Reliability study on rheological and cracking behavior of mastics using different sized particles

The objective of this present work was to evaluate the rheological and cracking behavior of mastics fabricated with different sized particles and asphalt types. Two asphalts (base and modified) and three fillers with different sized particles (C, M and F) were investigated. The rheological functions including the creep and recovery property and the fatigue property were measured by multiple stress creep-recovery (MSCR) and time sweep (TS) tests, respectively, while the cracking behavior of asphalt materials was investigated using extensile (ES) tests. Moreover, the dispersion characteristics of various sized particles inside a bituminous matrix and the associated mastic morphology were assessed by a scanning electron microscope (SEM). The results revealed that the presence of coarse-sized particles in matrix increased the non-recoverable compliance and fracture energy but decreased fatigue life of the mastics. The presence of medium-sized particles in matrix enhanced the high-temperature elasticity recovery behavior and degraded the low-temperature fracture energy and the cohesive strength of asphalt mastics, regardless of asphalt types. When filler particle size dropped down to a minimum dimension, asphalt type had negligible effects on the fatigue response and exerted a positive effect on the low-temperature cohesive strength for the related mastics. In addition, the dispersion characteristics of various sized particles inside a bituminous matrix and the associated mastic morphology can account for the rheological response and cracking behavior of the corresponding mastics.     

基于关联性的玄武岩纤维沥青胶浆及其混合料性能研究

为全面提升玄武岩纤维沥青混合料性能,研究了纤维类型及玄武岩纤维长度、掺量等因素对沥青胶浆抗裂性能、抗剪性能及流变特性的影响规律;基于纤维胶浆与纤维沥青混合料性能的关联性分析,揭示了玄武岩纤维对沥青混合料性能的细观增强机制。结果表明:玄武岩纤维对沥青胶浆的抗裂性能及流变特性影响显著,其极限拉力和车辙因子分别达到原沥青胶浆的4.5倍及1.08倍;纤维沥青胶浆高温流变特性与其沥青混合料高温稳定性变化规律存在差异,而前者抗裂性能与后者低温抗裂性能关联性较强;玄武岩纤维与沥青胶结料、集料之间形成三维网状结构,有利于抑制裂缝扩展。     

Fatigue and healing properties of nano-reinforced bituminous binders

The research work focused on fatigue and healing properties of bituminous binders containing carbon nanotubes (CNTs) and nanoclays (NCs) as reinforcing additives. Investigations were carried out by means of a dynamic shear rheometer and by employing specifically devised testing protocols. Experimental results were analysed with the specific goal of highlighting the role played by additive type and base bitumen. Although fatigue response of base bitumens was always improved by nano-modification, effectiveness of nano-particles was found to be highly dependent on the physico-chemical properties of blend components, which strongly influence the morphological configuration assumed by additives within bituminous media. Results obtained in healing tests were processed in order to discern between self-healing of cracks induced by fatigue damage and other artefact phenomena which are related to viscoelastic changes occurring in the bulk of the material. Outcomes of fatigue and healing tests were found to be coherent with interaction mechanisms which take place at the nano-scale.     

Exploring the recovery of fatigue damage in bituminous mixtures: the role of rest periods

Recovery capability of bituminous materials plays a significant role in the development of new technologies for extending the service life of asphalt pavements. This capability originates from various phenomena such as thixotropy, cooling, relaxation of hardening, or healing. However, their real effect on mechanical response is not clear. This article aims to investigate how rest periods (RPs) available between traffic loads can contribute to the damage recovery of bituminous materials. For this purpose, different types and durations of RPs were applied during the laboratory evaluation of fatigue resistance of these materials using the University of Granada Fatigue Asphalt Cracking Test method. The results indicate that the addition of RPs to the loading regime could lead to an extension in the fatigue life of bituminous materials. Additionally, an increase in the RP duration showed a positive impact on the resistance of the materials against cyclic loading. Nonetheless, these benefits are not only related to the recovery of lost properties during RPs, but also a growth in the amount of plastic deformations as a result of the applying RPs could delay the appearance of damages (i.e. cracking). Consequently, the bituminous material can tolerate a higher number of load cycles during fatigue test.     

