Fatigue Crack Growth Monitoring Research Articles

Distributed Backface Strain Sensing of Composite Adhesively Bonded Joints under Mode II Fatigue Loading

When investigating the fracture behaviour of adhesive joints, accurate crack tip localization is under mode II loading is particularly challenging due to the absence of crack opening. The crack front often has a curved shape, so methods that inspect the side of the specimen cannot capture the crack length throughout its width. In this study, distributed backface strain sensing using Optical Backscatter Reflectometry fibre sensors is used to monitor fatigue crack growth in adhesively bonded composite End Notched Flexure Specimens. The obtained backface strain profiles were compared with analytical and Finite Element solutions to understand their relationship with the crack length. Results show that the crack tip position can be found by a local extremum in the first derivative of the strain profile. The proposed monitoring technique was compared with more established techniques for crack length measurement, such as Visual Testing, Compliance Monitoring, and Phased Array Ultrasonic Testing, and was shown to provide good accuracy. The evaluation of these techniques not only plays a crucial role in ensuring accurate testing, but can also contribute to the development of effective strategies for inspection and monitoring of adhesive joints.

Fatigue Crack Growth Monitoring and Investigation on G20Mn5QT Cast Steel and Welds via Acoustic Emission

The fatigue crack growth properties of G20Mn5QT cast steel and corresponding butt welds, using compact tension specimens, were monitored and investigated via acoustic emission (AE) techniques. Fatigue crack growth is a combination of cyclic plastic deformations before the crack tip, tensile crack fractures, and shear crack fractures. The cyclic plastic deformations release the maximum amount of energy, which accounts for half of the total energy, and the second-largest number of AE signals, which are of the continuous-wave type. The tensile crack fractures release the second-largest amount of energy and the largest number of AE signals, which are of the burst-wave type. The shear crack fractures release the least amount of energy and the lowest number of AE signals, which are similar to the burst type, albeit with a relatively longer rise time and duration. Crack tip advancement can be regarded as a discontinuous process. The critical area before the crack tip brittlely ruptures when the fatigue damage caused by cyclic plastic deformations reaches critical status. The ruptures produce a large number of tensile crack fractures and rare shear crack fractures. Through fractography observation, the shear crack fractures occur probabilistically around defects caused by casting or welding, which lead to stress and strain in the local complex.

Monitoring of a fatigue test on a full-size railway steel bridge with acoustic emission

Within the frame of the Rail4Future project (https://www.rail4future.com) co-funded by the Austrian Research Promotion Agency, a railway steel bridge was removed from the track and transported to a workshop in one piece. Artificial defects were introduced at different sections of the bridge structure as per advice of the scientific project partners in order to initiate fatigue cracks in the course of the subsequent fatigue test. One objective of the fatigue test was to identify suitable measurement and testing techniques for monitoring of fatigue crack growth. Among other techniques utilised by different project partners, acoustic emission monitoring was applied by TÜV AUSTRIA. This contribution describes the preparation as well as the performance of the acoustic emission monitoring and the results obtained. It is shown that this non-destructive testing technique is suitable for fatigue crack monitoring.

Monitoring fatigue crack growth using magnetic stray field measurements

Monitoring fatigue crack growth using magnetic stray field measurements

Particle filter for fatigue crack growth prediction using SH0 wave on-line monitoring

Fatigue crack is one of the main failure modes of pressure vessels. Online monitoring and predicting methods of crack growth play an important role in the operation of important pressure vessel. The SH0 wave is non-dispersive, and it is not disturbed by internal media of pressure vessel and very sensitive to cracks, therefore it is suitable for fatigue crack growth monitoring. Moreover, fatigue crack growth in industry is affected by material properties, loads, which usually shows some uncertainty. And the particle filter (PF) is well suited to deal with prediction problems affected by uncertainty. Hence, the prediction method of crack growth based on SH0 wave monitoring and PF is proposed (short for SH0-PF). The basic theory of crack monitoring method using SH0 wave is introduced, and the signal feature extraction using the damage index is studied. The state equation characterizing the fatigue crack growth is established by Paris model, and the observation equation is established based on the normalized correlation moment damage index according to monitoring signal using SH0 wave. The prediction reliability of the fatigue crack growth applying SH0-PF is verified by experiment with the single edge notched specimen. The experimental results indicate that the prediction accuracy of SH0-PF is better than that of the traditional Paris model.

