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。     

Thermomechanical transformation fatigue of TiNiCu SMA actuators under a corrosive environment – Part I: Experimental results

In this two-part paper, the thermomechanical fatigue of TiNiCu shape memory alloy (SMA) wire actuators undergoing thermally induced martensitic phase transformation in a corrosive environment is investigated. The main objective of this work is to evaluate the cyclic response and fatigue behavior of TiNiCu SMA wire under corrosive conditions and to compare it to results obtained for fatigue testing in a corrosion-free environment. Part I focuses on the various experimental aspects of this work, including the presentation of fatigue results as a function of various testing parameters. The variable test parameters are five applied stress levels from about 50 MPa to about 250 MPa, and two different actuation strains, one corresponding to full actuation or complete transformation and the other to partial transformation. The results from fatigue testing in a corrosive environment show a consistent reduction of the fatigue life compared to corrosion-free fatigue results, in both complete and partial transformation conditions. It is also observed that corrosion-assisted fatigue leads to more scattered fatigue data and this spread is mostly attributed to enhanced and accelerated damage mechanisms due to corrosion. From these conclusions, a microstructure evaluation is performed to understand the damage that contributes to lower fatigue limits under corrosion and is presented in Part II of this work. Fracture surfaces, development of fatigue cracks and effect of corrosion are presented and discussed. The conclusion from the microstructure analysis has led to the formulation of a damage accumulation model accounting for a cyclic corrosion mechanism. This modeling approach allows for determining the fatigue life reduction of SMA wire actuators in a corrosive environment. All results of the microstructure analysis and fatigue life modeling are presented in Part II.     

Einfluß korrosiver Umgebung auf die Lebensdauer zyklisch beanspruchter Bauteile

Influence of a corrosive environment on the fatigue life of cyclic loaded structures Fatigue life of structural components in practice is effected by corrosive environments, too. The failure mainly is caused by the interaction of cyclic straining and corrosive attack. Extensive research and test programmes during the last two decades had been carried out to investigate the parameters influencing corrosion fatigue. Results of the most effective parameters on corrosion fatigue are given in the present paper. The amount of degrading effect on fatigue properties depends on the susceptibility of the material to corrosive environment, the structural shape and the environment itself. The various parameters determing fatigue behaviour are superimposed in a synergistic manner, so their effects cannot simply be added. That may be the reason for very different conclusions on the effect of corrosion on fatigue life from ?disastrous”? to ?neglegible”?. Though there is still a number of unclear points, the good agreement of results of welded specimens (V-shaped specimens) and welded tubular joints indicates a positive outlook for the application of small specimen corrosion fatigue data for the design of structural components.     

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.     

Environmental effect on the fatigue performance of bare and oxide coated 7075-T6 alloy

Fatigue behaviors of bare and anodic oxide coated 7075-T6 alloy have been investigated in laboratory air and 3.5%NaCI solution environment by using smooth cylindrical specimens. Presence of corrosive attack during fatigue test drastically reduced fatigue performance of the alloy. The deleterious effect was observed to be pronounced at high-cycles fatigue region, where the fatigue strength of the bare specimen was lowered by a factor of 2.9. However, the oxide coated specimens having a thickness of 23 μm showed a modest reduction in fatigue strength. Corrosion fatigue (CF) strength of the bare specimens was predominantly controlled by pitting-induced crack nucleation. Examinations on the surfaces of the corrosion-fatigued and immersed test specimens revealed that cyclic loading stimulated corrosion pit formation during CF tests. Also, corrosion behaviors of both the coated and bare specimen shave been investigated by potentiodynamic test. Despite superior corrosion resistance of coated specimens, fatigue performance was adversely affected under the combined action of corrosion attack and cyclic loading.     

Corrosion fatigue life prediction of a steel shaft material in seawater

Corrosion fatigue behaviour of a medium strength structural material was studied in air and in 3.5% NaCl solution. Emphasis was placed on the study of corrosion pit formation and the development of cracks from pits. Pitting and crack propagation were quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pitting and the pit-to-crack transition in order to predict the fatigue life. The results showed that a large number of corrosion pits with small size form at a very early stage in the fatigue lifetime. The number of pits and subsequent cracks was found to be higher at higher stress levels leading to multiple crack development and coalescence. When compared to air, fatigue life in a corrosive environment was significantly reduced at low stress levels due to pitting damage, indicating a dominant role of corrosion over that of mechanical effects. The corrosion fatigue model proposed shows good agreement with the experimental test data at lower stress levels but predicts more conservative lifetimes as the stress increases. Kitagawa–Takahashi diagram was produced for both test environments where it is indicated that the fatigue limit can be eliminated in a corrosive environment.     

