Flaw Width Research Articles

Crack coalescence prediction and load-bearing mechanism of defective specimen based on computer vision recognition model

The coalescence of cracks in rock, triggered by external stress or geological activities, plays a pivotal role in determining the mechanical properties and stability of rock structures. Consequently, the prediction method of crack coalescence become critical tools in ensuring the safety and stability of geotechnical engineering facilities. In this paper, based on the strain field data obtained by digital image correlation (DIC), a set of programs was developed to automatically identify the values and percentage changes of different strain intervals in the strain field of the specimen. Then, a computer vision recognition (CVR) model is established to study the process and prediction of crack coalescence in sandstone specimens with open flaws. This method overcomes the limitations, subjectivity and unpredictability of the traditional method of identifying cracks through artificial vision. The results show that the increase of the flaw width causes the displacement trend to be squeezed and deflected along the width direction, thereby changing the angle of crack initiation and exhibiting different forms of crack coalescence. The increase in the inclination angle of the flaw leads to an increase in the effective compressive area (ECA) of sandstone, and the magnitude of ECA is positively correlated with the uniaxial compressive strength (UCS) of the sandstone. Additionally, the CVR model identifies two types of numerical fluctuation signals before crack coalescence, namely the plastic characteristic signal (PCS) in the early stage of the experiment and the early-warning signal (EWS) near the period of crack coalescence, where EWS can be used as a predictive signal for crack coalescence. The research results provide a new algorithm technical support for the process analysis and prediction of crack coalescence, and provide a basis for early warning of rock mass engineering disasters.

Accuracy variation problem in an incremental magnetic encoder caused by surface flaws of electrodeposited measuring scales

This paper presents the direct relationship between the positioning accuracy of linear magnetic encoders and the surface flaws on their measuring scales. Measuring scales having recording media composed of electrodeposited Co-rich coatings are imprinted to form mimic surface flaws of particulate or stripe shapes. Effects of flaw sizes and stripe directions on the encoder positioning accuracy have been investigated. Benchmarked with an un-flawed scale which has an accuracy error of ± 2.0 μm in a 6.0 cm length, the flawed scales with a flaw width > 130 μm and a flaw depth > 4 ∼ 5 μm present accuracy variation in the 2-centimeter flaws regions on the measuring scales. Compared with particulate flaws, stripe flaws are more detrimental, especially the stripes being perpendicular (i.e., 90°) to the moving direction of read head. Accuracy variation behaviors due to surface flaws of electrodeposited measuring scales have been unambiguously demonstrated for long-length high-accuracy magnetic encoders.

Frost-Heaving Cracking Sensitivity of Single-Flaw Rock Mass Based on a Numerical Experimental Method

A novel numerical experimental method is developed herein to study frost-heaving cracking in fractured rock masses by reproducing the physical experimental frost-heaving process. The failure mode of a preexisting closed and water-saturated single-flaw in a rock mass during the frost-heaving process is affected by the inclination angle, flaw width, crack length, and cooling rate. Additionally, a regression model for predicting secondary crack formation and propagation is established by combining multiple stepwise regressions. Overall, the results indicate that preexisting flaw with various inclination angles, flaw lengths, and cooling rates mainly propagates along the flaw-coplanar direction. The secondary crack failure mode is most affected by the flaw width, and the length of the secondary crack increases with increasing inclination angle, flaw width, and flaw length, but decreases with increasing cooling rate. The contributions of the investigated factors to the secondary crack length follow the order: flaw width > inclination angle > cooling rate > flaw length. The results presented herein provide crucial theoretical guidance for engineering and construction projects in cold regions.

Experimental Evaluation of Multiple Frost Heaving Parameters for Preflawed Granite in Beizhan Iron Mining, Xinjiang, China

Ice-driven mechanical weathering in cold regions is considered a main factor impacting the stability of rock mass. In this work, the response surface method (RSM) was employed to evaluate and optimize the multiple frost heaving parameters to seek the maximum frost heaving force (FHF), in combination with experimental modeling based on a specially designed frost heaving force measurement system. Three kinds of rocks were prepared with parallel flaws in it having different flaw width, length, and cementation type, and these factors were used to fit an optimal response of the maximum FHF. The experimental results reveal five distinguished stages from the frost heaving force curve, and they are inoculation stage, explosive stage, decline to steady stage, recovery stage, and sudden drop stage. The sensitivity analysis reveals the influential order of the considered factors to peak FHF, which is the rock lithology, flaw width, flaw cement type, and flaw length. For low-porosity hard rock, increasing flaw width, flaw length, and flaw cement strength can improve the probability of frost heaving failure. It is suggested that rock lithology determines the water migration ability and influences the water-ice phase transformation a lot.

