Fatigue life prediction for boom structure of concrete pump truck

Frequent premature fatigue failure of construction machinery has spurred the demand for fatigue life prediction of such equipment. In this study, fatigue cracks were investigated in a concrete pump truck boom typically a high-strength steel plate welded box girder structure. The focus was two-tip corner cracks which frequently initiate at the weld toe between the top flange plate and the web plate because these are commonly observed in the field. A fatigue crack growth numerical approach for three-dimensional shell problems was proposed to simulate fatigue crack growth in cracked structures. Fatigue experiments were performed on a full-scale cracked boom to validate the effectiveness of this approach. The influence of the initial crack length was examined using the proposed approach. Numerical results revealed that variation in the initial crack length can affect the remaining fatigue life. For engineering convenience, an interpolation method was employed to determine the remaining fatigue life of a cracked boom with an arbitrary initial crack length. The comparisons between interpolation and numerical results demonstrate that the interpolation method can be used to guide repair decisions with reasonable precision.     

有限板共线多孔MSD疲劳裂纹扩展有限元模拟

介绍了二维断裂分析有限元软件FRANC2D/L在疲劳裂纹扩展模拟方面的基本步骤．利用该软件对有限板中心孔边对称裂纹的疲劳裂纹扩展进行模拟计算,对比模拟结果和试验数据,发现两者吻合良好,证明了利用该方法模拟疲劳裂纹扩展的可靠性．将FRANC2D/L应用到有限板共线多孔MSD疲劳裂纹扩展的有限元模拟上,得到了各孔边裂纹的长度和疲劳扩展寿命之间的关系曲线．模拟计算结果表明,在相同条件下,有限板中心孔边对称裂纹的裂纹扩展寿命要远远高于MSD结构中中心孔边裂纹的疲劳扩展寿命;由于MSD结构中影响各孔边裂纹的因素有所差异,各条裂纹的疲劳扩展寿命也会有所不同．另外,还给出了不含主裂纹的MSD和含主裂纹的MSD两种情况下的疲劳裂纹扩展历程,通过比较得知,含主裂纹的MSD结构更容易发生裂纹的合并和贯穿致使结构发生破坏．     

VARIABLE AMPLITUDE LOADING OF SMALL FATIGUE CRACKS IN 6261-T6 ALUMINIUM ALLOY

Abstract— Constant and variable amplitude (VA) loading fatigue studies were carried out on a 6261 aluminium alloy using cylindrical plain hour-glass specimens. Crack growth was monitored via surface replication using cellulose acetate.
Crack growth results at constant amplitude loading show the typical intermittent high and low periods of growth rate associated with crack-microstructure interactions. Acceleration in growth rate during an overload block depends on crack length and stress amplitude ratio. It appears to pass through a maximum at a crack length corresponding to the first microstructural barrier. Microstructural-based modelling is therefore required for small fatigue cracks, rather than solely closure-based modelling. The Navarro-de los Rios model of short fatigue crack growth appears able to provide good indications of crack growth rates under VA block loading, and gives reasonable life predictions.
For short cracks (surface length < 80 μm) and a small overload ratio (6.7%), crack growth may show severe retardation during the overload block. This is ascribed to crack tip blunting being more important than the increase in stresses when closure is low. It appears from a Miner's rule type exercise, that VA block loading has its major effect on growth at a surface crack length of 20 μm. This means that the crack initiation period cannot be ignored in life prediction models for small fatigue cracks.     

SMALL FATIGUE CRACKS IN AN AUSTENITIC STAINLESS STEEL

The present study concerns nucleation and growth of small surface cracks during the low-cycle fatigue of a nitrogen-containing austenitic stainless steel. Metallographic replicas as well as longitudinal sectioning were used to record the developing crack pattern on the specimen surface. The influence of grain size and nitrogen content is considered. Small surface cracks are observed after about 10% of the fatigue life. The nucleation of cracks continues until about half of the lifetime, when the crack density saturates. This saturation phenomenon is related to the local unloading effect of growing cracks.
The mean crack length increases continuously as a power-law until specimen failure. However, small grains and a low nitrogen content amplify the effect of crack–grain boundary interactions resulting in an intermediate retardation in growth.
At high nitrogen contents, the crack growth characteristics are very much related to the slip bands formed. This results in a more simultaneous growth of cracks, a more jagged feature of the cracks introducing a higher roughness-induced crack closure effect, and, consequently, better fatigue properties.     

