Stefan Pastrama | University Politehnica of Bucharest (original) (raw)
Address: Bucharest, Romania
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Key Engineering Materials, 2015
In this paper, the compounding technique for the calculation of stress intensity factors is conci... more In this paper, the compounding technique for the calculation of stress intensity factors is concisely presented and illustrated with the example of a plate subjected to tensile load and having two collinear cracks perpendicular to the remote stress. The application of the compounding method for determining the stress intensity factor in the case of a thin plate, having two symmetric stiffeners and subjected to uniform remote tensile stress, is then dealt with. Two types of cracks are considered: a central crack and a crack crossing a stiffener. A parametric study is carried out for obtaining compounded stress intensity factor for different crack lengths, by decomposing the structure into ancillary configurations whose stress intensity factors are taken from the scientific literature. The finite element method is used in order to check the compounded results and to draw conclusions about the reliability of the compounding technique.
Page 1. COMPETITIVE AND SUSTAINABLE GROWTH Project No. GRD1-2000-25069 New Perspectives in Aerona... more Page 1. COMPETITIVE AND SUSTAINABLE GROWTH Project No. GRD1-2000-25069 New Perspectives in Aeronautics Contract No. G4RD-CT-2000-00396 ADMIRE CONTRACT Nº: G4RD-CT-2000-0396 PROJECT Nº: GRD1 2000-25069 ACRONYM: ADMIRE ...
The quality of seedlings colonized by Tuber melanosporum is one of the main factors that contribu... more The quality of seedlings colonized by Tuber melanosporum is one of the main factors that contributes to the success or failure of a truffle crop. Truffle cultivation has quickly grown in European countries and elsewhere, so a commonly shared seedling evaluation method is needed. Five evaluation methods are currently published in the literature: three are used in Spain and two in France and Italy. Although all estimate the percentage colonization by T. melanosporum mycorrhizae, they do it in different ways. Two methods also estimate total number of mycorrhizae per seedling. Most are destructive. In this work, ten batches of holm oak seedlings inoculated with T. melanosporum from two different nurseries were evaluated by means of the five methods noted above. Some similarity was detected between the percentages of T. melanosporum mycorrhizae estimated by each method but not in their ability to assess the suitability of each batch. We discuss the advantages and disadvantages for each method and suggest approaches to reach consensus within the truffle culture industry for certifying mycorrhizal colonization by T. melanosporum and seedling quality.
The mechanical behavior of laminated composite structures can be estimated both numerically and e... more The mechanical behavior of laminated composite structures can be estimated both numerically and experimentally. One of the phenomena that are widely studied is delamination. Starting from an initial deterioration due to the shear stresses that appear between the laminae, this phenomenon extends in different modes, leading to a loss of strength and finally to the failure of the composite material. Together with the analytical approaches, the numerical methods and especially the finite element method are widely used today for the determination of the stress and strain state. In order to validate the numerical model and to prove the reliability of the new finite element, experimental tests were performed. This paper presents different numerical analysis, where a new finite element type has been developed by the authors to be used for the analysis of the stresses and strains in structures made from laminated fiber reinforced composites.
An algorithm based on the finite element results is used in this paper to obtain stress intensity... more An algorithm based on the finite element results is used in this paper to obtain stress intensity factor (SIF) values in some cracked structures frequently encountered in engineering practice. The procedure is based on the overdeterministic approach, used previously in fracture mechanics for processing photoelastic data in experimental determination of SIFs. The values of the stresses in an unlimited number of points around the crack tip can be used in order to fit a multi-term series expansion of the stress field. A system of equations is obtained in which the coefficients of each term are the unknowns. The number of equations is equal to the number of the considered points while the number of unknowns, which is much lower, is equal to the number of terms chosen in the series expansion. This overdeterministic system of equations is solved with an iterative procedure based on the Newton-Raphson method. To the difference of other procedures, the overdeterministic method has the advantage that it uses unlimited number of data points. In this way, errors can be minimized and the accuracy of the calculations increases. The examples presented in this paper show good accuracy between the values obtained with the overdeterministic algorithm and those from the scientific literature or obtained with other procedures.
