Initial Flaw Size Research Articles

Quantitative Evaluation of the Flaking Strength of Rolling Bearings with Small Defects: (Part 2: Evaluation of the Flaking Strength of Rolling Bearings with Small Drilled Holes, based on the Stress Intensity Factor)

Rolling contact fatigue (RCF) tests were conducted on rolling bearings, with holes micro-drilled at the mid-point of the tracks. In all of the RCF tests, fatigue cracks initiated at the edge, near the base of the drilled holes, later propagating by shear-mode. Even in the un-flaked specimens tested up to N = 2×108 cycles, short fatigue cracks were discovered at the edges. Using the stress intensity factor (SIF) range, as calculated for the initial defect size, fatigue life data were uniformly gathered inside a narrow band, irrespective of the diameters and depths of the holes. In addition, it was determined that the crack-size dependency of the threshold SIF range, well-known for Mode I fatigue cracks, also exists for Mode II fatigue cracks, as produced after rolling contact. The values of the threshold SIF range obtained by the RCF tests were in good agreement with those obtained in the torsional fatigue tests under static compression.

Fatigue crack growth analysis of steel elements reinforced with shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patches

This paper describes a numerical framework to simulate the fatigue crack growth of single edge-notched steel elements patched with shape memory alloy (SMA)/fiber reinforced polymer (FRP) composite patches. The framework includes the small crack propagation. The small crack propagation stage was simulated using the analytical model based on the equivalent initial flaw size method. The bridging effects of the SMA/FRP composite patch were simulated by two consecutive finite element simulations considering the interfacial debonding during fatigue loading. Numerical predictions were validated by comparison with experimental results. A parametric study was conducted using the validated model. The considered parameters were the number of SMA wires used in the patch, the applied stress range, and the length of the crack in the steel substrate at the time of installing the patch. The numerical results indicated that the SMA wires increased the critical crack length at which fracture occurred and that the patch was more effective at lower stress ranges. Further, the SMA/FRP patch can substantially delay the failure of a member with very little remaining fatigue life. The results demonstrate the potential of this technique and highlight the importance of early intervention when repairing fatigue sensitive details.

Risk-Informed Approach to Structure Integrity Prediction and Optimization of In-Service Inspection of Heat Exchange Equipment with High Defect Statistics

A physically and statistically based approach to steam generator heat exchange tube integrity assessment is proposed in the paper. The method is based on stochastic laws of crack dimensions distribution taking into account their growth, limit load model of cracked tube and SG plugging statistics. Based on the history of plugging tubes of specific SG, three statistical parameters have to be found: initial number of defects, initial defect size and defect growth rate. The developed method was used to predict HET failure for all Ukrainian SGs and to justify pressure reduction of hydrostatic test (HT) for primary side of NPPs with VVER. It is shown that pressure reduction does not practically increase the fracture probability during operation.

Local stress analysis of a defective rolling bearing using an explicit dynamic method

This article provides insights into the localized stress in a defect zone of the rolling bearings when the rolling elements pass through the defect. A dynamic finite element model of a rolling bearing with an artificial round defect in its outer raceway is solved numerically using the explicit dynamic finite element software package, ABAQUS. The maximum Mises stress and maximum contact pressure in the defect zone are obtained in the simulation. The effects of radial load, rotation speed, and initial defect size on the stress level are investigated. The results show that higher stresses are generated during the rolling balls passing through the defect. The radial load and defect size have significant effects on the stress level of defect, while low-level rotation speed has neglected effects.

Fracture mechanics modelling of constant and variable amplitude fatigue behaviour of field corroded 7075‐T6511 aluminium

AbstractFor ageing airframe structures, a critical challenge for next generation linear elastic fracture mechanics (LEFM) modelling is to predict the effect of corrosion damage on the remaining fatigue life and structural integrity of components. This effort aims to extend a previously developed LEFM modelling approach to field corroded specimens and variable amplitude loading. Iterations of LEFM modelling were performed with different initial flaw sizes and crack growth rate laws and compared to detailed experimental measurements of crack formation and small crack growth. Conservative LEFM‐based lifetime predictions of corroded components were achieved using a corrosion modified‐equivalent initial flaw size along with crack growth rates from a constant Kmax‐decreasing ΔK protocol. The source of the error in each of the LEFM iterations is critiqued to identify the bounds for engineering application.

