Danial Ashouri - Academia.edu (original) (raw)

Papers by Danial Ashouri

International Journal of Solids and Structures, 2015

Under transverse tensile loading, fibers oriented perpendicular to the tensile direction can unde... more Under transverse tensile loading, fibers oriented perpendicular to the tensile direction can undergo fiber/ matrix debonding. Experiments show that the first stage of fiber/matrix interface debonding is mode-I dominated fracture with very fast crack growth rate. Subsequent stable crack propagation along the interface is due to mixed mode I/II fracture. The aim of this study is to explore ways to stabilize the early stage of debonding so that it becomes possible to determine the mixed mode interfacial fracture properties for the entire mode-mixity range by in-situ observations. Therefore, the objective of this study is to stabilize crack initiation in the dominant mode-I fracture by changing the position of one fiber with its neighboring fiber or hole using the finite element analysis. The progressive fiber/matrix debonding is studied by focusing on the interaction of one fiber with its neighboring fiber or hole. The results show that decrease of the position angle stabilize the crack growth at the interface in the ligaments. This effect is more significant in the cases with small ligament thickness. In the two-fiber model and at very small ligaments the results show that the crack growth stops when the crack tips meet each other in the ligament and further crack growth is under dominant mode-II fracture. In the fiber-hole model, both the crack initiation and propagation are stabilized by decrease of the position angles at very thin ligaments. This paper suggests to use two fibers instead of a single fiber in order to ease the characterization of interfacial properties.

Composite Structures, 2015

Abstract The mechanical response of porous unidirectional composites under transverse normal and ... more Abstract The mechanical response of porous unidirectional composites under transverse normal and longitudinal shear loading is studied using the finite element analysis. The 3D model includes discrete and random distribution of fibers and voids. The micromechanical failure mechanisms are taken into account by considering the mixed-mode interfacial debonding and pressure-dependent yielding of the matrix using the modified Drucker–Prager plasticity model. The effect of the micromechanical features on the overall response of composite is discussed with a focus on the effect of microvoids and interfacial toughness. Finally, the computational prediction of the porous composite in the transverse normal-longitudinal shear stress space is obtained and compared with Puck’s model. The results show that both interfaces with low fracture toughness and microvoids with even small void volume fraction can significantly reduce the macroscopic strength of composite. The size and shape of microvoids can also microscopically lead to different crack paths.

The fibre/matrix interfacial debonding is found to be the first microscale failure mechanism lead... more The fibre/matrix interfacial debonding is found to be the first microscale failure mechanism leading to subsequent macroscale transverse cracks in composite materials under tensile load. In this paper, the micromechanical interface failure in fiber-reinforced composites is studied experimentally and by numerical modeling by means of the finite element analysis. Two fibers embedded in the matrix are subjected to a remote transverse tensile load (see Fig. 1a). The trapezoidal cohesive zone model proposed by Tvergaard and Hutchinson [14] is used to model the fracture of the fiber-matrix interfaces. This study is based on the comparison between the results of numerical modeling and those corresponding to the experimental tests by employing two parameters: The angle from the load direction to the crack tip and the crack normal opening. This comparison aims to investigate the interfacial properties and also assess the progressive fiber-matrix debonding by focusing on the interaction of two fibers with dissimilar interfacial strengths.

and it is a condition of accessing publications that users recognise and abide by the legal requi... more and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

