Short Crack Growth Research Articles

Mechanistic insights into twinning and detwinning during fatigue short crack growth in zirconium alloys

Mechanistic insights into twinning and detwinning during fatigue short crack growth in zirconium alloys

On the prediction of fatigue life of WAAM-processed Ti-6Al-4V under consideration of manufacturing defects

PurposeThere is still a need for a comprehensive investigation into how wire and arc manufactured (WAAM) parts fail under cyclic loading. This study investigates the effect of process-induced defects on the high-cycle fatigue performance of WAAM-processed Ti-6Al-4V with dedicated thermal treatment. Furthermore, the study assesses the applicability of the fatigue life prediction model, which was originally developed and validated for laser beam-welded Ti-6Al-4V joints, to WAAM-fabricated structures.Design/methodology/approachThe fatigue life assessment model was adapted to WAAM-fabricated Ti-6Al-4V. This model is based on the NASGRO equation, which considers short crack growth from internal and surface defects, such as lack of fusion defects and pores. The model was used to create artificial Wöhler curves, and the results from stress intensity factor range-decreasing fatigue crack propagation rate tests are compared to the experimental data in the form of a Kitagawa–Takahashi (KT) diagram.FindingsThe results demonstrate that the model can accurately predict the minimum fatigue life of specimens extracted from WAAM structures. When considering that the crack from internal defects grows in a vacuum-like environment, accurate Wöhler curves are predicted. The experimental data does not follow the expected trends of the KT diagram. Nonetheless, the measured long crack threshold stress intensity factor range produces a suitable estimate of defect severity.Originality/valueThe study results in a model with which a probabilistic computation of the fatigue life of additively manufactured samples based on the defect size distribution is possible. Furthermore, the KT diagram as a criterion for the assessment of defect severity is evaluated.

Influence of surface integrity on short crack growth behavior in HFMI-treated welded joints

This study investigates the influence of surface integrity and the localized fatigue phenomena on the initiation and propagation of short fatigue cracks in high-frequency mechanical impact (HFMI)-treated welded joints. The treated surface region, characterized by a compressive residual stress field, smooth notch geometry, and work hardening layer, improves welded joints’ fatigue strength. However, how these surface conditions influence the fatigue damage process zone during short crack initiation and growth is not yet well known. Therefore, this study systematically investigates the influence of different surface characteristics on fatigue life modeling of HFMI-treated welded joints made of high-strength steel. This is achieved using a non-local continuum damage mechanics-based approach of crack growth and elastic–plastic finite element simulation, explicitly modeling treated surface conditions. The simulated fatigue life is first verified with experiments and then applied to various surface conditions. The simulation results show that most of the fatigue life is spent until a crack size of 0.2 mm. The compressive residual stress field greatly extends both short crack initiation and propagation life, with its degree of contribution highly dependent on loading history and residual stress change. The role of the work hardening layer is mainly concentrated on improving fatigue life during short crack initiation and the very beginning of short crack growth.

Microstructural insights into short fatigue crack growth in particle-reinforced Al-matrix composite sheet

Microstructural insights into short fatigue crack growth in particle-reinforced Al-matrix composite sheet

Steel Catenary Riser Fatigue Assessment: Fracture Mechanics Approach Versus S-N Curve Method.

In this paper, the fatigue resistance of a full-scale Steel Catenary Riser (SCR) girth weld is investigated using the Strength-Number of cycles (S-N) curve method based on weld formation quality and fracture mechanics approaches. The test results, presented in the form of S-N curves, are superior to the design curve E in BS 7608. Compared with the S-N curve determined by a resonant bending rig, the analytical fracture mechanics, i.e., engineering critical assessment (ECA) based on BS 7910, can provide a rational estimation of full-scale girth welds. For the numerical methods, the short crack growth phase is crucial to improving the accuracy and reliability of the assessment. For the girth weld with a concave root, the geometries of the weld cap are the predominant factors for fatigue life. Although the crack initiation site is always located at the outer surface regardless of the flushed or welded caps, the weld grinding treatment is still effective in promoting fatigue life.

Short fatigue crack growth sensitivity to thermo-mechanical fatigue loading

Most high-temperature components are subject to out-of-phase thermomechanical fatigue (OP-TMF), which induces crack growth at low temperatures. However, OP-TMF has been little studied in the context of short cracks. This study focuses on the experimental sensitivity of OP-TMF loading conditions playing on temperature range, gradient, and dwell time for thin sheet superalloy specimens. The material of interest is a Co-based superalloy, HA188. It is widely used in combustion chambers.The experimental analysis is based on full-field measurements for temperature, strain and damage by infrared thermography, digital image correlation and high resolution images from 300 to 900°C. The main conclusion is that the temperature gradient, together with the temperature amplitude, largely determines the strain amplitude and subsequent fatigue crack growth rate (FCGR) of short cracks. In situ measurements of damage and crack closure were obtained using supervised machine learning based on images. This clarifies that crack closure is only partial and that the crack network growth rate is consistent with the individual short crack growth rate. Finally, 3D finite element analysis considering realistic temperature field and strain energy based FCGR model was able to evaluate the fatigue life in this context. It is shown that the OP-TMF FCGR is very close to the FCGR of the maximum temperature of the TMF cycle due to partial crack closure.

