How to tell if carbon frame is knackered

[Drago]

My structural knowledge may be deficient here, as my somewhat limited knowledge is restricted to.launch systems, launch vehicles and orbital structures, so I'd be grateful for a reference with some reasonable degree of authority that describes how a crack always starts on the exterior of a hollow form. Cheers.

 

[Yellow Saddle]

My structural knowledge may be deficient here, as my somewhat limited knowledge is restricted to.launch systems, launch vehicles and orbital structures, so I'd be grateful for a reference with some reasonable degree of authority that describes how a crack always starts on the exterior of a hollow form. Cheers.

Go the mirror. Pull your pants down slightly and have a look. Tell me if that crack starts inside or outside. If your neck is stiff, hire a plumber to work under the sink and observe.

 

[Yellow Saddle]

Orbital structure.....hmmmm, let me go and throw one for my dog.

 

[Drago]

Or you could be sensible and provide a reasonable response to a reasonable question. I'm genuinely interested in the phenomenon to which you refer and the best you can do is act like a plum.

 

[gbb]

Quite clearly there is a gulf between percieved or possible damage to a frame, the probability that it WILL be ok....and the insurers reluctance (quite rightly IMHO) to assume any responsibility for future failure, however remote. Thats why theyll pay up in my experience, they dont want that responsibility, they dont need it, just pay up and forget it. Same for any LBS, why in the world would anyone assume responsibility in our world of claims for just about anything.
The price of a bike, even a good one is small beer to an insurance company.
The nub of it ? consider a carbon frame as effectively disposable...and move on.

 

[DTD]

The word back from the shop is the bike shows no sign of damage to the frame, even though it's had a jolt. They'll sort out the wheel. My friend's reassured to ride the bike.

 

[mythste]

My structural knowledge may be deficient here, as my somewhat limited knowledge is restricted to.launch systems, launch vehicles and orbital structures, so I'd be grateful for a reference with some reasonable degree of authority that describes how a crack always starts on the exterior of a hollow form. Cheers.

I would have thought its something to do with most of the outer faces are convexed. If you have any stress itll be the convex side that will "give" first?

I've said convex too many times in my head and now im trying to work out if i do indeed mean convex. convex convex convex.

 

[Yellow Saddle]

I would have thought its something to do with most of the outer faces are convexed. If you have any stress itll be the convex side that will "give" first?

I've said convex too many times in my head and now im trying to work out if i do indeed mean convex. convex convex convex.

The word you are looking for is rounded.

 

[mythste]

The word you are looking for is rounded.

I was trying to be clever for the bigger boys.

Am I essentially correct though? I have a I have a C-Tech in long distance relative theoretical mechanical sympathy.

 

[Yellow Saddle]

I was trying to be clever for the bigger boys.

Am I essentially correct though? I have a I have a C-Tech in long distance relative theoretical mechanical sympathy.

Cracks, almost always, start or end at the surface of the material.A carbon frame is made up of tubes with very thin walls. The likelihood of a crack running around the circumference of the tube, just under the surface, is nil. The likelihood of a crack running along the length of a tube, just under the surface of the 0.8mm thick wall and never surfacing, is nil. If you want to be fancy, I'll extend the scenario: it holds true for convex, concave, linear, polyhedrals, ellipses, robuses (rombi??) and other fancy or simple shapes, including...ahem....orbital structures.

This is true for all frame materials, spokes, saddle vinyl, bar tape, tyre rubber, vinyl wiring insulation, plastic mudguards, aluminium mudguards, steel mudguards....you get the picture.

On bicycles we don't have to go to an aircraft factory to find cracks, we look and listen.

 

[PhilDawson8270]

I'd be grateful for a reference with some reasonable degree of authority that describes how a crack always starts on the exterior of a hollow form. Cheers.

A crack in a frame starts at the surface.

Cracks, almost always, start or end at the surface of the material.

With cylinders, it depends almost entirely on the external loading of the structure, in the absence of axial or radial loading on the thin walled cylinder, then hoop stress is the greatest.

And with hoop stress, the inside of the cylinder has the greatest stress. This is due to receiving the same total strain, but the outside having a greater circumference in which to distribute it over. So with the absence of axial or radial loading, a fracture in a cylinder should always begin on the inside of the cylinder.

However, a cycle frame isn't really exposed to pressure differentials, so the hoop stress should remain fairly constant.

Though, this same difference in circumference, is what increases the strain externally in axial and radial loading.

It is incorrect to say that a crack always starts on the inside, as with no increases in axial or radial loading, cracks will nearly always form internally. However, with axial and radial loading, if they go beyond the UTS before the hoop stresses reach it, then the crack will likely form on the outside.

However, the effect is negligible on something with wall thickness as little as they are on bicycles, any impact strong enough to fracture a well manufactured carbon tube, is likely to fracture completely through inside and outside.

 

[Yellow Saddle]

With cylinders, it depends almost entirely on the external loading of the structure, in the absence of axial or radial loading on the thin walled cylinder, then hoop stress is the greatest.

And with hoop stress, the inside of the cylinder has the greatest stress. This is due to receiving the same total strain, but the outside having a greater circumference in which to distribute it over. So with the absence of axial or radial loading, a fracture in a cylinder should always begin on the inside of the cylinder.

However, a cycle frame isn't really exposed to pressure differentials, so the hoop stress should remain fairly constant.

Though, this same difference in circumference, is what increases the strain externally in axial and radial loading.

It is incorrect to say that a crack always starts on the inside, as with no increases in axial or radial loading, cracks will nearly always form internally. However, with axial and radial loading, if they go beyond the UTS before the hoop stresses reach it, then the crack will likely form on the outside.

However, the effect is negligible on something with wall thickness as little as they are on bicycles, any impact strong enough to fracture a well manufactured carbon tube, is likely to fracture completely through inside and outside.

The accident in question and it's would-be cracks weren't due to hoop stresses. There is pressure equilibrium in bicycle tubes. It "cracked" (or not) from something similar to buckling, but without the axial overload. Such cracks breach the surface and are visible. To put it simply, the tubes were squashed.

 

[PhilDawson8270]

The accident in question and it's would-be cracks weren't due to hoop stresses. There is pressure equilibrium in bicycle tubes. It "cracked" (or not) from something similar to buckling, but without the axial overload. Such cracks breach the surface and are visible. To put it simply, the tubes were squashed.

I did say exactly that

 

[ayceejay]

Go the mirror. Pull your pants down slightly and have a look. Tell me if that crack starts inside or outside. If your neck is stiff, hire a plumber to work under the sink and observe.

When it gets all technical and scientifical like this I can't follow. Is the plumber under the sink there to observe the crack in my arse or am I to study his?

 

[MossCommuter]

Are carbon frames solid?