An experimental study was undertaken to investigate and optimize the fabrication processing conditions of the sandwich structures designed for flexural load bearing applications. Sandwich beams, each with two glass/epoxy faces and a rigid polyurethane foam core were constructed under four different processing conditions. Wet and dry faces were applied at two temperatures to atteine these four conditions. It has been shown that in comparison with the first processing condition (wet at 25oC) the specific flexural strengths of the second one (dry at 25oC), the third one (dry at 70oC) and the fourth one (dry at 70oC with a thin layer of an adhesive) are increased by 38% and 209%, 267%, respectively. The reason behind this significant enhancement is demonstrated to be related to the debond strength of the core-face interface. It has been demonstrated that the debond strength of the core-face and core plays an important role in enhancing the flexural rigidity and controlling of the failure mechanisms. If the core material be polymeric foam, the core and debonding strengths at the core-face interface will almost entirely dictate the structural sandwich composite performance especially under flexture. Results showed that the core strength decreases, but the debonding strength of the core-face interface increases with the increment of the processing temperature during the preparation of the rigid polyurethane foam core. We also observed that by increasing the debonding strength of the core-face interface, the failure mode changes from debonding of the core-face interface to the failure of the face. Finally, the sandwich structures were obtained with the excellent performance under flexural loading.

In this paper impact of rigid cylindrical projectiles on aluminum honeycomb panels is studied. Considering various failure modes of target structure (including cell walls tearing and their folding and shearing of plug) by applying energy balance the ballistic limit velocity of panel is determined analytically. It is assumed that the honeycomb behaviour is rigid, perfectly plastic and the projectile diameter is greater than the cell size. The results show that the ballistic limit velocity is increased by increasing the panel thickness, the compressive and shear strength of honeycomb material and the walls thickness. However increasing the cell size, density, and the length of projectile the velocity decneases. The results of this analysis is in good agreement with experimental data.

This article investigates the problem of simultaneous attitude and vibration control of a flexible spacecraft to perform high precision attitude maneuvers and reduce vibrations caused by the flexible panel excitations in the presence of external disturbances, system uncertainties, and actuator faults. Adaptive integral Sliding mode control is used in conjunction with an attitude actuator fault iterative learning observer (based on Sliding mode) to develop an active fault tolerant algorithm considering rigid-flexible body dynamic interactions. The discontinuous structure of fault-tolerant control led to discontinuous commands in the control signal, resulting in chattering. This issue was resolved by introducing an adaptive rule for the Sliding surface. Furthermore, the utilization of the sign function in the iterative learning observer for estimating actuator faults has not only enhanced its robustness to external disturbances through a straightforward design, but has also led to a decrease in computing workload. The strain rate feedback control algorithm has been employed with the use of piezoelectric sensor/actuator patches to minimize residual vibrations caused by rigid-flexible body dynamic interactions and the effect of attitude actuator faults. Lyapunov's law ensures finite-time overall system stability even with fully coupled rigid-flexible nonlinear dynamics. Numerical simulations demonstrate the performance and advantages of the proposed system compared to other conventional approaches.

Reaction to fire of fire-retarded rigid PUR foams and two types of metal faced rigid polyurethane foam core sandwich panel was evaluated by using cone calorimeter test method. The tests were carried out in various radiative heat fluxes from 15 to 75 kW/m2. The radiation rate effect on reaction to fire parameters, including time to ignition (TTI), peak of heat release rate (PRHR), total heat release (THR), average heat release rate (Av.RHR) and average heat of combustion (Av.EHC) was investigated. The phenomenon of char forming, when the foam is exposed to heat, leads to the formation of a protective layer on the surface of foam and hence no direct relation exists between AV.RHR and average specific mass loss rate (Av.Spec.MLR) of foam with increased radiation rate. In addition, the increased PRHR with foam density was also very smooth. The relation between TTI and heat flux was investigated for the foam and its corresponding correlation has been achieved with a specified density. Fire hazard assessment of foams and sandwitch panels was carried out by adopting Petrella and Richardson fire risk classification methods. The assessment results showed that rigid PUR foam and PUR sandwich panels may have a high contribution to bring the room to critical flashover condition, but the risk is intermediate from the viewpoint of fire endurance. The reasons of these risk levels are attributed to a very short TTI, relative high PRHR and an intermediate amount of THR. Decrease in foam density reduces heat release but it shows no significant effect on reducing flashover hazard.

This paper by reviewing the main examples of small to medium-scale retractable roof structures covering building courtyards is to present an innovative rigid retractable roof system employing spatial frames for a courtyard of an existing building in Tabriz Islamic Art University. The courtyard is currently used for temporary exhibitions and gatherings whenever permitted by environmental conditions. The proposed retractable roof will extend the application of the building throughout whole year period and also adds to its beauty and functionality. One of the main advantages of this design that makes it as a good alternative for this building is the way that the roof is retracted in different segments separately in a regulated deployment process and its potential in being used in different stages of the deployment process. The proposed roof consists of four retractable zones all covered with transparent rigid material and a fixed central part being inspired by the patterns of Iranian historic architecture. The retractable parts are placed at four corners and composed of rigid panels Sliding across each other. An actuating force is applied to the first panel of each module and consequently makes the other panels move throughout the associated fixed track.

