Characterize Fatigue Crack Research Articles

Plastic CTOD as fatigue crack growth characterising parameter in 2024‐T3 and 7050‐T6 aluminium alloys using DIC

AbstractThe plastic range of crack tip opening displacement (CTOD) has been used for the experimental characterisation of fatigue crack growth for 2024‐T3 and 7050‐T6 aluminium alloys using digital image correlation (DIC). Analysis of a complete loading cycle allowed resolving the CTOD into elastic and plastic components. Fatigue tests were conducted on compact tension specimens with a thickness of 1 mm and a width of 20 mm at stress ratios of 0.1, 0.3 and 0.5. The range of plastic CTOD could be related linearly to da/dN independent of stress ratio for both alloys. To facilitate accurate measurements of CTOD, a method was developed for correctly locating the crack tip and a sensitivity analysis was performed to explore the effect of measurement position behind the crack tip on the CTOD. The plastic range of CTOD was demonstrated to be a suitable alternate parameter to the stress intensity factor range for characterising fatigue crack propagation. A particularly innovative aspect of the work is that the paper describes a DIC‐based technique that the authors believe gives a reliable way to determine the appropriate position to measure CTOD.

A concise and novel binomial model for creep-fatigue crack growth behaviors

A concise binomial model including time-dependent term and cycle-dependent term is proposed in this paper in order to characterize fatigue crack growth rate at high temperature. The specific functional forms of the time-dependent term are derived for several materials, and then the characteristic dwell times corresponding to the most significant interaction effect are determined. Besides, the contribution of the time/cycle-dependent term to the total crack growth rate of the model at different stress intensity factor range ΔK levels is discussed. The research achievements are helpful to support the damage tolerance analysis of high temperature structures in service.

1Pb2-4 Study on the characteristics of aluminum-alloy fatigue cracks and the behavior of subharmonic generation

1Pb2-4 Study on the characteristics of aluminum-alloy fatigue cracks and the behavior of subharmonic generation

Determination of the geometric characteristics of fatigue cracks by digital image processing method

The article deals with the problem of determining the rate of crack growth in the process of fatigue failure of helicopter part samples. The studies were conducted using a video recording of the experiment. Methods of digital image processing were applied to the frames of the video recording to process the data of the object destruction process. Based on the developed algorithm, computational experiments were carried out, the geometric parameters of the cracks were determined for calculating the rate of change in their sizes. The use of digital image processing methods allows significantly improve the accuracy and reduce the processing time of experimental data.

Nonlinear crack assessment method in beams based on bispectrum-normal cloud model

Fatigue damage in engineering structures is universal. The occurrence of fatigue cracks brings unpredictable hidden dangers to a structure in terms of safety and service performance. Traditional damage identification methods, such as power spectrum analysis, are mostly based on linear elasticity theory that cannot reflect the typical nonlinear characteristics of fatigue cracks and cannot meet the higher requirements of the signal analysis method put forward by current mass detection data. To solve this problem, a numerical model of a cantilever beam with a breathing crack is established in this study. A method for diagnosing fatigue damage is studied by combining bispectral analysis and a statistical normal cloud model, which characterize the nonlinear characteristics of the structure. This method can effectively describe the nonlinear characteristics of the structure and reasonably evaluate the degree of fatigue damage in the structure. The bispectrum-normal cloud model method proposed in this study overcomes the limitations of existing linear damage detection methods in nonlinear damage detection, and can improve the efficiency of signal analysis from a statistical point of view. It has good prospects for structural nonlinear damage assessment.

