Fatigue Fracture of Concrete Subjected to Biaxial Stresses in the Tensile C–T Region

In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression–tension (C–T) biaxial stress are presented. In the tensile C–T region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor.     

Influence of residual stresses and loading frequencies on corrosion fatigue crack growth behavior of weldments

The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress and the loading frequency effects.     

Analysis of Scaling and Instability in FRP-Concrete Shear Debonding for Beam-Strengthening Applications

The debonding mode of failure, which is observed in girders strengthened using externally attached fiber-reinforced polymer (FRP) sheets, is studied in this paper. A numerical analysis of the direct-shear response of FRP attached to concrete substrate is performed to study the initiation, formation, and propagation of an interfacial crack between the two adherents. The material response of the bimaterial interface, which includes postpeak softening, is incorporated into the numerical model. The load response obtained numerically is shown to be in close agreement with that determined experimentally from direct shear tests on concrete blocks strengthened with FRP sheets. An instability in the load response is predicted close to failure and the arc-length method is used to obtain the entire load response past the displacement-limit point. The instability in the load response is shown to be a result of snapback, where both the load and the displacement decrease simultaneously. The effect of the bonded length on the stress transfer between the FRP and concrete and on the ultimate failure is also analyzed. It is shown that there is a scaling in the load capacity when the bonded length does not allow for the establishment of the full stress-transfer zone associated with interface crack growth. From the results of the numerical analysis, a fundamental understanding of interfacial crack propagation and instability at failure in concrete members strengthened using externally bonded FRP is developed. Using a simple energy based formulation; it is shown that in strengthened girders, the instability at complete debonding of FRP from concrete translates into an explosive failure associated with a sudden release of energy.     

Growth of small fatigue cracks in PH 13-8 Mo stainless steel

The growth of small fatigue cracks in PH 13-8 Mo (H1050) stainless steel under constant amplitude loading at different mean stresses (R=0.1 and −1) under generally high cycle fatigue conditions was investigated. Small cracks were allowed to initiate naturally at the root of a single edge notch specimen and were monitored using a surface replicating technique. It was found that the initiation and growth of surface cracks up to 100 μm encompassed 70 to 90 pct of the total fatigue life at stress amplitudes just above the fatigue limit. Cracks of length less than 100 μm were subject to strong influences of the microstructure and exhibited stage I (shear-dominated) growth, which was manifested in oscillatory crack growth rates. The oscillations diminished as the crack transitioned to stage II growth. The higher stress ratio (R=0.1) resulted in a more rapid transition from stage I to stage II growth in comparison to R=−1. After transitioning to stage II, the crack growth could be well characterized by conventional long crack tools even when the crack was still physically small. The small crack growth behavior is shown to be similar to that of a quenched and tempered AISI 4340 steel having a comparable strength.     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.      

Experimental Analysis of Crack Growth in GFRP Reinforced Concrete

For decades, bridge slabs have been troubled by the corrosion of steel reinforcement. The unique corrosion resistance of glass fiber-reinforced polymer (GFRP) bars makes them a promising alternative to steel bars. Experiments have been conducted to investigate the bond performance of GFRP reinforced concrete under constant amplitude cyclic fatigue loading. Each specimen was an identical length beam with a single GFRP bar at the bottom, intended to simulate a transverse strip of a typical bridge deck slab. The crack growth was monitored for specimens of different widths, simulating different transverse reinforcement spacings. Up to 2?million?cycles of cyclic loads were applied at 100% typical service load levels. No fatigue failure was encountered in the testing. The effects of moderate overloads were also investigated.     

A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展及其数值模拟

为探究不同加载角度下A7085铝合金Ⅰ-Ⅱ复合型疲劳裂纹扩展机理,在MTS疲劳试验机上采用紧凑拉伸剪切试件（CTS）对A7085铝合金进行不同加载角度的疲劳实验;用有限元分析计算不同裂纹扩展长度的裂纹尖端应力强度因子,通过七点递增多项式法对数据进行处理,计算出A7085铝合金Paris公式中的参数C和m.结果表明不同加载角度的裂纹基本沿着与外载荷垂直的方向扩展,裂纹扩展路径近似为一条直线,裂纹扩展角测量结果基本符合最大环向拉应力理论;Ⅰ-Ⅱ复合型裂纹一旦发生扩展,Ⅱ型应力强度因子K_Ⅱ所占比例急剧减小,Ⅰ型应力强度因子K_Ⅰ不断增大,此后K_Ⅱ远远小于K_Ⅰ,有效应力强度因子（K_Ⅰ和K_Ⅱ的组合）基本等于K_Ⅰ,相当于裂纹扩展主要受Ⅰ型应力强度因子控制,研究结果有助于对Ⅰ-Ⅱ复合型疲劳裂纹扩展机理的理解.      

