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Experimental and numerical study on the indentation behavior of TBM disc cutter on hard-rock precutting kerfs by high-pressure abrasive water jet

Cheng Jian‑Long, Yang Sheng‑Qi, Han Wei-Feng, Zhang Zuo-Ru, Jiang Zi-Hao, Lu Jun-Fu

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 1

art. no. e37, 2022

Tunnel boring machine (TBM) excavation of high strength or highly abrasive rock strata has some limitations, such as slow advance speed, low rock-breaking efficiency, and significant increase in the disc cutter changes and construction cost. To improve the rock boreability, a novel breakage method for hard rocks using a TBM disc cutter penetrating into kerfs precut by a high-pressure abrasive water jet is explored. With a confining pressure of 5 MPa, a series of cutter indentation tests and particle flow simulations of granite with two precutting kerfs are carried out to investigate the indentation behavior and the breaking efficiency. The effects of the kerf depth and the kerf spacing on the normal indentation force, rock chip volume, and specific energy are studied. The initiation, propagation, and coalescence modes of the surface and internal cracks and the failure mechanism are analyzed. The results show that the average peak force decreases significantly with the increase of the kerf depth, and the maximum rock chip volume and minimum specific energy are obtained at a kerf depth of 18.14 mm. The failure mode of kerf specimens after two indentations could be divided into the flat and slow shallow failure, one-sided inclined failure, and two-sided inclined failure. The micro-crack distribution of a single shallow kerf under low confining pressure is similar to that of intact rocks, while it is oblate and semi-elliptical under high confining pressure. However, for a single deep kerf, the breakage consists of a wedge-shaped crushed zone, a failure zone, and a damage zone around the kerf boundary and the bilateral inclined cracks, which are almost not affected by the confining pressure.

Relation between geometry of FSW tools and formation of nano- dispersed zones in macrostructure EN AW 6082-T6 alloy welded joints

Krasnowski Krzysztof, Khokhlova Julia, Khokhlov Maksym, Kostin Valery

Biuletyn Instytutu Spawalnictwa w Gliwicach

2021

Vol. 65, No. 5

7--16

An article present the result of macrostructure formation with distribution of mechanical properties in cross-sections of 8 mm-thick one-sided butt-welded FSW joints of EN AW 6082-T6 alloy which were obtained using three types of specially designed tools: C-type – conventional tool consisting of a housing, cylindrical threaded probe and a shoulder with a grooved spiral, T-type – Triflute-type tool consisting of a housing, cylindrical threaded probe with three grooves and a shoulder with a grooved spiral, S-type – simple tool consisting of a housing, smooth cylindrical probe without a thread and a flat shoulder. Friction stir welding was performed using equipment of the Institute of Welding in Gliwice of Poland, and mechanical tests in the E.O. Paton Electric Welding Institute of the NAS of Ukraine. Mechanical test by indentation was performed using Micron-gamma device, which allows experimental identification of structural state of metal and determination of the strain hardening presence by limiting values of ratio of hardness to Young’s modulus of elasticity. It was found that for all three specimens the HAZ hardness decreases, and in the zone of thermomechanical effect the hardness increases. Maximum hardness values are inherent to the central part of welded joint nugget, as well as to light-coloured oval concentrated fragments of structure in the nugget upper and lower part. Judging by the presence of nanosized hardened structure and uniformity of its distribution in the nugget, as well as good dispersion of oxide films and absence of discontinuities, the friction stir welding with C-type tool can be regarded as the optimum variant. An assumption was made that formation of a uniform structure in welds can be achieved at three–four revolutions of the tool in friction stir welding in one place. The model of thermal fields distribution in Al-plate during FSW using a C-type tool visualized the metal’s thermal condition when formated hardened nano-dispersed weld zones.

W artykule przedstawiono wyniki badań nad tworzeniem się nanostruktury z rozkładem właściwości mechanicznych w przekrojach poprzecznych złączy FSW o grubości 8 mm ze stopu EN AW 6082-T6 zgrzewanych jednostronnie doczołowo. Połączenia uzyskano przy użyciu trzech rodzajów specjalnie zaprojektowanych narzędzi. Stwierdzono, że dla wszystkich próbek twardość w SWC zmniejsza się, a w strefie oddziaływania termomechanicznego wzrasta. Maksymalne wartości twardości są charakterystyczne dla środkowej części jądra zgrzeiny, a także dla jasno zabarwionych, owalnych fragmentów w strukturze części górnej i dolnej jądra zgrzeiny. Oceniając obecność utwardzonej struktury nanometrycznej i równomierność jej rozmieszczenia w strefie zmieszania oraz dobre rozproszenie warstewek tlenkowych i brak nieciągłości, zgrzewanie tarciowe z przemieszaniem narzędziem konwencjonalnym można uznać za wariant optymalny. Można przypuścić, że podczas zgrzewania tarciowego z mieszaniem zgrzeiny uzyskuje się jednorodną strukturę zgrzein przy czterech i więcej obrotach narzędzia FSW w jednym miejscu. Model rozkładu pól termicznych w płycie aluminiowej podczas FSW, przy użyciu narzędzia konwencjonalnego, zwizualizował stan termiczny metalu podczas formowania utwardzonych stref nanodyspersyjnych w zgrzeinach.