Modified Gluteal Fold V-Y Advancement Flap for Reconstruction After Radical Vulvectomy.

To describe the surgical technique of the V-Y cutaneous supra-fascial (modified) gluteal advancement flaps for reconstruction after radical vulvectomy and to assess the outcome of patients according to their clinical characteristics. Between January 2006 and July 2012, 36 V-Y flaps were performed in 21 patients to cover the defect after radical surgery of primary vulvar cancers. Surgery duration, blood loss, hospital stay, and wound healing were assessed according to patient age, body mass index (BMI), American Society of Anesthesiologists (ASA) score, and the initial defect size. Median patient age, BMI, and ASA score were 80 (range, 31-91), 28 (range, 18-36), 3 (range, 1-3), respectively. Median surgery duration and blood loss were 180 minutes (range, 60-275) and 400 mL (range, 100-1000), respectively. Median operating time was higher in patients ASA3 than ASA less than 3, 200 versus 120 minutes (P = 0.038). Median initial defect size was higher in patients with BMI greater than 28 than 28 or less, 92 versus 55 cm (P = 0.004). Local scar defect was observed in 16 patients (76%), mild, less than 10 cm in 10 patients. Median wound healing duration was higher in patients with bilateral than unilateral flap, 16 versus 9.5 days (P = 0.034). The V-Y cutaneous suprafascial gluteal advancement flap for vulvar reconstruction after vulvectomy is an easy, safe, and reliable procedure. However, even mild local scar defect after bilateral flap may impact on wound healing and hospital stay, in elderly and ASA3 patients.

A superimposed cohesive zone model for investigating the fracture properties of concrete–asphalt interface debonding

AbstractA cohesive zone model is proposed to simulate the interface debonding, a preponderant cause of failure for bonded concrete overlay of asphalt (BCOA). The model is constructed by superimposing four root models, each representing the mechanism of one subcritical failure at the interface zone observed in laboratory experiments. The model parameters are established through an inverse analysis of wedge splitting tests performed on BCOA specimens. These inputs are mainly a function of the materials at the interface zone, such as microtexture and macrotexture, and thus can be expected to be applicable to the numerical simulation of a full scale BCOA slab. For modeling across scales, the impact of specimen size, milling depth and initial flaw size on the model, in terms of peak traction and fracture energy, is also discussed.

Effect of the defect initial shape on the fatigue lifetime of a continuous casting machine roll

The article deals with the influence of the defect initial shape on the residual lifetime of a continuous casting machine roll made of 25Cr1MoV steel. Based on this approach, previously proposed by some authors, the growth of the surface fatigue crack was modeled in a roll under loading and temperature conditions that are close to operational ones, taking into account the statistical distribution of the C parameter of Paris' equation. Dependencies of the continuous casting machines roll fatigue lifetime on the initial defect shape and critical defect sizes are obtained.

Sampling method in probabilistic S-version finite element analysis for initial flaw size

The aim of this paper is to demonstrate the accuracy of the sampling method in probabilistic fatigue surface crack growth analysis. A finite thickness plate with surface crack subjected to a fatigue load is considered for fracture analysis using the newly developed Probabilistic S-version Finite Element Model (ProbS-FEM). Two sampling methods, Monte Carlo and Latin Hypercube are employed in the ProbS-FEM. Two different sampling methods are used to generate the random variables in the analysis. The results from the strategies are compared to highlight the advantages of each sampling strategy. The distribution of the initial surface crack length and depth are extracted from the fatigue tests. The accuracy and consistency between the two sampling methods are evaluated. The Latin Hypercube sampling (LHS) shows advantages compared to the Monte Carlo. Then, the developed ProbS-FEM is demonstrated on a four-point bending model. The developed ProbS-FEM shows that the code is capable of modelling the uncertainty in the fatigue surface crack analysis with better accuracy and consistency of sampling process.

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.

