Location Of Fatigue Cracks Research Articles

Effect of welding residual stresses on the fatigue life assessment of welded connections

Welding residual stresses impose significant influences on the fatigue behavior and structural integrity of welded connections in various engineering applications. Understanding the effects of these residual stresses on fatigue life is crucial for ensuring the reliability and safety of welded structures. This paper proposes a damage-incorporated thermal–mechanical simulation method to examine the effect of welding residual stresses on the fatigue life prediction of welded plates and circular hollow section (CHS) X-joints. Compared to the experimental X-ray diffraction measurements and the fatigue tests, the predicted residual stresses and fatigue life demonstrate good agreement with experimental results for both high-cycle and low-cycle fatigue. The location of fatigue cracks resembles closely the estimation from a uniform fatigue damage indicator. The fatigue life prediction without incorporating the welding residual stresses does not affect significantly the life estimation for low-cycle fatigue but can cause substantial over-predictions for high-cycle fatigue.

Thermal stress simulation and fatigue life of commercial vehicle disk brakes under emergency braking conditions

Commercial vehicle disk brakes operate at a high-temperature and in a heavy-load environment within the braking system. The primary cause of failure is the cracking of the brake disk. In order to study its fatigue damage and service life, a finite element model of disk brake fatigue life was established, and thermal stress coupling simulation analysis was carried out from a practical problem. Based on the temperature and stress fields of the brake disk under emergency braking conditions obtained from the simulation results, the effects of vehicle load, initial speed, temperature, and other factors on brake fatigue life are explored. The fatigue life of the hazardous node can be calculated using the Manson–Coffin model, and then the strain–life (ɛ–N) curve of the material can be fit at high temperature. The fatigue life of brake disks was predicted using the fatigue analysis software FE-SAFE and verified by testing. The results showed that the maximum stress on the surface of the disk brake was the same as the area of the minimum fatigue life, accurately analyzing the fatigue life of the region and predicting the location of fatigue cracks. The results of the research can provide a reference for the design of disk brake engineering and fatigue failure.

Pipe geometry effect on fatigue cracking location of 90° elbows under in-plane bending

In this technical note, the criterion for fatigue cracking location for 90° elbows under in-plane bending is presented. The criterion is found using published elastic solutions and elastic-plastic FE analysis results. It is found that the most important parameter affecting fatigue crack location is the mean radius-to-pipe thickness ratio, rm/t. The circumferential crack at outer intrados is predicted for rm/t <4–5, and the axial crack at inner crown for rm/t >4–5. The exact value of rm/t depends slightly on the bend radius-to-mean pipe radius ratio. This conclusion is found to be appropriate from high cycle fatigue to very low cycle fatigue loading, and to be independent on the material.

Fatigue weld crack detection using distributed fiber optic strain sensing

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for early detection and location of fatigue cracks in welds in steel tubular test specimens. To do so, we have subjected welded tubular specimens instrumented with surface-mounted optical fiber sensors to resonant bending load, and we have measured the strain distributions in the test samples continuously and without any interruption of the test throughout its whole duration, with a 2.6 mm spatial resolution and a 1 με strain resolution. We show that the fatigue cracks, which initiate from the inner surface of the wall of the specimens, can be successfully detected and located in real time using our measurement method. We conclude that the detection of the crack initiation may provide relevant information serving the estimation of the remaining lifetime of the component.

Nonlocal approach to fatigue analysis of a welded bicycle frame joint

AbstractThe paper attempts to predict the probabilistic fatigue strength scatter bands of a welded bicycle frame joint. The experimental data of the base material and the weld material were approximated by a nonlinear probabilistic model of the Weibull distribution. The fatigue strength for the material at the fatigue crack location was estimated considering the mean stress, residual stress, fatigue notch factor, and hardness profile. The output finite element numerical data of the real welded joint were compared with a nonlocal multiaxial fatigue criterion. The implementation of the volumetric method takes into account the stress gradient effect in the three‐dimensional geometry of the weld toe. The comparative localization of the minimum stress and the real crack indicates the validity of the numerical model. The predicted fatigue strength determines the possible failure of a welded joint at 5·105 cycles.

