Materials Science Forum Vols. 768-769

Abstract: An Abaqus Extension, in the form of an Abaqus/CAE Plug-In, is presented that enables an easy, efficient, model-tree based approach to setup all aspects of a welding model from within Abaqus/CAE. Specifically, the paper describes the extension to 3-D of a similar capability, namely the 2-D Abaqus Welding Interface (AWI), that currently exists for automating most of the repetitive, time-consuming tasks associated with building a welding model in a traditional CAE environment to facilitate weld simulations. The tasks supported by the 3-D AWI include weld-bead definitions based on simple cross-sectional geometric entity picks, options for geometry- or element-based bead-chunking in the weld direction, automatic definition of weld passes, steps and boundary conditions for each pass, and set-up of both the thermal and stress analysis models. The 3-D AWI is expected to make the normally time-consuming welding analysis setup significantly faster. The use of the AWI is demonstrated on a six-pass plate welding problem described in the literature.

Abstract: Recently, reliable and hermetic joining of stainless steel to niobium pipes has been achieved with the explosive bonding technique. Joining of these two materials are essential to ensure production of a bimetallic transition element of pipe-type for its further use as a part of charged beam acceleration systems of the new generation. A non-destructive neutron diffraction investigation of the tri-axial strains along a radial cross-sectional line through the joint section has been performed. Residual stress results indicate inherently different natures in the residual stress values within the respective pipe sections. In the external stainless steel pipe the residual stresses are tensile, showing a sudden increase to 600 MPa as the interface is approached, whilst being compressive in the internal niobium pipe, not exceeding 650 MPa. A characteristic abrupt stress discontinuity exits at the interface region.

Abstract: Twin disc tribological tests were performed in wheel and rail materials, with specimens taken from a Spanish AVE train wheel and a UIC60 rail, in a program intended to characterize their contact fatigue behavior. The X-ray diffraction technique was used to characterize the residual stress distribution at the initial and damaged stages, as well as in intermediate stages, since existing residual stresses in the surface layers of the railways steels and its evolution during contact loading can have a major influence on crack initiation and propagation.

Abstract: Mixed friction acting in a rolling contact increases the v. Mises equivalent stress and shifts the maximum towards the surface. Tangential stresses are superimposed to the stress distribution. The resulting position of the maximum v. Mises stress depends on the magnitude of the friction coefficient and is located directly on the surface from values of about 0.25 upwards. The impact of three-dimensional machine vibrations on rolling bearings in operation can cause severe mixed friction running conditions. Residual stress distributions measured on indentation-free raceways indicate high friction coefficients of up to greater than 0.25. The surfaces reveal smoothing of the finishing structure but no adhesive wear. The simulation of the vibrationally loaded rolling-sliding contact is based on the tribological model of localized friction coefficient. This approach avoids seizing by allowing for increased friction only in intermittently changing subareas of the contact at low sliding speed. The macroscopic friction coefficient, meeting a mixing rule, does not exceed 0.1. The finite element method (FEM) is used for the stress analysis. In the first step, a simplified FEM model involves a circumferentially oriented band of high friction coefficient from 0.2 to 0.5 within a cylindrical roller contact. The resulting depth distributions of the v. Mises equivalent stress during overrolling and the corresponding residual stresses are evaluated below the inner ring raceway of the bearing. The features of the FEM model are discussed in detail. The increased sliding friction in the band shifts the maximum of the v. Mises equivalent stress to the surface. Compressive residual stresses are induced in the edge zone. Depending on the applied Hertzian pressure, an additional subsurface peak occurs. First results of the finite element analysis are presented.

Abstract: Rolling bearings in wind turbine gearboxes occasionally fail prematurely by so-called white etching cracks. The appearance of the damage indicates brittle spontaneous tensile stress induced surface cracking followed by corrosion fatigue driven crack growth. An X-ray diffraction based residual stress analysis reveals vibrations in service as the root cause. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are compared with residual stress patterns and a relevant fracture strength, respectively. White etching crack failures are reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from residual stress measurements. Cold working compressive residual stresses are an effective countermeasure.

Abstract: This paper presents a novel experiment to quantify both the initial residual stress state in a specimen and its redistribution due to plasticity induced by in-situ loading. The rate of relaxation of the residual stress with respect to permanent deformation is a measure of the elastic follow-up associated with the residual stress field. Residual stress measurements were made using high energy dispersive X-ray diffraction. Digital image correlation, verified by strain gauges, was used to measure full-field deformation on the specimen. The specimen was loaded and unloaded in-situ incrementally to promote plasticity, allowing the relaxation rate of the residual stress to be quantified. An elastic follow-up factor was calculated for the residual stress field, that indicated loading conditions of the residual stress field between fixed-displacement and fixed-load.