Key Engineering Materials, 2015
In this paper, the compounding technique for the calculation of stress intensity factors is conci... more In this paper, the compounding technique for the calculation of stress intensity factors is concisely presented and illustrated with the example of a plate subjected to tensile load and having two collinear cracks perpendicular to the remote stress. The application of the compounding method for determining the stress intensity factor in the case of a thin plate, having two symmetric stiffeners and subjected to uniform remote tensile stress, is then dealt with. Two types of cracks are considered: a central crack and a crack crossing a stiffener. A parametric study is carried out for obtaining compounded stress intensity factor for different crack lengths, by decomposing the structure into ancillary configurations whose stress intensity factors are taken from the scientific literature. The finite element method is used in order to check the compounded results and to draw conclusions about the reliability of the compounding technique.
Page 1. COMPETITIVE AND SUSTAINABLE GROWTH Project No. GRD1-2000-25069 New Perspectives in Aerona... more Page 1. COMPETITIVE AND SUSTAINABLE GROWTH Project No. GRD1-2000-25069 New Perspectives in Aeronautics Contract No. G4RD-CT-2000-00396 ADMIRE CONTRACT Nº: G4RD-CT-2000-0396 PROJECT Nº: GRD1 2000-25069 ACRONYM: ADMIRE ...
The quality of seedlings colonized by Tuber melanosporum is one of the main factors that contribu... more The quality of seedlings colonized by Tuber melanosporum is one of the main factors that contributes to the success or failure of a truffle crop. Truffle cultivation has quickly grown in European countries and elsewhere, so a commonly shared seedling evaluation method is needed. Five evaluation methods are currently published in the literature: three are used in Spain and two in France and Italy. Although all estimate the percentage colonization by T. melanosporum mycorrhizae, they do it in different ways. Two methods also estimate total number of mycorrhizae per seedling. Most are destructive. In this work, ten batches of holm oak seedlings inoculated with T. melanosporum from two different nurseries were evaluated by means of the five methods noted above. Some similarity was detected between the percentages of T. melanosporum mycorrhizae estimated by each method but not in their ability to assess the suitability of each batch. We discuss the advantages and disadvantages for each method and suggest approaches to reach consensus within the truffle culture industry for certifying mycorrhizal colonization by T. melanosporum and seedling quality.
The mechanical behavior of laminated composite structures can be estimated both numerically and e... more The mechanical behavior of laminated composite structures can be estimated both numerically and experimentally. One of the phenomena that are widely studied is delamination. Starting from an initial deterioration due to the shear stresses that appear between the laminae, this phenomenon extends in different modes, leading to a loss of strength and finally to the failure of the composite material. Together with the analytical approaches, the numerical methods and especially the finite element method are widely used today for the determination of the stress and strain state. In order to validate the numerical model and to prove the reliability of the new finite element, experimental tests were performed. This paper presents different numerical analysis, where a new finite element type has been developed by the authors to be used for the analysis of the stresses and strains in structures made from laminated fiber reinforced composites.
An algorithm based on the finite element results is used in this paper to obtain stress intensity... more An algorithm based on the finite element results is used in this paper to obtain stress intensity factor (SIF) values in some cracked structures frequently encountered in engineering practice. The procedure is based on the overdeterministic approach, used previously in fracture mechanics for processing photoelastic data in experimental determination of SIFs. The values of the stresses in an unlimited number of points around the crack tip can be used in order to fit a multi-term series expansion of the stress field. A system of equations is obtained in which the coefficients of each term are the unknowns. The number of equations is equal to the number of the considered points while the number of unknowns, which is much lower, is equal to the number of terms chosen in the series expansion. This overdeterministic system of equations is solved with an iterative procedure based on the Newton-Raphson method. To the difference of other procedures, the overdeterministic method has the advantage that it uses unlimited number of data points. In this way, errors can be minimized and the accuracy of the calculations increases. The examples presented in this paper show good accuracy between the values obtained with the overdeterministic algorithm and those from the scientific literature or obtained with other procedures.