Durability of welded aluminium extrusion profiles and aluminium sheets in vehicle structures

Different approaches - the nominal stress, structural hot spot stress and notch stress approach - to analyse the fatigue strength of welded structures made from wrought aluminium alloys were studied. Experimental and numerical investigation was carried out for this purpose on detail specimens and components. The results shown here were generated during the research project “Extrusion profile and sheet metal structures of wrought aluminium alloys in vehicle construction” [1].The studies show that due to the existing guidelines, welds on structures made from aluminium alloys are sometimes designed very conservatively. It is possible to optimise and reliably design welded joints of thin sheet structures by applying the notch stress approach using the reference radius r_ref = 0.05 mm and the reference SN curves derived here.     

Consideration of mean-stress effects on fatigue life of welded magnesium joints by the application of the Smith–Watson–Topper and reference radius concepts

Three types of welded joints have been assessed with regard to their fatigue strength based on the mean-stress damage parameter model according to Smith, Watson, and Topper (P_SWT) and on the reference notch radius concept. These analyses were performed with three different stress ratios, R = −1, R = 0 and R = 0.5, under axial loading. For each stress level, the corresponding Neuber-Hyperbolas, Masing-loops and their maximum stress and maximum strain values were determined in order to calculate damage parameter (P_SWT) values. For a given weld geometry, this damage parameter is able to unify the fatigue results for different R-values within at a tight scatter band and therefore to consider the mean-stress effect. The unification of the results for different weld geometries is performed by applying the reference radii r_ref = 0.05 and r_ref = 1.00 mm as suggested by the IIW-Recommendations.     

Fatigue strength of laser beam welded automotive components made of thin steel sheets considering size effects

The applicability of and the quality of assessment using the nominal stress, structural stress and notch stress approaches for calculating the fatigue strength of laserbeam welded components made of thin steel sheets has been investigated. For this purpose, the fatigue lives of a longitudinal carrier, an injector and two tube-flange specimens have been determined by tests under constant amplitude loading. Fatigue cracks initiated at sharp root notches on all of these components. While the nominal stress is derived by theory of structural mechanics, the determination of structural and notch stresses is performed using 3D finite element models and solid elements. The structural stress is derived by an extrapolation of surface stress to the fatigue critical notch and the notch stresses by rounding the sharp root notch with a reference radius of r_ref = 0.05 mm. For all of the concepts used, the endurable stresses have been compared to the design SN-curves recommended by the International Institute of Welding (IIW).On comparing the quality of assessment of the different concepts, the notch stress approach shows the highest scatter. The highest endurable notch stresses occur in specimens with crack initiation at weld ends. These specimens have a very small highly loaded weld length. The lowest endurable stresses are determined for specimens with a long, equally loaded weld. The reason for these findings can be explained by statistical size effects. For an improved fatigue assessment, an easily applicable method is introduced, which takes into account the highly stressed weld length.     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.     

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

In the context of the German joint research project “Applicability of fatigue analysis methods for seam welded components”, fatigue tests were performed by five universities and institutes on welded components, welded parts of larger structures as well as component-like samples of weld details. The sheet thickness t was in the range 1 mm ? t ? 20 mm. The welding parameters for all test coupons and structures tested were chosen according to the industrial production process. Based on the data acquired, nominal, structural and notch stress approaches were analysed with regard to applicability and quality of assessment. The actual weld geometry except the real notch radii was taken into account within the notch stress approach. For the notch radii various values, the reference radii 0.05, 0.3 and 1 mm, were applied.Experimental and numerical results for welded steel components are presented.Approximately equivalent scatter ranges were obtained when applying the various approaches based on the current state of the art. It should be noted that both the nominal and the structural stress approaches are limited in their application compared to the notch stress approach. A comparison of the scatter bands obtained for the various approaches is subject to limitations because it was necessary, in each case, to use different test series as the basis for determining the scatter bands.     

