Plastic zone size in fatigue cracking

Fatigue tests were performed on the specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. To validate fatigue crack propagation in terms of plastic zone size, elastic plastic fracture mechanics (EPFM) are studied in this investigation. Results show that FCGR is a function of specimen thickness, which worsens as the specimen thickness increases. It is considered that plastic zone size is an important fatigue crack propagation parameter in conjunction with applied stress level, specimen thickness and crack closure.     

Residual stress due to welding and its effect on the assessment of cracks near the weld interface

An experimental/analytical hybrid-type investigation of the effects of residual stress on crack propagation due to welding has been performed. The residual stresses in the SAW welded A533B plates and electron beam welded plates that consist of HT80 and A533B steels were detected by an acoustoelastic technique. The measured residual stress was incorporated into a finite element procedure, which simulated stable crack growth in 1T compact specimens, where the effects on far-field crack parameters and on near-field crack parameters were examined. Also investigated was the effect on fatigue crack propagation with the hypothetical residual stress of the identical distribution to that in the electron beam weld. The significance of the residual stress distribution ahead and behind the crack tip in relation to the plastic zone size was identified.     

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.     

Some aspects of thermal fatigue in stainless steel

This paper is concerned with the analysis of failures in the moderator circuit branch piping of the ATUCHA-1 pressurized heavy water reactor (PHWR), which is made of austenitic steel to DIN 1·4550 specification (similar to AISI 347). These failures are considered to result from thermal fatigue processes induced by fluctuations in a zone where stratified temperature layers occurred, the fluctuations being associated with variations in moderator flow.The first section evaluates the possibility of cracking due to thermal fatigue phenomena and concludes that under service conditions a crack may initiate and grow through 7 mm thickness of the branch pipe. In laboratory thermal fatigue tests that simulated the thermomechanical conditions for such a component, the number of cycles required to initiate a thermal fatigue crack in a notched modified standard fatigue specimen was about 10³. This value may be used to give a conservative prediction of the number of thermal cycles for crack initiation in actual branch pipes, including those subject to the cold plug condition which is produced in some emergency shut-down and valve testing situations.It was also demonstrated that beyond a crack depth of 7 mm stress corrosion cracking is the main process in further crack propagation. The relevance of this prediction is confirmed by microfractographic observations, since the brittle nature of the fracture surfaces under service conditions appears very different from the transgranular ductile striations found in both thermal and mechanical fatigue test specimens as a result of interacting environmental effects.     

FATIGUE CRACK PROPAGATION UWDER OSCILLATING TEMPERATURE DISTRIBUTIONS

A method has been presented for the estimation of fatigue crack growth in structures with harmonically varying temperuture distributions. The number of cycles required to extend a fatigue crack of a given depth to a certain amount has been calculated as a function of the temperature amplitude and frequency of the oscillating temperature distribution. The concepts from linear elastic fracture mechanics have been used in these evaluations.     

Fatigue crack initiation and propagation in austenitic stainless steels for light water reactors

The determination of fatigue life of components containing defects usually takes into account crack propagation only. In a real situation, a number of cycles are often required to reach fatigue crack initiation and predictive evaluation of fatigue crack initiation phases of real defects in austenitic stainless steel welded joints are presented. Fatigue crack growth rates in wrought and cast austenitic stainless steels and associated welds are also presented. Effects of various mechanical parameters (R ratio and variable amplitude loading) of a PWR environment and of metallurgical factors (δ ferrite content and ageing in cast austenitic stainless steels) are discussed.     

Experimental observations on mixed mode fatigue crack propagation

Fatigue crack propagation experiments were conducted on D16AT Al alloy to study the effect of load ratio on mixed mode fatigue crack propagation. The experiments were conducted on a Compact Tension Shear specimen at different crack angles in both mode I and mixed mode conditions. At each angle tests were conducted at three different load ratios of 0·1, 0·25 and 0·5. It has been observed that the crack growth rate increases with decrease in load ratio. The fracture surfaces were analysed under the scanning electron microscope. The effect of load ratio was studied in terms of striation spacing.     

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.

Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.

The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK*, based on notional crack lengths. Since the results of da/dN vs ΔK* for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for da/dN have been developed using various crack tip parameters.     

