Development Of Fatigue Cracks Research Articles

Fatigue cracking detection in steel bridge girders through a self-powered sensing concept

Development of fatigue cracking is affecting the structural performance of many of welded steel bridges in the United States. One of the main sources of fatigue cracking is out-of-plane distortion occurring at connections of transverse structural members and longitudinal girders. Distortion-induced fatigue cracks mostly occur in older bridges with members prone to fatigue. Prediction of secondary stresses in these members is difficult using conventional design specifications. This limitation suggests the necessity of utilizing new strategies to analyze the damage caused by distortion-related cracking. This study presents a new approach for detection of distortion-induced fatigue cracking of steel bridges based on the interpretation of the data provided by a newly developed self-powered piezo-floating-gate (PFG) sensor. The PFG sensors are empowered using piezoelectric transducers through harvesting energy from the mechanical loading experienced by the structure. In order to assess the performance of the proposed sensing system, three-dimensional finite element models were developed and the structural response of the girder was subsequently obtained. The fatigue life of the girder was determined based on J-integral concept and Paris Law. Several damage states were defined by extending the fatigue crack lengths. Thereafter, features representing the PFG sensor output were extracted from the strain data for different sensing nodes to detect the damage scenarios. Furthermore, a new data fusion concept based on the effect of group of sensors was proposed to improve the damage detection performance. The results indicate that the proposed method is capable of detecting different damage progression states. This is specifically evident for the sensors that are located close to the damage location. The acceptable performance of the proposed sensing system implies its applicability for other modalities of infrastructure/structural health monitoring (I/SHM).

Influence of the holes spacing in bolted connections on propagation of fatigue cracks in steel bridge girders

The analysis of the development of fatigue cracks conducted using a computer program requires a simple 2D surface model to be applied. The paper presents an example of creating four calculation models of a steel girder, two of which have been implemented in FRANC2D software. A 400 mm-high and 5.60 m-long steel I-beam was studied. Analysis of the development of fatigue cracks around the I-beam fastener holes was conducted for two selected computation models. What has been obtained as the result of the analysis was an answer to the question about the influence of the location of fastener holes on the propagation rate of hypothetical fatigue cracks. This answer was obtained indirectly, i.e. through the analysis of the value of stress intensity factor K.

Features of fatigue crack development in cast steel parts of railroad structures

The paper presents the results of an experimental study on the features of fatigue crack development in 20GFL grade low-alloy steel of a freight car bolster with simulation of the operating process of loading. The experiment results are compared with calculations using the fatigue failure diagram for this steel.

Accelerated pavement testing of thin RCC over soil cement pavements

Three full-scale roller compacted concrete (RCC) pavement sections built over a soil cement base were tested under accelerated pavement testing (APT). The RCC thicknesses varied from 102mm (4in.) to 152mm (6in.) and to 203mm (8in.), respectively. A bi-directional loading device with a dual-tire load assembly was used for this experiment. Each test section was instrumented with multiple pressure cells and strain gages. The objective was to evaluate the structural performance and load carrying capacity of thin RCC-surfaced pavements under accelerated loading. The APT results generally indicated that all three RCC pavement sections tested in this study possessed very high load carrying capacity; an estimated pavement life in terms of equivalent single axle load (ESAL) for the thinnest RCC section (i.e., RCC thickness of 102mm) evaluated was approximately 19.2 million. It was observed that a fatigue failure would be the primary pavement distress type for a thin RCC pavement under trafficking. Specifically, the development of fatigue cracking was found to originate from a longitudinal crack at the edge or in the center of a tire print, then extended and propagated, and eventually merged with cracks of other directions. Instrumentation results were used to characterize the fatigue damage under different load magnitudes. Finally, based on the APT performance of this experiment, two fatigue models for predicting the fatigue life of thin RCC pavements were developed.

