Roughness-induced Fatigue Crack Closure Research Articles

Quantification method for parameters affecting multi-scale roughness-induced fatigue crack closure

Our challenge is to clarify the relationship between crack roughness and microstructure. Roughness-induced crack closure (RICC) is known to be one of the main factors decelerating fatigue crack growth. Two factors trigger RICC: 1) nanometer-scale roughness on the crack surface (nano-roughness) and 2) degree of crack deflection (micro-roughness). These factors affect the friction stress acting on the crack planes and the stress intensity factor range for crack closure. For instance, S. Suresh and R. O. Ritchie discussed the effects of geometrical mismatch between fatigue crack planes on crack closure. We further attempt to measure multi-scale crack roughness to quantitatively estimate RICC with respect to both friction and crack closure effects. In this study, we examine the multi-scale crack roughness of a lamellar-structured Fe-9Mn-3Ni-1.4Al-0.01C steel sample as a case study. We verify the effect of crack surface friction. Here, we assume that the basic effect of nano-roughness on friction is significant when the inclination angle of micro-roughness against the loading direction is less than the angle of nano-roughness. If the inclination angle of micro-roughness against the loading direction is larger than the angle of nano-roughness, the nano-roughness effect does not occur because the crack surfaces do no contact with each other. To further discuss the underlying effects of multi-scale roughness, we will present more details on the microstructure- and mechanical condition-related roughness parameters and their quantification techniques.

Effect of temperature on the fatigue-crack growth behavior of the high-entropy alloy CrMnFeCoNi

Near-equiatomic multi-component high-entropy alloys (HEAs) have engendered much attention of late due to the remarkable mechanical properties of some of these new metallic materials. In particular, one of the first reported HEAs, the equiatomic, single-phase, face-centered cubic (fcc) alloy CrMnFeCoNi, often termed the Cantor alloy, has been shown to display an exceptional combination of strength, ductility and fracture toughness, i.e., damage tolerance, at room temperature, properties that are further enhanced at cryogenic temperatures. Despite this alloy being the most studied HEA to date, its resistance to crack growth under cyclic fatigue loading has not yet been characterized. Here, we examine its fatigue-crack propagation behavior, primarily at lower, near-threshold, growth rates, both at room temperature (293 K) and at 198 K. At 293 K, the alloy shows a fatigue threshold, ΔKTH, of ∼4.8 MPa√m, which increases by more than 30% to ΔKTH ∼6.3 MPa√m with decrease in temperature to 198 K; additionally, the Paris exponent m was found to increase from roughly 3.5 to 4.5 with decreasing temperature. Examination of the fracture surfaces and crack paths indicate a transition from predominantly transgranular crack propagation at room temperature to intergranular-dominated failure at the lower temperature. Such a change in crack path is generally associated with an increasing degree of physical contact between the two fracture surfaces, i.e., roughness-induced fatigue crack closure, which is likely to be the main reason for the difference in the measured thresholds. Additionally, we believe that the higher thresholds found at 198 K are associated with the alloy's higher strength at lower temperatures, which both reduces the crack-tip opening displacements at a given stress-intensity range and prevents plastic deformations of the grains in the wake of the crack. At room temperature, such plastically deformed grains can be associated with a loss of contact shielding of the crack-tip through closure, resulting in a lower threshold compared to 198 K.

Experimental and Numerical Simulation Study of Plasticity-Induced and Roughness-Induced Fatigue Crack Closure

In this study, fatigue crack closure behavior was investigated in the aluminum alloy 6061-T6 and the carbon steel JIS. S25C. It was found that crack closure in the aluminum alloy 6061-T6 showed the characteristics of plasticity-induced fatigue crack closure (PIFCC), whereas the carbon steel JIS. S25C showed the characteristics of roughness-induced fatigue crack closure (RIFCC). The experiments included the determination of the crack-opening levels Kop as a function of stress intensity factor range ΔK and the effect of surface removal on the crack-opening level. In order to simulate the behaviors of the plasticity-and the roughness-induced fatigue crack closure, the finite element method was adopted. The results of FEM were in good agreement with the experimental results. It was cconcluded that at a given yield strength level , a low Youngs modulus and a low work-hardening coefficient will favor PIFCC, whereas a high Youngs modulus and a high work-hardening coefficient will favor RIFCC.

