Surface Crack Length Research Articles

Machine Learning Based Prognostics of Fatigue Crack Growth in Notch Pre-cracked Aluminum 7075-T6 Rivet Hole

Constant stress amplitude fatigue tests were conducted on the notch pre-cracked Aluminum 7075-T6 rivet hole dog-bone coupons. Monitoring of visible surface crack length by special surface engraving using digital microscope images and by ultrasonic sensors signals was carried out to yield fatigue crack length measurements in relation to number of fatigue cycles applied. The experimental results provide ultrasonic sensor validation for fatigue crack length measurements. Fracto-graphic examination of failed fatigue surfaces has provided further confirmation of notch pre-crack length, crack initiation process, and crack growth marker bands. These experimental inputs were used in NASGRO and AFGROW software fatigue crack growth simulations. The simulation results did not match the crack initiation fatigue life measured by experiments. However, there was good agreement with crack growth simulations of larger cracks. Hence, we plan to develop a machine learning application that will learn the fatigue crack initiation and crack growth processes from data obtained from our own experiments and other fatigue data available from AFGROW databases. Nonlinear AutoRegressive models with eXogenous input (NARX) artificial neural network were used to predict crack growth longer than 5.0-mm. Particle filtering modeling with Bayesian updating was applied to these experimental data for prognostics of fatigue crack growth. A concept design and preliminary implementation results will be presented.

Effect of Wear on Surface Crack Propagation in Rail–Wheel Wet Contact

The interaction between wear and rolling contact fatigue in wet contact was investigated by means of experiments on a two-disk test bench. Specimens of various railway wheel steels were coupled with specimens of the same rail steel, subjected to rolling and sliding wet contact with varying sliding/rolling ratio. Some tests with a dry rolling–sliding contact phase preceding wet contact were carried out as well (dry–wet tests). The pressurization of the fluid entrapped in the surface cracks was the cause of a rapid and severe damage in the dry–wet tests, due to the nucleation of surface cracks by ratcheting in the dry phase, which subsequently propagate in the wet phase. In the wet tests, the fluid pressurization effect was much mitigated due to the absence of initial surface cracks; the specimens subjected to higher sliding/rolling ratio showed the best performance against rolling contact fatigue, due to the effect of wear in reducing the length of surface cracks. A model for assessing the interaction between wear and rolling contact fatigue was proposed, based on the correction of the classical Paris law for crack propagation by taking into account the effect of wear on crack length reduction. The application of the model to the experimental tests allowed finding a correlation between the model predictions and the occurrence of fatigue failure.

Influence of Weld Toe Radii on Fatigue Life Prediction

A study was carried out to investigate the influence of the weld toe radii on the fatigue strength of butt welded joints loaded in bending and tension. Fatigue analysis starting from weld toe cracks in marine and offshore welded pipe specimens were conducted using cut-out four-point bend fatigue test specimens. Fatigue life can be enhanced by reducing the local stress concentration generated by weld toe radius effects. This study investigated the effect of different weld toe radii on the stress intensity factor at the region of the weld toe through Finite Element Analysis (FEA). FEA was used to model a butt welded steel plate extracted from a pipe subject to tension and four-point bend loading. Semi-elliptical surface (SESC) cracks were modeled at the weld toe region with different SESC crack depth and surface crack length. Four weld toe radii and two modes of loading were investigated. The stress intensity factors, weald toe magnification factors, or Mkc and Mka are reported and used for fatigue life prediction.

Fatigue crack growth and damage characteristics of high-speed rail at low ambient temperature

Abstract Low temperature can be a significant problem affecting safety and maintenance of railway. In this study, the fatigue crack growth rate and rolling contact fatigue damage behaviors of high-speed rail material under different temperature conditions were investigated by a series of experiments. The results indicate that the stress and strength of rail material increase with the decrease of ambient temperature. The crack growth rate at 0 °C and − 20 °C is similar with that at 20 °C. While, when the temperature decreases to − 60 °C, the growth rate of crack increases sharply. The promotion of rail embrittlement at low temperature accompanied with the action of high stress causes the rapid failure and increase of surface crack length and subsurface crack damage. Meanwhile, three crack growth mechanism models at different temperatures can be inferred. The brittle fracture mode is increasingly apparent with the temperature decreasing.

