Fatigue Crack Growth Retardation Research Articles

Phase field modeling of fatigue crack growth retardation under single cycle overloads

A zone-based crack retardation model that responds to single cycle overloads is applied to a phase field fatigue framework. Fatigue crack growth is incorporated through the degradation of fracture toughness in the phase field fracture model where a representative loading strategy is followed instead of explicit cyclic loading. The model is demonstrated to capture Paris-Erdogan law type behavior. Crack growth retardation following an overload cycle is simulated through a zone ahead of the crack tip taking inspiration from existing plastic zone-based retardation models. The retardation zone is described through a strain energy density limit, with the overload ratio controlling the extent to which the fatigue damage accumulation rate is slowed inside the retardation zone. The model is implemented utilizing user subroutines in Abaqus with coupled-temperature displacement elements where temperature acts as a stand in for the phase field parameter. The model is found capable of reproducing experimental levels of fatigue life gains and reduction in crack growth rate for compact tension and center-cracked tension panel specimens when various overload levels are applied. Furthermore, the model is demonstrated to be able to capture complex crack paths with great accuracy.

Effect of laser peening without coating (LPwC) on retardation of fatigue crack growth in SM490 plates

Laser peening without coating (LPwC) is a well-known technique to improve high-cycle fatigue properties by introducing compressive residual stress (RS) near the surface of metal components. In this study, X-ray diffraction (XRD) and flexural fatigue tests were applied to pre-cracked 12 mm thick SM490A welding structural steel specimens that were subjected to LPwC nearly 20 years ago with a pulse energy of 200 mJ, a spot diameter of 0.8 mm and a pulse density of 36 pulse/mm2. XRD revealed that the compressive RS has remained stable to date, with approximately 400–500 MPa remaining at the surface and a compressive depth of approximately 0.9 mm from the surface, which is comparable to the data measured by XRD immediately after LPwC. In the flexural fatigue tests with a stress ratio of 0.1 and stress rages of 100, 150 and 200 MPa, LPwC extended the fatigue life by more than 1.6 times, depending on the stress range and individual specimens. Crack restarting cycles were significantly increased by a factor of at least 1.8, and the crack growth rate was suppressed by a factor of about 0.7 or less.

Roles of microstructures in high-cycle fatigue behaviors of 42CrMo high-strength steel under near-yield mean stress

In this study, high-cycle S-N characteristics and fatigue cracking behaviors under near-yield mean stress of 42CrMo steel with two different fine/coarse-grain tempered sorbite/bainite microstructures (FGM/CGM) were investigated. The results show that fatigue strength is greater for FGM than for CGM as expected, but fatigue crack growth (FCG) threshold is higher for the latter. Compared to FGM with a single crack source, CGM has multiple fatigue sources together with higher-degree ratchetting-fatigue damage, i.e., plastic-strain, dislocation cells and sub-grains, thus accelerating fatigue crack initiation. In the near-threshold regime, roughness-induced crack closure in coarse-microstructures of CGM leads to FCG retardation behavior, which increases FCG threshold. In the Paris regime, CGM presents a similar FCG rate to FGM. Microstructure barriers (i.e., crystal interfaces, carbides and interfaces with different orientations), secondary cracks, crack-path fluctuations, ratchetting and plastic strain jointly result in severer crack-tip stress shielding effects in CGM than in FGM. However, fatigue crack tends to propagate through the high Schmid factor region with {110}<111> slip system with lower slip resistance in FGM but not in CGM, which facilitates fatigue cracking of the former. Under the combined effects of the above factors, similar FCG rates are thus exhibited in both steels with different tensile strengths.

Retardation effect on sheet steel under a mixed I–II mode overload revealed by experiment, DIC and FEM methods

As a high strength and commercial steel, Q420 is widely employed in many engineering applications. In this paper, a series of experiments was conducted to investigate the fatigue crack growth retardation mechanisms under mixed I–II mode overloads with loading angles 0, 30, 60 and 90°. The baseline of loading spectrum is a pure mode I cyclic loading with a mixed overload applied after pre-cracking. The results show that the crack path is always straight for each case. Both the retardation level and fatigue life decrease with the increasing loading angle. A combined model that considers the residual stress, plasticity-induced and roughness-induced crack closure effects was proposed to predict the fatigue crack growth rate and fatigue life. The predicted results agree well with several experimental results.

