CHAPTER 3 EXPERIMENTAL INVESTIGATION

Transcription

1 66 CHAPTER 3 EXPERIMENTAL INVESTIGATION 3.1 PREAMBLE In this section, the preparation of an FRP Honeycomb core sandwich panel with different cell sizes, and the experimental methods adopted for assessing the mechanical characteristics, are explained. They are broadly as under: Manufacture of FRP Facings Manufacture of FRP Honeycomb core Manufacture of Sandwich panels Testing of facing sheets Static flexural and compression testing of Sandwich panels Low velocity Impact testing of Sandwich panels Dynamic testing of Sandwich panels 3.2 FLOW CHART OF THE RESEARCH E-glass fabrics, epoxy thermosetting resin based honeycomb core and facesheets were used to fabricate composite sandwich panels. The reinforcement constituents of the composite facesheets and core are E-glass 0 /90 fabrics, and LY 556 epoxy resin with HY 951 hardener used as the

2 67 matrix material. The core material having regular hexagonal cell configuration (with four different cell sizes of 8, 16, 20 and 25mm) was used in the fabrication of the composite. Dissertation on The Static, Impact and Vibration Behaviour of FRP Honeycomb Core Sandwich Composites Introduction, Literature survey & Problem definition 1.0 & 2.0 Experimental Investigation - Manufacture of Sandwich panels with different cell sizes - Physical & mechanical characterization of Face sheet - Flexural testing of sandwich panels - Compression testing of sandwich panels - Low velocity Impact testing of sandwich panels - Experimental Modal analysis of sandwich panels under different boundary conditions 3.0 Analysis of Static, Impact and vibration behaviour of Sandwich panels 4.0 Analysis of Static Behaviour 4.1 Analysis of Impact Behaviour 4.2 Analysis of vibration Behaviour 4.3 Correlation between cell size and core density, elastic constants, compressive strength, shear strength, stiffness, of Sandwich panels Correlation between cell size and low velocity impact strength of Sandwich panels Correlation between cell size and Modal parameters of Sandwich panels under different boundary conditions Evolution of design criteria of sandwich panels Conclusions 5.0 Figure 3.1 Flow chart of the research work

3 68 The physical and mechanical properties of the facesheets have been determined as per the appropriate ASTM standards. To assess the static behaviour of the sandwich panels compression, Flexural 3-Point and 4-Point tests have been carried out as per the guidelines of the ASTM standard to determine the elastic constants of the core, such as the compression and shear modulus, as well as on the compression and shear strength of the core. To assess the low velocity impact behavior of the sandwich panels under falling weight, impact tests have been carried out at different incident energy levels. To assess the dynamic behaviour of the sandwich panels, experimental modal analyses under different constraint conditions have been carried out. The details of the tests are provided in the respective sub-sections. Figure 3.1 shows the flow chart of the research work carried out. 3.3 DEVELOPMENT OF THE SANDWICH COMPOSITE The fabrication is carried out through the vacuum bag molding technique, in which the refinement of the hand lay-up, uses the vacuum to eliminate the entrapped air and excess resin. The process flow chart for the manufacturing of FRP composites is shown in Figure 3.2. The Vacuum bag hand lay-up process offers many benefits, when compared to the conventional hand lay-up techniques. As it is a closed molding process, it virtually eliminates potentially harmful volatile organic compound (VOC) emissions. It also allows unlimited set-up time, because the resin system is not introduced until all the reinforcements and core materials are in place. This method is of particular benefit, when producing large facings, due to the weight savings that can be gained, greater structural strength and efficiency gains against the Hand Lay-up process. The vacuum system also facilitates good resin distribution and consolidation of layers of the laminate. As a result, the resulting mechanical properties of the Facings are likely to be markedly

4 69 higher, than would be the case with hand laminating (Malin Akermo et al 1999). Epoxy E-glass fabric Core (8, 16, 20 & 25mm) Lamination of face sheet and core by wet lay up Vacuum Bagging Post Curing (2 hours at 100 o C) Curing (1 day at room temperature) Figure 3.2 Process flow chart for the manufacturing of sandwich composites Manufacture of face sheet Bi-woven glass E cloth, which is commercially available is used for making the face sheet and is shown in Figure 3.3. The cloth ply was trimmed to the correct size and impregnated in an adhesive made from a mixture of LY556 epoxy resin and HY 951 hardener, mixed in the ratio of 100:10. The ply was stacked in 0 /90 orientation and was built to a thickness of around 2.0 mm. The Vacuum hand lay up technique was used to make the facings and is shown in Figure 3.4. A Vacuum level of 450Hg/mm² is maintained for 1 hour to avoid surface undulations and also to avoid air pockets at the interface. The coupons were allowed to cure for about 24 hours

5 70 at room temperature. The glass and resin content in the Facings was respectively around 61.5 % and 38.5 %. Figure 3.3 Bi-woven glass E fabric Figure 3.4 Manufacture of Face sheet using Vacuum hand lay-up technique Manufacture of Honeycomb core For the manufacturing of the honeycomb core the matrix used is epoxy resin LY 556 mixed with a hardener HY 951 and the reinforcement is glass E fabric. The resin and hardener are mixed in the weight ratio of 10:1.

