Characterization of fatigue damage for paving asphaltic materials*

This study investigates the fatigue characteristics of typical bituminous materials used in road applications. Fatigue testing was performed in a four‐point bending beam test apparatus under controlled strain and stress conditions. Fatigue life was defined using the classical approach as the number of cycles, N_f, to 50% reduction in the initial stiffness modulus. It has also been defined in terms of macro‐crack initiation, N₁. A different approach, based on the linear reduction in stiffness during a particular stage of a fatigue test, was introduced to define a damage parameter, and the evolution of this damage parameter with number of cycles was used to characterize fatigue life. Furthermore, refinements to the linear damage model were introduced to take into account the difference in the evolution of dissipated energy between controlled strain and stress testing modes. These modifications have enabled the identification of a unique fatigue damage rate for both controlled strain and stress test modes.     

Application of continuum damage mechanics model in terms of fatigue life assessment for low cycle and quasi static loadings

The aim of this study is to test the classical Lemaitre model based on continuum damage mechanics (CDM) approach in the range of low cycle and quasi‐static fatigue life. Study is carried out with the use of results of experimental tests for C45 steel (according to AISI: 1045 steel) carried out under variable‐amplitude loading. Loading programs are of two‐step character and include blocks of cycles of different lengths and R = ‐1 coefficients. Fatigue lives are calculated according to Lemaitre model from experimentally obtained stress and strain histories recorded during fatigue tests. The results are compared with experimental tests results and with fatigue lives calculated with the use of by traditional fatigue approach based on Palmgren‐Miner damage summation hypothesis. Experimental test of fatigue life calculation results for C45 steel reveals that continuum damage method, using the recorded stresses and strains, predicts fatigue life better as compared to the remaining methods. The study also contains many detailed analyses of experimental results.     

Numerical modeling and simulation of wheel radial fatigue tests

A computational methodology is proposed for fatigue damage assessment of metallic automotive components and its application is presented with numerical simulations of wheel radial fatigue tests. The technique is based on the local strain approach in conjunction with linear elastic FE stress analyses. The stress–strain response at a material point is computed with a cyclic plasticity model coupled with a notch stress–strain approximation scheme. Critical plane damage parameters are used in the characterization of fatigue damage under multiaxial loading conditions. All computational modules are implemented into a software tool and used in the simulation of radial fatigue tests of a disk-type truck wheel. In numerical models, the wheel rotation is included with a nonproportional cyclic loading history, and dynamic effects due to wheel–tire interaction are neglected. The fatigue lives and potential crack locations are predicted using effective strain, Smith–Watson–Topper and Fatemi–Socie parameters using computed stress–strain histories. Three-different test conditions are simulated, and both number of test cycles and crack initiation sites are estimated. Comparisons with the actual tests proved the applicability of the proposed approach.     

循环荷载作用下钢桥面铺装疲劳损伤失效行为研究

针对钢桥面铺装工程中普遍采用的改性沥青(Stone Matrix Asphalt,SMA)、浇筑式沥青(Guss asphalt,GA)、环氧沥青(Epoxy asphalt,EP)混合料双层铺装结构,进行了循环车载作用下钢桥面与沥青混凝土铺装疲劳损伤特性理论分析与试验研究。基于疲劳损伤度,研究了钢桥面铺装疲劳损伤失效行为和疲劳开裂过程中损伤场、应力和应变场动态演变机制,推导出疲劳失效时的损伤场、应力和应变场计算表达式,并给出钢桥面铺装疲劳寿命理论公式。以三座钢箱梁桥桥面铺装(润扬长江大桥2005,南京长江三桥2005,苏通大桥2008)为例,对不同铺装结构组合方案下的复合梁进行疲劳试验分析和使用寿命理论预测。实例研究结果表明,钢桥面铺装疲劳损伤失效行为预估模型合理可行;相较于改性沥青、浇筑式沥青,环氧沥青混合料具有较强高的强度低变形能力,更适合于大跨径钢桥面铺装抗疲劳的设计要求;由环氧沥青混合料组合而成的“双层环氧沥青混凝土”和“浇注式沥青混凝土(下层)+环氧沥青混凝土(上层)”的抗疲劳性能优于其它沥青混合料铺装结构组合方案,同等厚度组合情况下疲劳使用寿命可延长1倍~2倍以上;“双层环氧沥青混凝土”已应用于润扬长江大桥、南京长江三桥和苏通长江大桥钢桥面工程,并已成功运行10年以上,其跟踪观测结果良好。     