Fatigue life prediction based on an equivalent initial flaw size approach and a new normalized fatigue crack growth model

Abstract A general procedure for fatigue life prediction of structural details based on Fracture Mechanics approach is presented in this paper, taking advantage of the new normalized fatigue crack growth model proposed by Castillo et al., here denoted as CCS model. An extension to the CCS model is proposed by adopting the cyclic J-integral range instead of the stress intensity factor range as reference parameter. This enables the generalized elastoplastic conditions resulting for the cracked geometry of the structural detail to be considered by means of the cyclic J-integral values obtained from a finite element analysis, for different loading levels and crack lengths. As a practical application, the proposed approach is applied to a notched plate made of P355NL1 steel, using the equivalent initial flaw size (EIFS) concept. Fatigue crack growth data for CT specimens from the literature is evaluated to estimate the modified CCS crack growth model parameters. The predicted fatigue propagation lifetime prediction is compared with the results and, finally, the goodness of the predictions is analysed and deviations discussed.

Research on Nondestructive Examination of Jacket Sections for CFETR Central Solenoid Model Coil

The central solenoid model coil (CSMC) is a large superconducting “model” coil, which is responsible for developing and verifying the key manufacturing methods of the China Fusion Engineering Test Reactor. Therefore, all subcomponents have very strict requirements, including the conductors which are cable-in-conduit conductor (CICC) type. The jacket section as an important component of the CSMC CICC is circular-in-square type, which is nonmagnetic stainless steel. Severe requirements not only high mechanical performances but also the size of initial defects in the jacket section were specified. Based on the linear elastic fracture mechanics, maximum acceptable defect sizes inside the jacket material have been estimated. Phased array ultrasonic test (PAUT) supplemented with eddy current test (ECT) were studied and developed for nondestructive examination. The defects inside the jacket section can be detected by PAUT with 11 runs by 4 probes. The defects on inner surfaces can be detected by ECT. Focal laws on PAUT and parameters of ECT are also discussed in this paper.

Microstructural effects on the R-curve behavior of WC-Co cemented carbides

The influence of microstructure on the R-curve behavior of hardmetals and its implication on their strength and reliability are studied. In doing so, an already validated model for R-curve description is implemented for rationalizing the fracture behavior of five microstructurally different WC-Co cemented carbides. Results indicate that hardmetals with R-curves developing smoothly over relatively long multiligament zones (i.e. lower slopes) exhibit reduced strength variability than those with steeper and shorter (i.e. higher slopes) ones. The reduced strength scatter associated with a low-slope R-curve behavior is discussed on the basis of the effectively developed toughness and the subcritical crack extension reached before unstable growth is triggered, both being dependent on microstructural length scale and initial flaw size. The results of this investigation highlight the relevance of an in-depth knowledge of R-curve characteristics of hardmetals in order to optimize the performance of engineering parts by means of microstructural design.

A Simple Fatigue Life Prediction Algorithm Using the Modified NASGRO Equation

A simple fatigue life prediction algorithm using the modified NASGRO equation is proposed in this paper. The NASGRO equation is modified by introducing the concept of intrinsic effective threshold stress intensity factor (SIF) rangeΔKeffth. One advantage of the proposed method is that the complex growth behavior analysis of small cracks can be avoided, and then the fatigue life can be calculated by directly integrating the crack growth model from the initial defect size to the critical crack size. The fatigue limit and the intrinsic effective threshold SIF rangeΔKeffthare used to calculate the initial defect size or initial flaw size. The value ofΔKeffthis determined by extrapolating the crack propagation rate curves. Instead of using the fatigue limit determined by the fatigue strength at the specific fatigue life, the fatigue limit is selected based on the horizontal tendency of theS-Ncurve. The calculated fatigue lives are compared to the experimental data of two different alloys. The predictedS-Ncurves agree with the test data well. Besides, the prediction results are compared with that calculated using the FASTRAN code. Results indicate that the proposed life prediction algorithm is simple and efficient.

Lifetime estimation of superheater header

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 the 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 ºС. 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 header, the registered operational data of steam temperature in header were used. The temperature range of header under quasi-stable operational mode was divided into three classes: the temperature range (1) T < 10 °C; (2) 10 °C < T < 30 °C; (3) T > 30 °C. The local minima and maxima were determined using the obtained steam temperature history. The stresses in the wall of header for the given temperature fluctuations were calculated by finite element method (FEM).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 at the crack tip in the ligament between the holes of superheater collector were estimated by FEM. Based on the analysis of header defects, the proposed front shape was taken in the form of semi-ellipse. The crack growth at 500 ° C was modelled by Paris equation.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.