6 71 To maintain the optimum strength, the resin glass ratio is found to be 35:65. The molding tool used is hexagonally machined split molding tool made of chromium plated mild steel, and is shown in Figure 3.5 (a). After ensuring that the surface is clean and free from foreign particles the application of release agents is done. (a) Split Moulding Tool (b) Wet laminate under load (c) Half honeycomb core (d) Joining of two halves of the core Figure 3.5 Fabrication of the honeycomb core

7 72 Figure 3.6 Honeycomb core A coat of resin mixture is applied on the molding surface and the plain weave glass E fabric is impregnated against the first half of the molding tool surface, by ensuring a thorough wetting of the glass fabric. Then the hexagonal mandrel is placed in the respective slots by pushing the glass cloth downwards into the half hexagonal slot of the molding tool. The Load is applied onto the wet laid-up laminate in order to improve its consolidation, as shown in Figure 3.5 (b). This is achieved by sealing the wet laid up laminate with a Peel-ply, perforated plastic film, and placing the breather over the perforated plastic film. Above this, a bag with a Vacuum Valve (returnable) is placed and sealed, which constitutes the vacuum bagging process. At one corner of the bag a port for the vacuum is arranged and subjected to Hg/mm 2 of pressure for 120 minutes to consolidate and to increase the inter laminar shear strength of the layers. After ambient curing, the laminate which takes the shape of half-hexagon as shown in Figure 3.5 (c) is taken from the molding tool, and a similar fabrication of the second half is done. The two halves are joined together by placing the epoxy resin putty on the contact surface, and allowing it to dry as shown in Figure 3.5 (d). Then it is cut to the required thickness to form the hexagonal honeycomb core. Figure 3.6 shows the honeycomb core.

8 Manufacture of Sandwich panels For the fabrication of the sandwich panel, the facings comprising of the glass E fabric impregnated with the above said resin mixture, are coupled with the open honeycomb structure using epoxy resin, compacted by means of the vacuum bagging technique. After curing, the sandwich panel is subjected to post curing in a hot-air oven at 100 o C up to 2 hours. The vacuum method provides higher reinforcement concentrations, better adhesion between layers, and more control over the resin/glass ratio compared to the hand lay-up process. Figure 3.7 shows the honeycomb sandwich panel under vaccum. Figure 3.7 Fabrication of honeycomb sandwich panel

9 74 Four types of sandwich panels of size 500 x 500mm are fabricated with different cell sizes, i.e., 8, 12, 16 and 20 mm. The cell shape of the honeycomb core is a regular hexagon. The membrane wall thickness of the core is 0.2 mm and the height of the core is fixed at 8 mm. The thickness of the top and bottom face sheets has been kept constant at 1 mm The cell sizes were selected, based on the ease of manufacture and testing requirements, stipulated by the relevant standards. A Cell size below 8 mm is extremely difficult, because the FRP sheets cannot be bent around the mould. A Panel with a cell size higher than 25 mm poses severe restrictions on flexural testing, as the width of the specimens become too unwieldy. 3.4 CHARACTERIZATION OF THE FACE SHEET A thorough understanding of the physical and mechanical properties of a material is paramount to determine whether the material is suitable for a specific application. Hence, the face sheet must be characterized and understood to determine how the selected material caters to the manufacturing of the sandwich panels. The physical and mechanical characteristics of the face sheet used for the fabrication of the sandwich panels are tested as per the ASTM standards Tensile test A Tensile test was conducted on the face sheet to determine the tensile strength and modulus of the composite face sheets, as per the ASTM D 3039M-08. The Test specimens were sectioned from the composite panels with the width of 25 mm, thickness of 2 mm and length of 220 mm. The Test specimens were bonded with composite tabs of 4 in length at both the ends. The tabs distribute the gripping stresses and prevent specimen failure caused by grip jaws. As the face sheet exhibits a similar behaviour for 0 and 90

10 75 directions, only one direction is tested. The specimens were tested using the universal test machine (Shimadzu) as shown in Figure 3.8. The test were conducted at a cross head speed of 2 mm/min. Figure 3.8 Tensile test setup for face sheet The tensile strength ( uts ) was calculated by using the Equation 3.1. uts = F/A (3.1) where F is the breaking load in N and A is the cross sectional area of the specimen in mm Compression test The Compression test method, according to ASTM D3410M-03 was used to measure the compressive strength and modulus values of the composite face sheet panels. For this purpose, compression test specimens