Experimental and theoretical characterization of the engineering behavior of bitumen mixed with mineral filler

The dynamic shear rheometer (DSR) and the bending beam rheometer (BBR) were used to characterize the rheological properties of bitumen mixed with mineral filler that is smaller than 75 μm in size. The study focuses on using a rheology-based model to assess the effect of two distinctly different fillers, quartz and calcite, on the engineering behavior of the bitumen-mineral filler mastic. Four conventionally different bitumens were selected to assess the filler effect. By mathematically modeling the rheological response, predicting the rheological behavior of mastics becomes simpler and more efficient in approach. The rheological properties of bitumen-mineral filler mastics are characterized using the time–temperature superposition principle after data obtained from DSR and BBR are converted to the same unit. The stiffening effects of the filler are relatively small at short loading times or low temperatures, but are larger at higher temperatures or long loading times. This stiffening effect is found to be bitumen dependent as well as filler dependent. The validity of a micromechanical model is confirmed in this study. The Nielsen model was selected since it employs rheological parameters that could explain the filler effect. The micromechanical model shows good agreement with testing data at the filler volume fraction up to 22%.     

Accelerated pavement testing and modeling of reflective cracking in pavements

In the field of reflective cracking initiation and propagation in pavement structures, an Accelerated Pavement Testing (APT) has been developed. The numerical method is also supported. Numerical results obtained by calculations made with the finite element CESAR-LCPC software are then compared with the experimental data. The reflective cracking APT experience is the first step in the development of a new laboratory equipment to evaluate the design of bituminous layers to repair pavements. The experiment was performed on a 30 m track where eight discontinuities were established by cutting-off the sub-bases in order to ensure a crack development in the bituminous layer. The conditions that promote crack propagation were reproduced by the application of heavy periodic loads using the Fabac ALT-APT test rig. The width of the bituminous layer was decreased to facilitate the observation of the crack propagation, while avoiding creep of the material. The structure has a large number of sensors that measure longitudinal and transverse deformations at the bottom and at the surface of the bituminous layer. The deflections and the displacements are measured at several levels of the structure, as well as the temperature and the lateral edge cracking which is monitored by cracking sensors on both sides of the track. Preliminary results on two joints were used to determine the mechanisms of rupture of the structure. Four more tests with improved sensors have completed the information. Experimental and numerical results are compared in order to validate the analysis of the fatigue process (debonding, damage and cracking) in full scale pavement.     

Analysis of the influence of binder properties on the mechanical response of bituminous mixtures

Asphalt mixtures are composed by a mass of aggregates (more than 90% of their total weight), which are bonded by a bituminous binder. Despite the fact that the binder is not the main component of these materials (around 5% of their total weight), it exerts a high influence on their mechanical response. In this sense, the service life of asphalt pavements will directly depend on the type of binder used, and thus an adequate choice is crucial to construct more durable roads. Because of this fact, it is necessary to know the characteristics of the bitumen in order to reduce the impact of different distresses that appear on roads. For this purpose, this paper studies the influence of the binder properties in the appearance of the main distresses that affect asphalt pavements around the world (stripping, fatigue cracking and plastic deformations). Five bitumens with different properties have been analysed during this research using diverse binder (UCL, multiple stress creep and recovery test and dynamic shear rheometer time sweep) and mixture (water sensitivity, wheel tracking and UGR-FACT) tests. The results obtained show that the properties of the binder influence the long-term performance of bituminous mixtures. In this sense, it can be said that flexible binders which are able to recover plastic deformations could extend the service life of the pavements.     

Modelling of cohesion and adhesion damage of seal based on dynamic shear rheometer testing

Most current seal designs are based on the volumetric properties of materials and voids. In order to improve seal design, the possibility of introducing mechanistic principles into seal design was investigated. Introducing mechanistic concepts into seal design means that principles such as elasticity and viscoelasticity could be used in terms of stress-strain to explain phenomena such as damage in the seal structure. Two main failure parameters of seals – cohesion failure (fatigue cracking due to ageing of binder and loss of elasticity) and adhesion failure or stripping (occurring between stone to bitumen or bitumen to base) – are investigated using the complex modulus (G ^*) which is one of the viscoelastic parameters of bituminous materials. This paper therefore investigates the testing procedure of cohesion fatigue damage (CFD) and Adhesion Fatigue Damage (AFD) of bituminous seal material using the Dynamic Shear Rheometer (DSR). The CFD and AFD modelling are based on the stiffness reduction principle of materials under the action of cyclic stress. Based on the Lifetime Optimisation Tool (LOT) research programme from Delft University of Technology, a DSR testing procedure and approach was adopted for seals. The tests were performed on 70/100 penetration grade bitumen columns (for CFD) and on stone columns constituted with dolorite glued together with 70/100 penetration grade bitumen (for AFD). It was observed that the model for CFD depends more on stress, while the model for AFD appears to depend more on temperature. This observation agrees with the fact that adhesion damage is more sensitive to temperature change, whereas cohesion damage is more prone to be influenced by applied fatigue stress. The CFD and AFD models provide an indication of non-linear development of the accumulated fatigue damage of seal. This is represented by the modelling of the change of G ^*, as suggested in this investigation.     

