Fatigue resistance of weld ends – Analysis of the notch stress using real geometry

Modern welded structures often contain weld start and end points which are the failure critical location. In particular, for special investigations into thin sheet structures, no approach for the determination of the fatigue life has been established thus far. In this research, the primary aim was to obtain the real geometry of weld ends with high precision using a three dimensional scanner to find a general approach using the notch stress concept. Going one step further, analyses have been performed regarding to unify the notch stress concept. The existing results of Olivier – who examined long welds with no start and end points – were re‐evaluated to unify the results of long regular welds with the local weld end under one scatter band.     

Analyse der Wirkung von Kerben,Mittel‐ und Eigenspannungen auf die Schwingfestigkeit des hochumgeformten Werkstoffbereichs von Spaltprofilen

Analysis of the effect of notches, mean and residual stresses on the fatigue strength of severely deformed material areas of linear flow split profiles The results of fatigue experiments on specimens extracted from the highly deformed area of linear flow split profiles are presented. The Focus is on the effect of mean stresses and notches, which is influenced by the previous forming process. The residual stress state inside a component as well as inside specimens is described. By numerical analyses considering residual stresses their effect on fatigue life can be analysed. Using Weibulls Weakest‐Link Appoach parameters, which were derived by testing smooth specimen are transferred to notched ones.     

Fatigue assessment of seam welds of automotive components by local stress approaches

In the present paper a method is described for the fatigue assessment of multi‐axial loaded welded joints in structures made from thin steel sheets. The focus is on seam welds under non‐proportional loading, assuming a constant angular phase shift between cycles leading to stress components acting at the seam welds. Fatigue analyses are using a local stress approach based on stresses derived by Theory of Elasticity. Furthermore, potentials of considering statistical size effects in order to analyse weld start and end points are outlined.     

Die Anrisslebensdauerabschätzung unter Berücksichtigung des statistischen Größeneinflusses am Beispiel der AlMgSi0,7‐Legierung

The fatigue lifetime estimation in consideration of the statistical size effect as example for the AlMgSi0,7 alloy The prediction of the crack initiation lifetime in consideration of the statistical and technological size effects will be introduced in course of an analytical model. To verify this method results from fatigue tests at constant and variable amplitude loads for unnotched and notched specimens of the aluminum alloy AA6009 will be considered.     

Sprühkompaktieren von Halbzeugen unterschiedlicher Geometrie aus unlegierten und niedriglegierten Stählen

Sprayforming of unalloyed and low alloy steel preforms Sprayforming offers the possibility to produce a variety of preforms in a single step directly from the melt. Numerous processing parameters ensure high flexibility but require detailed knowledge concerning the microstructure of sprayformed material depending on chemical composition and processing parameters. Within the current study fundamental investigations using unalloyed and low alloy steel grades (C15, C105, 100Cr6) for sprayforming of plates and billets have been performed concerning the influence of chromium and carbon content and selected processing parameters. Sprayformed preforms revealed austenite grain sizes up to 500 μm depending on the development of porosity. Different types of porosity have been detected which can be associated with incomplete densification due to high cooling rates, solidification shrinkage and entrapped nitrogen. Atomisation gas pressure and carbon content are parameters of primary importance.     

Festigkeitsberechnung scharf gekerbter Bauteile

Strength calculation of sharp notched components In this report the strength calculation of sharp notched components under static and cyclic loading is investigated and compared to the FKM code “Analytical strength assessment for machine components”. The calculation of the strength of sharp notched components according to the guideline does not lead to satisfying solutions. Therefore different concepts of strength calculation with consideration of scale and notch effects are examined e.g. the gradient concept of Siebel/Stieler. This concept is implemented in the FKM code for the calculation of the notch sensitivity factor. In order to evaluate the numerical results, experimental tests with various sharp notched specimens under static and cyclic loading were carried out. Based on these experimental results the FKM code is extended to sharp notched components.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

Untersuchung der Einflüsse von Substratrauheit und Spritzpartikelgröße auf die Haftung thermisch gespritzter Schichten

About the influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings The influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings was researched systematically. In addition to established spray materials (Cr₂O₃, WCCo, NiCr) and spraying processes (atmospheric plasma spraying (APS), high velocity flame spraying (HVOF)) different substrate materials (steel, stainless steel, aluminum) were included in the research work as well.     

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.     

Design methods for reliable fatigue assessment of PM components

The use of PM materials is rapidly expanding with an increasing concentration on highly loaded structural parts such as synchroniser hubs, gears, sprockets or shifting forks. The successful implementation of PM materials for such parts depends on a reliable fatigue design concept. Such a design concept has to consider the local durability, especially in fatigue critical sharply notched areas, depending on the local density of the material and stress gradients. This paper summarises different design methods in order to transfer the fatigue behaviour of specimens to components by considering sharply notched areas. Four different local approaches have been investigated: the highly stressed volume approach, the stress gradient approach, the critical distance method and the stress averaging method according to Neuber. The design methods have been analysed on the basis of fatigue testing results of unnotched and notched fatigue specimens and of synchroniser hubs made from a 4% Ni diffusion‐alloyed steel material (Distaloy AE+0.6%C). The transferability of characteristic fatigue properties from specimens to a sharply notched component, a synchroniser hub, is presented and the practicability of the design methods demonstrated and discussed. These investigations showed that the most reliable concept was the highly stressed volume approach. The accuracy of the approach can be comprehended separating statistical and so called material support effect.     

