Stable Fatigue Crack Research Articles

Fatigue crack growth rate under mixed-mode loading conditions (I+III) of a carbide-free bainitic steel designed for rail applications

Carbide-free bainitic steel was designed for railway infrastructure application, focusing on high-speed and heavy-loaded freight tracks. Considering the complex state of stresses occurring on the rails running surface, mode III plays a significant role in the initiation and propagation of fatigue cracks of rails during service. Thus, the Fatigue Crack Growth Rate (FCGR) under mixed-mode loading conditions (I+III) was evaluated. It was revealed, that fatigue lifetime increases with loading angle modes. In the area of fatigue fracture, transgranular cracking mechanisms dominated. For the stable fatigue crack growth, a trend was observed related to the decrease in the fraction of intergranular fracture with the increasing loading angle modes (α). Secondary cracks indicated privileged cracking directions related to the crystallographic structure of bainite. The influence of the mechanical stability of retained austenite during mixed-mode FCGR requires further in-depth research. These studies contribute to understanding the factors influencing the reliability of railway tracks in terms of designing new materials and modeling the rate of crack growth to precise assessment of the life cycle of rails.

Effect of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel

When designing against fatigue, the surface condition of steel is of utmost importance. Since hot-dip galvanizing is a surface treatment primarily aimed at preventing corrosion, it is critical to assess its influence on the fatigue properties of steel. Given the challenges in producing large structural galvanized steel without imperfections, it is crucial to understand the influence of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel. In this study, to gain such insights, tension-tension fatigue testing was conducted on hot-dip galvanized steel, followed by a detailed analysis of the crack initiation sites to determine any correlation with manufacturing-induced microscopic surface defects. It was observed that under low-cycle fatigue, pre-existing surface defects did not appear to have influenced fatigue mechanisms. However, under high-cycle fatigue, crack initiation sites exhibited evidence of pre-existing manufacturing induced defects which were significantly smaller than the total crack initiation area. This indicates that the defects could not immediately penetrate the substrate but expanded within the layer and only penetrated the substrate after reaching a critical size. Therefore, it is discovered that while manufacturing-induced defects may be challenging to eradicate entirely, those below a threshold size may not immediately evolve into a stable fatigue crack. Hence, if manufacturing induced defects are kept below a critical size, they may not significantly influence the transition of stage I crack into a stable stage II crack.

Crack growth in high strength mooring line steel under variable amplitude loading

Mooring lines are of key importance for the safe and reliable operation of numerous floating offshore structures. Due to the constantly changing wind and wave conditions acting on such structures, any fatigue crack growth in a mooring line will occur under cyclic loading of distinctly variable amplitude. This paper considers a long-term load history for mooring lines on a North Sea offshore platform. For fatigue testing purposes, a characteristic load sequence representing the actual load history in a condensed and simplified form is developed and presented. Fatigue crack growth rate test results for a grade R4 high strength mooring chain steel subjected to this variable amplitude load sequence is furthermore presented, including tests performed in air as well as under free corrosion conditions in artificial seawater. Based on comparison testing performed under variable- and constant amplitude conditions, the validity of a linear damage rule hypothesis for stable crack growth under the characteristic load sequence is investigated. This damage rule converts the load spectrum to an equivalent constant load range, making it a potentially useful tool for estimating fatigue crack growth in mooring chains when test data for a representative characteristic load sequence are unavailable. Results from the comparison testing are used to show that under stable fatigue crack growth conditions, while subjected to the characteristic load sequence developed for this offshore platform, the linear damage rule can be a reasonable assumption.

Acoustic Emission Monitoring of Fatigue Crack Growth in Hadfield Steel.

Evaluating the condition of a Hadfield steel crossing nose using existing inspection methods is subject to accessibility and geographical constraints. Thus, the use of conditional monitoring techniques to complement the existing inspection methods has become increasingly necessary. This paper focuses on the study of acoustic emission (AE) behaviour and its correlation with fatigue crack growth in Hadfield steel during bending fatigue tests. The probability density function for acoustic emission parameters was analysed based on the power law distribution. The results show that a sharp increase in the moving average and cumulative sum of the AE parameter can give early warning against the final failure of Hadfield steel. Two parts (Part 1 and Part 2) can be identified using the change in the slope of duration rate (dD/dN) vs. ΔK plot during the stable fatigue crack growth (FCG) process where Paris's law is valid. The fitted power law exponent of AE parameters is smaller in Part 2 than in Part 1. The novelty of this research lies in the use of the fitted power law distribution of AE parameters for monitoring fatigue damage evolution in Hadfield steel, unlike existing AE fatigue monitoring methodology, which relies solely on the analysis of AE parameter trends.

