Crack propagation and fatigue in zirconia-based composites

This paper reviews existing published studies on crack propagation behavior of zirconia-based composites. The first part of the paper is concerned with slow crack growth (SCG) under static loading. SCG in zirconia ceramics is shown to be a consequence of stress corrosion by water molecules at the crack tip. The influence of transformation toughening on SCG is discussed in terms of a stress intensity factor acting to reduce the net driving force for propagation. This proposition is in agreement with results obtained on 3Y-TZP and Mg-PSZ ceramics. A master curve is proposed which could be applied roughly to all zirconia ceramics. The influence of zirconia addition to alumina ceramics (ZTA ceramics) is also discussed. The second part of the paper deals with SCG under cyclic loading. A mechanical degradation of all zirconia-based composites is observed by a decrease of crack shielding. This degradation of zirconia-based composites under cyclic loading leads to increased velocities as compared to the static fatigue case. A master curve is also obtained, as in the case of static fatigue. Cyclic fatigue results are interpreted in terms of stress corrosion at the crack tip assisted by a decrease of the reinforcement.     

A relative crack opening time correlation for corrosion fatigue crack growth in offshore structures

A considerable amount of research has been carried out on the prediction of mean stress effects on fatigue crack growth in structures. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. Therefore, the extent to which mean stresses can enhance corrosion‐assisted fatigue damage in these structures needs to be better understood. A new theoretical model that accounts for mean stress effects on corrosion fatigue crack growth is proposed. The model is developed based on the relative crack opening period per fatigue cycle and by considering only the damaging portion of the stress cycle. The baseline data for the modelling exercise are the data obtained at a stress ratio of 0.1 in air and seawater tests conducted on compact tension specimens. The model is validated by comparison with experimental data and with other fatigue crack propagation models. The proposed model correlates fairly well with experimental data and the other models examined.     

A MICRO-MECHANICS MODEL OF CORROSION-FATIGUE CRACK GROWTH IN STEELS

Principles of Microstructural Fracture Mechanics (MFM) are used to develop a model for the characterization of environment-assisted short fatigue crack growth. Fatigue cracks are invariably initiated at corrosion pits formed at inclusions, hence the analysis includes stress concentration effects at pits that lead to the propagation of fatigue cracks the rates of which are considered to be proportional to the crack tip plastic displacement. This plasticity is constrained by microstructural barriers which are overcome in a non-aggressive environment at critical crack lengths only when the applied stress is higher than the fatigue limit. However, the superposition of an aggressive environment assists fatigue damage via crack tip dissolution, enhancement of crack tip plastic deformation, the introduction of stress concentrations at pits and a reduction of the strength of the microstructural barrier. These environment effects are manifested in a drastic reduction of the fatigue limit and higher crack propagation rates. The model is compared with fatigue crack propagation data of a BS251A58 steel tested in reversed torsion when submerged in a 0.6M NaCl solution.     

Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, \(\hbox {K}_{\mathrm{max}}\). This work shows that \(\hbox {K}_{\mathrm{max}}\) correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that \(\Delta \hbox {K}\) correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with \(\hbox {K}_{\mathrm{max}}\) with constant parameters.     

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.     

Prediction of corrosion fatigue lives of aluminium alloy on the basis of corrosion pit growth law

Tension‐compression and rotating‐bending fatigue tests were carried out using aluminium alloy 2024‐T3, in 3% NaCl solution. The corrosion pit growth characteristics, and also the fatigue crack initiation and propagation behaviour were investigated in detail. The results obtained are summarized as follows: (i) Most of corrosion fatigue life (60–80%) is occupied with a period of corrosion pit growth at low‐stress amplitude. The corrosion pit growth law can be expressed as functions of stress amplitude σ_a and an elapsed time t. (ii) The critical stress intensity factor for crack initiation from the corrosion pit was determined as 0.25 . This value is the same as the threshold stress intensity factor range for crack propagation. (iii) Corrosion fatigue life can be estimated on the basis of corrosion pit growth law and crack propagation law. The estimated fatigue lives agree well with the experimental data.     

