Fatigue Behavior Of Cracks Research Articles

Fatigue crack propagating behavior from inner surface crack on bent sheet of ultra-high strength steel

Fatigue fracture of structures made of bent sheets often occurs from cracks on the sheet inner surface. This study investigated fatigue crack growth and non-propagating behavior in bent sheets of ultra-high strength steel (UHSS) using artificial cracks introduced by a picosecond pulse laser. A newly-designed fatigue test showed that the internal compressive residual stress introduced by bending arrests crack propagation in the depth direction. A crack propagation analysis using the software SCANP revealed that this internal compressive residual stress suppressed not only the stress intensity factor in the depth direction, ΔKa, but also that in the width direction, ΔKc, even though the surface residual stress was tensile. A stress relief heat treatment test demonstrated that the crack arrest behavior disappeared and the propagation life was shortened in the absence of this compressive residual stress. Non-propagating cracks were observed in the specimens after 1×107 test cycles. The relationship between the fatigue limit and the artificial crack depth (Kitagawa-Takahashi diagram) was generally fitted by the Line Method with a critical distance of 46.7 μm. This study provides insight into the fatigue behavior of cracks on bent sheets of UHSS, which are widely used in many industries such as automobile manufacturing.

The interaction of fatigue cracks with a residual stress field using thermoelastic stress analysis and synchrotron X-ray diffraction experiments.

This article presents an experimental study on the fatigue behaviour of cracks emanating from cold-expanded holes utilizing thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques with the aim of resolving the long-standing ambiguity in the literature regarding potential relaxation, or modification, of beneficial compressive residual stresses as a result of fatigue crack propagation. The crack growth rates are found to be substantially lower as the crack tip moved through the residual stress zone induced by cold expansion. The TSA results demonstrated that the crack tip plastic zones were reduced in size by the presence of the residual compressive stresses induced by cold expansion. The crack tip plastic zones were found to be insignificant in size in comparison to the residual stress zone resulting from cold expansion, which implied that they were unlikely to have had a notable impact on the surrounding residual stresses induced by cold expansion. The residual stress distributions measured along the direction of crack growth, using SXRD, showed no signs of any significant stress relaxation or redistribution, which validates the conclusions drawn from the TSA data. Fractographic analysis qualitatively confirmed the influence on crack initiation of the residual stresses induced by the cold expansion. It was found that the application of single compressive overload caused a relaxation, or reduction in the residual stresses, which has wider implications for improving the fatigue life.

Grain size effects on notch sensitivity

This work presents experimental results on the fatigue behavior of cracks growing from circular notches under axial loading conditions for a very wide range of notch size to grain size ratios, including cases where the notch size is of the order of, or even smaller, than the grain size. Notch sensitivity is evaluated and the well-known dependence on grain size is clearly demonstrated. It was found that for the same specimen geometry and loading conditions, the influence of the stress concentration is directly dependent on the relationship between the notch size and the grain size.

Assessment of Fatigue Cracks in Rails

The fatigue behaviour of cracks at the foot region of a rail subjected to bending load has been investigated. Depending on the position of initiation of a small semi-elliptical surface crack in a rail, the crack front changes shape during propagation to failure due to the large variation of the stress intensity factor from point to point round the crack front due to differences in the local stress field as each point round the crack front lies at different distances from the neutral axis of the rail. This condition implies a variable crack growth rate that transforms the crack front shape during fatigue crack propagation. SIF values have been estimated by means of both the finite element method and analytical solutions derived for a semi-elliptical crack in a finite rectangular cross-section beam. The SIF value predictions obtained with the two methods show good agreement suggesting that the analytical solutions can be used for a rapid assessment of the severity of a flaw in a rail. A predictive model for crack growth has been derived for an initial small crack at an initiation point at the foot/web corner of a rail tested under four point bending fatigue in the laboratory, showing a reasonably good prediction of both the shape and size of the crack at failure when compared with experiment.

Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials

Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods to predict small crack fatigue behavior. In this paper small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and was used (i) to determine the bridging stress profile for the material and (ii) to make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, it has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.

Development of Predictive Models for Fatigue Crack Growth in Rails

Fatigue failures of rails often occur at the rail foot, since the geometry of this zone gives rise to stress concentrations under service loads or defects during rail manufacture and installation. In this paper, the fatigue behavior of cracks at the web/foot region of a rail is analyzed numerically. Analytical models in the literature for a semi-elliptical surface crack in a finite plate assume that the geometry of the front remains semi-elliptical during the whole propagation phase and the ellipse axes do not undergo translations or rotations. Fatigue tests show that this is not the case for such cracks in rails. A predictive model for crack growth has been developed by assuming an initial small crack at one probable initiation point between the web and foot of the rail in reference to a service condition loading. SIF values have been estimated by means of the finite element method and the plastic radius correction. The results attained were compared with crack growth experimental data.

Micromechanical aspects of fatigue in a MIG welded aluminium airframe alloy: Part 2. Short fatigue crack behaviour

Short crack fatigue behaviour of a metal inert gas (MIG) welded 2024-T351 aluminium alloy has been studied in terms of crack initiation and propagation, and the microstructural features of the weld. Extensive crack initiation was noted within the weld fusion zone. Relatively small interdentritic defects are seen to be more damaging (in initiating cracks) than the larger gas bubble defects that exist in this region. Cracks were also seen to initiate within the weld heat affected zone where tensile residual stresses were found to be high, however, rapid crack growth and coalescence within the fusion zone is seen to dominate failure. Multiple crack interactions are seen to have a significant effect on failure up to relatively large crack lengths (of the order of the fusion zone width), enhancing the variability in short crack growth. Secondary electron and back-scattered electron imaging, in association with electron back-scattered diffraction (EBSD) mapping, were carried out on failed samples to assess the associated microstructural interactions.

