Fatigue life estimation for 2017A-T4 and 6082-T6 aluminium alloys subjected to bending-torsion with mean stress

This paper proposes a model for estimating fatigue life under multiaxial stress states, based on critical plane concepts, taking into account the effect of mean shear stress. The fatigue life test results, calculated on the basis of the proposed model, are compared to the experimental ones related to 2017A-T4 and 6082-T6 aluminium alloy specimens under constant-amplitude bending, torsion and proportional combinations of bending and torsion. For the results obtained a statistical analysis is performed by comparing the calculation results with experimental data.     

Kerbgrundkonzepte für die schwingfeste Auslegung von Aluminiumschweißverbindungen am Beispiel der naturharten Legierung AlMg4,5Mn (AW‐5083) und der warmausgehärteten Legierung AlMgSi1 T6 (AW‐6082 T6)

Fatigue design of aluminium welded joints by the local stress concept exemplarily shown on the naturally aged wrought aluminium alloy AW‐5083 and the artificially aged wrought aluminium alloy AW‐ 6082 T6 Local fatigue design concepts based on material‐ and microstructural‐related parameters, e.g. the microsupport‐concept, cannot be regarded as easily applicable. The investigations, which compared the micro‐support‐concept with the local stress concept with a fictitious notch radius r_f, were carried out with different types of MIG‐welded joints of the aluminium alloys AW‐5083 and AW‐6082 T6 under fully reversed and pulsating axial loading. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to aluminium welded joints from plates with thicknesses t ≥ 5 to 25 mm independently from the alloy and weld geometries (fully or partially penetrated butt welds, transversal stiffener). Master design curves are proposed for different stress ratios, i.e. R = ‐1, 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 70 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations     

Validating generality of recently developed critical plane model for fatigue life assessments using multiaxial test database on seventeen different materials

The present studies are aimed at validation of a newly developed critical plane model with respect to large variety of engineering materials used for different applications. This newly developed model has been recently reported by present authors. To strengthen general applicability of this model, multiaxial test database consisting of a wide variety of multiaxial loading paths have been considered. The strain paths include pure axial, pure torsion, in‐phase axial‐torsion, out‐of‐phase axial‐torsion with phase shift angles varying from 30° to 180° having sine/trapezoidal/triangular strain waveforms, with/without mean axial/shear strains and asynchronous axial‐torsion strain paths of different frequency ratios etc. The materials covered in present study are mainly categorized as ferrous and nonferrous alloys. In ferrous alloy category, material grades from plain carbon steel (mild steel, 16MnR, SA333 Gr. 6, E235 and E355), low‐alloy steel (1Cr‐Mo‐V and S460 N) and austenitic stainless steel (SS304, SS316L and SS347) have been considered. In nonferrous alloy category, aluminium alloys (2024T3‐Al, 7075T651‐Al, and PA38‐T6‐Al), titanium (pure titanium and TC4 alloy), cobalt base super‐alloy (Haynes 188), and nickel alloy (Inconel‐718) have been considered. The predicted and test fatigue lives are found in good agreement for all these materials and complex multiaxial loading paths.     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Synergistic effect of environmental media and stress on the fatigue fracture behaviour of aluminium alloys

The present study aims at explaining the synergistic effect of environmental media and stress/strain on fatigue lives of aluminium alloys. Rotating bending fatigue tests were carried out using four different aluminium alloys LY12‐CZ, 2024‐T4, 7475‐T7351 and 7075‐T651, at air state, 3.5% and 5.0% NaCl aqueous solutions. These results indicated that synergistic actions of the environmental media and cyclic loading accelerated the fatigue crack propagation of aluminium alloys. Furthermore, various influence factors (such as solution concentration, cyclic numbers, high (low) strength aluminium alloys etc.) of the fatigue life at synergistic actions of the environmental media and stress were quantificationally discussed in this paper.     

Fatigue assessment of laserbeam‐welded aluminium joints under multiaxial loading

This paper presents the results and evaluation of the multiaxial fatigue behaviour of laserbeam‐welded overlapped tubular joints made from the artificially hardened aluminium alloy AlSi1MgMn T6 (EN AW 6082 T6) under multiaxial loadings with constant and variable amplitudes. Several fatigue test series under pure axial and pure torsional loadings as well as combined axial and torsional proportional and non‐proportional loadings have been carried out in the range of 2·10⁴ to 2·10⁷ cycles. The assessment of the investigated thin‐walled joints is based on a local notch stress concept. In this concept the fatigue critical area of the weld root is substituted by a fictitious notch radius r_ref = 0.05 mm. The equivalent stresses in the notch, considering especially the fatigue life reducing influence of non‐proportional loading in comparison to proportional loading, were calculated by a recently developed hypothesis, which is called the Stress Space Curve Hypothesis (SSCH). This hypothesis is based on the time evolution of the stress state during one load cycle. In addition, the fatigue strength evaluation of multiaxial spectrum loading was carried out using a modified Gough‐Pollard algorithm.     

