Sharif Digital Repository

An architectural modification method was utilized to improve fracture toughness of discontinuously reinforced aluminum (DRA) composites. Al-DRA composites having a structure similar to that of reinforced concrete were fabricated. The number of reinforcing DRA rods within Al matrix and volume fraction of SiC particles in DRA were altered to evaluate their effect on fracture behavior of these materials. It was found that architectural modification does not have any destructive influence on elastic modulus and yield strength of the composite. Moreover, the success of this method on toughness improvement strongly depends on the occurrence of debonding between Al and DRA regions upon loading  

This paper presents the results of an experimental study on retrofitting RC beams using FRP strengthening and RC jacketing. Three-point loadings are applied to twelve strengthened beams and three reference specimens to put the two techniques into perspective. Formation of first cracks, FRP debonding, and onset of concrete crushing are compared and discussed. Load-deflection behavior, failure modes, and crack propagation patterns are studied extensively. The experiments demonstrate that the RC jacketed beams exhibit a flexural failure identical to the reference beams. However, the FRP strengthened beams fail in a relatively small deflection due to the premature FRP debonding. Furthermore, the... 

In this study, homo- and co-polypropylene were modified by nanometric calcium carbonate and the effect of additive on scratch resistance was evaluated. Results of this study show that the length of scratch curvature can be considered as an acceptable criterion to evaluate the scratch resistance of polymers. Also, it was found that addition of calcium carbonate to homo-polypropylene increases scratch resistance of this polymer, but its effect on co-polypropylene is insignificant. Based on the results of this work, one may claim that incorporation of nanometric particles with good adhesion to the matrix and well dispersion might be effective in enhancement of scratch resistance. Formation of... 

Much research has been conducted and published on the examination of the behavior of reinforced steel and concrete structures with a FRP system. Nevertheless, the performance of FRP differs from that of FRCM, particularly at high temperature and ultimate strength. The present study provides a review of previous research on structural elements (viz. beams, columns, arches, slabs, and walls) retrofitted with FRCM systems, taking account of various parameters, such as layers, composite types, configurations, and anchors for controlling or delaying failure modes (FMs). Additionally, this paper discussed the details of different FMs observed during experimental tests, such as crushed concrete or... 

A modified analytical model is developed for analysis of 3-D elastic stress fields in short fibre composites subjected to an applied axial load. Two sets of exact displacement solutions for the matrix and fibre are derived based on the theory of elasticity. The superposition of these two solutions are then used to obtain the analytical expressions for the 3-D stress field components over the entire composite system including the fibre end region which is modelled by the use of imaginary fibre technique. The main difference with the previous works here is that the stress field is considered to be dependent on both radial and axial directions. Such an assumption made it possible to calculate... 

A modified analytical model is developed for analysis of 3D elastic stress fields in short fiber composites subjected to an applied axial load. Two sets of exact displacement solutions for the matrix and fiber are derived based on the theory of elasticity. The superposition state of these solutions are then used to obtain the analytical expressions for the 3D stress field components over the entire composite system including the fiber end region, which is modeled by the use of imaginary fiber technique. The main difference with the previous works here is that the stress field is considered to be a function of both radial and axial directions. Such an assumption made it possible to calculate... 

The efficiency of fiber/matrix interface bonding in composites has an important influence on their overall performance. This article deals with the simulation of interface imperfection effect as it grows under fatigue loading. The relations have been derived based on the constituents' properties for the primary damage mode in unidirectional composites and boundary conditions defined for elements of the micromechanics model. Conformation of model stiffness predictions with the best described residual stiffness model indicates the way to find the remaining constants in the simulated relation. Finally through this relation, cyclic corrected stress components in constituents could be determined.... 

The overall mechanical properties of composite materials are dependent on the mechanical response of individual constituents and their interactions while they may be relatively easy to determine. This paper represents a simulation process by which the cyclic stresses and fatigue loadings on its constituents could be predicted for an under fatigue loading lamina. Hence, the unidirectional composites fatigue would be studied through its constituents. The proposed model introduces a new coupled stiffness/strength technique by relating lamina stiffness to the stress field in its constituents. Therefore, the stress field and strength considerations in its constituents could be studied when the... 

