Modares Mechanical Engineering
Year:2020 | Volume:20 | Issue:1 |Papers Count:26

Rehabilitation is a process in which the patient achieves his/her lost ability and individual independence in performing their daily activities using numerous facilities and equipment. About 30% of human life-threatening injuries are related to their hand. The human hand, as one of the most important organs of the human body in interacting with the environment, has the greatest role in maintaining individual independence in daily work. In this article, a rehabilitation system has been designed for hand tendon injury using observations of traditional rehabilitation of hand injuries after surgery and recovery period, and through a mechanism based on structures restricting undesirable degrees of freedom. The mechanism used in this design has been selected by considering conventional tendon injury rehabilitation exercises. In this way, the system can easily bend the finger over the marked joint by using a tendon shape mechanism, which applies force on the tip of the finger. The process of system designing is completed using a prototype to examine the method of operation as well as to obtain the required forces for choosing electrical elements.

Service life and safety of a steel jacket platform is influenced by vibrations generated by environmental loads, waves and winds. Vibrations of the structure and deck may cause fatigue in the structural elements and joints. Also may disrupt the operation of the drilling equipment and facilities as well as the operation of the platform. Therefore, the main aim of this research is to control the vibrations of the steel jacket platform through shape memory alloys dampers. Shape memory alloys have two important properties of shape memory as well as superelastic behavior and are quite suitable for damping applications. In these alloys, crystal structures transition from the austenite to the martensite phase, and vice versa are accompanied by the energy dissipation. In this research, a 90m steel jacket structure equipped with SMA dampers installed in 80m water depth has been modeled as a multi-degree-of-freedom system and analyzed under the time history of wave loads. For solving the differential equations of system vibration and modeling the hysteresis behavior of the shape memory alloys elements, the direct integration alpha method and multi-linear idealized constitutive model have been used, respectively. Jacket platform equipped with the shape memory alloys dampers shows the better result with 42% reduction in deck displacement, 62% reduction in deck acceleration and 32% reduction in shear force of platform base.

In this research, the effect of tool rotation speed and dwelling time on the strength of the welds produced by protrusion friction stir spot welding (PFSSW) was investigated. This simple novel technique involves the use of a designed circular protrusion on the backing anvil. Welding was performed by pinless tools on the AA5053 sheets with a thickness of 1 mm at tool rotation speeds of 630-2000 rpm and dwelling times of 6 s, 12 s, and 18 s. Appearance surface of produced welds was the smooth and free keyhole in comparison with conventional friction stir spot welding. Tensile-shear test results showed that all welds were failed in circumferential failure mode. Maximum and minimum peak loads were obtained at 1600 rpm, 18 s (4. 9 kN) and 1000 rpm, 12 s (3. 5 kN), respectively. Maximum and minimum elongations were obtained at 2000 rpm, 18 s (5 mm) and 2000 rpm, 6 s (2. 3 mm), respectively. Maximum and minimum failure energies were obtained at 2000 rpm, 18 s (12. 3 J) and 1000 rpm, 12 s (3. 1 J), respectively.

In the conventional casting process, the presence of porosity in the structure is inevitable. Compocasting method is one of the processes for composite production. Performing friction stir processing as a complementary process will modify the microstructure and good distribution of reinforcing particles in the matrix. Therefore, in this study, friction stir processing was used to improve the composite properties of A390 / 10wt% SiC composites. The FSP process was performed at rotational and traveling speeds of 800rpm and 40 mm / min, respectively. Three ratios of shoulder diameter to pin diameter (D/d) of 2, 2. 5 and 3 were used, each of them was processed in one to three passes. An optical microscope (OM) was used to examine the microstructure of the processed samples. Microstructural data and its association with the results of the hardness and tensile test yielded the desired parameter. The results showed that FSP modifies the microstructure including resizing and distribution of SiC particles, primary silicon as well as changes the grain size of aluminum. The uniform distribution of particles on one side and the reduction of the grain size of aluminum, on the other hand, is effective in determining the desired parameter. The highest strength and toughness in the D/d ratio was 2. 5 and in the third pass were 260MPa and 10. 8M J/m3, respectively. Also, the average particle size of SiC, silicon and aluminum grains in the optimum parameter were 2. 98, 14. 98 and 16. 3 μ m, respectively.