Fatigue cracking behaviour of epoxy-based marine coatings on steel substrate under cyclic tension

Strain controlled fatigue tests have been performed on two types of heavily filled epoxy corrosion protection coating sprayed onto a 6mm steel substrate. Fatigue cycling was performed at R ratios of 0 and −1. The two coatings differed in their formulation and the major differences in mechanical performance were in their static strain to first crack development and their fracture toughness, where Coating A was significantly tougher than coating B. During strain cycling coating crack development was monitored using optical observations and surface replicas. It was found that in both coatings surface crack development began soon after the onset of cycling and proceeded via growth of surface channelling cracks and multiple initiation of new cracks. Detailed studies were made of crack development morphology and its relation to coating type and to the applied strain range. A definition of coating life as the first appearance of a 2mm surface crack length was used. This represented the end of the life where the coating protected the substrate. Before this life was achieved, crack growth rates of single cracks were invariant with crack length. After this point further crack growth, multiple cracking and crack to crack interactions took place. Cracking in this region could be characterised with a new total crack length parameter shown to be strongly dependent on applied strain range.

钢轨裂纹深度对脉冲涡流热成像的影响

我国高铁的快速发展对钢轨裂纹的巡检提出了越来越高的要求，基于电、磁、热等多物理场耦合的脉冲涡流热成像(ECPT)技术，可实现对钢轨表面裂纹的非接触、快速检测，对保障高铁安全运行意义重大。相对于表面裂纹长度和宽度，裂纹侵彻深度和侵入角度更加隐蔽，也更加难以检出和区分。本文通过电磁感应加热有限元仿真和钢轨表面人工裂纹检测实验，对比研究了垂直裂纹侵彻深度和斜裂纹侵入角度对钢轨表面热分布的影响，及其红外成像的区别。研究表明垂直裂纹侵彻深度超过1 mm时，高温区由励磁线圈附近趋向于裂纹两个顶端；斜裂纹则当侵彻深度超过2 mm时高温区发生迁移，且高温区沿裂纹非对称分布。因此，根据高温区分布位置及形状可实现多种类型钢轨表面裂纹的检测与识别。 The demand of rail track inspection is increasing with the development of high speed railway in China. The surface cracks of track can be detected non-destructively and fleetly by eddy current pulsed thermography (ECPT) technology, based on multi-physics such as electricity, magnetism and thermography. This technology is important to high speed railway security. Comparing with the length and width of surface crack, depth of vertical crack and angle of lean crack are hard to be detected and classified. By finite element simulation and artificial track crack inspection, this paper proposes the difference of IR images and the influence of vertical crack depth and lean crack angle on heat distribution. The results indicate that high temperature regions move to the ends of crack when the crack depth exceeds 1 mm. For lean cracks, high temperature regions distribute asym-metrically along cracks and move when the depth increases more than 2 mm. Therefore, different surface track cracks can be detected and classified by the shape and distribution of high tempera-ture regions.

Experimental Study on Evolution Process of Short Fatigue Crack in 2A12 Alloy under Complex Stress State

The fatigue behavior of 2A12 Aluminum alloy was experimentally studied through different annular notched specimens under symmetrical triangle with the frequency of 0.5 Hz. The experimental result showed that the microstructure played an important role during the entire fatigue life of 2A12 Aluminum alloy. The short fatigue crack only initiated due largely to the second-phase particle such as the S phase (Al2CuMg), the θ phase (CuAl2) and especially the black impurity phases debonding from the basal body when the fatigue cycle was sufficient. The cracks propagated separately along the circumferential direction of the notch, crack coalescence and interaction of cracks were not common at early and middle stage of short crack’s fatigue life. Cracks tended to propagate along direction different from the original one after crack coalescence. Surface crack length at early and middle stage of short crack’s fatigue life were presented. These curves showed crack growth rate increased relatively as the reducing of notched radius or the increasing of nominal strain amplitude which implied that geometry and loading conditions were the factors of the crack propagation.

Engineering criticality analysis on an offshore structure using the first- and second-order reliability method

Due to the uncertainties related to the flaw assessment parameters, such as flaw size, fracture toughness, loading spectrum and so on, the probability concept is preferred over deterministic one in flaw assessment. In this study, efforts have been made to develop the reliability based flaw assessment procedure which combines the flaw assessment procedure of BS7910 and first- and second-order reliability methods (FORM/SORM). Both crack length and depth of semi-elliptical surface crack at weld toe were handled as random variable whose probability distribution was defined as Gaussian with certain means and standard deviations. Then the limit state functions from static rupture and fatigue perspective were estimated using FORM and SORM in joint probability space of crack depth and length. The validity of predicted limit state functions were checked by comparing it with those obtained by Monte Carlo simulation. It was confirmed that the developed methodology worked perfectly in predicting the limit state functions without time-consuming Monte Carlo simulation.

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.

The thermal shock resistance of Y2O3 ceramics

Y2O3 ceramics samples with various grain size were tested in non-symmetric thermal shock conditions in two methods. In the first test the samples were heated in a furnace up to the predetermined temperature (530°C) and then the polished surfaces with Vickers indents were subjected to sudden temperature change using a high-velocity nitrogen jet at room temperature. The gas was channelled onto the disk centre for about 20s. In the second test samples were quickly inserted into a hot furnace (850°C). The polished surfaces with Vickers indents were sheltered by thick alumina disks. The length of surface cracks before and after thermal shocks were measured with an optical microscopy and then a parameter Rm (an indicator of thermal stress resistance) was obtained. Hence the thermal stress σT, which caused crack growth, was calculated using the value of fracture toughness KIc. These stresses were similar for both shock cases (about 50MPa) and they were not dependent on grain size of the ceramics.