Bayesian analysis of acoustic emission data for prediction of fatigue crack growth in concrete

Acoustic emission (AE) is a valuable technique for non-destructive testing, enabling real-time monitoring and measurement of fatigue crack growth in engineering structures. The present study focuses on investigating the fatigue crack growth behaviour in plain concrete specimens through Bayesian analysis of AE data collected during the testing of beam specimens under three-point bending. A log-linear relationship is established between parameters extracted from AE waveforms and the fatigue crack growth in concrete. Additionally, AE energy is identified as the most suitable parameter for characterising the fatigue behaviour of concrete. Bayesian regression is employed for estimating model parameters and their posterior distributions. The proposed model is validated through post-processing and Bayesian analysis of experimental data from the literature. Furthermore, it is demonstrated that the estimated model parameters remain unaffected by the frequency of fatigue loading and the size of the specimen.

Detection and monitoring of the fatigue crack growth on welds – Application-oriented use of NDT methods

Early detection of fatigue cracks and accurate measurements of the crack growth play an important role in the maintenance and repair strategies of steel structures exposed to cyclic loads during their service life. Observation of welded connections is especially of high relevance due to their higher susceptibility to fatigue damage. The aim of this contribution was to monitor fatigue crack growth in thick welded specimens during fatigue tests as holistically as possible, by implementing multiple NDT methods simultaneously in order to record the crack initiation and propagation until the final fracture. In addition to well-known methods such as strain gauges, thermography, and ultrasound, the crack luminescence method developed at the Bundesanstalt für Materialforschung und -prüfung (BAM), which makes cracks on the surface particularly visible, was also used. For data acquisition, a first data fusion concept was developed and applied in order to synchronize the data of the different methods and to evaluate them to a large extent automatically. The resulting database can ultimately also be used to access, view, and analyze the experimental data for various NDT methods. During the conducted fatigue tests, the simultaneous measurements of the same cracking process enabled a comprehensive comparison of the methods, highlighting their individual strengths and limitations. More importantly, they showed how a synergetic combination of different NDT methods can be beneficial for implementation in large-scale fatigue testing but also in monitoring and inspection programs of in-service structures - such as the support structures of offshore wind turbines.

Acoustic Emission Monitoring of Fatigue Crack Growth in Hadfield Steel.

Evaluating the condition of a Hadfield steel crossing nose using existing inspection methods is subject to accessibility and geographical constraints. Thus, the use of conditional monitoring techniques to complement the existing inspection methods has become increasingly necessary. This paper focuses on the study of acoustic emission (AE) behaviour and its correlation with fatigue crack growth in Hadfield steel during bending fatigue tests. The probability density function for acoustic emission parameters was analysed based on the power law distribution. The results show that a sharp increase in the moving average and cumulative sum of the AE parameter can give early warning against the final failure of Hadfield steel. Two parts (Part 1 and Part 2) can be identified using the change in the slope of duration rate (dD/dN) vs. ΔK plot during the stable fatigue crack growth (FCG) process where Paris's law is valid. The fitted power law exponent of AE parameters is smaller in Part 2 than in Part 1. The novelty of this research lies in the use of the fitted power law distribution of AE parameters for monitoring fatigue damage evolution in Hadfield steel, unlike existing AE fatigue monitoring methodology, which relies solely on the analysis of AE parameter trends.

A novel ultrasonic non-destructive testing methodology to monitor fatigue crack growth in compact tension specimens.

Detection and evaluation of fatigue cracks in structural materials are extremely important for life prediction analysis as part of the structural integrity procedure. In this article, we present a novel ultrasonic measurement methodology, based on the diffraction of elastic waves at the crack tips, to monitor the fatigue crack growth near the threshold regime in compact tension specimens at different load ratios. The diffraction phenomenon of ultrasonic waves from the crack tip is demonstrated using a finite element 2D wave propagation simulation. The applicability of this methodology has also been contrasted with that of the conventional direct current potential drop method. In addition, the crack morphology obtained in the ultrasonic c-scan imaging showed a variation in the crack propagation plane as a function of the cyclic loading parameters. The results suggest that this novel methodology is sensitive to fatigue cracks and can form the basis of in situ ultrasonic-based crack measurements in metallic and non-metallic materials.

