Composite Testing with Strain Gauges

New materials, new measurement challenges?

There is a conception within engineering that strain gauges are some kind of art practised by magicians in dark laboratories using alchemy. This seems especially true as soon as plastics and composites are mentioned.

Composites, and especially carbon composites (CFRP, Carbon Fibre Reinforced Polymer), are often considered modern miracle materials. In reality carbon fibres have been around since the 1860s, and in their modern high-strength versions since the 1960s. Impregnated with a polymer matrix they offer strong, light alternatives to traditional engineering materials.

Strain gauges have been around since the 1930s, greatly improved with the advent of foil construction in the 1950s, and further enhanced with our Advanced Sensors Technology in recent years. In other words, strain gauges and composites have been part of the engineers’ toolbox for many decades.

Considering how long they’ve both been around there still seems to be a misconception when it comes to bonding. In reality strain gauges are easy to bond to virtually every type of composite; in this case, “virtually” means all that we’ve tried so far! It just requires selection of the correct materials and techniques. This article is not for repeating all of this information as there’s plenty of literature available covering every aspect from sensor selection to data interpretation. Further reading is listed below should you wish to learn more about this. For the tl;dr version I’ve listed a few considerations below.

Gauge selection: generally choose a high resistance pattern (350, 1000 or even 5000 ohms) with a grid size around 5x the tow, weave or fibre size. Pre-attached leadwires are preferred by some practitioners but in reality our CEA-series Advanced Sensors can easily be soldered once bonded. We have a range of gauges specifically designed for certain configurations of test coupons such as our V-notch Iosipescu and compact shear modulus gauges or our “UB” pattern for short specimens.

Adhesive: 100% solids epoxies are ideal as they will gap-fill textured surfaces. M-Bond-AE10 is ideal and can be used from -195°C to 205°C (recently retested and certified to this higher temperature) and readily bonds to surfaces that are considered difficult to bond. M-Bond-200 cyanoacrylate may suit some tests but it is not the first or best choice for composites.

Surface Preparation: just the surface shine needs removing. This is easily achieved with SCP-3 400-grit abrasive on resin-rich surfaces, or a fibreglass pen on textured finishes. Use of high-purity GC-6 IPA (isopropyl alcohol/isopropanol) is ideal; water-based cleaners are sometimes used but should be avoided if possible.

Strain gauges, like any technology, just require a little tuition ideally through a recognised training workshop. Strain gauges are ubiquitous, with applications in every aspect of our modern lifestyle, so it’s well worth taking time to gain familiarity with this simultaneously cutting-edge yet mature technology.

Strain Gauge Measurements on Plastics and Composites  (Application Note VMM-1)

Surface Preparation of Composites (Application Note VMM-19)

Composites Checklist

 

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Anton Chittey

United Kingdom