Mode Fatigue Crack Propagation Research Articles

Deformation behavior at fatigue crack tip in nickel-based alloy

The microstructure and deformation behavior around the fatigue crack tip can significantly affect the early stage of the fatigue crack propagation, but currently, the evolution and deformation mode of the microstructure in the fatigue crack tip at the nanoscale are still unclear. Therefore, it is important to study the microstructure and deformation behavior at the fatigue crack tip in nanoscale to further reveal the fatigue failure mode of metal materials. In this study, the microstructure evolution and deformation mode of the fatigue crack tip of nickel-based alloy are studied by HRTEM technique. The results show that a large number of dislocation, stacking fault and micro/nano-twin are formed around the fatigue crack tip. The formation of these microstructures can promote the release of strain in the matrix during cyclic deformation. The main deformation mode of the fatigue crack tip includes slip, twinning, and dislocation shearing micro/nano-twin.

On the fatigue resistance assessment of friction stir welded joints affected by heat input

In view of the fact that composite friction welding parameters cannot independently reflect the service life of automotive joints, the evolution of fatigue behavior caused by process changes was studied by integrating their effects into unified heat input. Taking into account the geometric mutation and mechanical modifications of post-weld Al alloys, the developed hot spot stress estimation model of such joint has the ability to represent the influence of thermal forming process on the alternating singular stress, and is further reflected in the predicted life. The results show that on average, every increase in welding heat input energy by 8515 J led to an improvement in fatigue life of approximately 25.79 %. It was found that fatigue strength benchmark specified by hot spot stress improved the goodness of fit by 29.14 % to 55.94 % over the nominal stress, while increasing the inter-process variation coefficient by 10.77 %, thus exhibiting more accurate failure calibration for this non-uniform spot-welded structure. It was further verified that the life span error predicted using the MC model was 17.67 % to 45.54 % lower than that of the SWT model and closer to the experiment. The fatigue crack propagation modes with respect to load levels were also revealed through fracture morphology. Overall, the study provided a process-regulated fatigue resistance assessment for vehicle joints.

I+II mixed mode fatigue crack propagation under combination of tensile loading and corrosion in a HSS specimen

Both fatigue and corrosion are two phenomena that can be very often found in metallic components. Thus, it is necessary to study their mutual effect. In this paper, a high strength steel specimen was modelled via finite element method in order to investigate the interaction of a fatigue crack and a corrosion pit. Particularly, a rectangular specimen with a corrosion pit and a nearby angled crack under remote tensile loading was modelled and the crack deflection angle was investigated via MTS and SED criteria for various crack lengths and various crack inclination angles. The results obtained clearly show how the crack behavior is affected by the presence of the corrosion pit.

Fatigue response of open hole plates: A finite element simulation investigating the influence of dynamic and static cold expansion processes

Cold Expansion (CE) techniques are extensively used in the aeronautical industry to enhance the fatigue life of open-hole plates. However, the availability of accurate Finite Element (FE) models to simulate the fatigue behavior of this process, particularly Dynamic Cold Expansion (DCE), is limited. This study introduces two novel methods for predicting the fatigue response of DCE and Static Cold Expansion (SCE) open-hole plates. The first method directly estimates the total fatigue life using Continuum Damage Mechanics (CDM) and the Theory of Critical Distance (TCD). The second method separates the prediction of fatigue crack initiation life and propagation life, incorporating CDM, TCD, and the Extended Finite Element Method (XFEM). Moreover, FE models are developed to simulate residual stress, stress under external cyclic loads, and fatigue crack propagation behavior for both DCE and SCE specimens. The proposed methods are evaluated, compared, and the mechanisms behind fatigue life enhancement and fatigue crack propagation modes in CE specimens are discussed. It is found that the prediction accuracy is enhanced by considering stress distributions along the thickness direction and improving the Line Method (LM) in TCD through the introduction of a novel CE parameter. The results demonstrate that both methods achieve good predictive performance, with an average error index within ±30%. Furthermore, it is observed that both DCE and SCE processes primarily improve the fatigue crack initiation life of open-hole plates, with the percentage of crack initiation fatigue life increasing as the expansion size increases. The majority of the fatigue crack propagation life in CE specimens is concentrated in the initial stages of crack propagation. In addition, the effects of DCE and SCE processes on reducing the fatigue crack propagation rate are more pronounced along the thickness direction compared to the width direction, leading to distinct crack propagation modes between CE and non-cold expansion (NCE) specimens.

