Wednesday, February 24, 2010

Trying to be a responsible mechanic...

The lowly bicycle mechanic has fallen under a vast load of technical crap in the past couple of decades.

First came the frantic "innovation" of the mountain bike era. As the avalanche of technology crushed the enthusiasm out of most casual recreational participants, the industry grabbed at the straw of advanced materials to try to stimulate addiction among a clientele with plenty of disposable income. And so, carbon fiber became commonplace. It mated well with the resurgence of road biking, as the surviving cyclists moved toward smoother rides and fewer mechanical problems.

Carbon fiber brings its own special issues which require dire warnings to the consumer. Blah blah blah, risk of serious injury or death, etc. The owner's manual and any supplemental bulletins always say, "If the bike suffers any impact, have it inspected by your Authorized Dealer."

Since the only qualification to be an authorized dealer is a credit rating acceptable to the accounts receivable department at Wonderbike, Inc., just exactly what makes the poor dingus at the Greasy Hub Bike Emporium more qualified than anyone else to say with any certainty that the expensive part under scrutiny is actually safe?

The warning sheet that came with a Specialized I just assembled says, "Damage to composite is difficult to visually identify." So I went looking --not for the first time-- for more advanced and reliable methods. With all the carbon fiber used in aerospace and other truly serious applications, someone must have better advice than, "Examine it carefully. If you have a bad feeling about it, throw it away and buy a new one."

Here is the result of about an hour trolling on the Internet for articles related to "inspection carbon fiber impact damage:"

From IEEEXPLORE

A high critical temperature SQUID magnetometer has been successfully employed in the evaluation of the behavior of multi-ply carbon fibers reinforced composite panels for aeronautical applications under low-velocity impacts.

Measurements of the induced magnetic field have been carried out above specimen damaged with energy impact from 1 to 40 J. A quasi-linear behavior in two different regimes between the SQUID's response and the energy of the impact has been found. This suggests a correspondence to the detection of intrinsically different damage that occurs in the laminates at different energies

I wants me a SQUID magnetometer, just so I can say I have one.

"Is my fork okay?"

"Quickly! To the SQUID magnetometer!"



From POLIMEROS:

Polímeros
version ISSN 0104-1428
Abstract

TARPANI, José R. et al. Thermographic inspection of impact damage in carbon fiber-reinforcing polymer matrix laminates. Polímeros [online]. 2009, vol.19, n.4, pp. 318-328. ISSN 0104-1428. doi: 10.1590/S0104-14282009000400012.

Continuous carbon fiber reinforced thermoset and thermoplastic composite laminates were exposed to single transversal impact with different energy levels. The damages impinged to the structural materials were evaluated by active infrared thermography in the transmission mode. In general, the thermoplastic laminate thermograms showed clearer damage indications than those from the thermosetting composite. The convective heating of the samples by controlled hot air flow was more efficient than via irradiation using a filament lamp. It was also observed that longer heating times improved the damage visualization. The positioning of the specimen's impacted face regarding the infrared camera and the heating source did not affect the thermo-imaging of thermosetting specimens, whereas it substantially influenced the thermograms of thermoplastic laminates. The results obtained allow concluding that infrared thermography is a simple, robust and trustworthy methodology for detecting impact damages as slight as 5 J in carbon fiber composite laminates.

[Impact damage of carbon fiber polymer–matrix composites, studied by electrical resistance measurement
Purchase the full-text article

References and further reading may be available for this article. To view references and further reading you must
purchase this article.] Dealer tax and title not included. Some restrictions apply.

Maybe infrared thermography is the affordable solution. I haven't priced the equipment yet.

AND AS CITED:

Shoukai Wanga, D.D.L. Chunga, Corresponding Author Contact Information, E-mail The Corresponding Author and Jaycee

H. Chungb

aComposite Materials Research Laboratory, University at Buffalo, State University of New York, Buffalo, NY 14260-

4400, USA

bGlobal Contour Ltd, 1145 Ridge Road West, Rockwall, TX 75087, USA
Received 24 June 2004;
revised 20 February 2005;
accepted 24 February 2005.
Available online 13 May 2005.

Abstract

Drop impact damage of continuous carbon fiber epoxy–matrix composite laminates, was studied by electrical resistance measurement, which was shown to be more sensitive than the ultrasonic method. The oblique resistance at an angle between the longitudinal and through-thickness directions was more effective than the surface longitudinal resistance in indicating damage, particularly interior damage. The oblique resistance values from longitudinal segments of a specimen were not additive, but the surface resistance values were.

In the case of a unidirectional
composite, electrical contacts at 45° from the longitudinal direction in the plane of the laminate were more effective than those at 90°. Even minor damage associated with negligible indentation was sensed. The spatial distribution of damage was also studied.

AND....

Shoukai Wang1, Daojun Wang1, D. D. L. Chung1 Contact Information and Jaycee H. Chung2
(1) Composite Materials Research Laboratory, University at Buffalo, State University of New York, Buffalo, NY

14260-4400, USA
(2) Global Contour Ltd., 1145 Ridge Road West, Rockwall, TX, 75087

Received: 7 April 2005 Accepted: 24 June 2005 Published online: 3 March 2006
Abstract The method of sensing impact damage in carbon fiber polymer-matrix structural composite by DC electrical resistance measurement was evaluated by measuring the resistance of the top surface (surface receiving impact). The resistance obtained by using the four-probe method is a more sensitive, more precise (less data scatter) and more accurate indicator of composite damage than that obtained by using the two-probe method. The data scatter is low for both four-probe and two-probe resistances for impact energy up to 5 J, but it is lower for the four-probe resistance than the two-probe resistance. The data scatter increases with damage. It is attributed to electrical contact degradation. The four-probe resistance of the 8-lamina composite increases upon impact, such that the fractional increase diminishes as the distance from the point of impact increases. The four-probe resistance of the 24-lamina composite increases upon impact for the specimen segment containing the point of impact, but decreases slightly upon impact for the segments within about 20 mm from the point of impact. The two-probe resistance has less tendency to decrease upon impact than the four-probe resistance.

Hmmm. If you really want to know, check the four-probe resistance.

Until we upgrade our testing equipment, all you can do is look for dings or cracks, listen for scary noises, wear a mouth guard, and if something makes you nervous, replace it, no matter how much it costs. The bike industry thanks you for your business. That noise, that gouge? They're probably nothing.

5 comments:

Rantwick said...

Hey man, great post and good points! What, if anything, does make your LBS expert in evaluating carbon fiber? You are dead right... they are expert in selling the replacements you probably don't want to risk not getting... I've never been so glad that I can't afford the stuff.

Steve A said...

In a humorous way, are you suggesting a post from an aero engineer that deals with this daily might be helpful?

Ham said...

You mean, I ought to be doing more than tapping and listening? Oooo-er.

On a serious note, although my roadie gets lots of tlc, my commute gets a bit of a thrashing about. It has a carbon fork and I was only thinking to myself last weekend (as I finally got around to changing the front bearings and putting enough in - gosh it is smoother) how do I know if the fork is weakening? Or, do I change is every 4/5 years - 20,000 miles?

cafiend said...

Steve A, I would love some info from someone who actually knows. The bike companies are so rightfully shy of litigation that they will tell you to junk it if it scares you. BTW, that's not under warranty.

Ham, maybe we'll have a better answer for you soon.

Steve A said...

Preface post to "Carbon for the Rest of Us" was made. Just get Ham out of that miles/time change mindset.