Fatigue Failure Analysis Research Articles

Risk and sensitivity quantification of fracture failure employing cohesive zone elements

Many structures are subjected to the risk of fatigue failure. For their reliability-based design, it is thus important to calculate the probability of fatigue failure and assess the relative importance of the involved parameters. Although various studies have analyzed the fatigue failure, the stage of fracture failure has been less focused. In particular, the risk analysis of fracture failure needs to be conducted considering its importance in actual structures. This article proposes a new probabilistic framework for the risk and sensitivity analysis of structural fatigue failure employing cohesive zone elements. The proposed framework comprises three steps, namely finite element analysis using cohesive zone elements, response surface construction, and risk and sensitivity analysis of fatigue failure, which require several mathematical techniques and algorithms. The proposed framework is tested by applying it to an illustrative example, and the corresponding analysis results of fracture failure probability with different threshold values of a limit-state function are presented. In addition, the sensitivities of failure risk with respect to the statistical parameters of random variables are presented and their relative importance is discussed.

SEM: An Invaluable Tool in Failure Analysis

Abstract Many broken parts look similar at a glance, but examination of the failed part using advanced equipment and methods such as scanning electron microscopy (SEM) reveals much more about the origins of the failure. This article presents a case history involving SEM analysis of a trailer hitch fatigue failure.

Fatigue Failure Analyses of Three Gas Cylinders of Long-Tube Trailers

Three cylinders of a long-tube trailer leaked during an inspection and typical quality test. Thus, failure analysis was performed to determine the cause of failure. Macroscopic observation, metallographic analysis, and fracture analysis were implemented. Cylinder 1 was found leaking during the inspection. The crack was from the inner to the outer surface of the wall, and stripes from tool marks were found on the inner surface. Metallographic analysis proved that the fatigue crack originated from the tool marks in cylinder 1. Cylinders 2 and 3, which are fiber-reinforced, failed during the typical quality test, with life of approximately 13,000 cycles. The inner surface of cylinder 2 had many folds, and the fatigue crack intensified from these folds. The folds were formed by the shot blasting treatment of the cylinder. The crack of cylinder 3 was found in the position of the cylinder bottle shoulder. The inclusion on the cylinder shoulder was observed, and that on the subsurface could move to the inner surface during the hot spinning process and propagate gradually under cyclic load. The three fatigue failure analyses showed that quality examination is important during the process of cylinder production.

Fatigue failure analysis of steel wire rope sling based on share-splitting slip theory

At present, Section integrity hypothesis is mostly used to analyze the mechanical properties and identify the damage of the cable. Based on the actual situation and considering the slip effect between the wires of the cable, two simplified calculation models of the cable are proposed based on the theory of share-splitting slip, and the model parameters are obtained through experiments. Finally, on the background of Lijiang Railway Suspension Bridge in Yunnan Province, the fatigue test of the suspension cable is carried out by using the self-developed testing device which can consider both tension and bending fatigue, and the broken steel wire is further analyzed by means of electron microscopy. The results show that two simplified calculation models of wire rope sling are proposed based on the theory of share-splitting slip, which are close to the results of the original model, but greatly improve the calculation efficiency. Comparing the results of three models, it is found that the three models can reflect the development of the slip and fatigue damage of the wire rope sling, but they have some errors with the test. The fracture morphology of the steel wire is shown by electron microscopy photographs.

Weibull and Generalized Extreme Value Distributions for Wind Speed Data Analysis of Some Locations in India

Wind velocity data modeling plays a crucial role for the estimation of wind load and wind energy. Apart from these, the same modeling must also be used in the load cycle analysis of fatigue failure in slender structures to address periodic vortex shedding. Most authors fitted the entire available range of wind velocities of various locations using Weibull models. However, they did not check the validity of the model in describing the range of extreme wind velocity. In this work, the validity of Weibull models for describing parent as well as extreme hourly mean wind velocity data for four places on the east coast of India has been checked. While it predicts lower wind speeds accurately, the Weibull model has been found to become inappropriate for describing wind velocity in the range of extremes, i.e., above a certain threshold value. Therefore, this article focuses on the techniques of determining a limiting wind velocity beyond which the Weibull distribution is rendered unsuitable. In the range where the Weibull distribution fails, various extreme value distributions, such as Gumbel, Fréchet and reverse Weibull distributions have been compared, thereby determining the best estimator for each location.