The failure of an armour faced conveyor connector

The combined use of modern metallurgical techniques for fracture examination, laboratory test data and fracture mechanics calculations allows metallurgical failures to be examined in a quantitative manner. Complex load histories and environments can result in more than one sub-critical cracking mechanism occurring in a component. Quantitative understanding of the rate determining cracking process is a necessary prerequisite to rectifying the problem. The following case study describes a connector from an armour faced conveyor which failed in service. The failure investigation involved fractography, stress analysis, material property evaluation and fracture mechanics calculations. Fractographic evidence indicated a stress corrosion failure mechanism. Calculations of critical crack sizes showed that stress corrosion cracking alone could not account for the fracture. It was concluded that the failure was due to a sequence of three cracking processes which preceded unstable ductile fracture. Firstly, frictional heating caused rubbing or quench cracks typically 0.5–1 mm deep. Secondly, corrosion fatigue cracks grew several millimetres allowing the third fracture process, stress corrosion cracking, (SCC) to initiate and grow. In the situation described here, this process was much faster than corrosion fatigue. The influence of defect size due to rubbing cracks and the influence of K_ISCC have been compared with the corrosion fatigue life of the component. An increase in K_ISCC and hence critical defect size for SCC has been shown to increase the corrosion fatigue life of the component by a large factor. A change in design would also alleviate the problem of SCC by reducing the static stress, which is the driving force for SCC.     

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel-free character, the high nitrogen nickel-free austenitic stainless steel (HNASS) becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deformation. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank’s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550 MPa, while corrosion fatigue limit is 475 MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre-straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300 MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.     

Corrosion fatigue of bio-ceramic sapphire in isotonic sodium chloride solution

Alumina (sapphire) is considered as a candidate material for bio-ceramics, because of its superior strength characteristics, corrosion resistance and bio-inert properties. These biomaterials are used in aggressive corrosive environments such as the human body under severe mechanical conditions superposing cyclic loading on constant load. Therefore, it is necessary to evaluate the static and cyclic fatigue characteristics in an environment as severe as the human body. In this paper, the effects of human body environment corrosion upon cyclic fatigue characteristics of single crystal sapphire and polycrystalline alumina have been investigated using in vitro testing in isotonic sodium chloride solution, whose composition is closely similar to body fluid. Also, degraded corrosion morphologies and fracture surfaces of single crystal sapphire specimens were examined in detail by SEM and AFM. The fatigue limit of single crystal sapphire was found to be extremely degraded in the isotonic sodium chloride solution compared with that in air. The importance of environment assisted slip deformation is indicated.     

Corrosion fatigue behavior of friction stir processed interstitial free steel

In this study, interstitial free (IF) steel plates were subjected to double-sided friction stir processing (FSP). The fine-grained structure with an average grain size of about 12 μm was obtained in the processed zone (PZ) with a thickness of about 2.5 mm. The yield strength (325 MPa) and ultimate tensile strength (451 MPa) of FSP IF steel were significantly higher than those of base material (BM) (192 and 314 MPa), while the elongation (67.5%) almost remained unchanged compared with the BM (66.2%). The average microhardness value of the PZ was about 130 HV, 1.3 times higher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furthermore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhibited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.     

Failure of aircraft propeller assembly

This paper analyses the causes of the incident of a Cessna trainer whose propeller was separated due to the cracking of the propeller blade hub during the take off roll. Beach marks and fatigue striations, typical of fatigue cracks, were observed on the fracture surface and corrosive oxides were detected in the center of beach marks that are considered to be the crack origin. The stress acting on the fracture surface under a corrosive environment forms corrosive oxides, such as mud cracks. By analyzing the fractography and metallography of the failed parts, it is found that the propeller blade hub nucleated stress corrosion cracking (SCC) as a result of residual stress and corrosive environment and the SCC was the cause of the fatigue crack. Moreover, a fatigue crack reaches its critical length by repeated cyclic stress, which occurs during the rotation of the propeller blade and then, the rest of the fracture occurred instantaneously.     

A study of frequency and temperature effects on fatigue crack growth resistance of CPVC

Excellent corrosion resistance of chlorinated polyvinyl chloride (CPVC) makes it an attractive material for piping systems carrying corrosive materials. The relatively high glass transition temperature of CPVC has increased its use in hot water distribution. Establishing a relationship that describes the effect of test frequency on fatigue crack propagation (FCP) rate of polymers is an interesting challenge. FCP rates can decrease increase or remain constant with increasing test frequency. Moreover, FCP sensitivity to frequency of some polymers is known to be dependent on test temperature. In this study, fatigue crack propagation in a commercial grade chlorinated vinyl chloride (CPVC) over the frequency and temperature ranges of 0.1-10 Hz and −10 °C to 70 °C, respectively, was investigated. FCP tests were conducted on single edge notch (SEN) specimens prepared from 100-mm injection molded CPVC pipefittings. The crack growth rate (da/dN) was correlated with the stress intensity range ΔK. The FCP rate was found to be insensitive to frequency at sub room temperatures. The fatigue crack propagation resistance of CPVC was enhanced with increasing cyclic frequency at 50 and 70 °C. Frequency effect on FCP rate was found to be higher in the low frequency range.Macro-fractographic analysis of fracture surface showed that stepwise crack propagation existed at 0.1 and 1 Hz for all temperatures of interest.     

