Short Crack Growth Rate Research Articles

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.

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.

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.

Fatigue short crack growth: Overload-induced acceleration/retardation behavior

In-situ test was conducted to investigate the effect of overload (OL) on fatigue short crack growth rate (FSCGR). Mechanisms of dual crack growth competitions and overload-induced acceleration/retardation were revealed using digital image correlation (DIC) technology. Eventually, the plastic zone sizes obtained by DIC and Irwin model were compared to propose an OL-induced model to describe FSCGR.

High temperature effect on short crack behaviour in hypoeutectic Al-Si alloys and its modelling

Hypoeutectic Al-Si alloys used for components facing the combustion chamber of automotive engines necessitate excellent high-temperature mechanical properties. In this study, the effect of high temperature on short crack behaviour of hypoeutectic Al-Si alloys is investigated experimentally. The results show that the short crack growth rate is affected by thermal softening and precipitate coarsening. At 200 °C, the slight thermal softening and the positive effect of short-term precipitate coarsening causes the crack growth rates remain unchanged compared to room temperature (RT). At 250 °C, the crack growth rate remains essentially constant during the early stage, while with increasing testing time, the crack growth rate increases rapidly due to the precipitate coarsening. At 300 °C, the crack growth rate is significantly enhanced due to thermal softening and precipitate coarsening. Due to thermal softening, the elastic modulus and yield stress (σys) decrease with increasing temperature. Due to time and temperature dependent precipitate coarsening, the σys drops sharply with the testing time at 250 °C and 300 °C. At 200 °C, the σys increases first and then decreases with the increase of testing time. The transgranular mode dominates the short crack growth at both RT and high temperatures. The grain boundaries have an apparent suppression effect on crack growth. Finally, the relationships between σys and temperature are established for crack tip opening displacement range (ΔCTOD) calculation. The short crack growth rates can be well characterized and predicted by ΔCTOD.

A microstructure-sensitive analytical solution for short fatigue crack growth rate in metallic materials

Short fatigue crack growth in engineering alloys is among the most prominent challenges in mechanics of materials. Owing to its microstructural sensitivity, advanced and computationally expensive numerical methods are required to solve for crack growth rate. A novel mechanistic analytical model is presented, which adopts a stored energy density fracture criterion. Full-field implementation of the model in polycrystalline materials is achieved using a crystallographic crack-path prediction method based on a local stress intensity factor term. The model is applied to a range of Zircaloy-4 microstructures and demonstrates strong agreement with experimental rates and crack paths. Growth rate fluctuations across individual grains and substantial texture sensitivity are captured using the model. More broadly, this work demonstrates the benefits of mechanistic analytical modelling over conventional fracture mechanics and recent numerical approaches for accurate material performance predictions and design. Additionally, it offers a significant computer processing time reduction compared with state-of-the-art numerical methods.

Achieving superior fatigue strength in a powder-metallurgy titanium alloy via in-situ globularization during hot isostatic pressing

Powder-metallurgy (PM) titanium alloys exhibit outstanding quasistatic-mechanical properties, but suffer from low fatigue performance, which severely limits their applications in aerospace. Here, we achieve a superior fatigue strength of 600 MPa in a near-α PM titanium alloy, using a two-step hot-isostatic-pressing scheme, during which more than 80 vol.% (volume fraction) randomly orientated equiaxed grains was obtained. The largely improved fatigue strength (∼ 25%) is mainly attributed to the in-situ globularization of the lamella-like microstructure, leading to higher crack nucleation resistance and lower growth rates of short cracks. The present findings offer a useful route for fabricating PM titanium alloys with high fatigue strengths.

The influence of microstructure on short fatigue crack growth rates in Zircaloy-4: Crystal plasticity modelling and experiment

Rates of fatigue crack growth in Zircaloy-4 are highly microstructurally sensitive and dependent upon texture. Crystal plasticity finite element modelling with the eXtended Finite Element Method and a stored energy density fracture criterion are used to simulate crack propagation in single crystals and polycrystalline microstructures for comparison with experimental crack growth rate data. Results demonstrate that growth rate fluctuations at microstructural features are driven primarily by elastic anisotropy and yield stress mismatch. Additionally, reduced rate of growth in soft hexagonal close packed grains (relative to the remote loading direction) can be linked with crack path tortuosity, which is shown to be controlled by the cyclic development of an in-plane shear back stress. Most importantly, stored energy density is shown to accurately capture major microstructure-driven differences in crack growth rate. Comparison of simulated fatigue crack growth rates with experimental data enables estimation of the critical stored energy density for crack propagation in Zircaloy-4 to be 250 J/m2.

Microstructurally short crack growth simulation combining crystal plasticity with extended finite element method

Accurate description of microstructurally short crack growth remains a long-standing challenging task, due to the fluctuation of crack growth rate induced by grain size and orientation, and grain boundary impediment. Combining crystal plasticity with extended finite element method, a novel framework for short crack growth prediction is proposed, where a new criterion for short crack propagation is established in terms of accumulated shear strain on specific slip system. Compared with three classic empirical models, the proposed method exhibits substantial advantage on the prediction of the fluctuation in FGH96 superalloy, serving as a promising tool for precise estimation of short crack growth rate in polycrystalline materials.