Deep learning enables nonlinear Lamb waves for precise location of fatigue crack

Localization of fatigue cracks imposes immense significance to ensure the health of the engineering structures and prevent further catastrophic accidents. The nonlinear ultrasonic waves, especially the nonlinear Lamb waves, have been increasingly studied and employed for identifying micro-damages that are usually invisible to traditional linear ultrasonic waves. However, it remains a challenge to locate the fatigue cracks using nonlinear Lamb waves owing to the enormous difficulties in decoding location information from acoustic nonlinearity. Motivated by this, this work presents a data-driven method for precise location of fatigue crack using nonlinear Lamb waves. A 1D-Attention-convolutional neural network is developed to correlate the fatigue crack location with the wavelet coefficients at the second harmonic frequency of Lamb wave signals. The introduction of the Attention layer enables the models to pay more attention to the desired nonlinear features which dominates locating the fatigue crack. In particular, a convenient dataset creation scheme guided by the relative value label is proposed to generate sufficient data commonly required for deep learning approach. In addition, a lightweight single-excite-multiple-receive signal acquisition method is adopted instead of full-matrix capture method used in the traditional research, which highly improves detection efficiency. Numerical simulation and experimental validation manifest that the trained network can be used to establish the complex mapping between the nonlinear ultrasonic signals and the fatigue crack location features, so as to locate barely visible fatigue cracks. Our work provides a promising and practical way to facilitate nonlinear Lamb waves to accurately locate fatigue cracks in large-scale plate-like structures.

Location and Length Measurement of Invisible Fatigue Crack in Metal Components Using Wave Mixing Methods

Abstract For the invisible fatigue crack embedded in metal components, wave mixing methods are applied for the fatigue crack location and length measurement. When two incident waves meet within a component, a nonlinear interaction occurs and a third wave is generated, which is related to the damage degree of the meeting position of the two incident waves. When the delay time of incident waves changed in collinear wave mixing experiments, different meeting positions along the length direction of components were detected. The relationship between the sideband amplitude at difference or sum frequencies and delay time is obtained, and the delay time corresponding to the maximum amplitude is applied to locate the fatigue crack along the length direction. Then, adjusting the placing locations of transducers in the noncollinear wave mixing experiment, the meeting position of incident waves is controlled at the fatigue crack decided by the collinear wave mixing experiment. Next, changing the separation distance of two transmitters, the detection positions would be along the vertical direction of the components. The mixing nonlinear parameters of different detection positions are calculated, and the length of the fatigue crack can be measured in a vertical direction based on the distribution of the mixing nonlinear parameter.

Automatically Welded Tubular X‐Joints for Jacket Substructures: Prediction of the Technical Fatigue Crack Location

ABSTRACTTo increase the competitiveness of jacket substructures compared to monopiles a changeover from an individual towards a serial jacket production based on automated manufactured tubular joints combined with standardized pipes has to be achieved. Therefore, this paper addresses fatigue tests of automatically welded tubular X‐joints focusing on the location of the technical fatigue crack. The detected location of the technical crack is compared to numerical investigations predicting the most fatigue prone notch considering the structural stress approach as well as the notch stress approach. Besides, the welding process of the automated manufactured tubular X‐joints is presented.

Effect of stress-induced magnetization on crack monitoring by self magnetic flux leakage method

Fatigue cracks could occur in the material of offshore and marine structures that are cyclically loaded. These cracks occur due to the cyclic stresses induced by the waves causing premature failure. Monitoring these fatigue cracks using a wireless system gives the opportunity to guarantee the integrity of the structure without manual inspection, making this a safer way of inspection. In addition, inspection is very costly due to the man hours and because of the fact that the locations of fatigue cracks are mostly not easily accessible. Also, visual evaluation of the crack length can be difficult to assess, which could result in early retirement of structures or unnecessary and costly maintenance. A wireless crack monitoring system based on the Self Magnetic Flux Leakage (SMFL) method could be a solution for this problem. This method suggests that the ferromagnetic material is passively magnetized by the Earth’s magnetic field and the magnetic signal changes when a crack appears in the material. The measured data is sent wirelessly to the control room of the marine structure. Here, the data can be assessed to determine the size of the fatigue crack. With this information, it can be decided what course of action should be taken to ensure the integrity of the marine structure as long as possible. This will result in a safer way of working and being economically more efficient. For accurate crack sizing, the SMFL must be interpreted correctly. During cyclic loading the stresses in the material change, affecting the magnetic signal. This is called the stress-induced magnetization. The aim of this research is to investigate the effect of stress-induced magnetization on the SMFL in the stress concentration zone of a structural steel plate, and its implications for crack monitoring by the SMFL method. The measured stress magnetization curves are obtained by means of an experiment. In this experiment, a steel plate with an elliptical hole is cyclically loaded up to the yield stress. The magnetic signal is measured in a grid around the hole. The results show a maximum variation of 25 µT. Depending on the application, the stress-induced magnetization may need to be considered for the interpretation of the measured signals for crack monitoring using the SMFL method.

