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.     

Fatigue behaviour of tubular AlMgSi welded specimens subjected to bending–torsion loading

This paper is concerned with an experimental and numerical study of the fatigue behaviour of tubular AlMgSi welded specimens subjected to biaxial loading. In‐phase torsion–bending fatigue tests under constant amplitude loading were performed in a standard servo‐hydraulic machine with a suitable gripping system. Some tests in pure rotating bending with and without steady torsion were also performed. The influence of stress ratio R and bending–torsion stress ratio were analysed. Correlation of the fatigue lives was done using the distortion energy hypothesis (DEH), based on the local stresses and strains. The applicability of the local strain approach method to the prediction of the fatigue life of the welded tubular specimens was also investigated. Static torsion has only a slight detrimental influence on fatigue strength. The DEH (von Mises criterion) based on local stresses in the weld toes was shown to satisfactorily correlate fatigue lives for in‐phase multiaxial stress–strain states. The stress–strain field intensity predictions were shown to have less scatter and are in better agreement with the experimental results than the equivalent strain energy density approach.     

High cycle fatigue damage mechanisms in cast aluminium subject to complex loads

This article is dedicated to the high cycle fatigue behaviour of cast hypo-eutectic Al–Si alloys. In particular, the AlSi7Cu05Mg03 alloy is investigated. It presents the results of a vast experimental campaign undertaken to investigate the fatigue behaviour, and more specifically the fatigue damage mechanisms observed under complex loading conditions: plane bending with different load ratios, fully reversed torsion and equibiaxial bending with a load ratio of R = 0.1. A specific test set-up has been designed to create an equibiaxial stress state using disk shaped specimens. A tomographic analysis is also presented with the aim of characterising the micro-shrinkage pore population of the material.It is shown that two distinct and coexisting fatigue damage mechanisms occur in this material, depending on the presence of different microstructural heterogeneities (i.e. micro-shrinkage pores, Silicon particles in the eutectic zones, Fe-rich intermetallic phases, etc.). Furthermore, it is concluded that the effect of an equibiaxial tensile stress state is not detrimental in terms of high cycle fatigue. It is also shown that the Dang Van criterion is not able to simultaneously predict the multiaxial effect (i.e. torsion and equibiaxial tension) and the mean stress effect for this material.     

On the investigation of the biaxial fatigue behaviour of unidirectional composites

The paper illustrates the preliminary activity of an extensive research program oriented to investigate the multiaxial fatigue behaviour of unidirectional composite laminates, with particular attention to the analysis of the damage mechanisms and their correlation with the local multiaxial stress state to be used then as the basis for the development of multiaxial fatigue criterion. The definition of an effective experimental procedure for multiaxial fatigue testing is carefully discussed in terms of specimen geometry, specimen manufacturing and local stress state. Once identified in the thin-walled tubular specimens under tension–torsion loading the best test configuration for the aims of the research, the results of comparative fatigue tests investigating the influence of the tubes geometry (wall thickness to diameter ratio) on the transverse fatigue response are presented. In the final part of the paper the effects of an increasing shear stress component (σ₆) on the transverse (σ₂) fatigue strength and damage evolution in UD glass–epoxy tubes are illustrated.     

Selection of the critical plane orientation in two-parameter multiaxial fatigue failure criterion under combined bending and torsion

The present paper is focused on engineering application of the algorithm of fatigue life calculation under multiaxial fatigue loading. For that reason, simple two-parameter multiaxial fatigue failure criterion is proposed. The criterion is based on the normal and shear stresses on the critical plane. Experimental results obtained under multiaxial proportional, non-proportional cyclic loading and variable-amplitude bending and torsion were used to verify the proposed two-parameter criterion and other well-known multiaxial fatigue criteria. Elastic–plastic behaviour of the bulk material was taken into account in calculation of the stress/strain distribution across the specimen cross-section. It is shown that the proposed two-parameter multiaxial fatigue failure criterion gives the best correlation between the experimental and calculated fatigue lives.     

