Testing stab resistance
New stab-resistant fabric developed at AgResearch offers a different level of protection to conventional stab-proof garments, so researchers developed their own test method to measure the different performance characteristics of the fabric. Here, Senior Scientist Dr Stewart Collie describes a standard test method and the rupture test they created.
Key terms to discuss: Optimise, rupture test, tensile testing, variable
Questions to consider:
Why is a standard test method with a pass/fail test result not helpful for developing a new material?
When would this type of test be appropriate to use?
Why was it necessary for the scientists to set up their own test method?
How may this help them in developing their material?
Transcript
Dr Stewart Collie (AgResearch)
We are actually doing two tests. One is very close to a standard method that was developed in the UK and also adopted in the United States.
And a stab-resistant test involves dropping a standard blade – and the blade is a particular geometry, it’s got a certain shape and weight – and it’s dropped from a certain height, so there is a certain amount of energy of impact. And then you are looking at the level of penetration into the fabric. What you look at is a foam layer that is underneath that fabric, and you look at how big the hole in that is, and that tells you how far the blade has penetrated in. And there is a certain allowable level, and you can add multiple layers to try and improve your protective performance.
The trouble with that standard method, from a research perspective, you are largely getting a pass or a fail type of result. You get some information about penetration depth. So what we've done more recently is set up a rupture test on a tensile testing machine that we have, and that involves pushing the different types of blades – and we have done it with the screwdriver type blade and also with the knife blade – pushing that into a fabric sample that is held. And then with that, we can see how the force builds up, and we can see how the fabric distends, how it’s deformed downwards and the energy that it takes to push through it.
So then we've got some quite nice variables that we can measure. We can see much better what effect our various attempts to optimise the fabric are doing. To a large extent, the amount of injury that the person will receive doesn't depend just on how much force it takes to get through the fabric, but how far the fabric can be pushed inwards before the blade will get through. So if the fabric can yield a very long way before breaking, then the person, the wearer, won't necessarily receive a stab injury, but they might receive a very bad impact injury and have serious bruising or cracked ribs or other internal injuries. So it’s important that that is also minimised.
