Fatigue crack growth for a surface crack in a round bar under multi-axial loading condition

In this paper, the fatigue life, surface crack extension direction and crack growth rate in an elastic bar with a circular cross section are determined through experiments under cyclic torsion with axial static and cyclic tension/compression loading. The effects of the loading type, loading value and stress ratio on the crack growth behaviour are discussed. The results show that, under pure fatigue torsion loading, the crack extension direction is almost the same whatever the value of torsion loading. Under fatigue torsion with cyclic tension loading, it is found that the crack extension direction is mainly determined by the alternating parts of the stresses and is almost independent of the average parts of the stresses, whereas the fatigue life is obviously dependent on the average stress.     

Non‐planar mixed‐mode growth of initially straight‐fronted surface cracks,in cylindrical bars under tension,torsion and bending,using the symmetric Galerkin boundary element method‐finite element method alternating method

In this paper, the stress intensity factor (SIF) variations along an arbitrarily developing crack front, the non‐planar fatigue‐crack growth patterns, and the fatigue life of a round bar with an initially straight‐fronted surface crack, are studied by employing the 3D symmetric Galerkin boundary element method‐finite element method (SGBEM‐FEM) alternating method. Different loading cases, involving tension, bending and torsion of the bar, with different initial crack depths and different stress ratios in fatigue, are considered. By using the SGBEM‐FEM alternating method, the SIF variations along the evolving crack front are computed; the fatigue growth rates and directions of the non‐planar growths of the crack surface are predicted; the evolving fatigue‐crack growth patterns are simulated, and thus, the fatigue life estimations of the cracked round bar are made. The accuracy and reliability of the SGBEM‐FEM alternating method are verified by comparing the presently computed results to the empirical solutions of SIFs, as well as experimental data of fatigue crack growth, available in the open literature. It is shown that the current approach gives very accurate solutions of SIFs and simulations of fatigue crack growth during the entire crack propagation, with very little computational burden and human–labour cost. The characteristics of fatigue growth patterns of initially simple‐shaped cracks in the cylindrical bar under different Modes I, III and mixed‐mode types of loads are also discussed in detail.     

Simplified model for the fatigue growth analysis of surface cracks in round bars under mode I

The fatigue growth of an edge flaw in a round bar under cyclic tension or bending loading is examined, using a two-parameter numerical model. First, it is shown that the crack front evolution is defined by a very small number of parameters, which varies during crack growth. Approximated solutions for both the crack propagation path and the stress intensity factor are derived, and the fatigue predictions using this simple analytical method are finally compared with the numerical results.     

Subcritical crack-growth and lifetime behavior of glass and SiG under static load

Crack initiation and subcritical crack growth in glass sheet and SiC bar specimen under static loading were investigated to study the failure process. It has been demonstrated that the lifetime process of brittle materials involves three possible forms of crack growth: subcritical crack growth, partly subcritical crack growth and instantaneous fracture without subcritical crack growth. Curves of upsilon -K obtained in step-by-step static fatigue tests and in constant loading rate tests showed different trends for borosilicate glass sheet. alpha -SiC that is generally considered immune to mechanical fatigue effect and environmental attack was also tested under static loading and the lifetime was measured. The results showed that the threshold load to damage effect was over 80% of the initial strength for the SiC.     

Stress intensity factors for surface fatigue crack in a round bar under cyclic axial loading

In this paper, the surface fatigue crack growth shape for an initial straight-fronted edge crack in an elastic bar of circular cross-section is determined through experiments under pure fatigue axial loading. Three different initial notch depths are discussed. The relations of the aspect ratio (b/c) and relative crack depth (b/D) are obtained, and it is shown that there is a great difference in the growth of cracks with different initial front shapes and crack depths. Further, using the three-dimensional finite element method, the stress intensity factors (SIFs) are determined under remote uniform tension loading. Since the relationship of b/c and b/D changes during the fatigue crack growth, the SIFs are determined for different surface crack configurations.     

Subcritical Crack-Growth and Lifetime Behavior of Glass and SiC under Static Load

［1］H.F.Hardath: ‘A Review of Cumulative Damage‘ paper for AGARD, USA, 1965. ［2］P. Ostojic: J. Mater. Sci., 1995, 30, 3011. ［3］T.L.Anderson: Fracture Mechanics, CRC Press, USA,1995. ［4］K.Reifsnider and J.Duthoit:Concept of Fatigue Strength in Brittle Polymer and Ceramic Composite,in Fatigue‘99, eds. X.R.Wu and Z.G.Wang, 1999, 3,1647. ［5］S.M.Wiederhorn, E.R.Fuller and R.Thomson: Met.Sci, 1980, 14, 450. ［6］S.J.Dill, S.J.Bennison and R.H.Dauskardt: J. Am. Ceram. Soc., 1997, 80, 773. ［7］J.Mencik: Strength and Fracture of Glass and Ceramics, Elsevier, Amsterdam, 1992 . ［8］J.A.Salem, J.L.Shannon and R.C.Bradt: J. Am. Ceram. Soc., 1989, 72(1), 20. ［9］R.H.Dauskardt, D.B.Marshal and R.O.Ritchie: J. Am.Ceram. Soc., 1990, 73(4), 893.     

