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September 10, 2015

Behind the Arc Rating: The Burst Test

Picture this: you’re getting ready for work, you grab your long sleeve T-shirt, and as you’re pulling it on you hear that ever-dreaded, riiiippppppp. Your elbow has just burst through the arm of your shirt! In order to avoid this happening again, wouldn’t it be great if there were some way to know how much pressure a fabric could withstand before it bursts open? Good news! There’s a test for that: ASTM D3786 Test Method for Bursting Strength of Textile Fabrics – Diaphragm Bursting Strength Tester Method. As a part of the ASTM F1506 performance specification, we are required to perform the ASTM D3786 Burst test on all knit fabrics.

Why do we only use this on knit fabrics?

The brief answer is: it’s about fabric construction. Woven fabrics have an over/under structure of machine direction (MD) and cross machine direction (XD) threads-produced on a loom that feeds thread in only two directions and are generally inelastic. Knit fabrics are constructed through series’ of interlocked loops, which make a knit fabric more flexible in multiple directions.

Fabric Structures

So how do we account for all that flexibility in strength testing? We test in all directions at the same time!

ASTM D3786 calls for the use of either a pneumatic or hydraulic burst tester. These devices use either hydraulic fluid or compressed air to force a rubber diaphragm through a specified round space until the fabric, well, bursts. Hydraulic bursting testers, with high pressure capabilities, are used for many materials, including some wovens like parachutes, geotextiles, and air bags.  The force, however, reported in Kilopascals (kPa) or Pounds per Square Inch (psi), is not actually the correct force to burst the fabric.

Remember when we mentioned the rubber diaphragm? There is a certain amount of force necessary to inflate that diaphragm alone (tare pressure) and that force needs to be subtracted from the overall force required to burst the fabric. SO, once testing is completed we run a diaphragm correction, where we inflate only the diaphragm and record the tare pressure: the force to inflate the diaphragm to the same distension recorded at bursting. The bursting strength for the fabric is finally determined by subtracting the tare pressure from the original reported force.

Why is burst testing important?

Personal Protective Equipment is only as good as the quality of the materials that make it up. One might think “Oh, it’s just a small elbow tear, no big deal” until they realize that, while unlikely, an arc flash could burn through the hole in the elbow where skin is exposed, causing injury. If a knit shirt cannot withstand the rigors of operational wear, it cannot be trusted to fully protect. The burst requirements from ASTM F1506 demand a durable knit structure, less prone to wearing open, which provides better protection to the wearer.

That’s all from us at ArcWear, see you next time at Behind the Arc Rating!

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