Initial Flaw Size Research Articles

Modeling for corrosion fatigue crack initiation life based on corrosion kinetics and equivalent initial flaw size theory

In present paper, a novel, effective and economical model based on corrosion kinetics and equivalent initial flaw size theory was proposed to predict the corrosion fatigue crack initiation (CFCI) life of E690 steel in simulated seawater. Comparison between the predicted S-N curves and the measured data showed that this model was of nice accuracy under high peak stresses, while non-negligible deviation existed under low peak stresses due to lack of enough measured data or consideration for the stress effect on corrosion rate. Despite of that, thought of this model still has a good practicability in predicting CFCI life of steels.

Statistical Inference of the Equivalent Initial Flaw Size Distribution Using the Boundary Element Method under Multiple Sources of Uncertainty

In this work, a method for determining the Equivalent Initial Flaw Size (EIFS) distribution using the Boundary Element Method (BEM) is proposed. Maximum Likelihood Estimation (MLE) is used to infer the EIFS distribution of a cracked stiffened panel under multiple sources of uncertainty, including uncertainty in the loading conditions, fatigue crack growth model parameters, and in the measurement of crack size found from routine inspections. Results suggest that MLE is an effective tool for estimating the parameters of an EIFS distribution when no prior knowledge is available regarding the EIFS distribution or its parameters.

Multiscale Simulation of Surface Defects Influence Nanoindentation by a Quasi-Continuum Method

Microscopic properties of nanocrystal aluminum thin film have been investigated using the quasicontinuum method in order to study the influence of surface defects in nanoindentation. Various distances between the surface defect and indenter have been taken into account. The results show that as the distance between the pit and indenter increases, the nanohardness increases in a wave pattern associated with a cycle of three atoms, which is closely related to the crystal structure of periodic atoms arrangement on {1 1 1} atomic close-packed planes of face-centered cubic metal; when the adjacent distance between the pit and indenter is more than 16 atomic spacing, there is almost no effect on nanohardness. In addition, the theoretical formula for the necessary load for elastic-to-plastic transition of Al film has been modified with the initial surface defect size, which may contribute to the investigation of material property with surface defects.

Comparison of brittle- and viscous creep in quartzites: Implications for semi-brittle flow of rocks

The co-existence and interaction between brittle and viscous deformation processes contributes to the integrated strength of the crust and results in a wide range of energy-release mechanisms ranging from earthquakes to creep. Here, we compare flow laws derived for quartz-rich rocks deforming by brittle creep, wet dislocation power-law creep and dissolution-precipitation creep. We investigate theoretically the conditions when both brittle and viscous processes contribute significantly to deformation provided that all processes act independently and in parallel. Utilizing a comprehensive data set for deformation experiments in quartz-rich rocks, we find that the transition between deformation mechanisms is strongly dependent on input variables such as initial flaw size and grain size. The transition can occur abruptly or over hundreds of MPa in differential stress and hundreds of degrees Kelvin at a constant strain rate. The transition is strongly dependent on grain-size and confining pressure. Limitations to this work are first that all three flow laws are poorly constrained by experimental data for conditions relevant for the comparison. Secondly, a need exists for new experiments to infill the knowledge gaps between high-temperature and low-temperature deformation experiments and deriving quantitative flow laws for low-temperature plasticity and high-temperature brittle creep.

Crack growth analysis and fatigue life estimation in the piston rod of a Kaplan hydro turbine

AbstractLinear elastic fracture mechanics (LEFM) approaches are seldom used to analyze fatigue problems in hydraulic turbomachinery, where extremely simplified fatigue models are normally considered. In one of the largest Kaplan hydraulic turbines in the world, the piston rod, which is an important load transfer component, was found broken twice at the same location. This event, which leads to extremely high direct reparation costs and indirect costs, is a very useful case for research of fatigue problems in hydraulic turbines by means of LEFM. For this purpose, the crack growth progress in the piston rod is simulated with numerical LEFM techniques. A probabilistic model for the initial flaw size is considered. A reliability criterion, consisting in the probability of not having a critical crack or a fracture in the piston rod after a specific service time, is defined. Based on this model, the influence of initial crack models and operating services of the machine on the reliability of the unit is discussed. The calculated crack path and predicted number of loading cycles as a function of the initial crack size agree with the actual fracture section and operating time to fracture.

