Stretch has always been a tricky thing to measure. There are so many factors that go into how a webbing stretches that it is very difficult to get a consistent and meaningful measurement. That, coupled with the fact that stretch testing in the lab is very disconnected with how a webbing stretches in the field. However, having data on how a webbing stretches is important for determining how a webbing will behave in the field under normal usage. Most importantly, it's critical to have data measured in the same way across all webbings to be able to have a meaningful way to compare these webbings.

In the past, prior to when we had a testing machine in-house, we would rely on stretch data from our manufacturers. Since we work with several webbing manufacturers all over the world, this stretch data that we would get from the suppliers would be all over the place in terms of how it was measured. Some data was given to us in huge force increments (every 1,000 lbf, for example), while others were given to us as a flat value at a certain force (stretch @ 10 kN, for example). Obviously, this data is hard to compare and very hard to get any value from.

Once we got our break test machine, which has a string potentiometer built-in (a tool for measuring distance traveled), we were able to bring stretch testing in-house. This allowed us to gather more consistent data from webbing to webbing in order to have a basis to compare the stretch between our webbings. Obviously, this does not help us have data for all the webbings on the market, so there are still discrepancies when comparing webbings between manufacturers. We do not yet have a standard way to measure stretch in the slackline world and as such, each manufacturer has their own method for doing so. This makes things very tricky as stretch is a common trait that folks use to base their decision on which webbing to buy.

Today we are discussing the method that we use to measure the stretch of our webbings. We are doing this in hopes to start the discussion on optimizing this measurement process in hopes that other manufacturers can adopt the same methodology in order to make all webbings more comparable on this metric. Our method is far from perfect, but we are able to be consistent with it, giving us comparable data across our webbing offerings.

Here is our stretch-testing method:

Number of samples: 5

Length of each webbing sample: 1.5 meters (5 feet)

Pull speed : 300 - 305 mm per minute

We then take each sample and sew a loop on both ends. The size of this sewn loop is roughly 210 mm (8.25 inches). That is the point at which we fold the webbing over on itself. The length of the sewing patch is not important, but we use our standard loop pattern, which is 95 mm (3.75 inches) long.

We then take each sample and pull it to failure, measuring both the force and the distance traveled along the way. We measure these 2 data points at 500 Hz using our S-Type Load Cell and a LabJack T7 PRO. Both of our sensors are analog and as such, we use a scaling equation to map the voltage outputs to kilonewtons for force and millimeters for distance traveled. Both of these scaling equations have been calibrated at regular intervals through the year.

We then take this data and build a stretch graph for each sample. To start, we first down-sample the data to 20 Hz in order to smooth it out and take as much of the noise that is inherent in our analog sensors out of the dataset. Down-sampling involves taking the average values for each sensor over that range. In our case, since we start with data at 500 Hz, we average every 25 readings to get 1 reading in the down-sampled data.

Next, we establish our zero point for the stretch. This involves setting a minimum force threshold, at which we will find the position of our sample. Our minimum force threshold is set at 0.20 kN, which is fairly standard in other stretch measuring contexts. At the first point when our force reaches this value (in the down-sampled data), that position becomes our zero point.

Next, we do the same, but for a max force value. If we let the stretch data collection go until the sample breaks, the graphs turn out very ugly and hard to read. So, we set a max force at which we stop calculating the stretch, which is 18 kN. We then trim our down-sampled data down to be just the data between the zero point and the max force point to have a clean graph that can be combined with other samples of the sample webbing and be somewhat comparable visually.

Next up, we need to calculate how long our sample is at the zero point. To do that, we first measured the spacing on our machine to find the distance between the moving pin and the fixed pin at various points. We can then use this data to find how long our sample is at zero. This data is critical for calculating the stretch of the sample as a percentage.

Next up, we use this initial length number to calculate the distance stretched at each data point in our down-sampled dataset. We can then use this data to find the stretch percentage at each data point by dividing the distance stretched by the initial length. That will give us the percent-stretch at the measured force value.

Next up, we want to try and build a chart of stretch percentages at fixed force values. Right now, we have somewhat obscure force measurements tied to each one of our data points. For example, one such data point might be 4.34511% stretch at 2.31563 kN. We really want to know what the stretch is at whole number force values to be a more presentable data set. So, we build a new table of stretch data by iterating over force values that increment by 0.05 kN and finding the stretch at the first point that is greater than or equal to this force value. In other words, we start at 0.2 kN and find the first row that has a force higher than or equal to that and get the stretch number. Then we look at 0.25 kN and do the same. We do that all the way up to 12 kN, which is where our standard stretch data stops. Now we have a table that looks like the following:

Now we have clean stretch data for this sample that we can use to build a chart. We then repeat all these steps for the other 4 samples for this webbing type. All 5 samples are then graphed onto a single graph. We then calculate the average stretch at each force value to get the stretch data that we report for each webbing. The end result is then made into a Stretch Report, which looks like the following:

We've run this stretch report for most of our current webbings and have listed them all below. We are missing a few as our string potentiometer broke near the end of our testing. We have sent it in for repairs and hope to finish up the rest of the webbings to be able to share the reports here for them as well. All the stretch numbers that are listed on our webbing pages represent the values we got in this testing.

We are certainly open to feedback on this testing and calculating methodology, so please feel free the reach out with feedback. We hope this data is helpful for other manufacturers looking to share more comparable stretch data on their webbings. Alternatively, we are happy to run these tests on other webbings in order to build a full stretch table that is comparable and consistent. Please reach out if you would like to go this route.

Some known faults of our current method:

1. There is a decent amount of length at the ends of each sample where the sewn loops are where the webbing is doubled. Doubled webbing stretches differently than single webbing. This definitely skews our stretch data as the doubled bit is a decent percentage of the full length of the sample

2. Even with down-sampling, there is still some noise in our datasets. This noise can lead to false results when we query for the first value that breaches certain force thresholds. This is mostly covered by having multiple samples, but still can skew results.

3. Even with five samples for each webbing, we can still see quite a bit of variance in our measured data, especially when the stretch is very small (like in high tech webbing). You can see this clearly in the Axiom results below.

Here are the remaining stretch reports we have run so far: