Biaxial tension fatigue response of concrete

Concrete structures such as rigid airport pavements are subjected to repeated high-amplitude loads resulting from passing aircraft. The resulting stress-state in concrete is a biaxial combination of compression and tension. It is of interest to understand the response of plain concrete to such loading conditions, which will enable development of realistic material models for implementation in mechanistic pavement design procedures.The objective of this work is to characterize the quasi-static and low-cycle fatigue response of concrete subjected to biaxial stresses in the biaxial tension region, where the principal tensile stress is larger than or equal in magnitude when compared with the principal compressive stress. An experimental investigation of material behavior in the biaxial tension region is conducted. The experimental setup consists of the following test configurations: (a) notched concrete beams tested in three-point bend configuration, and (b) hollow concrete cylinders subjected to torsion.Failure of concrete in the biaxial tension region is shown to be a local phenomenon under quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the equivalent elastic crack concept. It is observed that the crack growth rate in constant amplitude fatigue loading exhibits a two-phase process: a deceleration phase followed by an acceleration stage. The crack growth in the acceleration stage is shown to follow Paris law. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the considered biaxial fatigue response.     

A design procedure for assessing low cycle fatigue life under proportional and non-proportional loading

This paper discusses the design assessment for structural components subjected to proportional and non-proportional loading. Multiaxial low cycle fatigue lives are influenced by stress and strain multiaxiality, their non-proportionality and a material property that relates to the degree of additional hardening. Many low cycle fatigue studies under proportional and non-proportional loading were carried out in laboratories, but a little study discussed the application of the results obtained in laboratories to an actual design for structural components. This paper proposes a fatigue life assessment for structural components subjected to proportional and non-proportional low cycle fatigue loading. The assessment provides a simple method for evaluating principal strain range, strain multiaxiality and strain non-proportionality. This paper also discusses low cycle fatigue parameters suitable for the life assessment of structural components subjected to multiaxial loading.     

Fatigue crack nucleation and growth in filled natural rubber

Rubber components subjected to fluctuating loads often fail due to nucleation and the growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This investigation explores the nucleation and growth of cracks in filled natural rubber. Both fatigue macro‐crack nucleation as well as fatigue crack growth experiments were conducted using simple tension and planar tension specimens, respectively. Crack nucleation as well as crack growth life prediction analysis approaches were used to correlate the experimental data. Several aspects of the fatigue process, such as failure mode and the effects of R ratio (minimum strain) on fatigue life, are also discussed. It is shown that a small positive R ratio can have a significant beneficial effect on fatigue life and crack growth rate, particularly at low strain range.     

Monitoring of Fatigue in Welded Beams Using Piezoelectric Wafer Based Impedance Technique

Real-life aerospace and mechanical structures are mostly prone to fatigue, which is the occurrence of localized but progressive damage due to continuous fluctuating stresses. Fatigue damage can be monitored by observing changes in the structural stiffness resulting from strength reduction as a function of the number of loading cycles. The problem is more severe if the welds are subjected to such fatigue behavior. This paper employed non-destructive testing (NDT) method such as dye-penetrant and advanced NDT method such as the piezoelectric wafer based electromechanical impedance (EMI) technique to study the fatigue behavior of fillet welds in three specimens. Two specimens were tested in one single day, whereas the third on three consecutive days by unloading the specimen at the end of each day. Generally in laboratory, fatigue test is conducted by continuous cyclic loading/unloading till failure. However, the third specimen was subjected to intermittent fatigue to understand the stiffness changes which are due to the crack and its growth, or material realignment and partial strength restoration. Furthermore, cracks leading to failure are captured using EMI based signature analysis in these specimens. The signatures were further analyzed using statistical index, numerical modeling and peak shift measurements. The uniqueness of this paper is to present a comprehensive study of both load and non-load carrying welds by means of intermittent fatigue tests, using a thorough admittance signature analysis. The major observation of this study is that the fatigue behavior can be viewed as fusion of axial and transverse load changes coupled with crack severity, as observed from signatures.     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE LIFE

Abstract— The effect of non-proportional overloading on both low cycle and high cycle fatigue life has been studied. Low cycle multiaxial fatigue tests were performed on EN 15R (a low alloy steel) using sequential loading blocks which comprised uniaxial "ordinary" cycles and torsion "overload" cycles, and vice versa. In high cycle fatigue, the behaviour of mode I crack growth in a medium carbon steel subjected to mixed (I and II) mode overloading was examined.
Under tension-torsion sequential overloading, crack growth behaviour shows an earlier transition from Stage I to Stage II with a pronounced reduction in accumulated fatigue life. Tensile overloading on torsion cycles was found to be more damaging compared to torsion overloading on repeated tensile cycles. The crack-load interaction in sequential overloading and its influence on crack growth and fatigue life is discussed. In low strain fatigue, Stage II crack growth retardation closely relates to the overload plastic zone size, crack tip blunting and crack surface shielding. Mixed mode overloading is shown to have a significant effect only if the mode I component of overloading is large enough to keep the crack open. Under both low cycle and high cycle fatigue conditions non-proportional overloading is shown to be more damaging than proportional overloading.     

不同超弹性本构模型和多维应力下开裂能密度的计算方法

橡胶隔振器的疲劳失效多属于大变形(有限变形)下的多轴疲劳问题。在多轴疲劳载荷下,有效用来驱动裂纹扩展的那部分应变能密度称为开裂能密度。基于开裂能密度和橡胶材料的裂纹扩展特性预测橡胶部件的疲劳寿命时,须计算在外载荷下橡胶部件的开裂能密度。为了由有限元软件ABAQUS默认输出的应变计算在有限变形下的开裂能密度,该文推导了不同超弹性本构模型下开裂能密度在主坐标系下的计算式和所需的积分方法。基于该文开裂能密度的计算方法,采用3次Ogden本构来描述大变形下橡胶材料的本构行为,计算分析了不同应变状态下开裂能密度的分布特点。通过分析计算得到的开裂能密度与应变能密度的关系,说明该文开裂能密度计算方法的准确性。最后将上述计算方法应用到橡胶隔振器的多轴疲劳寿命预测中。     

Towards Prediction of Failure in Ti‐6AI‐4V Aerospace Materials by Surface Observations

Abstract: This paper deals with the development of a methodology for the prediction of material failure in metallic aerospace alloys by evaluating changes in surface characteristics directly prior to unstable fatigue crack propagation. The study is based on in situ nondestructive characterisation of the depression zone ahead of the crack tip of fatigue‐pre‐cracked titanium alloy specimens subjected to static loading. A relationship between the surface characteristics of the deformation zone ahead of the crack and the stress intensity factor of the material was obtained. This relationship was common to a variety of microstructural conditions such as mill‐annealed and β‐annealed microstructures. Based on the analysis, prediction of the impending fracture in cracked samples of the material was enabled. The outcome of this study can be used for optimising the service life of structural components.     

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.     

Fatigue of sandwich structures with inserts

The main purpose of the study was to evaluate fatigue in sandwich structures with inserts. Cross-linked PVC foam with closed cells as the core material was used in this investigation because this foam type has gained widespread use for maritime applications. Fatigue tests for a four-point bending test on a sandwich-beam containing an insert were investigated using numerical fatigue calculations in addition to the finite element method. A small crack initiated in the numerical model was propagated by reducing the stiffness of the finite elements at the crack tip. The J-integral was used when estimating the energy release rate. The stiffness of the insert material was used as a design parameter. It was seen from the investigation that the stiffness of the insert material had almost no influence on the fatigue crack propagation.     

Mixed-mode fatigue crack growth under biaxial loading

Studies on crack growth in a panel with an inclined crack subjected to biaxial tensile fatigue loading are presented. The strain energy density factor approach is used to characterize the fatigue crack growth. The crack growth trajectory as a function of the initial crack angle and the biaxiality ratio is also predicted. The analysis is applied to 7075-T6 aluminium alloy to predict the dependence of crack growth rate on the crack angle. The effect of crack angle on the cyclic life of the component and on the cyclic life ratio is presented and discussed.     

CRACK GROWTH AND PREDICTION OF ENDURANCE IN THERMAL-MECHANICAL FATIGUE OF 12Cr—Mo—V—W STEEL

Abstract Fractography and fracture mechanics were used to study the behaviour of a crack that emanated from a starter notch on a specimen surface subjected to thermal cycles, and which grew in the direction perpendicular to the surface, under out-of-phase loading conditions. A 12Cr–Mo–V–W steel was used as representative of an elevated temperature material, and one that is frequently employed for vessels or heat-exchanger tubes. A low power travelling microscope observed crack length changes on the specimen surface. It was shown that the growth rate of a part-through crack could be expressed in terms of a strain intensity factor range and two constants, and that the striation spacings agreed with the microscopically observed crack growth rate. The integration of the crack growth rate equation gave an approximate evaluation of thermal fatigue life of smooth specimens.     

Static analysis fatigue and fracture of cracked beams on elastic foundation

In this paper an analysis of a long edge-cracked beam resting on elastic foundation and with rotational and translational restraints at the ends is presented. The simplified Euler-Bernoulli beam theory is employed with the cracked section represented as an elastic hinge. By using the Paris-Erdogan crack propagation law, the fatigue life of the cracked beam is represented as a function of the constraints at the ends, the initial depth of the edge-crack, its location along the beam and the stiffness of the elastic foundation. The bending moment redistribution caused by the crack growth is analyzed and graphically illustrated. The retarding effect on the crack growth rate in the case of redundant structures subjected to repeated loading is discussed.     

Durability assessment via residual strength and viscoelastic observations on filled rubber compounds

Because rubber compounds are widely used under cyclic loading conditions, such as tire applications, their fatigue behaviour has been studied for a long time. Two main stages are commonly considered in studying rubber fatigue: crack nucleation and crack growth. Not many studies exist on the residual strength of rubber subsequent to fatigue loading. In this study, the residual strength method is used to model fatigue behaviour of carbon black‐filled and silica‐filled styrene‐butadiene rubber (SBR) compounds. Samples are subjected to repeated fatigue loading (ie, nonzero mean stress) using different strain amplitudes and then subjected to uniaxial constant crosshead rate, relaxation, and creep tests to assess their residual strength and viscoelastic behaviours respectively. The residual strength results are compared with typical S‐N curves. Initial relaxation rates, initial creep rates, asymptotic relaxation values, and secondary creep rates are plotted as functions of fatigue cycle number to understand the viscoelastic behaviours of carbon black and silica‐filled SBR compounds as affected by fatigue processes.     

Corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads

The corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads was examined. This aluminum alloy is typically used in aerospace structural components such as the wing spars of aircraft. Axial fatigue specimens were subjected to a loading spectrum that consisted of a fully reversed periodic overload of near-yield magnitude followed by 200 smaller cycles at high R-ratio. The specimens were fatigue tested while they were fully immersed in an aerated and recirculated 3.5 wt% NaCl simulated seawater solution.The results for the corrosion-fatigue testing were compared to data obtained for the same overload spectrum applied in laboratory air. A damage analysis showed that the presence of the corrosive environment accelerated the damage accumulation rate to a greater extent than that observed in air, particularly at low stress ranges. This resulted in a reduction in the fatigue strength of the material when it was simultaneously subjected to overloads and a corrosive environment. It is believed that the reduced fatigue life was due primarily to corrosion pit formation and a combination of anodic dissolution at the crack tip and hydrogen embrittlement. For practical purposes, the endurance-limit of the material disappears under these conditions.     

Fatigue characterization of a polymer foam to use as a cancellous bone analog material in the assessment of orthopaedic devices

Analog materials are used as a substitute to cancellous bone for in vitro biomechanical tests due to their uniformity, consistency in properties and availability. To date, only the static material properties of these materials have been assessed, although they are often used in fatigue tests. Cancellous bone exhibits complex material behavior when subjected to fatigue loads, including modulus degradation, accumulation of permanent strain and increasing hysteresis. Analog materials should exhibit similar fatigue behavior to cancellous bone if they are to be used in cyclic loading tests. In our study, a polymer foam (commercial name HEREX C70.55) has been studied for its static and fatigue behavior and compared with that of cancellous bone. In compression, the foam exhibited qualitatively similar mechanical behavior, but the degree of modulus degradation and accumulation of permanent strain was lower than expected for cancellous bone. In general, the tensile properties of the foam were greater than found in compression, the opposite to the mechanical behavior of cancellous bone. The methodology employed here could form the basis of selecting suitable analog materials for cancellous bone in the future.     

