Composites: A Brief Description of their Structure and Mechanical Properties

Location:

Concepts Shown:

polymeric composite materials

Equipment:

The four panels that have been made and a sample of the unidirectional carbon fabric.

Procedure:

The panels and the unidirectional carbon weave that are provided should be passed out to the class in order to give the students a look at composite materials and their components. Additionally, showing the panels will allow people to see the matrix and fiber alignment up close. The first panel has carbon fibers arranged in such a way that they are longitudinally stressed when held at one end. The second panel however, has carbon fibers that are stressed laterally when the panel is held at one end. The third panel consists of a polystyrene spacer between two longitudinally aligned plies. The fourth panel consists of a broken ply with visible matrix and fiber damage.

Science:

A composite is a mixture of two or more distinct materials that act as a unified combination. The students will be able to see how the reinforcing carbon fibers in the loosely woven fabric are arranged in the polyester resin, which forms the matrix. One way to measure how the individual properties of the resin and carbon change when they are combined, is by first looking at the values of the elastic modulus of them individually and then the values for the panels themselves. Remember that the elastic modulus is the slope of the stress/strain curve. First of all, by bending the first two panels, it will be evident that the panel with longitudinally aligned fibers (i.e. fibers that run the length of the panel) is harder to bend than the panel with laterally arranged fibers (i.e. fibers that run along the width of the panel). Using this test, one can guess that E parallel > E perpendicular. Behavior such as this, where the composite�s properties (mechanical) are not the same in all directions, is termed anisotropic. It is important to understand that in order for the fibers to carry most of the load, they must have a higher elastic modulus than the polyester matrix. The bending test can be supplemented with derivations and computation below that will yield the elastic modulus of the two panels. For the longitudinally stressed panel, the strains of both the matrix and reinforcing fibers are assumed to be equal, if they are to deform elastically as a unit. The following derivation of E parallel is for a glass/polyester resin composite system. [eq] For the laterally stressed panel (transverse loading), the applied stresses on both the matrix and the fibers are assumed to be equal. In this panel, the matrix will support most of the load. [eq] For E parallel, (panel 1 with longitudinal loading) the value was found to be 705.7 GPa For E perpendicular, (panel 2 with lateral loading) the value was found to be 3.94 GPa The third panel demonstrates how the stiffness of the composite increases with the spacing of two plies. This third panel is also a crude example of how composites are used in industry. For instance, a polystyrene sheet not much thicker than the one used in the panel can be coated with composite plies and molded to give a lightweight yet strong wind surfer. The fourth panel is important in that it gives the students a first hand look at fiber and matrix damage, when the composite is broken. When a composite is bent beyond its limit, it will ultimately fail. The first signs of such failure is the propagation of minute cracks in the matrix, without significant damage to the fibers themselves. However, when the composite eventually gives, the fibers will almost seem to have been ripped out of the matrix. Additionally, it can be seen that although the carbon fibers are very strong in tension, they are surprisingly weak under compression, and will break quite easily under the latter conditions. Remarks: The carbon fabric has a high elastic modulus but is of little use alone, as it is too flimsy. The polyester resin on the other hand is stiff, yet too brittle to use by itself. The fascinating thing about composites is that they are lightweight yet strong materials, that are made by combining components that have few if any uses by themselves. Polymeric composites are not uncommon in today�s world. Everything from racquets to boat hulls are being made of these materials. If anyone is interested in making composites and calculating the volume fractions for them, then he/she can turn to the appendix where the process is outlined in some detail.

References:

Author:
Rahul Pinto
Credits:


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