Post-impact Fatigue Performance of 2198-T8 Aluminum-Lithium Alloy Sheet with Pre-crack

Abstract The fatigue test with the digital image correlation system is applied to 2198-T8 aluminum-lithium alloy sheets with preset cracks and impact dents. The influence of different inserts and impact energies on residual fatigue life and failure behavior is analyzed. The results show that the shape of inserts is the main factor of the residual fatigue life at the same impact energy following the order: U-shaped > hemispherical > conical. Moreover the fatigue life decreases with the increase of impact energy. The hemispherical and U-shaped inserts cause local hardening at low impact energy 5 J, resulting in a maximum increase in fatigue life of 12.09 %. In the initial stages of fatigue failure, high strain areas are always around the preset crack where the propagations initiate. There is a multi-axial loading area around the impact dent in the middle stage. Its fatigue crack propagation mode is type I+III, resulting in a deceleration period.

Low-temperature fatigue behavior and damage analysis of Al-7Zn-2Mg-1.5Cu alloy based on hysteretic energy

The fatigue performance of Al-7Zn-2Mg-1.5Cu alloy was investigated using strain-controlled low-cycle fatigue (LCF) tests at various low temperatures focusing on the effects of hysteresis energy on fatigue life and the analysis of corresponding parameters. The results show that the alloy exhibits different cyclic hardening ratio depended on the applied total strain amplitude. The cyclic hysteresis energy displays cyclic stabilization at different temperatures. The initiation of fatigue crack occurs on the surface of specimens and many fatigue striations can be found on fatigue crack propagation region. In addition, the hysteresis energy model can be used to evaluate fatigue life at different temperatures. Wherein, intrinsic fatigue toughness (W0) decreases obviously with the decrease of temperature, while damage transition exponent (β) increases slightly. Comparing with low temperature tensile test results, Wo is related to the static toughness and β has a relation with the yield strength ratio. The relationships between the dislocation configuration, fatigue crack propagation mode and two fatigue parameters are discussed from microscopic viewpoint.

Significant reduction of fatigue crack non-propagation limit caused by damage accumulation mode fatigue crack propagation in a precipitation-hardened punched steel plate

Precipitation-hardened steel exhibits specific characteristics due to the punching process that deteriorates its strength. To investigate these characteristics, fatigue tests were conducted with artificial micro defects on the punching processed surface that imitate the flaws caused by the process. The authors found that changes in material properties and the flaws change the fatigue strength properties. Specifically, the fatigue crack extends in the damage accumulation mode, which does not have the non-propagation mechanism and reduces the crack non-propagation limit. Furthermore, the damage accumulation mode fatigue crack propagation changes the fatigue limit's physical meaning from the fatigue crack non-propagation limit to the initiation limit. Finally, processing conditions that improve the fatigue strength characteristics were proposed.

Fatigue life enhancement mechanism and lifetime prediction of AA6061-T6 open-holed sheet treated by electromagnetic driving dynamic cold expansion

In this study, mechanisms of fatigue life enhancements of electromagnetic driving dynamic cold expansion (DCE) open holed sheets were investigated by testing fatigue life and obtaining residual stress fields experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distributions was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The differences in stress distribution led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Moreover, the predicted fatigue life results were in accordance with experimental data. • Dynamic cold expansion extended larger fatigue life for holed-sheets. • More uniform residual stress was induced by dynamic cold expansion. • Fatigue cracks propagated horizontally in dynamic cold expanded specimens. • Proposed method to predict fatigue life had good agreement with experiments.

Residual stress distribution and fatigue performance investigations of electromagnetic force-based dynamic cold expansion open-holed sheet

In this study, residual stress distributions and fatigue performances of electromagnetic driving dynamic cold expansion (DCE) open-holed sheets were investigated. Fatigue behaviors were tested, and residual stress fields were obtained experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distribution was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The stress distribution differences led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Uniform stress distributions along axial direction in DCE specimens contributed to random initiations and horizontal propagations of fatigue cracks, while the fatigue cracks in SCE specimens usually initiated at the mandrel entrance layer and propagated to the mandrel exit layer. Moreover, the predicted fatigue life results agreed well with experimental data.

Digital-image-correlation observation of cyclic plastic strain field during the damage-accumulation mode of fatigue crack propagation under pure cyclic mode II loading for cold-rolled SUS430 steel

Fatigue cracks propagate intermittently by the cyclic initiation and coalescence of voids in front of the fatigue crack tip during fatigue crack propagation in the damage accumulation mode. In this study, digital image correlation is used to measure the evolution of the measured plastic strain distribution near the crack tip with loading and cyclic loading during one cycle. The evolution is estimated based on strain distribution measurements results and mechanical considerations. The strain is set at the unloading point because results indicate that the crack propagated in the region where the experimental shear plastic strain is the maximum in the first quarter cycle (at the first maximum load); the measured plastic strain is almost constant in the region where the crack propagates, and the crack is initiated by a subsequent cyclic strain localization and coalescence with the main crack. This study clarified the damage accumulation mode fatigue crack propagation mechanism with the measured strain behavior, and presents a method to use continuum plastic strain at the crack tip as a mechanical driving force for this mode.