European Journal of Mechanics - A/Solids, 2013

Composite materials under loads normal to the fiber orientation often fail due to debonding betwe... more Composite materials under loads normal to the fiber orientation often fail due to debonding between fibers and matrix. In this paper a micromechanical model is developed to study the interfacial and geometrical effects in fiber-reinforced composites using generalized plane strain by means of the finite element method. Assuming a periodic distribution of fibers in the matrix, a unit cell is chosen including two quarter-circular fibers. By using this unit cell approach the composite material is modeled rather realistically as the possibility of having different fiberematrix strength exists. In the present study two different cases are considered: I) Two perfectly bonded interfaces. II) Two debonding interfaces of uneven strength. The fibers are purely elastic while the matrix is considered as isotropic with an elastoplastic behavior. To model the fracture of the fiberematrix interfaces, a trapezoidal cohesive zone model is used. A parametric study is carried out to evaluate the influence of the interfacial properties, fiber position and fiber volume fraction on the overall stressestrain response as well as the end-crack opening displacement and the opening crack angle. All the results presented are compared with corresponding perfectly bonded interfaces. Generally, different crack initiations and propagations at the two interfaces are seen, which result in an overall stressestrain response of the material that often first depict a rather smooth stress drop followed by a second sudden stress drop. This behavior is shown to be very sensitive to interface parameters as well as geometrical parameters. The interfacial dissimilarity shows for all the investigations, that decreasing the maximum cohesive strength leads to more stable interfacial crack growth, whereas increasing the critical interfacial separation causes a less distinct debonding at one interface before debonding at the other.

Composites Science and Technology, 2014

This thesis is submitted in partial fulfillment of the requirements for obtaining the degree of P... more This thesis is submitted in partial fulfillment of the requirements for obtaining the degree of Ph.D. in mechanical engineering at the Technical University of Denmark (DTU). The Ph.D. project was funded by the Danish Council for Strategic Research (grant no.: 09-067212) under the Danish Center for Composite Structures and Materials for Wind Turbines (DCCSM) and carried out at the Department of Mechanical Engineering, Solid Mechanics, and the Department of Wind Energy at DTU in the period April 1 st 2011-March 31 st 2014. Supervisors on the project were Associate Professor Ph.D. Brian Nyvang Legarth, Associate Professor Ph.D. Christian F. Niordson and Associate Professor Ph.D. Christian Berggreen from the Mechanical Engineering Department and Professor Dr.techn. Bent F. Sørensen from the Department of Wind Energy, section of Composites and Materials Mechanics. I am very grateful to my supervisors for their inspiring support and for always taking their time to discuss the work and the results during the project. I would also like to thank Professor Javier Llorca and Dr. Carlos González who were hosting me at IMDEA Materials Institute, Madrid, Spain, during my PhD external stay in the period October 2013-January 2014, for our beneficial collaboration.

International Journal of Solids and Structures, 2015

Under transverse tensile loading, fibers oriented perpendicular to the tensile direction can unde... more Under transverse tensile loading, fibers oriented perpendicular to the tensile direction can undergo fiber/ matrix debonding. Experiments show that the first stage of fiber/matrix interface debonding is mode-I dominated fracture with very fast crack growth rate. Subsequent stable crack propagation along the interface is due to mixed mode I/II fracture. The aim of this study is to explore ways to stabilize the early stage of debonding so that it becomes possible to determine the mixed mode interfacial fracture properties for the entire mode-mixity range by in-situ observations. Therefore, the objective of this study is to stabilize crack initiation in the dominant mode-I fracture by changing the position of one fiber with its neighboring fiber or hole using the finite element analysis. The progressive fiber/matrix debonding is studied by focusing on the interaction of one fiber with its neighboring fiber or hole. The results show that decrease of the position angle stabilize the crack growth at the interface in the ligaments. This effect is more significant in the cases with small ligament thickness. In the two-fiber model and at very small ligaments the results show that the crack growth stops when the crack tips meet each other in the ligament and further crack growth is under dominant mode-II fracture. In the fiber-hole model, both the crack initiation and propagation are stabilized by decrease of the position angles at very thin ligaments. This paper suggests to use two fibers instead of a single fiber in order to ease the characterization of interfacial properties.

Composite Structures, 2015

Abstract The mechanical response of porous unidirectional composites under transverse normal and ... more Abstract The mechanical response of porous unidirectional composites under transverse normal and longitudinal shear loading is studied using the finite element analysis. The 3D model includes discrete and random distribution of fibers and voids. The micromechanical failure mechanisms are taken into account by considering the mixed-mode interfacial debonding and pressure-dependent yielding of the matrix using the modified Drucker–Prager plasticity model. The effect of the micromechanical features on the overall response of composite is discussed with a focus on the effect of microvoids and interfacial toughness. Finally, the computational prediction of the porous composite in the transverse normal-longitudinal shear stress space is obtained and compared with Puck’s model. The results show that both interfaces with low fracture toughness and microvoids with even small void volume fraction can significantly reduce the macroscopic strength of composite. The size and shape of microvoids can also microscopically lead to different crack paths.