Influence of microstructural deformation mechanisms and shear strain localisations on small fatigue crack growth in ferritic stainless steel

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.

Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel

R4-grade mooring chain steel specimens were subjected to alternating phases of saltwater corrosion and air fatigue to replicate seasonal load variations on chains in service. No significant difference in fatigue lifetimes was registered between precorroded specimens subjected to continuous fatigue and those periodically interrupted by phases of accelerated corrosion. Moreover, eight months of natural corrosion of specimens with fatigue cracks did not have a large effect on their remaining fatigue lifetimes. Retardation of fatigue crack growth by crack tip blunting from corrosion is deemed unlikely based on observations from the current work and the literature. Corrosion accelerated by anodic polarization is demonstrated to be unsuitable for replicating natural corrosion of fatigue cracks.

Application of strain energy based approach for evaluation of fatigue crack growth retardation effect under random overload

Fatigue crack growth in structural components subjected to random amplitude loading is a significantly complex problem. Many models have been proposed to estimate the retardation effect due to a single overload, though for the random overload sequence, there is still a lack of effective quantitative description methods. In this research, a new time to crack initiation (TTCI) type for the calculation of equivalent initial flaw size (EIFS) is proposed based on the change in net-section strain energy density. For a certain number of fatigue cycles and current crack lengths, the value of EIFS can be obtained by solving the corresponding integral equation. The proposed EIFS calculation method was applied for the prediction of fatigue crack growth under single overload and random overload sequences. The prediction results agree well with the experimental data for different conditions. For a certain single overload or overload spectrum, the retardation effect is reflected in the decrease in EIFS, which appears to propagate from a smaller initial crack length. The proposed method shows that the new TTCI type of EIFS can be used as the quantitative characterization of the retardation effect due to the random overload sequence.

Analysis of near-tip fatigue crack path deflection in metallic materials

This article studies the plasticity-induced fatigue crack growth retardation in a steel caused by crack path deflection in the near-tip region. To this end, the finite element method was used to model the crack with its deflected kink. The results allow a calculation of the fatigue crack propagation rate for the two analyzed crack configurations, i.e.: (i) locally-multiaxial fatigue (main crack plus deflected kink) and (ii) uniaxial fatigue (only main crack). It is seen that the retardation effect rises with the increase of the initial deflected crack tip angle.

Analysis of near-tip fatigue crack path bifurcation in metallic materials

This article studies plasticity-induced fatigue crack growth retardation in metals caused by crack path bifurcation in the near-tip region. To this end, the finite element method was used to model the crack with its bifurcated tip under several stress intensity factor (SIF) ranges. The results show the appearance of a retardation effect in the fatigue crack growth rate of the bifurcated crack in relation to the growth rate of the fully straight crack.

Investigation of Laser Treatment as a Method for Fatigue Crack Growth Retardation in Aluminum Alloy 2198-T851

Among the third-generation Al-Li alloys, AA2198 stands out for its lower density, formability and increased stiffness, being suitable for use in aircraft fuselage sheets and other inner structures in order to reduce weight and improve performance. An important topic related to damage tolerant structures is the development of techniques to retard fatigue crack propagation, such as the localized heating by a laser source. The aim of the present work was to find the most suitable parameters for the production of laser heating lines in 2198-T851 alloy sheets in order to reduce the fatigue crack growth rate in this material. Laboratory tests using C(T) specimens under two loading conditions (R = 0.1 and 0.5) provided a useful dataset on the laser heated material. The experimental results indicate a 200 W laser beam power at treatment speeds of 1 and 10 mm/s was sufficient to retard crack growth in the current setup. The more expressive results were obtained for 200 W laser power with a speed of 1 mm/s and cyclic loading with stress ratio R = 0.1.