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

A study of the crack initiation angle in the substrate of spur gear teeth subjected to rolling contact fatigue

Fracture failure of engineering structures is caused by cracks that extend beyond a safe size. For modeling the stable fatigue crack propagation it is necessary to determine the angle of crack initiation, i. e. the angle under the crack growth takes place. There are several criteria that try to model this aspect, such as: Sih, MTS, M, T, Ip.The S-criterion (Sih) was used to simulate the fatigue growth of an internal crack situated in the substrate of a certain spur gear, taking into account the rolling contact fatigue. S-criterion states that the direction of crack initiation coincides with the direction of minimum strain energy density along a constant radius around the crack tip. Since the stress intensity factor Ki is negligible (because of the compression stresses in the substrate of the gear tooth), calculations were made for these conditions and it was obtained a constant and unique angle of crack initiation angle 9 ∼ 80°, that is in concordance with the experimental observations. This study serves to determine the fatigue crack initiation angle in a compression stresses field, as well as its use in modeling the propagation of the fatigue cracks.

Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, $$\hbox {K}_{\mathrm{max}}$$ . This work shows that $$\hbox {K}_{\mathrm{max}}$$ correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that $$\Delta \hbox {K}$$ correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with $$\hbox {K}_{\mathrm{max}}$$ with constant parameters.

The mode I fatigue behaviours of notched freestanding Al-3Mg foils

The effect of the thickness on fatigue crack propagation behaviour of rolled and annealed Al-3Mg alloy foils was investigated using notched rectangular specimens with thicknesses of 30µm and 100µm and elastic finite element method analysis. A specialized apparatus in combination with a force sensor and a video graphics array digital camera to record the crack growth was employed to investigate the mode I fatigue behaviour of freestanding Al-3Mg alloy foils. Tests were conducted at ambient temperature, with a loading frequency of 2Hz and a loading ratio R of 0.1. The foil thickness was found to play a distinct role in the fatigue properties of micron-foil metals. Compared to the 100-μm-thick foil, the 30-μm-thick foil exhibited higher average yield stress (σ0.2), but lower elongation, fracture toughness and fatigue life. The crack propagation rate da/dN of both foils as a function of the stress intensity factor ΔK followed the Paris relationship. The Paris constant m, increased slightly, from 4.6 to 5.6, as the foil thickness decreased from 100µm to 30µm. A smaller plastic zone size at the crack tip and a higher value for the self-similarity parameter z were found in the thinner foils, resulting in the higher Paris constant m value. With typical fatigue striations, the 30-μm-thick foils demonstrated failure in a brittle mode during the stable fatigue crack growth stage. However, the 100-μm-thick foils had abnormal striations and dimples in the stable fatigue crack growth stage, they failed in a mixed ductile and brittle mode.

Lifetime assessment of additive manufactured polymer materials by means of the rolling ring test using cyclically loaded notched ring specimens

Compared to other methods of fatigue testing which are in need of expensive testing machines and often highly time-consuming, the developed rolling ring tester is cost-effective, easily to handle and allows relatively fast data acquisition. Within only a few days essential information for fast quality assurance of laser-sintered parts are available such as about the influence of the processing conditions on the lifetime under cyclic loading. The number of cycles at break has been found to increase nearly logarithmically with decreasing deformation of the laser-sintered rolling rings made of polyamide 12. This behavior corresponds to a transition from fully stable fatigue crack growth to mixed stable–unstable crack propagation. Furthermore, the number of cycles at break increases linearly with increasing energy density used during the laser sintering process.

On the Short Surface Fatigue Crack Growth Behavior in a Fine-Grained WC-Co Cemented Carbide

In the present study, the fatigue crack growth (FCG) behavior of short surface cracks in a fine-grained cemented carbide with a length of less than 1 mm was investigated. The rotating bending and the four-point bending fatigue tests were carried out at stress ratios of R = −1 and R = 0.1 (R = maximum stress/minimum stress). It was found that a short surface crack had a longer stable fatigue crack growth area than a long through-thickness crack; the FCG behaviors of the two types of crack are clearly different. Furthermore, the FCG path of short surface cracks was investigated in detail to study the interaction between fatigue cracks and microstructures of the cemented carbide such as WC grains and the Co phase. At a low Kmax (Kmax = the maximum stress intensity factor), it was found that fatigue crack growth within WC grains is difficult because of a small driving force; instead, crack growth is along the brittle WC/WC interface. On the other hand, at a high Kmax, WC grain breakage often occurs, since the driving force of FCG is large, and the fatigue crack grows linearly.

