Offshore fatigue crack repair by grinding and wet welding

A fatigue crack repair option based on crack removal by grinding and subsequent filling of the resultant groove by wet welding is presented. Author's experimental and fracture mechanics analysis experience on fatigue crack removal by grinding has demonstrated that a fatigue life extension factor of 2 to 3 is feasible for crack removal done before crack penetration has reached 30% of the plate thickness. However, it has also been demonstrated when groove is left empty, that fatigue life extension after crack removal is mainly due to the stage of fatigue initiation because crack propagation after removal has a higher rate than in as‐welded condition. To overcome this situation, this work proposes for an underwater application like in the offshore industry, that filling the grinded groove by means of wet welding would produce a crack repair with a longer fatigue life than the empty groove case.     

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。     

Life prediction by ferrite–pearlite microstructural simulation of short fatigue cracks at high temperature

This paper dealt with fatigue behavior simulation based on ferrite–pearlite microstructure modeled by correctional Voronoi-polygons. The model took grain size, grain orientation and the percentage of pearlite and ferrite into consideration. The basal energy was proposed to represent the inherent energy for slip-band and grain boundary to cracking. The driving force for crack initiation and propagation caused by load condition was considered as the energy increment of slip-band and grain boundary. The fatigue behavior including crack initiation, propagation, coalescence and interference were simulated based on Monte Carlo method. The simulation results show a satisfying agreement with the experimental ones.     

Initiation criteria in fretting fatigue with spherical contact

This paper analyses different methods used to estimate fatigue life adding initiation with propagation in fretting fatigue tests with spherical contact. In particular, the discussion will be centred on the initiation phase. Some of the methods presented define a crack length in advance, after which crack propagation is considered and calculated. Stresses close to the surface are then used to estimate initiation. The initiation life is estimated by using either the stresses averaged up to a certain depth or the stresses at a certain depth. Yet another method combines initiation with propagation, although in this case the initiation length is not a fixed quantity, it is obtained as a result of analysing each specific case. These methods are applied to a series of fretting fatigue tests with spherical contact, performed with 7075-T6 aluminium alloy and the results are discussed.     

Fatigue life prediction for stainless steel welded plate CCT geometry based on Lawrence''s local-stress approach

In the present study, the effect of welding process and procedure on fatigue crack initiation from notches and fatigue crack propagation in AISI 304L stainless steel welds was experimentally investigated. Full penetration, double-vee butt welds have been fabricated and CCT type specimens were used. Lawrence's local-stress approach (a two-stage model) is used to predict the fatigue life. The notch-root stress method was applied to calculate the fatigue crack initiation life, while the fatigue crack propagation life was estimated using fracture mechanics concepts. The fatigue notch factor is calculated using Lawrence's approach. Constant amplitude fatigue tests with stress ratio, R=0 were carried out using 100 kN servo-hydraulic DARTEC universal testing machine with a frequency of 30 Hz. The predicted lives were compared with the experimental values. A good agreement has been reached. It is found that the weld procedure has a stronger effect on lives to initiation than on propagation lives.     

CORROSION FATIGUE OF AN HSLA STEEL

Abstract— Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.
The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found.     

Fatigue of fillet welded A515 steel

A study has been made of the fatigue behavior of fillet welded ASTM A515 steel. As-welded and stress-relieved skip fillet weld specimens were tested under pulsed tension and altering cyclic load to determine stress-life and crack propagation behavior. Crack initiation and propagation features were determined from sectioned surfaces. All fatigue cracks were semi-elliptic and initiated from weld end toes. The length/depth ratio was approximately constant during propagation. There was no consistent effect of tensile residual stress on fatigue life under pulsed tension but there was a detrimental effect under alternating loads. An equivalent crack model has been proposed to quantify the stress concentration effect at the crack initiation site based on the application of the Paris equation. The test results show that the equivalent crack can give a reasonable prediction of the fatigue life of a welded structure and is a potentially convenient tool in fatigue design.     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.     