The Effect of Porosity on Fatigue of Die Cast AM60

AM60 high-pressure die castings are known to contain significant porosity which can affect fatigue life. We have studied this using samples drawn from prototype AM60 shock towers by conducting strain-controlled fatigue tests accompanied by X-ray computed tomography analysis. The results show that the machined surface is the preferential location for fatigue crack development, with pores close to these surfaces serving as initiation sites. Fatigue life shows a strong inverse correlation with the size of the fatigue-crack-initiating pore. Pore shape and pore orientation also influence the response. A supplemental study on surface roughness shows that porosity is the dominant factor in fatigue. Tomography enables the link between porosity and fatigue crack initiation to be clearly identified. These data are complemented by SEM observations of the fracture surfaces which are generally flat and full of randomly oriented serration patterns but without long-range fatigue striations.

Fatigue reliability model for steel railway bridges

Railway bridges are exposed to repetitive high stress due to the live load which may lead to failure even when the stress level is lower than the allowable stress. Therefore, components and connections need to be analysed for possible damage caused by fatigue. The basic approach for estimating the remaining fatigue life of a structure element is to use S–N curves. However, those laboratory specimens tested with a constant-amplitude stress range show a wide range variability in the results. This means that fatigue resistance has to be considered as a random variable. If load and resistance parameters are random variables, structural performance should be measured in terms of reliability. The objective of this study is to present a reliability model for the fatigue limit states demonstrated on a typical steel railway bridge. The results from the reliability analysis for the fatigue limit state are presented for various time periods from 10 to 100 years and three cases of operating conditions. In each considered case of load, the lowest reliability indices were obtained for the riveted angle in the stringer-to-floor-beam connection which means that this member has the highest probability of fatigue crack development in the entire bridge.

Rozwój pęknięć zmęczeniowych w cyklicznie zginanych próbkach z pachwinowym złączem spawanym

Rozwój pęknięć zmęczeniowych w cyklicznie zginanych próbkach z pachwinowym złączem spawanym

Fatigue crack development in a low-carbon steel. Microstructure influence. Modelling

Fatigue in a low-carbon steel with ferrite and pearlite microstructure is investigated through testing of three groups of specimens. Two of the groups consist of cylindrical specimens subjected to tension-tension and rotating-bending fatigue; in this case surface microstructurally-short crack propagation is monitored by acetate-foil replica technique. The third group of specimens includes flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure. The obtained data for pure-bending fatigue show higher crack growth rates and shorter fatigue lifetimes than those found for rotating-bending fatigue. In comparison, the registered tension-tension fatigue data present the lowest crack growth rates, due to much lesser loading than that applied at rotating-bending and pure-bending fatigue. Based on data obtained, a Parabolic-linear model “Crack growth rate – Crack length” is used for describing and predicting adequately short crack propagation under the specified three types of fatigue. The model is supported by a comparison between the predicted and the actual fatigue lifetimes.

К вопросу о совершенствовании расчетов рельсов на прочность

From the middle of the last century, beginning with the works of Professor M. F. Verigo, rejection of yield strength at static loading and transition to the fatigue strength under cyclic loading were accepted and made as the main conceptual provisions in calculations of rails for strength when choosing allowable stress levels. Thus, calculation of rails for the strength should be carried out as all strength calculations of machine parts under loadings, variable in time. Further progress in the development of methods for calculating the strength of rails under varying stresses correspond with the implementation in practice of calculations of probability concepts. In article three types of calculations of rails for strength, having the greatest practical value now, are considered namely: calculation of cyclic strength of rails at prevention of formation of fatigue cracks in a foot and a neck; calculation of cyclic contact strength at prevention within a resource (or probability regulations) of formations of cracks of contact fatigue in a head of regulated shape and size, and also calculation of durability of rails taking into account existence of fatigue cracks at prevention of complete brittle fracture of all section. The main attention is paid to determination of allowable stresses when calculating cyclic strength, as well as to the accounting of a large-scale factor when calculating cyclic contact strength, and also to determination of duration of development of fatigue cracks and their critical size when calculating durability. The existing normative documents concerning calculations of rails on strength have become outdated and demand development of new ones.