Macromechanics study of stable fatigue crack growth in Al–Cu–Li–Mg–Ag alloy

AbstractThis study was made on a fresh variety of Al–Li base alloy to investigate the role of ageing precipitates and microstructure dimensions in the fatigue crack growth resistance. The fatigue crack growth rate was measured in three different states of the material (i.e. base metal in T8 condition, friction stir weld and laser beam weld in full‐aged condition). Metallurgical analysis showed that the base metal in T8 temper is precipitation hardened by an equivalent amount of δ′ (AL3Li), T1 (AI2CuLi) and θ′ (AI2Cu) precipitates. The friction stir weld retained the morphology of strengthening precipitate; however, coarsening of Cu containing precipitates has occurred. On the other hand, laser beam weld showed a different type of CuAl phase morphology, which is characteristic of cast metal. The results of fatigue tests confirmed that fatigue crack growth resistance largely depends on microstructural features, specifically the strengthening phases. The fatigue crack resistance was in the order of base metal > laser beam weldment > friction stir weldment. The CuAl phase played a vital role in the crack closure of the laser beam weldment, thus enhancing the fatigue life as compared with the friction stir weldment, which was evident from the plot between log of da/dN (crack growth in each cycle) and log of ΔK (stress intensity range).

Low Cycle Fatigue Behavior of TA29 Titanium Alloy at Different Temperatures

The low cycle fatigue behaviour of TA29 titanium alloy blisk forging at 300°C, 400°C, 500°C, 600°C were studied. The cyclic stress response, the stress-strain behaviour, the fatigue life and the fractograph were observed and analyzed. Through the double logarithm linear regression, the Coffin-Manson model parameters of TA29 titanium alloy at different temperatures were obtained. The results showed that the fatigue life decreases as the strain amplitude increases. Also, the testing temperature has a significant effect on the low cycle fatigue properties of TA29 titanium alloy. At the same level of the strain amplitude, the fatigue life and the peak value of cyclic stress decrease, while the plastic strain component in total strain amplitude increases with the increase of testing temperature. On the basis of the fractograph of fatigue specimens, the initiation of crack is mainly on the surface of the specimen. Fatigue striations can be seen clearly at fatigue crack propagation area. The fatigue striations in stable fatigue crack propagation area were broadened, and the propagation area was reduced and the final rupture area was increased with the increase of testing temperature.

Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

AbstractThe very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔKGBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔKGBF for uncharged specimens are close to crack growth threshold ΔKth, which indicates that ΔKGBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔKGBF.

Fatigue Crack Propagation of Notched Steel Rebar in RC Beams Repaired with Externally Bonded CFRP

AbstractThis paper presents the results of an experimental study on the fatigue performance of RC beams strengthened with different externally bonded carbon fiber-reinforced polymer (CFRP) systems. Seven specimens were fabricated; three had no CFRP, while the remaining four had one of two CFRP systems. The objective of the research reported in this paper was to determine the effect the CFRP repair had on the growth rate of a fatigue crack initiating from a notch in the tensile rebars. The steel reinforcement was selected for focus because similar studies have found this reinforcement to be the limiting fatigue component in RC beams. The experimental results reported in this paper showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were used to determine material constants for the Paris law, which describes growth of a stable fatigue crack. These results are then used to propose recommendations for design of CFRP repair systems...

Finite Element Simulation of Stable Fatigue Crack Growth Using Critical CTOD Determined by Preliminary Finite Element Analysis

Finite element simulation of stable fatigue crack growth using critical crack tip opening displacement (CTOD) was done. In the preliminary finite element simulation without crack growth, the critical CTOD was determined by monitoring the ratio between the displacement increments at the nodes above the crack tip and behind the crack tip in the neighborhood of the crack tip. The critical CTOD was determined as the vertical displacement at the node on the crack surface just behind the crack tip at the maximum ratio. In the main finite element simulation with crack growth, the crack growth rate with respect to the effective stress intensity factor range considering crack closure yielded more consistent result. The exponents m in the Paris law were determined.