The probability of failure estimation of composite structures under applied loads is an inevitable requirement given the uncertainties related to the material properties, loads, and boundary conditions. The properties of composite materials have more scatter due to non-homogeneity and anisotropic characteristics, and manufacturing defects. In this paper, the reliability of S2-Glass/Epoxy laminate composite materials containing a central circular hole under static tensile load is presented. Failure in fiber-reinforced plastic structures, since individual faults in each ply cannot be traced, is a random process due to the scatter caused by the behavior of the material. According to the... 

The probability of failure estimation of composite structures under applied loads is an inevitable requirement given the uncertainties related to the material properties, loads, and boundary conditions. The properties of composite materials have more scatter due to non-homogeneity and anisotropic characteristics, and manufacturing defects. In this paper, the reliability of S2-Glass/Epoxy laminate composite materials containing a central circular hole under static tensile load is presented. Failure in fiber-reinforced plastic structures, since individual faults in each ply cannot be traced, is a random process due to the scatter caused by the behavior of the material. According to the... 

A finite difference technique is developed to predict the second stage creep displacement rates and stress analysis of a short fiber metal matrix composite subjecting to a constant axial load through a micromechanical approach. The technique is capable to take into account the presence of interfacial debonding as one of the main factors affecting the creep performance of short fiber composites. The exponential law is adopted to describe the matrix creep behavior. Also, a model for prediction of interfacial debonding at fiber/matrix interface is developed using a stress based method. The obtained results could greatly help to better understand the flow pattern of matrix material and the load... 

An architectural modification method was utilized to improve fracture toughness of discontinuously reinforced aluminum (DRA) composites. Al-DRA composites having a structure similar to that of reinforced concrete were fabricated. The number of reinforcing DRA rods within Al matrix and volume fraction of SiC particles in DRA were altered to evaluate their effect on fracture behavior of these materials. It was found that architectural modification does not have any destructive influence on elastic modulus and yield strength of the composite. Moreover, the success of this method on toughness improvement strongly depends on the occurrence of debonding between Al and DRA regions upon loading  

This paper examines the electro-mechanical fields for a circular anisotropic piezoelectric fiber sensor inside an anisotropic piezoelectric or non-piezoelectric elastic matrix with imperfect interface under remote in-plane uniform tension. The interface imperfection is posed on the mechanical fields only. The present formulation admits different boundary value problems in a unified manner, so various fiber-matrix interface conditions are considered: (1) perfect bonding; (2) pure debonding; (3) in-plane pure sliding; (4) out-of-plane pure sliding; (5) full debonding; and (6) partial debonding. An interface condition is modeled by a specific layer of mechanical springs with vanishing... 

Prediction of the creep behavior of metal matrix whiskers reinforced composites is the subject of many analytical and experimental research works. However, application of the FEM modeling to this area has not been so successful. The present research work highlights the shortcomings of the previous FEM studies and combines the analytical findings with the power of FEM modeling. More specifically, the fiber/matrix debonding parameter ξ is incorporated into the modeling. In contrary to the analytical results, it is shown that the value of ξ could only change in the range of zero to 0.5. The results indicate that the exponential creep law along with the modified ξ-σ relationship, and the newly... 

In this study the effect of stress field on delamination and fiber/ matrix debonding in laminated composite panels is investigated from the micro-mechanical point of view by means of 3-D Finite Element Models. Specifically, the behavior of two-layer cross-ply symmetric laminates made up of SCS-6/Ti-24Al-11Nb Intermetallic Matrix Composite (IMC) during cooling from the processing temperature is studied. The results show that large plastic strains occur at the fiber/ matrix interface at the fiber end on the laminate free edge which may eventually extend to the interface of the layers of the laminate inflicting delamination damage. This phenomenon is more serious at the corner areas of the... 

A model for prediction the stress-strain behavior of particulate composite over wide ranges of filler concentration and composite deformation has been developed through combination of Anderson's and Yilmizer's model. The constitutive equations are extracted from first law of thermodynamic and nonlinear dilatational effects which are produced by filler-matrix debonding process. In addition to nonlinear behavior that has been resulted by filler-matrix debonding and was presented by Yilmizer, the formation and growing of void or cavitations has been also introduced in this model, whereas Anderson's model, most important reason for deviation of linear behavior is filler-matrix debonding and has...