Short Fatigue Cracks Research Articles

Advancing the understanding of short fatigue crack propagation: Leveraging ultrasonic testing device to approach rolling contact fatigue

Advancing the understanding of short fatigue crack propagation: Leveraging ultrasonic testing device to approach rolling contact fatigue

Critical damage events of 3D printed AlSi10Mg alloy via in situ synchrotron X-ray tomography

Fish-scale-like melt pool structures and internal defects are unique characteristic in additively manufactured (AM) metals. These features play a critical role in the damage and fracture processes under different service conditions, governing the material's performance. However, the relationship between these damage features and loading conditions, as well as the spatial interactions between melt pool structures and internal defects remains incompletely understood. By in situ time-lapse synchrotron X-ray tomography and diffraction, we identify the initiation and growth events of life-limiting damage under tensile, low cycle fatigue (LCF), and high cycle fatigue (HCF) loading. A novel transition from meso-structure insensitive, defect-dominated short fatigue crack propagation to a meso-structure sensitive mechanism occurs as the plastic zone expands of a growing crack. Under tension and LCF, the damage accumulation gradually increases and micro-voids nucleate at the melt pool boundaries (MPBs) after which the crack path follows the MPBs. In contrast, under HCF, surface defects initiate fatigue cracking and the MPBs have a very limited effect on the crack propagation path. Finally, physics-informed machine learning method is introduced to develop a novel methodology for predicting fatigue life by including three-dimensional features of defects in AM parts.

Modelling the notch-induced anomalous growth of short fatigue cracks and the growth of long fatigue cracks with unified phase-field formulas

Modelling the notch-induced anomalous growth of short fatigue cracks and the growth of long fatigue cracks with unified phase-field formulas

A closed form expression of stress intensity factor for an arbitrarily shaped planar crack in 3-D under tensile loading

A closed-form equation was developed in this work to quantify the stress intensity factor (SIF) along an arbitrarily shaped planar crack under mode-I loading by substituting the ellipse-related parameters in Irwin’s solution with geometry-related parameters fi of the crack. This allowed incorporation of the local crack curvature effect on SIF at each discrete point along the crack front and its relative weighted contribution to SIF of the neighboring crack point i, where either stress concentration or shielding took place, into computation of the SIF distribution along the crack front. Good agreement was obtained between the KI values derived in the current work and previous numerical simulation-based studies. The current method could capture the KI distributions for surface and embedded cracks having the same shape, with their maximum values being consistent with Murakami’s area method. This solution of KI allows for highly efficient calculation of ΔKI for quantification of the 3D growth behaviors of a fatigue crack with a complex shape, paving the way for studying the statistical nature of short fatigue crack growth.

Crack-tip plasticity mediated grain refinement and its resisting effect on the fatigue short crack growth

Fatigue short crack growth governed by the crack-tip plasticity dominates the fatigue life and strength of metallic materials or structural components. Here, for the first time, we discover a new mechanism of resisting fatigue short crack growth by grain refinement near the crack-tip driven by dynamic recrystallization in a Ni-based superalloy during high-cycle fatigue. The local cumulative plastic strain plays a determining role in the crack-tip grain refinement and concurrent dissolution of nanoprecipitation. Comprehensive microstructural analysis provides the evidence that the refined grains reduce the plastic micro-strain gradient in the vicinity of the crack-tip, which causes the crack blunting and deflection towards the interface of coarse-fine grains, hence decelerating the short crack growth. Although the grain refinement reduces the local stress threshold in the fine-grained areas (FGA), the dominant effects of FGA are identified to provide additional microstructural resistance to the propagation of short cracks.

Short fatigue crack growth and retained austenite in steels processed via quenching and partitioning

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

Conduction thermography for non-destructive assessment of fatigue cracks in metallic materials

Metallic materials are widely used in many applications of mechanical and aerospace engineering. Due to severe environmental and loading conditions, components and structures can experience cracking phenomena. In this regard, different non-destructive techniques were developed in the last years to detect and monitor cracks that can propagate during the actual loading conditions.This work deals with the application of the conduction thermography as a non-destructive technique for the offline control and inspection of different notched metallic specimens characterized by short fatigue cracks. The potentials of the technique have been demonstrated considering a low-cost set-up consisting of a power supply of low current and limited voltage, and a microbolometer sensor. Moreover, to further simplify the inspection, the specimens were not black coated. Quantitative results, in terms of crack lengths, have been obtained using a new procedure of data analysis and then compared to those provided by thermoelastic stress analysis, a well-established technique adopted for the detection and monitoring of cracks.

