Strain Conversion Experiments

Embedding simple machines in a matrix allows the material scientist to design a material with specific properties. For example, when "Z-type" machines are employed the material distorts in shear when compressed. Converting compressive-to-shear displacements, and vice versa, can be useful in applications where it is advantageous to change the size of a part when it is under load. In one example, gaskets and seals may be made to increase in thickness when under shear forces caused by thermal expansion of the sealing surfaces.

To display the displacement conversion effect, samples were made using 1-centimeter size machines that were made using rapid prototype technology. These large 1-centimeter specimens are tested (rather than the 1-millimeter machines used in the actual material) because they can be manufactured quickly. The machines were embedded in a polyurethane matrix. As shown in the figure below, two samples are placed on top of each other with an aluminum plate in between. The lower sample is inverted with respect to the upper one. Using this arrangement, the shear displacement can be measured with respect to the compressive displacement by measuring the movement of the center aluminum plate.

Deformation characteristics of a rapid prototype MAC sample as a function of compressive strain.

The results of these tests are shown below. In the plot, the horizontal axis is the compressive displacement of each sample as determined from the crosshead motion of the testing machine. Plotted on the vertical axis is the horizontal displacement of the aluminum plate. This displacement results from the shear motion of the samples. The data is plotted from three samples each containing machines with a different angle for the side walls, 90 degrees being vertical. As shown, the 60-degree machines give approximately a one-to-one conversion ratio between compressive displacement and shear displacement. For larger angles, the ratio is even larger than one-to-one.

To aid in understanding how machine augmented composites respond to forces, a finite element model of the structure of the material was developed. The model, shown below, included all of the material properties of the machines and the matrix material.

The plot below compares the results of the finite element model with the results of experimental data. The results compare well with the only difference caused by a variation in one of the material properties inputted in the model.

Finite element analysis grid of a prototype with Z-shaped machines. The red color denotes the matrix material, the blue represents the machines, and the white are areas of space.	This plot shows a finite element comparison with our experimental results. As shown, there is excellent correlation.