Effect of Temperature on the dynamic performance of the core material, face-sheets and the sandwich composite

The performance of sandwich structures is highly affected by the varying environmental temperature during service, especially when they are subjected to blast loading. Typically, sandwich panels consist of polymer based composites (face-sheets) and polymer foams (core material), and the properties of its components change substantially under different temperatures. Dynamic properties and failure mechanisms of composite structures under blast loading can be greatly influenced by exposure to different temperature. In this paper, high strain rate constitutive behavior of E-glass Vinyl ester composites and Corecell M100 foam at different target temperatures has been studied. The five different target temperatures chosen were 25°C, 40°C, 60°C, 80°C and 100°C. The results show a significant decrease in flow stress and a substantial change in the densification strain value with the increase in the temperature of the specimen of core material. High fiber-matrix delamination was observed in face-sheets at elevated ambient temperatures with no measurable change in the value of flow-stress. INTRODUCTION Composite materials have important applications in the marine and aerospace industry. Marine structures undergo mechanical loadings under varying ambient temperatures especially when operating in extreme environments such as the Arctic Ocean and gulf areas. The environmental temperature has an adverse effect on the blast resistance of the structure. As a core material, polymeric foams are extensively used for energy absorption for high strain rate applications against ballistic impacts and blast waves [1-2]. Recent studies [3] observe significant and complex effects of environmental temperature on the dynamic compressive behavior of syntactic foams. Composite materials, such as sandwich structures, have important applications in ship structures due to their advantages, such as high strength/weight ratio and high stiffness/weight ratio. Tekalur [4] studied the dynamic behavior of sandwich structures with reinforced polymer foam cores. The most recent research only focuses on the blast resistance of various composite structures without elevating the temperature. The numerical study on the response of a large aspect ratio sandwich panel subjected to the elevated temperatures on one of the surfaces shows that the maximum deflection of the panel increases with the increase in surface temperature [5]. These studies investigate the performance of sandwich panel under static loading conditions only. Dutta [6] tested the energy absorption and brittleness of graphite/epoxy composites at room and low temperature under low velocity impact. Similarly, Erickson et al. [7] performed low-velocity experiments on sandwich composites and both found that the temperature can have a significant effect on the energy absorbed and the peak force endured by the specimen. To the best of the author’s knowledge, there are no results of the blast performance of composite structures under different temperatures. An in-depth analysis on the effects of temperature is needed. The present paper experimentally studies the dynamic behavior of Corecell M100 foam core and E-glass Vinyl Ester composite face-sheets under high temperature environments using a Split Hopkinson Bar apparatus. A controlled temperature environment was designed in order to achieve the target temperatures. The shock-tube facility will be used to study the dynamic behavior of sandwich composites under blast loading. A special fixture will be designed to heat the sandwich specimen to different temperatures. A high-speed photography system with three cameras will be utilized to capture real-time motion images. Digital Image Correlation (DIC) techniques will be utilized to obtain the details of the deformation of the sandwich panels during the events. Post mortem visual observations of the test samples will provide more evidence to indentify the failure modes. These results will be used to analyze the mechanism of dynamic failure of the sandwich composites subjected to elevated temperature Proceedings of the IMPLAST 2010 Conference October 12-14 2010 Providence, Rhode Island USA © 2010 Society for Experimental Mechanics, Inc. environment. The shock tube experiments are currently in progress and will be discussed in detail during presentation. 2. MATERIAL AND SPECIMEN 2.1 Split Hopkinson Bar Specimen: Cylindrical samples were cut from the foam material using a die. The first batch had a diameter of 15.5 mm and a thickness of 6.4 mm. The second batch had a diameter of 11.5 mm and a thickness of 3.8 mm. Sample dimensions were chosen carefully to achieve the desired strain rate and obtain uniform deformation while minimizing inertia effects. For face-sheets, cylindrical specimens with a thickness of 3.18 mm and a diameter of 10.16 mm were used. 2.2 Sandwich Specimen: The skin material that was utilized in this study is E-Glass Vinyl Ester (EVE) composite. The woven roving E-glass fibers of the skin material were placed in a quasi-isotropic layout [0/45/90/-45]s. The fibers were made of the 0.61 kg/m area density plain weave. The resin system used was Ashland Derakane Momentum 8084 and the front skin and the back skin consisted of identical layup and materials. The core material used in the present study was Corecell M100 styrene foam, which was manufactured by Gurit SP Technologies specifically for high temperature marine applications. Table 1 list important the material properties of this foam from the manufacturer’s data [9]. Foam Type Nominal Density (kg/m) Compression Modulus (MPa) Shear Elongation (%) Corecell M100 107.5 107 52% The VARTM procedure was carried out to fabricate the sandwich composite panels. The overall dimensions for the specimen were 102 mm wide, 254 mm long and 33 mm thick. The foam core itself was 25.4 mm thick, while the skin thickness was 3.8 mm. The average areal density of the samples was 16.66 kg/m. Fig. 1 shows a real image of a specimen and its dimensions. 