Fatigue life estimation after crack repair in 6005 A-T6 aluminium alloy using the cold expansion hole technique

In this paper, the hole drilling (HD) and the cold expansion (CE) processes, which were used as a technique for crack repair, were investigated in order to estimate the beneficial effects on fatigue crack initiation (FCI). The FCI life is defined as the number of cycles to initiate a new crack of 0.2 mm on the surface of the specimen. Three hole radii and three degrees of cold expansion (DCE%) values were tested after a crack propagation period. Crack retardation after the CE process was observed. This phenomenon is due to two mechanisms: retardation owing to both geometric and mechanical effects, which is produced by the stress concentration at the drilled hole, and the large strain‐induced compressive residual stresses around the hole. In this report, the influence of the loading conditions was studied. For high values of the stress intensity factor range ΔK_ρ around the hole (based on the pseudo crack length a + ρ), the number of cycles corresponding to crack initiation N_i is low. At the edge of the hole, the maximum stress range can be approximated by the following formula: Δσ_max = 2ΔK_ρ /√πρ , where ρ is the hole radius and ΔK_ρ is the related stress intensity factor range.The FCI life extension, defined by the number of cycles corresponding to crack re‐initiation N_i , is related to the relative maximum stress range ratio R_σ = [(Δσ_max )/(Δσ_max )_th ] where (Δσ_max )_th is the value of the threshold maximum stress range obtained when N_i = 2 × 10⁶ cycles. The relationship between N_i and R_σ may be written as a power function.     

Frequency-domain fatigue life estimation with mean stress correction

Two frequency-domain fatigue life calculation methods are presented which take into account the impact of the mean stress effect. The emphasis is set on the algorithm for fatigue life assessment of the method proposed by the authors. It is supplemented with a mean stress effect correction. Correction method is based on the direct transformation of the zero mean stress Power Spectral Density (PSD) due to mean stress. The method is verified on the basis of own results for the S355JR steel. The authors analyze five models for the designation of the probability density function used in the calculation process. The results are presented in the form of probability distributions after PSD transformation and the calculated fatigue life is being compared with the experimental life in fatigue comparison graphs. An analysis of the choice of a mean stress correction model is also widely discussed and a fatigue life estimation is also performed. The method proposed by the authors is being compared with the Kihl–Sarkani method for mean stress correction in frequency domain.     

Fatigue failure of adhesively patched 2024-T3 and 7075-T6 clad and bare aluminium alloys

The fatigue behaviour of adhesive patches used for repairing aircraft components was investigated. Adhesive patches were simulated using single‐lap shear specimens on clad and bare 7075‐T6 and 2024‐T3 aluminium alloy substrates. Stress–life curves were generated under constant amplitude loading at three stress ratios: R=?1, 0 and 0.5. In the bare materials, failure always occurred in the adhesive itself leaving the substrates intact. At fatigue lives below about 100 000 cycles, the clad alloy specimens also failed in this manner. However, at lower stress levels, the clad alloys failed by cracks initiating in the cladding layer along the end of the lap and subsequently propagating through the substrate. The fatigue strength of the substrate, due to the adhesive patch on the clad materials, was reduced by an order of magnitude compared to the Military Handbook values.     

Fatigue limit and life evaluation formulae for compressive mean stress states

Considering the difference of stress concentration behaviour near micro-defects in materials, a unified fatigue limit evaluation formula has been developed, which can be applied to compressive mean stress states without the necessity of introducing additional material’s properties except for the modification parameter of mean stress. A generalised life evaluation formula has also been proposed and the dependence of fatigue life on mean stress can be simply expressed by the effect of fatigue limit corresponding to the mean stress. By considering the effect of fatigue limit corresponding to the compressive mean stress, the fatigue life can be evaluated by the same generalised formula as that developed for tensile mean stress state, without the necessity of carrying out additional S–N curve fatigue tests.     

A new algorithm for estimating fatigue life under mean value of stress

This paper discusses two problems: allowing for mean value of torsional stress and the variability of material properties with out of‐parallel fatigue characteristics. The effect of normal mean stress and shear mean stress is modified by reduction coefficients, which, to a large extent, depend on the value of existing loads. These coefficients have been developed experimentally on the basis of an analysis of the findings from fatigue tests on 2017A‐T4 and 6082‐T6 aluminium alloys and S355 alloy steel. The methods of calculation, suggested in this paper, are applicable to the materials in elastic–plastic state. The suggested algorithm for estimating fatigue life for the combination of bending or tension and compression and torsion under shear stress is based on Kluger's stress criterion. The usability of the algorithm was verified by comparing the calculation results with the results of own experimental tests on 2017A‐T4 and 6082‐T6 aluminium alloys, which have been noted to indicate sensitivity to shear mean stress, and the tests found in the professional literature (tests on S355, 30CrNiMo8 and 30NCD16 steel and Ti‐6Al‐4 V titanium alloy). A comparative analysis of the calculation and experimental results proved that there is a satisfactory correlation between them.     