Using a Beta (β) Correction to Improve the Life Predictions of Cold-Expanded Holes in 2024-T3 & 7075-T6 Aluminum Alloys

Fastener holes represent the most common fatigue details found in airframe structures. In order to minimize the impact fastener holes have on the fatigue behavior of critical aircraft components, many are processed by cold expansion (Cx). Cx imposes a residual compressive stress field around a hole that retards fatigue crack growth (FCG) and increases the fatigue life of the component. In order to take analytical advantage of the Cx process, a “one size fits all” method of reducing the initial flaw size to a minimum of a 0.005 in. (0.127 mm) symmetric corner crack is often employed. This approach leads to predictions that do not represent the physics of crack growth behavior within materials that have been processed using Cx. A physics-based approach was investigated to take analytical advantage of the Cx process. The authors developed an empirical-based factor (β) to characterize the effect of Cx on the FCG behavior within 2024-T351 and 7075-T651 aluminum alloys. This method takes into account the interaction of the fatigue crack and the residual stress field. This β can be used in FCG analysis to provide an accurate model of the crack growth behavior at Cx holes. In order to develop accurate da/dN versus ΔK plots for the Cx configuration, finite element modeling was used to calculate stress intensities along the crack front. By using the fracture-mechanics-based similitude principle, the nonCx and Cx da/dN versus ΔK data were compared and β corrections representing the residual stress field in the body were calculated.

Examination on Fatigue Limit and Crack Growth Characteristic of SBHS700 Base Metal

The object of this study is to clarify fatigue limit and fatigue crack growth characteristic of SBHS700 base metal which is 780 MPa class steel. This study carried out the fatigue tests of SBHS700 base metal containing different defect size, and the fatigue limit was compared with that of the conventional steel. Test results indicate that the fatigue limit increased with decrease initial defect size, and can be precisely evaluated by using $$\sqrt {area}$$ parameter model. This paper also presents that crack growth characteristic was almost the same as that of the conventional steel from the observation of striations by using Scanning electron microscope and length of beach marks.

Failure mechanism of fiber-reinforced polymer-confined concrete column with initial defects

The effect of initial defect size on the mechanical behavior and failure mode of carbon fiber-reinforced polymer- confined concrete column was investigated through theoretical analysis, finite element software simulation and experiment validation. Qualitative theoretical analysis was firstly explored to study the effect of initial defect size on the mechanical behavior of confined concrete column from macro to micro perspective. Numerical simulations and experimental investigation were then carried out and compared to investigate the mechanical behavior of carbon fiber-reinforced polymer-confined concrete column with initial defects under axial compression and eccentric compression. The variation of defect criticality was investigated by varying the layer number of carbon fiber-reinforced polymer and cross-section size of concrete columns. The effect of initial defect size on the failure mode of carbon fiber-reinforced polymer-confined concrete column was finally demonstrated.

Initial Fatigue Quality Assessment for Aircraft Wing Panel Fastener Hole

An effective initial fatigue quality assessment method is presented in order to verify aircraft wing panel fastener hole whether to satisfy the design requirements. Firstly, after finishing fatigue test of bolted specimens and fatigue fracture interpretation, the time to crack initiation distributions under 3 stress levels are obtained and then a general equivalent initial flaw size distribution is established. Secondly, a method of fatigue life prediction with 95% reliability is proposed. Finally, the initial fatigue quality of aircraft wing panel fastener hole is evaluated based on the economic life criterion and double 95% EIFS value. The results show that the initial fatigue quality of the given aircraft wing panel fastener hole satisfies the design requirements.