Experimental evaluation of the short and long fatigue crack growth rate of S355 structural steel offshore monopile weldments in air and synthetic seawater

Welded steel structures used in the offshore wind industry are exposed to harsh marine environments, which can result in corrosion-induced fatigue damage. Of particular concern is the heat affected zone (HAZ) of welded joints, a region known for its altered microstructure and mechanical properties, which can significantly influence the initiation and propagation of fatigue cracks. This study investigates the short and long fatigue crack growth rates, and the effect of seawater exposure, for the HAZ in S355 steel weldments. Single-edge notch bend (SENB) specimens are used, with a shallow notch in the HAZ. A series of specimens is immersed in synthetic seawater that is continuously circulated at a controlled temperature to assess the synergistic effects of corrosion and fatigue. The experimental method integrates a novel application of front face strain compliance for monitoring short cracks, alongside an extended back-face strain compliance approach for monitoring long crack propagation. It is concluded that the short fatigue crack growth rate of the HAZ is 2.7 to 3.5 times higher in seawater as compared to air. As the crack propagates and enters into the long crack regime, the ratio decreases to 2.2 times at the transition point of the two-stage crack growth curve and further decreases to 1.5 times when the notch advances towards fracture. The findings indicate that the fatigue crack growth rates documented in standards tend to be on the conservative side. This study significantly enriches the fatigue crack growth data available in literature, which will contribute to a more accurate lifetime assessment offshore wind turbine structures.

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.

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Analysis of the crack initiation and short crack propagation of laser cladding IN718 enduring fretting fatigue at 650 ℃

In order to reveal the behaviors of crack initiation and short fatigue crack propagation in laser cladding IN718 under high temperature conditions, very high cycle fretting fatigue (VHCFF) experiments were conducted at 650 ℃ with a transverse clamping force of P=400N and different axial stress loading conditions to obtain S_N data. The surface wear morphology and fracture morphology characteristics were analyzed by scanning electron microscope (SEM) and white light interferometer. Additionally, the microstructure characteristics of grains in the short fatigue crack region were examined through electron back scatter diffraction (EBSD). Parameters such as equivalent grain size, grain elastic modulus, Schmid factor, geometric compatibility factor, twist angle, and tilt angle were considered in the analysis of the characteristics of fatigue crack initiation and propagation. The results indicate that fretting fatigue cracks tend to initiate on grains with higher Schmid factors, larger equivalent grain size, and lower grain elastic modulus on the fretting contact surface. Moreover, the short fatigue cracks tend to propagate along the grains with the microstructural features of the high Schmid factor, the geometric compatibility factor of m′>0.4, the twist angle of α<50°, and the tilt angle of β<50°.

Ex-situ observation of the short crack initiation and propagation behaviour of nickel-base wrought alloy Inconel 718 for critical rotating components under fatigue conditions at elevated temperatures

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400°C and 600°C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50μm, while cracks larger than 100μm showed similar growth characteristics.

In-situ investigation of fatigue crack growth behavior of 34CrNi3Mo high-strength steel

In this study, the short fatigue crack growth behavior of 34CrNi3Mo high-strength steel was investigated through in-situ fatigue testing under scanning electron microscopy. An innovative method was proposed to determine the fatigue crack growth threshold by a changing load strategy, which shows high consistency with the standard test for compact tensile specimens. The difference in misorientation along distinct directions at the crack tip has been identified as a factor controlling crack deflection regardless of slip system types.

Microstructural insights into fatigue short crack propagation resistance and rate fluctuation in a Ni-based superalloy manufactured by laser powder bed fusion

The microstructural sensitivity of fatigue short crack path and its propagation rate in a Ni-based superalloy GH4169 manufactured by laser powder bed fusion (LPBF) was investigated at room temperature. In-situ digital image correlation (DIC) observation and post-mortem microstructural analysis around the crack path were performed. The results show that the intragranular cracks developed in the shear cracking mode are closely aligned along the activated slip bands in the γ-matrix grains with the crystallographic characteristics of parallel to the γ-{111} slip planes. Multiple slip was also activated, causing the crack retardation or deflection. Low-angle grain boundaries and subgrain boundaries were shown to cause deflections of intragranular cracking, while high-angle grain boundaries significantly arrested the short crack propagation. Moreover, the resistance of grain boundaries to short cracking was assessed using combined metrics including the crystallographic and microstructural parameters of twist angle, the Schmid factor and the geometrical compatibility factor. These site-specific microstructural analyses around the crack path provide insights into the microstructural origins of resistance to the short crack propagation as well as an interpretation of the observed significant fluctuations in the crack propagation rate.

A detailed study of short fatigue crack directions for carbon steel specimens with circular holes subject to cyclic biaxial loads

The effect of notches and biaxial fatigue loading on the crack paths was studied in detail for thin-walled tube specimens with a passing-through hole, paying special attention to the short-crack period. The study was focused on the high cycle fatigue regime. The material was a carbon steel and the tests were under load control, at Rσ=−1. The crack initiation point on the notch surface and the crack direction were studied with an optical microscope on the specimen outer surface and with a scanning electron microscope and a non-contact 3D optical profilometer on the fracture surface. The crack direction was analyzed for several crack lengths, ranging from the length of one average grain to the length of twenty average grains, all lengths within the short-crack regime, in order to carefully observe the evolution of the crack direction in the Stage I and during the transition from Stage I to Stage II. A statistical analysis of the crack initiation point and the experimental crack directions was carried out. In general, the crack initiation point was close to the maximum principal stress point. The crack direction during the first grains was approximately the Mode I direction. There was no initiation in Mode II. The crack continued in the Mode I direction as it got longer, within the short-crack period. The experimental fatigue limits were compared with the predictions calculated with two models from the literature. The direction of the straight lines used by the models to make the predictions were compared with the average crack directions measured experimentally. The goodness of the models, not only from the point of view of the fatigue limit value prediction but also from the closeness of the direction of the line used for the prediction to the experimental crack direction, was discussed.

