Crack, ping, pop! The last of the bioplastics you were tensile testing just failed! You analyze the data to get a stress-strain curve and observe clear distinctions between the various combinations of glycerol and gelatin. 

A stress-strain curve is a valuable tool in understanding material properties. It graphically illustrates the relationship between stress - the force per cross-sectional area, and strain - the deformation of the specimen. You can get insight into the material’s elasticity, impact resistance, and ductility from this simple test. 

Stress is important because it measures the force ‘flowing’ through a material. It takes into account the area, too! A thicker material experiences less stress than a thinner one because the force is divided among more materials. Because stress equals force over area, it has units N/m2 or pascals (Pa).

Strain measures the deformation of a material. A value of 1 means no stretching or compression has occurred. A value of 2 means that the stressed sample is twice as long as its resting state.

Knowing these concepts, you look at the data your tests produced.

Looking at the plot, we see three distinct curves from each bioplastic. High gelatin, in yellow, fails at the highest stress but the lowest strain. Meanwhile, in blue, high glycerol gives a bioplastic that fails at the lowest stress but the highest strain. The control is somewhere in between.

The yellow line shows brittle characteristics because it does not flex much at all as suggested by the low strain. On the other hand, yellow demonstrates a ductile nature because it stretched to almost 2.5 times its original length. Typically, brittle materials sustain a higher stress before they fail which is exactly what we see on the graph.

Engineers frequently use these graphs when designing buildings, cars, or anything that requires a heavier understanding of material science. In fact, these graphs can illustrate the different behaviours between the very famous iron-carbon metal alloy known as steel.

Pure iron is ductile. As you increase the carbon content, carbon atoms fit themselves in the little gaps between iron atoms. The alloy becomes more rigid and brittle as a result, but it can carry larger loads without failing. You can actually add so much carbon that the yield point becomes difficult to define because the alloy fractures without an obvious yield point.

The following graph shows the effect carbon has on the material's behavior: http://billavista.com/atv/Articles/Steel_and_Materials_Strength_Bible/index.html

This curve looks similar to the bioplastics plot. Pure iron experiences the largest amount of strain but the lowest stress before failure. On the contrary, high-carbon steel can carry a lot of stress but not much strain before failure.

Pure iron is a lot like a bioplastic high in glycerol. These materials will be the most stretchy but can hold the least amount of stress. A high gelatin bioplastic or high carbon steel will stretch much less but can support more stress before it finally breaks.

Understanding the interplay between ductility and ultimate strength is important because you will be able to balance these two properties. A disadvantage of a high-strength, brittle material is that it gives no warning that it is about to fail because it cannot stretch. This means that a structure can quickly go from looking perfectly fine to collapsed. Having a slightly weaker material that is more flexible can allow for deformation, which will warn those nearby that it is failing.

A material’s elasticity is determined by the slope. More formally, this value is called the Young’s modulus and it represents the proportionality between stress and strain within the linear region. A higher Young's modulus means a steeper slope, which means the material is more rigid.

Another consideration is impact resistance. Impact resistance is the energy a material can absorb before it fails. It is measured using the area under the curve. Brittle materials usually have a lower area than their ductile counterparts. As a result, ductile materials perform better when the goal is to absorb the energy from an impacting object.

When designing cars, the crumple zone is never made from brittle metal. Instead, it is designed to bend and buckle so as much energy as possible can be dissipated before the passengers come to a full stop.