A finite element analysis of fretting fatigue crack growth behavior in Ti-6Al-4V

There is a need for methodology(ies) to analyze the crack growth behavior under fretting fatigue condition since its experimental determination is a challenging task. A finite element sub-modeling method was used to estimate the crack propagation life in titanium alloy, Ti-6Al-4V specimens. Two contact geometries, cylinder-on-flat and flat-on-flat, were analyzed. The computed crack propagation lives were combined with the results of an experimental study where total fatigue lives were measured. The combined numerical-experimental approach provided the crack initiation lives. The crack propagation life increased with increasing applied cyclic bulk stress in similar manner for both contact geometries. Almost 90% of the fretting fatigue life was spent during the crack nucleation and initiation phases in the high cycle fatigue regime. A parametric study was also conducted to investigate the effects of contact load, coefficient of friction and tangential force on the crack growth behavior. The crack propagation life decreased with increase of these three parameters. This decrease was similar for the contact load and the tangential force in both contact geometries, however, the decrease in the case of coefficient of friction was relatively more in the cylindrical pad than in the flat pad.     

Crack evaluation and subsequent solution of the last stage blade in a low-pressure steam turbine

A crack of the last stage blade in a low-pressure steam turbine is studied. From non-destructive inspection, the crack was found at the trailing edge fillet of the last stage blade. On further examination using the replication of the crack and scanning electron microscopy, the crack was induced by stress corrosion cracking. The blade integrity is evaluated for several crack-removal conditions to repair the cracked blade. Finite element analysis is utilized to determine the stresses and dynamic characteristic of the blade for these conditions. The assessment of high cycle fatigue is performed by modified Goodman diagram. From the results, it is found that the crack-removal blade satisfies the design criteria for the fatigue life and has the sufficient margin from resonant condition of vibration. Repair for the cracked blade would be recommended on the basis of the careful analysis. For the repaired blade, periodic inspection is needed and replacement of the blade will be necessary when an indication is found.     

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.     

航空铝合金原位腐蚀疲劳性能及断裂机理EI北大核心CSCD

为了研究不同腐蚀条件下2024铝合金的疲劳性能,首先设计搭建原位腐蚀疲劳平台,然后分别进行无腐蚀疲劳、预腐蚀疲劳和原位腐蚀疲劳实验,分析不同腐蚀疲劳条件下2024铝合金的疲劳断裂行为,最后利用扫描电镜(SEM)表征宏、微观断口特征,探究失效机理。结果表明:相同腐蚀环境和时间下,预腐蚀和原位腐蚀疲劳寿命分别为无腐蚀疲劳寿命的92%和42%;在原位腐蚀疲劳条件下,滑移带挤入、挤出导致表面粗糙度增加,吸附较多腐蚀介质,加剧蚀坑演化,易于裂纹萌生并形成多个裂纹源。裂纹的连通形成更大尺寸的损伤,并在材料内部快速扩展。预腐蚀和原位腐蚀疲劳试件断口观察到大量脆性疲劳条带,并且原位腐蚀疲劳条带平均间距约为无腐蚀疲劳条带间距的2倍,说明原位腐蚀疲劳条件下裂纹扩展速率更快。     

Nd:YAG laser surface melting of aluminium alloy 6013 for improving pitting corrosion fatigue resistance

Laser surface treatment of aluminium alloy 6013 was conducted with the aim of improving the alloy’s resistance to pitting corrosion fatigue. The study showed that laser melting using a high power Nd:YAG laser increased the resistance of the alloy to pitting corrosion and pitting corrosion fatigue. As corrosion pits are favourable sites for the initiation of fatigue cracks, and the process of crack initiation often takes up most of the fatigue life, especially at low stress levels, a high pitting corrosion resistance resulted from the laser treatment improved fatigue crack initiation resistance. With regard to fatigue crack propagation, although interdendritic boundaries are vulnerable to corrosion attacks due to the presence of second phase particles, nonetheless, due to the nature of the rough and undulating fracture surface, fatigue growth would be retarded. Under the present experimental conditions, the improvement in corrosion resistance brought about by laser surface melting was found to prevail over the adverse effect caused by the residual stresses induced by laser melting.     

Fatigue behavior of surface cracked filament wound pipes with high tangential strength in corrosive environment

The aim of this study is to examine the corrosion fatigue behavior of filament wound composite pipes with a surface crack under alternating internal pressure. The filament wound pipes are composed of multi-layered E-glass/epoxy composites with a [±75°]₃ lay-up. The surface notches were formed on the outer surface of the pipe along the pipe axis. Dilute (0.6 M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The results of an experimental investigation into the corrosion fatigue tests are conducted to observe the oil leakage failure and the crack propagation of the composite pipe subjected internal pressure loading with an open ended condition in which the pipe can be deformed freely in the axial direction. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The fatigue tests are performed at 0.42 Hz frequency and a stress ratio of R = 0.05 in accordance with ASTM D-2992 standard. The oil leakage from the crack tip was observed after the crack propagation reached to the critical stress intensity level. The fatigue crack propagation behavior with the environment exposure was strongly dependent on the crack parameters such as crack-depth ratio and crack-aspect ratio. The micro structure of the fracture surface with the effect of environment and the fatigue loading were also observed.     