Fatigue behaviors of ultrasonic surface rolling processed AISI 1045: The role of residual stress and gradient microstructure

In the study, AISI 1045 steel were treated by ultrasonic surface rolling processing (USRP) with different parameters in terms of coverage rate. The gradient structures characterized with refined polished surface, the sub-surface nano-lamellar microstructure with a maximum compressive residual stress of 453.4 MPa was obtained, which shall be responsible to the superior fatigue properties to the pristine. Furthermore, the comparative investigation of fatigue behavior demonstrates that the surface roughness has the pronounce effects on the resistance to crack initiation, though the excessive USRP with the surface roughing may result in the premature crack growth; in contrast to the gradient microstructure, the compressive residual stress may delay effectively the incubation and increase the growth resistance of the physically short crack. Besides, the benefit of gradient microstructure is relatively limited in the resistance of fatigue short crack propagation.

Short fatigue crack growth mechanism in Ni-Co based superalloy at elevated temperatures and in oxidative atmospheres

In this study, to elucidate the oxidation effect on the short fatigue crack growth (SFCG) characteristics of Ni-Co based TMW-4M3 superalloy, fatigue tests were conducted at room/elevated temperature in air/vacuum and three-dimensional microscopic observation of the SFCs using a plasma focused ion beam – scanning electron microscope (PFIB – SEM) system. Fatigue lives tested under vacuum at elevated temperature were comparable to those at room temperature while those tested at elevated temperatures in air showed shorter fatigue life in higher stress regions and longer fatigue life in lower stress regions than the others. In situ observation of SFCs at elevated temperatures in air revealed SFCG deceleration in the small ΔK regions and acceleration in large ΔK regions. SFCs opening/closing behaviours at elevated temperatures measured by digital image correlation (DIC) showed the crack opening stress to be higher at elevated temperature in air, possibly due to oxide-induced crack closure. However, the crack closure effect did not fully explain the difference in FCG rate between room and elevated temperatures in air. Three-dimensional investigations revealed SFCs to form at elevated temperatures in air, showing straight transgranular FCG to be insensitive to microstructure in slow growth regions, and intergranular FCG to precede that in the surrounding material in fast growth regions, in contrast to microstructural SFCs features at room temperature. It appears that slow and straight unique SFCG at elevated temperatures might occur due to intermittent brittle fracture of oxides formed at the crack tip. This suggests a possible role of the oxide layer at the crack tip in broadening the plastic deformation distribution, thus contributing to FCGR deceleration.

Rotating bending fatigue mechanisms of L-PBF manufactured Ti-6Al-4V alloys using in situ X-ray tomography

Lightweight titanium alloys are being increasingly used in high value structures, but in-depth knowledge of the rotating bending fatigue (RBF) mechanism remains insufficient, particularly for additively manufactured components. In response, an in situ RBF test rig was developed for the first time, using time-lapse synchrotron radiation X-ray microtomography (SR-μCT) to identify the failure behavior of laser powder bed fused Ti-6Al-4V alloys. The RBF damage evolution was characterized by combining the non-destructive SR-μCT with post-mortem scanning electron microscopy, and electron backscatter diffraction. Compared with RBF life data and fatigue long crack growth from standard specimens, in situ RBF tests proved to be reliable, in revealing the fatigue mechanism. However, fatigue short cracks in the representative mini RBF testing campaign were found to initiate from microstructural features of outer surfaces, instead of gas pores or lack of fusion defects. It is demonstrated that neither the standard nor the improved Kitagawa-Takahashi diagrams are unable to correlate the fatigue strength to manufacturing defects. The equivalent defect should include the effect of surface microstructural defects, in addition to internal defects, when studying additively manufactured alloys.

Microstructure relevant fatigue short crack propagation behavior of Al0.3CoCrFeNi high entropy alloy: in-situ SEM study

The microstructure effects on fatigue short crack propagation behavior of Al0.3CoCrFeNi high entropy alloys (HEAs) were studied by in-situ fatigue testing under scanning electron microscope (SEM). The experimental results indicated that fine grain (FG) specimens had better fatigue crack propagation resistance and higher fatigue life compared with coarse-grain (CG) specimens. Fatigue short crack propagation followed transgranular mode, associated with octahedral slip. The fatigue small crack growth is primarily microstructurally sensitive, showing evident fluctuations related to the blocking effects of grain boundaries and twin boundaries. Furthermore, a modified fatigue crack growth model has been proposed, which can successfully evaluate the crack growth rates of HEAs with different grain microstructures under different fatigue stresses.

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.