Grain design is the most important part of a solid rocket motor. The aim of this study is finocyl grain design based on predetermined objective function with respect to ballistic curves in order to satisfy various thrust performance requirements through an innovative design approach using a genetic algorithm optimization method. The classical sampling method has been used for design space-filling. The level set method has been used for simulating the evolution of the burning surface in the propellant grain. An algorithm has been developed beside the level set code that prepares the initial grain configuration using Pro/Engineer software to export generated models to level set code. The lumped method has been used to perform internal ballistic analysis. Two meta-models are used to surrogate the level set method in the optimization design loop. The first method is based on adaptive basis function construction and the second method is based on the artificial neural network. In order to validate the proposed algorithm, a grain finosyl sample has been investigated. The results show that both grain design method reduced the design time significantly and this algorithm can be used in designing of any grain configuration. In addition, data have more accuracy in grain design based on the artificial neural network, so this method is the more effective and practical method to grain burn-back training.

In the present research, the steam generation performances of nanofluids containing titanium dioxide have experimentally been examined. For this purpose, a solar simulator with a xenon lamp as the radiation source, and a pyranometer as a light intensity measuring device are used. Then, the water based-nanofluids in five nanoparticle mass fractions of 0. 001, 0. 002, 0. 004, 0. 04, and 0. 08% exposed to the light intensity of 3. 5Suns (3. 5 kW/m2) were investigated to compare their evaporation performances with water (H2O). Finally, the effects of the solar power intensity on the steam generation were examined. The results showed that the titanium dioxide nanostructures are more efficient to directly absorb the solar energy than the water so that the maximum total evaporation efficiency of 77. 4% and 54% were obtained at 3. 5 kW. m-2 for nanofluid and water, respectively. Furthermore, it was found that light absorption increases as the nanofluid mass fraction increases. Also, increasing the light intensity from 1. 5 to 3. 5 kW. m-2 enhances the thermal efficiency, while it reduces the evaporation efficiency.

In this research, an example of the results obtained from the combination of the vibration monitoring program and the root cause analysis approach for the electromotor roller element bearings of the cement factory’ s mill fan has been examined and presented. By registering the inspectors’ reports on the release of abnormal sound from the bearings, the vibration data recorded in the monitoring program indicates the change of vibration trends and sensible increase in the bearing condition index (BC). By matching the vibration frequency with defect frequency of the elements of roller bearing, the defect in the bearing cage was predicted. Diagnosis of the root cause of failure was on the agenda for this aim. The defect of the bearing was detected after investigation of the root cause of failure in the bearings. The type of bearing used in the electromotor by the electromotor manufacturer was not suitable considering its operating characteristics, and the proposed periodic lubrication interval has not suitable for the type of related cage bearing and has resulted to its destruction.

The New Wave theory has recently applied for predicting wave forces on marine structures including offshore wind turbines. However, the validation of the theory in determining wave force has not been fully confirmed. Therefore, evaluation of the validity and accuracy of the results of New Wave theory to determining the stability parameters of various marine structures including base shear and the overturning moment is necessary. In the present article the results of wave generation by New Wave theory in determining water surface profiles, wave kinematics and offshore wind turbine monopile pier responses to the wave, including base shear, overturning moment and maximum displacement are compared to the experimental data and results from linear irregular wave time series generated from the wave spectrum. The results show that the New wave theory with a very short time of structural analysis calculations can be the reliable substitute for prediction of wave forces on offshore wind turbines in comparison to real irregular time series. The comparison of the results with the conventional 5th Stokes regular waves shows that the new wave theory is significantly more accurate in predicting wave kinematics and wave loads on offshore wind turbine monopiles.

In the process of manufacturing, the operation of improving the quality of the surfaces is important due to the different working conditions, the resistance to corrosion and fatigue life, friction, the type of contact between the surfaces and appearance. The purpose of this research is the experimental investigation of burnishing process on the flat surface by ultrasonic vibration in order to investigate the initial surface roughness as an input variable as well as its interaction effect on the final surface roughness of aluminum Al6061-T6 alloy. Response surface methodology (RSM) was utilized to correlate the empirical relationship between input and output variables and their interaction effects. Experimental tests with a constant frequency of 20 kHz were done to find the effect of the initial maximum surface roughness, ultrasonic vibration amplitude and static load on the surface roughness. The results show that the initial surface roughness has no direct effect on the output surface roughness, but the effect of vibration amplitude and static load on the final surface roughness depends on the initial surface roughness. The higher static load is needed for the high surface roughness, and the increase of static load has decreased the effect of initial surface roughness on the surface roughness. Also, in high vibration amplitude by increasing the initial surface roughness, the surface roughness is increased and at low vibration amplitude by increasing the initial surface roughness, the surface roughness is decreased. By increasing the vibration amplitude and the static load, the surface roughness is increased. Furthermore, the amplitude of vibration, the interaction effect of static load and the initial maximum surface roughness and static load have the highest effect on the final surface roughness, respectively.

