Development of High-Fidelity Imaging Procedures to Establish the Local Material Behavior in Friction Stir Welded Stainless Steel Joints

Abstract

Friction stir welded (FSW) 304 austenitic stainless steel (SS) joints are studied using a range of microstructural characterization techniques to identify various sub-regions across the weld. A high-resolution (HR) 2D-digital image correlation (DIC) methodology is developed to assess the local strain response across the weld surface and cross-section in the elastic regime. The HR-DIC methodology includes the stitching of multiple images, as it is only possible to partially cover the FSW region using a single camera with the high-resolution optical set-up. An image processing procedure is described to stitch the strain maps as well as strain data sets that allow full-field strain to be visualized and interrogated over the entire FSW region. It is demonstrated that the strains derived from the DIC can be associated with the local weld geometry and the material microstructure in the region of the FSW. The procedure is validated in the material elastic range and provides an important first step in enabling detailed mechanical assessments of the local effects in the FSW process.

Application of Imaging Techniques to Determine the Post-Yield Behaviour of the Heterogeneous Microstructure of Friction Stir Welds

“…Therefore, it is necessary to examine the strain distribution on the weld surface in addition to that on the weld cross-section. In [18] it was confirmed that tungsten was present in the weld nugget caused by wear of the FSW tool, and also reported in [20][21][22]. In the present paper, the spatial distribution of tungsten over and throughout the entire stir zone is revealed using X-ray CT and linked to mechanical behaviour of the weld sub-region, by establishing the effect of tungsten on the local strain response of the stir zone.…”

“…Therefore, it is more appropriate to perform the DIC measurements on the specimens that have not been machined. A procedure for compensating for the different thickness was developed in [18], and validated under the material's elastic response region.…”

“…A new procedure, based on the work in [18] is developed in the present paper so that the post-yield behaviour can be studied in detail. To the authors' knowledge, only one previous study [14] has used high-resolution DIC optics to study the post-yield behaviour of FSWs, but the local stress-strain response of the individual FSW weld sub-regions was not reported.…”

“…DIC studies attempted to date on the FSW welds have been traditionally focused on the weld cross-section. However, in [18,19], it was evident that both the FSW weld surface and cross-section have completely asymmetric microstructural features caused by the different levels of thermo-mechanical effects experienced during the FSW process. Therefore, it is necessary to examine the strain distribution on the weld surface in addition to that on the weld cross-section.…”

Development of an integrated sacrificial sensor for damage detection and monitoring in composite materials and adhesively bonded joints

“…As the required field of view is small (approx. 4.3 3 6.4 mm), it is necessary to use a high resolution DIC setup to capture data relating to damage initiation similar to Crammond et al 25 and Ramachandran et al 26 Macro lenses at five times magnification and high-resolution cameras as specified in Table 3 were used either side of the specimen as shown in Figure 10. 2D DIC was used since the short standoff distance (~10 mm) required to obtain the desired field of view did not permit the use of two cameras and hence stereo (3D) DIC.…”