Fatigue Life Estimation of Structural Components Using Macrofibre Composite Sensors

Abstract: Recently, a number of different structural health monitoring (SHM) techniques have been developed for the online inspection of air, land and sea engineering structures. Various smart materials are employed for detecting eminent damage in situ. Fatigue cracks in structural components are the most common cause of structural failure when exposed to fatigue loading. Fatigue design of structural components is typically accomplished either using a set of stress cycle (S‐N) data obtained from prior fatigue tests or using the fracture mechanics approach. The fracture mechanics approach considers the fatigue life of structures as a summation of crack initiation life and crack propagation life. The stress intensity factor (SIF) is required for the estimation of fatigue crack propagation life from the linear elastic fracture mechanics (LEFM) perspective. However, the accurate prediction of the SIF is difficult especially when the geometry or the boundary conditions of a structure becomes complex. In this study, a SHM application of macrofibre composite (MFC) sensors is presented. A set of MFC sensors is used for the real‐time measurement of the SIF. The measured values of the SIF are later used for the prediction of the crack propagation life. The impedance‐based SHM technique using the same set of MFC sensors is employed for the detection of crack initiation life.     

In situ analysis of delayed fibre failure within water-aged GFRP under static fatigue conditions

A stress corrosion model has been applied to the microscopic analysis of the delayed fibre failure processes occurring within a water-aged unidirectional glass/epoxy composite under static fatigue loading (i.e. relaxation). By means of in situ microscopic observations, the individual fibre failures within an elementary volume located on the tensile side of the flexural specimens have been quantified as a function of time under various applied strain levels. It was found that the time dependence of the in situ fibre failure processes obeyed a stress corrosion model. From the microscopic observations, it was possible to assess consistent values of the parameters characterising the in situ fibre strength distribution and the subcritical crack propagation law. A comparison with separate static fatigue experiments using unimpregnated fibre bundles demonstrated that the specific physico-chemical environment encountered by the glass fibres within the aged epoxy matrix can induce significant changes in the subcritical crack propagation rates, as compared to stress corrosion cracking data collected in humid air.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

Effect of crack-growth mechanism on the prediction of fracture load of adhesive joints

This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone, rather than a single, sharp crack, and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy Cybond 4523GB (American Cyanamid), there was approximately 5 mm of subcritical crack propagation prior to final fracture. Fracture-load predictions based on the initial uncracked geometry made in previous papers were unaffected by this small change in geometry. For joints bonded with the rubber-toughened epoxy Permabond ESP 310, approximately 50 mm of subcritical crack propagation was observed. It was again found that predictions made in previous papers on the basis of the initial geometry gave a good estimate of the final fracture load even though this subcritical crack propagation significantly altered the geometry, and thus the applied energy release rates. The effect of shear deformations of the adherends was also investigated, and it was found that shear deformations could be neglected in engineering calculations for joints subject to remote tensile loading.     

ENVIRONMENTAL EFFECTS ON CRACK PROPAGATION IN ALUMINIUM ALLOYS

Abstract Crack propagation rates have been measured in two aluminium alloys under cyclic and static loading, in air, and in salt solution. On the basis of these results, a model is proposed, whereby corrosion fatigue crack propagation may be interpreted in terms of fatigue and static stress corrosion characteristics. Two interacting processes are operative; one is "stress assisted dissolution", which tends to inhibit mechanical failure by crack blunting and microbranching. The other is "environment assisted fracture" which occurs too rapidly for dissolution to occur. One or other of these processes is always observed to be dominant. This proposal is discussed in relation to other recent models for corrosion fatigue cracking. The effects of frequency, waveform and mean stress variations are also considered.     

Effect of Cold Drawing on Microstructure and Corrosion Performance of High-Strength Steel

This paper deals with the effect of cold drawing on a high-strength steel in wire form with pearlitic microstructure. Cold drawing produces a preferential orientation of the pearlite lamellae aligned parallel to the cold drawing direction, resulting in anisotropic properties with regard to fracture behaviour in air and aggressive environments (stress corrosion cracking). While the hot rolled bar has a randomly oriented microstructure in both transverse and longitudinal sections, the fully drawn wire presents a randomly oriented appearance in the transverse cross-section, but a marked orientation in the longitudinal cross-section. These microstructural characteristics affect the time-dependent behaviour of the steels when a crack is present in a corrosive or hydrogen environment and influences both the subcritical crack growth rate, the time to failure and the crack propagation path. It is shown that in the strongly drawn steels the crack changes its propagation path, and a micromechanical model is proposed to explain this behaviour. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of different fluids on the static fatigue of a porous rock: Poro-mechanical coupling versus chemical effects