Influence of frequency and exposure to a saline solution on the corrosion fatigue crack growth behavior of the aluminum alloy 2024

The aim of this study is to identify domains where the interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024. The scope considered encompasses the influence of frequency and of alternate immersion in saline solution. Corrosion fatigue crack propagation tests have been carried out under sinusoidal and saw-tooth waveforms, and at different frequencies, load ratios, grain orientations and tempers, in air, distilled water and 3.5% NaCl in permanent and alternate immersion. The stress corrosion cracking behavior of the alloy 2024 has also been considered in order to evaluate the possible contribution of this type of damage during corrosion fatigue crack growth. In 3.5% NaCl, growth rates were found to decrease with decreasing frequency. In alternate immersion, growth rates were increased by up to an order of magnitude for the ΔK values considered compared to permanent immersion and air. The possible mechanisms that govern the corrosion fatigue behavior of the 2024 alloy are discussed in terms of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. Finally, it is proposed that the fatigue crack growth enhancement observed during permanent immersion is related to a crack-tip hydrogen embrittlement mechanism. Hydrogen would be produced by anodic dissolution in relation with film rupture periodicity and then be dragged into the process zone. In alternate immersion, precipitate-free zone dissolution would govern crack advance, as during stress corrosion cracking.     

AF1410与300M钢的腐蚀冲击疲劳行为

根据舰载飞机起落的服务条件提出了腐蚀冲击概念和试验方法，考察了两种起落架材料在盐水中的腐蚀冲击疲劳行为，包括冲击疲劳寿命，裂纹萌生与扩展速率。尽管两种材料在空气中的冲击疲劳寿命几乎相等。但300M钢在盐水中的冲击疲劳寿命下降幅度较大。在盐水介质中，氢脆加速300M钢冲击疲劳裂纹的萌生和扩展。局部塑性变形区优先腐蚀促使AF1410钢的裂纹萌生，盐水对AF1410钢的裂纹扩展速率没有影响。     

The effects of corrosion on fatigue crack propagation in 2024‐T3 Aluminium

Abstract

The effects of salt water with 0.5 percent nitric acid on fatigue crack propagation in 2024‐T3 aluminium alloy are studied experimentally. First, the plastic zone size ahead of the crack tip is measured. It is found that the plastic zone size as well as the closure stress in corrosion fatigue is smaller than that in ordinary fatigue. Secondly, the crack propagation rate of corrosion fatigue is much higher than that of ordinary fatigue due to the creation of microcracks ahead of the main crack. Thirdly, the effective stress range governs the crack propagation rate in both ordinary and corrosion fatigue.     

Critical stress intensity factors in steel cracked wires

In the conceptual framework of fracture mechanics analyses, the study of cracked wires axially loaded has the highest interest since numerous structural elements (e.g. wires, cables, cordons or tendons) work under such a type of loading during their service lives. So, a method that allows the determination of stress states at the crack front should be welcome as a useful way for ensuring the structural integrity of those components for different environmental conditions (air, stress corrosion cracking, hydrogen embrittlement,…). To fill this gap, an engineering estimation of the critical stress intensity factor (SIF) is proposed in this paper for eutectoid steel cracked wires under axial loading. The critical SIF is calculated by considering, apart from the fatigue precrack, the subcritical crack propagation before final fracture. Such a subcritical crack propagation is the process zone (by micro-void coalescence MVC) in the case of fracture in air, the subcritical cracking by localized anodic dissolution (LAD) in stress corrosion cracking (SCC) and the tearing topography surface (TTS) in hydrogen assisted cracking (HAC). In addition, different SIF solutions are used in the analysis so as to have a more complete picture of the different phenomena leading to failure and to provide the designer with sound scientific tools. This method allows the engineer to design in the framework of structural integrity and damage tolerance.     

