Page 55 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
52-53
in height. When this beam reaches the workpiece, the diffraction signal is generated over its entire height. The use of a secondary vertical collimator, combined with an optical element in the form of a cadmium slit placed in front of the detector for horizontal resolution, allows the selection of precise gauge volume and a depth-dependent peak position. This new strain-imaging approach generates spatially resolved data about the local strain state along the height of the workpiece with only one single exposition for each x, y-position. The experimental set-up is shown in figure 1. For this measurement mode, resolution in depth depends no longer on gauge volume size and distribution but on the size of slit in front of
the detector, according to a geometrical pinhole optics formula published by Young [3]. Additionally, the system can reduce influences from process fluctuations, since multiple short measurements are substituted with a single, longer count-time result and the sample surface is always clearly defined, as can be seen in the diffraction image in figure 2. Both the strain and the depth below the surface can be recalculated from the detector rows and columns, using Braggs’ law and the optical magnification of the system, respectively.
The three-dimensional measurement data confirm
the presence of high compressive strain fields in the longitudinal and transversal direction, generated through the elasto-plastic mechanical contact. Overall, the technique allowed an evaluation of both the distribution and amount of strain in each of the different zones
that exist during the deep rolling processing, as the reconstructed longitudinal 2D-strain map in figure 3 shows: the elastic reaction of the material in front of the roller, the elasto-plastically deformed volume followed by the transition region with superposition of residual strains and loading fields, and finally the achieved residual strain state. This study sheds new light on material response to mechanical loads, analysed using a novel neutron diffraction strain-imaging approach for in situ process evaluation to enhance our knowledge on the interdependence between process parameters, internal material load fields and the resulting material state for deep rolling.
Figure 3
Reconstruction of longitudinal 2D-strain map along the machined track.
Figure 2
Diffraction image on the detector for single measurement, showing surface cut-off and peak shift due to strains for the measurement position directly under the contact point.
www.ill.eu