Recent Advances in Fatigue Crack Growth

Many of the recent advances in the understanding of the fatigue crack growth process have resulted from an improved realization of the importance of fatigue crack closure in the crack growth process. Two basic crack closure processes have been identified. One of which is known as plasticity-induced fatigue crack closure (PIFCC), and the other is roughness-induced fatigue crack closure (RIFCC). Both forms occur in all alloys, but PIFCC is a surface-related process which is dominant in aluminum alloys such as 2024-T3, whereas RIFCC is dominant in most steels and titanium alloys. A proposed basic equation governing fatigue crack growth is (1) where where Kmax is the maximum stress intensity factor in a loading cycle and Kop is the stress intensity factor at the crack opening level. is the range of the stress intensity factor at the threshold level which is taken to correspond to a crack growth rate of 10-11 m/cycle. The material constant A has units of (MPa)-2, and therefore Eq. 1 is dimensionally correct. Eq.1 has been successfully used in the analysis of both long and short cracks, but in the latter case modification is needed to account for elastic-plastic behavior, the development of crack closure, and the Kitagawa effect which shows that the fatigue strength rather than the threshold level is the controlling factor determining the rate of fatigue crack growth in the very short fatigue crack growth range. Eq. 1 is used to show that The non-propagating cracks observed by Frost and Dugdale resulted from crack closure. The behavior of cracks as short as 10 microns in length can be predicted. Fatigue notch sensitivity is related to crack closure. Very high cycle fatigue (VHCF) behavior is also associated with fatigue crack closure.

On the Distinction Between Plasticity- and Roughness-Induced Fatigue Crack Closure

A series of experiments has been carried out to determine why some alloys display plasticity-induced fatigue crack closure (PIFCC), whereas other alloys display roughness-induced crack closure (RIFCC). Two alloys were studied, the aluminum alloy 6061-T6 (PIFCC) and a steel of comparable yield strength, S25C (RIFCC). The experiments included the determination of the crack-opening levels as a function of ΔK, da/dN as a function of ΔK eff – ΔK effth, removal of the specimen surface layers, removal of the crack wake, the determination of crack front shapes, crack surface roughness profiles, and the degree of lateral contraction in the plastic zone at a crack tip. Based on crack tip opening displacement (CTOD) considerations, it is concluded that PIFCC is favored in alloys of low modulus and relatively low yield strength. In addition, a low strain-hardening rate such as for the 6061 alloy will favor PIFCC. Steels with a higher modulus and a higher strain-hardening rate than 6061 will, in general, exhibit RIFCC, even at comparable yield strength levels. In ferritic steels, the fracture surface roughness and consequently the crack-opening level will increase as the coarseness of the microstructure increases.

Comparison of Crack Geometry Determined with Phase Contrast Radiography and with Microtomography

AbstractX-ray microtomography of the three-dimensional spatial distribution of crack opening as a function of applied load has helped clarify important processes of roughness-induced fatigue crack closure. Fracture mechanics dictates sample geometries which limit the crack opening sensitivity that can be obtained with microtomography, and there is a real need to be able to quantify smaller openings. Synchrotron phase contrast radiographs of an AA 2090 T8E41 (central 2 mm of a 12.7 mm thick plate) compact tension sample show the fatigue crack geometry clearly while absorption contrast radiographs of the same sample have difficulty even detecting the crack. The phase contrast expected in the projection images is determined from the microtomography-derived, three-dimensional crack geometry and compared to that observed experimentally. Good agreement is obtained between the phase contrast radiographs and the expected pattern of contrast.

Numerical modelling of combined roughness and plasticity induced crack closure effects in fatigue

The incidence of roughness induced fatigue crack closure has been studied by finite element modelling. Closure analyses in the literature have been reviewed and been shown to lack a reasonable treatment of: (a) propagating elastic-plastic cracks, and (b) the influence of the characteristically inhomogeneous plastic deformation associated with near-threshold crack growth in many materials. Based on an analysis of both overall specimen compliance and node behaviour along the crack path, the present modelling shows: (a) an increasing effect of crack path angle on roughness induced closure levels in keeping with the simple analytical model of Suresh and Ritchie; (b) the mechanism by which closure occurs is more strongly dependent on residual plastic strains in the wake than global shear displacements of the fracture surfaces due to mixed-mode behaviour at the crack tip; and (c) the closure levels are relatively low compared to experimental data, consistent with the absence of environmental irreversibility in the finite element models and the idealised crack path morphologies that were studied. Slip band simulations show a significant increasing effect of inhomogeneous deformation on closure levels, improving the apparent accuracy of the modelling results.

Numerical Modelling of Roughness and Plasticity Induced Crack Closure Effects in Fatigue

The incidence of roughness induced fatigue crack closure has been studied by finite element modelling. Closure analyses in the literature have been reviewed and been shown to lack a reasonable treatment of: (a) propagating elastic-plastic cracks, and (b) the influence of the characteristically inhomogeneous plastic deformation associated with near-threshold crack growth in many materials. Based on an analysis of both overall specimen compliance and node behaviour along the crack path, the present modelling shows: (a) an increasing effect of crack path angle on roughness induced closure levels in keeping with the simple analytical model of Suresh and Ritchie; (b) the mechanism by which closure occurs is more strongly dependent on residual plastic strains in the wake than global shear displacements of the fracture surfaces due to mixed-mode behaviour at the crack tip; and (c) the closure levels are relatively low compared to experimental data, consistent with the absence of environmental irreversibility in the finite element models and the idealised crack path morphologies that were studied. Slip band simulations show a significant increasing effect of inhomogeneous deformation on closure levels, improving the apparent accuracy of the modelling results.