Computer modelling application in life prediction of high temperature components

Defects introduced in pressure vessel components during fabrication processes act as potential sources for damage accumulation and subsequent catastrophic failure. Cracks nucleate at these stress risers and propagate aided by fatigue type of loading, corrosion and creep. Analysis of crack growth under conditions of ‘time-dependent fatigue’ is very important for the life prediction of pressure vessel components. In this paper the interaction of creep-hot corrosion and low cycle fatigue is analyzed based on the energy expended for the nucleation of damage at the advancing crack front. The total damage accumulation is divided into that due to (i) fatigue, (ii) corrosion and (iii) creep for modelling purpose. The analysis yields a relation in terms ofJ-integral which is applicable to both crack propagation and final failure. A corrosion-creep parameter (F _i ) has been introduced at the crack propagation stage and raw data from different sources have been analyzed for different types of loading and compared with the theoretical predictions. The total energy in tension which includes the tension going time, appears to be a good parameter for the prediction of time-dependent fatigue life.     

Fatigue crack propagation in steels

From previous investigations of the mechanisms of both fracture and fatigue crack propagation, the static fracture model proposed by Lal and Weiss may be thought as reasonable for describing fatigue crack propagation in metals at both low and intermediate stress intensity factor ranges $\text{ΔK}$. Recent progress in fatigue crack propagation indicates that it is not only possible, but also necessary, to modify this static fracture model. Based on the modified static fracture model, the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$, which is defined as the difference between $\text{ΔK}$ and the fatigue crack propagation threshold value $\text{Δ}{}_{\mathrm{th}}$, is taken as the governing parameter for fatigue crack propagation. Utilising the estimates of the theoretical strengths of metals employed in industry, a new expression for fatigue crack propagation, which may be predicted from the tensile properties of the metals, has been derived. The correlation between the fatigue crack propagation rate and the tensile properties is thus revealed. The new expression fits the test results of fatigue crack propagation of steels below 10^?3 mm/cycle and indicates well the effect of stress ratio on the fatigue crack propagation rate.     

Environmental dynamic fatigue crack propagation in high density polyethylene: an empirical modelling approach

Part of a programme to study environmental dynamic fatigue crack propagation in engineering thermoplastics is presented. High density polyethylene has been studied in terms of its stress cracking properties under dynamic loading conditions in detergent. This work complements previous investigations on stress cracking in detergent under static loading. The stress cracking was found to vary according to loading conditions and in conclusion, a dependence of crack growth rate on test frequency, amplitude and level of stress intensity factor is reported. An emperical model describing environmental fatigue crack propagation is proposed which adequately represents the experimental results of high density polyethylene, Nylon 66 and which, it is suggested, may be suitable for use with other polymers.     

Theoretical analysis of influence of crack dimensions and geometry on mass transport in corrosion–fatigue cracks

Abstract

Theoretical models are developed to describe the concentration of dissolved species in a corrosion–fatigue crack. The models are based on two separate reaction processes: the cathodic reduction of oxygen on the crack walls, and the anodic dissolution of metal at the crack tip. The concentrations of species in a trapezoidal crack are calculated for a range of crack depths from 0·25 to 20 mm, and for different minimum/maximum load ratios, stress–intensity factors, and frequencies, assuming single–edge–notch geometry. The relative importance of diffusion and fluid flow as functions of these parameters is analysed. The influence the geometry of the specimen has on the composition of the solution is determined, and the use of parallel–sided cracks is discussed. The implications of the conclusions for corrosion–fatigue crack growth are assessed.

MST/73     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Acoustic emission study of corrosion fatigue crack propagation mechanism for LY12CZ and 7075-T6 aluminum alloys

Acoustic emission signals were continuously monitored during fatigue crack propagation for LY12CZ and 7075-T6 aluminum alloys in laboratory air and 3.5% NaCl solution. The results showed that the acoustic emission count rate was as a linear function of crack propagation rate during fatigue and corrosion fatigue. The acoustic emission activity for LY12CZ was smaller in solution than that in air; but for 7075-T6, greater in solution than that in air. The acoustic emission waveform parameter, the frequency centroid ratio, was tried to use as a criterion to distinguish the corrosion fatigue crack propagation mechanism for anodic dissolution of LY12CZ and hydrogen embrittlement of 7075-T6.     

