Crack Closure Effect Research Articles

Fatigue life prediction for Q420qFNH weathering steel welded joints considering the effect of multiple cracks

Multiple semi-elliptical cracks frequently develop at the weld toes of welded joints. These interfering multiple cracks significantly accelerate crack growth rates and reduce the fatigue life of a welded joint compared to a single crack. This study proposes a method for predicting the fatigue life of a welded joint with multiple cracks, considering weld toe amplification effect, multiple crack interference amplification effect, and crack closure effect. The method is validated through fatigue tests on Q420qFNH weathering steel welded joints. The main factors influencing the interference amplification effect are investigated, and theoretical formulas for the interference amplification factor of the stress intensity factor are derived using the finite element method and the superposition principle. The initiation, aggregation, and morphological evolution during the propagation process of multiple cracks is accurately simulated, and the fatigue life of both cruciform-welded and butt-welded joints is predicted. Additionally, a quantitative analysis is conducted to assess the impact of the number of fatigue cracks on fatigue strength. The results indicate that the interference amplification effect of cracks depends on their distance and mainly affects the surface point where the crack front is closest. The proposed fatigue life prediction method can accurately estimate the fatigue life of Q420qFNH steel welded joints considering the effect of multi-cracks. The interference and coalescence of multiple cracks significantly reduce the fatigue life of welded joints. An increase in the number of fatigue cracks notably decreases the fatigue strength.

On the fatigue crack growth behavior of nanocrystalline CrMnFeCoNi

The fatigue crack growth (FCG) behavior of the equimolar CrMnFeCoNi Cantor-alloy in the nanocrystalline grain size regime has been explored with special focus on the influence of specimen orientation and the effect of additional heat-treatments. For the synthesis severe plastic deformation using high-pressure torsion (HPT) was used. The results were critically discussed regarding the impact of grain size on the FCG-behavior and compared to a conventional 316L stainless steel in the nanocrystalline state: The reduction of grain size in the Cantor alloy from the micrometer to the nanometer regime invokes crack growth rates being about one order of magnitude higher paired with significantly lower threshold values. Furthermore, the sensitivity of thresholds and crack growth rates in the Paris regime on the applied load ratio in the NC material state is also profoundly reduced. This can be explained based on a diminishing contribution of crack closure effects, especially roughness induced closure. Even though the chemical composition differs distinctively, the crack growth rates and fatigue threshold values of the nanocrystalline Cantor alloy and the stainless steel are comparable with the latter being slightly lower for the CrMnFeCoNi alloy. Moreover, the extent of anisotropy is more pronounced than for 316L. Heat-treatment leading in the NC-Cantor alloy to an increasing hardness, has also an effect on the FCG-rate and depends on the specimen orientation. In addition, an unusual self-annealing behavior affecting the FCG behavior of the Cantor alloy was found.

Closure Effect of I + II Mixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing. A mixed-mode I + II loading fatigue crack growth test was performed using EA4T axle steel specimens. The variation of the plastic-induced crack closure (PICC) effect and the roughness-induced crack closure (RICC) effect during crack deflection in the mixed-mode is examined in this study. The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading, and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC. The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC. The roughness of the crack surface before and after the crack deflection is different, and the spatial torsion crack surface is formed during the crack propagation process. With the increase of the crack length, the roughness of the fracture surface increases. During the crack deflection process, the PICC value fluctuates around 0.2, and the RICC value is increased to 0.15.

Effect of cooling rate in the mid-temperature region on fatigue crack propagation rate of bainitic steel

Controlling continuous cooling is an important means to control the most important mechanical properties of bainitic steel for frogs in railroad applications. In this paper, a bainitic steel for railway frogs is taken as the research object. By designing different cooling rates in the bainitic transformation medium temperature region, the effect of cooling rate on fatigue crack propagation rate was studied. The results show the sample of 4 °C/s cooling rate exhibits higher strength and impact toughness, while the crack propagation rate near the threshold is significantly lower and the threshold is relatively higher at 0.1 °C/s cooling rate. This is related to the fact that more massive blocky retained austenite and coarse microstructure in the sample at 0.1 °C/s cooling rate are more favorable to the occurrence of crack closure effect, especially in the near-threshold region. The fatigue crack propagation rate of the tested steel in the stable propagation region at 4 °C/s cooling rate is slightly higher than that at 0.1 °C/s. This is related to its complex interface distribution characteristics. However, with the increase of Δ K, the tested steel at 0.1 °C/s cooling rate tends to preferentially fail, which is related to its poor toughness.

