Ermüdungsverhalten von Betonstahl unter Korrosionsbedingungen

In the 2001 completely revised version of DIN 1045‐1 for the first time an explicit proof against fatigue, more a fatigue analysis, have been published. A limit on the fatigue strength at two million cycles, as it was formerly common in civil engineering, is no longer in use. The influence of corrosion on the fatigue behaviour has been investigated insufficiently. It is thus not satisfyingly clarified. To fill this gap of knowledge, a research program was launched, in which fatigue tests at steel samples with a diameter of 16 mm were performed under going corrosion. The fatigue behaviour of the reinforcing steel was determined for four different corrosive media. The aim of the investiagtions was to show that the SN lines, which are the result of the fatigue tests, have exposure caused very different patterns. On the other hand it is examined, whether crack initiation and crack growth of the steel specimen can be recorded separately from each other. Therefore, different testing methods for crack detection were used. They differ especially with respect to their methodology. The article briefly describes the use of the testing methods for crack detection, explains the fatigue tests with the different corrosive liquids and discusses the test results.     

Elektrodenauswahl für das WIG‐Unterwasser‐Schweißen

Selection of electrode for GTA‐Underwater Welding Reproducible good weld quality and economical benefit of underwater‐welding require a complete automation. For this purpose Gas Tungsten Arc‐welding (GTA or TIG) offers numerous advantages, especially for the root and the following hot pass run. Disadvantages of GTA‐welding are the low weld deposit rate and the limited lifetime of the tungsten electrode. Already small wear damages cause wide alterations of the arc under high surrounding pressure, so that a suited choice of the electrode enables to increase the productivity considerably. Therefore the influence of the electrode features on welding process, arc stability, arc ignition, weld geometry and electrode wear has been investigated. For quick and elementary selection of electrode an assessment catalogue was elaborated.     

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.     

Erweiterungen des Fließstreifenmodells hinsichtlich verschiedener Einflussfaktoren auf das Ermüdungsrisswachstum

Das Fließstreifenmodell ist ein verbreitetes Hilfsmittel zur Berechnung der zyklischen Rissspitzenöffnungsverschiebung und des Ermüdungsrisswachstums von Strukturen unter isothermer zyklischer Belastung. Im Zuge einer Modellerweiterung kommt das rheologische Masing‐Modell zum Einsatz, das aus parallel geschalteten Reihenschaltungen von Feder und Reibelement besteht. Indem den Reibelementen unterschiedliche Fließspannungen zugeordnet werden, kann mit dem Masing‐Modell die zyklische Spannungs‐Dehnungs‐Kurve des Werkstoffs beschrieben werden. Anstelle einer konstanten Fließspannung werden diese verschiedenen Fließspannungen zufällig den einzelnen Elementen, in die die Riss‐Ligament‐Linie im Fließstreifenmodell unterteilt ist, zugewiesen. Neben einer besseren Modellierung des mechanischen Materialverhaltens ist auf diese Weise die Berücksichtigung mikrostruktureller Aspekte des Ermüdungsrisswachstums möglich, wenn auf die Werte der Fließspannungen im Bereich des frühen mikrostrukturellen Risswachstums Einfluss genommen wird. Zur Untersuchung thermomechanischer Ermüdung werden die Fließspannungen in Abhängigkeit von der Temperatur formuliert.     

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.     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Ermüdungsverhalten und Dauerfestigkeit von Graphit und Aluminium – infiltriertem Graphit

Fatigue behaviour and endurance limit of graphite and of aluminium‐infiltrated graphite Fatigue properties of polycrystalline, isotropic graphite FU2590 and of FU2590 infiltrated with AlSi7Mg (FU2590/AlSi7Mg) were investigated in reversed bending tests at 25 Hz at numbers of cycles below 10⁷ and in tension‐compression tests at 20 kHz below 10⁹ cycles. The open porosity of Graphite (10‐11 Vol.‐%) was infiltrated with the aluminium alloy using the squeeze casting infiltration method, which led to an increase of the bending strength by 50 %, increase of tensile strength by 30 % and increase of stiffness by 15 %. Fully reversed tension‐compression loading of FU2590 delivers a mean endurance limit at 10⁹ cycles at the normalized maximum stresses (i.e. maximum tension stress of a cycle divided by the static strength) of 0,65±0,03. Mean numbers of cycles to failure of 10⁴ were found in fully reversed bending tests at the normalized maximum stress of 0,78. The infiltrated material shows approximately 30 % higher cyclic strength in reversed bending tests, and the mean endurance limit under tension compression loading increases by 15 %. The increased endurance limit of the infiltrated material is caused by the increased stiffness. The increased toughness of graphite due to the infiltration with aluminium is of additional beneficial influence at the higher cyclic stresses investigated in reversed bending tests and in static tests.     