Synchrotron 3D characterization of arrested fatigue cracks initiated from small tilted notches in steel

High resolution synchrotron X-ray tomography has been used to obtain 3D images of arrested cracks initiated at small artificial defects located on the surface of cylindrical steel specimens subjected to mode I fatigue loading. These defects consist in small semi-circular slits tilted at 0¢X, 30¢X or 60¢X with respect to the plane normal to the loading axis; all of them had the same defect size, area = 188 £gm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. Arrested cracks initiated from the notch were observed for all tilt angles at the surface of samples cycled at the fatigue limit (stress amplitude at which the specimen does not fail after 1¡Ñ107 cycles). High resolution synchrotron X-ray tomography has been used to obtain 3D images of those small defects and non-propagating cracks (NPC). Despite the fact that steel is a highly attenuating material for X rays, high resolution 3D images of the cracks and of the initiating defects were obtained (0.65 ƒÝm voxel size). The values of surface crack length measured by tomography are the same as those obtained by optical microscope measurements. The area values present the same tendency as the surface length of NPC, i.e. larger non-propagating cracks areas were observed in the softer steel. In the extreme case of 60o tilted defect, the crack fronts appear much more discontinuous with several cracks propagating in mode I until arrest.

Stress concentration factors and fatigue behavior of square bird-beak SHS T-joints under out-of-plane bending

In this study, fatigue tests of square bird-beak square hollow section (SHS) joints were conducted under out-of-plane bending. Nine orthogonally designed T-joints were fabricated. Static loading was first conducted, during which elastic strain distributions near the potential hot spots were measured. Stress concentration factors (SCFs) at weld toes were calculated using the quadratic extrapolation approach. Critical locations were also identified. Based on the high-cycle fatigue testing that followed, the fatigue crack initiation, propagation and fatigue failure mode were obtained. Variations of the surface crack length and out-of-plane rigidity against number of load cycles were also revealed. The cycle numbers that correspond to through-thickness cracks were observed to be greater than the numbers obtained from S–N curves provided by the IIW code, which indicates that the IIW curves/formulations for conventional joints could also be used to estimate the fatigue lives of square bird-beak joints if the hot spot stress ranges have been properly determined.

Crack propagation in Large Scale Yielding (LSY) Conditions

In Industry, some components are subjected to fatigue thermal loadings due to thermal fluctuations coming from the meet of a cold and a hot fluid. A Large Scale Yielding (LSY) conditions zone can be created: plasticity is not only located at the crack tip, but extends to a larger area of the component. Thus, traditional hypothesis of elasticity cannot be used for numerical predictions. Nevertheless, crack propagation needs to be estimated with adapted numerical models.Thermomechanical fatigue crack propagation tests are performed with an EDF facility in the desired large scale yielding conditions. During the thermal fatigue tests, temperatures, strains and surface crack lengths are measured and registered. Microscopic analyses are carried out in order to observe the influence of the microstructure on the fatigue crack path.This paper presents first experiments performed with the PACIFIC test rig and first results get for crack propagation in Large Scale Yielding conditions.

Grain size effect on the small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169

The aim of this paper was to identify the grain size effect on the small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169. Results showed that there was a transition of fatigue crack initiation mechanism between fine-grained material and coarse-grained material. Small fatigue crack growth rates were almost constant across a wide range of crack lengths. Once the surface crack length reached the critical size of 200μm, the crack would propagate fairly quickly. At a given experimental condition, the scattering of fatigue lives of the parallel specimens was dependent on the crack initiation mechanisms.

Inclined stress corrosion cracks in gas pipeline steels: morphology and implications

A systematic morphological survey was carried out on 120 stress corrosion cracks (SCC) from an ex service X65 Australian gas pipeline. Out of these cracks, 81% showed a tendency to grow into the material away from the perpendicular. The inclined crack path was suggested to be due to anisotropic residual stress/strains present in the material introduced during pipe manufacturing processes. The overall depth of inclined SCC cracks was found to follow a logarithmic relationship to the SCC surface crack length. Hoop travel was defined as the distance that the crack tip travels in the pipe hoop direction as it grows into the material. The specific hoop travel distance was found to follow a linear relationship as a function of the SCC surface crack length. The relationships between surface crack length to through‐wall crack depth and hoop travel could allow pipe operators to formulate a worst case scenario for the purposes of SCC assessment and management.

