Volume 51, Issue 6

This first part aims to study the correlation between the cyclic softening observed during low-cycle fatigue in a 2507 duplex stainless steel (DSS) alloyed with 0.17% N and the dislocation structure development. Tension-compression and tension-compression- torsion fatigue tests were carried out under total strain control at Δε teq /2 = 0.5%. The dislocation structures were characterized by transmission electron microscopy during cycling with interrupted tests. The austenite showed planar arrangements of dislocations during all the fatigue life, the ferrite showed evolution from a homogeneous distribution of dislocations to a structure of dislocation bundles in tension-compression, and to a refined wall structure in tension-compression-torsion. It is proposed that the evolution of the dislocation structure in the ferrite plays an important role during the cyclic softening in the DSS.

This second part aims to propose an extension of the bi-phased polycrystalline model shown in a previous contribution [1], in order to simulate the cyclic softening observed in an austenitic-ferritic stainless steel under uniaxial and non-proportional biaxial cyclic loading [2]. Assuming that the cyclic softening for uniaxial and biaxial non-proportional cyclic loading is due to a rearrangement of dislocations in hard (rich in dislocations) and soft (poor in dislocations) zones in ferritic grains [3], the ferritic single crystal law is modified. The new model parameters are identified from TEM observations [3] and by inverse procedure. Finally, the new model is validated for other cyclic loading paths. A good agreement is observed between simulation and experimental data.

Low cycle fatigue of SAF 2507 duplex stainless steel has been studied at different temperatures in two thermal conditions: unaged (as-received) and aged (100 hours at 475°C). The corresponding cyclic flow stress components, the friction, σ F , and the back stresses, σ B , have been analyzed in relation to the observed microstructure. σ F attains higher values at room temperature than in the intermediate range of temperatures in both thermal conditions due to the influence of the lattice friction stress on screw dislocations in the ferrite phase. The cyclic softening observed at all temperatures in aged samples is caused by the decrease of the back stress σ B . The increase of σ B in unaged samples at 300°C is consistent with the high dislocation density observed in the ferrite phase ascribed as a typical manifestation of dynamic strain aging.

Surface relief evolution during the fatigue life has been studied by Atomic Force Microscopy in a duplex stainless steel fatigued at a total strain range Δε t = 1.6%. In austenite, hill-like extrusions, observed since the beginning of the fatigue life, were the basic morphology. For ferritic grains, the modification took place later and the disposition of the persistent slip markings was irregular in a given grain. Highly rugged areas were the more developed surface morphology in ferrite after 60 percent of the fatigue life. These areas seemed to be connected with the austenitic neighbour grain in terms of surface activity and disorientation, and are proposed as the possible crack nucleation sites in the ferrite, near the ferrite austenite interfaces. It is shown that the origin of crack nucleation in ferrite is strongly dependent on the relative crystallographic orientation of the austenite and its plastic activity.

Monotonic and cyclic plastic deformation of metastable austenitic steels lead to a phase transformation from paramagnetic austenite into ferromagnetic α′-martensite. The deformation-induced changes of the magnetic properties are directly related to the accumulated plastic strain and therefore to the actual fatigue state. This paper includes a detailed characterization of the deformation-induced austenite-martensite-transformation in metastable austenitic steels. The cyclic deformation behaviour was evaluated by mechanical stress-strain hysteresis and temperature measurements. With in-situ Ferritescope magnetic measurements, the development of the α′-martensite, and the change in the magnetic induction due to the Villari effect were investigated. On the basis of far-reaching cross effects of mechanical and magnetic properties, measurements of magnetic-mechanical hysteresis loops were performed. Hence a method for fatigue life calculation based on the change in magnetic properties was developed.

The low cycle fatigue response of EUROFER 97 has been investigated at different total plastic strain ranges and temperatures between 20—450°C. The cyclic behaviour has been analyzed through the decomposition of the flow stress in back stress and friction stress, and different microstructural mechanisms associated with cyclic softening were proposed to explain the differences observed in those stress components. At 20°C, the cyclic softening could be attributed to a progressively unpinning of dislocations from carbon atoms. At higher temperatures, the mechanical softening effects agree with the evolution from initially very fine martensitic laths to a coarser equiaxial cells structure. A decrease of dislocation density inside the micrograins is observed for all the temperatures, to a higher degree for higher temperatures.

Kurzfassung Im vorliegenden Beitrag wird dargestellt, wie das Normungsbedürfnis aus verschiedenen europäischen Länder zu einem neuen genormten Verfahren zur Bestimmung nicht metallischer Einschlüsse in Stählen mit Richtreihenbildern geführt hat. Die Anwender der Reinheitsgradbestimmung werden über die grundlegende Verfahrensweise informiert und es werden dazu Beispiele gegeben.

In the work for this contribution, homogenization heat treatment was evaluated with respect to its effect on deformation ability of AA6063 aluminum alloy which is a widely used extrusion alloy. The samples machined from extrusion billets were homogenized at 540°C, 560°C and 580°C for a time period of 2, 4, 6, 8 and 10 h. The homogenization state of the samples was assessed via microstructure examination. The hardness of the samples was determined using Brinell hardness method. Tensile test was carried out in order to evaluate the deformation ability. Tensile strength and elongation at rupture were compared depending on the homogenization state of the alloy. Depending on the homogenization grade, the elongation at rupture was increased, and samples showed a gradual shift from brittle fracture to ductile fracture.

A stress concentration due to the presence of holes is a problem often encountered in the design of structures. The aim of the current investigation is to optimize the central distance between two holes in a plate subjected to pressure in x-direction in case of isotropic material plate, laminated plate, and FGM plate to minimize stress concentration around holes. The finite element analysis (FEA) and optimization technique are used to calculate the optimal central distance between two holes in a plate subjected to pressure in the x-direction for different types of materials and laminates. The investigations have shown that as long as the central distance is small, the minimum the stresses around the hole are. In addition, for all the material types used, the optimal central distance is not affected by the type of material.

The method of Pulsed Thermography has been used for the investigation of carbon as well as glass fibres reinforced plastics. We have studied material properties like porosity and density, on the one hand, and defects, on the other hand. The defects under investigation have been inclusions of foreign materials. Test samples with simple geometries as well as components with complex geometries have been surveyed. In the case of complex geometries, Finite-Element-Simulations have been performed additionally and used to interpret the experimental data with respect to geometry effects.

In non-destructive testing, Computed Tomography (CT) is increasingly used for dimensional measurement tasks such as the industrial inspection of work pieces. Besides the determination of the measurement uncertainty, the improvement of the image quality is subject of current research in order to establish CT as a measuring device.