Study of Allowable Initial Flaw Size of Rotating Main Shaft

Study of Allowable Initial Flaw Size of Rotating Main Shaft

Towards a temperature dependent and probabilistic lifetime concept for nodular ductile cast iron materials undergoing isothermal and thermo-mechanical fatigue

In this investigation, the fatigue behaviour of a ductile cast iron with high content of silicon and molybdenum, was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests as well as out-of-phase thermomechanical fatigue (OPTMF) tests within the temperature range RT – 500 °C. The studied material shows an embrittlement at temperatures nearby 400 °C. A possible explanation for the observed lifetime reduction is intergranular embrittlement (IE). A mechanism based lifetime model is proposed for assessing the lifetime. The model is based on the assumption that the crack advance per cycle is correlated with the cyclic crack tip opening displacement (ΔCTOD) attributed to the crack tip blunting caused by accumulation of plastic and creep deformations ahead of the crack tip. Intergranular embrittlement is accounted for by introducing a temperature and strain rate dependent prefactor in the crack growth law, which only acts in a certain temperature range. The model is calibrated for a GJS material and successfully applied to predict the lifetime of this material when undergoing isothermal and non-isothermal mechanical loadings. A probabilistic interpretation of the scatter of the investigated material is presented in conjunction with the random nature of the initial defect size distribution.

Estimation of P‐S‐N curves in very‐high‐cycle fatigue: Statistical procedure based on a general crack growth rate model

AbstractExtensive experimental investigations show that internal defects play a key role in the very‐high‐cycle fatigue (VHCF) response of metallic materials and that crack growth from internal defects can take place even if the stress intensity factor associated to the initial defect is below the threshold for crack growth. By introducing a reduction term in the typical formulation of the threshold for crack growth, the authors recently proposed a general phenomenological model, which can effectively describe crack growth from internal defects in VHCF. The model is able to consider the different crack growth scenarios that may arise in VHCF and is enough general to embrace the various weakening mechanisms proposed in the literature for explaining why crack can grow below the threshold.In the present paper, the model is generalized in a statistical framework. The statistical distributions of the crack growth threshold and of the initial defect size are put into the model. The procedure for the estimation of the Probabilistic‐S‐N curves and of the fatigue limit distribution is illustrated and numerically applied to an experimental dataset.

Residual lifetime assessment of thermal power plant superheater header

Abstract The safe operation of thermal power plants (TPPs) is largely determined by the reliability of main components of steam systems of power units, to which belong the headers of boiler superheaters. The aim of this study is to estimate the residual lifetime of superheater header starting from the initial defect size and up to the maximum allowable one. The above-mentioned lifetime is affected by the fluctuations of temperature under steady mode of operation of TPP superheater header. Superheaters headers operate in a steamy environment under the pressure of 15.5 MPa at temperature of 545 °С. The header is a thick-walled cylinder made of 12Cr1MoV steel with a length of 2314 mm, outer diameter of 325 mm and thickness of 50 mm. To estimate the residual lifetime of the header, the steam temperature in header was measured and digitized. The temperature range of the header under quasi-stable operational mode was divided into three classes: (1) Δt Δt Δt > 30 °C. The local minima and maxima were determined based on the obtained steam temperature history. The residual durability was evaluated taking into account the effect of thermo-mechanical stresses and also the stresses caused by internal steam pressure. The stress intensity factors (SIFs) at the crack tip in the ligament between the holes of superheater collector were estimated by FEM. The SIF of mode I (K I ) were determined along the fronts of modelled semi-elliptical cracks. The SIF correction function at the midpoint of the semi-elliptical crack front in the ligament between holes of the superheater header versus the crack depth and the temperature difference between the external and internal surfaces under a constant internal pressure of steam was obtained. Based on the analysis of header defects, the proposed front shape was taken in the form of the semi-ellipse. The crack growth at 500 °C was modelled by Paris and NASGRO equations. With the increase of temperature difference between the external and internal header surfaces from 10 °С to 50 °С, the number of cycles, that is necessary for the crack to reach 35 mm in depth, decreases approximately in 25 times. It was calculated, that the average value of temperature fluctuations is 15 °С for the Class 1, and is 46.2 °С for the Class 2. The lifetime of header can be extended due to the decrease of fluctuations of temperature range and their frequency.