Critical plane approach for the assessment of the fatigue behaviour of welded aluminium under multiaxial loading

ABSTRACT Multiaxial stress states occur in many welded constructions like chemical plants, railway carriages and frames of trucks. Those stresses can have constant and changing principal stress directions, depending on the loading mode. Latest research results on welded steel joints show a loss of fatigue life for changing principal stress directions simulated by out‐of‐phase bending and torsion compared to constant directions given by in‐phase loading. However, aluminium welds reveal no influence of changing principal directions on fatigue life compared to multiaxial loading with constant principal stress directions. This behaviour is not predictable by any conventional hypothesis. A hypothesis on the basis of local normal and shear stresses in the critical plane has been developed and applied to aluminium weldings.     

Assessment of the fatigue behaviour of welded aluminium joints under multiaxial spectrum loading by a critical plane approach

Multiaxial stress states occur in many welded constructions like chemical plants, railway carriages and frames of trucks. Depending on the loading mode, those stresses can have constant and changing principal stress directions. For welded fine grained steel, research results show a severe loss of fatigue life for changing principal stress directions simulated by out-of-phase bending and torsion compared to constant directions given by in-phase loading. However, aluminium welds reveal no influence of changing principal directions on fatigue life compared to multiaxial loading with constant principal stress directions under constant amplitude and spectrum loading. This behaviour is not predictable by any conventional hypothesis. A hypothesis on the basis of a combination of local normal and shear stress in the critical plane has been developed and successfully applied to aluminium weldings.     

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.     

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.     

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.     

Development of a new device to perform torsional ultrasonic fatigue testing

The interest in gaining experimental knowledge on fatigue strength of materials over 10⁹ cycles is rapidly increasing as evidenced for the large amount of investigations on this subject presented at the last very high cycle fatigue meeting (VHCF-3), held on September 2004. Most of the fatigue results presented at this conference were obtained under tension-compression, rotating bending, flexion and bending cyclic loading (some attaining 10¹⁰ cycles), using ultrasonic devices whose design was based on the natural frequency principles. In general, very little literature concerning the metallic alloys behavior under torsion cyclic loading using ultrasonic is available; however, in order to perform an accurate component design under multi-axial loading and VHCF, the material behavior under torsion cyclic loading is required. This investigation presents the development of a new mechanical device for testing and characterizing metallic alloys in the range of 10⁹–10¹⁰ cycles in torsional cyclic loading and the first experimental results for medium carbon steel (38MnSV5S). The new device was designed to excite the components under testing with pure torsional vibration mode at a frequency of 20 kHz.     

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.     

Hot workability of an Al-Mg alloy AA5182 with 1 wt% Cu

A comparative study of the hot workability of two aluminium alloys, alloy AA5182 used for automotive applications and a variant modified with 1 wt% copper, has been carried out. Hot torsion tests were performed on both alloys subjected to two different heat treatments: a low temperature preheat to 450 °C and a high temperature preheat at 540 °C. The results from the torsion experiments are interpreted in terms of microstructural features. Both treatments produce the same strength, but the high temperature preheat leads to better ductility. This improvement is related to the homogenization of solute elements in the matrix; and, concerning AA5182 + Cu, also to the dissolution of a non-equilibrium Al-Mg-Cu ternary eutectic present in the as-cast microstructure. The precipitation of (Fe, Mn)Al₆ precipitates in the matrix of both alloys is induced by the high temperature heat treatment. Comparison of the results obtained by hot torsion shows that at low deformation rates AA5182 + Cu has better ductility than the classical alloy, but its ductility is lower at strain rates above 0.6–0.8 s^–1. The null ductility transition temperature is lower compared with that in the classical alloy, restricting the range of hot working temperatures. Inside this range the strength of both alloys is approximately the same, although the strain rate sensitivity coefficient is increased by copper additions. The experimental strength values follow the classical sinus-hyperbolic constitutive equation for hot working.     

