How to test the elasticity of your shock cord in the field

Shock cords are hugely valuable assets, performing a variety of useful roles across multiple key sectors – many of which we have a long history of serving here at Ibex Marina Ropes. And whatever application you are using your own cords for, it is vital to test them periodically to ensure that they are safe to use.

Testing your shock cord’s elasticity in the field gives you a clear idea of whether it is performing as it should, especially when you depend on it for theatre rigging, marine uses, or military applications. Shock cords work by storing and releasing energy through stretch, so ensuring that you know their overall condition and stretch behaviour can help keep your setup safe and secure. Thankfully, you do not need specialist workshop equipment to do this, so as long as you have got the right training at your disposal, you can quickly and easily ascertain which of your shock cords have the integrity they need to be kept in circulation.

Make sure you are prepared

Before you start, you need to ensure you have a flat, stable working area to firmly secure one end of the cord. This could be a post, a hook, or even a solid part of a vehicle or structure, depending on where you are working. The cord needs to be anchored so that you can stretch it without risk of slipping or damaging it. Always inspect the cord visually first, looking for fraying, cuts, or areas where the sheath is separating from the core. Any visible damage can alter the results or signal that the cord needs replacing altogether.

You will need a few basic tools to get accurate results. These include:

• Tape measure or ruler
• Marker pen or chalk
• Spring scale or luggage scale
• Notebook or phone for recording measurements
• Gloves for hand protection

A tape measure or ruler is essential for recording length changes, and a marker pen or chalk helps you mark reference points on the cord before stretching. A spring scale or handheld luggage scale can help you apply and measure force if you want to test stretch under specific loads. A notebook or phone app for logging measurements ensures you can refer back to your results later, which is vital for spotting changes over time. Gloves are worth wearing if you are working with heavier cords under tension to protect your hands.

Stretch and measure your shock cord to test its elasticity

Here are the main stages in brief:

• Measure and mark the cord’s relaxed length
• Anchor one end securely and stretch steadily
• Record the extended length under tension

So, to go into that in a little more detail: start by measuring the cord’s relaxed length without any tension applied. Mark two points on the cord at a known distance, such as one metre apart. This makes it easier to measure how much the cord stretches without confusion about where the measurement begins and ends. Then, with one end anchored securely, pull the cord steadily and smoothly (rather than yanking it), so you can feel how it stretches and avoid any jerky motion that might damage it. Make sure to watch how the material reacts as you apply force – a smooth, consistent stretch indicates good elasticity, while stiffness or uneven resistance can suggest deterioration in the internal rubber strands.

Use your tape measure to record the new length once it is under tension – either by measuring between your marked points, or from the fixed end to your reference mark. If you are using a spring scale or luggage scale, you can apply a specific load and measure the amount of stretch at that force, which is useful for comparing cords or testing whether a cord is still within its expected performance range. Repeat the stretch a few times to ensure you are getting consistent results. This gives you a clearer idea of how the cord performs under repeated use, which is often how it is used in practice.

Compare your results to expected performance

Once you have recorded the stretched length, work out how much the cord has elongated. This is typically expressed as a percentage of the original length. For example, if a one-metre cord stretches to 1.5 metres, that is a 50% elongation. Many shock cords are designed to stretch by a certain percentage – often between 50% and 100% – depending on their construction and intended use. Ideally, make sure you know this figure in advance for the specific cord you are using, as this helps you decide whether it is still performing as it should. If you don’t know the exact specification for your cord, you can often estimate by comparing to similar cords in good condition or looking up general performance expectations for cords of that diameter and type.

If your results are outside the expected range, it could mean the cord has degraded, either through UV exposure, wear, or age. This does not always mean the cord is unusable, but it does mean you need to consider whether it is still safe for the job you want it to do. Testing also helps you identify cords that have uneven elasticity, where some sections stretch more than others. That can be a sign that part of the internal core has failed, which often means replacing the cord is the safest option.

Record and track elasticity changes over time

Once you have tested your cords, it is wise to keep a log of their performance, as this helps you spot patterns of wear. Note the date, the starting and stretched lengths, the force applied if you measured it, and any visual observations about the cord’s condition. If you are testing multiple cords, don’t forget to label them so that you can match your notes to the right piece of equipment later. This can make it a lot easier to decide when cords are nearing the end of their useful life.

If your cords are used in demanding environments, such as outdoor setups exposed to weather or in marine conditions, schedule checks at regular intervals. Keeping these records also helps you refine how often you need to replace cords, based on real data.

And of course, if you ever need any input or advice about the condition of your shock cords – or any replacements – this is where we can help here at Ibex Ropes Marina. We have unrivalled experience with shock cords, and serve a wide range of sectors across the UK and beyond, including aerospace, military and defence, subsea and offshore, and sport and leisure – just to name a few. If you require any expertise or advice, feel free to give us a call on 01706 360 363, and we will be happy to help however we can.