Rissfortschritts‐ und Ermüdungsverhalten der Aluminiumlegierung EN AW‐6060 nach ECAP und nachgelagerter Wärmebehandlung

Crack growth and high cycle fatigue behaviour of an AA6060 aluminium alloy after ECAP combined with a subsequent heat treatment Crack growth properties of the Al‐Mg‐Si alloy AA6060 as well as the high cycle fatigue behaviour have been investigated after equal‐channel angular pressing (ECAP). In our study, experiments have been conducted on different stages of microstructural breakdown and strain hardening of the material as they were present after different numbers of ECAP passes. A bimodal condition, obtained after two pressings, and a homogeneously ultrafine‐grained condition after eight repetitive pressings have been investigated. Furthermore, optimized conditions with an enhanced ductility, produced by ECAP processing combined with a following short‐time aging treatment were included into the study. Crack growth experiments have been conducted in the near‐threshold regime and the region of stable crack growth, covering a range of load ratios from R = 0.1 up to 0.7. It was found that the lowered fatigue threshold ΔK_th of the as‐extruded material can be enhanced by the combination of ECAP and short‐time aging, owing to the increased ductility and strain hardening capability of this material. By means of SEM investigations and tensile tests, the crack growth properties of the different conditions were related to microstructural and mechanical features. In fatigue tests, load reversals up to failure and the fatigue limit for an as‐extruded condition and an optimized condition after two ECAP‐passes have been compared to the coarse grained initial condition and a remarkable increase in fatigue strength was noted.     

Biaxiale Ermüdung reibrührgeschweißter Strukturelemente

Biaxial fatigue of friction stir welded stiffened panels Within the framework of the European WelAir project, cruciform specimens made from stiffening FSW overlap joints were fatigued in the DLR biaxial test rig. To resemble the loading situation of pressurized fuselage structures, proportional loading without any phase shift, but with different load ratios λ between the loading components in both directions was applied.Natural crack initiation and subsequent crack growth were governed by the stiffness gradient caused by introducing the stringer. Cracks initiated and propagated at run‐in and run‐out locations in a direction perpendicular to the weld seam. The shortest fatigue life was observed for uniaxial loading in welding direction (λ = 0). An additional stress component perpendicular to the joint line (λ > 0) resulted in a higher number of cycles to failure. Similar to single stringer panels, increasing the load ratio also increased the number of cycles to failure for FSW clip‐stringer structural members, but additionally gives a different location of the fatal crack.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.     

Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.     

Zwillingsbildung bei der thermischen Ermüdung der Mg‐Basislegierung AZ31

Twinning at thermal fatigue of magnesium alloy AZ31 In this paper results of thermal fatigue tests of the magnesium base alloy AZ31 carried out in a temperature range between ‐50 °C and +290 °C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stresses. The resulting materials behaviour is described by stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that AZ31 shows different stress‐strain behaviour during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning depends on the loading direction. However untwinning processes may occur during unloading and reloading in the opposite direction. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins are observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.     

Fatigue behaviour of welded joints from magnesium alloy (AZ31) according to the local strain concept

In the present study, the results of fatigue tests with the magnesium alloy AZ31 (ISO‐MgAl3Zn1) in the material states base metal, heat affected zone and weld metal obtained under strain control at room temperature within a range from 2·10² to 5·10⁶ cycles are presented. The fatigue behaviour was characterized by the Coffin–Manson–Basquin equations and the stress – strain behaviour by the Ramberg–Osgood equation. The data can be used to assess welded magnesium joints according to the local strain concept.     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

Mechanical,Chemical and Tribological Properties of the Nickel‐free High‐Nitrogen Steel X13CrMnMoN18‐14‐3 (1.4452)

Mechanische, chemische und tribologische Eigenschaften des Ni‐freien, hoch stickstoffhaltigen Stahls X13CrMnMoN18‐14‐3 (1.4452) Aufgrund der Probleme mit möglichen Ni‐Allergien besteht in der Medizintechnik die Forderung nach Ni‐freien Werkstoffen. Neben den bekannten Co‐ und Ti‐Basis Legierungen bieten die Ni‐freien hochstickstoffhaltigen Stähle eine kostengünstige Alternative. Der Stahl X13CrMnMoN18‐14‐3 (1.4452) ist ein Vertreter aus dieser Gruppe und wird in diesem Beitrag im lösungsgeglühten Zustand auf seine Korrosions‐, Ermüdungs‐ und Verschleißeigenschaften in Ringer Lösung untersucht und diskutiert. Im Vergleich zum üblichen medizinischen Edelstahl X2CrNiMo18‐15‐3 zeigt der 1.4452 unter allen Beanspruchungen bessere Eigenschaften. Das günstigere Ermüdungsverhalten wird durch die niedrige Stapelfehlerenergie bestimmt, während die besseren Korrosionseigenschaften auf den gelösten N zurückzuführen sind. Das sehr gute Gleitverschleißverhalten im artgleichen Kontakt wird durch das Zusammenwirken von beiden erreicht. Berücksichtigt man weiterhin das nicht‐zytotoxische Verhalten, kann man den Stahl 1.4452 als tauglich für medizinische Anwendungen bezeichnen.     

