I recently experienced the failure of the carbon fork on my 2001 K2 Mod3 road bike at 20 miles per hour. Yes, I was injured. My question is the bonding agent that secures the carbon fiber fork to the aluminum fork as strong and durable as the carbon fiber or the aluminum. Also I have been told by a bicycle manufacturer that the workable lifespan of "high performance bikes to be less than 10yrs, which seems to not agree with the article on lifespan of carbon fiber bikes that gives them an unlimited lifespan. I have some concern because I like to purchase my bikes slightly used thinking I can afford a much nicer bike. Example, I just purchased a Serotta Legend Ti with carbon forks & seat stays, hardly used for $3000 but am now somewhat nervous about material failure. Whats one to do to know if a bike is safe?
Carbon fiber is a bit tougher to determine the safety of than steel or aluminum (where a thorough visual inspection normally reveals the serious flaws, along with checks of headset tightness and what not) - aside from a thorough visual inspection for cracks and damage all around, you really need a professional evaluation. Bikeradar has some tips for inspecting carbon fiber bikes - key points are bonding areas and essentially places where stress builds up and making sure the bike hasn't been crashed before. There are plenty of guides on buying used bicycles online and safety checks to perform on them.
As for lifetime, especially in the case of a high end carbon fiber bike, the lifetime is a function of how much it has been used. A crash can do a lot more damage to a high end carbon fiber bike than to a lower end steel/aluminum bike (due to likely higher speeds and weight savings). There was a previous question here which highlighted the lifetime of carbon fiber bikes (especially forks), which you should read. In particular, the article by Zinn is useful.
You can do a thorough inspection of the bike and components, including a tear down. I have only ever bought one bike used. After I did, I tore it apart down to the frame and inspected everything. I ended up replacing a wheel because of cracks around a spoke hole on that rig.
As far as mixed material forks, that is a pretty common practice. I don't have any specific numbers to support this, but I haven't ever heard of the failure rates for such forks being different than all carbon forks.
As far as "carbon" is concerned, there is as much variation out there in the market as there is with anything. Years ago, the first carbon frames were assumed to be a bit more disposable and mainly for the elite race crowd who were replacing their bikes every other year or so anyway. There are currently companies making downhill race bikes from extremely thick and durable carbon that will take the same beating an aluminum frame would and last just as long. I'd say a place to look is at the company's warranty. Many companies are now offering lifetime frame warranties, even on their carbon frames.
I should quickly define a few words to avoid ambiguity in what follows:
Strong == Force / Cross-Section Area.
Durable == Strain (deformation) energy in the material before failure.
Endurance Limit == Asymptote where alternating stress does not decrease with additional load cycles
To your question: Bonding Agent
The bonding agent is (in theory) as strong as the carbon fiber epoxy, but structurally, it is the weakest over-all material in the joint.
Carbon Fiber >> Aluminum >>> Bonding Agent or Epoxy
Remember, carbon fiber is composed a the cloth-like carbon fiber soaked in an epoxy (another bonding agent), effectively gluing carbon sheets together. At the macro scale, the carbon fibers transfer most of the force and the epoxy is just there to hold the carbon fibers together. At the micro scale, only the epoxy and
bonding agent are bonded. I cant speak with respect to specific bonding agents or bike makers, but from an engineering perspective, the bonding agent should be as strong and durable as the carbon fiber epoxy, though weaker than individual carbon fibers or aluminum.
While the bonding agent and epoxy are close in strength, the bonded area/agent is the weakest material in the joint: At the micro scale, this is where deformations (strain) and stress are largest and failure begins at the macro scale.
To your next question: Life Span
Materials with an endurance limit have an unlimited fatigue life (for a max rider weight) in theory. Carbon fiber and Steel have an infinite fatigue life... In theory. If stresses are below the endurance limit, the material does not weaken and can undergo theoretically infinite stress/load cycles. Other non-ferrous alloys like Aluminum do not have an endurance limit, the material continues to weaken with additional stress/load cycles; eventually they are destined to fail- bike makers design aluminum frames to last far in excess of the what someone might ride during the bikes useful life.
In reality, many things affect fatigue life and strength in materials. Scratches (even small ones) are a Very common cause for significant failures to develop/propagate in all materials. Alloy frames suffer from incomplete welds or poor heat treatments during manufacture as possible causes of failure; typically alloy failures are not catastrophic (and safer) because the metal bends (giving obvious and ample warning) before breaking in half.
Carbon fiber is increasingly sensitive to impacts. Where alloys bend, carbon is brittle and might crack or de-laminate (where layers separate, first at the micro scale, leading to macro scale cracks and failure. As cracks develop, surrounding areas take more load, causing the existing crack to widen. Carbon (and composite failures in general) are more catastrophic in nature because carbon is more brittle and there is little warning or time before the carbon breaks in half. Voids (small air pockets trapped in between carbon layers) during manufacture, if present, are likely causes of failure. UV radiation and extreme heat, or oils and solvents can degrade the epoxy/bonding agent.
In practice, normal bike use leads to failures - ie. Handling the frame with dirt and sand on it, A rock kicked up from a tire impacts the frame, You fall, The frame gets scratches in the car or on the rack, Take your pick. So while the materials themselves have an infinite fatigue life, life happens and failures happen; bike makers continually tweak their designs and techniques to avoid real-life failures but they happen.
In buying a used bike, inspect it in person, and gain trust that the owner took good care of it. I bought my bike used as well - personally, I think thats the way to go - but there is some level of unavoidable risk.
The answer here is not about the strength of carbon vs aluminium but with bonding. I worked with (aftermarket) K2 Carbon mountain bikes in the late 1990's and they had aluminium dropouts at the rear (to hold the rear wheel on) bonded to layered carbon rear swingarm.
During testing for one of my products we mounted disk brakes to the dropout but suffered failures where the dropout would unbond from the carbon - this was due to excessive heat from the disk brake that weakened the "glue" bonding the carbon to aluminium. We redesigned the product to prevent heat transfer.
So answering the question: the bonding agent for layered carbon to aluminium has a time and heat based weakness - however, the good news is that they can be rebonded so long as they survived the crash.