Investigation of roughness induced crack closure effects in fatigue

The incidence of roughness induced fatigue crack closure has been studied by finite element modelling. Results show an increasing effect of crack path angle on closure levels with the closure mechanism being strongly dependent on residual plastic strains in the wake rather than global shear displacements of the fracture surfaces. An analytical expression for the functional dependence of roughness induced closure has been produced which shows reasonable agreement with the finite element results.

Fracture surface roughness and roughness-induced fatigue crack closure in Ti-2.5 wt% Cu

Abstract Fatigue crack closure and fracture surfaces in the alloy Ti-2.5 wt% Cu at low Δ K (d a /d N −8 m cycle −1 ) were examined. For a solution-annealed equiaxed microstructure as well as for a fine and a coarse lamellar microstructure, roughness-induced crack closure occurs due to serrated crack path, which influences the crack growth rates significantly. The fracture surface roughness is found to be a measure of roughness-induced crack closure in lamellar microstructures where planar slip prevails. The height distribution and tilt angle distribution of the crack path facets are the decisive factors for roughness-induced crack closure. In the solution-annealed equiaxed microstructure, ductile fracture areas by the side of crystallographic facets can act as obstacles to crack tip sliding along planar slip planes, causing less mode II displacement and thus resulting in a much lower level of crack closure.

A model for roughness-induced fatigue crack closure

A model for predicting the crack closing stress intensity factor for roughness-induced closure of fatigue cracks is developed based on a two-dimensional approach considering crack opening and closure of an idealized crack path. The model highlights the contribution of irreversible cyclic planar slip at the crack tip, and is extended to real cases describing roughness-induced crack closure as a function of fracture surface roughness parameters at low {Delta}K levels where planar slip prevails. The resulting equation indicates that roughness-induced crack closure depends on the maximum stress intensity factor, the standard deviation of heights as well as the standard deviation of angles of the crack profile elements, and the yield stress of the material. Comparison between the prediction of the model and experimental data of K{sub cl} for lamellar microstructures of Ti-2.5Cu as well as TIMETAL 1100 shows good agreement.

Growth and coalescence of fatigue cracks at weld toes in steel: Otegui, J.L., Burns, D.J., Kerr, H.W. and Mohaupt, U.H. Int. J. Pressure Vessels Piping48 2 (Nov 1991) 129–165

Near-equiatomic multi-component high-entropy alloys (HEAs) have engendered much attention of late due to the remarkable mechanical properties of some of these new metallic materials. In particular, one of the first reported HEAs, the equiatomic, single-phase, face-centered cubic (fcc) alloy CrMnFeCoNi, often termed the Cantor alloy, has been shown to display an exceptional combination of strength, ductility and fracture toughness, i.e., damage tolerance, at room temperature, properties that are further enhanced at cryogenic temperatures. Despite this alloy being the most studied HEA to date, its resistance to crack growth under cyclic fatigue loading has not yet been characterized. Here, we examine its fatigue-crack propagation behavior, primarily at lower, near-threshold, growth rates, both at room temperature (293 K) and at 198 K. At 293 K, the alloy shows a fatigue threshold, ΔKTH, of ∼4.8 MPa√m, which increases by more than 30% to ΔKTH ∼6.3 MPa√m with decrease in temperature to 198 K; additionally, the Paris exponent m was found to increase from roughly 3.5 to 4.5 with decreasing temperature. Examination of the fracture surfaces and crack paths indicate a transition from predominantly transgranular crack propagation at room temperature to intergranular-dominated failure at the lower temperature. Such a change in crack path is generally associated with an increasing degree of physical contact between the two fracture surfaces, i.e., roughness-induced fatigue crack closure, which is likely to be the main reason for the difference in the measured thresholds. Additionally, we believe that the higher thresholds found at 198 K are associated with the alloy's higher strength at lower temperatures, which both reduces the crack-tip opening displacements at a given stress-intensity range and prevents plastic deformations of the grains in the wake of the crack. At room temperature, such plastically deformed grains can be associated with a loss of contact shielding of the crack-tip through closure, resulting in a lower threshold compared to 198 K.

A geometric model for fatigue crack closure induced by fracture surface roughness under mode I displacements

A geometric model dealing with the roughness-induced fatigue crack closure under mode I displacements is proposed. It is shown that roughness-induced crack closure can also be acquired without mode II and III displacements at the crack tip. In this mechanism, the severe contact region is near the crack tip. The average crack closure ratio is the result of discrete asperities which come into contact.