Volume 57, Issue 2

This publication is focused on a critical plane approach to assess the influence of the multiaxial stress state on the lifetime of welded crane runway girders cyclically stressed by travelling wheel loads. Cyclic stress-strain curves of specimens both from the weld region and the base material of the girder are evaluated in strain-controlled reversible fatigue tests as a basis for the assessment of the lifetime. Finite element analyses are performed to estimate the multiaxial and non-proportional local stress condition acting on sharp notches of the girder, i. e. the weld toes and the weld root. The MPA AIM-life concept is used to evaluate the lifetime of crane runways on the basis of advanced fatigue parameters and the cyclic material properties. Furthermore, the notch stress approach according to IIW guideline [1] is applied to investigate the fatigue behavior. The estimated lifetimes are compared to experimental results of component fatigue tests conducted with full-scale welded crane runway girders.

Nickel-based cast IN 738LC superalloy has been cycled at increasing strain amplitudes at room temperature and at 800 °C. Hysteresis loops were analyzed using general statistical theory of the hysteresis loop. Dislocation structures of specimens cycled at these two temperatures were studied. They revealed localization of the cyclic plastic strain in the thin bands which are rich in dislocations. The analysis of the loop shapes yields effective stresses of the matrix and of the precipitates and the probability density function of the critical internal stresses at both temperatures. It allows to find the sources of the high cyclic stress.

The low density of magnesium alloys makes them attractive for light weight constructions. However, important aspects are the rather poor mechanical properties of the most commonly used Mg alloy AZ91 (approximately 9 wt.-% Al, 1 wt.-% Zn), which limits its use in, e. g., power train applications. Alloying AZ91 with Ca leads to a significant improvement of creep resistance. To investigate the influence of the Ca addition on the fatigue behavior, symmetric push pull tests have been performed on AZ91 alloys with an addition of 0, 1 and 3 wt.-% of Ca, respectively. At relatively high amplitudes, the Ca containing alloys show a decrease in fatigue life compared to Ca-free AZ91. However, with a decrease in amplitude, the fatigue life of the alloy with 3 wt.-% Ca are comparable with the Ca-free AZ91 alloy and with a further decrease in amplitude the alloy with an addition of 3 wt.-% Ca has an even higher fatigue life than the Ca-free alloy. All alloys contain pores due to the casting process.

The following contribution deals with the experimental investigation and theoretical evaluation of fatigue crack growth under isothermal and non-isothermal conditions at the nickel alloy 617. The microstructure and mechanical properties of alloy 617 are influenced significantly by the thermal heat treatment and the following thermal exposure in service. Hence, a solution annealed and a long-time service exposed material condition is studied. The crack growth measurement is carried out by using an alternate current potential drop system, which is integrated into a thermomechanical fatigue (TMF) test facility. The measured fatigue crack growth rates results in a function of material condition, temperature and load waveform. Furthermore, the results of the non-isothermal tests depend on the phase between thermal and mechanical load (in-phase, out-of-phase). A fracture mechanic based, time dependent model is upgraded by an approach to consider environmental effects, where almost all model parameters represent directly measureable values. A consistent description of all results and a good correlation with the experimental data can be achieved.

Laser cutting is widely used in the metal industry, particularly when components of high strength steel sheets are produced. However, the roughness of cut edges produced by laser differs from that obtained by other methods, such as mechanical blanking, and this fact influences the fatigue performance. In the present investigation, specimens of two grades of high strength austenitic steels, i. e. AISI 301LN and TWIP17Mn, were cut by laser and tested in the high cycle fatigue regime to determine their corresponding fatigue limits. A series of fatigue specimens were tested without polishing and other series after a careful polishing of the cut edges, in order to assess the influence of the cut edges condition. Results indicate a significant influence of the edge roughness, more distinctive for AISI 301LN than for TWIP steel.

