Tensile Strength

We used the tensile strength tester in the FoodTech lab at BCIT. The machine clamps on to the sample and pulls it until it breaks. The amount of force applied, the time over which the force is applied, and the distance the clamps have moved are all recorded. This machine provides far more precise data than our original self designed apparatus. The greater amount of precision obtained by the machine enabled us to produce more accurate graphs that show the stress-strain curve.

click to enlarge
When comparing all of the samples together in a force vs. time graph, we see that the tensile strength of the control sample is indeed much higher than the reinforced samples. However, with the control samples, the point of breakage was a clear defined high point characterized by a gradual increase in force followed by a drastic decrease. With the reinforced samples, there were several of these peaks, and the curves were often rounder without the drastic decrease in force characterized by the control samples. This indicates that the reinforced samples were not as brittle as the control samples and did not tend to break all at once. Instead, a certain part of the sample would fracture, likely the matrix, forming the first peak on the graph, and then as force was continued to be applied the fibres would break or be pulled from the matrix, forming successive peaks.

The ultimate tensile strength is the amount of force applied to break the sample. On the graph, it would be the highest point.

Ultimate Tensile Strength of samples

At 5% fibre content, the flax sample had a higher ultimate tensile strength than the hemp samples, but at 25% and 45% fibre content, the hemp samples were stronger. This could mean that the flax samples had better adhesion with the matrix, but were themselves weaker, so that as fibre content increased, the strength of the fibres themselves became more and more influential in the overall tensile strength of the composite.

Ultimate tensile strength when comparing fibre quantity

The general trend is that the less the fibre content, the greater the tensile strength. However, 25% hemp, at 18.1775N had a higher average tensile strength than either the 5% or the 45%, though the 5% sample is very close at 17.7455N.

The tensile modulus is a measure of how brittle a material is. The higher the tensile modulus, the more brittle a material is. It is calculated using the the equation:
tensile modulus = force / (final length -- initial length)

Chart ranking tensile moduli of samples from most brittle to least brittle.

The tensile moduli of the different samples indicate that the 45% flax sample is the most brittle. However, this may be because of the weakness of the sample; it broke too quickly before it was able to be stretched. As expected, the control sample is also among the most brittle. There is a large amount of variation within the flax samples, as the 25% fibre content samples are the least brittle. This may be due to flax's interactions with the matrix.