The European regulations for specimen fabrication prior to performance testing of asphalt mixtures

For fundamental performance testing of asphalt mixtures, specimens have to be produced from an asphalt mixture that is compacted in laboratory. For this purpose, different laboratory compaction methods are available today, which are based on the effects of impact, kneading or rolling action and/or vibration. It is well known that the method used for compaction considerably influences the properties of the specimen, and in turn, the specimen properties significantly influence the outcome of fundamental asphalt mix testing. The choice of compaction method is therefore of utmost importance. Hence, there is general consensus among experts that high priority must be given to the harmonisation of laboratory methods for the production of specimens prior to mix testing. Asphalt mix requirements must not be specified independently from the detailed compaction requirements. In Europe, a harmonised quality assurance system for asphalt mix properties was established recently. In order to guarantee its practicability, also a harmonised approach for the fabrication of specimens for fundamental testing becomes indispensable. In this paper, an overview is given on how standardisation of specimen production has progressed in Europe so far, and the need for further harmonisation is pointed out from a European point of view.     

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.     

老化对水泥乳化沥青胶结料动态力学性能的影响

基于分数导数修正Burgers模型,建立了水泥乳化沥青胶结料（简称复合胶结料）的本构方程。结合不同材料配比的复合胶结料的老化试验及频率扫描试验,分析了老化时间及材料配比等因素对复合胶结料黏弹性力学参数的影响。研究表明,复合胶结料的存储模量与损耗模量均随老化时间的增加而增大,在老化时间为0~8 h时,存储模量及损耗模量增加较快,当老化时间超过8 h后,其增长趋势减缓。对于乳化沥青与水泥的质量比（m_A/m_C）为1.2和1.4的复合胶结料,在不同的加载频率下,其相位角均随老化时间的增加而减小。对于m_A/m_C为1.0的复合胶结料,其相位角随着老化时间的增加呈现先增加后减小的变化规律。随着老化时间的增加,复合胶结料趋向于弹性材料的力学性质。分数导数修正Burgers模型可以较好的描述老化后复合胶结料的黏弹性动态力学行为,模型参数弹性模量E₁、黏度η₁和分数导数r可以描述老化情况下材料黏弹特性的变化规律。     

Thermal fatigue resistance characterization and ranking of materials using the V‐shape specimen testing method

Thermal fatigue resistance of materials is an extremely important criterion for the long‐term durability and reliability performance of very high‐temperature components and systems, such as advanced auto engine and exhaust systems. There is a broad range of material choices for thermal fatigue resistance applications. The final selection of the materials depends on the balance of engineering performance of the materials and the cost. To optimize the thermal fatigue resistance and cost of those materials, a reliable testing procedure for material thermal fatigue characterization and a material evaluation/selection matrix must be established. In this paper, the V‐shape specimen testing method in evaluating thermal fatigue resistance performance is introduced first. The influence of several factors, such as the thickness of specimens, operating temperature and hold time, on the thermal fatigue resistance is experimentally investigated. Subsequently, the statistical and probabilistic characteristics of the thermal fatigue failure data are analysed to reveal the possible failure mechanisms. Finally, a general rational approach for thermal fatigue resistance characterization and ranking is demonstrated, and a simple parameter λ = kσ_f/Eα, which combines the material strength, thermal conductivity and thermal expansion, is found to be the new breakthrough parameter, correlating to V‐shape thermal fatigue test results. Results on four currently used stainless steels verify the correlations and indicate the validity of this approach.     

The effect of initial notch shape of compact specimen on fatigue pre-cracking and fracture toughness testing

The circular notched compact specimens, along with standard specimens having straight or chevron notch are provided for fatigue and fracture toughness testings in order to study the crack observation capability during fatigue pre-cracking, skewness of the crack front, and the resulting fracture toughness K_Q. The test results indicated that circular notched specimens significantly facilitate the crack observation during fatigue testing as the cracks initiate on both surfaces of the specimen. No remarkable differences were observed on geometries of the fatigue crack front obtained and the resulting fracture toughness among these three types of specimen. The macroscopic observation of beach marks on the fracture surfaces revealed that, in the present material Ti-6Al-4V (ELI), the advance of only 1.3% of the whole crack length corresponded to the load level at which fracture toughness K_Q was evaluated.     

Numerical estimation of the fatigue crack front shape for a specimen with finite thickness

An iterative process for the estimation of a fatigue crack front based on linear elastic fracture mechanics using values of the stress singularity exponent is presented. Based on the assumption of a constant stress singularity exponent along the crack front, a numerical approach leading to crack shape determination is suggested and applied. The crack front was approximated by a spline curve. In each node defining the crack front the stress singularity exponent was estimated and a complete crack front shape was found. The difference between thin and thick specimens is then described and discussed. The approach presented leads to better estimation of the crack front shape for structures with different thicknesses and a more accurate determination of fatigue crack fracture parameters. The results presented can be helpful for a better understanding of fatigue failure and more reliable prediction of residual lifetime.     