Imperfect Inspection Policy for Systems with Multiple Correlated Degradation Processes

This paper develops an imperfect inspection policy for systems subject to multiple correlated degradation processes. The degradation processes are characterized by a multivariate Wiener process, and the dependency among the processes is described by a covariance matrix. A failure occurs when any of the degradation level exceeds a specific threshold. As degradation-based failures are usually dormant, inspection is required to detect the failures and prevent potential losses. The inspection is imperfect, in such a way that a failure may not be discovered at an inspection. Optimal inspection interval is obtained by minimizing the long run cost rate. In addition, the properties of the optimal inspection interval are studied and the upper and lower bounds of the optimum value are achieved. An example of fatigue crack development is used to illustrate and evaluate the inspection policy. The results imply that inspection accuracy contributes significantly to reducing the operating cost and more efforts should be made to improve the inspection quality.

Fatigue cracking of aluminium alloy AlZn6Mg0.8Zr subjected to thermomechanical treatment

The paper presents the results of experimental test of the fatigue of aluminium alloy type AlZn6Mg0,8Zr exposed to various low temperature thermomechanical treatment. Basquin’s characteristics of fatigue have been determined in mechanical test on smooth specimens at a simple state of loading in conditions of alternating bending. The tests were carried out on a fatigue test stand constructed by the authors – MZGS 100. The development of fatigue cracking has been described based on metallographic and fractographic investigations of the fractured samples making use of a scanning electron microscope (SEM). The results of qualitative microfractography of the tested samples in the low-cycle temporal range of fatigue strength revealed fractures of the transcrystalline quasi-cleavage type. It has also been found that local effects of intercrystalline brittle cracking of this type do occur.

Specification of metal damageability mechanism of long-operated steam line welds

The features of metal damageability of welded joints of steam lines, long operated under creep conditions, made of steels 15H1M1F and 12H1MF were specified. Damageability classification, consisting of five stages was proposed. The formation mechanisms of creep pores and fatigue cracks were considered. It was found that creep pores are formed most intensively in the area of incomplete recrystallization of heat-affected zone of welded joints made of heat-resistant steels. It was revealed that the metal destruction of welded joints has predominantly brittle intergranular nature. In the destruction area, the body of α-phase grains is affected by individual or locally grouped creep pores. It was found that the origination and development of fatigue cracks on the inner surface of steam lines have locally grouped nature. Their development is characterized by the absence of branching. Specification of metal damageability features of long-operated welded joints of steam lines will allow to reduce the damageability intensity, which provides increased lifetime.

Анализ моделей развития усталостных трещин

Анализ моделей развития усталостных трещин

Monitoring of Fracture of Welded Joints in Hazardous Facilities by Acoustic Emission under Static and Cyclic Loadings

Background/Objectives: Acoustic emission under static and fatigue failure of steel welded joints 9MnSi5 in a zone of thermal influence is investigated at temperatures of 296 K and 233 K. Methods/Statistical Analysis: Flat samples with a welded seam, weakened by a notch in the Zone of Thermal Influence (ZTI), were made for static and cyclic tests of steel 9MnSi5 monoaxial stretching. Type of welded connection: end-to-end, double-sided with full pen of chamfers (C255 type). Stress risers in tension were manufactured by a sparkerrosion wire-cutting machine Sodick AG400L LN2W with high precision of positioning and repeatability of the shape of notches. All static and dynamic tests were carried out by servohydraulic machine Instron 8802. Findings: The received results show that origin and development of fatigue cracks are significantly easier at positive temperatures, than at negative. This fact testifies that origin of fatigue cracks on real objects, most likely, happens during the summer period, and their sudden development (break down) - in winter. That means that the low temperature of operation is not the defining factor for catastrophic failure. This fact should be born in mind in the analysis of AE registered by monitoring systems during the summer and winter period in case of controlling objects working in the conditions of cyclic loadings. Applications/Improvements: It is shown that at all types of tests; the range of AE-signals is shifted towards lower frequencies as the moment of failure approaches. Criteria parameters based on the signal wave form and its power spectral density are suggested with the aim of using them in automatic monitoring systems.