THE INFLUENCE OF CRACK-FACE FLUID PRESSURE ON THE FATIGUE CRACK PROPAGATION DUE TO ROLLING CONTACT WITH FRICTIONAL HEAT

A three-dimensional surface planar crack problem in a half-space is considered under rolling/sliding contact with frictional heat and hydraulic pressure by the entrapped fluid within the crack. Rolling contact is simulated as a line load with both normal and shear components, moving with constant velocity over the surface of the half-space. The body force method for three-dimensional fracture mechanics is utilized to determine the three modes of stress intensity factors along the crack contour. To account for mixed-mode propagation, the modified Paris power law is used. Numerical results for the stress intensity factors and the simulations of fatigue crack propagation are given for 30-degree inclined planar surface cracks of semicircular shape. The effects of the frictional coefficient, sliding/rolling ratio, and the crack-face fluid pressure on the crack propagation life are considered for a high carbon-chromium bearing steel.     

Hydrogen induced blister cracking and mechanical failure in X65 pipeline steels

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm² for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.     

Hydrogen enhanced fatigue in full scale metallic vessel tests – Results from the MATHRYCE project

This study investigates the fatigue life of CrMo pressure vessels for hydrogen storage by hydraulic and hydrogen pressure cycle tests. Two different sized cylinders have been tested; 35 L inner volume and 28 MPa working pressure (WP) and 198 L volume and 41 MPa WP. On the inner surface of the cylinders U-shaped notches of different depths were machined by electro discharge machining technique. The initial notch sizes were designed based on a two stage fatigue predictive model based on fracture mechanics to develop through wall cracks in the deepest notches after about 50,000 hydraulic cycles together with crack propagation of the intermediate notches and crack initiation in the smallest. The cylinders were cycled between the nominal pressure of 2 MPa and the WP until leak before break (LBB). Strain gauges were placed at the external surface of the cylinders in correspondence of the internally machined notches. On the notches which developed through wall, the strain showed a progressive decrease followed by an increase of the hoop strain during the final stage of crack propagation until LBB failure. Hydrogen effect was clearly identified by the reduction in the number of cycles to failure comparing tests in hydrogen and in oil. Subsequent failure analysis at the end of each test revealed a typical trans-granular fatigue crack surface morphology (with fatigue striations) for tests in oil, while quasi cleavage and intergranular fracture appearance were found for hydrogen tests.     

Fracture toughness of spun cast SG iron pipe

The fracture toughness properties of two spun cast, ferritic SG iron pipe samples have been investigated and the results compared with previous work. It was found that the COD at fibrous crack initiation ($\text{δ}{}_{\mathrm{<mtext>i</mtext>}}$) was independent of: (a) specimen size; (b) machined or fatigue precracking and (c) graphite nodule number and spacing. This is discussed with respect to the fracture initiation mechanism which was found to be independent of graphite nodules. The maximum load COD values and COD transition were similar for fatigue cracked and machined slit specimens. The COD values and COD transition temperature obtained from spun cast pipe are lower than those obtained in previous work on SG iron and it has been shown that the maximum load COD decreases with decrease in graphite nodule spacing. The effect of graphite nodule spacing on the ductile fracture propagation energy is discussed.     

冻融损伤混凝土梁抗力性能演化的裂缝分形特征

为分析冻融循环作用下钢筋混凝土梁受压区的表面裂缝分布特征,采用自主设计的混凝土梁反力试验架对受压区受冻融循环作用的梁进行抗弯试验,取得各级荷载作用下梁表面的裂缝图像。依据分形理论计算表面裂缝分布的分形维数,并讨论分形维数与梁的荷载、跨中挠度、屈服荷载和冻融循环次数之间的关系。研究表明,受压区冻融损伤的钢筋混凝土梁表面裂缝的分布符合分形特征,其分形维数与荷载、跨中挠度、屈服荷载和冻融循环次数之间均有一定关系,裂缝的分形维数可作为钢筋混凝土构件安全性能预测的指标。研究成果可为冻融损伤混凝土梁安全预测提供参考。     

Green's function approach for crack propagation problem subjected to high cycle thermal fatigue loading

An efficient numerical approach using Green's function for the analysis of crack propagation under thermal transient load has been presented. The present approach based on multi-Green's functions pre-determined for each stage of the incremental crack growth substantially shortens the calculation time of the stress intensity factor (SIF) ranges. It was shown that the Green's function method (GFM) can be efficiently used to evaluate not only thermal stresses for fatigue analysis but also the SIF for crack propagation analysis. The crack propagation analysis results have been compared with those of the actual observation for the piping structure subjected to thermal striping load in a liquid metal fast breeder reactor. It was shown that the function determined at a fixed temperature can be applied to a relatively wide range of temperatures because of the compensation effect of the material properties, that is, some properties increase while the others decrease as the temperature increases.     

