Carbon forks. Lateral movement - Page 2
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  1. #26
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    Quote Originally Posted by tlg View Post
    The coin tap test is useless on a carbon fiber bike. It only works (potentially) on large consistent shaped objects. It'll never work reliably on an irregular shaped bicycle with compound shapes.

    carbonbikerepair.com.au/articles/82-the-limitations-of-tap-testing]The Limitations of Tap Testing
    The Limitations of Tap Testing
    You will often see reference to the tap test or coin test method for finding damage in composite materials. It is used in aerospace, so it must be good, right?
    As a qualified quality assurance technician having held CASA, FAA and a host of aerospace quality certifications, I am able to clarify this information.

    How does the tap test work?
    The method involves gently tapping the part with a special tap hammer or even a coin, hence the coin tap reference, you don't want to tap too hard though as this may create damage. The impact energy travels through the part causing it to resonate or ring in the same way a tuning fork rings at a certain frequency or note. If there is an inconsistent condition such as an unbond or major delamination, the audible ring will be different. By listening to the tonal changes, an indication that there may be damage can be noted. The method is dependant on the skill and experience of the operator. It is cheap and simple and works reasonably well for these type of defects. In aerospace Non Destructive Inspection it is a backup (known as a secondary) method or used in non critical applications, the primary method is ultrasound, so parts that are critical get an ultrasound scan.

    What can it find?
    Typically the method is used for finding flaws in bonded parts such as unbonds on flat panels between the composite or metal skin and the honeycomb core. Because it is an acoustic method, it is very dependant on the geometry of the part in the same way a different length tuning fork provides a different note. Thus the operator needs to be aware of how the sound wave or vibration travels through the part. Large consistent flat areas with thin skins are the most suitable, as the skin thickness increases and the curvature increases the results are less reliable. It is typically ok for finding unbonds in the composite skin to the honeycomb core greater than about 10mm in diameter. It is simple for an experienced technician to give a part a quick "tap" at any obvious visual damage indications, any suspected damage found is then scanned with the primary ultrasound method to validate.

    What can't it find?
    Tap testing is typically not recommended for laminate testing as it is unreliable in finding common laminate defects. For bonded parts if the damage is below a critical size or the material thickness is above a limit or located within geometry constraints it will not be able to find the damage. Porosity and voids, resin dry and resin rich areas also cannot be identified unless they are so bad it is obvious visually anyway. Light impact damage known as BVID, (barely visible impact damage) will also not be detectable. Background noise etc can also interfere with any results.

    How is this related to assessing damage in bikes?
    Bike frames are typically made from thin laminate with localised thicker areas at the higher stress joins. Bonded parts are limited in modern frames and typically no honeycomb core is used in the way some aircraft panels are constructed, with the exception of some disc wheels. On frames with bonded parts at the tube junctions such as imternal or external lugs, the compound curvature geometry and laminate thickness at the lug areas as well as the tube shapes contribute to getting unreliable information from a tap test. The damage also needs to be above a critical size dependant on the geometry of the inspected area, hence a 5mm defect in a seat stay that is only 10mm wide would probably not be found but will have a large effect on the structural integrity. Other factors such as internal joins, fillers and inserts will also affect the ability to get meaningful results.

    So as you can see, the tap test method on its own is not able to reliably detect damage in these type of parts.

    Conclusions
    Typically damage to bike frames that is able to be found with the tap test can mostly be identified visually as the primary method and then may be confirmed with a tap test as a secondary method. If an indication "taps" it is likely to be damage, if it does not "tap" there still may be damage however due to the limitations explaned above. Overall it is useful for confirming certain types of damage as a secondary method as long as you understand the limitations of the method.


    Rest assured we won't be giving up on our ultrasound scans any time soon, as it is a proven reliable primary method.
    coin tap by itself is not conclusive. But the article also concludes:

    Conclusions


    Typically damage to bike frames that is able to be found with the tap test can mostly be identified visually as the primary method and then may be confirmed with a tap test as a secondary method. If an indication "taps" it is likely to be damage, if it does not "tap" there still may be damage however due to the limitations explaned above. Overall it is useful for confirming certain types of damage as a secondary method as long as you understand the limitations of the method.
    It doesn't say useless. It's a worthwile secondary check, especially when the cost is almost nothing to try both in terms of effort and time.
    Last edited by aclinjury; 4 Weeks Ago at 03:17 PM.

  2. #27
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    Quote Originally Posted by tlg View Post
    The coin tap test is useless on a carbon fiber bike. It only works (potentially) on large consistent shaped objects. It'll never work reliably on an irregular shaped bicycle with compound shapes.

    carbonbikerepair.com.au/articles/82-the-limitations-of-tap-testing]The Limitations of Tap Testing
    The Limitations of Tap Testing
    You will often see reference to the tap test or coin test method for finding damage in composite materials. It is used in aerospace, so it must be good, right?
    As a qualified quality assurance technician having held CASA, FAA and a host of aerospace quality certifications, I am able to clarify this information.