On the influence of different in-plane biaxial loading conditions over FCG lives

In-plane biaxial loading conditions have a strong influence over fatigue crack growth trajectories and lives. Under equal and proportional loadings on both axes, pure mode I propagation occurs. Mixed mode conditions were simulated using different biaxial load ratios, and mode II propagation can be introduced when different loading phases or mean load values are used. Using an automatic algorithm, different in-phase and out-of-phase loading conditions were simulated in a cruciform specimen. Fatigue life is proportional to initial notch angle β, but inversely proportional to the load phase shift ϕ. The present results increase mixed mode fatigue crack propagation understanding.

Fatigue crack propagation direction under different loading conditions using MTS and MSS criteria

Fatigue Crack Growth plays a major role on structural integrity. When mixed mode fatigue crack propagation is introduced by complex geometry or loading conditions, numerical methods are essential for fatigue life determination. This paper explores the differences between two crack propagation models. Using an automatic fatigue crack growth algorithm and the finite element method, crack propagation on CTS and four-point bending specimens was simulated. Both specimens allow for pure mode I, mixed mode and pure mode II loading conditions. According to the maximum tangential stress criteria, fatigue crack propagation occurs along the direction where tangential stress is maximized. Comparing crack propagation results with literature, it was possible to conclude that this criterion can be used under pure mode I or mixed mode loading conditions. The maximum tangential stress criterion was also compared to the maximum shear stress criterion, where crack propagation occurs along the direction that maximizes shear stress. Crack propagation directions between these criteria were compared and it was possible to conclude that the maximum shear stress criterion should be used for pure mode II loading conditions. Overall, this methodology allows for fatigue crack growth simulations under complex loading conditions.

Effects of rivet hole arrangement on fatigue performance of thin sheets for fuselage: DIC and numerical calculation

The fatigue performance and fracture mechanics of 2198-T8 Al-Li thin sheet with multi-diameter rivet holes were studied experimentally and numerically, which is mainly used in the new generation of C919 aircrafts. Firstly, the fatigue crack propagation mode under uniaxial fatigue load was investigated based on digital image correlation and virtual extensometer. Then, the Stress Intensity Factor (SIF) formula was modified and the applicable range was determined based on the tests and finite element method simulation results for the specimen with single rivet hole. Finally, the modified SIF formula was applied to predict the fatigue life of specimen with multi-diameter rivet holes. The results show that the crack propagation mode transforms from mode I to mode I+II, and the modified SIF formula could be used to obtain a more conservative fatigue result for single rivet hole when the angle of pre-crack is less than 5°. For the specimen with multi-diameter rivet holes, the fatigue life decreases with the rise of diameter of the larger rivet hole Φ1, and it also decreases with the increase of difference between larger and smaller hole Φ1−Φ2 when larger diameter Φ1 is constant. The fatigue life of HH-type specimen is generally smaller than that of the SS-type under the same rivet hole arrangement. The prediction result is conservative with a maximum relative error less than 7.0%

Fatigue behavior of bird-beak SHS X-joints under cyclic brace axial forces

This paper presents an experimental investigation on the fatigue behavior of bird-beak X-joints under brace axial force. Two square bird-beak and two diamond bird-beak specimens were fabricated by using 80% partial joint penetration (PJP) welds. Static loading was firstly carried out to measure the hot spot stresses at weld toes, by which stress concentration factors (SCFs) were extrapolated and subsequently the critical locations were identified. A cyclic axial loading was then implemented on the brace end. The fatigue behavior investigated includes crack initiation, crack propagation, failure mode, fatigue life and stiffness degradation. The results show that the SCFs calculated from test data could well predict the location of crack initiation. Furthermore, the through-thickness crack lives ( N 3 ) of all are found to be greater than the fatigue lives derived from the IIW (International Institute of Welding) design S N curves, which indicates that these curves for conventional joints can be conservatively used for estimating the fatigue life of bird-beak SHS X-joints. • High-cycle fatigue tests for square and diamond bird-beak SHS X-joints under cyclic brace axial force were conducted. • Fatigue crack propagation and failure mode of bird-beak SHS X-joints under cyclic brace axial force were investigated. • Variations of crack length and joint stiffness with number of load cycles were revealed. • Comparisons between experimental fatigue data and IIW fatigue design curves were carried out.