Study on material property changes of mild steel S355 caused by block loads with varying mean stress

The paper presents the results of research on material property and structure changes of S355 steel samples induced by fatigue of material subjected to block bending loads, with varying mean load value. In the tests, the mean load was increased and decreased in subsequent blocks, where the amplitude varies in accordance with the accepted maximum bending moment value. The changes of strains recorded during the tests show higher maximum mean strain for the case, where the mean load increases in block. Otherwise, the maximum mean strain is three times less, than before. Some hardening occurs, which affects the mechanical properties of the material. This is visible in the results of tensile tests, where ultimate strength increases for load path with increased mean load value. Metallographic examinations revealed no fatigue cracks at this level of fatigue life.     

轨道交通用6082-T6铝合金MIG焊接接头组织与疲劳性能

对轨道交通用6082-T6铝合金进行MIG焊接,使用光学显微镜、X射线衍射仪、显微硬度计分别对焊接接头的显微组织、相结构与显微硬度进行观察与分析。结果表明,熔合区为柱状晶组织,焊缝主要由树枝晶和胞状晶组成,焊缝中心为等轴晶,焊缝体现出联生结晶的特点。母材相组成为基体α-Al固溶体、β-Mg2Si以及单质Si,焊缝金属相组成主要为α-Al固溶体。热影响区的宽度达31mm,且存在一个软化区。设定应力比R=0,测试了铝合金的疲劳寿命,通过拟合试验数据得到S-N曲线,得到MIG焊接6082-T6铝合金焊接接头的条件疲劳极限为99MPa,为母材的72.26%。用扫描电镜对疲劳断口进行观察和分析,结果表明,疲劳断口分布的二次裂纹促进了疲劳裂纹萌生和扩展,第二相粒子对疲劳裂纹的萌生起着重要的作用,焊缝中的气孔则容易成为疲劳源。稳态扩展区出现大量呈平行趋势且具有规则间距的疲劳条带,瞬断区存在大量韧窝和撕裂棱,体现出韧性断裂的特征。     

A mean stress correction model for tensile and compressive mean stress fatigue loadings

A new mean stress fatigue model based on the distortional strain energy is proposed to account for the mean stress effects on fatigue life. The proposed model is compared with the Morrow and the Smith‐Watson‐Topper (SWT) mean stress correction models using a number of experimental data sets for one cast iron, two steels and two aluminium alloys under tensile and compressive mean stress loadings. It is found that both the proposed mean stress correction model and the SWT model yield similar results and provide very good correlation for positive mean stress data and moderate negative mean stress data. For high compressive mean stresses, the proposed model shows reasonably good correlations, while the SWT model fails to correlate the fatigue data. The Morrow model was found to give poor correlations for all fatigue data analysed by yielding conservative results for compressive mean stresses and non‐conservative results for tensile mean stresses.     

Determination of structural stress for fatigue analysis of welded aluminium components subjected to bending

The results of extrapolation procedures for the determination of structural stresses are often questionable due to the fact that the stresses at extrapolation points obtained with finite element analyses can be strongly dependent on the mesh size of finite element model and loading mode. Also, existing design S–N curves are derived mostly on the basis of fatigue testing of joints loaded axially. In the present paper the influence of the finite element mesh size on the structural stress value determined by a linear extrapolation method is analysed. Also, the paper examines the possibility of using existing design S–N curves for cases of bending induced by a force on the welded stiffener. Fatigue test results from aluminium welded components with longitudinal or round pad stiffeners subjected to bending loads have been assessed using a structural stress range approach, and compared with the structural stress design S–N curve FAT 40 (IIW) and the structural stress design S–N curve FAT 44 (Eurocode 9). It is concluded that the more precise estimation of fatigue life of aluminium components subjected to bending can be achieved with structural stress design S–N curve proposed by Eurocode 9. The conclusions also include recommendations for regarding component finite element modelling for the determination of structural stresses in case of bending.     

Experimental measurement of crack opening and closure loads for 6082-T6 aluminium subjected to periodic single and block overloads and underloads

Crack closure and opening stresses have been determined for 6082-T6 aluminium, subjected to single and block overloads and single and block overloads and underloads, using an optical method. The distance between the overloads was varied in order to investigate the effect that crack growth distance between overloads has on crack growth rate. The optical method provided high magnification allowing crack closure away from the crack tip to be determined and the crack advance to be monitored dynamically. The data obtained gives insight into the significance of periodic overloading and underloading as well as what effect interactions, such as crack branching, have on crack growth retardation. The results obtained also enable numerical and analytical codes to be evaluated.     