Fatigue strength of PF-1 coil helium inlet with weld defects.

Helium inlet (HI), used for supply of liquid helium into a superconductor, is a critical element of PF-1 coil of the fusion reactor ITER. The superconductor is placed into internal bore of a steel jacket. TIG welding without subsequent heat treatment is used for connection the HI to the jacket. Deviations from the established technology of welding may cause some crack-like defects. Therefore possibility of occurrence the through wall cracks and inlet leak under cyclic loading was evaluated.Calculations of growth of initial weld defects with various depths under cyclic loading were performed using Paris equation. It was obtained that critical size of initial defects, which can growth up to through wall after 30,000 cycles is about 1 mm.For experimental study a cyclic strength and tightness fatigue tests of full-scale 1.5 m length sample of helium inlet have been conducted in liquid nitrogen (77K). Visual control, leak testing, X-ray and liquid penetration inspections were performed after passing the given base test 30,000 cycles. Defects were not found by all used control methods. Reliability of the adopted welding technology was confirmed.

Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size.

Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given.

Crack-closure based method for modelling the fatigue behavior of superelastic NiTi

One remarkable trait reportedly exhibited by NiTi is the improvement of its fatigue performance with increasing mean strain; this is in stark contrast to typical engineering materials where a decrease in fatigue behavior is expected. Clarification into this phenomenon, however, still remains incomplete in literature. In this study, the computational methodology developed by Bruzzi et al. (2002) is adapted to predict the complex fatigue behavior of superelastic NiTi. This defect tolerant approach correlates local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. Computationally derived predictions of fatigue life are attained for stable austenitic, stable martensitic and superelastic NiTi specimens for a multitude of loading conditions and initial defect sizes. Predicted stress-life curves (S–N curves) are compiled and converted to the more relevant strain-based constant life diagram. Further insight is therefore achieved into the individual contribution of the stable austenitic and stable martensitic phases on the unique fatigue behavior of superelastic NiTi. In this manner, this study offers a possible quantitative explanation toward the experimentally exhibited increased fatigue performance of superelastic NiTi.

Fatigue damage tolerance of two tapered composite patch configurations

To avoid high peel stresses around the perimeter of a bonded composite patch repair, the patch thickness often tapers to a fine edge. This paper investigates and compares the damage tolerance of two different configurations of taper design under fatigue loading with different size of initial bondline flaws. The bondline damage at the tip of the doubler repair was simulated by an initial flaw which was created through a Teflon tape. A constant amplitude fatigue loading was applied to all specimens. The crack propagation against the number of cycles was recorded and the fatigue life when the crack reached 100mm from the doubler tip was reported. Microscope investigations were conducted and provided micro scale evidence on the effects of initial flaw size on the crack initiation pattern of the two doubler design configurations. Finally, the damage tolerance of the two configurations were compared and conclusions were drawn with implications for the design of composite patch repairs. It was found that ply-drop doubler joints showed better damage tolerance than stepped doubler joints under fatigue loading. The experimental results suggested a threshold size of 5mm for the doubler tip flaw, if any inspection technology should be adopted for damage detection.

“Size effect” in the fatigue behavior of Friction Stir Welded plates

Comparative fatigue tests were carried out on Friction Stir Welded specimens of a 2195-T8 aluminum–lithium alloy that differed significantly in width. The width of the larger specimens was over thirteen times greater than that of the small specimens. Fatigue results showed a clear “size effect”, i.e. fatigue life of large specimens was about 40% of the corresponding value of small specimens. The Equivalent Initial Flaw Size methodology was adopted to correlate the two sets of results. Fatigue crack initiation life was disregarded with respect to crack propagation life, and fatigue life was evaluated only as propagation of a small pre-existing defect. Following this methodology, test results of small specimens were used to evaluate the initial equivalent flaw contained in each specimen. It was assumed that this data followed a normal distribution. The equivalent initial flaw in larger specimens was evaluated by simple geometrical considerations. A very good assessment of mean fatigue life and scatter in the fatigue results of large specimens was obtained by simulating the propagation of these defects. Calculations were carried out by taking also welding residual stresses into account, but the results demonstrated that this effect was not significant.