Schwingfeste Auslegung von Schweiß‐ verbindungen aus der Magnesiumknetlegierung AZ31(ISO‐MgAl3Zn1) mit dem Konzept der fiktiven Ersatzradien von rf = 1,0 mm und 0,05 mm

Fatigue design of welded joints from the wrought magnesium alloy AZ31 (ISO‐MgAl3Zn1) by the local stress concept with the fictitious notch radius of r_f = 1.0 mm and 0.05 mm The investigations were carried out with three different types of MIG‐ and TIG‐welded magnesium joints of the alloy AZ31. 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 magnesium welded joints from plates with thicknesses t ≥ 5 mm independently of the weld geometries (fully or partially penetrated butt welds, transversal stiffeners). Design curves are proposed for different stress ratios, i.e. R = ‐1 as well as 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 = 28 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations. Further, the FAT‐value Δσ = 73 MPa for the fictitious radius of r_f = 0.05 mm to be applied to welded thin magnesium joints is derived, too. These FAT‐values are compared with already known data for steel and aluminium joints. A linear relationship between the FAT‐values and the Young’s modulus is determined.     

Fatigue strength assessment of Invar alloy weld joints using the notch stress approach

The aim of this study investigated the fatigue strength of Invar alloy weld joints. Invar steel (Fe-Ni 36%) is widely used in the primary and secondary barriers of membrane-type liquified natural gas (LNG) containment vessels. The fatigue test was carried out for two different types of welded joints with raised edge specimens and with overlap joint specimens based on the nominal and notch stress approaches. The thickness of the Invar plate is less than 1.5 mm, so the notch stress approach with r_ref = 0.05 mm was applied. Our evaluation of the results in terms of the FAT value and the slopes of the design curves are compared with steel, aluminum, and magnesium weld joints in accordance with International Institute of Welding (IIW) recommendations.     

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     

Fatigue predictions of welded joints and the effective notch stress concept

Several research papers on a new concept of the effective notch stress of a welded joint were recently published. The toe of the weld is modelled with a prescribed radius. A parameter for the severity of the stress distribution at the weld can then be expressed as a K_t-value to be calculated with FE analysis. The new concept offers interesting applications for designing against fatigue crack initiation and predictions on the fatigue limit. It is argued that the radius concept should be replaced by a ratio of two dimensions and a proposal is made for this purpose. The fatigue assessment should be based on the calculated K_t-value. Limitations of the present codes are discussed.     

Fatigue notch factor and short crack propagation

This paper addresses the problem of high cycle fatigue at notches and the role of short crack propagation in the fatigue notch factor k_f. Ahead of a V-notched feature, the stress field is characterized by two parameters, i.e. the stress concentration factor k_t and the normalized notch stress intensity factor k_n. Whether fatigue strength at a given life is controlled by crack initiation (k_f = k_t) or by short crack propagation (k_f < k_t) depends on k_t, k_n and the material resistances to crack initiation and to short crack propagation. The analysis accounts for the effects of notch acuity, notch size, material and fatigue life on the fatigue notch factor k_f. It opens the door to a new method for predicting fatigue life using two S-N curves for a given material; one being measured from a smooth specimen, the other from a severe V-notch.     

Fatigue strength of laser beam welded thin steel structures under multiaxial loading

The multiaxial fatigue behaviour of thin laser beam welded tube–tube specimens of the structural steel St35 was assessed according to the methodology of the fictitious weld root radius of r_f=0.05 mm and the application of the Effective Equivalent Stress Hypothesis (EESH), especially considering the fatigue life reducing influence of out-of-phase loading in comparison to in-phase loading. The results are applicable for the fatigue design of laser beam welded car body and chassis structures of thin steel sheets (t<3 mm).     