Crack Growth Monitoring with Structure-Bonded Thin and Flexible Coils.

Structural health monitoring with thin and flexible eddy-current coils is proposed for in situ detection and monitoring of fatigue cracks in metallic aircraft structures, providing a promising means of crack sizing. This approach is seen as an efficient replacement to periodic inspections, as it brings economic and safety benefits. As such, printed-circuit-board eddy-current coils are viable for in situ crack monitoring for multi-layer, electrically conductive structures. They are minimally invasive and could be attached to or embedded into the evaluated structure. This work focuses on the monitoring of fatigue crack growth from a fastener hole with structure-bonded, thin, and flexible spiral coils. Numerical simulations were used for optimization of the driving frequency and selection of crack-sensitive coil parameters. The article also demonstrates the fatigue crack detection capabilities using spiral coils attached to a 7075-T6 aluminum coupon.

Fatigue crack monitoring of steel bridge with coating sensor based on potential difference method

• A coating sensor for fatigue crack monitoring using the potential difference method is proposed. • The theoretical model for crack monitoring using coating sensor is established. • The accuracy and applicability of coating sensor are validated by the fatigue tests and crack monitoring. Fatigue cracks are common in steel bridges. For the real-time monitoring of fatigue cracks during the service of steel bridges, this study proposes a coating sensor for fatigue crack monitoring of steel bridges based on the potential difference method. First, the coating sensor was designed with three components including driving, sensing, and protective layers, and the characteristics of each part were measured. The equation for the spatial potential distribution of the coating sensor was derived and subsequently solved using a numerical simulation method. A theoretical model for crack growth prediction was proposed, and the parameters of the theoretical model were fitted using regression analysis associated with the simulated results. Six samples with artificial cracks were used to validate the accuracy of the theoretical model for crack-growth monitoring. Finally, the accuracy and applicability of the coating sensor for monitoring fatigue crack growth were verified using fatigue tests. The results indicate that the coating sensor can easily crack as fatigue cracks grow in steel plates. The predicted and actual crack lengths in both the artificial crack growth tests and real fatigue crack growth experiments demonstrate good consistency. Moreover, comparing crack monitoring by coated sensors with image recognition crack monitoring using high-definition cameras shows that the fatigue crack monitoring of them matches well, demonstrating the accuracy and applicability of the coating sensor and corresponding theoretical model for fatigue crack monitoring.

A feasibility study on monitoring of weld fatigue crack growth based on coda wave interferometry (CWI)

Fatigue induced cracks in welds may result in the failure of welded structures and therefore it is important to detect the fatigue crack in the early stage. In this paper, we proposed a coda wave interferometry (CWI)-enabled fatigue cracks detection method using piezoceramic transducers. Because of its high sensitivity to the tiny change in the medium, the CWI method employs the multiple scattered coda waves to detect the slight change of welding fatigue cracks. Firstly, a theoretical model is developed by combining the CWI theory with the acoustoelastic effect. The derivation shows that there is a linear relationship between the crack width change and the coda wave relative velocity variation. To verify the correctness of the derived result, the CWI technique is applied to the detection of micro fatigue cracks of three butt welded specimens under fatigue loading. Experimental results show that the relative velocity variation of coda wave increases linearly with the increase of the width of welding fatigue cracks. In addition, energy-based active sensing (EAS) approach was also used as a comparison. Compared with the EAS method, the CWI technique is more sensitive to slight changes of welding fatigue cracks and therefore holds a great potential for the monitoring of micro fatigue crack in the early stage.

The Location Monitoring of Fatigue Crack Damage by Using the Spectral Area Extracted from FBG Spectra.

