Surface Crack Paths Research Articles

Fatigue crack initiation behaviour of notched 34CrNiMo6 steel bars under proportional bending-torsion loading

Fatigue crack initiation behaviour of notched 34CrNiMo6 high-strength steel bars with transverse blind holes and lateral U-shaped notches subjected to proportional bending-torsion loading is studied. Crack initiation at the geometric discontinuities is examined in situ using a high-magnification digital camera. Crack initiation mechanisms and failures modes are investigated after the fatigue testing program via scanning electron microscopy. Fatigue crack initiation lifetime is predicted using an averaged Total Strain Energy Density approach in conjunction with the Theory of Critical Distances. Crack initiation sites, crack surface angles and surface crack paths at the geometric discontinuities are successfully determined a priori from the maximum value of the first principal stress, the first principal direction, and the first principal stress field at the notch surface, respectively. Overall, fatigue lifetime predictions are very well correlated with the experimental observations.

Shear fatigue behavior of AW2099-T83 aluminum-lithium alloy

Monotonic shear and strain-controlled shear fatigue behavior of AW2099-T83/SHP Aluminum-Lithium alloy were investigated. Torsion fatigue testing was performed at strain amplitudes ranging from 0.5% to 1.5%. The material strain hardened under monotonic shear loading but cyclically softened after few fatigue cycles for all the ranges of applied strain amplitudes. Under torsion fatigue, macroscopic plastic deformation was observed only for strain amplitudes higher than 0.7%. Cyclic plastic strain was found to increase with the number of cycles and the mean stress remained very small during most of the specimen life. Coffin-Manson relation was used to determine the shear fatigue constants of the material. The developed properties were used to arrive at a good correlation between estimated and experimental fatigue life under shear strain loading. The shear fatigue properties were compared to values reported for traditional Al alloys 2xxx, 6xxx and 7xxx series in axial fatigue after the conversion of the shear fatigue parameters to axial equivalent. The surface crack paths were observed to be mostly in the longitudinal direction with a 90° bifurcation at low strain amplitudes. Resulting crack morphologies were analyzed by SEM.

Effect of loading orientation on fatigue behaviour in severely notched round bars under non-zero mean stress bending-torsion

This paper studies the effect of different loading orientations on fatigue behaviour in severely notched round bars under pulsating in-phase combined bending-torsion loading. Bending moments are applied in three different planes with respect to the notch root considering two bending moment to torsion moment ratios. Complementary, beach-marking tests are also performed for different loading scenarios to evaluate the crack shape changes during the crack growth. After fatigue testing, specimen surfaces are examined by scanning electron microscopy to identify the main failure micro-mechanisms. Fatigue life is predicted via the Coffin-Manson model, and the notch effect is evaluated using the Equivalent Strain Energy Density concept. Overall, experimental and predicted lives are very reasonably correlated. Furthermore, initiation sites, initiation angles and surface crack paths are successfully predicted from the principal stress field at the notch.

Effect of different environmental conditions on surface crack growth in aluminum alloys

Fatigue surface crack growth is studied through experiments and computations for aluminum alloys D16T and B95AT (analogue of 2024 and 7075 aluminum). Subjects for studies are cylindrical hollow specimens with external semi-elliptical surface crack. The variation of fatigue crack growth rate and surface crack paths behavior was studied under cyclic loading for different environmental conditions. Uniaxial tension tests were carried out at low (-60°C), room (+23°C) and high (+250°C) temperature. For the same specimen configuration and the different crack front position as a function of cyclic loading and temperatures conditions the distributions of governing parameter of the elastic-plastic stress fields in the form of In-factor along various crack fronts was determined from numerical calculations. This governing parameter was used as the foundation of the elastic-plastic stress intensity factor (SIF). Both elastic and plastic SIF approach was applied to the fatigue crack growth rate interpretation. It is found that there is a steady relationship between the crack growth rate and the plastic SIF in the form of general curve within a relatively narrow scatter band for all tested specimens at different temperatures.

Surface flaws behavior under tension, bending and biaxial cyclic loading

Fatigue surface crack growth and in-plane and out-of-plane constraint effects are studied through experiments and computations for the aluminum alloy D16T. A tension/bending central notched plate and cruciform specimens under different biaxial loadings with external semi-elliptical surface cracks are studied. The variation of the fatigue crack growth rate and surface crack paths is studied under cyclic tension, bending and biaxial tension–compression loading. For the experimental surface crack paths in the tested specimens, the T-stress, out-of-plane Tz factor, local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of the In-integral are calculated as a function of the aspect ratio by finite element analysis to characterize the constraint effects along the semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface, as well as at the deepest point of the semi-elliptical surface crack front, of the tested tension/bending plate and cruciform specimens. From the results, characteristics of the fatigue surface crack growth rate as a function of the loading conditions are established.

Surface and through thickness crack growth in cruciform specimens subjected to biaxial loading

Fatigue surface crack growth are studied through experiments and computations for aluminum alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading with external semi-elliptical surface crack. The variation of fatigue crack growth rate and surface crack paths behaviour was studied under cyclic uniaxial tension, equal biaxial tension and biaxial tension-compression loading. For the experimental surface crack paths in tested specimens the T-stress, the out-of-plane TZ factor, the local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. Both elastic and plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the cruciform specimens as well as in the deepest point of the semi-elliptical surface crack front. As result principal particularities of the fatigue surface crack growth rate as a function of loading conditions were established. The experimental and numerical results of the present study provide an opportunity to explore the suggestion that crack growth rate may be represented by the plastic stress intensity factor, rather than the magnitude of the elastic SIF alone.

