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Bursting strength is a measurement that shows how much force an object can take before it ruptures. This measurement is applied only to sheet materials, such as cloth, paper or plastic. When discussing non-sheet materials, such as how much force a chain can withstand, it is more common to use tensile strength as a measurement. Bursting strength is commonly referenced when packaging goods. Shippers will then know their packaging is strong enough to handle the weight of the materials inside.
The only materials that have a bursting strength are sheets, and paper products are the most commonly tested material. These paper products are typically used to ship heavy materials or goods with irregular shapes that are hard on their packaging, such as sharp edges or pointy corners. Other materials, such as cloth or plastic sheets, also have bursting strength, but there is rarely a large need to test them.
A substance’s bursting strength is typically found using a Mullen test. The sheet is held between two clamps and pulled tight. At this point, the clamps are creating uniform horizontal pressure on the material. The sheet is held taut, but it isn’t pulled. When it is in this state, the sheet can withstand more pressure than when in any other state.
An inflatable bladder slowly fills with air, creating uniform pressure along the entirety of the sheet. As the air pressure inside the bladder increases, the pressure placed on the sheet does as well. Eventually, the pressure on the sheet is too much and the material ruptures. The pressure in the bladder is checked to calculate the pressure on the paper at the time of rupture. The final amount is usually measured in pounds per square inch (psi).
When a Mullen test is used to find the strength of a paper shipping material, the results of the test are often printed directly on the container. This data, along with a handful of other metrics, makes up the block of information commonly found on commercial packaging. Often, this printed material is part of a proprietary system and is difficult for the average consumer to read.
There are several factors that affect the bursting strength of a material. One of the main factors is the underlying structure of the sheet. Sheets with long continuous fibers or lines are usually stronger than those made of shorter or composite materials. In packaging, corrugation improves the overall bursting strength of the cardboard, especially when going against the grain of the corrugation.