here, here, here
here
here

 

Common types of defects

 

Common defects can typically be identified within two main categories, manufacturing flaws and in - service damage. Using Non Destructive Inspection methods it is usually straightforward to determine the diference between something like a manufacturing void versus an impact delamination.

 

Typical Manufacturing flaws include:

 

Inclusions (Also known as FO), Foreign material  within the laminate, e.g. pre-preg backing paper, peel ply or other foreign object, accidentally included in material during manufacture, can promote delamination/unbonds and significantly reduce mechanical properties.

Improper fibre splicing/ply joining - Insufficient ply overlap at ply joins, creates non uniform load paths and internal or external cracks

Fibre wrinkling/kinking/defects - Individual fibers not aligned with the load path, can form during layup i.e. tight radii corners, poor layup techniques, uneven consolidation, material inability to drape etc.

Voids - An inclusion of air between the laminate layers, can be caused by poor layup and compaction as well as contamination during mixing of resin or the cure cycle. Can act as stress concentrations and will have an effect on some of the mechanical properties i.e. lower transverse and through-thickness tensile, flexural, shear and compression strengths.

Porosity -  Similar to voids, except being very small in size and often more dispersed. Void content considered negligible if less than 1-2%.

Resin rich areas  Excess resin trapped within the plies, can cause stress concentrations and reduce mechanical properties. Caused by inadequate resin bleed and/or non uniform compaction.

Fibre misalignment, distortion - As with fibre wrinkling, especially prevalent at sudden section change, such as inserts.

Fibre breakage - Individual fibers broken, either due to cutting or can be caused by excessive fibre curvature at sharp radii corners.

Unbonds  Plies or components not bonded, often due to contamination, poor surface treatment or poor consolidation or as a result of an inclusion. 

Imperfections due to machining  Delamination and fibre breakage caused by operations such as hole drilling and trimming. Can also introduce defects such interlaminar cracking and unbonds if not performed correctly.

 

Common in service damage includes:

 

Delaminations Separation of the individual plies within the laminate. This type of defect typically occurs in-service due to impacts or crush type loads, and can have a severe detrimental effect on mechanical properties, particularly in compression.

Fibre breakage - Fibres broken from overload, more common with higher modulus fibre grades.

Cracks - Typically caused by fibre breakage or manufacturing flaws such as wrinkling as well as resin rich area.

Disbonds - A bonding failure due to overload or reduced strength join from a manufacturing flaw such as a void, poor surface treatment, poor joint design etc.

Resin Microcracking - Overload of the resin or thermal overload leading to cracking within the resin matrix.

BVID - Barely Visible Impact Damage is a low energy impact leading to delamination without significant surface indication of damage.

 

 

There is a perception that working with carbon is some kind of "Black art", that it is done by people who have some kind of magic touch to get things looking right again.

Let's dispel that myth right now, it is NOT a black art, it is Engineering pure and simple.

Repairing a carbon bike is engineering because it is a loaded structure. Riding a bicycle particularly at speed such as descending or sprinting places large loads on the individual parts and these parts need to be engineered to deal with those loads. A repair is no different, the repair needs to be engineered and designed just as the frame is engineered and designed or it will fail just like a poorly designed frame fails.

When repairing a carbon bike the first most obvious step is to know what damage exists, how does one repair something without knowing the full extent of the damage. As in aerospace the best method for this on bikes is an Ultrasound scan.

Once the damage has been  assessed the repair needs to be designed so that the part will perform in a similar way under load as it did pre-repair. The number of plies, the ply angles, the grade of carbon, resin and the compaction level, all need to be considered. Substituting these factors will have an effect on the bikes ability to perform under load.

If these factors are not fully considered, for example by simply wrapping extra layers of carbon around the damage, the load paths will be significantly different. The repair may not fail itself, as it is probably significantly stronger and stiffer than the originally designed frame however the frame will fail next to the repair. This is obviously not a well engineered approach.

As discussed in previous technical articles, the resins ability to perform at different temperatures is also critical. Use the wrong resin and the repair may not perform on a hot day, causing a failure. The weight of the repair should also not be significantly different to the original. For many common repairs the frame should be within 10 grams of the original weight.

Of particular concern is the idea that as long as the repair looks good visually, everything is ok. When people pick up their bike from us they say "it looks good", because this is what they see. However the underlying structure is what really matters. A poor structural repair can be painted to look nice, but will it have the right structural properties to perform as required? When descending a mountain at 90 kph, do you think of the bling paint job or if the bike will hold together?

The thing about failures is that they typically occur when the loads are the highest and thus the consequences of the failure are also the highest. Sprinting at full power or high speed descents down mountains can cause serious injury or even death if the frame fails. This is serious, repairs just like bikes needs to be engineered to perform.

In summary a good repair is engineered to perform just as the frame is engineered to perform, consider this when choosing a repair service.

On this page you will find a link to a paper showing some of the frames we have sectioned.

Sectioning frames gives us the opportunity to get a better understanding on the bike brand and model in terms of their manufacturing processes, it provides a visual representation of the structural make up of the bike. This allows us to show you how your bike is made and also explain why it may have failed in a particular region. The frames are also a useful reference and a snapshot of how the manufacturing technology is improving over time.

As you can imagine there are many thousands of dollars of high end bikes here that have been put to the diamond saw along with a hefty investment of time required to actually cut the frames. The majority have been donated to us for a number of reasons, many have suffered damage and are significantly structurally compromised, others had manufacturing faults that were discovered during an ultrasound scan.

We will continue to develop our frame library as more frames become available to us, so if you have a frame that is no longer useful to you, let us know and it may be lucky enough to be added to the list.

This paper is another feature unique to us, and highlights our commitment to providing the best, unbiased quality information on carbon bikes.

The PDF of the paper can be found here. (1.2Mb)

After the recent hail storm here in Melbourne at Christmas, I had a local rider contact me asking for a bike assessment. He said his road bike was on the roof rack on the car as he was driving home when the storm front hit. There was significant damage to his car, so he wanted to be sure his bike was still safe to ride.

He did a visual inspection and couldn't see any damage but thought he would bring it in to us to provide a professional assessment. I examined the bike first visually and also could not see any obvious damage, however when I did the Ultrasound scan a number of delamination indications became very obvious on the scan. All up there were 8 delaminations spread out across the top tube. The damage was akin to small hammer hits, which is pretty much what golf ball sized hail is, the top tube was the only tube on the frame damaged which makes sense as it was fully exposed to the hail.

It was recommended that the frame was unsuitable for use in this condition. Once again this shows the value of Ultrasound scans to find non visible damage and determine the safety of your bike.

I have uploaded the seminar that I presented at the Ausbike show in 2010.

The seminar gives some background info on carbon bikes, composite materials and some repair and inspection examples.

Click here to see a PDF  version of the powerpoint file.