A computer simulation of four-point bending fatigue of a rear axle assembly

The bending fatigue test of a rear axle assembly is simulated by using a FE-integrated fatigue analysis methodology. The presented technique is based on local stress–strain approach in conjunction with two critical plane damage parameters. The stress–strain response at a material point is computed with a cyclic plasticity model coupled with a notch stress–strain approximation scheme. Linear elastic FE stress analyses are used in the calculation of local fatigue loading. All computational modules are implemented into the software tool and used in the four-point bending fatigue test simulation of rear axles made of a high-strength alloy steel. In fatigue models, proportional loadings with a static preload are considered, and the effects of residual stresses are neglected. The fatigue test cycles and crack initiation locations are predicted using Smith–Watson–Topper and Fatemi–Socie fatigue damage parameters. Both damage parameters provided conservative test cycle estimates for the test conditions simulated. It is also observed that von Mises stress distributions cannot be used to predict fatigue crack initiation locations while Smith–Watson–Topper critical plane parameter estimated the cracking location suitably. Comparisons with the prototype tests showed the applicability of the proposed approach.     

Fatigue resistance potential for hot mix asphalt using viscoelastic continuum damage analysis

This paper describes testing and evaluation of the fatigue resistance potential of hot‐mix asphalt mixtures using viscoelastic continuum damage analysis, which is based on dynamic modulus determination, a state‐variable approach and damage calculation. The dynamic modulus test for stiffness characterization and the direct tension test for fatigue resistance characterization were used in the testing procedure. The state‐variable approach can be used for numerical computation of a viscoelastic convolution integral. A Nelder–Mead simplex search was used in this study to determine the damage parameter of a stiffness reduction function. The fatigue resistance was evaluated as a function of loading rate, asphalt binder content, modifier (e.g. usage of hydrated lime), and temperature, and was found experimentally to have a strong dependence on these factors.     

Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling

Fatigue cracking is a major form of distress in asphalt pavements. Asphalt binder is the weakest asphalt concrete constituent and, thus, plays a critical role in determining the fatigue resistance of pavements. Therefore, the ability to characterize and model the inherent fatigue performance of an asphalt binder is a necessary first step to design mixtures and pavements that are not susceptible to premature fatigue failure. The simplified viscoelastic continuum damage (S-VECD) model has been used successfully by researchers to predict the damage evolution in asphalt mixtures for various traffic and climatic conditions using limited uniaxial test data. In this study, the S-VECD model, developed for asphalt mixtures, is adapted for asphalt binders tested under cyclic torsion in a dynamic shear rheometer. Derivation of the model framework is presented. The model is verified by producing damage characteristic curves that are both temperature- and loading history-independent based on time sweep tests, given that the effects of plasticity and adhesion loss on the material behavior are minimal. The applicability of the S-VECD model to the accelerated loading that is inherent of the linear amplitude sweep test is demonstrated, which reveals reasonable performance predictions, but with some loss in accuracy compared to time sweep tests due to the confounding effects of nonlinearity imposed by the high strain amplitudes included in the test. The asphalt binder S-VECD model is validated through comparisons to asphalt mixture S-VECD model results derived from cyclic direct tension tests and Accelerated Loading Facility performance tests. The results demonstrate good agreement between the asphalt binder and mixture test results and pavement performance, indicating that the developed model framework is able to capture the asphalt binder’s contribution to mixture fatigue and pavement fatigue cracking performance.     