基于声发射监测的316LN不锈钢的疲劳损伤评价

疲劳裂纹的萌生与扩展容易导致压力容器及管道的严重疲劳失效.因此就设备的安全可靠性而言,非常有必要对疲劳裂纹扩展过程进行监测,并对疲劳损伤程度进行评估.本文针对316LN不锈钢材料进行疲劳实验研究,利用直流电位法测量实验中的裂纹长度,得到了材料的疲劳裂纹扩展曲线.利用声发射技术对疲劳裂纹扩展过程进行监测,通过声发射多参数分析对疲劳损伤状态进行评价,同时建立了声发射参数与线弹性断裂力学参数之间的关系,并进行寿命预测.研究表明:声发射能够对316LN不锈钢的疲劳裂纹损伤进行有效评估,声发射累积参数如累积计数、累积能量和累积幅值曲线上的转折点标志着疲劳裂纹进入快速扩展阶段,这可以为工程人员提供失效预警;声发射波形和频谱分析表明,噪声信号的幅值较小且信号持续时间较长,信号包含的频率成分比较复杂,而裂纹扩展信号是突发型信号,衰减较快,信号频率主要集中在80~170 kHz范围内;声发射计数率、能量率和幅值率与应力强度因子幅度以及疲劳裂纹扩展速率之间呈线性关系,裂纹长度预测结果与实测值接近.本研究工作对于工程结构的疲劳失效预警和剩余寿命预测具有重要意义.      

Significance of the small crack growth law and its practical application

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σ _a ⁿ l (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σ _a ⁿ l and ΔK. The applicable conditions of the two parameters were discussed and manifested.     

Effect of compression point load on the path and life of fatigue crack growth in mixed mode loading

In this study, fatigue crack growth of edge crack in plate made from 5083 aluminum alloy under mixed mode fracture was numerically investigated. Effect of various parameters include initial crack direction angle (30°, 60°, 90°), initial crack length (20 and 25 mm) and load ratio (R=0 and 0.25) were considered. We also consider the effect of point compression load on crack growth life with 20 mm initial crack length under 30°, 60°, 90° at various parts of crack growth path. Mentioned models were simulated using the finite element softwares, ABAQUS and FRANC2D and their results were compared together. For validation we compare the numerical results for a specimen with its experimental data. Numerical results show good agreement with the experimental data for fracture path. It is observed that the point compression load can affect the path and life of crack growth in fatigue loading. If this load was applied in the first cycles of crack growth, its effect is more than another cases. For instance in the crack growth with initial angle 30o applying point load in 5, 10 and 15 mm of crack growth length, increase the life 79.4%, 18.9% and 6.2% respectively.     

Matrix cracking in unidirectional ceramic matrix composites under quasi-static and cyclic loading

Experimental data associated with the development of matrix cracks in unidirectional continuous silicon carbide fibre/calcium alumino-silicate matrix laminates under quasi-static loading are presented, including crack density, residual strain and hysteresis behaviour as functions of applied stress. Simple models are developed, based on an assumption of purely frictional load transfer between the fibre and matrix, which describe reasonably well the quasi-static stress-strain behaviour in the presence of cracks. Under tension-tension mechanical fatigue cycling it is found that the crack density stabilises at a relatively early stage in the test. Based on the quasi-static model, the changes in fatigue hysteresis loops on fatigue cycling are interpreted in terms of a reduction in the effective frictional interfacial shear stress.     