Advancements in fracture toughness testing of ultra-high-strength steel sheets: Unraveling the crack-closure effect and unanticipated thickness independence

A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 ∼ 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness.

Mean Stress Correction of S45C Carbon Steel Based on Crack Growth Concept (Proposal of Non-Closure Model)

Abstract Influence of the tensile mean stress on fatigue life and fatigue limit was investigated for carbon steel. Uni-axial fatigue tests were conducted under stress and strain-controlled conditions at room temperature. The fatigue life and fatigue limit were reduced by applying the mean stress for the same stress amplitude. The fatigue life exhibited a better correlation with the strain range than with the stress amplitude. Increase in strain range caused by applying the mean stress correlated well with the decrease in the fatigue life. It was proposed that the mean stress effect on the fatigue life was brought about by the change in crack growth rate caused by applying the mean stress. The mean stress enhanced crack opening and accelerated the crack growth. The reduction in the fatigue limit was also brought about by the same effect. It was shown that the effective strain range considering crack closure effect gave good prediction of fatigue life and fatigue limit with and without the mean stress.

Small crack growth behavior and correlated microstructure-sensitivity during cyclic deformation of a high-chromium/carbon wrought tool steel

The macro to micro-mechanical mechanisms of cyclic deformation behavior of a carbide-containing ferritic steel was studied, and the exceptional properties obtained in low-cycle regimes were discussed, relying on the sensitivity of the small crack propagation to the microstructural characteristics. In this respect, the evolution of boundaries, sub-boundaries, carbides, and micro-textures in the vicinity of the crack growth path has been characterized in detail. Regarding this matter, the subtle alteration of grain size gradient, resulting in the decrement of incompatibility between mechanical properties of grains, is the main factor that halts the crack growth temporarily. Furthermore, both pre-existed and cyclically developed substructures could tortuous the crack propagation path and induce the crack closure effect, respectively. The advantages and disadvantages introduced by primary carbides, serving as initiators of small cracks, and dispersed high volume fraction of submicron carbides, inducing the formation of an effective strengthening layer (ESL) and acting as deflectors of small cracks, have been investigated, respectively. The {001} <110> oriented grains have been identified as preferred micro-texture contributing to small crack propagation. In total, the small crack sensitivity of the elaborated microstructure has resulted in a prolonged lifetime.

Stability Analysis of Yebatan Underground Cavern Surrounding Rock based on Crack Evolution Theory

The deformation properties of rock are closely related to the effect of crack closure, generation and expansion in the rock. In this paper, the stress-crack strain evolution constitutive model is used to determine the relaxation damage zone of the surrounding rock based on the change regulation of the crack closure ratio during the whole process of surrounding rock damage, and the model is embedded into FLAC3D software, and then the stability evaluation of the surrounding rock of the cavern group under unsupported and supported conditions is carried out for the underground plant of Yebatan hydropower station. The excavation simulation shows that the displacement of the cavern surrounding rock increases with the excavation step, and the influence of the fault on the displacement of the surrounding rock is very significant. The high stress concentration area is distributed in the arch top of the plant, the tailwater connection hole, the arch top and the bottom of the tail chamber, and the tensile stress area is mainly distributed in the high side wall of the underground plant and the tail chamber. The crack closure ratio index was used to quantitatively evaluate the degree of relaxation damage of the surrounding rock, indicating that the relaxation damage area is influenced by the fault, and the volume of the surrounding rock damage area under the system support is significantly reduced compared with the unsupported working condition, showing the effectiveness of the support measures.

Effect of loading conditions on short fatigue crack propagation behaviours in cast Al[sbnd]Si alloys

The effect of loading conditions on short crack propagation behaviours in cast AlSi alloys was investigated by a combination of photo-microscopy monitoring and post mortem analysis. The findings suggest that the path of short cracks is minimally influenced by loading conditions, whereas loading conditions significantly affect the fatigue crack growth rate (FCGR) and its statistical characteristics. Crack closure effect is observed under zero and negative stress ratio conditions. Roughness-induced crack closure occurs due to the coarse grain microstructure of cast AlSi alloys and the Stage-I mode growth of cracks within grains in the microstructural short crack regime. The high level of plasticity around the crack tip under negative stress ratio conditions has a significant impact on FCGR, resulting in a negative crack opening stress. When the stress ratio exceeds approximately 0.3, short cracks exhibit no crack closure. The load-level effect is observed only at high load conditions with negative stress ratios. Finally, a probabilistic short crack growth model was established using a Bayesian linear regression model to analyse the effect of stress ratio on the uncertainty of FCGR. After considering the crack closure effect, the difference in distributions of FCGR model parameters decreases significantly among different stress ratio conditions.