Fatigue life approach methods based on general local concepts for welded joints assign universal material properties to geometrically well-defined weld toes or weld roots, in order to evaluate local stresses and strains, ignoring any metallurgical discrepancies. In this study, analysis of the main influences on these concepts, linear damage accumulation and cyclic material behavior, of the high-strength and ultra-high-strength steels S960QL, S960M and S1100QL was performed for the base material and for butt-welded specimens. Results of stress-controlled tests under constant amplitude loading (CAL) and variable amplitude loading (VAL) show that a linear damage accumulation with a damage sum of D al = 0.5 is conservative. Effects of the geometrical and metallurgical notch on the cyclic material behavior are shown by strain controlled testing of both base material and welded specimens. Cycles to crack initiation, presented in strain-life curves, are consequently reduced in the as-welded state. Cyclic stress-strain curves show a cyclic softening of the base material as well as of the welded joints. Furthermore, the advantages of the ultra-high-strength steel S1100QL disappear in welded connections.

A major safety concern in commercial civil aviation is losing a fan blade from jet engine during its normal operation either due to bird strike or limited fatigue. The assessment of fatigue strength of the support structures like bearing and intermediate connection in aero engine during windmilling has become an essential requirement for safe-fly-home situation. Windmilling is the rotation of a non-operating engine due to the airflow induced forces on the blades caused by the forward motion of the aircraft. The imbalanced rotation during windmilling post fan blade off (FBO) creates unavoidable vibratory loads in transient nature which leads to shorter fatigue life of the support structures. Since the repeated imbalance loads beyond elastic limit of the structure can cause the accumulation of plastic deformation, which exceeds the critical strain limit of the structure. This article deals with the ratcheting behavior (low cycle fatigue) of bearing support structures during windmilling post FBO in aero engine. When the accumulated plastic strain exceeds the critical strain limit of the bearing structure, then it experiences ratcheting fatigue failure.

Presently, AlSi based alloys, consisting up to 12 element systems, are used in the manufacture of automotive pistons for diesel engines. The pistons combustion wall is subject to complex superimposed transient mechanical and thermal loading with peak operating temperature representing a homologous temperature range T hom of 0.8 to 0.9. Using a special superimposed thermomechanical bench test apparatus, engine-like TMF loading has been reproduced and a number of semi in-situ experiments have been carried out to evaluate key microstructure damage mechanisms. The evolution of microstructural damage at the interface between hard Si inclusions and the softer Al matrix has been documented using scanning electron microscopy. The deformation characteristics at the Si/Al interface have been recreated using FEA techniques incorporating non-linear elasto-viscoplastic properties for the matrix material. Comparisons of bench test fatigue lives for transient superimposed high frequency and microstructural TMF loading for two different thermal cycles are then compared to isothermal mechanical load histories.

This publication presents an expression for the equivalent strain based on fatigue tests under complex strain state. The respective aluminum alloy was tested under a very low number of cycles. The criterion uses the critical plane determined according to the material kind function. The determined equivalent amplitude for complex proportional and non-proportional loading states is included into the scatter band of the same coefficient for simple loading states. The model based on the proposal by Carpinteri for determination of the critical plane position under a complex strain state can be a useful tool for fatigue life assessment in the case of a very low number of cycles. The expression for the equivalent strain includes both the normal and shear strain histories on the critical plane with suitable weight functions. The determined equivalent amplitude for complex proportional and non-proportional loading states is also included into the scatter band of the same coefficient for simple loading states.

New possibilities of low cycle fatigue material properties testing with the use of micro-specimens are presented in this publication. The discussed solution is of great significance for analysis of local properties of a material, e. g. in welded structures. Properties of a material determined in the manner presented in this publication find application both in fatigue life assessment (fatigue life curves) and in material modeling (cyclic stress-strain curves), e. g. in numerical analysis of local strain and stress that occur in structural components.

In the present work, a novel approach is adopted to evaluate the low cycle fatigue (LCF) damage at a mesoscopic scale. The fatigue behavior of a ferritic steel has been studied and damage criteria were assessed by a metallurgical approach that combines two analysis techniques: transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). After characterizing the cyclic behavior of the material for different loading parameters at a total strain range varying from 0.3 % to 0.7 %, the microstructure evolution induced by the cyclic plasticity was studied in a multi-scale investigation. Here, TEM and SEM-EBSD give complementary information about the link between macroscopic fatigue behavior and microstructure changes. TEM shows a clear indication of the dislocations structures related to fatigue damage. SEM-EBSD completes the investigation by giving quantitative data associated to local deformation. This relation enables to determine plastic strain assessment, for example, of industrial components.