The fibre/matrix interfacial debonding is found to be the first microscale failure mechanism lead... more The fibre/matrix interfacial debonding is found to be the first microscale failure mechanism leading to subsequent macroscale transverse cracks in composite materials under tensile load. In this paper, the micromechanical interface failure in fiber-reinforced composites is studied experimentally and by numerical modeling by means of the finite element analysis. Two fibers embedded in the matrix are subjected to a remote transverse tensile load (see Fig. 1a). The trapezoidal cohesive zone model proposed by Tvergaard and Hutchinson [14] is used to model the fracture of the fiber-matrix interfaces. This study is based on the comparison between the results of numerical modeling and those corresponding to the experimental tests by employing two parameters: The angle from the load direction to the crack tip and the crack normal opening. This comparison aims to investigate the interfacial properties and also assess the progressive fiber-matrix debonding by focusing on the interaction of two fibers with dissimilar interfacial strengths.

and it is a condition of accessing publications that users recognise and abide by the legal requi... more and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

European Journal of Mechanics - A/Solids, 2013

Composite materials under loads normal to the fiber orientation often fail due to debonding betwe... more Composite materials under loads normal to the fiber orientation often fail due to debonding between fibers and matrix. In this paper a micromechanical model is developed to study the interfacial and geometrical effects in fiber-reinforced composites using generalized plane strain by means of the finite element method. Assuming a periodic distribution of fibers in the matrix, a unit cell is chosen including two quarter-circular fibers. By using this unit cell approach the composite material is modeled rather realistically as the possibility of having different fiberematrix strength exists. In the present study two different cases are considered: I) Two perfectly bonded interfaces. II) Two debonding interfaces of uneven strength. The fibers are purely elastic while the matrix is considered as isotropic with an elastoplastic behavior. To model the fracture of the fiberematrix interfaces, a trapezoidal cohesive zone model is used. A parametric study is carried out to evaluate the influence of the interfacial properties, fiber position and fiber volume fraction on the overall stressestrain response as well as the end-crack opening displacement and the opening crack angle. All the results presented are compared with corresponding perfectly bonded interfaces. Generally, different crack initiations and propagations at the two interfaces are seen, which result in an overall stressestrain response of the material that often first depict a rather smooth stress drop followed by a second sudden stress drop. This behavior is shown to be very sensitive to interface parameters as well as geometrical parameters. The interfacial dissimilarity shows for all the investigations, that decreasing the maximum cohesive strength leads to more stable interfacial crack growth, whereas increasing the critical interfacial separation causes a less distinct debonding at one interface before debonding at the other.

Composites Science and Technology, 2014

This thesis is submitted in partial fulfillment of the requirements for obtaining the degree of P... more This thesis is submitted in partial fulfillment of the requirements for obtaining the degree of Ph.D. in mechanical engineering at the Technical University of Denmark (DTU). The Ph.D. project was funded by the Danish Council for Strategic Research (grant no.: 09-067212) under the Danish Center for Composite Structures and Materials for Wind Turbines (DCCSM) and carried out at the Department of Mechanical Engineering, Solid Mechanics, and the Department of Wind Energy at DTU in the period April 1 st 2011-March 31 st 2014. Supervisors on the project were Associate Professor Ph.D. Brian Nyvang Legarth, Associate Professor Ph.D. Christian F. Niordson and Associate Professor Ph.D. Christian Berggreen from the Mechanical Engineering Department and Professor Dr.techn. Bent F. Sørensen from the Department of Wind Energy, section of Composites and Materials Mechanics. I am very grateful to my supervisors for their inspiring support and for always taking their time to discuss the work and the results during the project. I would also like to thank Professor Javier Llorca and Dr. Carlos González who were hosting me at IMDEA Materials Institute, Madrid, Spain, during my PhD external stay in the period October 2013-January 2014, for our beneficial collaboration.