Abstract: Insulated rail joints (IRJs) are an integral part of the rail track signaling system and pose significant maintenance and replacement costs due to their low and fluctuating service lives. Failure occurs mainly in rail head region, bolt- holes of fishplates and web-holes of the rails. Propagation of cracks is influenced by the evolution of internal residual stresses in rails during rail manufacturing (hot-rolling, roller-straightening, and head-hardening process), and during service, particularly in heavy rail haul freight systems where loads are high. In this investigation, rail head accumulated residual stresses were analysed using neutron diffraction at the Australian Nuclear Science and Technology Organisation (ANSTO). Two ex-service two head-hardened rail joints damaged under different loading were examined and results were compared with those obtained from an unused rail joint reference sample in order to differentiate the stresses developed during rail manufacturing and stresses accumulated during rail service. Neutron diffraction analyses were carried out on the samples in longitudinal, transverse and vertical directions, and on 5mm thick sliceed samples cut by Electric Discharge Machining (EDM). For the rail joints from the service line, irrespective of loading conditions and in-service times, results revealed similar depth profiles of stress distribution. Evolution of residual stress fields in rails due to service was also accompanied by evidence of larger material flow based on reflected light and scanning electron microscopy studies. Stress evolution in the vicinity of rail ends was characterised by a compressive layer, approximately 5 mm deep, and a tension zone located approximately 5- 15mm below the surfaces. A significant variation of d0 with depth near the top surface was detected and was attributed to decarburization in the top layer induced by cold work. Stress distributions observed in longitudinal slices of the two different deformed rail samples were found to be similar. For the undeformed rail, the stress distributions obtained could be attributed to variations associated with thermo-mechanical history of the rail.

Abstract: Aircraft engine components are subjected, voluntarily or not, to the influence of residual stresses (RS). These RS may evolve in service conditions and may have an influence on fatigue life of the component. This paper presents a method to take into account the RS and their relaxation in a finite element calculation to obtain the fatigue life. This method is applied to a representative high-pressure turbine disk specimen made of N18 Nickel-based superalloy. Firstly, residual stresses are measured using X-Ray diffraction technique on the surface and the thickness of specimens. The influence of different surface finishing processes on the intensity and distribution of RS is compared to as-received specimen. Then, using the experimental profile as an initial state, a fatigue life analysis is performed (on fatigue specimen) by applying a multiaxial extension of the Smith-Watson-Topper model. Numerical and experimental results are discussed in detail and it appears that residual compressive stresses have almost no influence for high strain range but they improve the fatigue life for lower ranges.

Abstract: Rolling contact fatigue is a very complex process. The mechanism can only be described by speculative considerations. Because the loading conditions during the elastic- hydro- dynamical contact are not clearly described. The loading cycle runs within extremely short rates and structural alterations occur under high hydrostatic pressure. Widely unknown is therefore, how the materials conditions are influenced by these processes. But by means of simplified considerations an approach to the rolling contact fatigue process can be obtained. Following these conceptions simplifying quasi-static conditions are drawn. A lubricant film inhibits the metallic contact of the revolved bearing components. A HERTZian load stress will be accumulated over an elliptical contact area and within and beneath this contact area three dimensional stresses are acting. The materials strengthening can be described by the hypothesis of alteration of shape. During the fatigue period, the microstructure will be changed by micro- and macro- plastically deformation. By this residual stresses occur. These are superimposed to the operational -loading –stresses which change the distressing conditions of the material. The progressive plastically deformations accompanying the growing fatigue procedure cause perpetually alterations in the distress- conditions of the material. Structural alterations of the rolling contact fatigue process are shown by means of metallography as followed: by dark etching areas (DEA), and by white etching areas (WEA) showing bands, which are positioned beneath the contact area at an angle of 30° (30°WEB) and for instance at 80° (80°WEB), and furthermore by so called butterfly structures (butterflies with “white etched” flanks). All these white etching areas, regarding their morphological structure and the etching conditions, are commonly originated by two axial distressing. The three dimensional materials distressing within the roller-bearing component on the one hand and the two dimensionally originating of the WEA on the other seem to be an antagonism. But when the changes of residual stresses during the contact rolling fatigue process are to be analyzed, it is clear that this antagonism rises only virtually because there exists a real tri-axial stress condition, which tolerates a two axial distressing of the material. By the concept, that the growing plastically deformations cause residual stresses superposing the operational load stresses, the temporary cycle of the structural alterations and the local and angular positions of the 30° WEB can be explained.