Issue 78

V. Pisarev et alii, Fracture and Structural Integrity, 78 (2026) 96-117; DOI: 10.3221/IGF-ESIS.78.06

E STIMATION OF THE DISTRIBUTIONS OF RESIDUAL STRESS COMPONENTS OVER THE COUPON THICKNESS

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he distributions of the residual stress components, which are shown in Fig. 7–9, reveal the presence of significant gradients of the residual stress components inside the contact dimple. Moreover, there is a change in the sign of the component 2  in the horizontal direction (Fig.7b) and the component 1  in the vertical direction (Fig. 8a). This fact makes us think about the uniformity of the residual stress field in the direction of the sample thickness. This issue can be solved quite effectively using an experimental approach, which is based on obtaining information on opposite sides of the specimen. For thin-walled metal specimens, a similar approach can be implemented using a single coupon when drilling a single hole with simultaneous record of interference fringe patterns inherent in opposite faces of the sample [23]. These sides are the entry and exit surfaces of the drill bit. Unfortunately, this methodology cannot be applied to composite plates. The fact is that the exit of the drill is accompanied by significant damage to the surface layers of the composite material. This makes it impossible to obtain the quantitative data necessary to determine the components of residual stresses using a single sample. The only way out is to use two identical specimens. The Sp_75-Back coupon was used for this purpose. The entry point of the drill bit P1B is located on the outer surface, opposite to the side where the contact dimple is applied (Back Side B). The coordinates of this point coincide with the coordinates of the center of the contact dimple. The obtained interference fringe patterns are shown in Fig. 10. It should be immediately noted that the configuration of the interferograms in Fig. 10c and 10d coincides with the shape of the fringe patterns in Fig. 3e and 3f, which are obtained in the center of the contact dimple. These images clearly illustrate the fact that it is impossible to reliably determine the magnitude of the absolute fringe order differences on the output side of the drill. The configuration of the interferogram in Fig. 3e coincides with the shape of the fringe pattern in Fig. 10a. The non-coincidence resides in the magnitude of the absolute fringe order difference u N  . The mismatch between the interferograms in Figs. 3f and 10b are both in the fringe configuration and in the sign of the vertical in-plane displacement component v . This fact is illustrated in Fig. 11, which contains the fringe patterns obtained by introducing an additional phase shift. The corresponding quantitative values are given in Tab. 2. For a more complete understanding of the nature of the distribution of residual stresses in the vicinity of the contact dimple caused by impact along the thickness, two more probe holes were made in the composite plate at points P5B and P6B. The drill entry surface for points P5B and P6B is located on the side B opposite the contact dimple, and their coordinates coincide with the coordinates of points P5 and P6, respectively. Interferograms, the source of which is the local material removal at these points, are shown in Fig. 12. The fringe patterns obtained by drilling the corresponding holes from the impact side are shown in Fig. 6. A comparison of the corresponding values of the absolute fringe order differences u N  and v N  indicates that they retain their signs on the back side, but decrease in absolute magnitude. Quantitative data related to this process are shown in Tab.2.

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