Isn't it fascinating how much deformation your skis can handle, and yet they suddenly break without warning? In this blog post PhD student Eivind Hugaas describes the properties of composite materials used in hydrogen storage tanks and his experience with testing them in the lab.
Composites are fairly complex materials. The testing of them is no exception.
First of all, carbon and particularly glass fiber reinforced epoxy (in popular terms just “glass fiber”) is a lot softer material than steel. Though “soft” is maybe not what people associate with their windows (made of glass), the opposite is true in relative terms when compared to most standard steel types. This makes for some interesting scenes in the lab. It can be a bit unnerving to see your sample stretched like a spaghetti compared to the metal rod that’s barely doing anything on the neighboring test rig while actually being close to breaking. Very likely your sample got a lot more residual strength, so no need to worry.
Looking at ski sports such as ski jumping or downhill in slow motion is a very good exercise to understand just how much deformation glass or carbon fiber can take before breaking. The skis often consist of glass or carbon fiber in areas that are subjected to large deformations. The skis sometimes bend ninety degrees or more, but try to bend them that much by hand and you need superhuman force. Though the softness obviously contributes to the fact that the glass fiber can handle so much deformation, it does not get any softer as it deforms, such as most metals; it just suddenly brakes. Through the years, countless ski enthusiasts have encountered this quality as their skis break without warning.
The skis sometimes bend ninety degrees or more, but try to bend them that much by hand and you need superhuman force.
From a testing perspective, dealing with big deformations is nice as it’s very visually obvious what’s going on in the material. To monitor the deformations, Digital Image Correlation (DIC) is a good and robust solution employed in my research. DIC uses images to recognize patterns on the sample, usually in the form of a black and white speckle pattern that is sprayed onto the sample. When the sample deforms, clever software calculates how much the speckles have moved and then gives you the deformation in the different areas of the sample. Figure 1 shows a raw image from a sample during testing and how it is processed by the software to show degree of deformation. As can be seen, the sample deforms a lot around the hole, as would be expected.
The big question is what happens to the material when it is subjected to large deformations over longer periods of time such as a ski used over many years. Does the ski have the same feel after say, two years? This question is my main topic of interest.
Based on what is known, quite a lot happens to the ski over time. Firstly, the epoxy will crack. This happens early on and for most skis has likely happened already after the first trip. Though the epoxy cracks, it still keeps the structure together, and keeps an even distance between the glass or carbon fibers, which are the actual loadbearing parts of the material. After the epoxy has cracked, the material loses a tiny bit of stiffness and reaches a “steady state” stiffness. Further load cycling will gradually make the bonding between the epoxy and the fiber shear and as a result deformations will in most cases be distributed more evenly, which is beneficial for the strength. Exactly what mechanisms are in play and how they interact is a hot topic for research and something I’m trying to solve. It is likely to assume that an expensive ski with high quality carbon and epoxy will be less prone to fatigue problems and will perform consistently over many years.