Characterization of time-dependent brittle deformation is important for estimating the long-term stability of rock structures in shallow environments such as underground mines which are exposed to water or in a water vapor environment. The poro-mechanical (pore pressure) and chemical (stress corrosion reactions) effects of fluids at crack tips are responsible for the subcritical propagation of microcracks. This brittle creep results in the weakening of porous rocks over time (i.e., static fatigue). The focus of this work was to characterize using very long duration uniaxial multi-step creep tests, both the fluids effects on subcritical stress corrosion cracking in a porous rock (oolithic iron ore). Different fluids with increasing levels of chemical influence (oil, ethanol and water) were tested and both acoustic and mechanical properties were investigated. The evolution of the cumulative number of AE (Acoustic Emission) events, which reproduces the shape of the creep curves very efficiently, and static elastic properties indicate that micro-cracking plays the main role in the creep process. The poro-mechanical effect, which is predominant under partially saturated (water or ethanol) conditions, decreases the rate of subcritical cracking through capillary attraction forces. These forces harden porous rocks by modifying the effective stress state, increasing the activation energy barrier of the stress corrosion process and the fracture toughness and decreasing the stress intensity factor. The chemical effect of fluids is related to minerals developing stress corrosion reactions at crack tips which enhance subcritical cracking. Immersion in water/ethanol annihilates capillary forces and decreases the activation energy of the chemical reactions, thus increasing dilatancy, the rate of stress corrosion cracking, AE activity and rock weakening. Under saturated conditions, the time-dependent strength and time to failure increase as the chemical influence of the interstitial fluid decreases but the cumulative number and energy of AE at failure do not seem to be influenced by the chemistry of fluids. The short-term strength and deformability are also influenced by the level of chemical influence of the interstitial fluid but to a less remarkable extent because of the slow rate of chemical reactions. Water is therefore the most effective chemical agent promoting stress corrosion of iron ore among the fluids tested in our study.     

Corrosion fatigue and electrochemical behaviour of steel wires used in bridge cables

In‐service bridge wires often fail prior to the design life subjected to alternating stresses and environmental erosion. In this paper, a novel corrosion fatigue test device, integrating fatigue testing machine and electrochemical accelerated corrosion assemblies, was developed to characterize the corrosion fatigue and electrochemical behaviour of the wires. Using the developed device, corrosion fatigue tests of corroded bridge wires under different corrosion and loading conditions were conducted. Electrochemical characteristics, corrosion fatigue behaviour, failure mechanism, and so forth were investigated according to electrochemical measurements, fracture morphologies and the lifetime of wires. Results evidence the synchronization of corrosion and fatigue and show the accelerated corrosion due to static and fatigue stresses. Additionally, cracking and fracture induced by multiple crack initiation was dominant in corrosion fatigue of corroded wires, and the coexistence of multiple corrosion pits decreased the lifetime significantly.     

Determination of the Stress Intensity Factor of an elliptical breathing crack in a rotating shaft

The failures due to the propagation of fatigue cracks are one of the most frequent problems in rotating machines. Those failures sometimes are catastrophic and are sufficient to provoke the loss of the complete machine with high risks for people and other equipments. When a cracked shaft rotates, the breathing mechanism appears. The crack passes from an open state to a close state with a transition in which a partial opening or closing of the crack is produced. In this work, a new general expression that gives the Stress Intensity Factor (SIF) along the crack front of an elliptical crack in a rotating shaft in terms of the crack depth ratio, the crack aspect ratio, the relative position on the front and the angle of rotation has been developed for linear elastic materials. By the moment, no expressions of the SIF in term of these variables have been found in the literature. To this end, a quasi-static 3D numerical model of a cracked shaft with straight and elliptical cracks subjected to rotary bending using the Finite Element Method (FEM) has been made. To simulate the rotation of the shaft, different angular positions have been considered. The SIF in mode I along the crack front has been calculated for each angular position of the cracked shaft and for different crack geometries. The expression results have been compared with solutions obtained from the literature. It has been found that they are in good agreement. The model has been applied to other crack geometries with good results. The obtained SIF expression allows studying the dynamic behavior of cracked shafts and can be used to analyze the crack propagation.     