Monotonic and low cycle fatigue behaviour of 2024‐T3 aluminium alloy between room temperature and 300 °C for designing VAWT components

In the present study, the results of the monotonic tension tests and low cycle fatigue tests performed on aluminium alloy EN AW‐2024‐T3 under various operating temperatures are presented in order to assess the fatigue behaviour of the aluminium alloy under evaluated temperatures. Monotonic tests were performed to determine the influence of temperature on mechanical properties of the material. The aim of cyclic tests was to acquire the parameters required for Manson–Coffin equation in order to plot strain–fatigue life curves. Moreover, stress–strain behaviour of the alloy and the cyclic hardening behaviour were evaluated using Ramberg–Osgood equation. Finally, P_SWT‐damage parameters for each temperature have been calculated for further investigation of the effects of the temperature on fatigue life using acquired data while taking the account of mean stress effect into calculations. Variations in the experimental data due to various test temperatures are presented for both monotonic and cyclic tests.     

A modification of UniGrow 2‐parameter driving force model for short fatigue crack growth

In this paper, a modification of the UniGrow model is proposed to predict total fatigue life with the presence of a short fatigue crack by incorporating short crack propagation into the UniGrow crack growth model. The UniGrow model is modified by 2 different methods, namely the “short crack stress intensity correction method” and the “short crack data‐fitting method” to estimate the total fatigue life including both short and long fatigue crack propagations. Predicted fatigue lives obtained from these 2 methods were compared with experimental data sets of 2024‐T3, 7075‐T56 aluminium alloys, and Ti‐6Al‐4V titanium alloy. Two proposed methods have shown good fatigue life predictions at relatively high maximum stresses; however, they provide conservative fatigue life predictions at lower stresses corresponding high cycle fatigue lives where short crack behaviour dominates total fatigue life at lower stress levels.     

Einfluss der Mikrostruktur auf das Verschleiß‐ verhalten der hartanodisierten Aluminium‐legierungen EN AW‐6082 und EN AW‐7075

Microstructural effect on the wear behaviour of the hard‐anodised aluminium alloys EN AW‐6082 and EN AW‐7075 The suitability of hard‐anodising of high‐strength Al alloys (EN AW‐7075‐T651) for the fabrication of protective coatings which are also applicable on screws was investigated. A medium‐strength AlSi1MgMn alloy (AA60682‐T6), generally rated as applicable for anodising, was used as reference material. After possible setting phenomena of a screw joint, the load‐bearing surface of the screw can be subjected to an oscillating relative movement. The damaging tribological load was simulated in an oscillation wear test. The resulting wear appearances have revealed that the untreated oxide coatings on the EN AW‐6082 substrate are not capable of providing protection against tribological load. Since hot‐water sealing increases the hardness of the coating but also contains the technology‐induced risk of softening the substrate material, other tribological protection methods have been looked for. The analysis of the tribological tests (characterisation of the structure and the resulting properties of the material, measurement of the wear amount and analysis of the wear appearance) have shown that the films sealed with wax emulsion on both substrate materials are the most promising candidates for the application of devices under oscillation wear. The obtained roughness, friction coefficients and hardness values confirm the positive behaviour of the anodically oxidised EN AW‐7075‐T651 alloy under the chosen tribological load.     

A modification of Morrow and Smith–Watson–Topper mean stress correction models

A modification of the Morrow and the Smith, Watson and Topper (SWT) mean stress correction models is proposed to account for the mean stress effect on fatigue life. The capability and accuracy of the proposed model are compared to those of the original Morrow and the SWT model using published mean stress fatigue test data. The proposed mean stress correction model was found to be superior to both the SWT and the Morrow model in the case of the Incoloy 901 superalloy and the ASTM A723 steel. On the other hand both the proposed and the original SWT model provided equally good correlation with experimental data in the case of 7075‐T561 aluminium alloy and 1045 HRC 55 steel. The Morrow model was found to give the least accurate predictions for all four materials analysed.     

Einfluss der Werkstoffzähigkeit auf das Festigkeitsverhalten unter mehrachsiger Beanspruchung am Beispiel von Schweißverbindungen aus Stahl und Aluminium

Influence of Ductility on the Multiaxial Fatigue Behaviour by the Example of Welded Joints of Steel and Aluminium The multiaxial fatigue behaviour of materials with different ductility under constant and changing principal stress directions is also applicable to welded joints of different materials. For this, welded flange tube connections of the fine grained steel StE 460 and the artificially aged aluminium alloy AlSi1MgMn T6 were investigated under constant amplitude combined bending and torsion. Out‐of‐phase loading, i. e. changing principal stress directions, of the steel joints led to a decrease of fatigue life, which is observed at ductile material states. However, for the aluminium joints out‐of‐phase loading resulted same behaviour as in‐phase loading, which indicates a semi‐ductile material behaviour. The results for the welded steel joints were evaluated on basis of local stresses by the integral hypothesis of the Effective Equivalent Stress EES (WVS). This hypothesis for ductile material states takes into account the life decreasing influence of out‐of‐phase loading by considering the interaction of the shear stresses on different planes. The fatigue behaviour of the aluminium welds is described by the critical plane based combination of shear and normal stresses (KoNoS), which is valid for semi‐ductile material states.     