Untersuchung von Schmelz‐ und Rekristallisationsprozessen sowie Riss‐, Oxid‐ und Porenbildung beim Schweißen

Investigation of melting and re‐crystallization processes and of crack, oxide and pore formation during welding In the paper some known investigation methods of the primary re‐crystallization and of the structure of the crystallization front are discussed. All these methods have disadvantages, in some cases they cause a limited validity or are expensive. The presented new method is based on the micrographic recording of the melting and solidifying metal surfaces. The method overcomes the above‐mentioned disadvantages. It is able to investigate in the welding seams not only the primary re‐crystallization processes but also the secondary ones including the formation of cracks and pores and the appearance of non‐metallic inclusions like oxides. The facilities of the method are demonstrated on some examples of use.     

Schweißen von Hohlstrukturen und offenporösen Metallschäumen für den Einsatz in Kombi‐Kraftwerken

Welding of Hollow Structures and Open‐Porous Metal Foams for Application in Combined Cycle Power Plants For applications within the scope of novel cooling concepts joining technologies for sandwich composites and open‐porous metal foams are researched in the context of the Collaborative Research Centre 561 “Thermally highly loaded, porous and cooled multi‐layer systems for combined cycle power plants”. The research motivation and application fields of the different structures are defined. Welding processes and strategies for manufacturing these structures are specified as well as the joining technologies’ characteristics. Planned future works for enhancements of the processes and structures are listed.     

Ermüdungsverhalten ausgewählter Werkstoffe und Bauteile bei sehr hohen Schwingspielzahlen

Very High‐Cycle Fatigue of Selected Materials and Components Results of several fatigue tests using a servohydraulic testing machine (VHF 50 D) and a spring testing machine are presented. Investigations were carried out at room temperature with. – smooth and notched specimens made of aluminium wrought alloy EN AW 6082,. – screws M 8 made of EN AW 6056,. – smooth specimens made of die cast magnesium AZ91 hp (here also tests at 125 °C) and. – shot‐peened helical compression springs of four different high strength steels. Tests were done with a frequency of 20 Hz (spring testing machine) and between 250 Hz and 400 Hz (VHF50D) up to a maximum number of cycles of N = 1,5 x 10⁹. Crack initiation sites were investigated and could be found at high number of cycles below the surface for smooth magnesium and aluminium specimens and also for most of the shot‐peened helical compression springs, but not for screws and other notched specimens.     

Einfluß von Überlasten auf die Ausbreitung von Ermüdungsrissen in der Aluminiumlegierung 7075 T 7351

Influence of overloads on the propagation of fatigue cracks in the aluminium alloy 7075 T 7351 For realistic lifetime predictions the crack propagation behaviour under variable amplitude spectrum loads has to be known. In this work the influence of single overloads on the fatigue crack propagation in the alloy 7075 T 7351 was investigated at a constant loading level (K_max and ΔK = const.). The experiments showed, that a single overload causes a short crack acceleration followed by a pronounced deceleration of the crack propagation rate. After this deceleration at higher overloads a zone where the crack propagation rate is slightly increased (lost retardation) was observed. It has been shown, that the minimum crack propagation rate as well as the length of the influenced region depends on the height of the overload. The observed crack acceleration, the deceleration as well as the lost retardation can be explained by internal stresses caused by those overloads which have to be added to the external stresses.     

Laserstrahlschweißen von Titanwerkstoffen unter Berücksichtigung des Einflusses des Sauerstoffes

Influence of the oxygen content in the shielding gas on microstructure and mechanical properties of laser welds of titanium and titanium alloys In the present work, a new tool concept for laser welding of titanium in high volume production has been presented and evaluated. Through the innovative application of a six‐layer metal web it is possible to calm the argon gas flow and avoid pernicious turbulences during welding. The integration of the mentioned metal web at the base of an open welding chamber allows the automated welding of highly reactive materials, such as titanium, under atmospheric pressure and inert shielding conditions. The higher density of argon relative to air offers the unique possibility to leave the chamber open on the top, so that a higher degree of flexibility than gas shielding devices for TIG welding, especially for industrial robots, is attained and can be successfully used for industrial mass production. Furthermore this device is important for welding three‐dimensional contours or to shield the regions of overlap (in overlapped joints) where shielding gas trailers are unsuccessful. By means of the presented gas shielding procedure and a modern laser welding process such as Nd:YAG laser welding, systematic investigations on the effect of oxygen on the microstructure as well as on the mechanical properties of reference bead‐on‐plate weldments could be performed for the first time. As a result of these welding trials it can be concluded that in order to avoid discolorations and hardness increase, lower restrictions to the purity of the shielding gas, in comparison to TIG welding condition, can be allowed. The maximum tolerable value of oxygen in the welding atmosphere was found to be approximately 1000 ppm for laser welding. On the contrary the maximum value for TIG welding is about 30 ppm. Further investigations on the microstructural and mechanical properties of the joints confirm that the optical quality assurance criteria for TIG welding due to the standards of aircraft construction transferable to Nd:YAG welding are.