Surface Crack Growth Rate under Tension and Bending in Aluminum Alloys and Steel

Fatigue surface crack growth is studied using FE-analysis and experiments for aluminum alloy D16 and low carbon steel under tension and bending. The subject for the study is central notched specimens with external semi-elliptical surface crack.Crack growth rate was determined on precracked specimens under tension and three point bend loadings. As experimental result the crack length increment on the specimen surface and crack edges opening displacement were obtained. A relationship between the crack form geometry and a number of loading cycles was obtained during tests by beach marks produced on each specimen.For the experimental surface crack paths in tested specimens the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral was calculated by finite element analysis along semi-elliptical crack front. The governing parameter of the elastic-plastic stress fields In-integral was used as the foundation of the elastic-plastic stress intensity factor. The plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the specimens and in the deepest point of the semi-elliptical surface crack front.A significant influence of material properties and loading type on crack growth rate characteristics was established in the present study.The experimental and numerical results of the present study shows that the elastic-plastic stress intensity factor, which is sensitive to elastic-plastic material properties, is attractive for fatigue crack growth rate characterization.

Surface crack growth subject to bending and biaxial tension-compression

Fatigue surface crack growth and the in-plane and out-of-plane constraint effects are studied through experiments and computations for aluminium alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading and bending central notched specimens with external semi-elliptical surface crack. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth rate and surface crack paths behaviour is studied under cyclic pure bending and biaxial tension-compression fatigue loading. This work is centered on the relations between crack size on the free surface of specimen considered configurations, COD and aspect ratio under different fatigue loading conditions. For the experimental surface crack paths in tested specimens the T-stress, the local triaxiality parameter h, the out-of-plane TZ factor and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface of the tested bending and cruciform specimens as well as the deepest point of the semi-elliptical surface crack front. As result fatigue surface crack paths or crack front positions as a function of accumulated number of cycle of loading are obtained.

The concept of the average stress in the fracture process zone for the search of the crack path

The concept of the average stress has been employed to propose the maximum average tangential stress (MATS) criterion for predicting the direction of fracture angle. This criterion states that a crack grows when the maximum average tangential stress in the fracture process zone ahead of the crack tip reaches its critical value and the crack growth direction coincides with the direction of the maximum average tangential stress along a constant radius around the crack tip. The tangential stress is described by the singular and nonsingular (T-stress) terms in the Williams series solution. To demonstrate the validity of the proposed MATS criterion, this criterion is directly applied to experiments reported in the literature for the mixed mode I/II crack growth behavior of Guiting limestone. The predicted directions of fracture angle are consistent with the experimental data. The concept of the average stress has been also employed to predict the surface crack path under rolling-sliding contact loading. The proposed model considers the size and orientation of the initial crack, normal and tangential loading due to rolling–sliding contact as well as the influence of fluid trapped inside the crack by a hydraulic pressure mechanism. The MATS criterion is directly applied to equivalent contact model for surface crack growth on a gear tooth flank.

Fatigue behaviour and life prediction of lateral notched round bars under bending–torsion loading

Fatigue behaviour of lateral notched round bars made of DIN 34CrNiMo6 high strength steel under single bending, single torsion and combined bending–torsion was studied. Crack initiation and crack growth were monitored in situ using a high-resolution digital system. Fracture surfaces were examined by scanning electron microscopy and were replicated with a three-dimensional laser scanner. The notch effect was analysed using the equivalent strain energy density concept and the fatigue life predictions were carried out through the Coffin–Manson model. Criteria based on the principal stress field were proposed to predict the most likely initiation sites, surface crack paths and surface crack angles. Finally, very good correlations between experimental and predicted fatigue lives were observed, particularly for lives greater than 104cycles.

Mechanisms of short crack propagation in austenitic–ferritic duplex steel

For applications requiring both high strength and high corrosion resistance, austenitic–ferritic duplex steels are often the material of choice. In this study, cyclic deformation experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462. By measuring the crack opening and crack sliding displacement in situ in a scanning electron microscope, the characteristics of the different crack propagation mechanisms in the two phases are determined. In the ferritic phase, two different appearances of short cracks can be observed, one exhibiting a very smooth and the other one a rather rough surface crack path. Electron backscatter diffraction measurements on the crack-containing grains in addition with high resolution imaging of the topography of the crack flanks reveal that contrary to common assumptions in the literature, short cracks in ferrite do not propagate via single slip. Instead, two different slip systems with an identical slip direction, but different slip planes, are activated. In this context, the specific appearance of different crack paths can be explained with the orientation of the respective grains. Furthermore, a model for discontinuous crack propagation especially of rough cracks in ferrite is developed. Finally, a correlation between the crack propagation rate and the plastic deformation of the crack tip is revealed and the possibility of determining the barrier effect of grain and phase boundaries via the measurement of the plastic deformation of the crack tip is investigated.

Low cycle fatigue of Ti-Mn alloys: Microstructural aspects of fatigue crack growth

Fatigue crack path behavior of a series of Ti-Mn alloys heat treated to produce volume fractions of the alpha phase ranging from 0 to 97.5 pct was investigated. Both Widmanstatten and equiaxed morphologies of the α phase were used in this study. Interior and surface crack paths are discussed in terms of slip behavior and microstructural details.