What is Flexibility?
Flexibility, the ability of a material to deform elastically and return to its original shape when the applied stress is removed.
Flexible materials can be stretched reversibly when they undergo elastic strain. The material property that characterizes the elastic limit is the yield strength, or maximum stress that a material can withstand before it breaks or deforms permanently.
Beyond thickness, the flexibility of materials can be compared using the ratio of yield strength, which reflects how strong or weak a material is, to the elastic modulus, which measures how stiff the material is.
Why are flexible materials important?
Flexibility is important because it enables parts or tools to give way, when force is applied, or when something bumps into them. This enables these parts or tools to perform tasks that require a gentle touch.
Common uses of flexible materials
The following applications are commonly 3D printed using flexible materials:
- Bumpers. Created with flexible materials, “bumpers” can push glass or other breakable materials to the side without breaking them. Heineken uses flexible materials for this purpose in its bottling plant in Seville, Spain.
- Sealing joints. With the ability to deform to the right geometry and allow for a tight fit, sealing joints created with flexible materials can be used in all industries that use or process liquids or oils in their factories.
- Grippers. Using flexible materials, some companies create grippers than allow gentle handling of products.
What Are the Different Types Of Flexibility?
There are two main types of flexibility encountered in everyday life: elasticity and plasticity. These two forms of flexibility may seem quite similar from the outside, but once you understand how they actually work, they actually become something quite different and distinct from each other’s. When we boil it down to the simplest form, you can separate the ways that materials bend into two distinct categories and forms of change:
- Elasticity means that materials can change their shape when a force is applied and will return to their original shape after this force is removed. Rubber is a good example of how this works- when you stop stretching a rubber band, it goes back to its original shape. Although the particles and their bonds between them are distorted when the force is applied, the internal structure of the material can return back to the original form.
- Plasticity means that materials can change their shape when a force is applied to them, but they do not return to their original shape when the force is removed and will require external force to bend it back into their original shape. Think about paperclips – we can bend and twist them into new shapes, and they hold these shapes until we bend them again. Although the internal structure remains a single unit, it’s permanently changed, some internal bonds may have been broken, and it won’t easily return to the exact structure that existed before the force was applied.
This is why even flexible materials can break – if the force applied to the material is too strong, it can damage the bonds between the particles so much that they completely split and the material breaks.
When we bend the paperclip, the bonds between the latticed metal particles are stretched, and if they’re put through too much tension, they snap. Even very elastic materials have an upper limit to how much they can be stretched before they break.
All materials are elastic and plastic up to a certain point – and it’s important for people to understand how much so when they’re trying to make something – a building needs materials with a certain amount of elasticity so that if it’s hit by something, it can absorb some of the impacts without breaking, and plasticity to make sure that if the impact is beyond what the material can absorb without bending, that it bends more than breaking entirely.
Modern cars are a good example of how plasticity works – the cars are designed with crumple zones made of materials that are plastic enough to crumple and bend – absorbing the impact so that the central structure of the car, which has to be more rigid, doesn’t break and seriously injure the passengers.
What’s the difference between flexibility and elasticity?
Flexibility and elasticity are two different concepts and properties. For example, rubbers (at room temperature) are mainly elastic and flexible (ductile) materials, whereas glass (again, at room temperature) is also elastic but rigid (brittle). The flexibility of a material is a character related to the toughness (which is resistance against impact loading) property of the material, not to its elasticity, .
So, elastic material can be highly tough (ductile) or can be very brittle. We can say that elasticity is the resistance of a material against permanent deformation (against plastic deformation), while ductility (flexibility) is related to the amount of energy that materiel can absorb during deformation. Glass is much stiffer and more elastic than rubber, but it breaks easier with much less energy. Rubber is tougher than glass but less stiff.
For materials A and B in the question: Material A with a high elastic modulus deforms much less than material B with a smaller modulus. Because B deforms with much less force (stress), we can say that it is more flexible than A. In other words, material B deforms more easily, while that deformation could be reversible/temporary (elastic) or irreversible/permanent (plastic).