In this paper, a new damage feature, spectral area, was extracted to effectively detect crack location by studying the deformation mechanism of fiber Bragg grating (FBG) reflection spectra. In order to verify the robustness and reliability of spectral area to detect crack location, the following work was carried out: Firstly, the strain information was extracted by extended finite element method (XFEM) with fatigue crack propagation. The transmission matrix method (TMM) was used to simulate FBG reflection spectra using numerical results. Secondly, the fatigue crack growth monitoring experiment based on FBG sensors was carried out, and the digital image correlation (DIC) method was used to measure the strain values at the placement of FBG sensors with crack propagation. The temperature characteristic test of FBG was carried out to investigate the influence of temperature variation on the spectral area. The results presented that the spectral area was insensitive to temperature variation and experimental noise, and was greatly sensitive to the complex non-uniform strain field cause by crack damage. Moreover, compared with the 5 mm FBG sensor, the 10 mm FBG sensor showed a larger critical detection range for crack damage. Therefore, the spectral area can be used as a reliable damage feature to detect the crack location quantitatively based on the simulated and experimental results.

Fatigue crack growth identification in bonded joints by using carbon nanotube doped adhesive films

An investigation on fatigue crack growth monitoring of bonded joints has been conducted employing a novel carbon nanotube adhesive film, chosen for their proven exceptional properties for sensing purposes. Single lap shear fatigue tests have been carried out at a 7 kN load, load ratio 0.1 and 10 Hz frequency. A correlation between the electrical response of the adhesive joints and the fatigue crack growth has been established by means of voltage acquisition through the thickness in combination with crack length monitoring by microscopy analysis. Three phases can be distinguished during the fatigue tests: phase one without crack initiation with a steady electrical response; phase two in which cracks start propagating, with a slight increase of the electrical resistance, and phase three, corresponding to final propagation to failure with a drastic increase of the electrical resistance. A correlation between the electrical response, the estimated crack area and the measured edge-crack size is achieved, allowing to get a deeper understanding of crack growth phenomenon as a function of the number of cycles. Thus, the potential and the applicability of the proposed technique for monitoring crack growth in lap joint fatigue tests have been demonstrated.

Determination of effective stress intensity factors under mixed‐mode from digital image correlation fields in presence of contact stresses and plasticity

AbstractDigital image correlation (DIC) is more and more popular to monitor fatigue crack growth and to determine the stress intensity factors. However, the posttreatment of the recorded displacement fields becomes tricky when the crack faces are not stress‐free and when crack tip plasticity becomes significant. Several posttreatment methods to locate the crack tip and measure the effective stress intensity factors in such cases are compared, using finite element method‐computed displacement fields, and then used on real DIC fields. An approach coupling DIC and finite element method is proposed to estimate the contact stresses along the crack.

A research on fatigue crack growth monitoring based on multi-sensor and data fusion

Fatigue crack propagation is one of the main problems in structural health monitoring. For the safety and operability of the metal structure, it is necessary to monitor the fatigue crack growth process of the structure in real time. In order to more accurately monitor the expansion of fatigue cracks, two kinds of sensors are used in this article: strain gauges and piezoelectric transducers. A model-based inverse finite element model algorithm is proposed to perform pattern recognition of fatigue crack length, and the fatigue crack monitoring experiment is carried out to verify the algorithm. The strain spectra of the specimen under cyclic load in the simulation and experimental crack propagation are obtained, respectively. The active lamb wave technique is also used to monitor the crack propagation. The relationship between the crack length and the lamb wave characteristic parameter is established. In order to improve the recognition accuracy of the crack propagation mode, the random forest and inverse finite element model algorithms are used to identify the crack length, and the Dempster–Shafer evidence theory is used as data fusion to integrate the conclusion of the two algorithms to make a more accountable and correct judge of the crack length. An experiment has been conducted to demonstrate the effectiveness of the method.

Observations of fatigue crack behaviour in proton‐irradiated 304 stainless steel

AbstractThermoelastic stress analysis (TSA) has been used to monitor fatigue crack growth in compact tension (CT) specimens, made from 304 grade austenitic stainless steel, that have been subject to proton irradiation. Several specimens had a 10 × 10 mm area ahead of a 1‐mm precrack irradiated with a 1.6 MeV proton beam up to 0.216C to 0.648C of accumulated charge prior to fatigue testing. Subsequently, specimens were loaded sinusoidally at 20 Hz with an R ratio of 0.5, and TSA data were collected both at the loading frequency and its second harmonic. Irradiation appears to cause an increase in the fatigue life, with a reduction in crack growth rate observed in the irradiated specimens compared with the unirradiated control specimens. Irradiation damage caused a moderately linear change in both the parameters of the Paris law with accumulated charge from the irradiation.