The influence of constant axial compression pre-stress on the fatigue failure of torsion loaded tube springs

This paper reports the results of a series of biaxial static compression and torsion experiments performed to evaluate the effects of static compression stress on the fatigue life those smooth tubes made of high strength spring steel. Compression pre-stress was introduced by a solid steel bar inserted into a hollow spring and loaded with a screw-joint. The experimentally obtained results show a significant extension of fatigue strain life as a result of combining axial compression loading with torsion. Cracking behavior was observed and it was noted that compression pre-stresses contribute to retardation of the fatigue crack initiation process and, consequently, contribute to the extension of fatigue life. The fatigue shear crack initiated in a transverse direction. This crack continues to propagate in the same direction until it starts to propagate as a macro-crack on the maximum shear plane.     

Fatigue crack paths for inclined cracks and surface flaws under biaxial loading

Two approaches are developed for geometrical modeling of crack growth trajectories for the inclined through thickness central cracks and the part-through surface flaw respectively. The principal feature of such modeling is the determination of crack growth direction and the definition of crack length increment in this direction. The damage process zone size concept is employed for calculations of mixed-mode crack growth trajectories and surface flaw shape and positions. Crack front behavior for a straight-fronted edge crack in an elastic bar of circular cross-section is studied through experiments and computations under axial tension loading. The elaborated theoretical model is applied for fatigue crack shape simulation of part-through cracks in a hollow thick- and thin-walled cylinders under different biaxial loading conditions. Suggested approach of crack paths modeling is used for an analysis and prevention of operation failures of existing in-service aircraft gas-turbine engine rotating components.     

Numerical simulation of fatigue crack propagation in compressive residual stress fields of notched round bars

In this paper, the influence of the residual compressive stresses induced by roller burnishing on fatigue crack propagation in the fillet of notched round bar is investigated. A 3D finite element simulation model of rolling has allowed to introduce a residual stress profile as an initial condition. After the rolling process, fatigue loading has been applied to three‐point bending specimens in which an initial crack has been introduced. A numerical predictive method of crack propagation in roller burnished specimens has also been implemented. It is based on a step‐by‐step process of stress intensity factor calculations by elastic finite element analyses. These stress intensity factor results are combined with the Paris law to estimate the fatigue crack growth rate. In the case of roller burnished specimens, a numerical modification concerning experimental crack closure has to be considered. This method is applied to three specimens: without roller burnishing, and with two levels of roller burnishing (type A and type B). In all these cases, the computational finite element predictions of fatigue crack growth rate agree well with the experimental measurements. The developed model can be easily extended to crankshafts in real operating conditions.     

FORMATION AND GROWTH OF FATIGUE CRACKS IN REINFORCING STEEL FOR CONCRETE

Abstract— –The da/d Nvs Δ Krelation for Torbar Steel is obtained by testing specimens cut from 32 mm bar. This relation is compared with available data for Unisteel 410 and with medium strength steels. Also, the pattern of fatigue cracking in Torbar is studied by inspecting bars retrieved from reinforced concrete beams which were subjected to repeated loading. It has been observed that fatigue crack locations in the bars coincided with the locations of concrete cracks traversing the bars. Tension tests on the fatigue cracked bars show that the cracks considerably reduce the ductility of the bars.     

Fatigue crack initiation in as forged Ti–6Al–4V bars with macrozones present

Macrozones are regions containing grains with similar orientations, which in titanium alloys are detrimental to the fatigue properties. In this study, an investigation regarding the effect of the macrozone characteristics on the crack initiation under four-point bending fatigue cycling tests on an as-forged Ti–6Al–4V bar with macrozones present has been carried out. The macrozone characteristics studied in this paper include the Schmid factors of grains within the macrozone, the misorientation between two macrozones or a macrozone and its adjacent area and the angle of the interface versus loading axis. In this study, fatigue crack was initiated within two adjacent macrozones or within a macrozone and its adjacent region with high Schmid factors. Misorientation crack factors are introduced to combine all the above influencing factors and crack initiation on the interface. Fatigue crack initiation mechanism around macrozones has been studied and discussed. The texture index is introduced as a single value to represent the sharpness of the microtexture of a macrozone.     

Eccentric Circumferential Cracks in Cylindrical Specimens

The effect of eccentricity on the stress intensity factors for circumferential cracks in cylindrical specimens subjected to torsional and axial loading is studied. BEM solutions are compared with proposed approximate methods and some agreement is found for the axial load cases. A simple method of how the eccentricity develops under axial loading as the crack grows is presented. It shows that the eccentricity accelerates as the crack grows and that the problem thereby can be characterized as unstable.     

