What is stress? What is strain?

The answer to the second question is part of the answer to the first question.
Most experts will advise to run your web at 10% of the tension that would damage the web, a safety factor of 10:1.

Definition: Stress is a force over an area (units are lbs/in2 or psi, N/m2 or Pa)

Stress and pressure have the same units. Tensile stress tends to elongate something. Compressive stress (a.k.a. pressure) tends to shorten something.

Definition: Strain is a shape change, usually described as a fraction or percent change calculated from dividing the change in a dimension by its original, unstressed dimension.

When you tension a web, it gets longer in the machine direction. If you stretch a 250mm (10-inch) sample to 252.5mm (10.1 inches), you have strained it 1 percent.

What is the Modulus of Elasticity?

Definition: Modulus is the initial slope of the stress-strain curve. Modulus is the ratio of the applied stress to the change in shape of an elastic body (also known as elastic or Young’s modulus ).

Tension elongation testing is commonly used to find a material’s break tension and break elongation. However, the same test will also measure a material’s modulus of elasticity or Young’s modulus.

Modulus will have units of force per area, such at MPa or psi.

Example modulii: (1kpsi = 6.9 MPa)

Material Modulus (SI) Modulus (American)
E(Steel) 200 GPa 30,000 kpsi
E(Aluminum) 69 GPa 10,000 kpsi
E(BOPET) 3.5-5.5 GPa 500-800 kpsi
Paper 3.5-4.2 GPa 500-600 kpsi
Polypropylene 0.7-1.7 GPa 100-250 kpsi
Polyethylene 0.2-1.4 GPa 25-200 kpsi

Materials like steel and aluminum will require high stresses (load over area) to get even the smallest strain or elongation, but will yield at relatively low strains, if you can apply the stress to get them there.
Other webs are elastic over a large strain range.

Once you’ve reached a web’s elastic limit, it will no longer fully recover when unloaded. In web handling, the goal is almost always to handle the web below the elastic limit and avoid any permanent damage to the web.

What is Poisson’s ratio?
When is Poisson’s ratio important?

When you tension a web, you change its length, but you may not make a significant change in its density or volume (volume = length x width x thickness). If the length increases, but the volume doesn’t change significantly, then the web must shrink in another direction (and it does). Most tensioned webs will deform and see a decrease in width and thickness.

When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson’s ratio (ν), named after Siméon-Denis Poisson, is a measure of this tendency. Poisson’s ratio is the ratio of the strain normal or perpendicular to the applied load divided by the strain in the direction of the applied load. In web tensioning, we often ignore the small thickness change from tensioning, but the width change can be significant. In many webs, the Poisson’s ratio is around 0.3.

Poisson’s ratio will be much higher in other webs, such as textile, porous films, tissues, and foams, where air is pushed out under tension. In high Poisson’s ratio web, managing width loss from tensioning can be a top concern. Poisson’s ratio will also contribute to curl problems in laminates and width changes or MD buckling in wound rolls.

Example: Web tension may stretch thin polyethylene or polypropylene films to 1 percent in their machine direction. This tension will also create a width change or necking of 0.3 percent. If the web is 1.5m (60-inches) wide, this would reduce the width by 4.5mm (0.18 inches or 180 mils).

What is elasticity?
What is viscoelasticity?

Definition: Elasticity is the property of returning to an initial form after deformation.

Definition: Viscosity is the property of having a resistance to flow.

Definition: Viscoelasticity is the property of having both viscous and elastic properties.

Rubber is the classic example of an elastic material. You push on it, it deforms; you let go, it recovers. An elastic material responds to load almost immediately (the load travels through the material at the speed of sound). The amount of deformation is proportional to the load and independent of time.

Molasses is the classic viscous material. When a force is applied to a viscous material, it will flow. The longer the load is on the viscous material, the more it will flow. When the force is removed, it will stop flowing, and the material won’t recover.

When a viscoelastic (V-E) material is loaded, it will respond with a mixture of viscous and elastic behavior. Upon loading, a V-E material will immediately stretch (elastic behavior) and begin to flow (viscous behavior). When the load is removed from a V-E material, it will recover, some immediately (elastic behavior) and some will recover more over time (viscous behavior). Vinyl electrical tape is a classic and easily observed V-E material.

For short times of web handling (seconds, hours), viscoelasticity can be ignored in most materials. For long times of winding (hours, days, months), viscoelasticity can NOT be ignored. CD differences in creep is the cause of most bagginess.

Try this test yourself: Pull out a 2-3 foot length of electrical tape. Hang a 1-2 lb weight on it. Note the initial elongation and that the tape will continue to elongate. Take the weight off. Note the initial recovery and ongoing recovery.

Congratulations, you’ve just completed your first creep test.