Natural Seawater Impact on Crack Propagation and Fatigue Behavior of Welded Nickel Aluminum Bronze

ABSTRACTNickel aluminum bronze (NAB) is a complex alloy used extensively in the marine environment. Fatigue strength of NAB is reduced by welding and prior seawater corrosion. This study investigated the combined effect of corrosion and plasma welding on the fatigue behavior of NAB. Natural seawater corroded samples were used in tension‐tension cyclic loading tests to observe fatigue crack initiation, propagation, and failure. Fatigue cracks initiated from corrosion pits at the weld toe. Stress corrosion and fatigue cracks propagated along the path of β′ and κIII phases. A short crack growth model (SCGM) predicted fatigue strength using experimentally obtained material properties and corrosion pit dimensions. Model predictions were used to develop S‐N curves and were within 30% of experimental results. The SCGM produced accurate and reliable fatigue life results that could be applied by industry to aid in revalidation decision making and inspection scheduling.

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.

Short Fatigue-Crack Growth from Crack-like Defects under Completely Reversed Loading Predicted Based on Cyclic R-Curve.

Understanding short fatigue-crack propagation behavior is inevitable in the defect-tolerant design of structures. Short cracks propagate differently from long cracks, and the amount of crack closure plays a key role in the propagation behavior of short cracks. In the present paper, the buildup of fatigue-crack closure due to plasticity with crack extension from crack-like defects is simulated with a modified strip yield model, which leaves plastic stretch in the wake of the advancing crack. Crack-like defects are assumed to be closure-free and do not close even under compression. The effect of the size of crack-like defects on the growth and arrest of short cracks was systematically investigated and the cyclic R-curve derived. The cyclic R-curve determined under constant amplitude loading of multiple specimens is confirmed to be independent of the initial defect length. Load-shedding and ΔK-constant loading tests are employed to extend the cyclic R-curve beyond the fatigue limit determined under constant amplitude loading. The initiation stage of cracks is taken into account in R-curves when applied to smooth specimens.

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.

Quantitative assessment of microstructural damage for interior crack initiation and high cycle fatigue life modeling

Axially loaded cyclic tests of a nickel-based alloy were conducted at 550 °C, 630 °C and 650 °C and stress ratios of −1, −0.3 and 0.5 up to high cycle fatigue regime. Multiscale characterizations and quantitative analyses of microstructural damage were carried out to illustrate the physics and mechanics of interior microstructure induced fatigue strength degradation and cracking. Results showed that the fatigue strength increased with the decreases of twin spacing, the size and aspect ratio of carbide, and γ′ phase size. Twin collapse due to slip band-twin interaction was found for the first time and confirmed as a form of crack initiation process, followed by short crack growth forming facets with significant contribution from shear stress. A new mechanistic fatigue life model of interior microstructure induced cracking was established with sound agreement with experimental data. All these combined into a design-manufacturing integrated approach which are capable of ensuring the safe operation and maintenance of service structures in long-life regime.

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

Modelling short crack growth under creep-fatigue interaction for different dwell times

The creep-fatigue interaction (CFI) short crack growth behavior of 316H steel under different dwell times at 550℃ was investigated in this work. The dwell time effect of the CFI short crack growth behavior is analyzed systematically, the results revealed that compared to the long cracks, the short cracks is more sensitive to the creep damage effect during the dwell period. Meanwhile, a CTOD-based CFI short crack growth rate predication model is proposed by introducing the creep effect during the dwell time. Compared with conventional fracture mechanics approach, the CTOD-based approach demonstrated a superior capability in providing a unified prediction result for the creep-fatigue interaction in short crack growth rates under different dwell times.

Statistical modeling of microstructurally short crack growth in high cycle fatigue

Capturing the behavior of short cracks under fatigue conditions has become a challenge for researchers. The early stage of crack propagation is influenced by microstructural features, such as grain boundaries and orientations, which leads to significant statistical variations. Models such as the ones developed by Navarro and De Los Rios have been able to describe the process by considering the average properties of the material and estimating the number of cycles related to the short crack stage. However, fatigue should be seen as an extreme value process. In the present work, a more realistic model was developed to consider the statistical nature related to grain orientations to estimate fatigue life dispersion. Eleven crack propagation criteria have been used to compute the number of cycles spent in the short crack stage. Parameters such as the Schmid factor, the potential crack length, and the various angles describing the misorientation between adjacent grains were considered. One thousand random scenarios for short crack growth were studied for each propagation criterion and some statistical analyzes were run to estimate the worst-case scenario out of one million. Results can be well described using the generalized extreme value distribution, capturing the stochastic nature of the fatigue life.

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.

Analysis of Crack Propagation Behaviors in RPV Dissimilar Metal Welded Joints Affected by Residual Stress.

In severe service environments, the presence of high local residual stress, significant organizational gradient, and nonlinear changes in material properties often leads to stress corrosion cracking (SCC) in dissimilar metal welded (DMW) joints. To accurately predict the crack growth rate, researching the initiation and propagation behavior of SCC cracks in DMW joints under residual stress (RS) is one of the most important methods to ensure the safe operation of nuclear power plants. Using the extended finite element method (XFEM), the crack propagation behaviors in DMW joints under different RS states are predicted and compared. The effects of RS, crack location, and initial crack length on crack propagation behavior are investigated. The crack in a DMW joint without RS deflects to the material of low yield strength. High residual stress urges the crack growing direction to deflect toward the material of high yield strength. Young's modulus has little impact on the crack deflection paths. The distance between the specimen symmetric line and the boundary line has little effect on the crack initiation and propagation within the RS field. A long initial crack is more likely to initiate and propagate than a short crack. To a long crack and the crack that is far from the interface of two materials, the impact of residual stress on the crack propagation path is significant when it is located in a material with high yield strength, while when the initial crack is located in the material with low yield strength, RS has a great influence on the deflection of a short crack growth direction on the condition that the crack is adjacent to the interface.