11 76 were cut from larger face sheet panels and tests along the ply-layup directions were performed using the universal test machine (Shimadzu) at a crosshead speed of 1.3 mm/min. The compressive stress was calculated by dividing the load with the cross-sectional area of the specimens. The modulus was estimated from the slope of the stress - strain curve Flexural test The flexural test method according to the ASTM D was used to determine the flexural strength and modulus of the composites. For this purpose, test specimens 20 mm in width, 2 mm in height and 160 mm in length, were sectioned from the face sheet panels using a diamond saw. Specimens were tested in a 3-point bending apparatus under loading, both in the longitudinal and tranverse directions of the fibers. The composite face sheets were tested using the Schimadzu universal test machine at a crosshead speed of 1.2 mm/min. During the test, the load versus central deflection was recorded, and the slope P/ w and the elastic modulus were determined. 3.5 TESTING OF SANDWICH PANELS While considering a sandwich construction for an application, care must be given to the directional mechanical properties, and the design must ensure that the best advantage of this attribute is incorporated. The biggest advantage of a sandwich construction is that it possesses high stiffness at very low weights. However, the stiffness behavior must be thoroughly understood, so as to make a comprehensive computation of the deflection arising out of the shear deflection because of low core densities as well as bending deflection. The panel can fail in any one of several different ways, depending upon the geometric and fabrication characteristics of the panel, and how it is

12 77 loaded. For example, a face sheet can fail in tension, compression, shear or local buckling. Additionally, the core can fail in shear or by crushing. A face sheet can separate from the core due to excessive shear or normal tensile stress in the adhesive bond. Test methods have been developed to isolate and simulate each of these specific failure modes Flexural test of Sandwich panels Flexural tests were carried out as per the ASTM C393M-06, to characterize the flexural properties of the sandwich composites. In accordance with this standard, a series of 3-point bending, 4-point bending and short beam shear strength (SBSS) tests are conducted. The three, four bending and SBSS bending test setups are shown in Figures 3.9 a, b, c. This test method was used to determine the flexural stiffness parameter (D), the core shear modulus (G) and the core shear strength ( core ). The load-displacement curves were plotted for all the test conditions. The SBSS test setup is similar to the 3-point bend test except the use of specimen span length. The Test parameters for the flexural testing of the sandwich composite specimens are given in Table 3.1. Table 3.1 Test parameters for the flexural testing of the sandwich composite specimens Type of bending test Span length Width Thickness (mm) (mm) (mm) Three-point Four-point SBSS

13 78 (a) (b) Figure 3.9 (c) Flexural test setup for (a) Three-point bending (b) Fourpoint bending (c) SBSS test By means of the flexure test, load can be applied to the specimen to produce constant or variable bending moments, and constant or variable shears. The concentrated load for all the specimens is applied by the movable cross head, gradually at the cross head displacement rate of 2 mm/min. The load is applied on the specimen till fracture and the maximum load at fracture

14 79 has been recorded for each case. Load - deflection curves are also recorded using a deflectometer placed at the mid-span length of the specimen. The slope of the load versus deflection curve is determined for each specimen. The following equations are used from the ASTM standard C393M-06 to compute D and G. For a point load at the mid-span of a simply supported beam, the overall deflection including bending and shear is: P1 L1 1-11L 2 /8L1 D = (3.2) P L /P L G = P L c 8L /11L b d+c 16P L /11P L -1 (3.3) where, P slope of the load deflection curve in three point bending P slope of the load deflection curve in four point bending L 1 - span length in three point bending test (160 mm) L 2 - span length in four point bending test (200 mm) The shear strength of the core( core ) and the bending stress ( facing ) of the face sheet are computed using the equation (3.4). core P = ; facing = 2bc PL 2t d+c b (3.4)

15 Flatwise compressive test of Sandwich core Flatwise compression tests of sandwich panels are conducted in accordance with the ASTM C365M-11 standard to determine the flatwis e compressive strength and modulus. In order to prevent local crushing at the edges of the honeycomb cores, the edges were stabilized with thin facings, so that the load which causes failure in the core does not cause any damage to the facings. The Setup used for compression tests is shown in Figure Figure 3.10 Flatwise compression test setup loaded with specimen A constant crosshead movement rate is maintained at 0.5 mm/min, as suggested by the ASTM standard. Specimens in this study have the dimensions of mm for length, width and height respectively, in accordance with the ASTM C365M-11. The Load-displacement data is obtained from the tests, and is used to carry out the calculations for the modulus and compressive strength.