The paper discusses the design of an observer-based fault-tolerant control algorithm and active vibration control for attitude stailization of a flexible spacecraft (as a rigid-flexible system) subject to external disturbances. An iterative learning observer has been developed in order to estimate the torque deviation caused by actuator faults. One of the main features of the proposed observer is the consideration of external disturbances in its structure. Next, a fault-tolerant Sliding mode control (SMC) law based on a proportional-integral-derivative (PID) structure with a time-varying switching gain is proposed in order to generate control signals with ideal performance. To minimize residual vibrations during and after the maneuver, the strain rate feedback (SRF) control algorithm is also activated simultaneously with fault-tolerant control. Using Lyapunov theory, the proposed control strategies guarantee global stability for the closed loop system. Numerical simulations as a comparative study have been used to demonstrate the effectiveness of the developed system compared to conventional algorithms, such as integral Sliding mode control, when handling actuator failures, external disturbances, and flexible body excitations in rigid-flexible dynamic systems.

Sandwich panels as composite materials have two external walls of either metallic or polymer type. The space between these walls is filled by hard foam or other materials and the thickness of different layers is based on the final application of the panel. In the present work, the extent of variation in core density of polyether urethane foam and subsequent flexural and compressive changes in sandwich panels with glass or epoxy face sheets are tested and investigated. A number of hard polyether urethane foams with different middle panel layers density 80- 295 kg/m3 are designed to study the effect of foam density on mechanical properties including flexural and compressive properties. Flexural and compressive test results show that increased core density leads to improved mechanical properties. The slope of the curve decreases beyond density of 235 kg/m3. The reason may be explained on the limitation of shear intensity in increasing the mechanical properties. In this respect an optimum density of 235 kg/m3 is obtained for the system under examinations and for reaching higher strength panels, foams of different core materials should be selected.

This paper proposes robust hybrid Sliding mode control with a time-varying Sliding surface and active vibration control for a flexible spacecraft during attitude maneuver. The fully coupled nonlinear dynamic model of the rigidflexible system includes the three-axis rotation of the rigid body in interaction with the transverse deformation of the smart PZT-mounted flexible appendages. The smooth control signal includes a hyperbolic tangent and a sharpness function to reduce the effects of chattering and high-frequency interactions of the flexible parts and external disturbances in interaction with the rigid body and controller. The structure of the variable Sliding surface with time has made it possible to adjust the effect of attitude parameters (quaternions and angular velocities) on the control performance. Also, the residual vibrations during and after the reaching phase are suppressed using a robust active vibration control algorithm. The simulations in the form of a comparative study show the performance and superiority of the proposed approach compared to conventional Sliding mode control approaches for systems with structural flexibility in the presence of external perturbations and uncertainties.

An active vibration control algorithm and robust integral Sliding mode control (SMC) are discussed to stabilize the attitude of the flexible spacecraft under external disturbances and actuator faults. As a coupled rigid-flexible dynamical system, the flexible spacecraft is modeled as a rigid hub with two solar panels equipped with piezoelectric (PZT) sensors and actuators. A passive fault-tolerant integral Sliding mode control algorithm using a nominal proportional-derivative control algorithm and an improved fault-tolerant algorithm with time-varying additive fault is developed to increase system’s performance, prevent the system's flexible modes excitations in the phase of reaching the Sliding surface. Therefore, when the system enters the Sliding mode, the closed-loop dynamic behavior, including actuator faults, will be identical to that of the system without faults. It is possible to reduce the residual vibrations caused by the attitude dynamics and actuator faults by simultaneously activating the strain rate feedback (SRF) vibration control algorithm during the maneuver. The performance of the proposed integral fault-tolerant control in terms of the flexible modes excitation, the control effort, and achieving the desired attitude parameters in a comparative study demonstrated its advantage and superiority over the conventional integral Sliding mode algorithms.

In this article two dimensional Sliding contact of a rigid cylindrical punch on a functionally graded (FG) semi-infinite medium is studied. The modulus of elasticity in the graded layer is calculated based on TTO model approximation. This model defines a parameter q which considers the microstructural interactions between the constituting phases. The governing equations are solved by finite difference (FD) method by means of MATLAB software. The effects of different parameters such as non-homogeneity, q, the dimensions of the punch, the thickness of the graded layer and the coefficient of friction are investigated on the force-displacement diagram of the punch, stress distribution and modes I and II stress intensify factors (KI and KII). The results show that the contact force and the stress distribution under the punch are affected by the thickness of the graded layer, radius of the punch and the non-homogeneity coefficient. But the variation of q has no effects on the contact force and the stress distribution under the punch. In the absence of the friction, KI is always non-positive and the crack has no tendency to grow under this mode. Increasing the friction coefficient, increases KI but decreases KII.