Experimental characterisation of fatigue crack growth based on the CTOD measured from crack tip displacement fields using DIC

The current work presents an experimental study on the use of the crack tip opening displacement (CTOD) to evaluate its ability to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements obtained by implementing digital image correlation (DIC) on growing fatigue cracks are used to measure the CTOD. Two fatigue tests at stress ratios of 0.1 and 0.6 were conducted on compact tension (CT) specimens manufactured from a 1 mm thick sheet of commercially pure titanium. A sensitivity analysis to explore the effect of the position selected behind the crack tip for the CTOD measurement was performed. The analysis of a full loading cycle allowed identifying the elastic and plastic components of the CTOD. The plastic CTOD was found to be directly related to the nonlinear zone (i.e., plastic deformation) generated at the crack tip during fatigue propagation. Moreover, a linear relationship between da/dN and ΔCTODp independent of the stress ratio was found. Results show that the CTOD can be used as a viable alternative to the stress intensity factor range (ΔK) in characterising fatigue crack propagation since the parameter considers the fatigue threshold and crack shielding in an intrinsic way.

2D and 3D characterization of rolling contact fatigue cracks in manganese steel wing rails from a crossing

Rail wheel contact at switches and crossings (S&Cs) induces impact stresses along with rolling contact stresses, resulting in plastic deformation and eventually crack formation. Damaged and deformed wing rails of a manganese steel crossing are studied and the microstructure, hardness and 3D crack network within the steel are characterized. It is found that the surface of the rail receives the maximum deformation resulting in a hardened top layer. The deformation is manifested by a high density of twins and dislocation boundaries in the microstructure. A complex crack network is revealed in high resolution by X-ray tomography.

Martensite variant localization effects on fatigue crack growth - The CuZnAl example

The paper underscores the difficulty in characterizing crack growth in shape memory alloys with conventional stress intensity approaches even when anisotropy is accounted for. The local displacements deviate significantly from the presumed K solutions because of transformation localization at macro scales extending from the crack tip. Modifications in stress intensity due to variant induced tractions are derived providing an explanation of the extrinsic behavior. Precise displacement and strain measurements are provided to highlight the complexity of deformation and the need for better fracture mechanics models, including crack tip opening displacements, to characterize fatigue crack growth in shape memory alloys.

Fatigue assessment of longitudinal rib-to-crossbeam welded joints in orthotropic steel bridge decks

Fatigue cracks at rib-to-crossbeam welded joints in orthotropic steel decks (OSDs) accelerate degradation of steel bridges. This research aims to investigate fatigue cracking characteristics of rib-to-crossbeam welded joints, and assess the fatigue strength of the welded joints through fatigue testing and finite element analysis. Nine large-scale specimens were tested under static and fatigue loading to explore the failure mode, fatigue life, and failure process. The fatigue test results were compared with the fatigue strength values recommended in different design specifications. Based on the hot spot stress and notch stress approaches, SN curves for rib-to-crossbeam welded joints were derived and compared with the existing fatigue test data. The results indicate that fatigue cracks that initiate from the weld end at the weld toe controls the fatigue resistance of the rib-to-crossbeam welded joints. A statistical analysis was performed to study the probability distribution of the fatigue strength.

Application of Local Mean Decomposition to Acoustic Emission Data for Natural Fatigue Cracks Feature Extraction in Rotating Shafts

This paper presents the results of applying recently developed local mean decomposition (LMD), a new iterative approach to acoustic emission(AE) feature extraction of natural fatigue cracks in rotating shafts providing an energy-frequency-time distribution with more adaptable precision. The method decomposes AE signals into a set of functions, each of which is the product of an envelope signal and a frequency modulated signal from which a time-varying instantaneous frequency can be derived. It has been found that LMD appears to be a better tool providing an energy-frequency-time distribution compared to Hilbert–Huang transform (HHT) for natural fatigue crack characterization in a rotating rotor in the experiment cases. It was concluded that LMD-based AE technology could more successfully extract the features of natural fatigue cracks induced on rotating shafts.