基于有限元方法的疲劳裂纹闭合行为

裂纹闭合行为将很大程度改变疲劳裂纹扩展行为。针对316L不锈钢,结合常幅加载和单个拉伸过载试验和动态数值模拟方法,对疲劳裂纹扩展行为中的裂纹闭合现象开展了一系列研究工作。详细对比了不同扩展阶段的裂纹闭合行为随裂纹长度、应力比和过载影响因素的变化,以及对裂纹扩展速率的影响。同时,研究了单个拉伸过载和裂纹闭合行为之间的内在联系和机理。结合裂纹闭合理论和有限元计算结果,等效应力强度因子被用来描述316L不锈钢的裂纹扩展过程,并提出316L不锈钢的裂纹扩展速率的预测模型。     

循环扰动荷载作用下花岗岩中裂隙萌生扩展过程的颗粒流模拟

从细观角度、采用颗粒离散元法开展了预制裂隙花岗岩循环加卸载的数值模拟试验。首先,使用图像处理技术识别花岗岩中的不同细观组分、结合室内单轴压缩试验结果对细观力学参数进行了标定。然后,通过编制颗粒流代码追踪裂隙的类型和扩展过程,分析岩石破坏过程中裂隙发展的阶段性特征。结果表明:不同倾角裂隙岩石的新生裂隙走向与预制裂隙贯通方向基本一致;根据新生裂隙的优势倾向分组得到裂隙起裂角与预制裂隙倾角的关系:倾角β≤45°时剪切和张拉裂隙的起裂角单调递减,倾角β≥60°时剪切和张拉裂隙的起裂角单调递增;循环扰动荷载增加了裂隙岩体的轴向变形,轴向累积残余应变曲线呈反S形、提高扰动荷载应力上限促使曲线进入加速阶段;试件峰值强度随裂隙倾角增大表现出先减小后增大的趋势,峰值强度为实验室完整岩石单轴抗压强度的63% ~ 89%,反映了较为明显的劣化现象;在循环荷载作用下,剪切裂隙和张拉裂隙增长曲线表现出明显的变化特点,在裂隙不稳定扩展阶段中张拉裂隙数目增长速率显著大于剪切裂隙,对分析岩石变形破坏过程具有一定的参考意义。      

Fatigue Crack Growth Analysis Under Spectrum Loading in Various Environmental Conditions

The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic–plastic stresses–strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress–strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith–Watson–Topper (SWT) fatigue damage parameter, D = σ_maxΔε/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, ΔK ^p K _max ^1?p . The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth model. The method can be also used to predict fatigue crack growth under constant amplitude and spectrum loading in various environmental conditions such as vacuum, air, and corrosive environment providing that appropriate limited constant amplitude fatigue crack growth data obtained in the same environment are available. The proposed methodology is equally suitable for fatigue analysis of smooth, notched, and cracked components.     

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

Micromechanism of accelerated near-threshold fatigue crack growth in Al2O3/Al-Cu composite at elevated temperature

Fatigue crack growth behavior of a peak-aged Al₂O₃/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.     

The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.     

The early stage of fatigue crack propagation in al 2048

Microcracks which develop at the surface of Al 2048 subjected to fully reversed fatigue loading are found to be partially open at zero load. The magnitude of such residual crack openings measured at the test specimen surface is determined as a function of the crack length and the relative humidity in which the specimens were fatigued. For a fixed crack length the residual crack openings are found to be largest if the relative humidity is low or if the crack path is particularly irregular. The present interpretation of the phenomenon is that the residual crack openings are indirectly related to the crack closure load associated with the microcracks.     

Failure Process of Granite

The initiation, development, and coalescence of cracks when a rock specimen is loaded have been studied by various researchers. The current consensus is that such complex process is however not yet fully understood. In the present study, the problem has been revisited by carrying out extensive testing on granite specimens. Each specimen was loaded under uniaxial loading condition to various load levels in both prepeak and postpeak ranges. After unloading, the crack patterns were printed onto the surface of a piece of resin. The images of the patterns were then captured using a digital camera through optical microscope or scanning electron microscope. Statistical analyses on these images were then carried out using an image analysis software. The study focuses on the cracking behavior of the various mineral components of the granite and their respective roles in the failure process. In addition, a numerical model was developed to simulate the test results and parametric study was carried out to investigate the effects of various factors (heterogeneity index, C/T ratio, grain arrangement, and grain size, etc.) on the failure behavior of the granite. The study has provided insight on the development of cracks, especially in the postpeak range. A better understanding on the roles of the major minerals in failure process of the granite has been achieved.     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University