Cyclic fatigue of ceramics

The fatigue behaviour of alumina, zirconia-toughened alumina (ZTA) and tetragonal zirconia (TZP) have been investigated using three different techniques. Direct push-pull testing has been used to generate both static and cyclic fatigue data. The results clearly show that all the materials studied are susceptible to both static and cyclic fatigue, and that the times to failure under cyclic loading are considerably shorter than under static loads. The fatigue failure origins have been identified and the influence of surface condition on fatigue life has been assessed. The slow propagation of cracks subject to cyclic tensile and compressive loads has been studied using compact tension specimens and tapered double cantilever beam specimens. These investigations have confirmed the existence of cyclic fatigue effects in coarse-grained alumina and have shown the crack increment per cycle (da/dN) to have a power-law dependence on the peak stress intensity factor. A technique, based on repeated indentation, has been used to investigate the propagation of sub-surface cracks subjected to cyclic loading in both fine-grained alumina and ZTA. The results of the investigation suggest that compressive or closure loads on the crack faces are factors which affect the cyclic fatigue crack growth in ceramics. Based on those observations, an explanation is proposed for the mechanical cyclic fatigue effects in the ceramics investigated.     

Estimating durability of steels at repeated bending impacts

Many components are subjected to repeated impacts, or in some cases these impacts can appear as additional loads. Repeated impacts define a fatigue phenomenon known under the name of Impact Fatigue. Because the strain rate changes the material characteristics it is to expect that the material properties at impact fatigue to be different in regard to those obtained at non-impact fatigue. This paper presents a classification of repeated impact tests, and starting from this a series of parameters used for durability estimation will be analyzed. The high number of parameters used by different authors creates difficulties in comparison the different laboratories results. The importance of the shape and dimensions of specimens, and the stiffness of supports are highlighted. In order to avoid these influences the authors proposed an experimental technique, based on testing of Charpy specimens, in similar conditions as single impact test. A new parameter η is proposed in order to correlate the durability at repeated impacts with the Charpy V Notch (CVN) impact energy.     

高孔率泡沫金属材料疲劳表征模型及其实验研究

通过基于高孔率开口泡沫金属材料结构特点的简化结构模型和受力状态分析,建立了此类材料在循环载荷作用下的负载结构-疲劳模型,分析得出了对应疲劳性能的衡量指标。在上述模型的基础上,运用由该模型得出的高孔率开口泡沫金属疲劳性能的衡量指标,以电沉积法所得泡沫镍为例,对此类材料的疲劳性能进行了相关的实验研究。通过压-压循环和弯曲循环两种载荷作用的实验,验证了理论分析所得疲劳性能衡量指标的可行性。结果表明:泡沫镍在压-压循环载荷作用下的类应力疲劳性能随孔率增大而降低,而在弯曲循环载荷作用下的类应变疲劳性能则随孔率增大和孔径减小而提高。     

FATIGUE CRACK GROWTH IN DUCTILE MATERIALS UNDER CYCLIC COMPRESSIVE LOADING

Abstract— Mode I fatigue crack growth has been studied in notched specimens of 7017-T651 aluminium alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent compressive cyclic loading in laboratory air, a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented.     

Creep fatigue of a directionally solidified Ni‐base superalloy – smooth and cylindrically notched specimens

Turbine blade life modelling is complicated by the presence of notches, dwells, high temperatures, thermal cycles and temperature gradients. Furthermore, directionally solidified (DS) Ni‐base superalloys are highly anisotropic. This work seeks to characterize the response of the DS Ni‐base superalloy CM247LC subjected to isothermal low cycle fatigue at either 750 or 950 °C. This study considers the effects of strain rate, dwells at the maximum temperature, and stress concentrations. Experiments were conducted under uniaxial loading on smooth and cylindrically notched round‐bar specimens in both longitudinal and transverse orientations. The location of the creep‐fatigue crack is at the maximum Hill's effective stress in the notched specimens. In addition, the notch behaviour is discussed in light of finite element analysis using an anisotropic elastic‐crystal viscoplastic material model.