Research of the crack development process in a nickel heat-resistant alloyin the process of testing

A method of direct observation of the process of low-cycle fatigue crack development during bend loading directly in the chamber of a scanning electronic microscope is described in the paper. It was found that the presence of slip bands near the apex of the crack is the determining factor of crack development in a specimen made of a nickel heat-resistant alloy with a monocrystalline structure. The fracture in the area of the crack apex was found to be of a brittle nature. In the case of crack propagation arrest before an obstacle an area of plastic deformation forms at some distance from the apex of the crack, with branches from the main crack.  Crystallographic orientation of the specimen was determined using the X-ray structure analysis and the Shmid factor was calculated for glide planes along which the fracture took place and secondary cracks were formed. The prevailing direction of the crack development was determined by slip systems with the value of Shmid factor maximum for the given conditions. It is shown that the described method of observation of the material structure and crack propagation under low-cycle fatigue makes it possible to obtain unique information about the fracture pattern and the impact of the material’s structure on its characteristics. The results can be used to determine the choice of the most favorable crystallographic orientation of the monocrystalline structure of the material of gas turbine engine blades relative to their geometry.

Research of the crack development process in a nickel heat-resistant alloyin the process of testing

A method of direct observation of the process of low-cycle fatigue crack development during bend loading directly in the chamber of a scanning electronic microscope is described in the paper. It was found that the presence of slip bands near the apex of the crack is the determining factor of crack development in a specimen made of a nickel heat-resistant alloy with a monocrystalline structure. The fracture in the area of the crack apex was found to be of a brittle nature. In the case of crack propagation arrest before an obstacle an area of plastic deformation forms at some distance from the apex of the crack, with branches from the main crack. Crystallographic orientation of the specimen was determined using the X-ray structure analysis and the Shmid factor was calculated for glide planes along which the fracture took place and secondary cracks were formed. The prevailing direction of the crack development was determined by slip systems with the value of Shmid factor maximum for the given conditions. It is shown that the described method of observation of the material structure and crack propagation under low-cycle fatigue makes it possible to obtain unique information about the fracture pattern and the impact of the material’s structure on its characteristics. The results can be used to determine the choice of the most favorable crystallographic orientation of the monocrystalline structure of the material of gas turbine engine blades relative to their geometry.

Models of initiation fatigue crack paths proposed by Macha

Professor E. Macha devoted his academic life to solving the problems connected with random multiaxial fatigue in components of machines and structures. In his studies he formulated stress, strain and energy criteria related to critical plane concept. He also proposed several methods to determine critical plane position. In particular, he formulated and verified weight functions applied in order to determine critical plane position. The variance method constituted another significant contribution to the development of methods for determining critical plane position. Apart from these criteria, Macha was exploring energy approach in fatigue of materials and the development of fatigue cracks. He has also observed that strain characteristics multiplied by stress amplitude determined at specimen half-life are applied to estimate fatigue life using energy criteria. However, for cyclically instable materials, stress amplitude value may differ a lot; therefore he proposed the method to determine energy fatigue characteristics directly from experimental research.