A combined finite element method and continuum damage mechanics approach to simulate the in vitro fatigue behavior of human cortical bone

The fatigue of bone, in particular the associated modulus degradation and accumulation of permanent strain, has been implicated as the cause of femoral neck fractures and the migration of total joint replacements. The objective of this study was to develop a technique to simulate the tensile fatigue behavior of human cortical bone. A combined continuum damage mechanics (CDM) and finite element analysis (FEA) approach was used to predict the number of cycles to failure, modulus degradation and accumulation of permanent strain of human cortical bone specimens. The simulation of fatigue testing of eight dumb-bell specimens of cortical bone were performed and the predictions compared with existing experimental data. The predictions from the finite element models were in close agreement with the experimental data. The models predicted similar development of modulus degradation and permanent strain as observed in the experimental tests. The technique is capable of predicting the accumulation of permanent strain without the need for simulating every single load step. These findings suggest that the complex fatigue behavior of human cortical bone can be simulated using the described approach and forms the first step for simulating the more complex mechanisms associated with femoral neck fractures and implant migration.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

Fatigue life prediction: A Continuum Damage Mechanics and Fracture Mechanics approach

Continuum Damage Mechanics (CDM) approach is used to predict crack initiation life and Fracture Mechanics approach predicts crack growth life. Strain controlled fatigue life of a ferrous alloy, EN 19 steel, has been determined using CDM and Fracture Mechanics approach. By combining these two approaches, life could be predicted with damage value in the material. All inputs required for the models have been determined by conducting monotonic, cyclic and fracture tests. Predicted life is also compared by conducting strain controlled fatigue tests. Predicted life in the strain amplitude range of 0.3–0.7% (fatigue life range of 10²–10⁵), compares well with the experimental results. All tests have been conducted at specimen level, stress ratio of −1 and at room temperature. The variation of crack initiation and crack propagation life with strain amplitude shows that maximum life is consumed by crack growth process at higher strain amplitude and at lower strain amplitudes, maximum life is spent for crack initiation process.     

Impact of asphalt concrete fatigue endurance limit definition on pavement performance prediction

Many well-constructed Hot Mix Asphalt pavements have been in service for 40 or more years without any evidence of fatigue cracking. This field experience suggests that there exists a strain level, known as the fatigue endurance limit (FEL), below which an asphalt concrete pavement will not exhibit fatigue cracks. Several studies have been conducted to define and verify this limit. Each of these methods is associated with certain assumptions regarding the nature of the FEL and heretofore a comprehensive comparison of each has not been made using a consistent set of mixtures. Likewise, the impact of any observed differences in FEL on the predicted pavement performance has not been made. This paper investigates and compares six different methods for identifying the FEL: NCHRP 9–44A approach, simplified viscoelastic continuum damage model, smeared-healing with continuum damage model, plateau value approach, pseudo-strain analysis method, and reduced cycles method. Each method is found to yield different values ranges from approximately 30–170 microstrains at 21.1 °C. The predicted FEL from each of the six methods are then used with the mechanistic empirical design algorithm to evaluate their effects on predicted pavement performance. Simulation outputs show different pavement performance and perpetual pavement structural design thicknesses from each of the methods. The study outcomes are expected to benefit future field verification research of FEL as it provides comprehensive analyses using six different methods. This future verification research may indicate the method that best represents actual perpetual pavement design and performance.     

A continuum damage model applied to high-temperature fatigue lifetime prediction of a martensitic tool steel

High‐temperature operational conditions of hot work tool steels induce several thermomechanical loads. Depending on the processes, (i.e. forging, die casting or extrusion), stress, strain, strain rate and temperature levels applied on the material are nevertheless very different. Thus, lifetime prediction models need to be able to take into account a broad range of working conditions. In this paper, a non‐isothermal continuum damage model is identified for a widely used hot work tool steel AISI H11 (X38CrMoV5) with a nominal hardness of 47 HRc. This investigation is based on an extensive high‐temperature, low‐cycle fatigue database performed under strain rate controlled conditions with and without dwell times in the temperature range 300–600°C . As analysis of experimental results does not reveal significant time‐dependent damage mechanisms, only a fatigue damage component was activated in the model formulation. After normalization, all fatigue results are defined on a master Woehler curve defined by a nonlinear damage model, which allows the parameter identification. Last, a validation stage of the model is performed from thermomechanical fatigue tests.     