Fatigue behavior of corroded steel plates strengthened with CFRP plates

This paper presents an experimental study performed to investigate the effectiveness of carbon fiber reinforced polymer (CFRP) plates in extending fatigue life of corroded steel plates. Specimens with five kinds of corrosion damage level, four kinds of strengthening configuration and four kinds of applied fatigue stress amplitude were tested and the primary fatigue behavior indications consist of failure mode, fatigue fractography, fatigue life and crack propagation were analyzed. Results showed that externally patching CFRP plates significantly strengthened the fatigue behavior of corroded steel plates, and even made the fatigue life of corroded steel plates recovered or exceeded that of the un-corroded ones, the fatigue life of the patched specimen which presented the most significant strengthening effectiveness even showed a fatigue life extension of more than 85.3 times with respect to the un-patched corroded steel plate and approximately 2 times over the un-corroded steel plate. Results also indicated that different strengthening configurations appeared to have diversity effect on the fatigue life extension and crack growth retardation of corroded steel plates, end anchorage measures effectively delayed the failure process of CFRP bonding interface and improved the fatigue strengthening effect, increasing CFRP strengthening stiffness or adopting prestressed CFRP plates significantly reduced the crack growth rate and extended the fatigue life of corroded steel plates. The outcome of the tests provided meaningful references and essential data for the reliable application of CFRP strengthening system in the fatigue performance improvement of corroded steel structures.

Fatigue crack-growth retardation after overloading in gaseous hydrogen: Revisiting the effect of hydrogen on crack-tip plastic-zone development

The impact of hydrogen on crack-tip plastic-zone development was revisited via a novel approach, utilizing the measurement of fatigue crack-growth retardation in a medium-strength martensitic steel after a single overloading in laboratory air and in 90-MPa-hydrogen gas. The plastic zone can be characterized according to the crack-propagation length for reverting from the retardation caused by plasticity-induced crack-closure ascribed to overloading (overloading-affected, crack-growth distance). Hydrogen sharpened the shape of overloaded crack-tip and suppressed the extension of the severely-deformed zone in the crack proximity. Besides, it enhanced frequent crack-tip branching, giving rise to a slower crack growth rate than the in-air situation at the initial stage of retardation. However, no change in the overloading-affected, crack-growth distance was detected between the in-air and hydrogen-gas conditions. Ultimately, hydrogen barely altered the overall plastic-zone size.

Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.

Numerical Modeling of Plasticity-Induced Fatigue Crack Growth Retardation Due to Deflection in the Near-Tip Area

This article studies the retardation effect in plasticity-induced fatigue crack growth rate for a low-medium strength steel, due to the appearance of microdeflections in the crack path. To this end, the finite element method was used to model the crack with its kinked tip under several stress intensity factor (SIF) ranges. The results allowed a calculation (after a small number of cycles) of the fatigue crack propagation rate for the multiaxial and uniaxial fatigue configurations at the microscopic level. It was observed that the retardation effect rose with an increase in the initial kinked crack tip angle, an increase in the initial projected kinked crack tip length, and with a decrease in the SIF range.

Effect of laser heating on mechanical properties, residual stresses and retardation of fatigue crack growth in AA2024

AbstractLocal laser heating treatment using a defocussed laser beam was applied to the surface of 2‐mm‐thick AA2024‐T3 sheets. Two different treatment designs—namely, lines and circles—as well as the positioning and number of treatments were investigated regarding their potential to retard the propagation of through‐thickness fatigue cracks. The highest fatigue crack growth life extension of 285% was achieved by the application of four laser heating lines or two circles on each specimen side. The induced compressive residual stress field through the LH process is primarily responsible for an improvement in fatigue crack growth life. An emphasis was placed on investigating the effect of the treatment on the possible reduction of tensile and fatigue strength (S‐N life). If only one line was applied transverse to the loading direction or only one circle was used, the reduction of fatigue strength was comparable to the reduction of fatigue strength resulting from the stress concentration introduced due to the presence of a rivet hole.