Probabilistic machine learning approach to bridge fatigue failure analysis due to vehicular overloading

With the rapid development of freight transportation, truck overloading becomes very common and severe, posing a great threat to the safety of bridges, and it can even result in bridge failure. Traditional approaches investigating the overloading-induced fatigue damage on bridges, such as finite element analysis (FEA) and reliability analysis, have proven to be computationally expensive and model dependent. In this study, the prediction of fatigue failure probability of bridges due to traffic overloading was investigated by using the feedforward neural network and the Monte Carlo method. Our results show that based on a finite set of training data for the bridge under consideration, the proposed machine-learning-based approach can assist in providing an instantaneous assessment of the fatigue failure probability with high accuracy.

Thermomechanical fatigue experiment and failure analysis on a nickel-based superalloy turbine blade

Thermomechanical fatigue (TMF) experiment with dwell times on a nickel-based superalloy turbine blade was conducted. The strain field and temperature field of the test section were simulated well through adjusting gripping fixture and induction coil. In addition to the loading, heating, cooling and control subsystems, high temperature strain measurement subsystem was introduced into the experiment to ensure the accuracy of the simulation results of service condition for the test section. In the whole experiment, the service condition of the test section was reproduced. The experiment result showed that TMF crack initiated at the trailing edge of the test section and propagated along the leading edge direction. Using the scanning electron microscope (SEM), multiple crack sources at the surface of turbine blade were observed and the crack surface was oxidized seriously. In addition, based on the fractographic and metallographic observation, the mixed features of intergranular fracture and transgranular fracture were found. The interaction of oxidation, creep damage and fatigue damage is an important reason for the TMF failure of the turbine blade.

Dynamic Bayesian networks based approach for risk analysis of subsea wellhead fatigue failure during service life

Subsea wellhead is a critical component of the drilling and production system in offshore oil and gas industry. Excited by cyclical fatigue loadings due to environmental forces, the wellhead is prone to fatigue failure, which could lead to the loss of well integrity and even catastrophic accidents. Although fatigue failure probability of the wellhead carries an elevated uncertainties, it will definitely increase with the accumulation of fatigue in wellhead. This paper presents a fatigue failure risk analysis approach based on dynamic Bayesian Networks, aiming to predict the fatigue failure probability of the wellhead during service life. The proposed model can use the previously accumulated fatigue of the wellhead to probabilistically predict the present failure risk under present dynamic conditions. The practical application of the developed model is demonstrated through a case study. Adopting the predictive, diagnostic analysis techniques in the Bayesian inference, the dynamic fatigue failure probability of the wellhead at any time slices was achieved, and the most influential factors were figured out. Finally, the corresponding safety control measures are proposed to effectively mitigate the fatigue failure risk of subsea wellhead during service life.

Fatigue failure analysis of roll steel pins from a chain assembly: Fracture mechanism and numerical modeling

The current study is focused on the investigation of the failure case history pertaining to the in-service fractured steel pins of a chain assembly installed in a continuous cold drawing machine. The findings of the present investigation showed that the pins failed due to multiple-origin rotating bending fatigue under low applied load, due to the application of tensile and/or bending stresses, initiated from the pin circumference. The multiple-crack initiation is consistent to severe stress concentration conditions, as imposed by the low fillet radius attributed to the original pin design. No microstructural abnormalities were detected, that could have been correlated to the principal cause of failure. In the context of the present evaluation, analytical methods such as visual inspection, optical microscopy, SEM fractography and microhardness testing were utilized. Moreover, a finite element analysis of the assembled chain was performed in order to investigate the stress distribution during the operation of the chain and to formulate a sound quantitative approach which will assist in better understanding of chain/pin loading conditions. The primarily insufficient component design resulted in high stress concentration factors interacted with the soft annealed material used for the component and caused fatigue under the presence of fluctuating stresses.

Fatigue failure analysis of express freight sliding side covered wagon based on the rigid-flexibility model

PurposeThe car body stiffness of express freight sliding side covered wagon decreased for the sliding and the resonance vibration based on the flexible car body has affected the dynamics performances. Dynamic loading will cause fatigue cracks and eventually lead to fatigue failure of the car body. This paper aims to investigate the influence of car body flexibility on the evaluation of the failure.Design/methodology/approachIn this paper, the railway vehicles random analysis procedure (RVRAP) is employed to study the fatigue failure of the rigid-flexibility model. Following the analysis process, the rigid-flexibility model is established and four contrastive schemes for simulation analysis are designed. To verify the results, an experimental test using the real car body structure on the vibration test plant is carried out; the RMS of dynamic stress is obtained and compared with the simulation results.FindingsThe results show that the flexibility of the car body has a crucial influence on the fatigue life.Originality/valueThe reliability is verified regarding the use of RVRAP at an appropriate stage on the antifatigue design of the vehicle.