    How does the tap test work?
    The method involves gently tapping the part with a special tap hammer or even a coin, hence the coin tap reference, you don't want to tap too hard though as this may create damage. The impact energy travels through the part causing it to resonate or ring in the same way a tuning fork rings at a certain frequency or note. If there is an inconsistent condition such as an unbond or major delamination, the audible ring will be different. By listening to the tonal changes, an indication that there may be damage can be noted. The method is dependant on the skill and experience of the operator. It is cheap and simple and works reasonably well for these type of defects. In aerospace Non Destructive Inspection it is a backup (known as a secondary) method or used in non critical applications, the primary method is ultrasound, so parts that are critical get an ultrasound scan.

    What can it find?
    Typically the method is used for finding flaws in bonded parts such as unbonds on flat panels between the composite or metal skin and the honeycomb core. Because it is an acoustic method, it is very dependant on the geometry of the part in the same way a different length tuning fork provides a different note. Thus the operator needs to be aware of how the sound wave or vibration travels through the part. Large consistent flat areas with thin skins are the most suitable, as the skin thickness increases and the curvature increases the results are less reliable. It is typically ok for finding unbonds in the composite skin to the honeycomb core greater than about 10mm in diameter. It is simple for an experienced technician to give a part a quick "tap" at any obvious visual damage indications, any suspected damage found is then scanned with the primary ultrasound method to validate.

    What can't it find?
    Tap testing is typically not recommended for laminate testing as it is unreliable in finding common laminate defects. For bonded parts if the damage is below a critical size or the material thickness is above a limit or located within geometry constraints it will not be able to find the damage. Porosity and voids, resin dry and resin rich areas also cannot be identified unless they are so bad it is obvious visually anyway. Light impact damage known as BVID, (barely visible impact damage) will also not be detectable. Background noise etc can also interfere with any results.

    How is this related to assessing damage in bikes?
    Bike frames are typically made from thin laminate with localised thicker areas at the higher stress joins. Bonded parts are limited in modern frames and typically no honeycomb core is used in the way some aircraft panels are constructed, with the exception of some disc wheels. On frames with bonded parts at the tube junctions such as imternal or external lugs, the compound curvature geometry and laminate thickness at the lug areas as well as the tube shapes contribute to getting unreliable information from a tap test. The damage also needs to be above a critical size dependant on the geometry of the inspected area, hence a 5mm defect in a seat stay that is only 10mm wide would probably not be found but will have a large effect on the structural integrity. Other factors such as internal joins, fillers and inserts will also affect the ability to get meaningful results.

    So as you can see, the tap test method on its own is not able to reliably detect damage in these type of parts.

    Conclusions
    Typically damage to bike frames that is able to be found with the tap test can mostly be identified visually as the primary method and then may be confirmed with a tap test as a secondary method. If an indication "taps" it is likely to be damage, if it does not "tap" there still may be damage however due to the limitations explaned above. Overall it is useful for confirming certain types of damage as a secondary method as long as you understand the limitations of the method.


    Rest assured we won't be giving up on our ultrasound scans any time soon, as it is a proven reliable primary method.
    Definitely not 'useless'. It is used often to verify damage that is first noticed visually.
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  3. #28
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    Quote Originally Posted by cxwrench View Post
    Definitely not 'useless'. It is used often to verify damage that is first noticed visually.
    i'm not sure I understand.

    If there is visual damage, what more information can coin-tapping provide? It's not going to give you any useful information about the integrity of the structure and what kind of forces it might withstand.

  4. #29
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    Quote Originally Posted by Finx View Post
    i'm not sure I understand.

    If there is visual damage, what more information can coin-tapping provide? It's not going to give you any useful information about the integrity of the structure and what kind of forces it might withstand.
    A coin tap will let you know whether or not it's cosmetic or possibly structural.

    You can have visual damage without structural damage.

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  5. #30
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    Quote Originally Posted by frdfandc View Post
    A coin tap will let you know whether or not it's cosmetic or possibly structural.

    You can have visual damage without structural damage.

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    Exactly ^this^. Customer brings bike in, it has a paint chip. Me: "Oh, there's some paint damage on the side of your top tube, has your bike fallen against something lately?" Customer: "Hmmm...yeah, some jackass knocked it over at the coffee shop and it fell against the bike rack, is that a problem?". Me: "Could be, let me do a quick test to see if I can determine if there is any structural damage." Tap tap tap thud tap tap. Me: "Yep, your frame is cracked."
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  6. #31
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    Quote Originally Posted by cxwrench View Post
    Exactly ^this^. Customer brings bike in, it has a paint chip. Me: "Oh, there's some paint damage on the side of your top tube, has your bike fallen against something lately?" Customer: "Hmmm...yeah, some jackass knocked it over at the coffee shop and it fell against the bike rack, is that a problem?". Me: "Could be, let me do a quick test to see if I can determine if there is any structural damage." Tap tap tap thud tap tap. Me: "Yep, your frame is cracked."
    And if no thud, you send them on their merry way?