Computational and Experimental Fatigue Analysis of Contoured Spinal Rods

Posterior fixation with contoured rods is an established methodology for the treatment of spinal deformities. Both uniform industrial preforming and intraoperative contouring introduce tensile and compressive plastic deformations, respectively, at the concave and at the convex sides of the rod. The purpose of this study is to develop a validated numerical framework capable of predicting how the fatigue behavior of contoured spinal rods is affected by residual stresses when loaded in lordotic and kyphotic configurations. Established finite element models (FEM) describing static contouring were implemented as a preliminary simulation step and were followed by subsequent cyclical loading steps. The equivalent Sines stress distribution predicted in each configuration was compared to that in straight rods (SR) and related to the corresponding experimental number of cycles to failure. In the straight configuration, the maximum equivalent stress (441 MPa) exceeds the limit curve, as confirmed by experimental rod breakage after around 1.9 × 105 loading cycles. The stresses further increased in the lordotic configuration, where failure was reached within 2.4 × 104 cycles. The maximum equivalent stress was below the limit curve for the kyphotic configuration (640 MPa), for which a run-out of 106 cycles was reached. Microscopy inspection confirmed agreement between numerical predictions and experimental fatigue crack location. The contouring technique (uniform contouring (UC) or French bender (FB)) was not related to any statistically significant difference. Our study demonstrates the key role of residual stresses in altering the mean stress component, superposing to the tensile cyclic load, potentially explaining the higher failure rate of lordotic rods compared to kyphotic ones.

Fatigue performance and evaluation of welded joints in steel truss bridges

While welded joints are extensively used in the connections of steel truss bridges, service life of the bridges is largely dependent on the fatigue resistance of the welded joints. Stress concentration and weld residual stress are two primary causes of fatigue damage in the welded joints. In this study, corner-fillet profile (CFP) and ultrasonic impact treatment (UIT) are used to improve the fatigue performance of the welded joints through relieving stress concentration and weld residual stresses, respectively. The fatigue resistance of welded joints is evaluated through experimentation. The results indicate that the use of CFP and/or UIT can alter the initiation location of fatigue crack. The fatigue resistance of welded joints was increased by 24% and 36% by using the CFP and UIT, respectively. The fatigue performance of welded joints was evaluated using three different methods, including the nominal stress method, effective notch stress method, and peak stress method. The peak stress method with a single fatigue resistance curve demonstrated the highest applicability and accuracy.

09.16: Edge clamping effect on the fatigue resistance of an orthotropic steel bridge deck

ABSTRACTOrthotropic steel decks are an excellent solution for long span bridges, except if the spreading of wheel loads through the pavement is poor. In the latter case they are heavily sensitive to fatigue cracking. The most critical locations of fatigue cracks are the connection of longitudinal stiffeners to the deck plate and the cutouts and cope‐holes in the webs of the transverse stiffeners. OSD with variable depth are seldom used, since the idea contradicts with one of the main assets to limit the bridge deck depth. However, in few cases variable depth may be meaningful as in the case of a railway bridge across river Dyle, recently built in Mechelen (Belgium). This 50 m span bridge consists of 2 lateral stubs of 1.55 m diameter connected by the OSD. The tubes are slightly bent to foster arch thrust action and rotated towards the bridge axis. In addition, the arch springs are encased in the concrete abutment. Due to this arrangement, the crossbeams are partially clamped in the lateral tubes of high torsion stiffness. The effect of the end clamping of crossbeams reduces the tension in the lower part of the crossbeam web, also near the outer cope holes, where the largest stress concentrations occur and fatigue resistance is the lowest. The reduction of the magnitude of these stress concentrations is significant and depends on the crossbeam span. However, the effect disappears if the crossbeam web is higher, thus confirming it is due to bending tension of the crossbeams. In addition, the influence length of crossbeams has been verified in the case of river Dyle bridge and complies well with Eurocode.

