Catálogo de publicaciones - libros

The effect of residual stresses generated by load history on fracture has been investigated through experimental and numerical studies. In this paper the application of a “local approach” to determine the change in applied load to cause fracture is described. Then the R6 defect assessment procedure is presented. The effect of tensile residual stress on brittle fracture of ferritic steel type A533B at -150ºC is explored. The modified J-integral, J has been also used to estimate the combined crack driving force during loading to fracture of the specimens containing tensile residual stresses ahead of a crack/notch. It is shown that by having more accurate estimates of driving force created by the residual stresses leads to increased agreement between experiments and assessment.

Thermal cycling, high heating rates, high temperature peaks and inter-pass and post weld cooling are parameters that largely affect residual stress generation in and around welds. A multi-pass weld joining two pipes made of different materials is simulated using 2-D axi-symmetric finite element analysis. The proposed methodology for weld simulation incorporates the well-known birth of elements technique and follows the prescribed temperature approach for heat input modeling. The effect of various aspects of modeling, on the accuracy of predicted residual strains, is investigated through a series of sensitivity tests, using a 2-D axi-symmetric model. Radiation, creep and heat input model selection, have a significant impact on results, but phase change, convective cooling and pipe contact are not as important. Results are also compared to neutron diffraction measurements obtained from the literature. Welding electrode start/stop effects on predicted residual strains are found to be significant, after a limited 3-D analysis, which justifies further investigation.

The influence of various modelling aspects on the prediction of residual stresses in a 3-bead letterbox-type repair weld is investigated in the present work. The repair is performed on a 2¼CrMo low alloy ferritic steel plate, containing a machined central groove of 9 depth, 200 length and 14 width. Three weld beads are deposited in the groove using AL CROMO S 225 2¼ CrMo electrodes. The repaired region is considerably long and narrow to enable a 2D plane strain analysis. Using the commercial finite element code ANSYS and the very well known “birth and death” technique, the effect of material hardening rule, different heat input models such as prescribed temperature and heat generation rate approach, radiation boundary conditions and coefficient of convective cooling on the evaluation of residual stress field is examined in a sensitivity analysis frame work. Finite element 2D mesh and time step size are optimised affording useful information for a future 3D analysis. Metallurgical phase transformation effects are not included in the model, although it is general knowledge that its role in the formation of a residual stress field might be quite significant for ferritic steels. Recorded data for temperature and thermal strain histories are used to validate predictions obtained by finite element computer simulation. Comparisons reveal a good agreement between predicted and recorded temperature and thermal strain histories. Material hardening rule affects remarkably the results whereas the implication of radiation boundary conditions has a small contribution on the predicted results.

The highly localized transient heat and strongly nonlinear temperature fields in both heating and cooling processes cause nonuniform thermal expansion and contraction, and thus result in plastic deformation in the weld and surrounding areas. Consequently, residual stress, strain and distortion are permanently produced in the welded structures. High tensile residual stresses are known to promote fracture and fatigue, while compressive residual stresses may induce undesired, and often unpredictable, global or local buckling during or after the welding. It is particularly evident with large and thick panels, as used in the construction of nuclear building. These adversely affect the fabrication, assembly, and service life of the structures. Therefore, prediction and control of residual stresses and distortion from the welding process are extremely important for the nuclear installation’s security.~~~This study focuses on the three-thermo-mechanical behavior of 316L stainless steel, during a TIG welding process. In this paper, we investigate the effect of the heat modeling source, thermal exchanges and viscous property on experimental and numerical results. Therefore, a parallel experimental and numerical study is carried out on an industrial 24-25 mock-up benchmark [4], a test more representative of a real welding operation, considering repair welding, is implemented to validate three-dimensional numerical effect. The TIG process, with 316L material filler, is considered. Comparative analyses through numerical simulations using finite element code (version 7.4 code_Aster from EDF) are performed.

The occurrence of cracks in – normally welded – components with safety relevance in, e.g. nuclear installations or in the (petro-)chemical industry, is not an unusual event. In most cases such cracking is detected in periodic inspections prior to complete failure of the component. Sometimes a detected defect necessitates repair of the damaged component to facilitate its further operation. Repairing of a crack would normally be performed by excavating of the material surrounding the crack and subsequent filling of the excavation by welding. However, such a repair welding process leaves the component in a sensitive state in that it generates a complicated residual stress pattern and that the heat affected zone of the weld might become very susceptible to the formation of new cracking [1]. Post weld heat treatment of a repaired component can be an option to mitigate the damaging impact of the welding process. Through heat treatments residual stresses can be severely reduced or redistributed to obtain stress fields around the weld deemed less detrimental. At the same time a heat treatment process could positively influence the HAZ sensitivity for further cracking. In any case, a thorough assessment of the welding process is necessary to ensure a safe continued operation of the repaired component.