  7. #32
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    Yes. If there is no change in pitch there is very little chance there is any structural damage. Carbon is either undamaged, or it's broken. There is no dent, or bend, or warp. It's either the way it came out of the factory or it's junk. If you've done it for 15-20 years you'd know what I'm talking about. If there is any doubt at all we ere to the side of safety and will recommend sending to the manufacturer for inspection.
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  8. #33
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    Quote Originally Posted by aclinjury View Post
    I got a bunch of forks here. Just tested them by squeezing them together using 2 fingers and a thumb. I tried to squeeze as hard as I could, I'm guessing the effort felt like trying to lock down on a skewer. Deflection was about 4mm.

    Carbon fiber is very resistant to fatigue, so if your fork is in fact still in good condition, you should be able to squeeze it pretty hard with 2 fingesr and a thumb over and over again and the legs would still be ok (i.e., it shouldn't give out any hint or wimper of sound).

    Another thing you could do is take a coin and tap the fork throughout. Tap both legs, inside and outside, all over. Assuming the fork is symmetrical, both legs should give out similar sound when "symmetrical areas" from both sides are tapped. So tap one side, then go to the other side and tap the same region, you should get the same sound. Of course there is always a slim chance that both same positional regions of the 2 legs are cracked, thus giving you the same sound, thus making this test give a false negative; however, the chances that there is a crack on both legs at the exact same locations is... very slim.
    Squeezing as hard as I can with my thumb and 2 fingers would be more pressure then I put on it. My fork blades are really thin. A new supersix fork. It has more deflection then my other forks becauaw of the blades but still was probably a few mm.

    Did the tap test. Both sides sound the same. Gives a hollow sound until I get above the blades.

  9. #34
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    There are also fluorescent dyes that you can apply to objects for non-destructive testing. The dye settles into cracks and you can see them with black light...
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  10. #35
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    Lbs believes it's fine. They were gonna reach out to Cannondale to see if I can get purchase a replacement. Never heard back from them. I'm assuming Cannondale won't sell me one? I did want something that would match the bike, glossy black.

    They said it wouldn't fall under there replacement program as they can't find any damage to the fork.

  11. #36
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    Measure point to point electrical resistance on a new frame/fork with a multimeter to set benchmarks and remeasure in the case of paint cracks/chips?

    There's some research out there indicating this may detect structural damage.

    https://www.semanticscholar.org/pape...da90dde5799fa0

    https://file.scirp.org/pdf/OJCM_2014011509365876.pdf

  12. #37
    tlg
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    Quote Originally Posted by Bremerradkurier View Post
    Measure point to point electrical resistance on a new frame/fork with a multimeter to set benchmarks and remeasure in the case of paint cracks/chips?

    There's some research out there indicating this may detect structural damage.

    https://www.semanticscholar.org/pape...da90dde5799fa0

    https://file.scirp.org/pdf/OJCM_2014011509365876.pdf
    That is an interesting study. But a brief read of the report, it's far more complicated than attaching a multimeter to your fork. For one, all the layers of carbon fiber are separated and sealed by (nonconductive) epoxy. You couldn't just stick a multimeter on it and get a resistance reading.

    For the cyclic loading test, a closed-loop hydrostatic servo material-testing machine is used. Tension-tension cyclic loading tests of stress ratio R = 0.1 are performed
    here. The applied maximum strain for the quasi-isotropic specimen is 2800 μ that is only 20% of the tensile fracture strain (fracture stress is 695 MPa). The frequency is50 Hz and the sine wave is used for loading

    To measure electrical resistance change, a LCR meter (type 3522) produced by Hioki Co. Japan was used. (They cost $4000)

    The testing requires Temperature Compensation and Cyclic Loading
    The abscissa is the number of cycles and the ordinate is the electrical resistance change ratio using the reference resistance at 10^3 cycles and 20˚C.

    The testing was performed on CFRP plates, rectangular plate specimens of 230 mm length and 12.5 mm width having 0 plies. It would not work on a complex CFC structure.
    Generally, CFRP laminates have angles plies such as 45˚-plies, the results of the electric current during cyclic loading of the quasi-isotropic laminate indicates that the reference electrical resistance R0 decreases with the increase of cyclic loading even when the loading does not
    cause damage. This shows that the simple monitoring with the electrical resistance measurement of the CFRP structures is not applicable for the actual CFRP structures.
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