Fatigue crack growth simulation in a first stage of compressor blade

A first-stage rotary compressor blade of a Model GE-F6 gas turbine failed due to vibration in early March 2008. Initial investigations showed that pitting on the pressure side of the blade caused micro cracks, leading to larger cracks due to high cycle fatigue. To assess this failure, a series of experimental, numerical, and analytical analyses were conducted. Fractography of the fractured surface of the blade indicated that two semi-elliptical cracks incorporated and formed a single crack. In this study, static and dynamic stress analyses were performed in Abaqus software. Moreover, fracture mechanics criterion was accomplished to simulate fatigue crack growth. This was carried out using a fracture analysis code for 3-dimensional problems (Franc3D) in two states. Firstly, stress intensity factors (SIFs) for one semi-elliptical surface crack and then SIFs for two semi-elliptical surface cracks were taken into account. Finally, the Paris and Forman–Newman–De Koning models were used to predict fatigue life. Since stress level and crack shape in both conditions are the same and the SIF at the crack tip reaches the fracture toughness of the blade, SIFs results indicate that insertion of a second crack has no effect on the final SIF, however, the second crack facilitates the process of reaching the critical length. So, fatigue life in two-crack condition is less than in the one-crack state.     

考虑结构裂纹扩展的振动疲劳寿命计算方法

工程实践中任何结构都存在不同程度的裂纹损伤,振动激励下动响应与疲劳裂纹扩展之间互相耦合,直接影响结构振动疲劳寿命.为了考虑结构振动疲劳耦合效应对疲劳寿命的影响,提出了一种考虑结构裂纹扩展的振动疲劳寿命计算方法.分析时,通过建立若干个含不同长度裂纹的结构有限元模型模拟结构裂纹扩展,采用Paris方程分段计算结构振动疲劳裂纹扩展寿命,通过试验确定的固有频率降变化规律反推结构裂纹萌生寿命,最后累计得到结构疲劳总寿命.结论表明,仿真计算结果与试验结果比较吻合.     

THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

Matrix fatigue cracking in fiber composites

A model is developed for fatigue growth of matrix cracks in metals reinforced with aligned continuous elastic fibers. The mechanics of elastic cracks bridged by frictionally constrained fibers is used to develop the model, which provides estimates of the tip value of the stress intensity factor amplitude, ΔK_TIP. It is found that when the applied load amplitude is held fixed during fatigue crack growth, ΔK_TIP, and thus the rate of growth approach an asymptotic value independent of crack length. The residual strength after fatigue crack growth is also discussed. In some cases, the residual strength is unaffected by prior fatigue growth. But, in another regime, the matrix crack length allows fibers to begin breaking before the matrix crack grows. The strength is then inversely proportional to the square root of fatigue crack length.     

Fatigue life and crack closure in specimens subjected to variable amplitude loads under plane strain conditions

The propagation of fatigue cracks in specimens subjected to variable amplitude loading under plane strain conditions was investigated experimentally and numerically, to find the influence of the variable amplitude loading on the stabilised crack closure level. Experiments on four-point-bend specimens with a Gurney block load scheme, showed that the crack closure level depends on the crack length but not on the stress range of the fluctuations. Numerical simulations performed in the fatigue crack growth program FASTRAN-II showed good agreement with the experimental results. In addition, statistical uncertainty analyses performed on the fatigue life show that, for technical applications, the uncertainties in initial crack length and load levels have a greater influence on the uncertainty in fatigue life, than the fluctuation level of the load.     

颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和扩展行为的影响

通过原位扫描电子显微镜(SEM)研究了粉末冶金制备的Cu/WCp复合材料的疲劳裂纹萌生和扩展行为,分析了颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和早期扩展行为的影响。结果表明:疲劳微裂纹萌生于WCp颗粒和基体Cu的界面;微裂纹之间相互连接并形成主裂纹,当主裂纹和颗粒相遇时裂纹沿着颗粒界面扩展。在低应力强度因子幅ΔK区域疲劳小裂纹具有明显的"异常现象",并占据了全寿命的71%左右。疲劳小裂纹的早期扩展阶段易受局部微观结构和颗粒WCp的影响,扩展速率波动性较大,随机性较强;当小裂纹长度超过150μm时,裂纹扩展加快直至试样快速断裂。裂纹偏折、分叉和塑性尾迹降低了疲劳裂纹扩展速率,而颗粒界面脱粘则提高了复合材料的疲劳裂纹扩展速率。通过数值模拟也可以发现颗粒脱粘增大了材料的疲劳扩展驱动力,从而提高了疲劳裂纹扩展速率。     

A MICRO-MECHANICS ANALYSIS FOR SHORT FATIGUE CRACK GROWTH

Crack initiation and early growth of fatigue cracks in a fully annealed 0.4% carbon steel was investigated using plastic replicas and torsion loading. In a structure consisting of a 70/30 mixture of pearlite and ferrite the cracks are seen to develop and grow initially along slip bands in the ferrite phase. Energetic considerations lead to the formulation of a model which, while characterizing short crack growth rate, also considers those microstructural variables relevant to fatigue crack initiation and early crack growth. The driving force for crack growth is provided by the energy of the slip band; correspondingly crack growth per cycle is proportional to the strength of the slip band. In the short fatigue crack region, cracks grow initially at a fast rate but deceleration occurs quickly and, depending on the stress level, they either arrest or are temporarily halted at a critical length. This critical length is shown to coincide with the value of the threshold length for crack growth under LEFM conditions.     