Generalised Neuber concept of fictitious notch rounding

The microsupport effect at sharp notches subjected to high-cycle fatigue can be described according to Neuber by averaging the maximum notch stress in a small material volume (microsupport length ρ^*) at the notch root (radius ρ). The averaged stress may be expressed by the maximum stress of a corresponding notch of an enlarged, fictitious radius, ρ_f = ρ + sρ^*, where s is the microsupport factor. The status of Neuber’s concept within his general theory of notch stresses is reviewed, followed by more recent theoretical and application-relevant developments. The theoretical developments refer to the notch angle dependency of the support factor, to its value for pointed versus rounded notches and to in-plane shear loading with out-of-bisector crack propagation. The application developments refer to the fatigue assessment of welded joints.     

Stress concentration effects of undercut defect and reinforcement metal in butt welded joint

In the present study, stress distribution of butt welded joints with various amounts of reinforcement metal and undercut defect has been investigated under uniaxial tension for a full penetration by systematically conducting a series of two-dimensional finite element (FE) models. The FE analysis indicated that the amount of reinforcement metal in weld zone has an important effect on stress distribution. For 120° of the reinforcement angle that designating reinforcement metal in weld joint, and 0.5 mm of toe radius, the value of stress concentration factor (SCF) exceeds 3.3σ₀. The analyses show that SCF takes much higher values in both low reinforcement angle and ratio of toe radius to plate thickness (R/t). As for joints with undercut defects, it is concluded that severity of SCF is mainly controlled by the ratio of depth to radius of undercut (h/r) and width (W). In addition to undercut defect, the presence of reinforcement metal, SCF noticeably increases with decreasing the reinforcement angle; it attains maximum value (7.4σ₀) for h/r = 5 and W = 3 mm. However, for the joints having wider undercut defects, the influence of reinforcement metal on SCF is found to be relatively lower; SCF is 6.7σ₀ for W = 6 mm. Finally, an attempt has been made to construct simple relationships among the SCF of the weld joint, reinforcement angle, undercut defect and dimensionless parameters defining weld toe detail.     

Fatigue behaviour of friction welded medium carbon steel and austenitic stainless steel dissimilar joints

This paper reports the fatigue behaviour of friction welded medium carbon steel–austenitic stainless steel (MCS–ASS) dissimilar joints. Commercial grade medium carbon steel rods of 12 mm diameter and AISI 304 grade austenitic stainless steel rods of 12 mm diameter were used to fabricate the joints. A constant speed, continuous drive friction welding machine was used to fabricate the joints. Fatigue life of the joints was evaluated conducting the experiments using rotary bending fatigue testing machine (R = −1). Applied stress vs. number of cycles to failure (S–N) curve was plotted for unnotched and notched specimens. Basquin constants, fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated for the dissimilar joints. Fatigue strength of the joints is correlated with microstructure, microhardness and tensile properties of the joints.     

A generalised notch stress intensity factor for U-notched components loaded under mixed mode

A novel notch stress intensity factor (NSIF) for U-notched specimens loaded under mixed mode is examined in this article. The concept is based on the averaged strain energy density criterion, or alternatively on the cohesive zone model, as well as the equivalent local mode approach. To a certain extent, it is a generalisation of Glinka’s NSIF for mode I, where σ_tip is replaced by σ_max.The applicability of a fracture criterion based on this new NSIF is checked against 171 fracture tests with PMMA (at −60 °C) performed on U-notched specimens, with different notch root radii and loaded under mixed mode. The asymptotic behaviour of the new NSIF as the notch becomes a crack (when the notch root radius tends to zero) or when the notch disappears (when the notch root radius tends to infinity) is also discussed.     

Influence of notch radius and microstructure on the fracture behavior of Al–Zn–Mg–Cu alloys of different purity

The influence of notch radius on the fracture behavior of two high-strength Al–Zn–Mg–Cu alloys with different Fe content in the T73 condition was investigated. The fracture toughness tests were performed on non-fatigue-precracked notched bending specimens with different notch radii ranged from 0.15 mm to 1.0 mm. The obtained data were interpreted using the concept of Notch Fracture Mechanics combined with finite-element method (FEM) calculations. It was found that both alloys are very sensitive to the notch radius. The fracture toughness increases with increasing notch radius. For a given notch radii, the increase in fracture toughness is more significant for the more pure alloy. The fracture behavior of investigated alloys with respect to microstructural features and their relation with the fracture micromechanisms were analyzed.     

Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

The local fatigue strength of a laserbeam weld of a complex engine component, which joins a PM with a formed sheet component, was assessed by the notch stress concept with the fictitious reference radius of r_ref = 0.05 mm. First, simplified specimens, following the main geometric dimensions of the parts, were manufactured. On these specimens the fatigue strength was identified by tests and the notch stresses calculated by finite element analysis. Based on these results a design SN‐curve was derived to assess the fatigue strength of the engine component. The numerical assessment of the welded joint was verified by proof tests with the component. The assessment could be improved by considering statistical and stress gradient dependent size effects according to the concept of the highly stressed volume.     

3-D stress intensity factors for arrays of inner radial lunular or crescentic cracks in a typical spherical pressure vessel

Many spherical pressure vessels are manufactured by methods such as the integrated hydro-bulge forming (IHBF) method, where the sphere is composed of a series of double curved petals welded along their meridional lines. Such vessels are susceptible to multiple radial cracking along the welds. For fatigue life assessment and fracture endurance of such vessels one needs to evaluate the stress intensity factors (SIFs) distribution along the fronts of these cracks. However, to date, only two 3-D solutions for the SIF for one inner semi-elliptical crack in thin or thick spheres are available, as well as 2-D SIFs for one through-the-thickness crack in thin spherical shells. In the present paper, mode I SIF distributions for a wide range of lunular and crescentic cracks are evaluated. The 3-D analysis is performed, via the FE method employing singular elements along the crack front, for a typical spherical pressure vessel with outer to inner radius ratios of η = R_o/R_i = 1.1. SIFs are evaluated for arrays containing n = 1-20 cracks; for a wide range of crack depth to wall thickness ratio, a/t, from 0.025 to 0.95; and for various ellipticities of the crack, i.e., the ratio of crack depth to semi crack length, a/c, from 0.2 to 1.5. The obtained results clearly indicate that the SIFs are considerably affected by the three-dimensionality of the problem, and the following parameters: the number of cracks in the array-n, the relative crack depth a/t, and the crack ellipticity a/c.     

Notch effects and crack propagation analysis on kenaf/polypropylene nonwoven composites

This paper has studied the open-hole and pin filled-hole effects on the tensile properties of Kenaf/Polypropylene Nonwoven Composites (KPNCs) in production of automotive interior parts. The influence of specimen width-to-hole diameter (W/D) ratios of 6, 3, and 2 on failure load was studied. Two sample thicknesses of 3 mm and 6 mm were evaluated. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. A preliminary model by extended finite element method (XFEM) was established to predict the failure load and simulate crack propagation of 3 mm thick open-hole and pin filled-hole specimens. Good agreement was found between experimental and simulation results. By calculating the stress concentration factor K_t for brittle materials, the net section stress factor K_n for ductile materials, and the strength reduction factor K_r, it was found that KPNC was relatively ductile and insensitive to the notch.     

Evaluation of incipient plasticity from interfaces between ultra-thin gold films and compliant substrates

Nanoindentation experiments were conducted for 30 nm-thick Au films on two types of substrates, polyimide (compliant) and glass (stiff), to clarify the dominant mechanics of incipient plasticity from the interface. A high resolved shear stress τ_r could be effectively applied to the Au/polyimide interface due to the compliant substrate, and plastic deformation was initiated at the interface. The critical resolved shear stress τ_crss at the interface was determined to have a value of 0.4 ∼ 0.5 GPa. On the other hand, in Au/glass, τ_r peaked within the Au film, and the maximum values were 1.1 ∼ 2.2 GPa depending on the tip radius, whereas the values of τ_r at the Au/glass interface were almost identical at 0.5 ∼ 0.7 GPa. Therefore, plastic deformation might be initiated from the Au/glass interface. The values of τ_crss for heterogeneous nucleation at the interfaces were smaller than that for homogeneous nucleation in the Au films.