Fatigue design of wire-wound pressure vessels using ASME-API 579 procedure

The wire winding of high pressure vessels is a technique usually applied to introduce initial compressive stresses in the inner core of the vessel, with the aim to improve the fatigue life under cyclic pressure conditions. In this work, the procedure followed to calculate the number of design cycles is presented, using the fracture mechanics approach and the structural integrity concepts. In particular, the API 579-1/ASME FFS-1 procedure has been used to analyse the structural integrity of the vessel through the crack propagation stage. Starting from a postulated internal semi-elliptical crack the number of design cycles is determined, the flaw aspect ratio is updated and the structural integrity of the cracked vessel is evaluated using the Failure Assessment Diagram (FAD). Different propagation laws, which take into account for negative stress intensity ratio factors R = K_min/K_max < 0, are reviewed, because of their high influence on the fatigue life of wire-wound vessels. In addition, this paper presents a number of useful expressions to calculate the stress intensity factor (SIF) for internal semi-elliptical cracks in wire-wound pressure vessels, in order to carry out the numerical integration of the number of cycles, updating the flaw aspect ratio, during the fatigue crack growth.     

Influence of Inhibitor Concentration on Corrosion Fatigue Crack Initiation and Propagation

The paper reports the effect of0.01,0.1 and 1%NaNO2, a passive inhibitor, on corrosion fatigue (CF) crack initiation and propagation for a low strength structural steel A537 in 3.5%NaCl aqueous solution. The experimental results show that inhibitor increases the required cycles of CF crack initiation effectively, and this effect increases with increasing inhibitor concentration.However, there is nearly no effect of NaNO2 on CF crack propagation. The same CF crack propagation rate was found in all kinds of solutions. The results also indicate that the passive time in 1%NaNO2 solution during plastic deformation is much longer than cyclic time. NaNO2 passivates the specimen sudece and repairs passive film damaged by cyclic loading during the crack initiation. while the passsive film is not formed fully due to continuous plastic deformation at the crack tip during the CF crack propagation, which is much different from that in the stress corrosion cracking and general corrosion     

Stress corrosion cracking model in 7075 aluminium alloy

The stress corrosion cracking is a typical fracture process in metals and alloys. Among aluminium alloys, the 7075 alloy, presents a high performance in the mechanical properties but it is susceptible to stress corrosion cracking. This paper presents a semiempiric model of crack growth by stress corrosion cracking for the above alloy. This model only uses macroscopic parameters from fracture mechanic theory and experimental tests which are easy to obtain. The model quantifies the fissure rate related to environmental condition, microstructure and loading level, permitting the evaluation of the crack growth process at different environmental conditions and heat treatments. The model results are compared with the experimental data obtained. The theoretical model reproduces adequately the stress corrosion cracking process for the 7075 alloy.     

金属裂纹板复合材料修补结构的超奇异积分方程方法

根据应力强度因子在线弹性范围内具有可叠加性,将金属裂纹板复合材料修补结构进行简化,在表面裂纹线弹簧模型的基础上,建立了基于超奇异积分方程的Line-Spring模型。利用第二类Chebyshev多项式展开的方法,将超奇异积分方程转化为线性方程组,推导出以裂纹面位移表示的应力强度因子表达式,得到了裂纹尖端应力强度因子的数值解,并利用虚拟裂纹闭合法加以验证。参数分析确定了影响对称修补裂纹板应力强度因子的两个主要参数：胶层界面刚度和补片与金属板刚度比,为胶接修补结构的承载能力分析以及改进设计提供理论依据。     

Fatigue and fracture paths in cold drawn pearlitic steel

This paper analyses the influence of microstructural anisotropy of a progressively drawn pearlitic steel (orientation of pearlitic lamellae in the drawing direction) on the microscopic and macroscopic evolution of cracking paths produced by fatigue and fracture. The fatigue crack path is always contained in the transverse section of the wires, i.e., the subcritical propagation develops under a global mode I, so that the main crack path is associated with mode I and some very local deflections take place to produce a roughness in the fatigue crack path depending on the drawing level. The fracture crack path evolves from a global mode I propagation following the transverse plane in slightly drawn steels (including the hot rolled bar that is not cold drawn at all) to a global mixed-mode propagation associated with crack deflection in intermediate and heavily drawn steels (the latter with a strong mode II component), the deviation angle being an increasing function of the drawing degree in the steel.     