Fatigue failure analysis of a Ø14 B500C steel rebar: Metallurgical evaluation and numerical simulation

In the present work, the investigation of an early fatigue failure of steel rebar samples, of dimensions of Ø 14 mm, during standard fatigue tests is performed. The rebar failed after approximately 1.67 × 106 fatigue cycles, much below of the specified 5.0 × 106 cycles according to BS 4449 standard. The rebar tested sample exhibited a crack initiation site close to the root of a surface geometric discontinuity (“nipple”), which is considered as a stress concentration point. Light optical microscopy, microhardness testing, SEM/EDS, and numerical simulation were employed as the main investigation methods. Fractographic examination depicted the presence of clearly resolved fine fatigue striations elucidating the continuous crack growth period, under Paris law regime, followed by a dimpled ductile zone as the instant ultimate fracture zone. An evaluation of the steel inclusions (MnS, Fe-Si-Mn-Al-Mg-O) was achieved by optical metallography and SEM/EDS analysis. In addition, the results of the numerical analysis, verified that the existence of manufacturing imperfections or geometric irregularities, such as the “nipple” marking of the bar, imposed a significant stress raise up to 2 to 3.3 times of the applied mean stress level, leading to an effective reduction of the expected lifetime of the specimen.

Fatigue Failure Analysis of Cracks near the Sole Plate of a Half-Through Steel-Arch Bridge

Fatigue Failure Analysis of Cracks near the Sole Plate of a Half-Through Steel-Arch Bridge

Fatigue Failure Analysis on Precracked 304 Stainless Steel Components Repaired by Laser with Addition of Nanocomposites

Abstract The service life of components can be extended by repairing fatigue crack with laser so as to improve resource utilization rate. However, the effect of laser repair may be influenced by the instability of laser treatment. To investigate the addition of nanocomposites on the laser repairing effect, fatigue test under uniaxial tension was carried out to evaluate the fatigue life of precracked 304 stainless steel components repaired by laser. According to the results obtained from fatigue tests, a probabilistic fatigue life distribution model considered the sampling size and discreteness of data was established based on the three-parameter Weibull distribution model and three-parameter stress-life equation. The results of probabilistic fatigue life distribution model show that the fatigue life of components repaired by laser with addition of WC nanocomposites can be enhanced significantly, especially the high cycle fatigue life. Moreover, the microstructure of repaired region show that the WC nanoparticles could refine the recrystallized grains and lead to the improvement of strength and toughness of material around crack tip, which are the main reason of improved fatigue life. Meanwhile, smooth surface of repaired region caused by WC nanoparticles can also prevent the initiation of microcracks.

Fatigue damage and failure analysis of honeycomb sandwich

Fatigue behavior of sandwich structures with honeycomb core and GFRP skins is studied and failure mode is investigated through tomographic observations. The first S/N curves are presented. The second part discusses the failure modes observed during fatigue tests and responsible for stiffness decrease during fatigue tests.

A Wind Climate Dynamic Modeling and Control Using Weibull and Extreme Value Distribution System

The dynamics of wind velocity data modeling plays a crucial role for the estimation of wind load and wind energy. Apart from these, the same modeling must also be used in the load cycle analysis of fatigue failure in slender structures to address periodic vortex shedding. Most authors fitted wind velocities of various locations using Weibull model. However, they did not check the validity of the model in describing the range of extreme wind velocity, which is not clear from the usual graphical representation. In this work, the validity of Weibull model for describing parent as well as extreme hourly mean wind velocity data for four places on the east coast of India has been checked; Weibull model has been found to become inappropriate for describing wind velocity in the range of extremes.