Reference-Free Breathing Crack Identification of Beam-Like Structures Using an Enhanced Spatial Fourier Power Spectrum with Exponential Weighting Functions

Detection of incipient damage of structures at the earliest possible stage is desirable for successful implementation of any health monitoring system. In this paper, we focus on breathing crack problem and present a new reference-free algorithm for fatigue crack detection, localization, and characterization for beam-like structures. We use the spatial curvature of the Fourier power spectrum as a damage sensitive feature for fatigue crack identification. An exponential weighting function that takes into account nonlinear dynamic signatures, such as sub- and superharmonics, is proposed in the Fourier power spectrum in order to enrich the damage-sensitive features of the structure. Both numerical and experimental studies have been carried out to test and verify the proposed algorithm.

Evaluation of a Digital Image Correlation Bridge Inspection Methodology on Complex Distortion-Induced Fatigue Cracking

Distortion-induced fatigue cracking in aging steel bridges is a primary concern for many bridge owners and stakeholders, accounting for the majority of fatigue cracks in bridges in the United States. Currently Departments of Transportation primarily use visual inspections to locate and characterize fatigue cracks. However, this approach has drawbacks, as recent studies have shown that visual inspections are not able to consistently identify realistically-sized fatigue cracks in highway infrastructure. Additionally, it requires significant time and cost to perform visual inspections, and both inspectors and the traveling public are placed at risk during the inspection. Bridge owners and stakeholders would benefit from the development of inspection techniques that do not rely on hands-on human visual inspection – to reduce the risks of harm to the inspectors and traveling public, increase reliability, and decrease the time and cost of performing inspections.Vision-based technologies are an active area of research in the field of structural health monitoring, aimed at developing alternatives to manual inspection for identifying damage to transportation infrastructure. The majority of these studies have focused on macro-indicators of damage, including identifying excessive corrosion, concrete deck deterioration, and large displacements due to substructure movement. Digital image correlation (DIC) is one such vision-based technology that shows promise for detecting and characterizing fatigue cracks, but investigations and applications have thus far been limited to simple, in-plane cracking.Three-dimensional DIC measurements have the ability to capture full-field displacements and surface strains, allowing for the potential of identifying and characterizing out-of-plane fatigue cracks, such as those occurring on steel bridges exposed to loading through differential girder displacement. Since the majority of fatigue cracks arise from out-of-plane loading, investigating the efficacy of DIC for detecting distortion-induced fatigue cracks is of clear value.This paper describes an experimental study in which a scaled steel girder-to-cross-frame specimen was cyclically loaded to produce geometrically complex distortion-induced fatigue cracking. To examine the potential usefulness of DIC for future automated bridge inspections the resulting fatigue cracks were characterized using DIC and visual inspection. The methodology applied proved successful at quantifying certain aspects of the complex, bifurcating cracks, but had difficulty characterizing crack segments that are no longer actively loaded. Additional work is needed to improve the accuracy and reliability of the crack detection results for this technology to potentially be used as an automated inspection tool in the future.

FINITELY SCOPED, HIGH RELIABILITY FATIGUE CRACK GROWTH MEASUREMENTS

ABSTRACT Classical procedures for characterizing fatigue crack growth behavior often suffer from uncertainties that make it difficult to plan for productive use of test instrument time, and that can result ultimately in too noisy measurements. An enhanced procedure has been implemented that is based on a fixed test time budget, and that establishes operating conditions that produce crack growth rates down to minimum measurable rates. The procedure features (1) a haversine pulse deformation test cycle followed by a rest period, (2) a strain peak that ramps linearly over time, (3) minimum and maximum limits on the strain peak chosen to avoid unproductive test time, and (4) a stress–strain probe cycle for purposes of observing strain energy density. A set of replicates of a carbon black filled, natural rubber bushing compound has been characterized via both procedures, and a statistical analysis is made to compare both. The new procedure significantly improves the quality of crack growth rate curve measurements.

Influence of Multi-Holes on Fatigue Behaviors of Cast Magnesium Alloys Based on In-Situ Scanning Electron Microscope Technology.

