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1

Study on mechanical characteristics of rock type I fracture and anchorage strengthening mechanism

Zhang Wei, Zhao Tongbin, Guo Weiyao, Xing Minglu

Journal of Theoretical and Applied Mechanics

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2022

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Vol. 60 nr 3

423--434

EN

Engineering rocks are easily affected by excavation unloading and are in uniaxial compres- sion or tension, forming a typical I-type tension crack. Anchor rods are often used for on-site support to ensure safety and reliability of the project. The study of propagation and pen- etration of type I tension cracks and quantitative evaluation of rock anchoring effects are of great significance for exploring mechanical properties of rock fracture and revealing the mechanism of rock failure. In this paper, combined with speckle light measurement, a rock fracture toughness test of different anchoring positions and pre-tightening forces is carried out, the deformation evolution law of the crack tip and the fracture mechanics characteris- tics of the anchored rock are obtained, and the anchoring strengthening mechanism of the rock is discussed based on the theory of the net stress intensity factor. The research shows that the rock fracture process is divided into four stages: elastic deformation, steady crack propagation, crack instability propagation and residual deformation. After anchoring, the time of crack instability growth can be prolonged by 172% and the final residual deformation can be increased by 148%. Compared with the unanchored rock, the fracture toughness of rock initiation and instability increased by 83% and 124% respectively, and increased with growth of the pre-tightening force, which shows that the bolt increases the critical value of rock initiation and instability to achieve the toughening effect. After the rock is anchored, the time required for the crack to propagate to the same length increases by 55%, and the lateral deformation area is reduced by 46%, indicating that the lateral closing force of the bolt inhibits crack propagation and delays the instability of the rock matrix.

2

Application of a geometrically nonlinear elastoplastic gradient-enhanced damage model with incremental potential to composite microstructures

Boussetta H., Dittmann J., Wulfinghoff S.

Archives of Mechanics

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2021

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Vol. 73, nr 5-6

499--540

EN

The prediction of inelastic processes like plastic deformations and cracks within the microstructure of modern man-made materials by realistic, yet simple and efficient continuum models remains a major task in material modelling. For this purpose, gradient-extended standard dissipative solids represent one of the most promising model classes, which is also formulated and applied in this work to investigate microscopic failure mechanisms in three exemplary three-dimensional composite microstructures. The model combines geometrically nonlinear isotropic elastoplasticity with an isotropic damage model with gradient-extension. For the numerical treatment, a variational constitutive update algorithm based on the exponential map is applied. The model is used to provide insight into the microscopic failure of a brittle woven composite material, a particle-reinforced plastic and a carbon fiber reinforced composite. The influence of different microstructural and material parameters on the overall failure behavior is characterized. Adaptive meshing is used to enable a refined numerical resolution of the cracked regions.

3

Study on PLA/PA11 Bio-Based Toughening Melt-Blown Nonwovens

Zhu Feichao, Yu Bin, Su Juanjuan, Han Jian

Autex Research Journal

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2020

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Vol. 20, no. 1

24--31

EN

With aim to improve the mechanical and thermal properties of poly (lactic acid) (PLA) melt-blown nonwovens (MBs), polyamide 11 (PA11) was melt blended with PLA at the weight proportions of PLA/PA11 (95/5, 90/10, 85/15, 80/20), and the corresponding PLA/PA11 MBs were also manufactured. The crystallization, thermal and rheological behaviors of PLA/PA11 blends were investigated. PLA/PA11 MBs were also characterized by morphology and mechanical properties. The results indicated that PA11, as globular dispersed phases, formed confined crystals and could improve the thermal stability of PLA matrix. The viscosity of PLA/PA11 blends was slightly increased but the rheological behaviors of “shear-thinning” kept unchanged in comparison with PLA. The average diameter of PLA/PA11 MB fibers was slightly increased, whereas the toughness of PLA/PA11 MBs including the strength and elongation were efficiently enhanced compared with those of PLA MBs.

4

The effect of non-metallic inclusions on the crack propagation impact energy of thoughened 35B2+Cr steel

Krawczyk J., Pawłowski B.

Metallurgy and Foundry Engineering

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2008

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Vol. 34, no. 2

115-124

EN

35B2+Cr steel was designed as a material for screws. The investigations were carried out on toughened samples, taken from three ingots of 35B2+Cr steel (delivered from three various steel suppliers), and of different content of non-metallic inclusions. In each case, the fraction of non-metallic inclusions was in agreement with the corresponding standard. This study presents the results of the research focused on the influence of non-metallic inclusions on the impact energy of crack development in 35B2+Cr steel. The role of non-metallic inclusions was considered both in relation to their total fraction and in relation to the fraction of oxides, sulfides, nitrides and exogenous inclusions separately. Assuming linear dependence between notch-root radius and the impact energy of crack nucleation, basing on the impact toughness tests, the impact energy of the crack development was evaluated. It was proved, that in spite of the level of fraction of non-metallic inclusions compatible with the corresponding standards, they directly or indirectly influence the impact energy of 35B2+Cr steel. This influence depends on the character of the inclusions.