Fatigue failure analysis of HSLA steel sheets holed by conventional and flow drilling processes

Flow drilling process produces bushes for removable joints that can be threaded by forming tapping process, reducing stages in production of components of automobile industry and in construction sheds. High strength and low alloy steels (HSLA) are employed on several applications in the processing industry. Benefits provided by these materials make them an efficient solution, since their high cold resistance allows reducing weight on structures, as well as maintaining the required mechanical properties. The present study evaluated the conventional and friction drilling processes in HSLA steel sheets with a thickness of 4.25 mm. Specimens were subjected to cyclic loadings, with no rework after holemaking, so the resultant characteristics of each process were maintained. Fractured surfaces were analyzed with a scanning electron microscope (SEM) to identify the fracture mechanism in different drilling processes. Fractographies obtained on both processes indicated that the failure mechanism occurred due to ductile fracture resulting from the nucleation, growth and coalescence of micro cavities. Keywords: High strength low alloy steel, conventional drilling, flow drilling, ductile fracture.

The fatigue failure analysis of shafts anchors of DUGG-28 generators after repair

The fatigue failure analysis of shafts anchors of DUGG-28 generators after repair

Fatigue failure analysis of HSLA steel sheets holed by conventional and flow drilling processes

ABSTRACT Flow drilling process produces bushes for removable joints that can be threaded by forming tapping process, reducing stages in production of components of automobile industry and in construction sheds. High strength and low alloy steels (HSLA) are employed on several applications in the processing industry. Benefits provided by these materials make them an efficient solution, since their high cold resistance allows reducing weight on structures, as well as maintaining the required mechanical properties. The present study evaluated the conventional and friction drilling processes in HSLA steel sheets with a thickness of 4.25 mm. Specimens were subjected to cyclic loadings, with no rework after holemaking, so the resultant characteristics of each process were maintained. Fractured surfaces were analyzed with a scanning electron microscope (SEM) to identify the fracture mechanism in different drilling processes. Fractographies obtained on both processes indicated that the failure mechanism occurred due to ductile fracture resulting from the nucleation, growth and coalescence of micro cavities.

Low Cycle Fatigue Failure Analysis of a Ni‐Based Single Crystal Superalloys at 850 °C

This paper presents a low cycle fatigue (LCF) failure analysis on [001], [011], and [111] orientations of a Ni‐based single crystal superalloys at 850 °C. Fracture mechanism and microstructure damage evolution are revealed through crystal plastic theory and fracture morphology analysis. Strain‐controlled LCF tests are conducted with the standard round bar samples; the fracture morphology and microstructure characteristics, such as the fracture mechanism and microstructure damage evolution, are observed via scanning electron microscopy (SEM), optical microscopy (OM). Crystal plastic theory is adopted to establish the crystal slip system framework, considering elastic and plastic strain rate, elastic orientation factors, and the resolved shear stress (Rss). The life prediction model is established based on Levkovitch damage theory, theoretical analysis, and test results has good consistency. The results demonstrate that fatigue life on [001] orientation is the longest, but considering the effects of stress amplitude and Young's modulus, [011] orientation exhibits the best fatigue performance. Under LCF loading, the initial crack starts to occur on surface of specimen if the internal defects are small and negligible; in contrast the initial crack starts to occur in subsurface or internal of specimen if the microstructure defects exist in material. The LCF cross‐section of SC belongs to the quasi‐cleavable type, the crack propagates in the direction of {111} plane and then extends along the vertical stress axis until the specimen fractures.

Fatigue life evaluation of welded joints in steel bridge considering residual stress

Welded joints are widely used to connect structural components in steel truss bridges. Weld residual stresses (WRS) and weld residual stresses relaxation (WRSR) have notable influences on fatigue crack propagation in welded joints, and therefore affect the fatigue life of welded joints. Failing to properly consider the effect of WRS and WRSR in fatigue evaluation may lead to inaccurate results. This study presents a fatigue life prediction model based on the elastic fracture mechanics, with consideration of the WRS and WRSR. The solution for stress intensity factor caused by cyclic loading and WRS is derived. The WRS-induced stress intensity factor is calculated using a weight function technique. Fatigue tests of eight welded joint specimens are implemented, and the fatigue failure analysis of specimens is conducted. The proposed fatigue life prediction model is validated against fatigue test results of welded joints. By considering both WRS and WRSR, the model provides a prediction of fatigue life with a maximum error of 14%. Finally, the validated model is employed to investigate the fatigue life of a real bridge. The fatigue life is underestimated by 17% by considering WRS but not considering WRSR; the fatigue life is overestimated by 49% by neglecting WRS and WRSR.