Fatigue crack propagation modes: plastic deformation mode and damage accumulation mode

To define the fatigue crack propagation mode for the prediction and classification of the experimental results based on its evolution mechanism, successive mesoscopic observations of a fatigue process under pure cyclic mode II loading are performed. Thus, damage accumulation, which is considered to be a vacancy accumulation, is found to be a mechanism. The phenomena occur not at the crack tip but ahead of it. Thus, “plastic deformation mode” and “damage accumulation mode” are the terms proposed to represent the fatigue crack propagation modes instead of “tensile mode” and “shear mode,” respectively. Moreover, as a method to classify both the fatigue crack propagation modes using the experimental results, it is proposed to identify the plastic strain in the wake of the crack tip.

In situ scanning electron microscopy analysis of effect of temperature on small fatigue crack growth behavior of nickel-based single-crystal superalloy

In situ observation and measurement of the high-temperature deformation of advanced hot-section materials using scanning electron microscopy (SEM) is beneficial for understanding their microscopic failure mechanisms. Through in-situ measurement and subsequent analysis, crack nucleation site, growth rate and propagation behavior can be determined as well as the interaction between crack and microstructure, such as inclusion, pore or grain boundary. However, these measurements are challenging above 700 °C as the SEM images are severely degraded by the thermal electron emission. In this work, an in situ mechanical testing system for extremely high temperatures (up to 1000 °C) inside an SEM was developed and used to investigate the small fatigue crack growth behavior of a nickel-based single-crystal superalloy from room temperature to 980 °C. The fatigue crack propagation modes and growth rates were investigated at different temperatures. The mechanism of the propagation mode transition was discussed in detail. Conventional Paris law was found no longer capable of characterizing the growth behavior of small crack. Subsequently, with the help of the digital image correlation technique, plastic zone size of the crack tip was obtained, and it was found that it could characterize the growth behavior of the small crack.

Effect of temperature on the probability and cost analysis of mixed-mode fatigue crack propagation in patched aluminium plate

By the present paper, a numerical model for cost and probability analysis of mixed mode fatigue crack propagation in a patched 2024-T3Aluminium plate is developed. The finite element method is used to analyse the temperature effect on the bonded composite repair performance of the Aluminium plate. Using a fine mesh, mid-plane finite element results are in good experimental results given by literature. Different patch shapes are proposed like; circle, rectangle, square, ellipse, regular octagon, and prolonged octagon. The stress intensity factors at the crack tip are chosen as fracture criteria in order to estimate the repair performance. According to the temperature variation, the lifespan of the repaired fatigue crack growth is estimated using Paris law. The repair done on a central inclined crack with a single-side composite patch show that, best results are given by prolonged octagon and patch is damaged under at elevated temperature. The life-cycle of the patched Aluminium plate is obtained by Monte Carlo simulation, and the probability of failure is quantified according to various geometrical and mechanical parameters. At elevated temperature, the most dominant input parameters are thicknesses of adhesive and patch. In sensitivity analysis effect of thickness of the adhesive layer reaches 35%.Finally, a code source has been developed and implanted in ANSYS software, to consider the manufacturing process and optimal maintenance costs to prove the performance of patch operation from both safety and economical point of view.

Evaluation of temperature‐sensitive fatigue crack propagation of a high‐speed railway wheel rim material

AbstractThe fatigue crack growth rate (FCGR) of ER8C high‐speed railway wheel rim material was tested at various service temperatures. The temperature sensitivity of fatigue crack propagation was evaluated, and the effect of temperature on the crack propagation mechanism was analyzed. The obtained results indicate a fatigue ductile‐to‐brittle transition (FDBT) point at −20°C for the ER8C wheel rim materials. A reverse relationship was found between FCGR and temperature for the near threshold and Paris regimes when the temperature was below the FDBT point. However, no evident changing rule was found when the temperature was above this transition point. An evident fatigue crack propagation mode transition was found from lamellar tearing to intergranular cracks, which was related to the FDBT for the near‐threshold regime.

Applications on Three-Dimensional Mixed Mode Fatigue Crack Propagation Using Fracture and Crack Propagation Analysis System (FCPAS)

In this study, results from two applications on three-dimensional fatigue crack propagation using the in-house developed “Fracture and Crack Propagation Analysis System (FCPAS)” are presented. FCPAS uses finite element models to calculate three-dimensional mixed mode stress intensity factors and other sub-modules to predict shapes of the incrementally growing crack fronts and the resulting fatigue crack propagation lives. The evolving crack surfaces are generated by successively adding the incremental growth surfaces and re-meshing and re-solving the finite element model using the standalone program FRAC3D, a finite element-based standalone program employing three-dimensional enriched finite elements. The results show that the fracture surfaces evolve into a mode-I type three-dimensional crack as expected. Very good agreements are obtained between the results of the current studies and those of numerical applications in terms of both evolving crack surfaces and crack propagation lives. Therefore, it is concluded that the applied method is capable of accurately predicting linear elastic three-dimensional mixed mode fatigue crack propagation problems.

What is shear mode fatigue crack propagation?

What is shear mode fatigue crack propagation?