Acquiring in situ Fatigue Crack Growth Curves by a Compliance Method for Micro Bending Beams to Reveal the Interaction of Fatigue Cracks with Grain Boundaries

To understand the interaction of dislocations with microstructural obstacles it is necessary to find test methods which are able to resolve the interaction of only a single or few defects with obstacles like interfaces. Therefore, the investigation of micro specimens has been established over the years as a suitable method to test the influence of microstructural features on the mechanical response. While quasi-static loading of micro specimens has been carried out extensively in the past decade and has given powerful insights on the mechanical behavior at small scales, cyclic loading and fatigue crack growth experiments still provide a challenge. In order to check the possibility to systematically initiate and monitor fatigue crack growth rates in micro specimens, we cyclically loaded micro bending beams made of a nickelbase superalloy. Furthermore, for grain boundaries of differing types we checked if the crack growth curves are suitable to measure crack - microstructure interactions. The fatigue cracks showed a deceleration when approaching the grain boundaries followed by an abrupt re-acceleration, which is in accordance to macroscopic experiments and connected to dislocation or slip transfer from the plastic zone of the crack through the grain boundary. Furthermore, we observed a dependency of the deceleration on the grain boundary type and the crystallographic orientation of the neighboring grains. A 3D-HR-orientation gradient map was gathered by HR-EBSD using the software CrossCourt in combination with a self-provided MATLAB tool to reveal detailed information about strain localization at the grain boundary in the process zone near the crack tip.

A large-area strain sensing technology for monitoring fatigue cracks in steel bridges

This paper presents a novel large-area strain sensing technology for monitoring fatigue cracks in steel bridges. The technology is based on a soft elastomeric capacitor (SEC), which serves as a flexible and large-area strain gauge. Previous experiments have verified the SEC’s capability to monitor low-cycle fatigue cracks experiencing large plastic deformation and large crack opening. Here an investigation into further extending the SEC’s capability for long-term monitoring of fatigue cracks in steel bridges subject to traffic loading, which experience smaller crack openings. It is proposed that the peak-to-peak amplitude (pk–pk amplitude) of the sensor’s capacitance measurement as the indicator of crack growth to achieve robustness against capacitance drift during long-term monitoring. Then a robust crack monitoring algorithm is developed to reliably identify the level of pk–pk amplitudes through frequency analysis, from which a crack growth index (CGI) is obtained for monitoring fatigue crack growth under various loading conditions. To generate representative fatigue cracks in a laboratory, loading protocols were designed based on constant ranges of stress intensity to limit plastic deformations at the crack tip. A series of small-scale fatigue tests were performed under the designed loading protocols with various stress intensity ratios. Test results under the realistic fatigue crack conditions demonstrated the proposed crack monitoring algorithm can generate robust CGIs which are positively correlated with crack lengths and independent from loading conditions.

Risk-based maintenance planning of subsea pipelines through fatigue crack growth monitoring

Research and development in the field of risk-based maintenance of offshore structures has recently attracted large attention due to the significant level of accident risk and the cost associated with maintenance in such remote facilities. The uncertainties associated with the deterioration of these facilities require a sound decision making methodology for maintenance planning. This paper presents a dynamic risk-based methodology for maintenance scheduling of subsea pipelines subjected to fatigue cracks. The developed method can assist the asset managers to select the optimum approach for mitigating the consequences of failure while minimizing the maintenance costs. A Bayesian network is developed to model the probabilistic deterioration process and then it is extended to an influence diagram for estimating the expected utility of each decision alternative. Observation of damage state is included in the model to enhance decision making capacity. To demonstrate the applicability of the methodology, three cases with different fatigue crack incidents on a pipeline are considered. Based on the monitoring results, the model is able to determine whether the maintenance should be performed or not. The economic risk associated with maintenance is also minimized by suggesting the optimum maintenance technique among multiple possible methods such as welding or major repair.