of a minimum radius that cannot sharpen, so such glass has longer fatigue life. Glass aged in water after abrasion has blunter crack tips, and consequently shorter life. Chemical conditions at the crack tip are the same over a wide range of external pH, and also influence the effect of surface treatment on fatigue times.