Strain accumulation in bituminous binders under repeated creep-recovery loading predicted from small-amplitude oscillatory shear (SAOS) experiments

The creep-recovery (CR) test starts out with a period of shearing at constant stress (creep) and is followed by a period of zero-shear stress where some of the accumulated shear strain gets reversed. Linear viscoelasticity (LVE) allows one to predict the strain response to repeated creep-recovery (RCR) loading from measured small-amplitude oscillatory shear (SAOS) data. Only the relaxation and retardation time spectra of a material need to be known and these can be determined from SAOS data. In an application of the Boltzmann superposition principle (BSP), the strain response to RCR loading can be obtained as a linear superposition of the strain response to many single creep-recovery tests. SAOS and RCR data were collected for several unmodified and modified bituminous binders, and the measured and predicted RCR responses were compared. Generally good agreement was found between the measured and predicted strain accumulation under RCR loading. However, in the case of modified binders, the strain accumulation was slightly overestimated (≤20% relative error) due to the insufficient SAOS information at long relaxation times. Our analysis also demonstrates that the evolution in the strain response under RCR loading, caused by incomplete recovery, can be reasonably well predicted by the presented methodology. It was also shown that the outlined modeling framework can be used, as a first approximation, to estimate the rutting resistance of bituminous binders by predicting the values of the Multiple Stress Creep Recovery (MSCR) test parameters.     

Fretting fatigue crack initiation mechanism in Ti–6Al–4V

Fretting fatigue crack initiation in titanium alloy, Ti?6Al?4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.     

Torsional fatigue resistance of plasma sprayed HA coating on Ti–6Al–4V

The torsional strength of plasma sprayed hydroxyapatite (HA) coatings was studied under static and cyclic loading. The torsional shear tests were conducted in a frustum test device developed in this laboratory, which adapted to various coating thicknesses. The interfacial fatigue resistance was measured in terms of interfacial fatigue strength defined as the average maximum stress (_fmax). A staircase fatigue method was employed to determine the interfacial fatigue strength; this method resolved the uncertainty in detecting coating failure during torsion fatigue. The values for coating shear strength and shear fatigue strength obtained from the torsional tests did not differ from those obtained by previous tensional shear tests in this laboratory. The fatigue strength of one million cycles was about 35% lower than static shear strength. This finding might be used for estimating fatigue life span without cyclic loading tests.     

Multiaxial fatigue damage parameter and life prediction for medium-carbon steel based on the critical plane approach

The tension–torsion fatigue characteristics were investigated under proportional and non-proportional loading in this paper. The fatigue cracks on the surface of multiaxial fatigue specimens were observed and analyzed by a scan electron microscope. On the basis of the investigation on the Kindil–Brown–Miller and Fatemi–Socie’s critical plane approaches, a shear strain based multiaxial fatigue damage parameter was proposed by von Mises criterion based on combining the maximum shear strain and the normal strain excursion between adjacent turning points of the maximum shear strain on the critical plane. The proposed multiaxial fatigue damage parameter does not include the weight constants. According to the proposed multiaxial fatigue damage parameter, the multiaxial fatigue life prediction model was established with the Coffin–Manson equation, which is used to predict the multiaxial fatigue life of medium-carbon steel. The results showed that the proposed multiaxial fatigue damage parameter could be used under either multiaxial proportional or non-proportional loading.     