Effect of periodic overloads and spectrum loading on fatigue life and microstructure in a Grade 900A rail steel

Fatigue life and crack growth retardation due to spectrum loading, which are superposed on constant amplitude cycles, have been investigated in this paper. For this purpose, numerical and experimental methods are used to investigate the behavior of fatigue crack growth in the rail specimens. A 3D nonlinear boundary element model has been applied to estimate the fatigue life of the rail specimens. Then, the effect of several parameters on the fatigue life in rail specimens is investigated using the suggested 3-D boundary element model and experimental tests. In the numerical analysis, the modified Paris model has been used. The analysis and experimental tests have been performed on C(T) specimens made of a pearlitic Grade 900A steel. In the experimental tests and numerical analysis, the periodic overloads have remained constant and the effect of overload ratio (OLR) and occurrence ratios (OCR) on the fatigue life has been investigated. Finally, microstructure analysis on microstructure morphology of the region where cracks grow for specimens was performed. Comparison between experimental results and numerical data indicates that the modified Paris model and the Forman_Newman_De Koning (FNK) model is actually accurate.

Experimental Study on the Fatigue Crack Growth and Overload Effect in Medium Density Polyethylene

The fatigue crack growth behavior of medium-density polyethylene (MDPE) under constant amplitude load was experimented on plate specimens. The effect of additional imposed single overload was also researched. An optimized crack front marking method was developed to obtain the actual crack front shape of polymer materials in fatigue crack growth, which was successfully applied to the studied MDPE. The results showed that the retardation of fatigue crack growth evidently appeared after the tensile overload was applied, which was similar to metallic materials. The passivation of the wedge-shaped plastic zone was a primary cause to crack retardation. The theory based on residual stress was introduced to describe the retardation effect, and the plastic zone size of crack tip was calculated by the Dugdale model. The Wheeler model was successfully used to fit the decreased crack growth rate.

The effect of grain size on the fatigue overload behaviour of nickel

The fatigue overload behaviours of coarse grain (~30 μm), ultrafine grain (360 nm) and nanocrystalline (~30 nm) Ni are compared under constant amplitude loading at R = 0.1 after a 100% overload. Synchrotron X-ray diffraction is applied to map the elastic crack-tip strain fields at the mid-thickness of a compact tension specimen of nanocrystalline Ni at various stages through the loading cycles, from which the variation in stress before, during and after overload is estimated. Digital image correlation is used to measure the crack length and the displacement fields at the specimen surfaces for both grain sizes, from which the fatigue crack growth (FCG) rate, crack opening displacement and stress intensity factor range are determined. The FCG for coarse grain Ni is most significantly retarded whereas the nanocrystalline Ni is least affected by the overload due to an increased yield stress and a more planar crack surface morphology. As a result, FCG retardation by plasticity, surface roughness and residual stress-induced crack closure are reduced.

Near-tip strain evolution and crack closure of growing fatigue crack under a single tensile overload

Fatigue crack growth retardation of AZ31 alloy under a single tensile overload was investigated by 3D synchrotron radiation, 2D in-situ SEM and digital image correlation techniques. Results showed a consistent evolution of crack closure level by both 3D and 2D methodologies. The straining and plastic zone ahead of crack-tip, and crack opening displacement behind, which were quantitative indicators of crack-tip shielding, residual stress formation and relaxation, were correlated well with the retardation behavior. Mechanistically, compressive residual stress played a dominant but time-limited role while crack closure was secondary, based on an intrinsic load transfer concept during crack growth.

Two engineering models for predicting the retardation of fatigue crack growth caused by mixed mode overload

The effect of mixed mode overload on the retardation of pure mode I cyclic loading has been investigated experimentally on Al5083-H111. Two models for extension of the Wheeler retardation model are proposed in order to calculate retardation of fatigue crack growth subjected to mixed mode overload. The first proposed model can predict retardation with high precision. The second proposed model is a simplified form of the first one and does not contain more experimental constants than the Wheeler model so, it may be more practical. The application of the proposed models has been examined for another material and testing conditions.