16 81 core P Face sheet area (3.5) E p c b L (3.6) sheet in mm 2. Where, C is the core height in mm and b x L is the area of the face Low velocity impact test of the Sandwich panel Low-velocity impact tests were carried out at different energy levels on honeycomb sandwich panels, using an instrumented falling weight apparatus to obtain information about the absorbed energy and maximum impact force. Indigenously developed instrumented low velocity impact test equipment was employed to perform the non-penetrating impact test. The maximum impact energy is limited by suitably adjusting the falling height and mass. The mass together with the height of the drop determines the energy of the impact. With an increase in the mass and height of the fall, the potential energy is converted to kinetic energy. The Instrumented falling weight impact testing machine is shown in Figure 3.11 (a). In accordance with the ASTM D the impact test is performed by sticking the specimen at the centre by a flat square dart. The square dart is made of mild steel, and sized 25mm 25mm. Figure 3.11 (b) shows the specimen clamping apparatus having a fixture with a square slot of 100mm. This is specifically designed in order to assure the consistency of the clamping force through the pre-loading of the four helical springs. The

17 82 vertical guides of the impact are power lubricated frequently to minimize any friction generated during the descent of the impactor. This machine is capable of impacting samples at energies of up to 140J. For this test, samples of the size mm were impacted. Table 3.2 lists the impact test parameters which give the range of the drop height and impact mass to achieve the required impact energy. An inbuilt data acquisition system along with an impact software was used to monitor the position and acceleration of the impactor. The incident energy is calculated based on the height history, while the dissipation of energy was derived from both the acceleration and height histories of the impactor, assuming rigid body motion. Tests were conducted under different impact energies, ranging from 7 to 50 J. Table 3.2 Impact Test Parameters S.No Drop height mm Drop mass kg Impact energy J

18 83 (a) Instrumented falling weight impact testing machine (b) Specimen Clamping Fixture Figure 3.11 Low velocity impact test setup

19 Vibration testing of the Sandwich panel The vibrational characteristics of the specimens are obtained by studying their impulse response. Assessing the modal characteristics of sandwich panels is very essential for design and manufacturing. The tests are carried out for two boundary conditions C-F-F-F (One end Clamped) and C- F-C-F (Two ends Clamped). In the cantilever (C-F-F-F) analysis, the honeycomb sandwich panel is clamped at one end using a suitable fixture, and the impact test is carried out. The accelerometer is placed at one of the corner nodes, while the hammer is made to impact at multiple nodes. For the Fixed- Fixed (C-F-C-F) analysis, the honeycomb sandwich panel is clamped at two opposite ends using a suitable fixture, and the impact test is carried out. The accelerometer is placed at one of the corner nodes, while the hammer is made to impact at multiple nodes. The traditional strike method is used to measure the vibration properties. The modal test setup is shown in Figure Figure 3.12 Modal test setup

20 85 The specimens are subjected to impulses through a hard tipped hammer which is provided with a force transducer with a sensitivity of 2.25mV/N and the response is measured through the accelerometer with a sensitivity of 10 mv/g. The hammer consists of an integral ICP quartz force sensor mounted on the striking end of the hammer head. The striking end of the hammer has a threaded hole for the installation of a variety of impact tips. The tip functions to transfer the force of the impact to the sensor and protects the sensor face from damage. The impulse and the response are processed on a computer aided fast fourier transformer (FFT) analyzer test system (LMS Inc.) in order to extract the modal parameters with the help of built in software (SMARTOFFICE). The types of specimens investigated in this study are in the form of plates. The specimens are cut with a nominal length of 170 mm, breadth of 150 mm and thickness of 10mm. In the total length of 170mm, 20mm were used for fixing the specimen to obtain a cantilever condition, thus maintaining the effective test dimension as 150 x 150 mm, as shown in Figure 3.13 a. Similarly, another specimen was prepared for the C-F-C-F condition, and is shown in Figure 3.13 b. The sandwich specimen is subjected to impulses at 25 station locations. The frequency response curves are obtained for various grid points located on the specimen.

21 86 (a) C-F-F-F condition (b) C-F-C-F Condition Figure 3.13 Specimens attached to the fixture for simulating the C-F-F- F and C-F-C-F condition

22 87 The FFT spectrum analyzer samples the time varying input signal, computes the magnitude of its sine and cosine components, and displays the spectrum of these measured frequency components. The FFT analyzer used consisted of 8 channels for input and output which helps to plot the spectrum with a linear, logarithmic, or db amplitude scale and a linear or logarithmic frequency scale. The FFT analyzer also helps in obtaining the frequency response function (FRF), through which the dynamic behaviour of a structure is determined. The Frequency Response Function (FRF) is a fundamental measurement that isolates the inherent dynamic properties of a mechanical structure. Experimental modal parameters (frequency, damping, and mode shape) are also obtained from a set of FRF measurements. FRF is defined as the ratio of the Fourier transform of an output response (X (w)) divided by the Fourier transform of the input force (F (w)) that caused the output.