Variables affecting the fatigue resistance of PVD-coated components

The effect of intrinsic properties of CrN coatings on fatigue behaviour was studied in this paper. The coating layer microhardness and the residual stresses characterising the surface film were measured and the obtained results were introduced in a numerical modelling predicting fatigue life procedure of coated components. The effect of a CrN monolayer film deposited on bulk samples, produced in 2205 duplex stainless steel, H11 tool steel or 6082 aluminium alloy was investigated. The fatigue limit of coated and uncoated samples was experimentally determined while the development of FEM models, confirmed by means of experimental tests, represents a powerful tool to predict fatigue life of coated components. The effects on the fatigue strength of coating and bulk material defects like droplets and non-metallic inclusions were considered along with the residual stress gradient characterising the coating and evaluated by means of X-ray measurements. The influence of the substrate material plastic deformation on the integrity of the coating was evaluated too.     

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.     

The significance of stress corrosion cracking in corrosion fatigue crack growth studies

A model is proposed to account for interactions between fatigue and stress corrosion crack propagation mechanisms in appropriate corrosion fatigue conditions. Tests on an alloy steel, and both wrought and cast aluminium alloys, are reported. Despite the use of very simple coefficients in the equations derived, encouraging results are obtained.     

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.     

The effect of porosity on the fatigue life of cast aluminium‐silicon alloys*

Fatigue strength optimization of cast aluminium alloys requires an understanding of the role of micropores resulting from the casting process. High cycle fatigue tests conducted on cast A356‐T6 show that the pore size and proximity to the specimen surface significantly influence fatigue crack initiation. This is supported by finite element analyses (both elastic and elastic–plastic) which demonstrate that high stress/strain concentration is induced by pores which are both large and near to the specimen surface. A new pore‐sensitive model based on a modified stress‐life approach has been developed which correlates fatigue life with the size of the failure‐dominant pore. The model prediction is in good agreement with experimental data.     

Elastic correction of fatigue crack growth laws

Fatigue crack growth (FCG) is usually studied assuming that ΔK is the driving parameter. An effective ΔK is considered in the presence of crack closure. However, after crack opening, there is an elastic regime that does not contribute to FCG. The objective here is to quantify this elastic range of ΔK, ΔK_el, for different loading conditions and material properties. The yield stress was found to be the most important material parameter, followed by the hardening exponent. A linear decrease of ΔK_el with ΔK was found for the 7050‐T6, 6082‐T6, and 6016‐T4 aluminium alloys, while the 304L stainless steel presented a slight increase. On the other hand, the increase of K_max was found to increase the elastic fatigue range. Relatively high values of elastic range were obtained for the plane strain state, compared with the plane stress state.     

Stabilität von Druckeigenspannungen unter zyklischer Beanspruchung für autofrettierte Bauteile aus Aluminium

Autofrettage provides the possibility for increasing the fatigue strength of internal pressurized components. This technique introduces compressive residual stresses, applying a single pressure overload prior to the service load. Whereas a lot of investigations for steel materials have been done so far, this report refers to the potential of the autofrettage process for the aluminum wrought alloy EN AW‐6082‐T5. In order for residual stresses to ensure a reliable increase of the stress amplitude, they must not be relaxed. A significant increase in lifetime at a load amplitude of Δp = 500 bar is shown for T‐shaped bore intersection specimens that are autofrettaged at a pressure of 1500 bar. The introduced residual stresses did not relax after 2.5×10⁶ number of cycles. Only for a load with a maximum equivalent stress above the yield strength the components initial compressive residual stress distribution is changed and even partially transferred to the tensile section. Therefore, using autofrettage is appropriate to increase the fatigue strength of aluminum components.     

铝合金6082T6中的疲劳不扩展裂纹

铝合金6082T6作为一种常用的航空材料具有很好的力学性能,使用这种材料对疲劳不扩展裂纹进行了研究。试样形状为单边缺口拉伸试样,并在裂纹尖端使用数控机床钻直径为1mm的止裂孔。通过分析可得：疲劳不扩展裂纹可以通过对试样几何尺寸的合理设置得到。在设计中可以在疲劳构件容易萌生裂纹的部位预留出不扩展裂纹的容许长度,从而更大限度的提高构件的寿命可靠性,并且可以抵抗一定程度的外界不确定载荷的冲击影响,减少在裂纹维修或检查间隔期间的裂纹扩展风险。