Fatigue life analysis of internal circumferential crack of tubular structures

In this paper, the fatigue life of cracked tubular structures subjected to remote cyclic tensile stress is studied by using the fracture mechanics and Paris’ law. To evaluate the flooded member detection, a reassessed leak-before-break approach is implemented, in which a more realistic crack development, by taking the partly through thickness crack stage into consideration, has been investigated, and that has never been reported in literature for tubular member with internal crack. Parametric study demonstrates that the time available to detect the presence of the crack in its flooded life depends particularly on the rate of accumulation of fatigue damage and on the initial length of surface crack presented. The results of the analysis have been applied by a case study of a particular offshore structure and loading situation, obtained to support inspection intervals of three years for flooded member detection. By comparing the current results with the conventional leak-before-break procedure in flooded member detection, it is found that for the situation studied in this paper, the flooded life predicted from conventional leak-before-break method for the crack is always on the safe side. Therefore, it is too conservative for setting up the inspection interval for flooded member detection.

An improved Vickers indentation fracture toughness model to assess the quality of thermally sprayed coatings

This study presents an improved approach to the quality assessment of thermally sprayed coatings. Measurements were carried out on five different coatings. Since it is the overall extent of surface cracking during Vickers indentation that is indicative of the volumetric damage, the surface crack length was measured, including the radial cracks, edge cracks, and other cracks around the indentation. It is concluded that the proposed model provides a way forward for determining the fracture toughness (K1c) of brittle materials where no radial cracks are developed. An elastic–plastic finite element simulation of the Vickers indentation test was conducted to locate the stress fields.

Effect of water uptake on the fracture behavior of low-k organosilicate glass

Water uptake in porous low-k dielectrics has become a significant challenge for both back-end-of-the-line integration and circuit reliability. This work examines the effects of water uptake on the fracture behavior of nanoporous low-k organosilicate glass. By using annealing dehydration and humidity conditioning, the roles of different water types and their concentrations are analyzed in detail. For as-deposited SiCOH films, annealing dehydration can enhance the resistance to crack occurrence, and these enhancements can be offset by higher humidity conditioning. It was found that the film-cracking threshold can be lowered by in-diffused water in the film as well as by water at the SiCOH/subtract interface. This occurs because the water decreases the film fracture energy and adhesion energy, respectively. By conditioning at high humidity, the variation of the film cracking threshold agrees well with the behavior of the film hardness and modulus of elasticity as a function of relative humidity. The crack morphologies of low-k porous films are also implicitly related to water uptake in the materials. Film cracking thresholds and crack morphologies of UV-cured low-k materials exhibit a weaker dependence on the water uptake, indicating a low degree of hydrophilicity of the SiCOH film after UV curing, which corroborates the previous results. Furthermore, by measuring the surface crack length, the material–fracture toughness can be found. The results demonstrate that neither annealing dehydration nor water uptake have significant effects on fracture toughness of as-deposited SiCOH, while for UV-cured SiCOH, annealing enhances the film-fracture toughness.

The probabilistic life time prediction model of oil pipeline due to local corrosion crack

A four-stage probabilistic damage model is proposed basis of cross-scale damage processes to deal with the local corrosion crack of oil pipeline. At first, some key parameters for life time prediction were determined; then the probabilistic damage model is formulated and numerically calculated by using Monte Carlo Simulation (MCS). Furthermore, the model is used to deal with an example in order to check its validity. The results show that the life-span of this pipeline is nearly 20.55years, and the pipe wall thickness, operating pressure difference and corrosion electric current density are the three key parameters to determine the span-life of this pipeline; the longest examination and repair period should be less than 4.71years for safety when the surface crack length of 10mm can be detected reliably.

Fatigue Crack Development in Epoxy Coatings on Steel Substrate: The Role of Coating and Substrate Properties in Determination of the Onset of Fatigue Cracks

Development of service cracks in epoxy based corrosion protective coatings limits the life of the substrate structure. If cracks develop, corrosion protection is lost and costs of repair and re-protection of large marine structures can be crippling. Factors controlling development of cracks in the coating are poorly understood, and predictions of coating lifetime approximate. For bulk tanker applications service strains imposed on coatings arise from both low frequency thermal mismatch strains and mechanical strains from wave action. In this work the number of cycles to crack initiation has been measured during strain controlled fatigue of two selected 300 μm thick coatings applied to a 5.5 mm thick steel substrate. Cycling was performed at frequencies of 1 Hz, and strain amplitudes between ±0.16% and ±0.5%. Coating crack development was monitored optically. It was found that cycles to crack initiation depended on both strain amplitude and on the static ductility of the coating. After initiation crack growth rates in the coating reduced with increasing surface crack length. The significance of the results is considered in the light of requirements for quantitative models for service life prediction of coatings on metallic substrates.