Investigating the history of production and dynamics of growing or collapsing bubbles under various environmental conditions plays an important role in the correct understanding of the process of boiling, evaporation, cavitation, and condensation. In this paper, the rising shape regime the air bubble injected into the water column was studied and simulated using numerical and experimental methods. For this purpose, a column filled with water was used in the laboratory as a host fluid and using the high-speed image recording method, the most important hydrodynamic properties of the bubbles, such as velocity, size, pathway, and other bubble properties were measured. Then, using the computational fluid dynamics and the volume of fluid two-phase flow model, ascent and deformation of the single-bubble injected into a stationary reservoir were investigated and compared with previous and current experimental and numerical results. The result of this validation with a good approximation was in accordance with the reference results and it proved the correctness of the solver’ s and its settings. Finally, the bubble shape regime was calculated by the non-dimensional numbers of Eö tvö s and Morton and compared with the numerical simulation and empirical test. The regime obtained from the Clift diagram is a spherical cap regime, which at the same conditions, is in accordance with the bubble shaped regime simulated by numerical and experimental methods and this confirms the validity of the numerical solution.

In the present research, a new model is presented to predict the burning rate of a solid rocket motor (SRM) in the presence of erosive burning phenomenon. This model is based on the Wang model and the major modification is adding the pressure change in the erosive burning rate. In addition, the necessary relations needed to calculate the velocity gradient on the propellant surface in a one-dimensional internal ballistics code was presented. To assess the new model, the test results of a laboratory motor designed in this research were used. Also, to compare the performance and accuracy of this model with the other models, this motor was simulated with the presented model and the six available models. The results of the comparison indicate that the new model has better accuracy than the other models. The advantage of introducing the pressure effect in the Wang model has been shown. Another advantage of the new model is that this model doesn’ t have any experimental constants dependent on the propellant composition or grain dimensions which is a common defect in popular models such as Lenoir-Robillard model.

Hydrodynamic coefficients have primary importance in determining the maneuvering characteristic of a marine vehicle. The use of computational fluid dynamics (CFD) methods due to the lower cost of these methods compared to laboratory methods in determination of hydrodynamic coefficients have always been considered. Validation of the CFD methods and enhancing their accuracy are the major topics in the application of CFD for the underwater vehicle. The hydrodynamic coefficients of an elliptical-shape underwater vehicle and the effect of motion amplitude and velocity parameters have been investigated by the STAR-CCM+ software and through dynamic overset meshing. The results of the simulations have been compared and analyzed and the error reduction criteria have been presented considering the amplitude dimensions and velocity values in the simulation. In addition, an innovative method for simultaneous calculation of hydrodynamic coefficients of surge motion has been presented which shows good accuracy by comparing the results with theoretical and laboratory data.

Modeling the movement of different parts of the body has been studied a lot in recent years. Body movement models such as fingers movements are good guides for designing different robots. Also, motion disability is one of the common diseases that have a great impact on patients’ life quality. To treat the rupture of finger tendon, individual rehearsal rehabilitation exercises for each phalanx is required. In order to achieve this aim and take control of each phalanx movement, the mathematical model of the desired trajectory for each joint is necessary. The angle of each joint is measured with the help of a gyro sensor installed on a novel wearable rehabilitation robot proposed in this paper. The mathematical models of the phalanges motions are obtained by curve fitting. The model is applicable not only in the rehabilitation robots but also in the other robotic works. In most of the works in this area, the desired trajectory diagram was drawn and tracking of the trajectory was investigated. Thus, the desired trajectory formula should be fined for the other works. But in this work, the corresponding formula was found and it can help other researchers to easily use of these formulas for their work. To ensure the accuracy and efficiency of the calculated trajectories, the trajectories are implemented in a control system. In order to control this system, a suitable sliding mode controller was designed and the results of system controlling and trajectory tracking using this controller was obtained.

