Materials Science Forum Vols. 524-525

Abstract: When a material is under mechanical load, the stresses modify the velocity of acoustic waves. This consists in acoustoelastic effect. This property can be exploited for stress measurement in the material itself when the stress concerns the surface of the material, or in the bulk material, as in bolts. The ultrasonic bolt tightening control allows knowledge of preload stress in screws. The accuracy is much better than in the case of torque wrench. In fact, the use of this mechanical-based measurement equipment leads to uncertainties due to random resistant torques, induced by mechanical manufacturing tolerances. The ultrasonic measurement of the pre-load gives the information about the tension inside of the bolt, independently of mechanical uncertainties. This consists in an in-situ measurement. More, in some cases, the ultrasonic method allows knowledge of the preload on bolts already tightened, without untightening. This method is called bi-wave method, and has many connections with birefringent method. Examples of application of the method are presented about the differential mode case, which is largely known and industrialized, and in the case of bi-wave method, which lead to several on site applications, performed by CETIM with a prototype equipment.

Abstract: The knowledge of residual stresses allows a reliable prediction of structure performances evolution, such as service life [1-3]. In this paper, we develop a new method for residual stresses determination combining Electronic Speckle Pattern Interferometry (ESPI) with the machining of a groove. The internal stress field is perturbed as the depth of the groove is increased incrementally. The structure finds a new equilibrium state generating displacements which are measured using ESPI. This method was tested on an aluminium alloy AU4G plate treated locally by an ultrasonic shot-peening. The investigation of the images obtained with the phase shifting technique and fringe patterns, makes it possible to analyze, simultaneously, the stress profile along two directions: along the depth of the structure, and along the groove direction.

Abstract: The photoacoustic piezoelectric images of Vickers indented silicon nitride, Al2O3-SiCTiC composite ceramic, nanonickel and nanocopper in unloaded and loaded states are presented. It is demonstrated the influence of mechanical stresses on the photoacoustic images of Vickers indented areas both for ceramics and metals. For Al2O3-SiC-TiC composite ceramic it is also shown that an increase of annealing time is accompanied by the decrease in the photoacoustic response from the radial crack tips. The obtained photoacoustic experimental results are explained within the framework of the model for the photoacoustic effect in stressed solids proposed by us earlier.

Abstract: It has been shown that the introduction of a trailing heat sink following a welding heat source at a short distance can reduce or even eliminate the buckling deformation of welded thin plate structures. To increase the insight into the stress development during welding with a trailing heat sink, residual stress measurements were carried out by means of laboratory X-ray and synchrotron X-ray diffraction techniques. Both the laboratory X-ray and the synchrotron X-ray experiments show a difference between the residual stress distributions for the cooled and the uncooled samples. The longitudinal stresses show a reduction in the compressive stress level towards the plate edges, to a level below the critical buckling stress. The transverse stresses are drastically reduced when the active cooling is applied.

Abstract: Electromagnetic compression of tubular profiles with high electrical conductivity is an innovative joining process for lightweight structures. The components are joint using pulsed magnetic fields which apply radial pressures of up to 200 MPa to tubular work pieces causing a symmetric reduction of the diameter with typical strain rates of about 104 sec-1. Since there is no contact between components to be joined and the joining machine, any damage of component’s surface can be avoided. The load, which the joint can transmit, is strongly dependent on the residual stresses in the region of the joint. In the present article, the influence of charging energy, gap width before joining and diameter on the development of the residual stresses is analysed using geometry changes during splitting of the components. Besides, the contact zone between the components is analysed and the pullout force of the joints is determined by tensile tests. This allows the evaluation of correlations between joining process, state after joining and mechanical properties.

Abstract: We have studied the effect of a post-weld heat treatment on plasma arc welds on Zircaloy 4 plates. The samples consist of two 100 mm long, 50 mm wide, and 6.25 mm thick plates, welded along the rolling (longitudinal) direction. The heat-treatment consisted of a steady increase in temperature from room temperature to 450oC over a period of 4.5 hours; followed by cooling with an equivalent cooling rate. Residual strains and stresses along the longitudinal, transverse and normal directions on an as-welded and a heat-treated specimen were measured by neutron diffraction on the ENGIN-X beamline at the Isis Facility, Rutherford Laboratory, UK. Peak tensile stresses of (105±25) MPa were found in the as-welded specimen, which were reduced to (70±20) MPa after the heat-treatment. Thermal compressive stresses of (-80±20) MPa were found along the normal direction, which were not affected by the heat treatment. The use of a full-pattern Rietveld refinement for the determination of bulk strains in Zircaloy specimens is also discussed.

Abstract: With recent issues related to stress corrosion cracking (SCC) and thermal fatigue of power plant components, including nuclear steam generator tubing, condenser tubing, boiler tubes steam turbine blades, the residual stress in materials plays an important role in determining those susceptible for failure caused by fatigue or SCC. These residual stresses, combined with the applied stresses inherited from manufacturing and assembly can be tensile or compressive. The SCC can act in a corrosive environment only when the stresses in the material are tensile, thus leading to cracking of the component and the structure. Thermal fatigue can be originated from thermal gradients caused during the operation of power plants. Proto Manufacturing Limited, leader in the residual stress measurement and equipment development tailored for these applications has developed instruments capable of measuring residuals tresses in confined areas such as inner diameter of tubes as small as 50 mm in diameter, non-destructively in the direction of interest, axial direction. In this paper interesting examples of new x-ray diffraction instrument and measurement validation are presented along with application inside a 50 mm I.D.

Abstract: The overall behaviour of metals composed of grains with different sizes is simulated as well as the evolution of their internal structure making use a self-consistent modelling for elasticviscoplastic materials. The Representative Volume Element is composed of isotropic spherical grains randomly distributed with a grain size distribution following a log-normal statistical function. Thus the heterogeneity of the RVE comes only from the grain size dependence of the local flow stress. The viscoplastic strain rate of the grains is modelled through a classic isotropic power law involving a reference stress depending on the individual grain size and the local plastic strain. Numerical results applied to IF steels firstly display that the overall yield stress depends not only on the mean grain size but also on the dispersion of the grain diameter distribution. The role of the grain size dispersion becomes significant when the mean grain size is on the order of "m, and, a decrease of the overall yield stress with an increase of the dispersion is observed. Secondly, prediction of the evolution of the internal structure indicates an increase of second order internal stresses with grain size dispersion. When this one is large enough and the mean grain size is on the order of "m, residual stresses due to heterogeneities arising from the grain size distribution are on the same order than the ones related to heterogeneities associated with plastic anisotropy found for polycrystalline IF steels.

Abstract: A two-level homogenisation approach is applied to the micro-mechanical modelling of the elasto-plasticity of polycrystalline materials during various strain-path changes. The model is tested by simulating the development of intragranular strains during different complex loads. Mechanical tests measurements are used as a reference in order to validate the model. The anisotropy of plastic deformation in relation to the evolution of the dislocation structure is analysed. The results demonstrate the relevance of this approach for FCC polycrystals.

Abstract: For modeling the deformation and the heat treatment related change of micro structural material properties, a crystal structure with several grains is analyzed using the molecular dynamics simulation. The generated atom arrangement has been equilibrated, sheared and tempered, and the resulting microstructures and stresses as well as their changes are presented. The shearing of the multiple grain model into the region of plastic deformation caused a significant change in its microstructure and introduced additional stress. On applying the heat treatment simulation, it was possible to show thermally induced relaxation processes in a microstructure using MD.