Introduction of a new notched specimen geometry to determine fracture properties by small punch testing

The small punch (SP) testing technique can be used to assess fracture properties from a very small volume of material. However, the correlation of fracture parameters obtained from SP testing with those produced using standard methods is non-trivial, not only due to the size effect, but also due to the absence of a notch in most existing types of SP specimens. In the present study, a novel SP specimen design is proposed, with a sharp circular notch, leading to the development of a near plane strain condition during the SP test. The new geometry is examined through SP testing of P91 steel, as well as finite element analysis. Optical and scanning electron microscopy is used to study the crack propagation from the notch tip in both ductile and brittle fracture modes.     

The use of asphalt low shear viscosity to predict permanent deformation performance of asphalt concrete

The permanent deformation performance of asphalt concrete is strongly dependent on the rheological properties of the asphalt binder. It has been recognized that the asphalt’s low shear viscosity (LSV) characterizes the mixture’s rutting resistance. At the same time, the pavement temperature is one of the main factors that significantly affect the mixture performance. In this work the rutting performance of mixtures prepared with the same aggregate gradation and different binders [conventional (C), multigrade (M) and polymer modified (PM) asphalts] were evaluated by using wheel tracking tests (WTT) performed at 50, 60, 70 and 80°C; in parallel, the LSV of asphalts were measured at the same temperatures. The relationship between the asphalt’s LSV and rutting, to predict asphalt mixture performance, was discussed and a criterion to consider the effect of LSV is proposed.     

Development of a new device to perform torsional ultrasonic fatigue testing

The interest in gaining experimental knowledge on fatigue strength of materials over 10⁹ cycles is rapidly increasing as evidenced for the large amount of investigations on this subject presented at the last very high cycle fatigue meeting (VHCF-3), held on September 2004. Most of the fatigue results presented at this conference were obtained under tension-compression, rotating bending, flexion and bending cyclic loading (some attaining 10¹⁰ cycles), using ultrasonic devices whose design was based on the natural frequency principles. In general, very little literature concerning the metallic alloys behavior under torsion cyclic loading using ultrasonic is available; however, in order to perform an accurate component design under multi-axial loading and VHCF, the material behavior under torsion cyclic loading is required. This investigation presents the development of a new mechanical device for testing and characterizing metallic alloys in the range of 10⁹–10¹⁰ cycles in torsional cyclic loading and the first experimental results for medium carbon steel (38MnSV5S). The new device was designed to excite the components under testing with pure torsional vibration mode at a frequency of 20 kHz.     

Improvements on asphalt mixtures rutting performance characterization by the use of low shear viscosity

The relationship between the rutting performance of dense asphalt concretes and the low shear viscosity (LSV) of different asphalt binders was analysed in a previous work. A LSV limit was found for the original asphalt to prevent the rutting of the mixtures, and in addition, a model to predict the rutting performance based on the LSV of the asphalt binder was validated. With the aim of amplifying the criterion previously found, the performances of micro and stone mastic asphalt mixtures are studied in this work. Conventional, multigrade and polymer modified asphalts were used as binders. Considering that the properties of original and aged asphalts must be taken into account for a better asphalt binder characterization, LSV measurements on aged asphalts were also done in order to analyse their relationship to the mixtures rutting performance. The micro and stone mastic asphalt mixtures showed a similar behaviour as the dense grade asphalt concrete in the previous study. Regarding the control of rutting, a LSV limit of 500 Pa.s was found for original asphalts, while 2,000 Pa.s was the limit for aged asphalt binders. The model to predict the rutting performance of asphalt mixtures was amplified, incorporating both original and aged asphalt LSVs as appropriate input data.     

Effect of load control mode on the fatigue performance of asphalt binder

Fatigue cracking is one of the major distresses found in asphalt pavements. The ability of asphalt binder to resist the accumulation of fatigue damage can have a profound effect on the service life of asphalt pavements. However, different from other materials, the fatigue characteristics of asphalt binder depend heavily on the load control mode. That is the fatigue characteristics of asphalt binder under controlled-stress mode differ from those under controlled-strain mode. This discrepancy affects the analysis results of asphalt binder fatigue performance. Therefore, this study is aimed at analyzing the effects of the load control mode on the fatigue performance of asphalt binder through a comparison of the fatigue characteristics under two different load control modes. The effect range of the load control mode was confirmed, and the method to differentiate the region affected by load control modes from the region not affected by load control modes was put forward. The relationship between the fatigue performances of asphalt binder under different control modes was analysed. Based on the results, it is concluded that the fatigue process can be divided into two regions using the curve of normalized dynamic modulus versus normalized phase angle. And, if the controlled stress and the controlled strain are equivalent levels, the fatigue characteristics of asphalt binder are independent of load control modes in the first region. The accumulated dissipated energy at the division point of the two regions is independent of both control modes and load levels, and serves as the effect threshold of the load control mode. Besides, the curve of accumulated dissipated energy versus number of cycles under controlled-stress mode is symmetrical to that curve under controlled-strain mode.     