Fatigue performance and fatigue damage parameter estimation of spot welded joints of aluminium alloys 6111‐T4 and 5754

In the present study, fatigue behaviours of spot welded joints of aluminium alloys 6111‐T4 and 5754 have been experimentally investigated. Fatigue results indicate that fatigue strength of spot weld primarily depends on specimen loading type and gauge thickness. Effects of base material and load ratio on fatigue resistance of welded specimen are insignificant. An equivalent stress based fatigue damage parameter is derived to consolidate empirical data and develop predictive capabilities for automobile designers. The fatigue damage parameter defined in this study is proven effective in consolidating a large amount of fatigue data into a narrow band and is especially suitable for the comparative fatigue strength evaluation of components and specimens.     

A critical assessment of methods for the determination of the shear stress amplitude in multiaxial fatigue criteria belonging to critical plane class

Multiaxial high cycle fatigue criteria based on the critical plane approach necessitate unambiguous definitions of the amplitude and mean value of the shear stress (τ_a and τ_m) acting on the material planes. Four of the existing definitions relate the values of τ_a and τ_m to a geometrical element of the curve described by the tip of the shear stress vector (curve Ψ), respectively, the radius of the Minimum Circumscribed Circle, the Longest Chord, the Longest Projection, the diagonal of the Maximum Rectangular Hull (MRH).In this paper a critical assessment of the above definitions is proposed, focusing on that based on the concept of MRH, which is the most recently developed. The main issues of the comparison are the uniqueness of the solution in the determination of τ_a and τ_m, the ability to differentiate proportional and non-proportional stresses, the differences of the values of τ_a obtained by each of the 4 methods for differently shaped curves Ψ.     

Influence of casting defect and SDAS on the multiaxial fatigue behaviour of A356-T6 alloy including mean stress effect

The effect of mean stress on the multiaxial High Cycle Fatigue (HCF) behaviour of cast A356-T6 alloy containing natural and artificial defects with varying Secondary Dendrite Arming Spacing (SDAS) has been investigated experimentally. Tension, torsion and combined tension–torsion fatigue tests have been performed for two loading ratios: R_σ = 0 and R_σ = −1. A Scanning Electron Microscopy (SEM) was used to perform fractographic analysis of the fracture surfaces to characterise the defect causing failure. In order to gauge the effect of mean stress and defects, the results are reported with standard Kitagawa and Haigh diagrams. A surface response method has been employed to characterise the influence of defect size and SDAS on the fatigue limit. Relationships and correlations describing the observed behaviour have been incorporated in the Defect Stress Gradient (DSG) criterion with the goal of determining the influence of defects on the fatigue limit through a stress gradient approach.Results clearly show that: (i) the mean stress has a detrimental effect on the fatigue limit. This effect is a function of the loading, which is most pronounced under tension, less under combined tension–torsion, and least pronounced under torsion conditions; (ii) in the absence of defects, the SDAS controls the fatigue limit of cast A356, this effect is much more important under torsion loading; (iii) the DSG criterion is improved by the mean of a parameter describing the microstructure effect through the SDAS.     

Analysis of high temperature plastic behaviour and its relation with weldability in friction stir welding for aluminium alloys AA5083-H111 and AA6082-T6

The influence of the plastic behaviour of two aluminium alloys, very popular in welding construction, on friction stir weldability, is analysed in this work. The two base materials, a non-heat-treatable (AA5083-H111) and a heat-treatable aluminium (AA6082-T6) alloy, are characterised by markedly different strengthening mechanisms and microstructural evolution at increasing temperatures. Their plastic behaviour, under different testing conditions, was analysed and compared. The two base materials were also welded under varied friction stir welding (FSW) conditions in order to characterise their weldability. The relation between weldability, material flow during FSW and the plastic behaviour of the base materials, at different temperatures, was analysed. It was found that the AA6082 alloy, which displays intense flow softening during tensile loading at high temperatures, and is sensitive to dynamic precipitation and overageing under intense non-uniform deformation, displays good weldability in FSW. Under the same welding conditions, the AA5083 alloy, which in quasi-static conditions displays steady flow behaviour at increasing temperatures, and is sensitive to moderate hardening at high strain rates, displays poor weldability.     