Fatigue crack growth in AA6082-T6/AA7050-T6 bi-materials: Effect of plastic zone ahead of crack tip

The main objective here is the understanding of the effect of plastic zone ahead of crack tip on fatigue crack growth (FCG). For that, a numerical simulation of FCG was made, based on cumulative plastic strain, considering a bi-material composed by the 6082-T6 and 7050-T6 aluminium alloys. The first one has a lower yield stress but a higher critical cumulative plastic strain. The values of da/dN are higher for the 6082-T6 which means that the effect of the easier plastic deformation dominates over the effect of the higher critical cumulative plastic strain. The bi-material 6082-7050 showed a pronounced decrease of da/dN near the interface, which starts when the monotonic plastic zone ahead of crack tip reaches the material transition. This indicates that the crack tip deformation is affected by the material inside the monotonic plastic zone. On the other hand, the bi-material 7050-6082 shows an accentuated increase of da/dN, which starts when plastic deformation is observed ahead of transition. The elimination of the contact of crack flanks reduces drastically the transient effects, which indicates that these are linked with plasticity induced crack closure effects. Crack tip blunting was proposed to be the mechanism of plasticity induced crack closure explaining the trends observed.

A multi-scale experimental investigation on fatigue crack propagation rate behavior of powder bed fusion-laser beam 316L stainless steel subjected to various heat treatments

In this study, three heat treatments referred to as heat treatment 1 (HT1), HT2, and HT3 were applied to tune the microstructure of powder bed fusion-laser beam (PBF-LB) 316L stainless steel. Furthermore, the fatigue crack propagation rate (FCPR) of as-built PBF-LB 316L specimens and specimens subjected to the three aforementioned heat treatments was thoroughly investigated. To this end, the FCPR was evaluated by a fatigue testing machine combined with an infrared thermal camera, and the Paris law was established to quantitatively assess the FCPR of PBF-LB 316L. In addition, a systematic investigation of microstructural evolution, residual stress, tensile properties, and fatigue crack propagation (FCP) fracture surfaces as well as their effect on the FCPR in function of various heat treatments were carried out. The results indicate that the release of residual stresses, the annihilation of the cellular structure, and the occurrence of recrystallization are obtained through the application of HT1, HT2, and HT3 respectively. Furthermore, it is shown that although all three heat treatments can improve the FCPR behavior of PBF-LB 316L to some extent, HT3 PBF-LB 316L presents the lowest FCPR mainly due to its most pronounced plasticity-induced crack closure effect, indicating its highest resistance to FCP even though the recrystallization obtained by HT3 results in the lowest yield strength (YS). Finally, some recommendations about adequate heat treatments concerning fatigue life are suggested.

Simulation of crack closure effect on cracked beam vibrations

Vibrations of beams containing cracks are usually investigated by using open or breathing crack models. Neither one of these approaches considers the plasticity induced crack closure (PICC) phenomena which occurs in fatigue crack growth. In this study, PICC is integrated into the vibration analysis of a cracked beam. First, fatigue crack growth at a notch is simulated with elastoplastic finite element analysis. Residual deformations, stresses and the contact pressures on crack faces are obtained. In the second phase, the model which includes these quantities is used for forced vibration analysis under harmonic loading by using implicit finite element method. For comparison, analyses are also running without PICC. Simulation results indicate that PICC has a significant effect on vibrations of cracked beams. For low load amplitude levels, cracked beam behaves like an intact beam. Nonlinear behavior appears only at rather high amplitude levels. It is concluded that PICC could suppress the crack breathing behavior and this could make it difficult to detect a fatigue crack in a beam from its nonlinear vibration response.