Effect of laser shock peening on boring hole surface integrity and conformal contact fretting fatigue life of Ti-6Al-4 V alloy

The service performance of hole surface is directly tied to its machining-induced surface integrity which is usually produced by sequential operations. Due to its conformal geometry, the assembly hole has a high risk of fretting damage when connecting parts. This paper investigated the effect of laser shock peening on the surface integrity of boring hole and fretting fatigue life of Ti-6Al-4 V alloy. The results showed that the fretting fatigue life of the specimens was dramatically improved at various fatigue stress levels after LSP. The fretting fatigue life was respectively increased by 14 %, 2.27, and 9.35 folds at fatigue stress levels of high (800 MPa), intermediate (500 MPa), and low (300 MPa). In addition, the fatigue stress limit was increased by 70.4 % at the fatigue cycle of 1.0 × 107. The improvement was resulted from the beneficial surface integrity which inhibited fretting wear, and decreased short crack growth rate. Besides, the LSP could not necessarily introduce a greater compressive residual stress (CRS) but should certainly increase the penetration depth of the CRS. The LSP process partially relaxed the CRS depending on whether the existed surface residual stress reached a certain threshold, which was –450 MPa under the selected conditions in this study. This research helps understand the surface integrity evolution in successive processing, which may contribute to planning appropriate processes for desirable surface integrity.

Machine learning based short fatigue crack growth rate prediction for aluminium alloys

As a result of different fatigue characteristics influenced by intricate microstructures, comparing with fatigue crack growth rate in long fatigue crack region, the growth in the short one is more complex to be fitted with fewer parameters. There have been more restrictions for traditional models in describing the nonlinearity between the fatigue crack growth rate and stress intensity factor range in short crack regime. Due to their outstanding ability in prediction with high accuracy and in description of nonlinearity with satisfactory flexibility, machine learning approaches have been payed more attention. The machine learning models have been the better choices to deal with the limitation in fatigue-related problems which traditional solutions cannot overcome. In this paper, two machine learning algorithms: k-nearest neighbour algorithm (KNN) and random forest (RF) are implemented to predict the short fatigue crack growth rate for 2024-T3 and LC9cs aluminium alloys. The testing outcomes of these applied machine learning algorithms are compared to evaluate their prediction abilities. The final results reveal that the values of Pearson correlation coefficient R2 of the KNN are generally higher than that of another method for each material. Each of them has an excellent performance in accuracy and effectiveness, and all of them have excellent extrapolation capabilities to predict the nonlinearity.

Study on the short fatigue crack initiation and propagation behavior of 42CrMo

A micro-tensile fatigue test of 42CrMo alloy steel was carried out. The result shows that the crack initiation and propagation of the material could be characterized with randomness and localization. There existed two sorts of short fatigue crack propagation behaviors for the tested material, that is, self-propagation and joint propagation with other short cracks. A short fatigue crack growth rate function was built considering the propagation characteristics. Moreover, a simulation model of short fatigue crack evolution was built. Total three samples were tested and simulated. The experimental result was in good consistent with that from simulation, both showing that the crack growth rate accelerated first and then decelerated during propagation.

Research on evolution behavior of 42CrMo microscopic fatigue short crack based on microstructure effect of material

To investigate the causes of damage dispersion in fatigue life of alloy materials, fatigue tests on 42CrMo are carried out under microscope with the aid of a micro fatigue testing device. The fatigue crack initiation and propagation evolution process of 42CrMo are studied on the micromechanical scale. The fatigue short crack and short crack growth rates are measured respectively under loads of 1400, 1550, and 1650 N. The experimental result shows that the initiation of fatigue short cracks is related to the randomness of microstructure defects. The initiation and propagation of fatigue short cracks on the surface of samples are random and local, while the propagation is characterized by first deceleration and then acceleration.

Fatigue crack initiation and growth behavior in a notch with periodic overloads in the low‐cycle fatigue regime of FV566 ex‐service steam turbine blade material

AbstractThe effect of periodic overloads on crack initiation, growth rate, and fatigue life, within a notch stress field representative of a turbine blade root fixing, has been investigated. Bend bars made from FV566 martensitic stainless steel were extracted from the root of ex‐service power plant turbine blades and representative notches introduced. These notched plain bend bars were loaded plastically in the low‐cycle fatigue regime and were tested with overloads up to 150% of the cyclic baseload every 150 baseload cycles. A periodic overload of 50% of the cyclic baseload increased the number of cycles to crack initiation and slightly retarded the crack growth rates of both short and long cracks, leading to a slight improvement in fatigue life. The results suggest that small overloads (less than 10%) are not expected to significantly impact fatigue lifetimes or service scheduling of components such as low‐pressure steam turbine blades.

