Materials Science Forum Vols. 524-525

Abstract: Finite element modelling has proved to be an effective tool for the investigation of trends effected by changing welding conditions. This is especially important in mechanical tensioning of friction stir welds because of the large number of parameters involved. In this paper, an FE model is used to examine the effectiveness of the mechanical tensioning technique for controlling residual stresses in FSWs by the investigation of trends caused by changes to the welding parameters. Comparisons between different geometries, traverse speeds, and welding off-axis angle all produced consistent results, and showed that the peak stresses are most strongly influenced by both the local tensioning and heat input, and not by the more global welding conditions. The results also showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~50% of the room temperature yield stress, tensile stresses were replaced by compressive residual stresses within the weld.

Abstract: This article presents the first part of a study on the interaction between residual stresses and crack driving force. Blunt notched CT specimens were pre-strained to introduce residual stresses at the notch, where a crack is subsequently introduced. FE modelling is used to model the specimen preload and pre-cracking. Modelling predictions are validated by two different methods. The total predicted surface residual strains are compared to image correlation measurements. The predicted residual strains were measured using neutron diffraction, both before and after fatigue cracking. The residual strain profiles show good agreement with the 3D FE model in the far field but the peak strains measured near the notch are smaller those predicted. This is a result of the low spatial resolution of the technique.

Abstract: Exact closed-form stress intensity factor (SIF) solutions have been developed for a mode- I through-thickness cracks in an infinite plate. Centre-crack problems have been analysed comprehensively in the literature, but the focus has been on the effect of simple loading about the crack centre. In the current work, the formula of Sih-Paris-Erdogan has been extended to consider the SIF difference on the left and right crack tips, under the local influence of general asymmetric and symmetric stress field. Exact SIF magnification factors convenient for computations have been derived that simultaneously circumvent the problem of crack-tip stress singularity. The solutions so obtained are applied to generate the residual SIFs that would act on a crack growing under the influence of the residual stress fields associated with welded plates and cold-worked holes using the measured residual stress profiles.

Abstract: This work deals with the numerical simulation of 3D guillotining of sheet metal using anisotropic elastoplastic model accounting for non-linear isotropic and kinematic hardening fully coupled with isotropic ductile damage and initial residual stresses. Both theoretical and numerical aspects are presented. A 3D finite element model is developed for the numerical simulation of the study state guillotining process. An explicit dynamic resolution strategy is used to solve the associated initial and boundary value problem. Results from the simulation of the guillotining process are given and the influence of residual stresses is investigated.

Abstract: In this study, residual stresses state at different scales in the 301LN unstable austenitic steel after deep drawing was determined. The first part of the work deals with the characterization of the martensitic transformation during uniaxial loading. The austenite/martensite content which was determined by X-Ray Diffraction increases until a maximum of 0.6 for 30% strain. Internal stress distribution was determined by coupling in-situ tensile tests with sin²ψ method. As soon as martensite appears, the magnitudes of the internal stresses in this phase were found to be 400 MPa higher than in the austenite. To establish a relation between the complex loading path effect and the phase stress state, deep drawing tests were carried out for different drawing ratios. Both macroscopic tangential residual stresses and residual stresses in the martensite were determined. It appears that the macroscopic tangential residual stresses are positive and increase with increasing drawing ratios and the maximum value is located at middle height of the cup. It is about 850MPa for the Drawing Ratio (DR)=2.00. The tangential residual stresses in the martensite were found to be positive in the external face and have a same evolution as the macroscopic ones.

Abstract: The overall plastic behavior of polycrystalline materials strongly depends on the microstructure and on the local rheology of individual grains. The characterization of the strain and stress heterogeneities within the specimen, which result from the intergranular mechanical interactions, is of particular interest since they largely control the microstructure evolutions such as texture development, work-hardening, damage, recrystallization, etc. The influence of microstructure on the effective behavior can be addressed by physical-based predictive models (homogenization schemes) based either on full-field or on mean-field approaches. But these models require the knowledge of the grain behavior, which in turn must be determined on the real specimen under investigation. The microextensometry technique allows the determination of the surface total (i.e. plastic + elastic) strain field with a micrometric spatial resolution. On the other hand, the white beam X-ray microdiffraction technique developed recently at the Advanced Light Source enables the determination of the elastic strain with the same spatial resolution. For polycrystalline materials with grain size of about 10 micrometers, a complete intragranular mechanical characterization can thus be performed by coupling these two techniques. The very first results obtained on plastically deformed copper and zirconium specimens are presented.

Abstract: A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses on the micron scale, using a one cubic micrometer spot. The experimental Kossel line patterns are obtained by way of a CCD camera and are then fully indexed using a home-made simulation program. The so-determined orientation is compared with Electron Back-Scattered Diffraction (EBSD) results, and in-situ tests are performed inside the SEM using a tensile/compressive machine. The aim is to verify a 50MPa stress sensitivity for this technique and to take advantage from this microscope environment to associate microstructure observations (slip lines, particle decohesion, crack initiation) with determined stress analyses.

Abstract: The ambiguity in determination of complete elastic strain tensor by convergent beam electron diffraction can be overcome by simultaneous use of multiple diffraction patterns. Numerical tests of strain determining procedure based on multiple patterns have been carried out. Patterns were simulated using both kinematic and dynamic approaches, and then they were used as input in the tested procedure. The tests indicate that, in practice, at least three patterns are needed in order to determine a complete strain tensor with reasonable accuracy. The strain resolution of two parts per ten thousand was achieved with five diffraction patterns. Moreover, the impact of errors in voltage and camera length is considered. It is shown that within the kinematic description, the deviations from the correct voltage are equivalent to errors in the isotropic part of strain.

Abstract: New challenges for design, manufacturing and packaging of MEMS/NEMS arise from the ongoing miniaturization process. Therefore there is a demand on detailed information on thermo-mechanical material properties of the applied materials. Because of size effects and the strong dependency of the thermo-mechanical behavior of active and passive components on process parameters often unsolved questions of residual stresses lead to system failure due to crack formation. With the fibDAC (Focused Ion Beam based Deformation Analysis by Correlation) method which is presented in this paper the classical hole drilling method for stress release measurement has been downscaled to the nanoscale. The ion beam of the FIB station is used as a milling tool which causes the stress release at silicon microstructures of MEMS devices. The analysis of the stress release is achieved by digital image correlation (DIC) applied to load state SEM images captured in a cross beam equipment (combination of SEM and FIB). The results of the DIC analysis are deformation fields which are transferred to stress solution by application of finite element analysis. In another step the resolution of the method has been improved by the application of trench milling instead of hole milling. Thereby deformation measurements in the nm range are established. The method is also a powerful tool for the analysis of sub-grain stresses of engineering materials.

Abstract: Laser shock peening is a very effective mechanical surface treatment to enhance the fatigue behaviour of highly stressed components. In this work the effect of different laser shock peening conditions on the residual stress depth profile and fatigue behaviour without any sacrificial coating layer is investigated for two high strength titanium alloys, Ti-6Al-4V and Timetal LCB. The results show that the optimization of peening conditions is crucial to obtain excellent fatigue properties. Especially, power density, spot size and coverage severely influence the residual stress profile of laser shock peened Ti-6Al-4V and Timetal LCB specimens. For both alloys, subsurface as well as surface compressive residual stress peaks can be obtained by varying the peening conditions. In general, Timetal LCB exhibits steeper stress gradients than Ti-6Al-4V for identical peening conditions. The main parameters affecting the fatigue life are near-surface cold work and compressive residual stresses.