3. EXPERIMENT SETUP AND PROCEDURE 3.1 Quasi-static Characterization: The quasi-static compression tests were performed using a standard compression test machine (Instron Model 5582). The tests were performed following the ASTM D 1621 – 04a standard [8] using rectangular specimens (50.8 mm×50.8 mm, 19.1 mm thick) at a crosshead speed of 1 mm/min for M100 foam. Table1. Material properties of the foam core [9] Fig. 2 Shock tube apparatus Shock tube Muzzle and Specimen Fig. 1 Real specimen and its dimensions 254 mm 33 mm 102 mm 3.2 Split Hopkinson Bar Apparatus: Split Hopkinson Pressure Bar setup was used to test M100 foam at high strain rates of deformation ranging from 4000 to 6000/s. Due to the low-impedance of Corecell foam materials, dynamic experiments for the core materials were performed with a modified SHPB setup. A hollow incident bar was used to increase the strain rate to achieve high end-strain values and a hollow transmitted bar was used to increase the transmitted signal intensity. It had a 50.8 mm long steel striker, 1828.8 mm-long incident bar and 1447 mm-long transmitted bar. Incident and transmitted bars were made of a 6061 aluminum alloy. The nominal outer and inside diameters of the both hollow incident/transmitted bar were 19.05 mm and 16.51 mm respectively. At each end of the hollow bars, end caps made of the same material were pressure fitted and pinned using aluminum pins. By applying lead pulse shapers, the effect of the end caps on the stress waves was minimized. The details of the analysis and derivation of equations for analysis of experimental data can be found in Chen’s paper [9]. 3.3 Heating Chamber for SHPB: Fig. 3 shows the schematic of the heating setup utilized for heating the specimen in SHPB. A small heating chamber (220mm x 130 mm, 170 mm high) made of wood was prepared and a standard resistance heating wire, Nickel-Chromium Alloy, 60% Ni / 16% Cr, was utilized to heat the chamber. An external DC power supply (0 – 30 V) with voltage controller was utilized to control the amount of heat supplied. A heat resistant borosilicate glass sheet was used as a transparent removable face for the chamber, which allows monitoring of the state of the specimen during heating. The chamber was calibrated using a series of experiments by supplying different voltages to the chamber and the saturation temperature value was estimated by using a K-type thermocouple. Fig 3: Heating Chamber for SHPB 3.4 Shock Tube: The shock tube apparatus will be utilized in present study to develop controlled blast loadings. The detail of this apparatus can be found in Ref [10]. Fig. 2 shows the shock tube apparatus with a detailed image of the muzzle. The final muzzle diameter is 76.2 mm. Two pressure transducers (PCB102A) are mounted at the end of the muzzle section with a distance 160 mm. The support fixtures ensure simply supported boundary conditions with a 152.4 mm span. Shock tube has an overall length of 8 m, consisting of a driver, driven and muzzle section. The high-pressure driver section and the low pressure driven section are separated by a diaphragm. By pressurizing the highpressure section, a pressure difference across the diaphragm is created. When this difference reaches a critical value, the diaphragms rupture. This rapid release of gas creates a shock wave, which travels down the tube to impart dynamic loading on the specimen. 3.5 High-Speed Photography Systems: Two high-speed photography systems will be utilized to capture the real-time 3-D deformation data of the specimen. Fig. 4 shows the experimental setup. It consists of a back-view 3-D Digital Image Correlation (DIC) system with two cameras and a side-view camera system with one camera. All cameras are Photron SA1 highspeed digital camera, which have an ability to capture images at a frame-rate of 20,000 fps with an image Incident bar Transmitted bar Specimen Heating Elements Circulation fan resolution of 512×512 pixels for one second time duration. These cameras are synchronized to make sure that the images and data can be correlated and compared. The 3-D DIC technique is one of the most recent non-contact methods for analyzing full-field shape, motion and deformation. Two cameras capture two images from different angles at the same time. By correlating these two images, one can obtain the three dimensional shape of the surface. Correlating this deformed shape to a reference (zero-load) shape gives full-field in-plane and out-of-plane deformations. To ensure good image quality, a speckle pattern with good contrast is put on the specimen prior to experiments. 3.6 Experimental Procedure for Shock tube: In the present study, the shock wave loading has an incident peak pressure of approximately 1 MPa and a wave velocity of approximately 1030 m/s. First, sandwich panels are subjected to blast loading under room temperature environment. The further heating experiments are performed on two target temperatures: 400C and 800C. 4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1 Quasi-static Results: Fig 5: Quasi-static compressive behavior of M100 foam core and face-sheets Shock tube