Experimental and numerical evaluations of stress relaxation in A356 aluminium alloy subjected to out-of-phase thermomechanical cyclic loadings

Abstract

In this study, the stress relaxation has been measured experimentally and has been also calculated numerically by the finite element method in the A356·0 aluminium–silicon–magnesium alloy, under out-of-phase thermomechanical cyclic loadings. To get this objective, strain based thermomechanical fatigue tests were performed on cylindrical specimens, at an out-of-phase condition. In this loading condition, when the temperature was maximum, the mechanical strain was compressive and vice versa. These fatigue experiments were repeated at various dwell times, in which the temperature was held at the maximum temperature. This hold time was considered as 5, 30, 60 and 180 s and then the stress relaxation was measured during the mid-life cycle of each test. Besides, the finite element analysis was also conducted on the material to simulate the stress relaxation numerically. A two-layer visco-plastic model was applied to simulate the high temperature cyclic behavior of the material. Finite element results showed a good agreement with experimental results, which were obtained from thermomechanical fatigue tests on the A356·0 aluminium alloy. The two-layer visco-plastic model could properly predict the stress relaxation at elevated temperatures, during various dwell times.     

The peak stress method applied to fatigue assessments of steel and aluminium fillet-welded joints subjected to mode I loading

The aim of this work is to present an engineering method based on linear elastic finite element (FE) analyses oriented to fatigue strength assessments of fillet‐welded joints made of steel or aluminium alloys and subjected to mode I loading in the weld toe region where fatigue cracks nucleate. The proposed approach combines the robustness of the notch stress intensity factor approach with the simplicity of the so‐called ‘peak stress method’. Fatigue strength assessments are performed on the basis of (i) a well‐defined elastic peak stress evaluated by FE analyses at the crack initiation point (design stress) and (ii) a unified scatter band (design fatigue curve) dependent on the class of material, i.e. structural steel or aluminium alloys. The elastic peak stress is calculated by using rather coarse meshes with a fixed FE size. A simple rule to calculate the elastic peak stress is also provided if a FE size different from that used in the present work is adopted. The method can be applied to joints having complex geometry by adopting a two‐step analysis procedure that involves standard finite element (FE) models like those usually adopted in an industrial context. The proposed approach is validated against a number of fatigue data published in the literature.     

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

The fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.     

Fatigue strength assessment of partial and full‐penetration steel and aluminium butt‐welded joints according to the peak stress method

In fatigue design of welded joints, the local approach based on the notch stress intensity factors (NSIFs) assumes that the weld toe profile is a sharp V‐notch having a tip radius equal to zero, while the root side is a pre‐crack in the structure. The peak stress method (PSM) is an engineering, FE‐oriented application of the NSIF approach to fatigue design of welded joints, which takes advantage of the elastic peak stresses from FE analyses carried out by using a given mesh pattern, where the element type is kept constant and the average element size can be chosen arbitrarily within a given range. The meshes required for the PSM application are rather coarse if compared with those necessary to evaluate the NSIFs from the local stress distributions. In this paper, the PSM is extended for the first time to butt‐welded joints in steel as well as in aluminium alloys, by comparing a number of experimental data taken from the literature with the design scatter bands previously calibrated on results relevant only to fillet‐welded joints. A major problem in the case of butt‐welded joints is to define the weld bead geometry with reasonable accuracy. Only in few cases such geometrical data were available, and this fact made the application of the local approaches more difficult. Provided the local geometry is defined, the PSM can be easily applied: a properly defined design stress, that is, the equivalent peak stress, is shown (i) to single out the crack initiation point in cases where competition between root and toe failure exists and (ii) to correlate with good approximation all analysed experimental data.     

Fatigue life prediction for butt‐welded joints considering weld‐induced residual stresses and initial damage,relaxation of residual stress,and elasto‐plastic fatigue damage

Fatigue damage of butt‐welded joints is investigated by a damage mechanics method. First, the weld‐induced residual stresses are determined by using a sequentially coupled thermo‐mechanical finite element analysis. The plastic damage of material is then calculated with the use of Lemaitre's plastic damage model. Second, during the subsequent fatigue damage analysis, the residual stresses are superimposed on the fatigue loading, and the weld‐induced plastic damage is considered as the initial damage via an elasto‐plastic fatigue damage model. Finally, the fatigue damage evolution, the relaxation of residual stress, and the fatigue lives of the joints are evaluated using a numerical implementation. The predicted results agree well with the experimental data.     

On the mixed Mode II/III fatigue threshold behaviour for aluminium alloys 2014‐T6 and 7075‐T6

This paper describes an investigation into the fatigue threshold behaviour of two structural aluminium aerospace alloys, Al 2014‐T6 and Al 7075‐T6, when subjected to Mode II, Mode III and mixed Mode II/III loading. A unique four‐point shear loading test rig was employed to cyclically load sharply edge‐notched square bar specimens using an increasing load technique. The main aim of the work has been to generate Mode II–Mode III interaction diagrams for the fatigue threshold in each case, in order to facilitate improved design procedures for components fabricated from these alloys, which are susceptible to fatigue cracking under predominantly shear type loading. Aircraft are subjected to structural loads consisting of: pressurization, tension/compression, bending, shear and torsion, both on the ground and in flight. Representative fatigue fracture surfaces have been examined using scanning electron microscopy.