THE EFFECT OF CONCRETE BOND ON THE STRESS INTENSITY FACTORS FOR AN EDGE CRACK IN A REINFORCING BAR

Abstract— The stress intensity factors at the tip of an edge crack in a round bar embedded in the tension zone of a concrete beam are found for several conditions of bond failure between the steel reinforcing bar and the concrete. A two-dimensional finite element technique is modified to represent the third dimension, the steel-concrete interface and debonding. The integrated form of the Paris crack growth law is used with the calculated stress intensity factors to compute the propagating fatigue lives for embedded bar and bar in air. The results indicate that embedded bars appear to have shorter fatigue propagation lives than bars in air if the crack initiators are less than 0.05 of the bar diameter, while for deeper crack initiators, the opposite is true. It has also been shown that the propagating fatigue life of the bar increases with the degree of bond.     

加载历史和裂纹闭合对疲劳裂纹扩展行为影响的数值模拟

采用不同应力比条件下的16MnR钢紧凑拉伸试样,设计了三种有限元分析模型,即不考虑加载历史效应的静态裂纹扩展模型,同时考虑加载历史和裂纹闭合的动态裂纹扩展模型以及仅考虑加载历史的伪动态裂纹扩展模型,对疲劳裂纹闭合过程、裂纹尖端的应力-应变迟滞环、疲劳损伤和裂纹扩展速率进行了数值模拟与分析,进而着重探讨了加载历史和裂纹闭合影响疲劳裂纹扩展行为的交互作用机制。结果表明:对于同类分析模型,应力比越大越不容易产生裂纹闭合;而在应力比相同的情况下,加载历史引起的残余压应力对裂纹闭合有明显的促进作用。裂纹闭合效应阻碍了平均应力的松弛,减小了裂纹尖端附近的应力-应变场强度、疲劳损伤和裂纹扩展速率,而加载历史引起的残余压应力则加快了平均应力的松弛和抑制了棘轮效应。与实验结果比较发现,只有同时考虑了裂纹闭合效应和加载历史影响的动态裂纹扩展模型,才能对疲劳裂纹扩展行为进行准确、定量的模拟。     

Subcritical crack propagation under cyclic stress impulse

An equipment has been designed to observe subcritical crack propagation under cyclic impulse (impact) loads. The equipment design uses the concept of stress wave propagation in bars. A four point bend notched specimen is struck by an incident bar with a known stress wave. The test specimens were machined from PMMA sheet (Lucite^®). The crack, initiated from the notch, was detected by a step wise increase of a graphitic grid imprinted on one side of the specimen. The data was analyzed using fracture mechanics theory and compared with that of conventional fatigue.Although the applied strain rate was quite high (1s^–1), stable crack propagation was significant. It appears that the elastic energy stored in the specimen within the duration of each impulse is dissipated in craze formation at the tip of the advancing crack. Furthermore, the magnitude of stable crack propagation was larger under impulse loading than under sinusoidal fatigue. On the other hand, cracks were slower under impulse loading. Fractographic evidence attributes these phenomena to the nature of craze growth under each loading condition.     

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.     

7050铝合金Ⅰ-Ⅱ复合型疲劳裂纹研究

为研究7050铝合金在Ⅰ-Ⅱ型复合加载下疲劳裂纹扩展规律,在Amsler HFP5000高频试验机上利用Richard加载装置,完成紧凑拉剪(CTS)试样疲劳裂纹扩展试验,利用有限元对Ⅰ-Ⅱ复合型裂纹进行数值模拟,采用APDL命令流计算不同裂纹长度的应力强度因子,并引入最大周向应力准则计算裂纹扩展角,用有限元计算等效应...     

Fatigue crack growth in adhesively bonded composite-metal double-lap joints

This paper presents experimental and numerical investigations of the fatigue crack initiation and growth mechanism in metal-to-composite bonded double-lap joints. Fatigue tests were conducted under tension dominated loading, with crack lengths being measured optically. Examination of the fracture surface using scanning electron microscope revealed that fatigue cracks were near the interface between the co-cured adhesive and the first ply of the composite adherend. The finite element method has been used to determine the strain-energy release rate of a fatigue crack growing along the first ply of the composite. The effects of spew fillet size and crack initiation modes have also been studied by the finite element method. Comparison of the present experimental crack growth results with those measured using double-overlap joints, where the fatigue cracks were driven by pure mode II loading, indicate that the tensile mode loading has a overwhelming effect on the fatigue crack growth rates. The present results suggest that fatigue failure of metal-composite double-lap joints is mainly driven by tensile mode loading due to the peel stress.     

Atomic force microscopy and the mechanism of fatigue crack growth

Fatigue crack test was performed using a grain-orientated 3% silicon iron under constant amplitude loading. Growth behaviour of the fatigue crack and slip deformation behaviour near the crack tip were observed in detail by using an atomic force microscope. In the lower K region, only one preferential slip system of this material operated and the fatigue crack grew along that slip plane. It was found that constraint of slip deformation due to cyclic strain hardening resulted in crack arrest and crack branching. The fatigue crack grew in a zigzag manner as a result of such successive crack branching and deflection. In the high K region, two preferential slip systems operated simultaneously to an almost identical extent and the fatigue crack grew in a direction perpendicular to the loading axis. The slipping distance in one loading cycle was measured quantitatively by using the image processing technique and the crack growth mechanism is discussed.