The multiaxial weld root fatigue of butt welded joints subjected to uniaxial loading

In this study, the fatigue strength of inclined butt welds subjected to a proportional multiaxial stress state generated by uniaxial loading is studied in nominal and local stress concepts. The local methodologies studied included principal stress hypothesis, von Mises stress hypothesis and modified Wöhler curve method. Nominal methodologies included modified Gough–Pollard interaction equation, the design equation in Eurocode3 and the interaction equation in DNV standard. Results are evaluated along with data published in relevant literature. It is observed that both local and nominal stress assessment methods are able to estimate multiaxial fatigue strength. No obvious difference in fatigue strength is observed in the nominal stress concept, but the notch stress concept is able to capture a decrease in fatigue strength in shear‐dominated joints. It is concluded that modified Wöhler curve method is a suitable tool for the evaluation of fatigue strength in joints dominated by both normal and shear stresses.     

Flight-by-flight fatigue crack growth life assessment

Conventionally, fatigue crack growth in aircraft structures under flight spectrum loading is often analysed and predicted based on crack growth rates obtained from constant-amplitude crack growth testing with cycle-by-cycle life prediction methods or models. Because the mechanism of fatigue crack growth under spectrum loading is yet to be fully understood, no matter how closely the models are able to account for the load interaction effects, the predictions generally have to be subjected to the validation by fatigue crack growth tests using either representative specimens or real structures under the representative flight spectrum. In view of this fact, it is not difficult to deduce that the predictions should be much more reliable if the predictions are made directly based on the flight spectrum crack growth data. Therefore, a new approach to fatigue crack growth life assessment has been proposed in this paper based on the analysis of flight-by-flight fatigue crack growth data measured by quantitative fractography for several common aircraft structural materials under various fighter aircraft flight spectra. Quantitative fractography was successfully used for titanium coupons to generate crack growth curves under flight spectrum loading. The crack growths were also shown to be exponential. As a demonstration, the flight-by-flight approach was used to determine fatigue crack growth lives of aircraft aft fuselage frames under a fighter aircraft usage.     

On the use of nominal stresses to predict the fatigue strength of welded joints under biaxial cyclic loading

In this paper, the modified Wöhler curve method proposed by Susmel and Lazzarin is employed to predict the fatigue life of welded connections subjected to biaxial cyclic loading. This criterion is reformulated here in order not to take into account the mean stress effect, as suggested by several design codes (at least when welded connections are not completely stress relieved). The accuracy of the proposed method in fatigue lifetime estimation was evaluated by using a number of data sets taken from the literature. The modified Wöhler curve method was applied in terms of nominal stresses and was calibrated using the uniaxial and torsional fatigue curve determined by reanalysing the experimental data, as well as using the standard fatigue curves of the Eurocode 3. The proposed approach was seen to be successful, giving multiaxial fatigue life predictions located within the widest scatter band related either to uniaxial or to torsional data, independently of both out‐of‐phase angle and load ratio value. Finally, the accuracy of the modified Wöhler curve method was compared to the one obtained by applying the procedure suggested by the Eurocode 3: the proposed criterion is demonstrated to be much more accurate and reliable than the standard one.     

Übertragbarkeit – ein zentrales Problem der Lebensdauervorhersage schwingbelasteter Bauteile

Transferability – a fundamental problem for fatigue life prediction Frequently specimen fatigue data have to be applied for fatigue life assessment of components and structures. It has to be assured that such data are relevant for the component under consideration. Equally it has to be assured that constant amplitude S-N data can be used for fatigue assessment in the case of spectrum loading. It is shown that application of transfer functions which are often of empirical nature requires an understanding of basic principles of material and component behaviour. Life prediction concepts should acknowledge the existence and relative portion of the crack initiation and crack propagation phases which are dominated by different mechanisms.     

Design of Emergency Aircraft Repairs Using Composite Patches

The increasing demand for fatigue life extension of both military and civilian aircraft has led to advances in repair technology for cracked metallic structures. Conventional structural repairs may significantly degrade the aircraft fatigue life and lower its aerodynamic performance. Adhesively bonded composite reinforcement is a new technology of great importance due to the remarkable advantages obtained, such as mechanical efficiency and repair time and cost reduction. In this article, bonded composite patch repairs were designed for quick application to aircraft under emergency conditions, such as aircraft battle damage repair (ABDR). A formulated method was developed, to be applied when damage has to be restored quickly, without restrictions to safety of flight. Different damage cases were investigated using finite-element analysis (FEA), taking into account specific parameters of the structure under repair. Based on the FEA results, a quick design procedure using composite patch repairs for the most frequent damage cases is proposed.     

