نشریه علوم و فناوری کامپوزیت
سال دوم شماره 2 (پیاپی 4، تابستان 1394)

In this study، initiation and evolution of damages in AS4/PEEK laminated composites with center open hole under in-plane loading are investigated using finite element analysis. Hashin failure criteria are used to obtain the damage initiation of composite laminates. In addition، in order to capture the various in-plane failure mechanisms in this laminate، Hashin progressive damage model، which is an energy based model، is used. As delamination is one of the most critical damage mechanisms in composite materials، initiation and propagation of delamination in the mentioned laminate are modeled utilizing cohesive zone model. In order to simulate progressive damages of laminated composites، Abaqus software is used and all applied damage models are available in this commercial software. It is worth to note that، in the proposed model، interlaminar and intralaminar damages are considered simultaneously. Comparison of the present numerical results and experimental ones shows that by using the cohesive zone model to simulate the delamination، experimental load-displacement curve can be predicted more accurately. Also different in-ply damages and delamination patterns that obtained by the proposed model are presented to clarify different damage mechanisms in composite materials..

Polymethylmethacrylate (PMMA) based dental and bone composites are used frequently in biomedical applications as implants، bone cements، restorative materials and etc. This is mainly because of some characteristics of PMMA such as biocompatibility، good mechanical properties and its low cost. However، in comparison with bone and dentin، PMMA based dentures or implants have lesser fracture toughness (i. e. smaller crack growth resistance) which makes them very susceptible to brittle and sudden failures. The use of nano additive materials like hydroxyapatite and alumina as filler particles may probably increase the crack growth resistance of such composites. Hence in this research the effects of adding different weight percentages of these two nano particles are examined on mode I fracture toughness of PMMA based denture composites. The experimental fracture toughness results demonstrated that the fracture resistances of manufactured nano-composites are increased in general relative to neat PMMA. The influence of alumina was more pronounced than the use of hydroxyapatite particle. The highest increase in fracture toughness was observed for composite containing 10% hydroxyapatite and 6% alumina.

In this paper، robust vibration control of a thin functionally graded beam with a variable cross-section has been investigated. For this purpose، piezoelectric patches are used as sensors to measure the displacement of the beam and as actuators to apply control forces. In this way، firstly، Euler-Bernoulli theory is used to derive the governing dynamical partial differential equation، through the Hamilton’s principle. Approximate solution of these equations is achieved using finite difference method، and the proper orthogonal decomposition is then used to obtain vibration mode shapes. After that، time-dependent ordinary differential equations are attained using Galerkin projection scheme and then represented in the state-space form. Since the data measurement is done in sampling intervals، the system is considered as sampled-data. In this way، direct digital control design methodology is used. For this purpose، based on its zero-order hold equivalent model، a robust discrete-time، observer-based، output feedback controller is designed. In this regard، controller and observer gains are designed by a Lyapunov-based method. This procedure is done by solving a set of linear matrix inequalities. Simulation studies show the effectiveness of the proposed method.

Due to high strength and stiffness in comparison with their weights، laminated composite materials are widely used in many structures such as aerospace. Therefore to predict their mechanical response، the understanding of their failure mechanisms is very important. The delamination between composite layers and adhesive joints is one of the main damage modes of these materials. In this research، the cohesive zone model is used to predict the damage evaluation of composite wing adhesive joints. The advantage of this method is the modeling of delamination growth without any requirements to the presence of initial crack and remeshing. Moreover to predict the probable damage in composite layers the Ladeveze progressive damage model has been implemented in Abaqus using user defined code (Umat) and also the importance of considering the intralaminar failure on the acceleration in damage initiation and propagation in adhesively bonded joints have been evaluated. The results verify the proper accuracy of implemented method. Furthermore، the results of solid cohesive elements showed to be more accurate in predicting damage initiation and evaluation in comparison to shell elements. Finally effects of adhesive properties such as thickness and quality of bonding in load capability of wing structure have been investigated.

In the present investigation، first، 48 fracture tests were conducted on the Brazilian disk specimens made of the quasi-brittle polymer Polymethyl-methacrylate and weakened by central V-notches with end holes (VO-notches) under combined tensile-shear loading with different mode mixity ratios. The fracture loads and the fracture initiation angles (FIAs) were experimentally measured. Then، two brittle fracture criteria were developed on the basis of the failure concepts of the maximum tangential stress (MTS) and the mean stress (MS) for predicting the experimental fracture loads and the FIAs، and the theoretical results were obtained in the form of the fracture curves and curves of fracture initiation angle. A very good agreement was shown to exist between the theoretical and the experimental results. Moreover، it was found that both the criteria provide almost identical predictions to the experimental results.

Although Al/SiC composite has attracted much attention of the researchers، no due attention has been paid so far to its physical properties particularly its dimensional stability. Therefore، the objective of this research is to investigate the thermal-physical properties of these composites and the presented models of these properties. Initially، the Al/4%Cu alloy was prepared by mechanical alloying، and then the aluminum composite was synthesized with different percentages of silicon carbide (with different particle sizes). To characterize the composite hardness and density tests were performed. Thermal test on the Al-4%Cu/SiC composites revealed that with increase of volume fraction of SiC particles up to 25%، thermal expansion coefficient of the composite has decreased linearly، which indicates the presence of ceramic particles with very low thermal expansion coefficient in the composite. Also، these results reveal that with increase of temperature up to 500°C، thermal expansion coefficient has increased up to 20. 6 for the composite with 15% of reinforcement particles، although this increase is not linear. In order to investigate the effect of the reinforcement particles size on thermal expansion coefficient of the Al-4%Cu/SiC composite thermal tests were performed which indicated that the reinforcement particles size has no conspicuous effect on thermal expansion coefficient of the composite but causes a thermal strain of 0. 8% in the composite to some extent.

Joints are the most important parts of composite structures، because of their more sensitivity to load transmission، stress concentration and adherents differences (in materials، geometries and boundary conditions). Numerical analysis of all mechanical joints in big structures is costly in time and memory usage. In big structures، precise numerical analysis of mechanical joints needs to an appropriate criterion to find the critical joint. In this paper different modeling methods on analysis of mechanical joints are compared to each others. The numerical analysis is done by means of ABAQUS-6. 11. 1 finite element (FE) code. Predicted elastic modulus، the joint strength (maximum force) and load-displacement trends are compared with experimental results and time cost of modeling are compared to each others. Result study shown that the elastic modulus and the joint strength are better predicted in the improved Solid-Shell model. Beam-Shell model predicted a good elastic modulus in the joint، although the joint strength is predicted less than experiments. The final suggestion of this paper to time saving purpose in joint analysis of a big structure with many joints is to start the analysis of all joints with Beam-Shell model and finding the failed mechanical joints. In second step، just the failed joints should be re-model with improved Shell-Solid model and the structure should be checked for joint failure again

In this paper، the influencing factors on hybrid adhesive-riveted joints were investigated. For this purpose، first a finite element model was validated based on experimental results، then the effects of influencing material and geometrical factors on the strength of the hybrid joints were studied using the explicit finite element analyses. The numerical model was able to simulate damage in the adhesive layer and in the rivets. The results showed that in a hybrid adhesive-riveted joint، it is better to position the rivets where a minimum reduction in the adhesive joint due to the rivet holes obtained i. e. as far as possible from the joint ends. This is in contrary to the riveted joints in which placing the rivets closer to the joint ends resulted higher joint strength. In general، incorporating rivets in an adhesive joint for manufacturing a hybrid joint can result higher or lower joint strength in comparison with the adhesive joint in different conditions that were studied in this research. It should be noted that in case that a low-strength adhesive is utilized in a hybrid adhesive-riveted joint then placing the rivets closer to the joint ends can results higher strength for the joint similar to the riveted joints.