Composites Experts for Failure Postulation

It looks like an overload failure. Anyone been using the bike to jump things?

At the failure surface is there any evidence or discoloration, melting, etching?

Most likely a manufacturing defect.

Steve S,

I only have pictures. Looking at whats available, there is no sign of discoloration. I dont know about melting or etching.

The point where the vertical snapped-carries no load.Only for steering.Bamboo would have been a FAR better choice.

And you were not negotiating a sharp bend at the moment(Poor Ron -I feel sorry!).

Poor design,Lack of sense of proportions, Negligence by a big exporter!

Sue them and tell them to visit my website www.innovation2survive.com-

CHANGE!

MukulMahant,

Not sure why you say there is no load at the interface of vertical (steerer tube) with blade.

Hello bicycledisciple

General:

In engineering work, or designing parts which fit together, it is normal to provide some gradual stress relief between the adjoining parts.

This is often done as a tapered transition between the two parts, or a "V", (with a rounded bottom of the "V"), cut out of opposite sides of the larger part, as in older steel-framed cycles.

This tapered or gradual transition is done the same when making off the end of EHV cables into connecting bushings, so as not to concentrate the electrical stress but spread it over a longer cable distance, and thus avoid failure of that cable termination.

Tapering of structures or assembled parts to relieve abrupt stresses is an important part of ensuring that damage does not occur later, while the structure or part is in use.

The Damaged Cycle:

The transition in the pictures of that carbon fibre cycle appears to be abrupt, and either the part was defective, or at least poorly designed, without any gradual stress transition zone which I am able to perceive.

Carbon Fibre stress-caused failure is generally catastrophic, without warning, and can lead to death, disability or other injury.

I can feel those facial injuries from here.

Kind Regards....

See this thread a few weeks back... it is on failure of composite handlebars and covers the topic pretty well.
I would also add that it seems to have failed at one of the bearings...Is there interaction between grease and the composite resin?

(Hope the face heals up ok..)

Del

FYI : I'm not the one who has suffered any injuries. I'm only an engineer trying to understand this failure by referring to someone's bicycle pictures.

My observations :

- Steerer tube seems to have been bonded to an alloy or ALU crown of fork, and then maybe sanded down.

- Cylindrical base of steerer tube must not have provided adequate surface area for bond. This way, any stress risers can really spell trouble.

- I also wonder if difference in coefficients of thermal expansion of the bonding sites somehow played a role (?)

- Manufacturing defect a likely possibility. I dont know too much about how these things are exactly made, so I can't offer an explanation in this area.

I'm not seeing much evidence of elongation, seeing from these pix evidence of shear.

Was there a sharp edge in the edge of the metal tube at the pointof failure?

I'd expect to see much more elongation in the carbon than i'm seeing if it were overloaded...

My guessis stress riser at the point of separation caused by notching due to sharp edge of tube, some one 'sanding to fit etc.

my 2 cents, i'm a steel guy, not a composite champ.

milo

Composite failures tend to be brittle fractures, and so it would not be expected to see elongation.. At least IMHO...

Thanks for the follw up.

I agree with the brittle characterization. Wasn't sure what I was seeing in the metallic sleeve surrounding the failure, what Del called a "bearing"

milo

Nor was I ! Ther is presumably a bearing somewhere in the steering head ...normally a race with loads of small diameter balls...I just wondered if the grease therein was a factor.
We really ned a real good close up shot.... and I too don't think you are going to get elongation in a composite... snap a piece of timber (nearest vaguely similar material to hand)...do you see elongation?... nope.

Del

I agree Del, I am wondering if maybe the bearing race failed resulting in scoring of the the composite tube. The pictures look like there could have been a clean cut of the tube to the point that there was sufficient diameter reduction that the final failure was simple brittle fracture.

One plausible theory anyway I suppose!

Hello friends

While it would not be the intention here to postulate or analyze the failure aspects of the bicycle frame system, the writer wishes to pinpoint certain fundamentals that are to be carefully considered in selecting engineering materials for specific applications. It is also mentioned here that the writer is working on various "alternate engineering materials" (including Polymer Composites), toward arriving at ecologically and environmentally benign product systems.

1. Metals have been known to be in existence for over 3000 years, and when Steel and other metallic Alloys were developed during the last 200+ years, engineering materials reached unprecedented levels of technological utility. Thus, a vast storehouse of knowledge and experiences were gained in using and converting these materials into specialty Engineered products that have all stood the test of time.

2. Polymer Composites are in vogue during the last 50+ years, and the science/ technology/ engineering are still in rudimentary stage (comparatively), although great amounts of works have been done and practically verified, in terms of utility and in many cases having superior durability.

3. The "realities" of actual performance in applications are better understood in case of Metals and Alloys than those of Composites, not so much due to any inferiority of these materials; but attributable due mainly to the lesser human experiences so far (in these last 50+ years).

4. Although there are many specific property-details that are to be considered before selecting any material for a particular application, a "FIRST SELECTION" criteria could be based on certain Fundamental Mechanical Engineering Properties:

http://www.adrive.com/public/2a1fc7f5244f3af332dd19618143bc2a9474181f5dd8cc6f4f1c2906b343ffb3.html

... it would be noted that metals are "tougher" than the Polymer Composites, as seen by the "Elongation" of almost all metals being higher. But, we have now learnt many lessons and have arrived at creative designs to overcome these "initial hiccups" and today's Composites can claim many property-aspects that the metals could never offer.

5. It would be necessary for Composite Engineers/ Designers/ Users to interact more often, looking at various "cases", and arrive at appropriate "corrective" actions, through more and more usage experiences. That alone would take us upward, to improve the ultimate utility and durability of Composites.

Best wishes

pvhramani

Thin-walled carbon fiber/epoxy tubes can easily be cut with a knife or scissors. Shear forces at the connection line between metal outer tube and the carbon fiber tube would act like a knife. Additional thickness, cross-reinforcing or a shear-resistant layer at the shear line should have been part of the design.

Many grades of carbon fiber are available. A lower-cost utility grade would have more impurities along the length of the fiber and less stress conditioning. A high performance, mil-spec (and high cost) carbon fiber would have received additional processing to remove impurities and additional stress conditioning. Inexpensive grades which are used for tripods and other low-impact and low-stress products are available as off-the-shelf commodities. The probability is that someone less knowledgeable about carbon fiber assumed that the simple, non-reinforced carbon fiber, off-the-shelf tube would outperform metal. Bad assumption.

Measuring tube wall thickness, chemical removal of the epoxy resin and tensile testing of the released carbon fibers would likely reveal the logic involved in choosing that particular tube for the bicycle.

The part wasn't made strong enough to withstand the loads imposed upon it.

The model airplane guys are including a layer(s) of Kevlar Aramid fiber cloth with the carbon cloth to increase "toughness".

As I understand it, using the carbon fiber by itself is technically sufficient, but experience out in the field is indicating the need for the Kevlar reinforcement.

The part wasn't made strong enough to withstand the loads imposed upon it.

Yup, I think the dynamic loads on this sort of thing vastly exceed what the designers expect. It is surprising what dynamic loads a human can exert.

Try picking up the bathroom scales, grip with your hands either side and squeeze... I can squeeze more than my body weight.
Imagine a guy on the bike standing on the pedals and kicking up hill pulling on those bars....loads of power pulling on and off, recipe for fatigue failure.