TLA改性沥青的动态蠕变性能研究

通过对不同掺量的TLA改性沥青进行动态蠕变实验,并基于Burgers模型的粘弹理论进行拟合分析,重点分析了模型参数随温度和掺量的变化规律以及不同掺量的蠕变曲线的形状变化规律。结果表明,基于Burgers模型拟合TLA改性沥青蠕变曲线相关性较高;任一掺量的TLA改性沥青,随着温度的升高,所有的参数值均明显变小;GV值随温度升高降低,当掺量>30%后,GV增大的幅度才较为显著;TLA改性沥青蠕变曲线3个阶段的形状与参数大小密切相关。     

Ductile shear fracture at the surface of a bent specimen

The development of shear bands at the stretched surface of a bent plate is analysed numerically, based on an approximate continuum model of a ductile porous material. This material model accounts for the nucleation and growth of voids as well as the effect of the yield surface curvature, which is represented by a combination of kinematic hardening and isotropic hardening. An imperfection in the form of an initial surface waviness is assumed, which triggers shear bands at the wave bottoms. The corresponding periodic pattern of shear bands is considered, and the growth of the bands is followed, until shear cracks develop from the void-sheets inside the bands. The delay of localization due to the nonuniform strain field is studied for different versions of the material model. Furthermore, the stability of the uniform growth of several adjacent shear bands is investigated.     

Employing dynamic fuzzy membership functions to assess environmental performance in the supplier selection process

The proposed system illustrates that logic fuzzy can be used to aid management in assessing a supplier's environmental performance in the supplier selection process. A user-centred hierarchical system employing scalable fuzzy membership functions implement human priorities in the supplier selection process, with particular focus on a supplier's environmental performance. Traditionally, when evaluating supplier performance, companies have considered criteria such as price, quality, flexibility, etc. These criteria are of varying importance to individual companies pertaining to their own specific objectives. However, with environmental pressures increasing, many companies have begun to give more attention to environmental issues and, in particular, to their suppliers’ environmental performance. The framework presented here was developed to introduce efficiently environmental criteria into the existing supplier selection process and to reflect on its relevant importance to individual companies. The system presented attempts to simulate the human preference given to particular supplier selection criteria with particular focus on environmental issues when considering supplier selection. The system considers environmental data from multiple aspects of a suppliers business, and based on the relevant impact this will have on a Buying Organization, a decision is reached on the suitability of the supplier. This enables a particular supplier's strengths and weaknesses to be considered as well as considering their significance and relevance to the Buying Organization.     

Optimized butterfly specimen for the fracture testing of sheet materials under combined normal and shear loading

The present paper is concerned with multi-axial ductile fracture experiments on sheet metals. Different stress-states are achieved within a flat specimen by applying different combinations of normal and transverse loads to the specimen boundaries. The specimen geometry is optimized such that fracture initiates remote from the free specimen boundaries. Fracture experiments are carried out on TRIP780 steel for four different loading conditions, varying from pure shear to transverse plane strain tension. Hybrid experimental–numerical analyses are performed to determine the stress and strain fields within the specimen gage section. The results show that strain localization cannot be avoided prior to the onset of fracture. Through-thickness necking prevails under tension-dominated loading while the deformation localizes along a band crossing the entire gage section under shear-dominated loading. Both experimental and simulation results demonstrate that the proposed fracture testing method is very sensitive to imperfections in the specimen machining. The loading paths to fracture are determined in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain. The experimental data indicates that the relationship between the stress triaxiality and the equivalent plastic strain at the onset of ductile fracture is not unique.     

一种新型非调质钢弯曲疲劳性能的试样尺寸效应

使用液压伺服疲劳试验机考察一种新型(汽车前轴用)Nb+V复合微合金非调质钢的疲劳行为,绘制出S-N曲线并分析了疲劳断日特征,研究了其三点弯曲疲劳性能的试样尺寸效应及其原因.结果表明,试样的尺寸对非调质钢的三点弯曲疲劳性能有显著的影响,其三点弯曲疲劳极限随着试样尺寸的减小而增加,但是试样尺寸对疲劳试样的断口形貌几乎没有影响;在三点弯曲疲劳试验中,试样尺寸效应源于试样内部的应力梯度,小尺寸试样的应力梯度比大尺寸试样的高.