Probabilistic fatigue assessment for railway axles and derivation of a simple format for damage calculations

This paper describes a procedure for the determination of railway axle risk of fatigue failure under service loading for a simple fatigue assessment compliant to modern structural recommendations.After an initial review of reliability assessment under fatigue, a fully probabilistic approach is outlined, whose input data for the fatigue damage obtained with the EURAXLES project are briefly summarized. Then, a series of Montecarlo simulations was carried out in order to determine the maximum allowable stress for a given axle made of EA4T and EA1N under service conditions identified by different load spectra from the literature.Results have been obtained in terms of a safety factor for damage calculations that allows designers to adopt a simple semi-probabilistic approach for designing axles for a target reliability against fatigue. The application of this procedure to a railway axle then shows how safety factors should be have to be further increased for taking into the prospective presence of impact damages.     

A simple approximate expression for finite life fatigue behaviour in the presence of ‘crack‐like’ or ‘blunt’ notches

In this note, we explore the possibility of simple extensions of the heuristic El Haddad formula for finite life, as an approximate expression valid for crack‐like notches, and of the ‘Luká? and Klesnil’ equation for blunt notches. The key starting point is to assume, in analogy to the Basquin power‐law SN curve for the fatigue life of the uncracked (plain) specimen, a power law for the ‘finite life’intrinsic El Haddad crack size. The approach has similarities with what recently proposed by Susmel and Taylor as a Critical Distance Method for Medium‐Cycle Fatigue regime. Reasonable agreement is found with the fatigue data of Susmel and Taylor for notches, and in particular the error seems smaller in finite life than for infinite life, where these equations are already used. In these respects, the present proposal can be considered as a simple empirical unified approach for rapid assessment of the notch effect under finite life.     

The fatigue of aircraft structures

Current procedures for fatigue design and for fatigue life estimation, substantiation and monitoring of aircraft structures are reviewed. Major gaps in the present state of knowledge are identified and further research directed towards filling these gaps is discussed.Brief reference is made to the major fields of basic or fundamental research connected with the fatigue of structures including, the application of fracture mechanics to the theory of crack propagation, elevated temperature behaviour of aircraft materials, atmospheric turbulence, reliability based design and aeroelastic behaviour.It is seen that with the present advances in aircraft design and utilisation the fatigue assessment of aircraft structures is an increasingly complex problem for which no general method of solution has yet been established. At present reliance is placed on extensive testing of design details and components, usually in conjunction with a full scale fatigue test, to validate the fatigue analysis and substantiate, or provide the essential data used for fatigue life monitoring. There is however a well developed trend for basic studies of the various aspects of fatigue behaviour, to find increasing application in the interpretation of experimental results for fatigue life assessment and improved methods of fatigue design.     

Größeneinfluß auf die Schwingfestigkeitseigenschaften von Schweißverbindungen

The size effect on the fatigue properties of weldments Constant amplitude fatigue tests (R = ? 1 and 0)have been carried out in order to investigate the influence of specimen size and manufacturing conditions on the high-cycle fatigue behaviour of large welded I-beams (web height 500 mm, total length 12 and 7 m respectively). The specimens contained transverse butt welds and longitudinal fillet welds, mainly produced by manual metal arc welding. The Δs?-concept, as proposed in international standards for fatigue related design is supported by the test results as far as transverse butt welds or longitudinal fillet welds in large sections are concerned. The HCF-behaviour of butt joints in large sections, welded under site conditions, are reasonably covered by Eurocode 3 category 90. EC 3-Category 100 does not conservatively cover the HCF-behaviour of longitudinal fillet joints in large sections.     

Interim fatigue design recommendations for fillet welded joints under complex loading

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIW S – N curve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out-of-phase (i.e. non-proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out-of-phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.     