Remaining fatigue life estimation of corroded bridge members

This paper presents a method to estimate remaining fatigue lives of railway bridge members subjected to time‐dependent corrosion. The method addresses effects of material loss due to general (uniform) corrosion and fatigue strength degradation of material due to corrosive environment. The method mainly consists of stress history, which is obtained by considering the effect of time‐dependent loss of material, full‐range S–N curve, which represents the corrosive environment, and sequential law, which takes the loading sequence effect more precisely in to account than Miner's rule. Initially, nonlinear behaviour of material loss over time (i.e. time‐dependent growth of corrosion wastage) is discussed, and hence, necessary formulae to calculate time‐dependent cross‐sectional properties are comprehensively presented. Then finite element analysis‐based procedure is clearly proposed to predict stress histories of corroded members. A technique is introduced to obtain the full‐range S–N curve for the corroded structural detail. The concept of sequential law is summarized with the algorithm, and then the proposed method is applied to predict the remaining fatigue lives of the corroded members of a railway bridge. The predicted remaining lives were compared with the previous method‐based estimations, and comparisons reveal the range of 16–47% reduction of fatigue lives of critical members when time‐dependent corrosion is taken into account. Also, the results reveal that the corroded members of smaller cross‐sectional area are most vulnerable for fatigue damage. Finally, significance of the proposed method is confirmed.     

Corrosion and Fatigue Testing of Microsized 304 Stainless Steel Beams Fabricated by Femtosecond Laser

The 304 stainless steel (SS) microcantilever specimens with dimensions of 30 μm×30 μm×50μm (thickness×width × length) were fabricated by femtosecond (fs) laser. The microsized cantilevers of good quality with structure and dimensions according commendably with that of the designed cantilever were obtained. The result shows that fs laser micromachining is a promising method for directly fabricating metallic microcomponents.Corrosion and fatigue properties of microsized specimens were carried out on the microsized 304 SS cantilever beams by a newly developed fatigue testing machine. The results show that the microsized 304 SS specimens appear to have an improved resistance towards localized corrosion compared to ordinary-sized 304 SS specimens after the static corrosion testing. The testing result shows that the presence of corrosive solution reduces the fatigue lifetime of the 304 SS specimen by a factor of 10-100. The maximum bending loads measured by fatigue testing machine decrease rapidly at the terminal stage of environment assisted fatigue testing.Corrosion fracture first occurred at the range of notch with a higher tensile bending stress, and exhibited clear evidence of trans-columnar fracture detected by SEM (scanning electron microscopy).     

Corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads

The corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads was examined. This aluminum alloy is typically used in aerospace structural components such as the wing spars of aircraft. Axial fatigue specimens were subjected to a loading spectrum that consisted of a fully reversed periodic overload of near-yield magnitude followed by 200 smaller cycles at high R-ratio. The specimens were fatigue tested while they were fully immersed in an aerated and recirculated 3.5 wt% NaCl simulated seawater solution.The results for the corrosion-fatigue testing were compared to data obtained for the same overload spectrum applied in laboratory air. A damage analysis showed that the presence of the corrosive environment accelerated the damage accumulation rate to a greater extent than that observed in air, particularly at low stress ranges. This resulted in a reduction in the fatigue strength of the material when it was simultaneously subjected to overloads and a corrosive environment. It is believed that the reduced fatigue life was due primarily to corrosion pit formation and a combination of anodic dissolution at the crack tip and hydrogen embrittlement. For practical purposes, the endurance-limit of the material disappears under these conditions.     

A study on corrosion‐fatigue behavior of AA7075‐T651 subjected to different surface modification treatments

Two corrosive media were used (3.5 wt% NaCl aqueous solution and distilled water) to examine the corrosion‐fatigue behavior of AA 7075‐T651, subjected to various surface modifications (wire‐EDM, blasting, and anodizing). An in‐situ corrosion‐fatigue device was used to test the corrosion‐fatigue durability. The apparatus is able to generate cyclic loads within a corrosive solution. The mechanical loading is simulated with the aid of finite element method (FEM). At both corrosive environments, a prolongation of the corrosion‐fatigue life was achieved by the blasting procedure, compared with the as‐machined specimens under same conditions. Anodizing had a deleterious impact in all examined cases.     