Numerical investigation of fatigue characteristics of concrete pavement

In concrete pavements, fatigue is one of the major causes of distress. Repeated loads result in the formation of cracks. The propagation of these cracks cause internal progressive damage within the structure, which ultimately leads to failure of the pavement due to fatigue. This paper presents a theoretical investigation of crack propagation within concrete pavement and its fatigue characteristics under cyclic loading. A numerical fatigue performance model has been developed for this purpose. The model is based on fictitious crack approach for the propagation of cracks and stress degradation approach for estimating the bridging stress under cyclic loading. Using the numerical model, a parametric study has been performed for a typical concrete pavement to evaluate its fatigue characteristics for different foundation strengths. The method can be used for prediction of crack propagation in concrete pavement under cyclic loading and gives an estimate of the incremental damage or the entire crack history of the pavement.     

加筋沥青路面APA试验及其非线性疲劳损伤分析

应用沥青路面分析仪(APA)对沥青路面结构模拟试件进行多种荷载下的往返轮载疲劳试验,对比研究土工布、玻璃纤维格栅对沥青路面疲劳性能的加筋效果.在此基础上,提出沥青混合料非线性疲劳损伤演化模型,应用基于疲劳损伤力学的非线性有限元方法模拟APA 试验过程,分析了试件应力、变形、疲劳寿命等,并与试验结果对比,论证了所提疲劳损伤演化模型的合理性,指出因加筋材料的桥联效应显著地改善了裂缝穿过筋材后路面的受力变形状况,有效地延长路面的疲劳寿命.同时,基于疲劳损伤分析,进一步验证了Paris 公式可用于描述路面结构的疲劳裂缝稳定发展过程,并获得了有关的参数.     

单个冲击对不锈钢管道焊接头低周疲劳寿命的影响

完成了单个冲击对１Ｃｒ１８Ｎｉ９１不锈钢管道焊接头试样低周疲劳寿命影响的试验研究。单个试验最大瞬时峰值应变率达４８０ｓ＾－１。试样未经消除焊接残余应力。采用成组法试验（每组７个试样）,对称加载模式,总应变幅为０．００２２８。结果表明,冲击影响受到焊接残余应力和冲击塑性导入机制的耦合作用。焊接残余应力与冲击应力叠加将增加材料损伤,而冲击塑性导入将减缓疲劳损伤和降低疲劳寿命分散性。前者扼制后者。考虑疲     

Evaluation of low cycle fatigue life in AZ31 magnesium alloy

Magnesium alloys are attracting engineers for their practical application to structural components. Here fatigue properties, which is essential for structural use, have been examined on extruded AZ31 bar under uniaxial cyclic loading by both strain and stress controlled conditions. Adding fatigue tests with mean stresses under stress controlling conditions, fatigue life evaluation method has been discussed along with the analysis of cyclic stress–strain behavior. The specimen is easy to yield in compression by twinning. This leads to the asymmetric hysteresis curves. It also tends to deform quasi-elastically during unloading from compression; this makes the plastic strain amplitude smaller to the maximum one in the hysteresis curve. These asymmetric features fairly disappear at half-life in the stress controlled tests. The fatigue lives and deformation characteristics can be expressed nicely by Manson–Coffin type equation. On the contrary, the strain controlled tests retain the asymmetry till the end and produce tensile mean stresses. The fatigue lives are unsuccessfully evaluated by the above equation. Various mean stress correction models for cubic metals are not operative in magnesium alloys. A new model has been devised adding a correction term of −σ_m/2E to the above mentioned Manson–Coffin type equation. Strain controlled test, as it retains pyriform shape till the end, could be evaluated more accurately with the maximum plastic strain amplitude in the hysteresis curve.     

