Dynamic response of a stiffened laminated sandwich composite plate under blast load

In this study, effectiveness of the damage resistance of sandwich composite plates against explosive blasts is determined. Composite manufacturing techniques and material properties are researched, data obtained from literature used in creating finite element models and modal tests for composite plates were conducted. Four layered carbon composite plate and polypropylene sandwich materials are produced in order to use for blast loading tests. The manufacturing process is made in Istanbul Technical University Faculty of Aeronautics and Astronautics Laboratory. Carbon fiber roll is cut by composite cutter tool. An aluminum plate is used as mold after sanded. Mold release wax is applied on aluminum plate. The cut square carbon fabric is placed on waxed aluminum. Double sided tape is applied on the sides of aluminum plate to fixate vacuum bag. Peel-ply, mesh and vacuum bag and vacuum tubes placed on plate. Epoxy resin and hardener are combined and applied on carbon fiber. Polypropylene and carbon fabric materials was used for the production of stiffened sandwich composite elements. For the process of producing sandwich material, manual lay-up method was preferred. With this method, two separate reinforcements were produced. The first stiffener element is produced in the form of a 1-layer carbon, a polypropylene honeycomb and a 3-layer carbon fabric, respectively, from top to bottom. The second stiffener element is made of 2-layer carbon, polypropylene comb and 2-layer carbon fabric. Modal analysis of four layer carbon fiber composite material has done on ANSYS Mechanical APDL. A 4-layer carbon fiber composite plate model was created. For this purpose Shell 281 elements are used. The plate modal frequencies obtained from the finite element software are given as table. The mode shapes obtained from the program are also shown. In the stiffened sandwich composite plate model, the carbon plate in the upper and lower layers is modeled with Shell 281 element. The Solid 186 model is used in the model of the polypropylene sandwich element in the stiffener. Single stiffener, two stiffener and three stiffener plates are modeled for use in the works. 2-layer carbon, polypropylene comb and 2-layer carbon stiffened plate mode shapes and frequencies are given. In order to determine the composite plate's dynamic response for blast loading, series of blast pressure experiments were conducted. To obtain blast pressure, air was pressured with a compressor to 500 litre capacity storage unit. Pressured air was sent to a hose through a regulator. The output of hose is connected to shock tube. The shock tube is a tube sealed with some kind of membrane to contain compressed air to intended pressure value. The blast load was obtained form the pressured air inside the shock tube. LQ125 pressure transducers are used for obtaining pressure data as electrical signals. Power units provide sufficient energy to transducers. Also strain gages placed on the plate for obtaining strain data. The VXI EX1629 data analysing system was connected to strain gages and transducers. 30x30 cm carbon composite plate was experimented with single, double and triple reinforcement components. 1-layer carbon, a polypropylene honeycomb and a 3-layer carbon fabric stiffener was used in these experiments. Also unreinforced composite plate was used for confrimation of results. Three pressure transducers placed in front of the plate and three strain gages placed back side. Four different experiments were conducted. The resultant data with graphics are presented. The effects of material and reinforcements on the dynamic behaviour are investigated. These experiments were performed without activating the filter option of the data acquisition device. New experiments were carried out by activating the filter option of the data acquisition device. Experiments were carried out on non-stiffened and single stiffened plates. 2-layer carbon, polypropylene comb and 2-layer carbon fabric stiffener element used in these experiments. Various filter values have been tested in order to ensure that the pressure data can be analyzed without noise. It was decided to analyze the experiment with 200 Hz filter. The obtained pressure data was tried to be approximated to the pressure function. The pressure approximation function was then analyzed by applying it to models created in ANSYS software. For analysis in ANSYS, Shell 281 element is used for carbon and Solid 186 is used for polypropylene sandwich. Rayleigh type damping coefficients were used during the pressure analyzes. Comparison of experiment and analysis data for non-stiffened plate and single stiffened plate is given. Analysis results are compatible with the maximum and minimum strains in the experiment. Parametric study has been carried out in ANSYS software to examine the various strain values that can be formed by using the reinforcement element in different configurations. In the single reinforced plate, the analysis was performed to measure the strain change by the number of carbon plates at the top and bottom of the reinforcing element. From the data it is observed that as the number of layers on the reinforcement element increases, the strain value decreases. A rigid sandwich composite plate with dimensions 29.5x29.5 cm was used for finding a pressure function to be used in subsequent studies. For obtaining pressure gradient optimally, five pressure sensors were placed on a rigid plate and the pressure values were examined experimentally. The sensors are placed on the line of the plate diagonal so that the pressure gradient can be obtained in an optimal manner while some of the available sensors were placed at an equal distance from the origin for verification. In these experiments, the data of the sensors used for verification show a good fit. A general analytical pressure function, which can be applied to all pressure values, has been derived from the data obtained as a result of the experiments. This pressure function is expressed in terms of point values by a Lagrange interpolation polynomial. In conclusion, dynamic response of sandwich composite plate under blast load was examined. Experimental results of reinforced composite plates are compared with numerical results. A new pressure equation for blast loading was found.