Finite element analysis of the effect of weld geometry and load condition on fatigue strength of lap joint

Many welded lap joints are subject to fluctuating loads, and fatigue failure plays a dominant role in the failure of such structures. Based on the concepts of linear elastic fracture mechanics, the effects of weld geometry, load conditions and the boundary constraints on fatigue strength of a ferrite–pearlite steel lap joint were investigated in this paper using the finite element method. Paris's power law was used to predict the fatigue life of the joints. Various weld geometry including the leg length, flank angle and the size of lack-of-penetration were considered during the calculation of fatigue strengths. The loads include tension, bending and their combinations. It was found that the existence of a root crack has no influence on the fatigue strength of the joint, under the relevant load conditions. The existence of a toe crack is also of no influence on the fatigue strength of the joint if the applied loads, e.g. DOB>0 in this paper, produce a compressive stress field at the top region of the main plate. For a lap joint with a free transverse (Y direction in this paper) boundary constraint at the main plate, a joint with a smaller size of lack-of-penetration, a reasonably large weld leg and smaller flank angle is recommended to be used in engineering practice, in order to obtain a higher fatigue strength. For a lap joint, with transverse fixed boundary constraint at the main plate, the fatigue strength increases with a decrease of weld size but the influence of flank angle depends on the type of load carried. It was also found that the size reduction in FE model is a significant influence on the calculated fatigue strength; the use of reduced size FE model gives a much higher overestimate of fatigue strength of the joint.     

Modeling the fatigue crack growth and propagation life of a joint of two elastic materials using interface elements

In this study, a new approach for the simulation of fatigue crack growth in two dissimilar elastic materials has been developed and specifically, the concept has been applied to a butt-welded joint. The phenomena of crack propagation and interface debonding can be regarded as the formation of new surface. Thus, it is possible to model these problems by introducing the mechanism of surface formation. In the proposed method, the formation of new surface is represented by an interface element based on the interface potential energy. The properties of this interface element represent the bonding strength of the material. As the cyclic load continues, the bonding strength decreases between the interacting surfaces and the crack propagates slowly. Based on this concept, an ANSYS code has been written for the simulation of crack propagation. Using this code, the fatigue crack growth rate and fatigue crack propagation life of a 2D Finite Element Method (FEM) model of a butt-welded joint for different stress ratios have been analyzed and presented in this paper. The variation of the crack opening displacement (COD) over crack lengths and crack tip stress and strain over crack tip stress have also been presented. The trend of fatigue crack growth rate is similar to results found in the literature. For validation, the predictions are compared with experimental results and reasonably good comparisons are found. The method is relatively simple compared to other conventional FEM methods and it eliminates the limitation of crack propagation of one element length per cycle and saves computer time significantly.     

Probabilistic fracture mechanics analysis of a nuclear pressure vessel for allocation of in-service inspection

To find whether there are significant differences in fracture probability between various regions in a reactor pressure vessel a limited probabilistic fracture mechanics (PFM) study was carried out. The loading was assumed to be deterministic, whereas some of the other quantities involved were assumed to be of random character. A simple, mainly analytical probabilistic model was developed. The critical event was taken as unstable crack growth without any ensuing crack arrest. All random information on the J_R vs Δa behaviour was assumed to be contained in a single scalar parameter. Various distributions such as the Weibull and the log-normal distributions were assumed for this parameter to judge the sensitivity of results to various assumptions. The initial size of pre-service defects was assumed to follow the OCTAVIA distribution. Possible time-dependent crack growth such as fatigue or stress corrosion cracking was treated in a simplified manner assuming deterministic material properties.