Effects of flaw width on cracking behavior of single-flawed rock specimens

Cracking behavior of flawed samples has been extensively studied in rocks and rock-like materials using experimental and numerical approaches. However, the existing research mainly focuses on the effects of flaw length l and inclination angle α on rock mechanical behavior, a limited attention has been given to understand the role of flaw width b on crack initiation and propagation in relation to mechanical parameters of rocks. In this paper, a two-dimensional particle flow code (PFC2D)-based synthetic rock model, which is calibrated by the laboratory sandstone properties, is used to investigate the effects of flaw width on cracking behavior of single-flawed rock samples under uniaxial compression tests. Numerical simulation results show that the flaw width b has significant effects on the crack geometrical parameters (crack initiation location d and crack initiation angle θ). The values of θ and d significantly decrease with the increase of flaw width when b < 1.0 mm and their values tend to be stable with the further increase of flaw widths. It is also found that the mechanical properties (uniaxial compressive strength UCS and deformation modulus Es) of flawed rock samples are related to the crack geometrical parameters (θ and d) and the effects of flaw width on the values of UCS and Es of single-flawed rock specimens is significant when b < 1.0 mm for lower crack inclination angle (α < 60°).

Failure assessment methodology for boiler tubes with localized external erosion defects

Boiler tubes used in power plants and manufacturing industries are prone to failure due to the harsh environment they operate in, usually involving high temperature, pressure, and some erosive-corrosive mechanisms. Among the wide range of failures associated with the tubes, localized external erosion continues to be a leading cause of tube leakages and unscheduled boiler outages in power plants and other utilities. This paper lays the foundation to develop a rapid decision-making tool to help prioritize the maintenance, repair, or replacement of these tubes. A failure assessment methodology is proposed, based on the analysis of failed localized thinned tubes from a power plant. Minimal geometric measurements are available for each failed tube (width of flaw, length and flaw and minimal remaining thickness). Finite element models of idealized flaws are created that match the measured data for all the failed tubes. Finite element models with higher remaining wall thickness than the measured values are also constructed. Using two material models, comprehensive nonlinear finite element analyses are conducted on the 160 modeled flawed tubes. The flawed tubes are assessed using the API-ASME fitness-for-service assessment protocol to demonstrate how these flawed tubes would be ranked from most severe to least severe. Allowable wall loss for some of the considered flawed tubes ranges from 44% to 81% of the original wall thickness, indicating that fitness-for-service assessments can establish safe operation even for substantially eroded boiler tubes.

Analytical Assessment of the Remaining Strength of Corroded Pipelines and Comparison With Experimental Criteria

Recently published analytical solutions for the remaining strength of a pipeline with narrow axial and axisymmetric volumetric flaws are described in this paper, and their experimental and numerical validation are reviewed. Next, the domains of applicability of each solution are studied, some simplifications suitable to steel pipelines are introduced, and an analytical model for the remaining strength of corroded steel pipelines is presented. This analytical solution is compared with the standards most widely used in the industry for assessment of corroded pipelines: ASME B31G, modified ASME, and DNV RP-F101. The empirical and analytical solutions are compared with respect to their most relevant parameters: critical (or flow) stress, flaw geometry parameterization, and Folias or bulging factor formulation. Finally, two common pipeline steels, API 5L grades X42 and X100, are selected to compare the different corrosion assessment methodologies. Corrosion defects of 75%, 50%, and 25% thickness reduction are evaluated. None of the experimental equations take into account the strain-hardening behavior of the pipe material, and therefore, they cannot properly model materials with very dissimilar plastic behavior. The comparison indicates that the empirical methods underestimate the remaining strength of shallow defects, which might lead to unnecessary repair recommendations. Furthermore, it was found that the use of a parameter employed by some of the empirical equations to model the assumed flaw shape leads to excessively optimistic and nonconservative results of remaining strength for long and deep flaws. Finally, the flaw width is not considered in the experimental criteria, and the comparative results suggest that the empirical solutions are somewhat imprecise to model the burst of wide flaws.

Study on the Optimization of Eddy Current Testing Coil and the Defect Detection Sensitivity

As an essential component in the eddy current testing system, the detection sensitivity of eddy current probe affects the performance of the system. Different probes such as cylindrical differential probe, single orthogonal rectangular probe and biorthogonal rectangular probe were modeled using Comsol Multiphysics in this paper. The electromagnetic characteristics of probes were analyzed through the finite element simulation. Comparing the different probe detection signals, the new biorthogonal rectangular probe was found to have a better inhibition effect on the lift-off effect. The detection capability of probes according to flaw's width, length and depth were also studied. Based on the results of simulation, a new type of biorthogonal rectangular probe with higher detection sensitivity was designed.