Load‐bearing capacity of slender unreinforced masonry compression members under biaxial bending

Dedicated to Prof. Dr.‐Ing. Carl‐Alexander Graubner on the occasion of his 60th birthday Masonry members have to resist vertical loads and bending moments about the weak axis due to rotation of adjacent slabs. If the compression member is part of the bracing system, there are also bending moments about the strong axis. This paper deals with the load‐bearing capacity of biaxially eccentrically compressed unreinforced compression members with rectangular cross‐sections. For linear‐elastic material, the principles of an analytical model is presented, which considers geometrical and physical (cracking) non‐linearity. The deflections of the wall can be determined by using moment‐curvature relations, making possible the analytical analysis of compression members considering the effects of 2nd order theory. For a non‐linear stress‐strain relation, the calculation of the load carrying capacity of rectangular compression members under biaxial bending is complex and has to be determined numerically. The good accordance of the results of the analytical model with the numeric calculations is also shown for various eccentricities. In addition, a simplified proposal for the calculation of the load‐bearing capacity of biaxially eccentrically compressed unreinforced compression members is shown. The proposal is based on the load‐bearing capacity of uniaxially eccentrically compressed unreinforced compression members. Therefore it is possible to use the proposal considering existing models, for example according to Eurocode 2 or 6.     

Fatigue life recovery in aluminium alloy aircraft structure

Abstract This paper presents the results of several DSTO research programmes which investigated the effectiveness of the fatigue life enhancement method used on RAAF F/A‐18 aircraft – glass bead peening. The research identified ways in which process improvement could enhance this effectiveness, and developed a procedure for mid‐life reworking of critical airframe parts to effectively restore the original fatigue life. The procedure included removing a very thin layer of material (and with it, any undesirable manufacturing features and accumulated fatigue cracking) from the surface. Further life recovery can be achieved, if required, by applying an optimized peening procedure. This process has been developed to allow restoration of fatigue life to critical airframe components which are thought to be accumulating fatigue crack damage faster than desired.     

Experiments and model predictions for fatigue crack propagation in riveted lap‐joints with multiple site damage

In this paper, the growth of long fatigue cracks up to failure in aircraft components is studied. A deterministic model is presented, able to simulate the growth of fatigue through cracks located at rivet holes in lap‐joint panels. It also includes criteria to assess the link‐up of collinear adjacent cracks in a MSD scenario. To validate the model, a fatigue test campaign was carried out on riveted lap‐joint specimens in order to produce experimental crack growth and link‐up data. Accurate measurements of naturally occurred surface cracks were automatically performed by the Image Analysis technique, thus allowing the tests to run 24 h a day. The comparison between experimental tests and numerical simulations is good, thus confirming the model as a useful tool for the assessment of fatigue life of aircraft riveted joints.     

THE FATIGUE RESISTANCE OF PEENED 7050-T7451 ALUMINIUM ALLOY—REPAIR AND RE-TREATMENT OF A COMPONENT SURFACE

Abstract— Metal and glass-bead peening treatments, widely used throughout the aircraft industry to enhance the fatigue performance of many steels and titanium alloys, are now being routinely applied to high-strength aluminium-alloy components. This paper describes the effects of peening on the fatigue life of 7050 aluminium alloy material, which is representative of alloys used for many components in modern military aircraft. Using simulated service loading, two proposed peening/re-peening procedures were evaluated and compared with both the original peened surface and a simple hand-polished surface. The results show that optimisation of peening parameters to reduce surface damage can provide a substantial improvement in fatigue life over both the original peening treatment and the polished surface treatment, however, poor control of peening procedures, or unnecessary "overpeening" can lead to a relatively poor fatigue life. For re-peened surfaces, a procedure incorporating a polishing step, designed to repair any damage from the severe peening applied initially, gave the best fatigue performance. Results are discussed in relation to the stability of the residual surface stresses under fatigue loading, the surface roughness, and the number and types of defects introduced by the peening treatments.     

