MetaCORE™

Stiffness, measured by the Young’s modulus E, expresses how much a material will deform in response to an applied load. We all intuitively recognize that stiffer materials are generally heavier, and compliant materials are generally lighter. As a result, we’re surprised when we find light materials that are very stiff (composites and technical ceramics) or delighted when we find heavy materials that are very soft (memory foam). This intuition is quantified when we plot the density of a material, ρ, against its Young’s modulus and observe a generally upward-leaning trend.

A material’s Specific Energy Absorption (SEA) tells you how much energy can be absorbed by crushing a given amount of the material. Often, materials reporting high values of SEA only absorb in one direction, while the other two directions offer little-to-no functionality. Plotting a material’s maximum SEA vs. its minimum SEA reveals the problem.

Before there were seatbelts in every car, researchers identified useful metrics for engineering vehicle safety systems. One metric, the Crush Force Efficiency (CFE), is particularly good for quantifying the transfer of force from a collision to a vehicle occupant. While CFE is useful, it doesn’t tell you how much energy is ultimately absorbed by the material mitigating impact. This is where the Specific Energy Absorption (SEA) comes in. Some materials, like foams, have a great CFE but absorb very little energy. Other materials like honeycomb have great SEA but allow for undesirable propagation of harmful de-acceleration forces. Plotting CFE versus SEA gives a good high-level perspective on these two critical – and distinct – aspects of crashworthiness.

By dividing the Young’s modulus by the material’s density, you derive its specific stiffness (sometimes referred to as “specific modulus” or “stiffness-to-weight ratio”). High specific modulus materials are widely applicable in aerospace applications where low-weight high-stiffness materials are desired since they resist deformation. Consider choosing a material for building an airplane. Aluminum seems obvious because it’s less dense than steel, but steel is stronger than aluminum, so maybe we should use a thinner steel plate to save weight without sacrificing tensile strength. However, even if we find the right weight-to-tensile-strength ratios, we end up sacrificing stiffness, which ultimately allows the wings to flex too much during flight. These trade-offs are all too common when selecting engineering materials.