A Fatigue Life Prediction Method Based on Strain Intensity Factor.

In this paper, a strain-intensity-factor-based method is proposed to calculate the fatigue crack growth under the fully reversed loading condition. A theoretical analysis is conducted in detail to demonstrate that the strain intensity factor is likely to be a better driving parameter correlated with the fatigue crack growth rate than the stress intensity factor (SIF), especially for some metallic materials (such as 316 austenitic stainless steel) in the low cycle fatigue region with negative stress ratios R (typically R = −1). For fully reversed cyclic loading, the constitutive relation between stress and strain should follow the cyclic stress-strain curve rather than the monotonic one (it is a nonlinear function even within the elastic region). Based on that, a transformation algorithm between the SIF and the strain intensity factor is developed, and the fatigue crack growth rate testing data of 316 austenitic stainless steel and AZ31 magnesium alloy are employed to validate the proposed model. It is clearly observed that the scatter band width of crack growth rate vs. strain intensity factor is narrower than that vs. the SIF for different load ranges (which indicates that the strain intensity factor is a better parameter than the stress intensity factor under the fully reversed load condition). It is also shown that the crack growth rate is not uniquely determined by the SIF range even under the same R, but is also influenced by the maximum loading. Additionally, the fatigue life data (strain-life curve) of smooth cylindrical specimens are also used for further comparison, where a modified Paris equation and the equivalent initial flaw size (EIFS) are involved. The results of the proposed method have a better agreement with the experimental data compared to the stress intensity factor based method. Overall, the strain intensity factor method shows a fairly good ability in calculating the fatigue crack propagation, especially for the fully reversed cyclic loading condition.

Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement

AbstractShot peening is a dynamic cold‐working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly used as a surface treatment technique within the aerospace industry during manufacturing to improve fatigue performance of structural components. The compressive residual stress induced during shot peening results in fatigue crack growth retardation, improving the performance of shot‐peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Because of the relatively soft nature of AA7050‐T7451, shot peening can result in cracking of the constituent precipitate particles, creating an initial damage state. The aim of this paper is to understand the balance and fundamentals of these competing phenomena through a comparative study throughout the fatigue lifecycle of baseline versus shot‐peened AA7050‐T7451. Microstructure and surface topology characterization and comparison of the baseline and shot‐peened AA7050‐T7451 has been performed using scanning electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and optical profilometry techniques. A residual stress analysis through interrupted fatigue of the baseline and shot‐peened AA7050‐T7451 was completed using a combination of X‐ray diffraction and nanoindentation. The fatigue life performance of the baseline versus shot‐peened material has been evaluated, including crack initiation and propagation. Subsurface particles crack upon shot peening but did not incubate into the matrix during fatigue loading, presumably due to the compressive residual stress field. In the baseline samples, the particles were initially intact, but upon fatigue loading, crack nucleation was observed in the particles, and these cracks incubated into the matrix. In damage tolerant analysis, an initial defect size is needed for lifetime assessment, which is often difficult to determine, leading to overly conservative evaluations. This work provides a critical assessment of the mechanism for shot peening enhancement for fatigue performance and quantifies how incubation of a short crack is inhibited from an initially cracked particle into the matrix within a residual stress field.