钢箱梁桥海量应变监测数据分析与 疲劳评估方法研究

海量监测数据的分析与应用是桥梁结构健康监测研究中的重要问题之一,以润扬大桥钢箱梁的长期应变监测数据为研究对象,研究了海量应变监测数据分析与疲劳评估方法。首先,基于Eurocode3规范的S-N曲线和Palmgren-Miner准则建立了焊缝疲劳损伤分析及寿命预测方法。其次,研究了应力循环的快速提取及应变数据中随机干扰成分的剔除方法。在此基础上,讨论了对公路钢箱梁桥开展长期持续疲劳监测的必要性,并给出了润扬大桥钢箱梁焊缝的疲劳寿命预测值。研究发现,采用MATLAB与C语言联合编程的雨流计数法可快速处理海量应变监测数据;根据对焊缝疲劳损伤的贡献可确定有效应力循环阀值,从而剔除由随机干扰引起的数量极多但幅值较小的应力循环;焊缝疲劳损伤在一年内会发生大幅度的变化,短期应变监测数据可能导致焊缝疲劳寿命评估值出现较大的偏差。     

Wechsellast‐Prüfung von Hüftgelenk‐Pfannen mit keramischen Einsätzen

Fatigue Test of Hip‐Joint Cups with Ceramic Inserts A safe design of hip‐joint components for the long‐term use in humans requires an evaluation of the fatigue lifetime under the in‐vivo loading conditions. For compound cups with ceramic inserts no standardized or commonly agreed procedure exists up to now. Therefore, a test is presented here which is practised by the authors and which possibly could be the basis for a standard. Because of the expensive testing procedure and the large number of possible cup‐insert combinations a failure test with only few samples was chosen.     

A method for assessing critical plane‐based multiaxial fatigue damage models

Fatigue failure is a complex phenomenon. Therefore, development of a fatigue damage model that considers all associated complexities resulting from the application of different cyclic loading types, geometries, materials, and environmental conditions is a challenging task. Nevertheless, fatigue damage models such as critical plane‐based models are popular because of their capability to estimate life mostly within ±2 and ±3 factors of life for smooth specimens. In this study, a method is proposed for assessing the fatigue life estimation capability of different critical plane‐based models. In this method, a subroutine was developed and used to search for best estimated life regardless of critical plane assumption. Therefore, different fatigue damage models were evaluated at all possible planes to search for the best life. Smith‐Watson‐Topper (normal strain‐based), Fatemi‐Socie (shear strain‐based), and Jahed‐Varvani (total strain energy density‐based) models are compared by using the proposed assessment method. The assessment is done on smooth specimen level by using the experimental multiaxial fatigue data of 3 alloys, namely, AZ31B and AZ61A extruded magnesium alloys and S460N structural steel alloy. Using the proposed assessment method, it was found that the examined models may not be able to reproduce the experimental lives even if they were evaluated at all physical planes.     

Definition and validation of Eurocode 3 FAT classes for structural steels containing oxy-fuel,plasma and laser cut holes

When the fatigue behaviour of structural components containing holes is analysed, Eurocode 3 only considers the fatigue performance of drilled holes, limiting the use of thermal cutting processes to produce, for example, bolt holes. This paper studies the fatigue performance of structural steel plates containing thermally cut holes. The research covers three thermal cutting methods: the traditional one (oxy-fuel cutting) and two more modern processes (plasma and laser cutting). An experimental program composed of 150 fatigue specimens has been completed, combining four steels (S355M, S460M, S690Q and S890Q), the three thermal cutting methods and two different thicknesses (15 mm and 25 mm). The S–N results obtained have been used to estimate the corresponding Eurocode 3 FAT classes, which have finally been validated by comparing them to additional experimental data found in the literature.     

具有隔震构造的核电站安全壳在强烈地震和大型商用飞机撞击下的振动响应分析

核电站安全壳的安全性和完整性在建造运营期间必须要得到保证,尤其是对于强烈地震和飞机撞击等超设计基准事故。基础隔震作为一种能有效降低上部结构地震响应并防止结构破坏的典型隔震措施,在未来的核电站设计建造中备受关注,因而在核电站隔震系统的建造设计时应考虑大型商用飞机撞击的影响。该文基于CPR1000核电站安全壳,在底板下侧布置了不同型号的高阻尼橡胶隔震支座,从结构的隔震率（DAR）、最大水平位移和加速度反应谱,对安全壳在强烈地震作用下的隔振性能进行了综合分析。并结合采用荷载时程法以A320商用飞机撞击荷载为例进行飞机撞击核电站安全壳的分析,着重从飞机撞击后的振动衰减过程的振动特性以及飞机撞击诱导振动的加速度反应谱的分析结果对具有隔震构造的安全壳振动规律进行了分析。