A shear stress-based parameter for fretting fatigue crack initiation

The purpose of this study was to investigate the fretting fatigue crack initiation behaviour of titanium alloy, Ti–6Al–4V. Fretting contact conditions were varied by using different geometries of the fretting pad. Applied forces were also varied to obtain fretting fatigue crack initiation lives in both the low- and high-cycle fatigue regimes. Fretting fatigue specimens were examined to determine the crack location and the crack angle orientation along the contact surface. Salient features of fretting fatigue experiments were modelled and analysed with finite element analysis. Computed results of the finite element analyses were used to formulate a shear stress-based parameter to predict the fretting fatigue crack initiation life, location and orientation. Comparison of the analytical and experimental results showed that fretting fatigue crack initiation was governed by the maximum shear stress, and therefore a parameter involving the maximum shear stress range on the critical plane with the correction factor for the local mean stress or stress ratio effect was found to be effective in characterizing the fretting fatigue crack initiation behaviour in titanium alloy, Ti–6Al–4V.     

Bulk and shear characterization of bituminous mixtures in the linear viscoelastic domain

Traffic loads and temperature variations produce three-dimensional stress–strain fields inside road pavements, and therefore the characterization of bituminous mixtures in different deformation modes is important for prediction of the performance of pavement structures. This paper presents a methodology for the bulk and shear characterization of bituminous mixtures in the linear viscoelastic domain, under the hypothesis of material isotropy, by means of uniaxial harmonic tests with the measurement of axial and transverse strains. The theoretical approach was based on the application of the elastic–viscoelastic correspondence principle and was validated by performing tension–compression tests at selected frequencies and temperatures on asphalt concrete specimens characterized by different volumetric properties. The results showed that since uniaxial tests induced both volume and shape variations, the simultaneous measurement of the complex bulk and shear moduli was possible. The validity of the time–temperature superposition principle was also verified for both deformation modes, allowing the construction of master curves for the bulk and shear moduli. The results also showed that the total dissipated energy could be decomposed into its volumetric and deviatoric fractions with excellent accuracy.     

Fatigue behaviour of fastener holes in high‐strength aluminium plates repaired by cold spray deposition

This work examines the fatigue behaviour of additive cold spray (CS) repairs of AA7075 and AA2024 fastener holes. Structural ring repairs around fastener holes were made by machining blend‐outs ranging from 1/8 to 1/2 the thickness of the plate, then refilling the section of removed material with CS deposition. The repairs were then tested in a lap shear geometry with the repair on both the free (outside) and the mating (inside) surfaces, as well as in remote uniaxial tension. CS repairs for the inside lap shear AA7075 repair configuration and the outside lap shear AA2024 repair configuration were found to have significantly increased fatigue lives even exceeding the number of cycles to failure of the undamaged, unrepaired control plates. Further, none of the CS repairs caused any detrimental impact on fatigue life, and microhardness results indicate that no thermal damage to the substrate occurred. Some interface cracking was seen in the CS repairs; however, no separation of the repair from the substrate was observed.     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

Quantification of different effects occurring during fatigue tests on bituminous mixtures

Quantification of different effects (nonlinearity, heating, thixotropy, and fatigue) occurring during fatigue tests on bituminous mixtures is presented in this paper. A focus is given on the nonlinearity phenomenon. Continuous fatigue tests and a test with specific protocol (called fatigue tests to estimate biasing effects) were performed in tension/compression mode on cylindrical samples of the same material. The analysis of results reveals that reversible effects (nonlinearity, heating, and thixotropy) are important (more than 90% decrease at 100,000 cycles for a strain amplitude of 100 μm/m at 10 Hz) and cannot be ignored when interpreting classical fatigue tests. The nonlinearity effects respect the time‐temperature superposition principle, and they are more pronounced at “high” temperature (at the same frequency). Direction of nonlinearity curve in the Cole‐Cole axes is shown to be independent of temperature and frequency for the considered range.     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

Relationship between absolute viscosity of polymer-modified bitumens and rutting resistance of pavement

New polymer-modified bituminous binders require verification of the relationship between binder properties and road pavement performance, formerly found for conventional road bitumens. The paper deals with one particular problem of polymer-bitumen (PB) viscosity and bituminous mixture rutting resistance. Non-Newtonian, shear-dependent behaviour of PB in a viscosity meassurement has been characterized by a new equation proposed by the author. This equation enables calculation of the absolute (zero shear rate) viscosity of PB and is applied for evaluation of test results at 60and 90°C. The absolute viscosity of PB at 60°C has been correlated with the bituminous mixture's rutting resistance at 45°C. Correlation with other rheological properties of PB is also examined by means of multiple regression analysis.