S‐N curve for riveted details in corrosive environment and its application to a bridge

A formula for stress‐life curve is proposed to predict the fatigue life of riveted bridges located in corrosive environments. The corrosive environment‐dependent parameters of the S‐N curve are determined based on the corrosion fatigue testing results of different types of steel specimens in air, fresh water, and seawater. Eurocode detail category 71 and UK WI‐rivet detail category represent the fatigue strength of riveted members. The proposed S‐N curve formula is compared with full‐scale fatigue test results of riveted joints, plate girders, and truss girders, which were tested in a corrosive environment. Thus, the validity of the formula is confirmed. The formula does not require any material parameter other than the code‐given fatigue curve of riveted details. The fatigue life of a riveted railway bridge is estimated by using the proposed formula, and the results are compared with conventional approaches. The applicability and significance of the proposed curve are confirmed.     

Schwingungsrißkorrosionsverhalten des Duplexstahles X 2 CrNiMoN 22 5 3 unter definierten Wärmeübergangsbedingungen

Corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 under heat transfer conditions The corrosion behaviour of metallic components is not only affected by the temperature of the corrosive environment but also by the heat transfer conditions between the heated material and the cooling agent. Therefore the corrosion fatigue behaviour of the austenitic-ferritic stainless steel X 2 CrNiMoN 22 5 3 (german material-number 1.4462) in 3% NaCl-solution is investigated for isothermal conditions and three different heat transfer conditions. The specimens are tested under cyclic tension load (R = s?_u/s?_o = 0) with a frequency off =25 Hz up to N_Grenz 10⁷. The isothermal fatigue strengths are 380 N/mm² for room temperature and 340 N/mm² for a temperature of 70°C. For heat transfer conditions between the sinusoidal loaded specimens and the corrosive agent a new developed testing equipment is presented. The corrosion fatigue strength for a heat flux of 45 KW/m² reaches a value of 410 N/mm², while the improvement relative to the isothermal room temperature strength is lower for higher heat flux values (100 and 150 KW/m²). The better corrosion fatigue behaviour for heat transfer conditions bases on the favourable conditions for the formation of the passive layer. The thickness of the layer is nearly twice as high as for isothermal room temperature corrosion and therefore the crack initiation is delayed. For higher values of heat flux local corrosion attack is found. With that the positive effect on corrosion fatigue strength is diminished.     

Dynamic fatigue behaviour of a steatite ceramic

Dynamic fatigue of a low dielectric loss steatite was investigated. To this end, the values of n and B, the so‐called subcritical crack growth (SCG) parameters were experimentally determined. The steatite exhibited the expected dynamic fatigue behaviour, so that the stress corrosion susceptibility parameter, n, of 24 was obtained. In addition, the material/environment parameter B, which is a constant for a given test environment, was also attained. These parameters are instrumental in predicting the lifetime of components under stress. When the applied load is such that the resulting strength equals half of the inert strength (σ_i), defined as the strength of a sample tested in an inert environment or at a fast stress rate, i.e. where no subcritical crack growth occurs, the time to failure (t_f) of the material was found to be ～140 h. Measurement of the fracture toughness of steatite is also of upmost importance and so it was determined using three test methods. A value of K_Ic = 1.91 ± 0.29 MPa m^1/2 was attained by the indentation fracture method through measurement of the cracks emanating from the Vickers indentation. This value is in good agreement with those determined using the K_Isc (surface crack in flexure) test method (2.21 ± 0.07 MPa m^1/2) and fractography analysis test method (2.00 ± 0.44 MPa m^1/2). Differences in test procedure and analysis causing the values from each test method to be different are discussed.     

Evaluation of the Effects of Corrosion on Fatigue Life of Clad Aluminum Alloy 2024-T3-Riveted Lap Joints with Acoustic Emission Monitoring

Corrosion affects the fatigue life of clad aluminum alloy-riveted lap joints, such as those found on an aircraft fuselage structure. Single-, double-, and triple-column-riveted lap joint specimens were fabricated and corroded in a Q-Fog accelerated corrosion chamber for five months using an ASTM G85-A5 prohesion test. Specimens were taken out of the chamber every 4?weeks, and the corrosion products which had been deposited on them were removed by immersion in concentrated nitric acid. For each corroded specimen, the mass loss with corresponding corrosion rate was determined. The specimens were fatigue loaded to failure on an MTS Universal Testing Machine with acoustic emission monitoring. Results indicate that exposure of lap joint specimens to this corrosive environment increased corrosion (mass loss), corrosion rate, and significantly reduced fatigue life. For a prolonged exposure in the corrosive environment, the fatigue life was reduced to zero, which has significant implication for aging aircraft. Acoustic emission monitoring successfully detected fatigue failure. Two failure modes, multisite crack damage and shear of the rivets, were observed.