In situ investigation of small fatigue crack growth in poly-crystal and single-crystal aluminium alloys

Abstract   In situ scanning electron microscope observations of short crack growth in both a poly-crystal and a single-crystal alloy revealed that fatigue cracks may grow in a shear decohesion mode over a length that is several times the grain size, far beyond the conventional stage I regime. In the poly-crystal aluminium alloy 2024-T351, fatigue cracks were found to continue to grow along one shear band even after two mutually perpendicular shear bands had formed at the crack tip. For the single-crystal alloy specimen with the loading axis being nearly perpendicular to its main shear plane, mode I fatigue cracks were found to grow along the shear band. These two types of fatigue crack growth pose a significant challenge to the existing fatigue crack growth correlating parameters that are based on crack-tip opening displacement. In particular, it has been found that the cyclic crack-tip opening displacement, which accounts for both large-scale yielding and the lack of plasticity-induced crack closure, is unable to unify the growth rates of short and long cracks in aluminium 2024-T351, suggesting a possible dependence of crack growth threshold on crack length.     

A SIMULATION OF THE FATIGUE CRACK PROCESS IN TYPE 304 STAINLESS STEEL AT 538°C

Abstract— Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, were obtained from fatigue tests on type 304 stainless steel at 538°C in a previous study. Using this information as input data and a condition for the connection and interactions of cracks, a statistical simulation and life prediction due to the high temperature fatigue crack growth process was performed and illustrated as output on a two-dimensional graphical display.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

Ermüdungsrißwachstum in Kerben

Fatigue Crack Growth in Notches Nowadays it is wellknown that an important part of the fatigue life time, usually differenciated in crack initiation and crack growth, is often controlled by fatigue crack growth of cracks in notches. An elastic-plastic on the J-integral based crack growth model considering the crack opening and closure phenomenon will be described to determine crack growth of cracks in notches between crack initiation and failure. Experimental results and finite element analysis were used to verify the developed model.     

Numerical simulation of thermal fatigue behavior in a cracked disc of AISI H-11 tool steel

Thermal fatigue is one of the key reasons of material failure in components which are exposed to high temperature cycles. Prediction of crack initiation site and crack propagation speed during thermal cycling helps us predict the life of a component in service. In this research numerical simulation of crack propagation due to thermal cycling on a circular disc has been carried out. The thermal profiles used in the simulation are taken from actual thermal fatigue experiments. The effect of the length of cracks and interaction between adjacent cracks has been investigated. 50 sets of numerical simulation models with different crack numbers and crack lengths have been simulated. The variation in Stress Intensity Factor (SIF), hoop stress and Crack Mouth Opening Displacement (CMOD) has been plotted as a function of primary/secondary crack lengths and number of cracks. Envelopes of possible crack growth have been identified and correspond well to the experimental observations. Results show a significant drop in hoop stress, SIF and CMOD with increase in number of cracks, thus limiting the number of cracks possible in a thermal fatigue crack network.     

A predictive fatigue life model for anodized 7050 aluminium alloy

The objective of this study is to predict fatigue life of anodized 7050 aluminum alloy specimens. In the case of anodized 7050-T7451 alloy, fractographic observations of fatigue tested specimens showed that pickling pits were the predominant sites for crack nucleation and subsequent failure. It has been shown that fatigue failure was favored by the presence of multiple cracks. From these experimental results, a fatigue life predictive model has been developed including multi-site crack consideration, coalescence between neighboring cracks, a short crack growth stage and a long crack propagation stage. In this model, all pickling pits are considered as potential initial flaws from which short cracks could nucleate if stress conditions allow. This model is built from experimental topography measurements of pickled surfaces which allowed to detect the pits and to characterize their sizes (depth, length, width). From depth crack propagation point of view, the pickling pits are considered as stress concentrator during the only short crack growth stage. From surface crack propagation point of view, machining roughness is equally considered as stress concentrator and its influence is taken into account during the all propagation stage. The predictive model results have been compared to experimental fatigue data obtained for anodized 7050-T7451 specimens. Predictions and experimental results are in good agreement.     

FATIGUE MICROCRACKS IN A LOW CARBON STEEL

Abstract— The behaviour of a low carbon steel has been studied, in particular the initiation, growth and coalescence of fatigue microcracks on the surfaces of smooth specimens via surface replicas and photomicrographs. From the study, quantitative information on the initiation period, growth and coalescence of cracks, statistical distributions of crack length and crack depth, density of cracks, distribution pattern and characteristics of the major crack, were obtained. Knowledge of these parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life assessment.     

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.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.