Hip stem fatigue test prediction

The accuracy of fatigue test prediction methods for the standard fatigue testing of hip stems was evaluated against the experimental results of static and fatigue tests. Axial unnotched strain-controlled material fatigue tests provided the required cyclic material properties. Finite element analysis of the hip stems predicted a maximum tensile stress to within 3–7% of strain gauge measurements. The four methods investigated accurately predicted hip stem fatigue strength at 5 million cycles (?1% to ?9% errors). The strain–life methods successfully predicted fatigue life (factors 1/7.0–9.2 of the test) at high and low stress amplitudes of 352 and 315 MPa, respectively. The classical stress–life method was only accurate (factor 1/1.9) for the low stress level. The current study has demonstrated that fatigue test prediction methods can be applied with confidence to support standard fatigue testing of hip stems. Further studies can expand the understanding of these methods and their clinical relevance by investigating effects due to variable amplitude loading and environment.     

An inverse analysis approach to determine fatigue performance of bituminous mixes

In pavement engineering, fatigue resistance is evaluated using different tests protocols and different specimen geometries. The dependency of the specimen shape geometry on fatigue performance does not allow the evolution of intrinsic material properties. This paper deals with the calibration of intrinsic fatigue damage parameters for bituminous materials. A fatigue damage model is implemented. The decrease of stiffness of the specimen during fatigue tests for different laboratory testing conditions is calculated from finite element computations. An inverse optimization technique is used in order to adjust the fatigue damage parameters on bending fatigue tests. A Levenberg-Marquardt algorithm is implemented to fit the finite element specimen global response on experimental results. An application on bending laboratory fatigue tests is presented to illustrate the applicability of the method for pavement engineering.     

A synthesis of the push‐pull fatigue behaviour of plain and notched stainless steel specimens by using the specific heat loss

The energy dissipated to the surroundings as heat in a unit volume of material per cycle, Q, was recently proposed as fatigue damage index, and it was successfully applied to rationalise fatigue data obtained by carrying out stress‐controlled and strain‐controlled fatigue tests on AISI 304 L stainless steel plain and hole specimens. In this paper, it is shown that the Q parameter is independent on thermal and mechanical boundary conditions occurring during experiments. After that, additional stress‐controlled fatigue tests on plain and notched specimens characterised by smaller notch tip radii than those tested previously have been performed. Present data have been compared with previous ones, and it was found that all available results can be synthesised in terms of the energy parameter Q into a unique scatter band, independently on the testing conditions (stress‐controlled or strain‐controlled) and on the specimens' geometry (plain or notched). About 100 data were included in the statistical analysis to characterise the energy‐based scatter band of the material. Finally, some limitations of applicability of the experimental technique adopted in the present paper are discussed.     

Analysis of Fatigue Damage under Block Loading in a Low Carbon Steel

Abstract: The fatigue damage accumulation behaviour of the P355NL1 steel is characterised using block loading fatigue tests. First, the constant amplitude low‐cycle fatigue behaviour of the P355NL1 steel is evaluated through strain‐controlled fatigue tests of smooth specimens. Both fatigue and cyclic elastoplastic behaviours are analysed. Then, block loading is applied to identify the key features of the fatigue damage accumulation phenomena for the P355NL1 steel. The block loading is composed of two distinct low‐cycle constant amplitude strain‐controlled blocks. The first block is applied for a predefined number of loading cycles, being followed by a second block which is applied until failure. The block loading illustrates that fatigue damage evolves nonlinearly with the number of load cycles as a function of the strain amplitude. These observations suggest a nonlinear damage accumulation rule with load sequence effects. The linear Palmgren–Miner's rule used extensively in design is not verified for the P355NL1 steel. Finally, using the generated experimental data, the cyclic elastoplastic behaviour of the P355NL1 steel is modelled using a continuum plasticity model with nonlinear kinematic hardening, available in the commercial finite element code ansys ®.