Various regions (and crack geometries) were considered and the fracture and leakage probabilities were calculated for a number of load cases. The fracture probabilities were strongly dependent on the assumption made for the fracture toughness distribution, but the order of the fracture probabilities of the regions seemed to be relatively unaffected by this. The leakage probabilities are in most cases much lower than the fracture probabilities, indicating that consequence considerations are not very important for non-destructive test (NDT) allocation purposes. It is also concluded that probabilistic methods of the present kind may be a useful tool for judging which locations should be the primary targets for NDT.     

The shape of a surface crack in a plate based on a given stress intensity factor distribution

A new method is developed for the evaluation of a crack shape based on a given stress intensity factor (SIF) distribution for a surface crack under Mode-I loading conditions. The SIF distribution along the crack front is investigated using a direct simulation technique, in which the effect of crack closure on fatigue crack growth is considered. Then a SIF distribution function is chosen based on the numerical results. Crack shape (and the SIF) is achieved based on the given SIF distribution function using a numerical iterative procedure. Empirical SIF equations for surface cracks in plates subjected to tension and pure bending fatigue load are determined by systematic curve fitting of the numerical results. The depth ratio and the aspect ratio are considered in the ranges of 0.1–0.9 and 0.2–1.2, respectively. The aspect-ratio variation of surface cracks under fatigue loading is predicted. The application of the new method to predict the shape of a surface crack in plates subjected to tension and pure bending and comparisons of the results obtained with the predictions of the empirical equations proposed by Newman and Raju are presented.     

Ⅰ型断裂尺寸效应及裂纹扩展阻力曲线的有限元分析

针对三点弯曲法在测定混凝土等材料断裂能时存在不同程度的尺寸效应现象,基于有限元软件ABAQUS,建立5种不同尺寸、几何相似的混凝土梁,采用线性内聚力模型模拟裂纹尖端微小内聚力区域的力学特性,分析单边带预制裂缝的三点弯曲梁的Ⅰ型断裂开裂过程,针对其尺寸效应和裂纹扩展阻力曲线开展研究。结果表明,随着试件尺寸的增大,试件断裂过程区的相对长度和名义强度减小,应力强度因子和峰值荷载及对应挠度值增大;小尺寸结构破坏模式的延性特性较显著,而大尺寸结构表现出更加显著的脆性特性;尺寸效应定律的线性形式能较好地反映名义强度和试件尺寸之间的关系;通过材料的裂纹扩展阻力曲线获得裂纹失稳扩展临界点所对应的宏观断裂能,其值与输入的数值模拟参数基本一致,采用裂纹扩展阻力曲线获取材料断裂能是可行的。     

Life cycle assessment of hydrogen energy facility by criterion for maximum load frequency

The paper deals with the topical issue of providing the base load for the Nuclear Power Plant (NPP) in the nighttime off-peak load hours. To find a solution to this issue, we analyzed energy storage technologies, such as hydroelectric power stations. Since the construction of this type of station is associated with various risks (technical, environmental, seismic, etc.), their deployment in the immediate vicinity of the Near Nuclear Power Plants is unacceptable. This implies the tariffs for the supply of power from the grid transmission system may exceed the nuclear generating costs by 2 or 3 times, and significantly affect the cost for the produced peak energy and competitive advantages of these stations. As more competitive technology of electric energy storage, the paper reviews the system based on utilizing hydrogen energy facilities with hydrogen and oxygen produced by water electrolysis due to excess power from nuclear power plants in the nighttime. The key advantage of these facilities is location in the vicinity of NPPs with the possibility of charging at the cost of the NPP energy. At the same time, hydrogen and oxygen production and their further utilization in the NPP steam cycle has the recurrent nature and connected with the daily startup and shutdown procedures of the main facilities. Thus, the aim of this research is to determine the life cycle of the main hydrogen energy facility under cyclic loads. The fatigue fracture theory is applied to analyze the performance of startup/shutdown cycles in the main hydrogen energy facility in combination with the NPP. We have conducted the estimation of fatigue crack growth depending on the load frequency for the critical components of electrolysis plants, compressors, metal hydrogen and oxygen storage tanks, as well as hydrogen-oxygen combustion chambers. The paper focuses on the impact of hydrogen corrosion on the rate of fatigue crack growth and proposes criterion defining the number of cycles occurred prior to the fracture extension process. Based on the criterion of maximum cycles prior to the fraction extension process, we have defined the boundaries for effective performance of the main hydrogen energy facility.