In this paper, oscillations of a thin high flexible strip attached to a three-dimensional body in viscous subsonic flow were simulated. The aim is to analyze the interactions of fluid and structure using a proper coupling algorithm that can couple the fluid and structure solvers and provide the proper data exchange between them. A computational fluid dynamics solver is used for fluid flow simulation and Euler-Bernoulli cantilevered beam model is used for structural analysis. For analyzing the fluid-structure interaction, iterative partitioned coupling algorithm is used for interrelation and data exchange between structure and fluid. Then, the results of vibration characteristics including the amplitude and frequency and forces and moments variations are presented with respect to different bending stiffness and strip masses. The simulation is done in 2D and 3D conditions which 3D case is for a cylinder and flexible strip attached to the bottom of the body. Results show that the developed framework captures the physics of fluid-structure interaction successfully. Also, parametric study shows that for the flexible thin strip attached to the end of the body in the specified regime of flow, three deformation types consist of static deformation, stable oscillations, and chaotic unstable oscillations will occur based on the strip characteristics.

Wear is one of the most detrimental mechanisms which can affect the performance of many industrial systems. TiN coating due to its unique properties such as resistance to wear, oxidation, and heat is widely used in mechanical elements. In this research, TiN coating has been coated on steel substrate using a physical vapor deposition method. The coating’ s properties have been obtained using nano-indentation test. Pin on disk wear test has been conducted while the disks are coated. The tests are conducted under three different loads and different speeds. It was shown that the samples with thicker coating show a better tribological performance. In this study, the relationship between wear and entropy has been investigated in order to predict the wear rate for different materials by predicting temperature. Also, temperature changes over time were predicted in two states of with and without coating. It was also shown that the samples with thicker coating have better wear resistance. One of the innovations of this research is the ability to establishing a correlation between the wear rate and produced temperature.

In this research, the deformation of a non-Newtonian leaky-dielectric droplet suspended in another non-Newtonian fluid under a uniform electric field is simulated. The aim of this research is the studying the effect of the electric field on the hydrodynamic of non-Newtonian droplets and also the comparison between the behavior of Newtonian and non-Newtonian droplets in the presence of an electric field. The power law model is used to describe non-Newtonian fluid behavior. The level set method is employed to determine the location of the interface. Also, the ghost fluid method is used to apply discontinuities at the interface. By applying an electric field, a non-Newtonian droplet deforms similarly to a Newtonian one. This deformation may occur either in the direction of the electric field or perpendicular to it. By increasing the electric Capillary number (ratio of electric force to surface tension force) the deformation of the non-Newtonian droplet with different power law constants increases. In this research, the behavior of different non-Newtonian droplets with different power constants was compared and it was observed that by an increase in the power law constant the drop deformation increases. According to the results, the deformation of a shear-thinning droplet under an electric field is less than the deformation of a Newtonian droplet and the deformation of a Newtonian droplet is less than the deformation of a shear thickening droplet.

Recently the use of reactive shaped charges with bimetallic liners are taken into consideration to increase destruction quality in water environments. In this research, according to the results of a series of valid experimental results, the analysis of a reactive shaped charge with a bimetallic liner made of copper-aluminum liner has been numerically verified. In this verification, a suggested theory for the cutoff velocity of bimetallic liners has been used to calculate the cutoff velocity. The amount of penetration depth using a numerical solution is in good agreement with the experimental value. These results have been compared with the values obtained from the analytical solution. Finally, the behavior of the shaped charge with bimetallic liner has been compared with a single metallic liner using the same target geometry in both and it has been shown that the overall penetration quality such as depth, diameter and the profile of reactive shaped charge with a bimetallic liner was found to be better than the single metal liner.

The important factor in turbine efficiency is turbine rotation. The higher the rotor time at different speeds, especially at low speeds, increases the turbine power. In this regard, first, the airfoil NACA0015 was selected and the K-ω SST turbulence method was used for numerical analysis. The validation was performed using experimental results. The wind turbine was designed and fabricated by CATIA software. The aluminum sheet used by a series alloy is used to make smooth, porous leaves from simple cards and diamond-shaped leaves, in a porous form with 0. 3 mm thick. The instrument used in measurement, testing and fabrication have been calibrated to compute more precisely and to generate wind flow from the four-fan blower. The results show that the darriues vertical axis wind turbine with porous and flat blades has begun to rotation at the speed of 2. 3 and 3. 9 m/s. At the speed of 2. 5 and 3 m/s, the rotation of wind turbine porous blade doubled and at the speed of 4 m/s, its rotation speed was 3 times higher than the speed of straight blade turbine. The rotation of wind turbine porous blade in speeds of 4. 5, 5, 5. 5, 6, 6. 5 and 7 m/s were 56. 25 %, 20 %, 22 %, 15 %, 7. 5 %, and 12% higher than the straight blade turbine and in speed of 8-10 m/s the rotation of the straight blade turbine and porous blade turbine is almost equal.

The idea of designing new geometries for catalytic bed in the decomposition chamber of monopropellant thrusters is introduced with numerical simulations of pore-scale turbulent flows. The LES numerical technique is used for simulation of turbulent structures in the flowfield. The efficiency and reliability of the results obtained from numerical simulation have been determined by solving a benchmark problem of turbulent flow over the pack of cubes. The results show very good agreement with the experimental data, indicating the accuracy of the used model and numerical solution process. The characteristics of turbulent flow over two different geometries have been investigated using the numerical method. The results have been analyzed to evaluate the effectiveness of geometrical changes on the parameters associated with the catalytic reaction. All simulations have been conducted for cold flow, and the exact effects of the geometrical design of porous bed on reactive flow have not been quantified. The eddy dissipation and length scales of turbulence have been considered as the main parameters, because of their effect on rates of turbulent mixing and rate of reaction. The difference between the turbulent dissipation and length scales in the investigated flows in two different geometries indicates the effectiveness of the geometrical changes of the porous bed on the flow characteristics. Coherent structures are seen in the new geometry and the wall shear stress is reduced significantly, which increases the life of the catalytic coating.

In the present research, the performance of a single-cylinder engine with a pre-chamber and natural gas fuel designed in Urmia University has been investigated and the effect of Exhaust Gas Recirculation (EGR) on engine performance has been analyzed. The results indicate that the simultaneous use of the pre-chamber and the EGR reduces significantly nitrogen oxides emission. Also, the amount of unburned hydrocarbons (HC) decreases in the low EGR, but the amount of HC increases significantly with higher EGR. EGR increases the carbon monoxide (CO) emission but does not have a significant effect on carbon dioxide (CO2) emission. Simultaneous use of EGR and pre-chamber can reduce the amount of emission while it can maintain the engine braking. The engine power and the indicated mean effective pressures (IMEP) which are the main indicators of the engine’ s performance, decrease by 3 to 4 percent for every 5 percent of the EGR. The results show that the EGR reduces the velocity of the jet flames out of the pre-chamber which ultimately reduces the advance of the flame front. Analysis of the results of the experimental test and the simulated model shows that an ideal range for EGR in an engine with a pre-chamber can be defined in which the emission is minimal and the engine power is maintained. In the engine used in this research, the exhaust gas reaction is in the ideal 10% range.

Electrowinning is the process of copper deposing from the intracellular electrolyte solution to the cathode by creating an electric current. In the present study, the hydrodynamic simulation of the electrowinning cell of Miduk Copper Complex is studied using computational fluid dynamics. Ansys-CFX software is used for this modeling. Navier Stokes and continuity equations are considered as the two-phase fluid and gas, turbulent, incompressible and steady states and the equation for copper concentration in the electrolyte will be solved with consideration of its specific boundary condition. Turbulence will be modeled using the k-ω method. The general and local simulations have been used together due to the large variation in the properties, close to the cathode and anode, and the large volume of the cell, to create a good mesh and increase the speed and accuracy of the solution. First, in general simulation, the entire geometry of the cell is modeled by creating a suitable mesh. Then in the local simulation, only the volume between the two cathodes of the cell is considered and modeled with higher precision. Data on boundary conditions in the local simulation of interface boundary are obtained with general simulation data, which increases the accuracy of modeling. Comparison of the results of general and local simulations shows an accuracy of up to 30% in close to the electrodes. The results of this simulation are the velocity vector, the concentration of acid and copper, the turbulence intensity, pressure and the volume fraction of the oxygen phase in the whole of the electrowinning cell. Finally, the model has been validated by experiments on the real cells. The results show the high accuracy of this modeling technique with less than 2. 5% deviation.

Isogeometric analysis is a new approach in computational mechanics where the geometry and computational modeling is carried out by using NURBS and B-spline functions. The main advantage of the isogeometric approach is in unifying the discretization and problem-solving processes that lead to saving of computational time and cost. In this research, the governing equations of buckling analysis of thin plates stiffened with stiffeners with various geometries are obtained by use of the variational accounting method and first-order shear deformation theory (FSDT). The geometry of stiffener and its position on arbitrarily plate are considered. The equation of buckling is derived by employing the total potential energy, and the obtained system of equations is solved by discretization with the isogeometric analysis method. One of the main advantages of this approach is that it does not need a fine mesh for unification of the connection between the plate and its stiffeners so that, it leads to more accurate answers in comparison with other numerical methods and commercial software with the same number of unknowns. Finally, In order to verification, a few examples are presented and the obtained results are compared with the available results of the analytical and numerical method.

Nowadays, the interaction of oil droplets with gas bubbles plays an important role in many industrial, environmental and biological processes. Therefore, in this paper, the outcome of a collision between a silicon oil droplet and an air bubble in water has studied in order to identify the effective parameters in this process. For this purpose, an especial setup was built and four series of experiments in both dynamic (in which the relative velocity of collision is equal to the bubble velocity due to the Buoyancy force) and static conditions were carried out. The results of these experiments were presented and discussed in the form of several tables and pictures. In these experiments, a high-speed camera and image processing were used to gain a better understanding about bubble-drop coalescence qualitatively, and to obtain some quantitative information such as contact time, velocity, and kinetic and interfacial energies of bubbles and drops during the impact. The results of this study show that in addition to the spreading coefficient, the kinetic energy of bubble/droplet in the collision and their contact time, are also determinative parameters in the determination of the outcome of a collision. In the dynamic and static states, the effect of kinetic energy and contact time are more effective, respectively.

Glass as a non-conductive material has special properties such as transparency, chemical resistance, and hardness. Traditional machining methods have noticeable limitations in their capability for shaping the glass parts. Electrochemical discharge machining (ECDM), as an advanced machining method, gives a chance to implement special processes on the glass. There are many effective parameters in the ECDM process and each of them has its special effect, but the influence of electrolyte type has been rarely evaluated in the literature. In this research, the effects of two types of NaOH and H2SO4 electrolytes on the glass have been studied. Electrolyte temperature, as another effective parameter on the chemical reactions, is also considered in these experiments. Surface quality, machining depth and overcut are considered as the machining outputs. The experimental results obtained in this research indicated that the application of H2SO4 acidic electrolyte after machining in NaOH electrolyte rather than machining solely in NaOH electrolyte has a significant effect on the walls of the holes. It is also observed that with a higher electrolyte temperature, the walls of the holes become smoother. It is also shown that, by applying two steps implementation of drilling and application of acidic electrolyte (NaOH/H2SO4), holes have a lower overcut, and the machining depth is improved up to 20. 5% in the hydrodynamic regime.

Today, composite materials have attracted much attention in many industries, such as aerospace, automotive and marine industries, due to the strength to weight ratio, as well as the ratio of stiffness to high weight. Thermoset resins are among the most widely used resins in making composites based polymer. In the meantime, phenolic resins are the oldest industrial and thermosetting resins, which have important applications in various industries. The distinctive feature of this resin has resulted to its application as industrial insulators, thermal shields composites and ablative composites, and abrasive parts such as brake linings and clutch plates, and many other components. One of the applications of phenolic resins, especially resole with a relatively high thermal resistance, is the use of thermal insulation composites in thermal shields and hot air nozzles. The thermal insulation composites are often phenolic resins and their reinforcement is mainly asbestos fibers with high thermal resistance. Today, due to the carcinogenicity of asbestos fibers and the problems caused by its use, these fibers are removed from the list of reinforcing insulators, and silica fibers, a new product with an asbestos thermal stability and no environmental problem, have replaced instead of them. In this study, talc mineral micro-particles with a mean particle size of 8 μ m as reinforcing the mechanical properties and thermal stability, were added to 25, 15 and 35 phr in phenolic resins in several layers of silica-phenolic composites.