Area and point approaches in fatigue life evaluation under combined bending and torsion loading

We present a new nonlocal approach to nonuniform stress distribution, consisting in reduction of stresses to representative local ones in the critical plane for fatigue life calculation. The shear and normal stresses are averaged in two overlapping areas of different sizes on the critical plane. The proposed method is compared with the point (in critical distance) method and both are verified by fatigue tests under combined bending and torsion. Verification is done for the experimental and calculated fatigue lives with use of two multiaxial fatigue failure criteria. __________ Translated from Problemy Prochnosti, No. 1, pp. 69–72, January–February, 2008.     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

Multiaxial fatigue behaviour of Sintered Steels under combined in and out of phase bending and torsion

This paper discusses the fatigue behaviour of sintered steels under multiaxial loading. These steels are the Fe-1.5% Cu and the Fe-2.0% Cu-2.5% Ni, sintered at low and high temperatures, in the densities 7.1 and 7.4 g/cm³, which are used in the production of several ready to assemble automotive parts. Fully reversed or pulsating combined loading with constant frequency and amplitudes acting in and out of phase, was applied to round notched specimens (K_tb = 1.49, K_tt = 1.24) in the finite fatigue life region (10⁴ ≤ N_f ≤ 2 · 10⁶). The mechanics of crack initiation and propagation as well as rupture were studied using fractography and microfractography. These analyses led to a mechanical model based on local normal stresses for the fatigue life evaluation. The fatigue life evaluation on the base of the local bending stress obtained under uniaxial loading describes the test results for in phase combined bending and torsion satisfactorily. But the increase of fatigue strength and life by out of phase loading is overestimated in the case of fully reversed loading. However for design purposes the out of phase loading can be neglected because of its beneficial effect in increasing fatigue life for this type of material. If the dependence of the different stress concentrations under combined in and out of phase loading on the supportable local bending stress obtained under uniaxial loadings is considered, then the calculation procedure covers all test results.     

Comparison of the fatigue characteristics for some selected structural materials under bending and torsion

A survey of the multiaxial fatigue criteria including the ratio of normal and shear stresses is presented. We also discuss the fatigue characteristics of some selected structural materials in bending and torsion. The ratio of normal stresses to shear stresses is determined for a given number of cycles N _f within the range 5∙10⁴ −2∙10⁶ . Moreover, it follows from the performed analysis of the fatigue equations and the relative difference R that the materials can be split into groups for which it is possible or impossible to apply a constant value of the considered ratio in the criteria including this ratio.     

A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube-to-flange joints subjected to combined loadings

Full penetration T butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V‐notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N‐SIF) and assess the high cycle fatigue strength of the welded joints. The N‐SIF‐based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N‐SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R_C and given in closed form as a function of the mode I and mode III N‐SIFs. The value of R_C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R_C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re‐analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine‐grained Fe E 460 steel and of age‐hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R_C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves.     

Versuchs- und Rechendaten zur Dauerschwingfestigkeit von metallischen Werkstoffen unter mehrachsiger Beanspruchung

Experimental Data and Calculated Results about the Fatigue Endurance Limit of Metals under Multiaxial Alternating Load An extensive catalogue of present available experimental data about the fatigue endurance limit of metallic materials under multiaxial loading conditions and thereupon determined deviation ratios between experimental results (long life fatigue tests) and calculated values by five newer computation methods is demonstrated (a further showed statistical analysis of these devations indicates, that in relation to the other failure criterions the ?Quadratische Versagenshypothese”? QVH is preferably recommended for a reliable application). The tabulated data-catalogue totally includes 530 referenced loading cases (limited to various biaxial states of combined normal and torsional alternating stress with sinusoidal synchronous or out-of-phase amplitudes and superimposed mean stresses) with experimental results (probability of survival of P_S = 50%) on metallic materials (unalloyed and alloy steels, non-ferrous metals and wrought aluminium alloys, cast irons and sintered metals).     

FATIGUE LIFE PREDICTION OF NOTCHED COMPOSITE COMPONENTS

Abstract— The local stress/strain approach has been used to predict the fatigue lives of notched composite components. The method was based on a microstress analysis and the application of a multiaxial fatigue parameter incorporating the alternating strain components on the critical plane. This parameter was able to correlate the fatigue lives obtained under a variety of multiaxial loading and geometrical configurations, enabling a generalized fatigue life curve to be determined on the basis of limited experimental data.
The ability of the multiaxial fatigue parameter to relate the fatigue behaviour of composites was illustrated by predicting the locations of crack initiation sites in a unidirectional silicon carbide fibre reinforced titanium plate containing a circular hole tested under constant amplitude cyclic loading. The same approach was also successfully employed to predict the fatigue lives of graphite reinforced epoxy composite tubes with circular holes tested under several combinations of cyclic tension and torsion.