Variability in room temperature fatigue life of alpha + beta processed Ti–6Al–4V

The variability in fatigue behavior is often what drives the design of components such as turbine engine blades and disks. These components are critical and must be designed with a very low probability of failure over the lifetime of the system. To meet that design criterion, the lower limit of fatigue life capability is typically used. The challenge is to reliably predict the lower limit of fatigue behavior. This study investigates the fatigue variability of an alpha + beta processed Ti–6Al–4V turbine engine alloy by conducting a statistically significant number of repeated tests at a few conditions. Testing includes three conditions including two maximum stresses, 675 and 635 MPa; and two surface conditions, electropolished and low stress grinding. All tests are constant amplitude with a stress ratio of 0.1. A similar approach has been performed on several other turbine engine material systems often revealing a bimodal behavior. It is proposed that crack propagation using small crack growth data can be used to predict the low life behavior mode and is demonstrated with the Ti–6Al–4V data.     

CORROSION FATIGUE FRACTURE BEHAVIOUR OF A SiC WHISKERALUMINIUM MATRIX COMPOSITE UNDER COMBINED TENSIONTORSION LOADING

We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under load-controlled conditions with a constant value of the combined stress ratio, α = τ_max /σ_max in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.     

Experimental crack propagation and failure analysis of the first stage compressor blade subjected to vibration

This paper presents results of experimental vibration tests of the helicopter turbo-engine compressor blades. The blades used in investigation were retired from maintenance under technical inspection of engine. Investigations were conducted for selected undamaged blades, without existence of preliminary cracks or corrosion pits. The blades during experiment were entered into transverse vibration. The crack propagation process was conducted in resonance condition. During the fatigue test, the growth of crack was monitored. In the second part of work, a nonlinear finite element method was utilized to determine the stress state of the blade during vibration. In this analysis a first mode of transverse vibration were considered. High maximum principal stress zone was found at the region of blade where the crack occurred.     

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10 V-2 Fe-3 Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10 V-2 Fe-3 Al alloy in very high cycle fatigue(VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/βdecohesion, occur when the stress ratio(R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10~7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from-0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.     

Fatigue behaviour of bare and pre-corroded magnesium alloy AZ31

Constant amplitude fatigue tests have been performed using smooth specimens of a rolled AZ31 magnesium alloy in order to assess the fatigue behaviour of the material. The tests were periodically interrupted and replicas were taken from the surface of the specimens in order to reveal crack initiation and early crack propagation. Based on the derived S–N curve a very high stress sensitivity of the fatigue life can be concluded; it may be attributed to the inability of the material to accumulate fatigue damage in terms of cyclic plasticity at the early stage of fatigue. Fatigue cracks initiate already after few fatigue cycles between strain incompatibility points (e.g. grain boundaries) due to difficulties in satisfying the von Mises criterion. The initiation and propagation mechanisms of the fatigue cracks are characterized as cleavage. Furthermore, the corrosion susceptibility of the material has been investigated in a salt spray environment. It becomes evident that the presence of corrosion damage, in terms of corrosion pitting, results in the development of stress concentration, facilitating essentially the initiation and propagation of fatigue cracks. Thus, the fatigue limit is reduced to 50% of the respective value of the un-corroded material.     

Fatigue behaviour prediction of steel reinforcement bars using an adapted Navarro and De Los Rios model

Fatigue cracks tend to initiate on the rebar surface and therefore, the surface conditions may control their fatigue behaviour. This study investigates the influence of surface microstructure and roughness dispersion on the scatter and fatigue life of hot rolled (HR)–cold worked (CW) and quenched and self-tempered (QST) rebars. The stochastic nature of the fatigue life is mainly affected by the scatter of short cracks in the crack initiation phase. A model adapted from Navarro and De Los Rios (N–R) was developed to predict the crack initiation, including short crack growth, and long crack propagation phases. The crack initiation phase includes the dispersion inherent to the grain size, grain orientation ratio and multiple phases i.e., ferrite–pearlite and martensite as well as the roughness dispersion determined on the rebar surface and the influence of the rib geometry. The stress concentration factor due to the rib geometry was considered as a constant parameter. In the long crack propagation phase, all microstructural features are considered as constants. The model results were compared to experimental data from the literature.