Theoretical and Experimental Investigations of Fracture Mechanical Graded Materials

With the focus on innovative manufacturing and application-oriented products considering lightweight construction functional graded materials become more and more interesting. Due to the application matched functional gradation different local demands such as absorbability, abrasion and fatigue of structures are met. However the gradation has a remarkable influence on the crack propagation behaviour. According to the different fracture mechanical graded materials the area of stable crack growth is more or less distinct and the crack velocity changes. In addition the crack propagation direction can be influenced by the gradation. Within this contribution the influence of a fracture mechanical gradation on the crack growth rate, the area of stable fatigue crack growth and the crack propagation direction is investigated. For the prediction of the crack growth behaviour in fracture mechanical graded materials considering different loading situations (Mode I, Mixed Mode) a new concept was developed. This TSSR-concept can be applied to sharp as well as smooth material gradations. In addition results of experimental investigations of crack growth considering fracture mechanical graded materials are discussed.

An Experimental Study of Fatigue Crack Propagation of 16MnR Pressure Vessel Steel in Mode-I Constant Amplitude Loading

An experimental investigation was performed on fatigue crack growth behavior of a 16MnR pressure vessel steel. Standard compact tension (CT) specimens with three specimen thicknesses and notch sizes were subjected to Mode I constant amplitude loading with several stress ratios and loading amplitudes. The results revealed that the stress ratio had an insignificant influence on the fatigue crack growth of the material. The stable fatigue crack growth rate (FCGR) was accelerated as specimen thickness increased. The fatigue crack was extended in terms of the curve crack shape. The crack front at the surface was retarded compared to that at the interior along thickness direction, and the crack front at the mid-thickness plane reached the maximum value of the crack length. The similar curve crack shape was obtained in the stable crack growth stage. The maximum difference of the crack front along thickness direction was increased with the increasing of the specimen thickness. The early crack growth from the notch was effected by the size of the notch, the stress ratio and loading amplitude.

An Experimental Study of Fatigue Crack Propagation of 16MnR Pressure Vessel Steel in Mode-I Constant Amplitude Loading

An experimental investigation was performed on fatigue crack growth behavior of a 16MnR pressure vessel steel. Standard compact tension (CT) specimens with three specimen thicknesses and notch sizes were subjected to Mode I constant amplitude loading with several stress ratios and loading amplitudes. The results revealed that the stress ratio had an insignificant influence on the fatigue crack growth of the material. The stable fatigue crack growth rate (FCGR) was accelerated as specimen thickness increased. The fatigue crack was extended in terms of the curve crack shape. The crack front at the surface was retarded compared to that at the interior along thickness direction, and the crack front at the mid-thickness plane reached the maximum value of the crack length. The similar curve crack shape was obtained in the stable crack growth stage. The maximum difference of the crack front along thickness direction was increased with the increasing of the specimen thickness. The early crack growth from the notch was effected by the size of the notch, the stress ratio and loading amplitude.

An Experiment of Fatigue Crack Growth under Different <i>R</i>-Ratio for 2024-T4 Aluminum Alloy

The fatigue crack growth tests of compact tension (CT) specimens of 2024-T4 aluminum alloy were conducted under constant amplitude loading with differentR-ratios, 0.05, 0.1, 0.5, 0.75, respectively. The thickness of the specimen is 3.8mm. All the fatigue crack growth experiments were carried out in ambient air without a pre-crack. The early crack growth region reflects the influence of the notch. An optical reading micrometer with a magnification of 40 was used to measure the crack length. The size of the notch together with the loading conditions has a great influence on the early crack growth within the notch influenced region. Beyond the notch influenced zone, the stable fatigue crack growth is reached and can be characterized by the Paris law. The experimental results indicate that fatigue crack growth rate increases with theR-ratio for a given stress intensity factor amplitude.

Simulation for Fatigue Crack Propagation Behavior Based on Low Cycle Fatigue Critical Damage

Based on low cycle fatigue critical damage, a numerical procedure for predicting stable fatigue crack propagation behavior of materials and structures was developed, which was named as LFF (LCF: Low Cycle Fatigue + FCP: Fatigue Crack Propagation + FEA: Finite Element Analysis) in this work. In the LFF method, stress and strain amplitude of nodes located at the plastic zone in the crack growing direction and fatigue damage of these nodes were firstly computed from a finite element code ANSYS. Then, by assuming a discontinuous step of propagation in the process of fatigue crack propagation, the stable fatigue crack propagation behavior of materials and structures could be estimated. Experiments on low cycle fatigue and fatigue crack propagation for Cr2Ni2MoV, TA12 and TC4 alloys were employed to certificate the applicability and validity of the LFF method. The results show that the LFF method is suitable for the estimation of stable fatigue crack propagation of a CT specimen with a straight crack under mode Ⅰloading and with a slant crack under mixed mode Ⅰ and Ⅱ loading.