rus
Issue #1/2021
D.K.Magomedova, А.А.Churakova
Distribution of stresses in fine- and coarse-grained cylindrical aluminum alloy 6101 samples subjected to static tension
Distribution of stresses in fine- and coarse-grained cylindrical aluminum alloy 6101 samples subjected to static tension
DOI: 10.22184/1993-8578.2021.14.1.30.34
Calculation of strength and durability of various metal structures presents one of the most significant tasks in the contemporary world. To achieve it, the different mechanical criteria of the material, such as strength, ductility, etc., should be known. The calculation data and t distribution pattern of critical stresses that define formation of pores in the material (in our case, Al-6101) under static loading are presented in this article. The first phase of material fracture is the pore formation and merging. Therefore, its subsequent fracture can be estimated using the data on the critical stresses of the material.
Calculation of strength and durability of various metal structures presents one of the most significant tasks in the contemporary world. To achieve it, the different mechanical criteria of the material, such as strength, ductility, etc., should be known. The calculation data and t distribution pattern of critical stresses that define formation of pores in the material (in our case, Al-6101) under static loading are presented in this article. The first phase of material fracture is the pore formation and merging. Therefore, its subsequent fracture can be estimated using the data on the critical stresses of the material.
Теги: aluminium alloy critical stress static loading алюминиевый сплав критические напряжения статическое нагружение
DISTRIBUTION OF STRESSES IN FINE- AND COARSE-GRAINED CYLINDRICAL ALUMINUM ALLOY 6101 SAMPLES SUBJECTED TO STATIC TENSION
Calculation of strength and durability of various metal structures presents one of the most significant tasks in the contemporary world. To achieve it, the different mechanical criteria of the material, such as strength, ductility, etc. [1, 2] should be known. The calculation data and t distribution pattern of critical stresses that define formation of pores in the material (in our case, Al-6101) under static loading are presented in this article. The first phase of material fracture is the pore formation and merging. Therefore, its subsequent fracture can be estimated using the data on the critical stresses of the material [3, 4].
INTRODUCTION
The need to develop and implement simple and, at the same time, effective fracture criteria that allow one to reliably assess the conditions for safe operation of metal structures, especially structures with stress concentrators, is obvious in connection with the creation and increase of the number of engineering structures of complex geometry. This problem is relevant for many industries. It is known that in the process of destruction of materials, the ultimate strength is exceeded at a certain characteristic distance for a certain characteristic time. The development of theoretical models that take into account the nonlocal nature of the fracture process and are algorithmized for use in standard computing packages (ABAQUS, LS-DYNA, ANSYS, Comsol), makes it possible to increase accuracy of predicting the moment of structure failure and reduce the cost of experimental support for the practical implementation of the research.
RESEARCH METHODS
Al-6101 alloy was used in the form of cylindrical samples with different cross-sectional diameters. A part of the original work pieces (coarse-grained alloy, CGA) was annealed at a temperature of 550 °C for two hours and then quenched in water at room temperature; after hardening, they underwent natural aging (NA) for six days. The other part was also subjected to annealing at a temperature of 550 °C for two hours and further annealing for 12 hours at a temperature of 170 °C, followed by quenching with water at room temperature – samples with artificial aging (AA). To obtain an ultrafine-grained (UFG) structure, part of the work pieces were treated by SPD by equal-channel angular pressing according to Conform (ECAP-K) scheme [2].
Identical samples were made of CGA and UFG blanks with a cylindrical working part 5.0 and 2.8 in diameter (an additional groove was made on a 5-mm diameter) and 34 mm high. The use of samples of different geometric shapes in the work was due to the need to see the influence of geometry on the strength and plasticity of the material. The experiment and the data obtained are presented in detail in [5].
Uniaxial tension tests of the samples were carried out on a Shimadzu AG-50kNX testing machine. Stretching of the samples was carried out at room temperature with a constant strain rate of 1.4 · 10–4 s–1. Stretching was carried out until the destruction of the samples. The surface of the samples in the fractional section was examined using an electron microscope.
The methodology for calculating critical stresses is based on paper [1]. We consider static loading at room temperature and constant tensile rate. The critical stress at the metal matrix / particle interface is the most widely used criterion (Fig.1).
The calculation of the critical stress σr at the metal matrix / particle interface was first performed by the group of Argon, Im and Needleman in [1], where the pore formation criterion for coarse-grained steel and copper rods with grooves is presented as:
σm + σeq ≥ qr, (1)
here – hydrostatic stress, and
is equivalent voltage, where σ1, σ2, σ3 – principal stress values. The calculation for Al-6101 was carried out using this criterion.
RESULTS
The package was used to calculate the distribution of triaxial (the sum of equivalent and hydrostatic) stresses obtained using the ANSYS 19.0 package. The calculation was carried out according to the data from the video extensometer and the testing machine for each CGA geometry of all types of aging and UFG samples.
DISCUSSION
Analysis of the triaxial stress distribution, taking into account the difference in geometries, shows that the groove leads to localization of stress values in the cross-section area of the minimum diameter, while the nature of the distribution of triaxial stresses depends on the geometry of the groove. The minimum values of triaxial stresses, 220 MPa, refer to the CGA specimen NA (cross-sectional diameter 5 mm), while the maximum values of triaxial stresses, 385 MPa, have been achieved in the UFG specimen.