800MPa级钢悬臂弯曲加载低周腐蚀疲劳表面裂纹扩展特性研究

在悬臂弯曲加载方式下,研究了800MPa级10CrNiMo结构钢在3.5%NaCl水溶液中的低周腐蚀疲劳表面裂纹扩展特性,采用金相显微镜、扫描电子显微镜及X射线能谱仪观察并分析了腐蚀疲劳试样断口,与空气中相同加载方式下该材料的低周疲劳表面裂纹扩展速率进行了比较,结果表明,在表面裂纹扩展初期,在3.5%NaCl水溶液中的表面裂纹扩展速率较空气中的快,但随着总应变范围的增加,这种快的趋势在逐渐减小,当总应变范围达到约1%时,在两种环境中的表面裂纹扩展速率基本相当;在表面裂纹扩展的后期,裂纹内部聚集了大量的腐蚀产物,该腐蚀产物主要为铁的氧化物,氧化物的存在增加了裂纹扩展的闭合效应,减缓了表面裂纹的扩展。     

Finite fracture mechanics analysis of crack onset at a stress concentration in a UD glass/epoxy composite in off-axis tension

The presence of stress concentrations at holes and notches is known to reduce the strength of composite materials. Due to complexity of the damage processes at a stress raiser in a composite, different modeling approaches have been developed, ranging from empirical point and average stress criteria to involved damage mechanics or cohesive zone-based models of failure. Finite fracture mechanics approach with a coupled stress and energy failure criterion, recently developed and applied mainly to cracking in homogeneous isotropic materials, allows predicting the appearance and propagation of a crack using material strength and toughness characteristics obtained from independent tests. The present study concerns application of the finite fracture mechanics to the analysis of cracking at a notch in a UD glass/epoxy composite subjected to tensile off-axis loading. Based on UD composite strength and intralaminar toughness characterized by separate tests, finite fracture mechanics analysis provided conservative estimates of crack onset stress at the notch.     

Phenomenology of fracture in sintered alpha silicon carbide

Crack propagation mechanisms in a sintered alpha silicon carbide were studied as a function of initial flaw size, temperature, loading rate and applied stress. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw (IIF) technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 40m were strength controlling. The flexural strength was found to be independent of temperature (20 to 1400° C) and the fracture faces did not show the presence of subcritical crack growth (SCG). Preliminary results from stress rate testing also failed to show the presence of SCG in tests made at 1200° C in air. In contrast, flexural stress rupture tests carried out at 1200 and 1300° C in air using precracked specimens indicated the materials susceptibility to time-dependent deformation and showed the presence of SCG. Fractographic evidence for transgranular crack propagation during fast fracture (catastrophic failure) and intergranular crack propagation during SCG is presented.     

Bursting of a corn silo – An interdisciplinary failure analysis

A corn silo made from corrugated sheet was the subject of a complete structural failure during standard operation. The main goals of the investigations performed were to identify possible causes for the accident (i.e., list of hypotheses), to choose the most probable one and to provide evidence by experimental and theoretical analysis. Metallographic analysis of the cracked sheet showed significant thickness reduction due to corrosion. The standard strength analysis resulted in a generally high stress level and a high likelihood of cracking in the corroded area. However, the bursting of the silo could not be explained alone based on this calculation. Therefore, with help of fracture mechanics it was assessed whether an unstable propagation of the initial crack was realistic. The general results of this study led to the conclusion that the bursting of the silo was most probably due to unstable propagation of a crack originated in the corroded area of the sheet metal near the inspection window.     

Near-threshold fatigue propagation of physically through-thickness short and long cracks in a low alloy steel

In this paper, the near-threshold fatigue behavior of physically through-thickness short cracks and of long cracks in a low alloy steel is investigated by experiments in ambient air. Physically through-thickness short fatigue cracks are created by gradually removing the plastic wake of long cracks in compact tension specimens. The crack closure is systematically measured using the compliance variation technique with numerical data acquisition and filtering for accurate detection of the stress intensity factor (SIF) at the crack opening. Based on the experimental results, the nominal threshold SIF range is shown to be dependent on the crack length and the characteristic of the crack wake which is strongly dependent on the loading history. The effective threshold SIF range and the relation between the crack propagation rate and the effective SIF range after the crack closure correction are shown to be independent on crack length and loading history. The shielding effect of the crack closure is shown to be related to the wake length and load history. The effective threshold SIF range and the relationship between the crack growth rate and the effective SIF range appear to be unique for this material in ambient air. These properties can be considered as specific fatigue properties of the couple material/ambient air environment.     

Sickle-shaped cracks in metallic round bars under cyclic eccentric axial loading

In the present paper, the fatigue propagation of an initial sickle-shaped surface crack in a metallic round bar under eccentric axial loading acting perpendicular to the crack plane is examined. Firstly, the stress-intensity factor (SIF) along the crack front is determined through a three-dimensional finite element analysis and the one-quarter point displacement method, for different values of the loading eccentricity. Then, the fatigue behaviour of the cracked bar is numerically analysed by a step-by-step procedure based on the Paris–Erdogan law. The results are plotted in terms of crack paths, intersection angle and crack depth evolution, by varying the loading eccentricity.