Cutting-edge insights: Crack growth rates in 9% Ni steel from room to cryogenic conditions influenced by crack closure and effective stress intensity factors

This study investigates the fatigue crack growth rate of 9% Ni steel at varying stress ratios under both room and cryogenic temperatures, attributing different growth rates to the phenomenon of crack closure. Traditional methods of studying crack closure, including the opening force, compliance ratio, and adjusted compliance ratio techniques, are assessed. Our results are juxtaposed with previous formulae for the opening force technique, leading to the introduction of a new formula that incorporates temperature effects by normalising the stress intensity factor with the yield strength at the respective test temperature. We highlight the adjusted compliance ratio (ACR) technique's limitations at cryogenic temperatures due to challenges in measuring notch open-crack compliance. As a solution, we present an innovative method that determines compliance using the effective elastic modulus. The mechanics behind the ACR technique are elucidated through the lens of the wedging mechanism. Our insights reveal that the ACR technique precisely calculates the effective stress intensity factor range (ΔKeff) under specific conditions. We address existing limitations by introducing a novel method, the triple compliance ratio (TCR) technique. A comparative analysis of effective stress intensity factors from all four techniques reveals the TCR technique's superior ability to correlate crack closure with crack growth. By eliminating the crack closure effect, the TCR technique offers a more accurate prediction of fatigue crack growth rate across diverse load ratios.

Mixed-mode I&II fatigue crack growth behaviors of 16MND5 steel: The role of crack driving forces and crack closure

Although numerous fatigue crack growth (FCG) tests have been conducted, the conclusions drawn regarding the mixed-mode FCG rate are inconsistent and even contradictory. A reasonable conjecture is that crack driving force and crack closure effects may play a crucial role in causing this situation. In this paper, the 16MND5 steel, commonly used reactor pressure vessel (RPV) steel, was employed to investigate the mixed-mode I&II FCG behaviors through the compact-tension-shear (CTS) experiments. The influence of crack driving force and crack closure effect was analyzed and discussed. The results show that the crack propagation path tends to maximize the tangential stress component at the crack tip, and the effect of ΔKII can be negligible once the Paris region is reached. Moreover, when considering the crack closure corrected ΔKI, named ΔKI eff, all the FCG rate data under different loading conditions can be collapsed into a narrower band of distribution. The premise for arriving at this result is to use the stress intensity factors solution that are applicable to specimens with real slanted propagating cracks, rather than applying equivalent methods based on single-edge straight crack assumptions. Finally, the fatigue fractography further confirms that the FCG mechanism during the Paris region all follow a similar quasi-mode I behavior.

Investigation of fatigue crack closure effect on the evaluation of edge cracks with the fundamental mode of edge waves

Fatigue cracks often initiate and propagate from edges of structural components. Detection and evaluation of edge cracks could be very challenging, specifically, due to the crack closure phenomenon, which makes fatigue cracks to be partially closed when the applied loading is removed; this usually corresponds to maintenance and NDE inspection conditions. Despite that fatigue crack closure is well investigated, past experimental and theoretical studies related to guided wave-based NDEs largely ignored this phenomenon. In this article, the fundamental symmetric mode of edge waves (ES0) is used to evaluate crack closure effects on the evaluation of fatigue cracks. The experimental studies have demonstrated that the reflected and transmitted signals at different frequencies correlate very well with the length of the open region of fatigue cracks. However, an accurate evaluation of the total crack length can only be conducted under an applied loading, which fully separates the crack faces. Finally, a new FE model has been proposed to simulate the fatigue crack closure and its effects on propagation of ultrasonic bulk and guided waves. The outcomes of FE modelling and experimental study were found in a good agreement.

Stress and frequency dependence of wave velocities in saturated rocks based on acoustoelasticity with squirt-flow dissipation

SUMMARY We perform seismic and ultrasonic measurements in carbonate and shaley sandstone samples as a function of differential pressure. The velocities show a strong frequency and pressure dependence. The dispersion disappears with increasing pressure and the squirt flow in turn inhibits the pressure dependence. To model these effects, we combine the Gurevich's squirt-flow model with the Mori–Tanaka scheme and the David Zimmerman model, and extend it with third-order elastic constants, to obtain a frequency-dependent acoustoelasticity model. Comparisons between measurements from this study and literature and modelling results show that the P-wave velocity increases non-linearly first and then nearly linearly, dominated by crack closure and acoustoelasticity, respectively. The pressure dependence of wave velocities is reduced by liquid substitution and further by the squirt-flow mechanism. The effects of fluid properties and crack closure on P-wave velocity decrease with differential pressure. The results will feed a new model and help better understanding the wave propagation in pre-stressed rocks at different scales.