Mechanical and fatigue properties of self-piercing riveted joints in high-strength steel and aluminium alloy

Static tensile and fatigue tests were performed on shear and tensile self-piercing riveted aluminium-steel structures to evaluate their mechanical and fatigue properties. The influences of the thickness and the strength of the high-strength steel on mechanical and fatigue performances were investigated based on the tensile and F-N curves of the joints. The results show that mechanical and fatigue properties of the shear self-piercing riveted joints are much better than those of the tensile self-piercing riveted joints. Mechanical and fatigue performances of the two joints were significantly influenced by the thickness and strength of the steel sheet, and were markedly improved when the thickness of steel sheet increased. The steel strength showed significantly different effects on shear and tensile riveted structures, i. e., when the steel strength increased, the strength of the shear structure greatly increased while the tensile structure just had a slight increase in the strength. Fatigue failure generally occurred in the sheet materials and the fatigue crack location changed with increasing the sheet thickness and the sheet strength.

Localisation of plastic strain at the microstructurlal level in Hastelloy X subjected to monotonic, fatigue, and creep loading: the role of grain boundaries and slip transmission

The localisation of plastic strain in polycrystalline metals is influenced by the microstructure. In this work, we study the localisation and heterogeneity of the plastic response using high resolution strain measurements via digital image correlation and microstructural characterisation using electron backscattering diffraction in monotonic tension, fatigue, and creep. We focus on the role of grain boundaries acting as barriers to dislocation motion. More specifically, we address slip transmission across grain boundaries and the importance of the residual Burgers vector in determining the cross-boundary reaction. We provide insight into how these cross-boundary reactions influence the accumulation of plastic strains across interfaces, dictate the locations of fatigue cracks, and affect the length of fatigue microcracks. The scope of the paper is partially a summary and overview of most recent published work as well as presenting new experimental results, particularly at elevated temperatures. The main outcomes of the agglomerate of experimental and analysis results are discussed with the intent to provide a deep understanding of the plastic response of the nickel-based superalloy, Hastelloy X, at different length scales and different loading and temperature conditions.

Very high cycle fatigue tests of quenched and self-tempered steel reinforcement bars

Investigations on the fatigue strength of steel reinforcement bars (rebars) mainly involves fatigue tests with hot rolled (HR) and cold worked (CW) steels. However, in the last few decades, HR and CW rebars were replaced by quenched and self-tempered (QST) rebars with hardened surface layer. There still remains a lack of research on fatigue strength of QST rebars especially in the very high cycle domain i.e., number of stress cycles surpassing 5 million. This study is part of a further detailed investigation on the fatigue behaviour of HR, CW and QST rebars in the very high cycle domain. It aims to investigate the fatigue performance of QST rebars axially tested at number of stress cycles in the range of 106–108. A preliminary study of the gripping method is followed by fatigue test results including non-destructive inspection of the rebar surface and fractographic analyses. The rebar surface is examined with liquid penetrant to reveal fatigue crack location and size in specific frequency interval monitored during the tests. Fractured surface analyses are performed by scanning electron microscopy to detect the location from where fatigue cracks initiate. Cross sectional area reduction resulting from fatigue crack propagation is also determined. Fractographic investigations are compared with the fractured surfaces of HR, CW and QST rebars from the literature.

Calculations of notch stress factor of a thin-walled spreader bracket fillet weld with the use of a local stress approach