Vergleichende Untersuchungen zur Anrisslebensdauervorhersage gekerbter Proben mit dem Örtlichen Konzept und dem Nennspannungskonzept am Beispiel des Stahls Cm15

Comparative Investigations on Service Life Assessment of Notched Specimens Based on the Local Strain and the Nominal Stress Approach to Fatigue for a Steel SAE 1017 It is still unclear whether the strain based approach to fatigue or the stress based approach to fatigue should be preferred for service life assessment of notched components. In order to clarify the similarities and differences between these concepts stress and strain controlled fatigue experiments have been performed with notched specimens. It has been found, that stress and strain controlled fatigue testing results in the same number of cycles until failure. Essential for this correlation is that the cyclic stable strain amplitude at the notch root is taken for the entry into the strain‐life diagram in both cases. Starting from an elastic‐plastic analysis of the material behaviour at the notch root it is shown, how the strain‐life curve can be converted into a stress‐life curve. Based on that result service‐life is calculated from both approaches mentioned above. The calculation gives nearly the same service‐lives for both cases, but overestimates the measured data. It becomes obvious, that a S‐N curve determined under one‐level loading doesn’t provide a proper basis for service life assessment. While strain or stress‐life curves always contain crack initiation phase as well as crack propagation phase, the fatigue process under irregular loads is mainly governed by crack propagation. As a consequence, the damage per cycle is underestimated for loads near the fatigue limit, if Miner’s rule is used.     

Determination of local characteristics for the application of the Weakest‐Link Model

The Weakest‐Link model is based on defects that are statistically distributed within the material with local stress. The failure at least at one defect causes the failure of the total structure. Based on this model, the so‐called statistic size effect can be evaluated in the case of cyclic loading and in the case of static loading the failure behaviour of ferritic steel within the brittle fracture range is highlighted. The application of the Weakest‐Link model requires the allocation of the local characteristics ‐ surface and / or volume ‐ to the discrete points of the stress. By using the method “SPIEL” which is independent from the FE code used, the allocation of couples of values ‐surface and stress and / or volume and stress ‐ by a suitable choice of unit load cases is possible. In consequence of the method “SPIEL” particularities are to be taken into consideration. In the present paper these particularities will be described exemplarily for the FE programs ABAQUS¹ and ANSYS².     

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     

A prediction method for endurance of carburized components based on fatigue of corresponding material states with different carbon contents

Carburizing generates different material properties and residual stresses in the carburized layer. A material mechanics based model for endurance prediction of carburized steel components has been developed, in which thin walled tubes are used under cyclic internal and monotonic external pressures allow the determination of the material properties of the carburized layers. An unexpected independence of endurance and mean stress sensitivity from carbon content and hardness has been found, contrary to existing scientific knowledge. Component like specimens were tested to validate the endurance prediction model. The deviations of the predictions from the experiment are in the range of ±8 percent.     

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.     

Materialbeanspruchungsanalyse und ihre Anwendung auf Prüfstandsversuche zum Oberflächenausfall (Nierlich‐Schadensmodus) von Wälzlagern

Material Response Analysis and its Application to Rig Tests for the Surface Failure (Nierlich Damage Mode) of Rolling Bearings The material response analysis according to Nierlich using X‐ray diffraction represents an important physical examination technique for the evaluation of material stressing and the lifetime estimation of rolling bearings and other highly loaded machine parts. The method is presented and employed for the evaluation of automobile gearbox rig tests. The extensively described damage modes of the practically predominating surface and the classical sub‐surface failure of rolling bearings can be distinguished that way. In gearboxes, lubricating oil contaminated by metal abrasion of the cogwheels usually appears. Penetrating foreign particles produce indentations at the ring raceways and rolling elements of the rolling bearings, which promotes surface fatigue. The results of the X‐ray diffraction measurements confirm this damage mode. Evaluation of the occurred material stressing permits a more detailed characterization of the surface failure of rolling bearings.     

Zur Statistik in der Betriebsfestigkeit

About statistics in fatigue strength Material strength values can be distributed over a wide range. The lower limit of this range (e.g. minimum strength) is always well above zero. It is the reference point for design purposes. Formulas to describe the strength distribution F must be apt to explicitly fix this point. Formulas assuming negative or zero minimum values, as Normal or Weibull's two parametric distribution formula, are unsuited. The standard deviation is dependent of size (e.g. volume) of the specimens and therefore can not serve as a characteristic of a material or of a type of machine element. And therefore estimating minimum values via mean value and standard deviation is quite unreliable. This is especially critical with notched specimens where the volume at risk is extremely small. To evaluate reliable minimum values it is necessary to have a number of test specimens as high as possible and to extrapolate the distribution curve to the failure free time (or strength). The number of specimens can hardly be reduced. On the other hand the fatigue test time can be reduced by abstaining from searching the standard deviation. Usually an engineering object has to be designed absolutely failure free in the s‐n‐diagram. The allowable design reference point has to be situated below or left of the failure‐free curve F = 0. Small remaining risks resulting from non‐material influences in field application can by no means be quantified out of laboratory material tests.