PL

Stal 35B2+Cr została zaprojektowana jako materiał do produkcji śrub. Badania wykonano na próbkach w stanie ulepszonym cieplnie, wykonanych z trzech wytopów stali 35B2+Cr (pochodzących od trzech różnych dostawców stali), charakteryzujących się różną zawartością wtrąceń niemetalicznych. W każdym przypadku zawartość wtrąceń niemetalicznych była zgodna z wymaganiami określonymi w odpowiednich normach. W artykule przedstawiono wyniki badań wpływu wtrąceń niemetalicznych na pracę rozprzestrzeniania się pęknięcia w stali 35B2+Cr. Rolę wtrąceń niemetalicznych rozpatrywano zarówno zbiorczo, jak i w podziale na tlenki, siarczki, azotki oraz wtrącenia egzogeniczne. Stosując założenie o liniowej zależności pomiędzy promieniem zaokrąglenia karbu a pracą zarodkowania pęknięcia, na podstawie prób udarnościowych wyznaczono pracę rozprzestrzeniania się pęknięcia. Wykazano, że pomimo poziomu zawartości wtrąceń niemetalicznych zgodnego z wymaganiami odpowiednich norm, wpływają one bezpośrednio lub pośrednio na pracę rozprzestrzeniania się pęknięcia w stali 35B2+Cr. Wielkość tego wpływu zależy od rodzaju wtrąceń.

5

Ocena struktury ulepszonych cieplnie stali 36HNM i 38HMJ metodą prądów wirowych

Pachutko B.

Obróbka Plastyczna Metali

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2006

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T. 17, nr 1

41-50

PL

W pracy przedstawiono wyniki oceny zmian struktury ulepszonych cieplnie stali 36HNM i 38HMJ na podstawie kontroli ich twardości, za pomocą przyrządu Wirotest 03 i czujnika stykowego przy częstotliwości prądu magnesowania 0,5 kHz. Próbki hartowano wg zaleceń podanych w normach: PN-EN 10083-1+A1:1999 i PN-EN 10085:2003. Odpuszczanie próbek wykonano w zakresie temperatury 180÷650°C. Strukturę próbek dokumentowano za pomocą mikroskopu świetlnego ECLIPSE L150. Pomiary twardości próbek wykonano metodą Rockwella HRC wg normy PN-EN ISO 6508-1:2002 za pomocą twardościomierza FR-3ATL (Future-Tech Corp.). Praktyczne stosowanie metody prądów wirowych do jakościowej oceny struktury badanych stali ograniczone jest do próbek hartowanych i odpuszczonych w zakresie niższych temperatur do ok. 400÷450 °C z powodu zmniejszania się przenikalności magnetycznej tych stali wskutek zwiększenia udziału objętościowego węglików i procesu ich koagulacji w temperaturach powyżej 500 °C.

EN

The paper presents the results of structure change assessment in 36HNM and 38HMJ toughened steels based on the check of their hardness by means of the Wirotest 03 device and a contact sensor with the magnetizing current frequency of 0.5 kHz. Samples were hardened as per the recommendations stated in the standards: PN-EN 10083-1+A1:1999 and PN-EN 10085:2003. Toughening of the samples was effected within the temperature range of 180¸650°C. The sample structure was documented by means of a metallurgical microscope ECLIPSE L150. Hardness measurements were performed by the Rockwell method, HRC, acc. to standard PN-EN ISO 6508-1:2002 by means of a FR-3ATL hardness tester (Future-Tech Corp.). The practical application of the eddy current method for qualitative assessment of the steel structures is limited to samples hardened and toughened within a lower temperature range of about 400¸450°C due to the decrease of the magnetic permeability of those steels as a result of the increase of volumetric content of carbides and the process of their coagulation at the temperatures above 500°C.

6

Łańcuchy kotwiczne kategorii 3 nie wymagające ulepszania cieplnego

Bugłacki H., Krzysztofowicz K.

Zeszyty Naukowe Akademii Marynarki Wojennej

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1998

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R. 39 nr 2 (137)

45--56

PL

Opracowano wzory korelacyjne wlaściwości mechanicznych i mikrostruktury prętów ze stali na łańcuchy kotwiczne kategorii 3. Zaprojektowano nowy gatunek stali 28GA na łańcuchy kotwiczne kategorii 3. w hucie Lucchini w Warszawie wykonano próbny wytop stali i z otrzymanego wlewka /720kg/ wykuto pręty o średnicy 76 mm. Przedstawiono wyniki badań tych prętów pokazujące, że uzyskane właściwości mechaniczne spełniają wymagania towarzystw klasyfikacyjnych dla łańcuchów kotwicznych kategorii 3 bez ulepszenia cieplnego.

EN

The correlation formuale of mechanical properties and microstructure of steel bars for anchor chain cables grade 3 weredeveloped. New steel grade 28GA for anchor chain cables grade 3 was designed. In Lucchini steel works in Warsaw experimental steel heat and forging of bars were carried out. The test results of these bars were presented and showed that the machanical properties of bars meet requiremenrs of clasification societes for anchor chain cables grade 3 without quenching and tempering.