The effect of silicon content on long crack fatigue behaviour of aluminium–silicon piston alloys at elevated temperature

The microstructure of aluminium piston alloys comprises primary and eutectic silicon together with numerous intermetallics. Previous research has shown that primary silicon strongly influences both fatigue crack initiation and subsequent propagation behaviour, however, the detailed effects of varying silicon volume fraction and morphology have not been fully addressed. Therefore, the fatigue properties of a number of candidate piston alloys with varying volume fractions of silicon have been studied. Long crack fatigue tests have been performed at room and elevated temperature typical of the gudgeon pin boss (200 °C) using a test frequency of 15 Hz (a typical engine frequency at engine idle condition). Microstructural characterisation using image analysis approaches combined with optical profilometry has been used to assess the fracture surfaces of test samples. The role of primary Si in enhancing crack growth rates at high Δ K levels, whilst affording improvements in crack growth rates at lower Δ K levels due to local crack deflections and shielding, has been confirmed. In the absence of primary Si (lower Si content alloys) the low Δ K level crack growth behaviour is dominated by matrix properties (intra-dendritic crack growth pre-dominates) whilst the high Δ K level crack growth behaviour is inter-dendritic and occurs along the weak path of the eutectic Si and/or intermetallic network.

Elevated temperature short crack fatigue behaviour in near eutectic Al–Si alloys

This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent short crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by cracking or decohesion (dependent on test temperature) and are also sought out preferentially during short crack growth.

Methodology for modelling the small crack fatigue behaviour of aluminium alloys

The objective of this study is to develop a computational modelling methodology of the small fatigue crack growth behaviour of: (1) a forged 2124 Al alloy in the T4 condition and (2) a cast 359 Al alloy in the T6 condition. In particular, the focus of this work is on correlating local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. A defect tolerant approach is assumed. The crack tip is modelled using the finite element method, and the correlating parameter, Δ J eff (the effective range of the J-integral), is calculated. An effective crack growth rate curve is calculated, and the Δ J eff is used to obtain the crack growth increment per cycle. Small crack growth rate curves for different stress levels and initial defect sizes are presented for each alloy for a stress ratio, R, of 0.1. Predicted S– N curves are then compared with experimental results for both the Al 2124 and Al 359 alloys. A good agreement with experimental results is achieved for an appropriate choice of defect size. Finally, as a means of validating the choice of defect size, the fatigue limit vs. defect size results are compared on a Kitagawa diagram with those obtained by linear elastic fracture mechanics, for the Al 2124 alloy.

Analysis of cracks at deep notches

This paper describes the fatigue behavior of cracks located at two deep “dovetail” notch geometries. Fadgue tests with transparent polymer models produced changes in crack size and shape as a function of elapsed cycles. Numerical stress intensity factor solutions were determined for both through-cracks (using a finite element code), and corner cracks (using a boundary element code) located at the notch bore. The numerical stress intensity factor solutions were verified by predicting fatigue lives, crack sizes and shapes for the experimental results. These predictions showed that the numerically determined stress intensity factors accurately predict fatigue lives, as well as crack size and shape for the dovetail geometries.

Response of Short Cracks in High-Strength Steels to Fatigue Loadings—Part I: Behavior in Air and Freely Corroding in Seawater

Experiments have been performed which determined the growth rate of short cracks (initial depth ~ 0.1 mm) for five high-strength steels (yield stress 370–570 MPa) in air and natural seawater. Attention was focused on regions I and II of the classical da/dN-ΔK curve with test conditions being relevant to offshore structure service. The results indicated enhanced growth rate for short fatigue cracks (a ⪝ 1 mm) in both environments. Parameters with which short crack fatigue behavior might best be correlated were investigated, and it was found that the enhanced fatigue crack growth rate associated with short cracks transitioned to long crack growth kinetics at a ΔK value which depended only upon environment. The significance of the results are discussed within the context of offshore structure integrity.

When is a crack not a crack? Some data on the fatigue behaviour of cracks and sharp notches: Taylor, D., Staniaszek, I.A.N. and Knott, J.F. Int. J. Fatigue Sept. 1990 12, (5), 397–402

When is a crack not a crack? Some data on the fatigue behaviour of cracks and sharp notches: Taylor, D., Staniaszek, I.A.N. and Knott, J.F. Int. J. Fatigue Sept. 1990 12, (5), 397–402

When is a crack not a crack? Some data on the fatigue behaviour of cracks and sharp notches

Long, sharp notches were produced by blunting and stress relieving precracks in A533B steel. A critical root radius could be defined below which the behaviour of a notch was the same as that of a sharp crack. This critical value is a function of stress intensity level and can be predicted approximately by a theory based on comparison of plastic zone sizes. The phenomenon is complicated by the existence of a fast-growth transient in the case of very sharp notches, which is presumed to be a crack closure effect. An understanding of this boundary between crack-like behaviour and non-crack-like behaviour is important for the practical assessment of defects as well as for a fundamental understanding of fracture mechanics.