The low cycle fatigue tests on the crack initiation and propagation of cast magnesium alloys with two small holes were carried out by using in-situ scanning electron microscope (SEM) observation technology. The fatigue crack propagation behaviors and fatigue life, which are affected by two small artificial through holes, including the distances between two holes and their locations, were discussed in detail based on the experimental results and the finite element analysis (FEA). The results indicated that the fatigue multi-cracks occurred chiefly at the edges of two holes and the main crack propagation was along the weak dendrite boundary with the plastic deformation vestiges on the surface of α-Mg phase of cast AM50 and AM60B alloys. The fatigue cracking characteristics of cast AZ91 alloy depended mainly on the brittle properties of β-Mg17Al12 phase, in which the multi-cracks occurred still at the edges of two holes and boundaries of β-Mg17Al12 phase. The fatigue crack initiation position of cast magnesium alloys depends strongly on the radius of curvature of through hole or stress concentration factor at the closed edges of two through holes. In addition, the fatigue multi-cracks were amalgamated for the samples with titled 45° of two small holes of cast Mg-Al alloys when the hole distance is less than 4D (D is the diameter of the small hole).

Investigation of effective stress intensity factors during overload fatigue cycles using photoelastic and DIC techniques

This work uses DIC and photoelastic techniques to investigate the effect of single overload cycles applied during constant amplitude fatigue. Effective values of the range of stress intensity factor were calculated using the CJP model of crack tip stress and displacement fields, as this model considers both wake contact and compatibility-induced influences on the applied elastic field arising from the plastic enclave generated around a fatigue crack. Values of the effective stress intensity factor are related to the observed crack growth acceleration and retardation. In addition, the paper compares the CJP results with those obtained using a compliance-based method. The present work demonstrates the utility of the CJP model in characterising fatigue crack growth rates during variable amplitude loading. It is also possible with the CJP model, through changes in the coefficient values and hence, for the first time, to shed explicit light on the contributions made by different mechanisms to the shielding effects of an overload.

Laboratory apparatus for in‐situ corrosion fatigue testing and characterisation of fatigue cracks using X‐ray micro‐computed tomography

AbstractThis paper presents the design, construction, and assembly of laboratory apparatus to undertake in‐situ corrosion fatigue tests in a sour corrosive environment under uniaxial fatigue loading. The bespoke test cell allows periodic nondestructive X‐ray micro‐computed tomography of the specimen in‐situ during fatigue testing and thus enables monitoring of material degradation in‐situ as it progresses and in particular the pit‐to‐crack transition. This approach provides more direct information on crack initiation than complementary ex‐situ techniques such as scanning electron microscopy of post‐test metallographic specimens. Moreover, the apparatus was designed to allow a fatigue cycle to be interrupted and maintain the sample under static tensile load, during X‐ray tomography scans. This process reduced the risk of premature crack closure during interrupted tests. Results presented herein demonstrate the performance and reliability of our approach and will hopefully stimulate other groups to use similar “lab‐scale” initiatives.

A numerical lattice method to characterize a contact fatigue crack growth and its Paris coefficients using configurational forces and stress-life curves

Paris’ law material constants have computationally been determined using the implications of Configurational Mechanics and total stress-life curves to characterize fatigue crack growth rate, which is a challenge in experimental programs for measuring these parameters. A 2D plane strain lattice approach with a brittle erosion algorithm was employed to characterize total fatigue life and fracture behavior of propagating cracks of a pad-substrate system under high-cycle fatigue loadings in the context of LEFM. The capability of the lattice in predicting the direction of crack tip extension was validated by comparing with the analytical results of a center inclined cracked domain under uniaxial direct tension and also the surface crack initiation angle of the contact fatigue pad-substrate system. The lattice not only does fairly predict the Paris’ coefficients comparing to experimental measurements in literature, but also obtains total fatigue life by assuming a failure value for the crack length and demonstrates a curved fatigue crack. Having a simple constitutive formulation and a straightforward erosion algorithm, the numerical lattice can be integrated with the proposed fatigue fracture approach to obtain the Paris’ material constants, contact fatigue life, and crack propagation path.

Characterisation of clustered cracks using an ACFM sensor and application of an artificial neural network

The alternating current field measurement (ACFM) technique can be applied for surface-breaking fatigue crack detection and sizing; the link between the ACFM signal and crack size is well understood for individual cracks. However, the ACFM response to multiple clustered cracks is significantly different to that of isolated cracks. In railway rails the high wheel-rail forces can lead to rolling contact fatigue (RCF) cracks. Often cracks appear together in small clusters or in long stretches. The accurate characterisation of such fatigue cracks is essential for carrying out efficient and safe repair and maintenance. This paper presents a method for sizing the important sub-surface section of multiple cracks using ACFM via the application of an artificial neural network (ANN). The approach is demonstrated using a railway case study: a simulation-based dataset of signal response covering the range of RCF cracks typically seen in in-service railway tracks has been generated to give a thorough representation of the effect of clustered crack parameters on the ACFM response. A 5 × 5 × 2 × 1 multi-layer ANN has been optimised and trained using the validated simulation database to learn the inverse relationship between the crack pocket length (desired output) and the ACFM signal for a given cluster of RCF cracks. The network has been evaluated on a set of experimental data to size cracks of known dimensions from ACFM measurements and also on unseen simulation data. Results from both simulation and experiment show that the approach presented can be used to size clustered cracks to approximately the same degree of accuracy as is possible for isolated cracks.

Analytical insight into “breathing” crack-induced acoustic nonlinearity with an application to quantitative evaluation of contact cracks

To characterize fatigue cracks, in the undersized stage in particular, preferably in a quantitative and precise manner, a two-dimensional (2D) analytical model is developed for interpreting the modulation mechanism of a “breathing” crack on guided ultrasonic waves (GUWs). In conjunction with a modal decomposition method and a variational principle-based algorithm, the model is capable of analytically depicting the propagating and evanescent waves induced owing to the interaction of probing GUWs with a “breathing” crack, and further extracting linear and nonlinear wave features (e.g., reflection, transmission, mode conversion and contact acoustic nonlinearity (CAN)). With the model, a quantitative correlation between CAN embodied in acquired GUWs and crack parameters (e.g., location and severity) is obtained, whereby a set of damage indices is proposed via which the severity of the crack can be evaluated quantitatively. The evaluation, in principle, does not entail a benchmarking process against baseline signals. As validation, the results obtained from the analytical model are compared with those from finite element simulation, showing good consistency. This has demonstrated accuracy of the developed analytical model in interpreting contact crack-induced CAN, and spotlighted its application to quantitative evaluation of fatigue damage.

A semi-baseline damage identification approach for complex structures using energy ratio correction technique

Summary Because damage identification results based on the Lamb waves propagation approach can be influenced by varying environmental and operational conditions, development of a robust monitoring system with no need of the prior measured data of the structure has gained much attention recently. The instantaneous baseline damage detection technique is one of the promising methods that overcome the mentioned obstacles. For this, the material properties, electromechanical characteristics, and the geometric features must be identical. Also, sensor distances in similar paths and the geometric dimensions of the transducers must be the same. So, implementing the instantaneous baseline damage identification in complex structures is rather complicated due to the inherent non-homogeneities involved. To ease the complexity, this article proposes a semi-instantaneous baseline damage identification approach by modifying an existing instantaneous baseline method. This is used to detect and characterize fatigue cracks that initiate around the rivet holes of lap joint structures. In this method, an active sensing network is mounted on the lap joint, and a robust and effective feature called energy ratio change is extracted from the collected time-domain signals using the wavelet transform. The introduced “identicality coefficient” for all the sensing paths in pristine condition of the structure is obtained and used to remove any inequalities that may occur to the signals of each path. The obtained results show that the proposed method can detect fatigue cracks around the lap joint rivet holes and estimate the crack size. An experiment as well as numerical simulations is performed to verify the method.