Mechanistic fatigue in Ni-based superalloy single crystals: A study of crack paths and growth rates

The mechanistic basis of microstructurally short crack paths and growth rates is investigated in Ni-based superalloy single crystals using Crystal Plasticity (CP) and eXtended Finite Element (XFEM) analyses of experimental edge-cracked samples over a range of crystal orientations. Crack paths are determined to be those along crystallographic slip systems within which the slip is highest. Crack growth rates are determined by the crack tip critical stored energy density. Experimental observations of tortuous crack paths and their dependence on crystal orientation are reasonably well captured by the mechanistic model. Key features of alternating and straight crack paths are reproduced. The experimentally measured crack growth rates as a function of crystal orientation are also captured by the mechanistic model and controlled by the crack tip critical stored energy density which was found to be 385 Jm−2 in the Ni-based superalloy single crystals analysed. A new methodology for determination of critical stored energy density and mechanistic quantification of short crack growth rates is presented.

A framework of modelling slip-controlled crack growth in polycrystals using crystal plasticity and XFEM

Short cracks tend to develop at high and irregular rates compared to macroscopic cracks, making the prediction of fatigue life a challenging task. In this work, a numerical framework combining crystal plasticity model and the Extended Finite Element Method (XFEM) is applied to study the slip-controlled short crack growth in a polycrystal superalloy RR1000. The model is calibrated from experiments and used to evaluate short crack growth paths and rates. Two fracture criteria are used and compared: the onset of fracture is controlled by the total and individual cumulative shear strain respectively, and the crack grows either perpendicular to the direction of maximum principal strain or along crystallographic directions.

Effects of Stress Amplitude Ratio and Phase Angle on Short Fatigue Crack Behavior and Fracture Mode of EA4T Steel

The loads on railway axles in actual service conditions are complex and variable, which may lead to local changes in the fatigue performance, which then affect the fatigue reliability and safety evaluation results of the components and structures. At present, studies on the short fatigue crack behavior and fracture failure mode of EA4T steel, a common material for high-speed trains, are limited. Therefore, fatigue tests on smooth funnel solid round bar specimens under different stress amplitude ratios and different phase angles were performed. Replica technology was used to record the initiation and propagation information of short fatigue cracks. Due to the homogenous and fine microstructure of all the specimens, the same short fatigue crack behavior was observed regardless of the loading condition. The fracture morphology observed by a scanning electron microscope shows that the cracks initiated on the surface of the specimens, and the specific fracture morphology was exceedingly distinct in each region. In addition, the short fatigue crack growth rate and fatigue life were compared and analyzed. The results show that the stress amplitude ratio and phase angle have a considerable influence on the fatigue life and short crack growth rate of the specimens. The results of this research can provide a reference for practical engineering applications.

Advantageous Description of Short Fatigue Crack Growth Rates in Austenitic Stainless Steels with Distinct Properties

In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polák model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress–strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a “master curve” was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data.

The practical need for short fatigue crack growth rate models

Accurate fatigue crack growth rate models in the short crack regime are essential to the design and sustainment of modern aerospace structures. In this paper we briefly recap the short crack problem space and offer a case study demonstrating the practical significance of these models in the service life prediction for combat aircraft primary structure under a complex manoeuvre plus buffet loads spectrum. The necessity of accurate fatigue crack growth rate predictions in the short crack regime is demonstrated for this practical application using two models assessed against coupon fatigue test results representative of a critical aircraft structural location. A fractography based fatigue crack growth rate model relying on direct, quantitative fractography measurements in the short crack regime, is shown to offer better predictions of both total crack growth life and crack growth rates at a range of crack depths than long crack-based models.

Effect of microstructural features on short fatigue crack growth behaviour in SA508 Grade 3 Class I low alloy steel

Aim of the paper is to understand the effect of microstructural features of SA508 Grade 3 Class I low alloy steel (LAS) on short crack propagation rate under cyclic loading. The complex upper bainitic microstructure of this LAS consists of low angle bainitic ferrite lath boundaries and high angle prior austenite grain boundaries (PAGBs). Compared to bainitic ferrite lath boundaries, the PAGBs provided major hindrance to short fatigue crack propagation in the subject LAS. The high angle PAGBs strongly resist the dislocation motion ahead of the crack tip as the crack tip approaches the PAGBs compared to that of low angle bainitic ferrite lath boundaries. This restriction of dislocation motion ahead of the crack tip based on hindrance provided by PAGBs resulted in retardation in short fatigue crack propagation rate along the crack path. The short fatigue crack propagated at stress intensity factor (SIF) range ‘ΔK’ values lower than threshold SIF range ‘ΔKth’ for the long cracks. The growth rate of short fatigue cracks cannot be predicted by Paris law which is applicable for long crack growth. This is due to the fact that crack growth rate undergoes acceleration and retardation in short crack regime because of microstructural effects.