Microstructure Evolution Associated with a Superior Low-Cycle Fatigue Resistance of the Fe-30Mn-4Si-2Al Alloy

The microstructure evolution responsible for the superior low-cycle fatigue (LCF) resistance (N f > 8000 cycles at a total strain range of 2 pct) was studied in the Fe-30Mn-4Si-2Al alloy susceptible to strain-induced martensitic transformation. To investigate the microstructure effect on the LCF behaviors of the alloy, a series of interrupted fatigue tests at total strain range of 2 pct were carried out. A characteristic softening stage followed by the secondary hardening was observed during cyclic loading of the studied alloy. This softening is associated with the strain localization caused by persistent Luders bands formation and the transformation of Luders bands into strain-induced e-martensite is found to have a key role in the delayed fatigue fracture of the alloy being studied. Therefore, the continuous transformation process involving Luders bands and e-martensite formation associated with intermediate stacking fault energy (SFE) (γ SF of 14 mJ/m2) is necessary to prevent the rearrangement of dislocations into walls/channels and substructures inherent to high-SFE (γ SF higher 20 mJ/m2) alloys capable to accelerated fatigue damage. However, sluggish martensite transformation kinetics is necessary to delay the formation of the e-martensite associated with the development and propagation of fatigue crack in alloys with very low SFE.

Failure of metals II: Fatigue

In this interpretive review, fatigue in metallic systems is considered primarily from the perspective of interactions between the microstructure, the deformation mode and the mechanical state at both low and high temperatures. In Part 1 the development and early propagation of cracks is considered in terms of the basic damage mechanisms and the relative size of the crack with respect to applicable microstructural feature(s). In this section, a multistage grain scale approach to microstructure-sensitive fatigue crack formation and growth is presented which uses Fatigue Indicator Parameters (FIPs) to correlate these processes. Various FIPs parameters are discussed in terms of their indication of the state of fatigue. The development and early crack propagation is considered in the context of microstructure and notches, and probabilistic aspects of the notch fatigue problem are discussed. These features are integrated into a systematic approach for the selection of fatigue resistant microstructures for given applications. In Part 2, attention is focused on Ni-base superalloys and the interaction between oxidation, creep and microstructure (including coatings) in the formation and propagation of cracks. This part of the overview addresses both experimental and modelling aspects. Methodologies based upon fundamental physical processes are presented for understanding and predicting the development and propagation of fatigue cracks, including effects of sequential oxide type formation and of creep on either restraining or accelerating damage by oxidation. The variable fatigue resistance of discs in jet engines is seen to depend upon the variability of microstructure and its influence on the severity of creep/oxidation interactions. All of these factors are considered in the practical case where both temperature and loading parameters vary simultaneously (thermomechanical fatigue). A physics-based life prediction model considering the interactions of deformation and environmental damage is reviewed in terms of its applicability to life prediction of components.

Simulation of multicentric destruction with regard for inhomogeneous distribution of rated voltage

The algorithm simulating the initiation and development of fatigue cracks in the joint weld on the example of the mount of the mobile platform and the supporting cowl of Gottwald NMK 170 EG (63 tons) crane rotator, the destruction of which occurred in 2011 in the territory of “Novorossiysk commercial sea port” OJSC (NCSP), is worked out. The proposed algorithm takes into account the non-uniform weld geometry affecting the value of the stress concentration factor and the uneven distribution of the rated stress related to the joint design features. The modeling process is based on the experimentally established fact that the geometry profile of the junction between the weld and the base metal along the welded joint changes randomly. As a result, the fatigue flaking along the weld lacks any regular pattern. Under the cyclic loading, the development of the destruction is a sequential process of the crack nucleation in different parts of welded joints along the weld, single surface cracks growth under the fracture mechanics laws, nucleation of new cracks, and crack coalescence in the adjoining sections. At the coalescence of the crack edges, the contour profile is changing which leads to increasing the growth rate of the crack depth. In the future, the development of the multicentric destruction continues until the critical crack. The damage accumulation and destruction development rate depends on the uniformity coefficient of the stress concentration along the weld. The implementation of the proposed model allows solving the problems of determining the frequency of node diagnosis and the points with the utmost probability of fatigue cracks initiation, as well as validating the possible methods of structural reinforcement.