Presence of geometric notches in welded joints causes concentration of strains and stresses, therefore reducing fatigue strength of such joints. This article presents an analysis of stress concentrations in a fillet weld of a spreader mounting bracket on a small sailing yacht. The aim of this article is to direct the attention of designers, manufacturers and regulatory bodies to issues of fatigue cracks that form in brackets fastening spreaders to masts. The issue concerns mainly yachts with overall length of a hull below 10m. Despite frequent defects the solution is still being chosen by designers, mainly for technological reasons. Current design regulations of classification societies and other regulatory bodies for small recreational yachts do not specify a technical solution for spreader fastening. With the use of finite element analysis it was shown that for short term design loads structural strength of a bracket is sufficient. Then, with the use of the notch stress approach, it was proven that the spreader bracket welded of thin plates is a structural solution that runs a risk of fatigue cracks. The below presented calculations of the geometric concentration factor of weld section in the locations of fatigue cracks were performed for different geometric cases of joints. The analysis results indicate the need for greater attention that should be paid to the problem of fatigue cracks that form in welds of brackets fastening spreaders to masts. The determined values of effective stress concentration coefficients show the need for conducting fatigue calculations of brackets welded of thin plates, or choosing an alternative solution for fastening spreaders to masts. In both cases it seems appropriate to conduct fatigue calculations for a structural element which is “a weak link” of the main drive on a sailing yacht.

ROLE OF SURFACE CONDITION ON THE PLANE BENDING FATIGUE BEHAVIOR OF NODULAR CAST IRON

Nodular cast irons are technologically important materials and are used extensively in automotive industry. Defects produced during casting process often play a dominant role in limiting mechanical properties and fatigue life under cyclic loading in cast alloy components. Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. In this study the effects of surface condition on the fatigue life of nodular cast iron were evaluated under cyclic plane bending with maximum stress reached at the surface of interest. The results show differences in fatigue strength, which are associated with the surface conditions. The characteristics of the surface layers of the test specimens with different surface condition were examined by metallography. Selected fracture surfaces were investigated to determine the initiation location of fatigue cracks.

Analysis on Crack Types and Causes of Junshan Bridge’s Steel Box Girder Orthotropic Plate

The fatigue cracking of orthotropic plate is a fatal problem for the long operation of steel bridge safely. There are numerous fatigue cracks appearing on Junshan Bridge, which locate mainly on diaphragm, U-Rib and fillet weld between the U-Rib and deck. Based on the location of fatigue cracks, the paper analyzed the causes of fatigue cracking from three aspects of mechanics cause, detail design and manufacturing factors, providing scientific basis of future health monitoring for bridges.

Fatigue crack location and fatigue life for riveted lap joints in aircraft fuselage

Effects of variables related to design and production of riveted lap joints representative of longitudinal sheet connections for a pressurized transport aircraft fuselage were experimentally investigated. Specimens from an aircraft Al alloy D16Cz Alclad sheets of three different thicknesses (1.9, 1.2 and 0.8mm) were assembled under load control using round head rivets and rivets with the compensator from a P24 Al alloy. For the joints from 1.9mm thick sheets fatigue tests indicated a dependency of the crack initiation site and crack path on the squeeze force level and on the rivet type. At the same time, increasing the squeeze force led to improved fatigue properties of the joints, specimens assembled using the rivets with the compensator showing fatigue lives longer than joints with the round head rivets. All observed trends have been explained based on hole expansion and load transfer measurements. For thin sheets connected using the round head rivets, local deformations under the rivet driven head arising during the rivet installation promoted crack initiation and failure in the adjacent sheet. Fatigue test results indicated that the detrimental effect of this type imperfections could outweigh the benefits associated with a decrease in secondary bending due to thinning the sheets. The rivets with the compensator were observed to cause significant local imperfections beneath the manufactured head, which adversely affected the fatigue performance of the joints from thin sheets.

Combined laser spot imaging thermography and ultrasonic measurements for crack detection

A surface crack, close to a small, well-defined heated spot, impedes lateral heat flow and produces alterations in the shape of the thermal spot image that can be monitored by thermography. Spot heating has been achieved using both cw and pulsed laser beam illumination on stainless steel and titanium samples and the technique has been found to be successful for determining the location of fatigue cracks. When using a pulsed laser beam one can also simultaneously generate wideband ultrasonic signals in the sample; this can be used to detect the presence of surface and sub-surface defects in the sample. Results are presented that have been obtained using a fixed camera and cw laser beam position with the sample being moved through the field of view of the camera. Results are also presented using a fixed camera and sample, with a raster scanned cw or pulsed lasers beam moving across the sample. A demonstration of how a non-contact ultrasonic measurement can be performed simultaneously is presented. Thermal imaging results obtained using cw laser beam heating close to a surface crack are compared with predictions made using a preliminary 2D numerical model.