An Overview of the mechanism-based thermomechanical fatigue method (DTMF) in the design of automotive components

The DTMF damage Parameter described in the paper is the generalization of the creep fatigue parameter (DCF) proposed by Riedel, which quantifies the amount of damage produced by a non-isothermal loading cycle. The entire design methodology is outlined in the text, starting with the calibration of Chaboche's viscoplastic model and then the determination of the thermomechanical fatigue (TMF) parameters. Chaboche is generally a good choice for its ability to describe well both kinematic and isotropic hardening behaviors, also enabling stress relaxation and strain rate dependency to be considered. The DTMF method is based on the fracture mechanics concept of cyclic crack tip opening displacement (ΔCTOD) where the effective stress range takes the crack closure effect into account. This method is typically used to describe the thermomechanical low cycle fatigue behavior of cast irons, cast steels, stainless steels, Inconel, HiSiMo and nickel-based alloys. These materials are used in components that need to withstand high temperatures (higher than half of the melting point) in service as exhaust manifolds, cylinder heads, valves, turbochargers and disk brakes. Three failure mechanisms are detailed here: oxidation, creep and fatigue. Oxidation is a complex phenomenon that may occur when the material is hot under tensile in-phase loading or hot under compressive out-of-phase loading. Creep is a time and temperature dependent diffusion process which is incorporated into the DTMF parameter as a multiplicative factor. In this context fatigue life is defined as the number of cycles to propagate an existent microcrack up to an arbitrary size that is defined on a case-by-case basis.

Plasticity-induced crack closure in the presence of loading irregularities in short cracks initiated at interior defects

A crack propagation finite-element model of interior defect-initiated short cracks under near-threshold loading conditions is used to investigate the effect of plasticity-induced crack closure in the presence of a single overload and underload. The effect of an overload predicted by the present model is qualitatively very similar to what is commonly reported, but in this case an underload produces a differing result to the literature consensus, yielding a similar but weaker effect as the overload. Extended acceleration of crack propagation due to a single underload might not be attributable to plasticity-induced crack closure under plane strain conditions. A distinct crack profile shape consequent of crack tip blunting is present with both loading irregularities. A new method for examining crack closure for the entire crack surface is presented.

Fatigue crack driving force of railway bogie frames using rigid-flexible coupled dynamics: A case for beam model

The stress intensity factor is essential to determine the residual fatigue lifetime of cracked components. However, the traditional method used to determine the stress intensity factor is very difficult to characterize the dynamic behaviors of structure arising from the fatigue crack. This paper proposed a methodology to determine the dynamic stress intensity factor (DSIF) of a crack incorporating with the rigid-flexible coupled dynamics. Firstly, a rigid-flexible coupled dynamic model of cantilever beam representing the typical structure of bogie frame with a straight crack was developed based on SIMPACK platform. In the model, the equivalent spring - contact element was employed to model the contact behaviors of crack interface. Subsequently, the DSIF considering the crack closure effect was calculated by the node displacement extrapolation. The validity of proposed method was demonstrated through comparing the DSIF with those obtained by other analytical methods. The results suggest that the proposed methodology can characterize the closure effect of crack, and yield more accurate estimation for the DSIF. This method establishes a link between the multibody system dynamics and the fracture mechanics, which enable us to calculate the DSIF of crack under the operating condition using a rigid-flexible coupled dynamic model.

The cyclic deformation behavior and microstructural evolution of 304L steel manufactured by selective laser melting under various temperatures

The low cycle fatigue (LCF) tests with strain-controlled are carried out to investigate the cyclic deformation behavior of 304L steel manufactured by selective laser melting (SLM 304L) at room temperature (RT), 300 °C, and 650 °C. According to the experimental results, the continuous cyclic softening behavior of SLM 304L is presented at different temperatures. Meanwhile, due to the external force during the fatigue test, the substructure morphology is changed from the elongated cellular substructure of the as-built SLM 304L to the cell substructure of the failure sample at different temperatures. Moreover, since the dislocation rearrangement process is a net decrease in dislocation density, the continuous cyclic softening behavior is presented. Besides, the dynamic recovery and recrystallization promote the cyclic softening behavior of SLM 304L at high temperatures. Additionally, when the multiplication and annihilation of dislocations research the equilibrium state, the dislocation density has no obvious change in the microstructure, leading to the steady state of the cyclic response curve. Finally, considering the influence of the crack closure effect, a new fatigue life prediction model is proposed and discussed.