In this context letterbox repair welds applied to thin ferritic steel plates to simulate repair of postulated shallow cracks have been manufactured. The excavations of postulated cracks for these experiments were filled with 20 to 30 welding passes. Components have been made available in the as welded state and after the application of PWHT. Two different heat treatment processes are compared: a. a full scale treatment, where the entire test piece has been subjected to an elevated temperature for several hours in order to significantly reduce the residual stresses, and b. an alternative treatment whereby the heat is applied locally for a short period of time in order to redistribute the stresses in a controlled manner.

In this paper the experimental determination of these residual stresses in the as welded and in the heat-treated states is presented. Such measurements have been performed by neutron diffraction at the High Flux Reactor (HFR) of the Joint Research Centre of the European Commission in Petten, the Netherlands.

The principle of residual stress measurements by neutron diffraction is introduced [2] and the particular considerations for performing such measurements in multi-pass butt welds are briefly outlined [3]. The experimental approach is presented and explained and an outline is given on the data analyses. Results are depicted in the form of comparison between the as received and the heat treated stress states. The derived data facilitate conclusions on the effects and effectiveness of the applied heat treatments and they also demonstrate that neutron diffraction is a very suitable tool for non-destructive analysis of internal residual stress fields in such welded components of considerable thickness. In addition, the method is well suited for the validation of predictive numerical models.

In this paper, attractive properties of unconventional and high-resolution neutron diffraction performances exploiting cylindrically bent perfect (BPC) Si-crystal monochromators documented by experimental results, are presented. They permit high or even ultrahigh-resolution of macro- and microstrain scanning of bulk polycrystalline materials. The diffractometer using a dispersive type multiple reflection monochromator can operate with the resolution of the backscattering device, however, at a rather small monochromator take-off angle.

In this investigation, ground surface integrity and fatigue behavior improvements of the AISI 304 SS resulting from the application of wire brushing at ambient and low temperatures were investigated. It was found that the cold work hardening generated by the cryogenic brushing increases the levels of the compressive residual stresses comparatively to the dry brushing and therefore, results on higher nucleation fatigue lifetime of mechanical components having undergone this treatment. On the other hand the propagation fatigue lifetime of these components was found to be extended by plastic induced martensite formed at the tips of the nucleated fatigue cracks. The realized improvement rates expressed in terms of endurance limits at 2x106 cycles comparatively to the ground state are 47% for the dry brushing conditions and 72% for the cryogenic brushing.

Surface integrity of commercial Ti-6wt.%Al-4wt.%V alloy after high speed machining has been investigated. Scanning electron microscopy reveals highly deformed grains in the obtained surface, turned in the direction of the cutting tool passage. X-ray diffraction confirms the intense deformation of the machined surface and shows a crystallographic texture modification. The metallurgical observations do not reveal any phase transformation and no evidence of formation of a has been noticed. A preliminary study of the residual stress reveals a strong influence of machining parameters on the residual stress in obtained surface.

A great deal of the properties of materials is influenced by phenomena taking place in the sub-micron region. Scattering techniques play an important role for obtaining structural information. Small-Angle Neutron-Scattering (SANS) is one such scattering technique by which one can obtain structural information of the material being studied. Structural information here means size and form of the object under investigation. In a SANS experiment one collects data as a function of the momentum transfer which is proportional to the scattering angle. The probed length scales by SANS varies from a few nanometres to 600 nanometres. In this paper we present the present status of the SANS facility with a preliminary measurement on core-shell particles. Furthermore, we look at the future perspectives of upgrading the facility to enhance the neutron flux at the sample position

The use of numerical techniques to simulate the welding process is not new and the increase in computing power has seen the size and complexity of the models increase. Such features mean that in some cases simplifications and assumptions have to be made to approximate residual stresses. A programme of work is now underway to develop a procedure for residual stress prediction which will account for how the various simplifications and assumptions affect the magnitude of predicted stresses, and to identify the limitations of the various modelling techniques. This paper describes how variations of the heat source representation and the material cyclic hardening behaviour affect the predicted residual stresses in an austenitic single weld bead-on-plate specimen.