DC electrical tree growth in epoxy resin and the influence of the size of inceptive AC trees

Electrical tree propagation is a precursor to dielectric failure in high voltage polymeric insulation. Tree growth has been widely studied under AC conditions, but its behavior under DC is not well understood. The aim of this work was to examine the importance of polarity and initiating defect size on DC tree propagation. Electrical tree propagation in epoxy resin under constant DC voltages of +60 kV and −60 kV was measured in samples with classical needle-plane electrodes, but with small initial trees (< 100 μm) incepted under lower AC voltages before the DC tests. Experimental results showed strong polarity dependence. In either polarity, the length of the initial AC tree had a major influence on the inception of the subsequent DC tree. The effect was attributed to the defect being influential if it is larger than the space charge injection region. As a consequence, there is a critical defect size that will accelerate failure of DC insulation dependent on space charge injection behavior of the polymer/electrode system in question — a critical finding for high voltage asset management.

Fatigue life prediction based on equivalent initial flaw size for Al-Li alloy 2297 under spectrum loading

Fatigue life prediction methods based on the Equivalent Initial Flaw Size (EIFS) concept under variable amplitude (VA) spectrum loading conditions were studied in this paper. Fatigue tests of Al-Li alloy 2297 under constant amplitude (CA) and VA loading cases were carried out. Based on the comparison of predictions and test results, different ways to determine the EIFSs and whether the load-interaction effects should be considered in VA fatigue life prediction were discussed to recommend their applicable cases. A good agreement is observed between EIFS model predictions and experimental data, especially in case of high maximum stresses in VA loading.

Determination method of equivalent initial flaw size for crack initiated at hole chamfering

Determination method of equivalent initial flaw size for crack initiated at hole chamfering

Effect of elevated temperature on the fatigue strength of casted AlSi8Cu3 aluminium alloys

In this paper, the fatigue strength of casted aluminium alloy AlSi8Cu3 T5 and T6 is investigated at room and an elevated temperature of 150 °C. The specimens are extracted from cylinder heads (AlSi8Cu3 T5) at one and from crankcases (AlSi8Cu3 T6) at two defined specimen locations. In addition, quasi-static tensile tests are executed for both temperature conditions. Extensive fractographic analyses of tested specimens are performed to characterise the failure mechanisms and measure the crack initiating defect size. This work is supported by computed tomography analysis to achieve an enhanced knowledge of the micropore morphology. The experiments demonstrate that the fatigue strength of the AlSi8Cu3 T5 (cylinder head) and the position 1 of the AlSi8Cu3 T6 (crankcase) significantly decreases at 150 °C by 25 % and 7 % respectively. These two specimen positions exhibit smaller micropore sizes (120 µm and 85 µm at room temperature) compared to position 2 of AlSi8Cu3 T6 (crankcase) and show a partially change in the failure mechanism from defect at room temperature to slip band induced crack initiation at 150 °C. Position 2 of AlSi8Cu3 T6 (crankcase) indicates a partial change of the fatigue strength level at 150 °C compared to room temperature. Additionally it illustrates no change in failure mechanism, whereby all specimens reveal a defect induced failure, which can be explained by significant higher micropore sizes (537 µm at room temperature) compared to the other extraction positions.

Effect of defect size on P-S-N curves in Very-High-Cycle Fatigue

It is well-known that internal defects play a key role in the Very-High-Cycle Fatigue (VHCF) response of metallic materials. VHCF failures generally nucleate from internal defects, whose size strongly affects the material strength and life. Therefore, S-N curves in the VHCF regime are defect size dependent and the scatter of fatigue data is significantly influenced by the statistical distribution of the defect size within the material.The present paper proposes an innovative approach for the statistical modeling of Probabilistic-S-N (P-S-N) curves in the VHCF regime. The proposed model considers conditional P-S-N curves that depend on a specific value of the initial defect size. From the statistical distribution of the initial defect size, marginal P-S-N curves are estimated and the effect of the risk-volume on the VHCF response is also modeled. Finally, the paper reports a numerical example that quantitatively illustrates the concepts of conditional and marginal P-S-N curves and that shows the effect of the risk-volume on the VHCF response.