Dependence of the magnetic field of a flaw on the distance to the surface of a tested article

The dependence of the magnetic field of a flaw on the distance to the surface of a tested article has been studied for rectangular slot-type flaws. For flaws with a small opening width, it can be considered that their magnetic field decreases in inverse proportion to this distance, but with an increase in the flaw width, the field decreases more slowly.

Study on the Fundamental Characteristics of the .THETA. Probe and Crack Monitoring Using This Probe Combined with the Caustic Method

The new Θ probe, which is used in eddy current testing, has various advantages compared to previous probes. In the present study, the fundamental characteristics of this probe are first investigated using commercially available aluminum plates. The effects of liftoff and the angle between the detecting coil and the flaw on the flaw signal are then clarified. The effects of the flaw dimensions on the flaw signal are also investigated, and it is shown that the amplitude reaches a constant value for a flaw depth of H>8 mm, a flaw width of W>7 mm, and a flaw length of L>20 mm, within the experimental conditions of the present study. Next, the monitoring of crack initiation under static loading and crack growth under the low cycle fatigue condition in the duralumin plates is attempted by applying the Θ probe combined with the caustic method. It is shown that the signal amplitude can be obtained when fracture is initiated under static loading. In addition, in the A7075 plate, the amplitude is increased at the fatigue crack growth rate of da/dN larger than 0.017 mm/cycle. Consequently, the possibility of crack monitoring using the Θ probe is confirmed.

The role of flaw geometry in thin film delamination from two-dimensional interface flaws along free edges

Delamination from planar interface edge flaws between a thin film and a semi-infinite substrate is examined to determine the roles of flaw width and depth relative to the film thickness. The flaws have curved and straight sections, and the crack front intersects the free edge at a right angle. Three-dimensional finite element models are used to extract local energy release rates and mode-mixity angles along the entire crack front. This paper focuses the crack dimensions required to reach steady state, wherein energy release rates are independent of flaw dimensions along the entire crack front. Results indicate that moderate elastic mismatch, although affecting mode mixity, plays a small role in determining the crack aspect ratios required to reach steady state. For wide cracks, the energy release rate for crack advance into the film interior approaches the plane-strain steady-state value when the half-width of the crack is approximately four times its depth (for cracks whose depths is several times the film thickness). For narrow cracks, the energy release rate near the free edge is significantly greater than the plane-strain steady-state result, and reaches a steady state when the depth approximately 10 times its width (for widths several time the film thickness). The results imply that delamination from wide cracks is reasonably accurately predicted via plane-strain analyses. Conversely, two-dimensional models are incapable of accurately predicting delamination from narrow cracks, which have a tendency to widen into flaws with more balanced aspect ratios (i.e. without growth in the depth direction).

Investigations by non-destructive inspection to determine the size of natural defects in large forgings of turbogenerators

For more than 10 years German turbine manufacturers and forgemasters have have been studying the fracture mechanics behaviour of natural flaws in turbine rotors. One approach of these studies consists of determining the size of flaws by conventional and computer-assisted ultrasonic inspection methods. The tests are being conducted on a total of 22 representative reject forgings which feature individual as well as resolvable and non-resolvable group indications. The fracture mechanics behaviour of the flaws is being studied on large notchless specimens under conditions of quasi-operational pulsating tensile stresses up to roughly 25,000 load cycles. Test results show that for a “worst case analysis” under a fracture mechanics acceptance check of small indications, the size of the flat-bottom hole determined by the DGS method can be assumed to be the maximum possible flaw width. Larger indications exceeding twice the wave length of the test frequency must be adequately defined by determining the half-value width of the echodynamics curve. Computer-assisted mechanised ultrasonic inspection methods have proved to be suitable for detecting large individual indications and flaw regions with group indications.

Infiltration of glass flaws by alkoxide coatings

Controlled flaws were produced in silica glass by Vickers indentations. The flaws were examined after formation, after coating with a tetraethoxysilane (TEOS) solution, and after removal of the coating using Ar + ion sputtering. Flaws with widths less than 35 nm were healed by the solutions, while wider flaws were unaffected by the coatings. For the healed regions, the solutions infiltrated into the flaw to a depth of at least 70 nm. A model of the strengthening of glass due to filling-in and healing by sol-gel coatings is modified to take into account the effect of flaw width on infiltration.