The strength and ductility of the material are influenced by geometry of the test specimens: specimens with a narrower working part turn out to be the least ductile, but show the higher strength. The strength and ductility are also influenced by the structure of the material: as the grain size decreases, the strength increases.
CONCLUSIONS
The use of the Argon model and the results of a theoretical study of the distribution of triaxial stresses in the samples during the tests made it possible to determine the critical stresses at appearance of pores for two states under study: σr ≈ 220 MPa for a short-circuit EC and σr ≈ 330 MPa for a short-circuit IC. A qualitative analysis of the experimental and theoretical results suggests that the critical stress during formation of pores in the UFG material has higher values.
■
The authors are grateful to the Russian Science Foundation grant (No. 17-19-01311) and the St. Petersburg State University project Activity 3 (id: 26130576).
Calculation of strength and durability of various metal structures presents one of the most significant tasks in the contemporary world. To achieve it, the different mechanical criteria of the material, such as strength, ductility, etc. [1, 2] should be known. The calculation data and t distribution pattern of critical stresses that define formation of pores in the material (in our case, Al-6101) under static loading are presented in this article. The first phase of material fracture is the pore formation and merging. Therefore, its subsequent fracture can be estimated using the data on the critical stresses of the material [3, 4].
INTRODUCTION
The need to develop and implement simple and, at the same time, effective fracture criteria that allow one to reliably assess the conditions for safe operation of metal structures, especially structures with stress concentrators, is obvious in connection with the creation and increase of the number of engineering structures of complex geometry. This problem is relevant for many industries. It is known that in the process of destruction of materials, the ultimate strength is exceeded at a certain characteristic distance for a certain characteristic time. The development of theoretical models that take into account the nonlocal nature of the fracture process and are algorithmized for use in standard computing packages (ABAQUS, LS-DYNA, ANSYS, Comsol), makes it possible to increase accuracy of predicting the moment of structure failure and reduce the cost of experimental support for the practical implementation of the research.
RESEARCH METHODS
Al-6101 alloy was used in the form of cylindrical samples with different cross-sectional diameters. A part of the original work pieces (coarse-grained alloy, CGA) was annealed at a temperature of 550 °C for two hours and then quenched in water at room temperature; after hardening, they underwent natural aging (NA) for six days. The other part was also subjected to annealing at a temperature of 550 °C for two hours and further annealing for 12 hours at a temperature of 170 °C, followed by quenching with water at room temperature – samples with artificial aging (AA). To obtain an ultrafine-grained (UFG) structure, part of the work pieces were treated by SPD by equal-channel angular pressing according to Conform (ECAP-K) scheme [2].
Identical samples were made of CGA and UFG blanks with a cylindrical working part 5.0 and 2.8 in diameter (an additional groove was made on a 5-mm diameter) and 34 mm high. The use of samples of different geometric shapes in the work was due to the need to see the influence of geometry on the strength and plasticity of the material. The experiment and the data obtained are presented in detail in [5].
Uniaxial tension tests of the samples were carried out on a Shimadzu AG-50kNX testing machine. Stretching of the samples was carried out at room temperature with a constant strain rate of 1.4 · 10–4 s–1. Stretching was carried out until the destruction of the samples. The surface of the samples in the fractional section was examined using an electron microscope.
The methodology for calculating critical stresses is based on paper [1]. We consider static loading at room temperature and constant tensile rate. The critical stress at the metal matrix / particle interface is the most widely used criterion (Fig.1).
The calculation of the critical stress σr at the metal matrix / particle interface was first performed by the group of Argon, Im and Needleman in [1], where the pore formation criterion for coarse-grained steel and copper rods with grooves is presented as:
σm + σeq ≥ qr, (1)
here – hydrostatic stress, and
is equivalent voltage, where σ1, σ2, σ3 – principal stress values. The calculation for Al-6101 was carried out using this criterion.
RESULTS
The package was used to calculate the distribution of triaxial (the sum of equivalent and hydrostatic) stresses obtained using the ANSYS 19.0 package. The calculation was carried out according to the data from the video extensometer and the testing machine for each CGA geometry of all types of aging and UFG samples.
DISCUSSION
Analysis of the triaxial stress distribution, taking into account the difference in geometries, shows that the groove leads to localization of stress values in the cross-section area of the minimum diameter, while the nature of the distribution of triaxial stresses depends on the geometry of the groove. The minimum values of triaxial stresses, 220 MPa, refer to the CGA specimen NA (cross-sectional diameter 5 mm), while the maximum values of triaxial stresses, 385 MPa, have been achieved in the UFG specimen.
The strength and ductility of the material are influenced by geometry of the test specimens: specimens with a narrower working part turn out to be the least ductile, but show the higher strength. The strength and ductility are also influenced by the structure of the material: as the grain size decreases, the strength increases.
CONCLUSIONS
The use of the Argon model and the results of a theoretical study of the distribution of triaxial stresses in the samples during the tests made it possible to determine the critical stresses at appearance of pores for two states under study: σr ≈ 220 MPa for a short-circuit EC and σr ≈ 330 MPa for a short-circuit IC. A qualitative analysis of the experimental and theoretical results suggests that the critical stress during formation of pores in the UFG material has higher values.
■
The authors are grateful to the Russian Science Foundation grant (No. 17-19-01311) and the St. Petersburg State University project Activity 3 (id: 26130576).
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