The application of microstructural fracture mechanics to various metal surface states

The fatigue crack initiation period, previously thought to be a necessary precursor to fatigue crack propagation and eventual failure, is considered here to be a negligible phase in the fatigue failure of polycrystalline metals. Rather this period is considered to be the propagation of a defect of microstructural dimensions by a variety of processes. The significance of this alternative view is examined in relation to corrosion fatigue, models of short crack growth, different loading modes, and the enhancement of fatigue resistance by surface shot-peening treatments. In both inert and aggressive environments, the fatigue lifetime of plain steel specimens of various strengths and treatments is predominantly determined by the early propagation of short cracks of microstructural dimensions. Microstructural fracture mechanics, rather than continuum mechanics, can quantify both pit growth and Stage I shear crack growth behavior before the defect reaches the dominant microstructural barrier which controls the fatigue behavior of the material. The important processes that determine lifetime are those that are strongly dependent on the synergism between the aggressive environment and cyclic stresses; these are the pitting, Stage I and the Stage I-to-Stage II crack propagation processes. A model has been produced to quantify these three important stages of lifetime named above. Under torsion loading, where Stage I cracks prefer to propagate along the surface, an intermittent series of deceleration/acceleration events of crack growth occur across the first few grain boundaries until the defect is blocked in its further development by a major microstructural barrier. When this barrier is breached, the environmentally-assisted Stage I crack rapidly becomes a Stage II crack. Under push-pull loading, the Stage I environmentally-assisted crack can propagate faster into the bulk material and, as a consequence, the transition to a Stage II environmentally-assisted crack is rapid thereby eliminating the need for the intermittent process observed under torsion loading. With no environmentally-assisted fatigue processes (i.e., testing in air) reversed torsion and push-pull loading test data can best be correlated by a von Mises criterion. Corrosion fatigue lifetimes can best be correlated by a Rankine (tensile stress) criterion. Shot-peening enhances the corrosion fatigue resistance of polycrystalline metals by inducing residual compressive stresses in the surface and creating numerous and more rapidly formed microcracks. This is probably caused by the presence of variously oriented plastically deformed bands within the surface microstructure and “short crack-short crack” interactions both of which delay the progress of the dominant crack toward its final Stage II phase. The present work is published according to the kind permission of the Royal Society in London. In 1997, Prof. K. J. Miller celebrated his 65th birthday. Congratulations of the Editorial Board on this occasion are presented at the end of this issue. Research Institute for the Integrity of Structures, Sheffield University (SIRIUS), England. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 1, pp. 9–32, January–February, 1997.     

6061-T6 铝合金激光焊接接头腐蚀疲劳裂纹扩展

目的研究6061-T6铝合金激光焊接接头的腐蚀疲劳裂纹扩展特性,并分析裂纹扩展的影响因素。方法利用光纤激光器,焊接尺寸为150 mm×100 mm×4 mm(焊接方向、横向、熔深方向)的6061-T6铝合金,采用SE(B)三点弯曲疲劳裂纹扩展试验并利用连续降K法,分别在空气和人工海水中进行疲劳裂纹扩展试验,通过使用金相显微镜(OE)和扫描电子显微镜(SEM),对金相结构进行观测分析。结果同样工艺参数的焊接接头,在海水中疲劳裂纹门槛值(4.063 016 MPa·m~(0.5))大于空气中的门槛值(3.479 166 MPa·m~(0.5));在疲劳裂纹扩展中速区(da/dN10~(-5) mm/cycle)时,海水焊接接头疲劳裂纹扩展速率大于空气中的,低速区(da/d N10~(-5) mm/cycle)则小于在空气中的。结论成形良好的焊缝、晶粒细小的焊缝组织有助于接头疲劳裂纹扩展性能的提高;中速区,海水中疲劳裂纹扩展速率偏大,主要是由腐蚀条件下焊缝裂纹尖端阳极溶解和交变载荷共同作用导致;低速区,海水中疲劳裂纹扩展速率偏小,主要原因是腐蚀产物堆积于疲劳裂纹扩展尖端,产生较强裂纹闭合效应。     

Review of stress corrosion cracking of pipeline steels in “low” and “high” pH solutions

This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the high pH stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the low pH stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied.