MATERIAL CHARACTERIZATION OF COMPOSITES FOR A VERTICAL WIND TUBRINE

An experimental investigation was conducted to evaluate the performance of composite materials for a vertical wind turbine. The materials used were polyester resins with fiberglass reinforcement, corrugated aluminum core composite, and a polypropylene honeycomb core composite. Quasi-static tensile experiments were conducted using an Instron 5585 and following ASTM Standard D638. Tensile Modulus, Tensile Strength and other material characteristics were calculated using digital image correlation data acquisition and MATLAB. Blast experiments were conducted on the materials using a shock tube apparatus to investigate the dynamic response and performance. The 6.35mm Polyester Resin composite had the lowest deflection when normalized with thickness. This material had the highest tensile modulus and yield strength as well showing that of the materials tested it is the optimal choice for the wind turbine.


Preface
This thesis is a prepared in manuscript format. Section 1 is a introduction for the read to understand the background behind and previous research in the characterization of composite materials. This is given in order conceptional grasp the di↵eneces between composite of varying material and even those made of the same material. Section 2 highlights the basic material properties as well as the experimental apparatuses used. This section include initial properties observed of the tested material and a brief description of the procedure for tensile testing using an Instron and blast loading using a shock tube. Section 3 outlines the results of tensile test to determine quasi-static properties.
The section also outline the results the blast loading experiment for each material. Section 4 proposes the overall conclusion of the thesis. iv Contents Abstract ii Acknowledgments iii Preface iv

Introduction
Impact response and mechanical behavior of honeycomb sandwiches and polypropylene honeycomb cores have been conducted within the past two decades. Honeycomb cores can be used in aircraft construction, as well as in a system trying to reduce weight while achieving similar structural strength [1]. The cores and faces are bonded to create a structure designed to handle tension, transverse shearing, compression and lateral stability while having a low density [2]. Polypropylene honeycomb cores have high strength to weight ratio and are also weather resistant [1]. Both characteristics make it a viable option for the vertical wind turbine. The compressive behavior of honeycomb cores is relatable to the density in that strength increases as density increases [2].
Mechanical properties of natural reinforced fiber composites were investigated by Gopinath et al., which can be used to understand the di↵erences between varying glass fibre composites. Two composites were studied, jute fibre with polyester and jute fibre with epoxy. During tensile testing using ASTM D3039 standards were found to have Youngs Moduli of 0.811 GPa and 1.064 GPa respectively. [3]. The composites are found to be 83 and 65 times weaker than aluminum. When compared to di↵erent variations of jute fibre composite, chemically treated with 5% or 10%, the tensile strength was greater for the 5% by 16% and for polyester and resin compared to the 10%. Gopinath et al. found that these composites can be applied in the automotive field.
An investigation of the dynamic mechanical properties of glass/bamboo fiber reinforced unsaturated polyester resin composites was done in 2011. The investigation used a dynamic mechanical analyser, scanning electron microscope, to determine three parameters. The parameters were storage modulus, loss modulus and mechanical dampening. The investigation compared the parameters to the composition of glass/bamboo fibers. Pure resin had a lower storage modulus than glass/bamboo fiber composite. Loss modulus was seen to decrease with an increase in bamboo fibers [4].
The mechanical performance of biofibre/glass reinforced polyester hybrid composite was studied at Ravenshaw College [5]. The composite was a matrix of pineapple leaf fibres (PALF), sisal fibres and glass fibres of varying weight percentage wt. %.
The tensile test conducted met ASTM -D638 standard [6]. The total fibre content of the composite was kept equal to 25% while the glass fibre portion varied from 0% to 12.9%. Glass fibre wt.% of approximately 8.6% increase the ultimate tensile strength by 66% but at wt. %of 12.9% the composite lost tensile strength by 10%. Mishra et al found that at low wt.% the increase strength was due to the sisal fibre transferring load from the glass fibre. When the wt.% exceeds the amount that the sisal fibre can transfer loading out of the strength decreases.
Blicblau et al. at the Swinborne University of Technology in Australia experimented on raw wool and polyester resin composites. The experiment conducted used specimen of varying mass fractions from 0% to 55%. The tensile test was conducted with 36 specimen that were conditioned at 22 degrees Celsius for 24 hours. All tests were conducted on the Instron 1114 and showed that an increase in mass fraction yielded little change in strength. The tensile strength for 0% is 33.9 MPa while a mass fraction of 55% is 41.9 MPa. The modulus of elasticity for the two percentages was 0.9 MPa and 2.8 MPa respectively [7] The use of natural fibre may be environmentally friendly however, the reduction of strength does not seem to be a fair trade-o↵.
In 2011 a study was done on the impact response of polypropylene honeycomb cores without fiber metal laminate faces. The impact response was determined from low velocity impact tests using a drop tower. It was determined that low impact would only indent the specimen while the higher impact energies created delamination as well as core crushing and bending. There was also a range between the upper and lower limits of impact energy where energy absorption increases with an increase in energy [1].
Encore in Leuven Belgium developed a production technology to manufacture high-end honeycomb structures at a lower cost. Previously honeycomb structures were primarily limited to aerospace applications but with changes in production technology the cost e cient applications can now include packaging, building and construction as well as automotive and much more. A redesign of the currently used twin walled corrugated composites was necessary due to the lack of mechanical strength in the transverse extrusion direction. The initial redesign was cup-shaped bubble cores, which only perform well at thickness of 3-5 mm. During the production process the material is stretched unevenly which result in negative e↵ect on performance.     Table 1

Material Characterization
Quasi-static tensile experiments were conducted using an Instron 5585 and following ASTM Standard D638. Tensile experiments were conducted for the following materials: LR, PT and PF. 2D DIC was utilized to obtain the strain data during the material characterization experiments.

Tensile Test
Quasi-static tensile experiments were conducted using an Instron 5585 and following ASTM Standard D638. Tensile experiments were conducted for the following materials: LR, PT and PF. The specimen were machined according to the ASTM D638 and clamped in the Instron. The testing speed was set to 5 mm/min and the data was recorded using the Instron load cells. One camera was setup for 2D DIC in order to get strain data.

Shock Tube Facility
A shock tube apparatus was used to generate a concentrated shockwave which provides a dynamic load on the composite specimens. The shock tube is 8 m in length and is composed of four separate sections: driver section, driven section, converging conical section, and a 38 mm diameter muzzle. A schematic of the shock tube is shown in Figure 2. The driver and driven sections of the shock tube are separated by a Mylar diaphragm, allowing for the pressurization of the driver section. When a critical pressure is reached, the diaphragm bursts, and the high pressure propagates down the length of the shock tube. The high pressure shock waves becomes a planar shock front and loads the specimen on the muzzle end.
When the shockwave meets the specimen, the shock wave is compressed and reflected back into the shock tube. The load that the specimen encounters is the

Tensile Test
The Alumicore and Honeycomb Polypropylene composites are not in conformance to any ASTM standard due to the fact that they are sandwich structures, with facesheets and a core, which are bonded together by some adhesive. This results in a modulus that is a lumped parameter of both of these features which does not adhere to the test standard. Material properties obtained during the tensile test can be seen in

Blast Loading
The requirement of the wind turbine is a material that can withstand a wind load of 0.0015 MPa. However, the minimum blast load achievable in the shock tube is 0.6 MPa (500 times the requirement). As shown in Table 1 at 500 times the required load, all materials, except for the Alumicore meet, the maximum deflection requirement.
The pressure profiles of each material can be seen in Figures 4,5,6,7,8and 9 below.
The deflection data was normalized using the thickness of each material in order to accurately compare the results.

Alumicore
Alumicore deflects at the groove interfaces, and depending on side of impact, the corrugation causes a greater reduction of deflection. Figure 12 shows that normalized deflection of both when both sides are tested. The material strength would be dependent upon which direction the blast load occurs from.

Polypropylene Honeycomb
The Polypropylene Honeycomb also has visible damage and delamination of outer layer. For the first test using the Honeycomb Polypropylene the recording software misfired and specimen was then blast loading 5 additional times to correct the software issue. This pre-damaged specimen deflected less than an inch after 5 blast loads.
Shown in Figure 13, the polypropylene honeycomb experienced less than 18 mm of out-of-plane displacement in the highest damaged induced test.

Laminate Resin Veil
The Laminate Veil had inconsistent thickness that contributed to the 13% error between tests. The Laminate Veil also had no visible damage.

Safe Plank
Due to the manufacturing process of the Safe Plank resulting in raised edges, seen in Figure 15, in the testing material this halted the shock tube from be flush with the specimen introducing error in the results. The Safe plank is the same material as the both polyester resin samples however, the di↵erence is in how it was provided for testing.

Conclusion
The quasi-static and dynamic behaviors of the composite materials considered for the wind turbine application were determined under uniaxial tension and blast loading.
In order to determine the quasi-static tensile behavior, the specimens were place in the