Inside a carbon bike frame inspection: the NDT methods, in order

A lab-grade carbon bike frame inspection runs a specific pipeline from surface triage to subsurface scanning. The order is documented, the methods read different physics, and the geometry of the frame decides which subsurface method is the right next step after surface triage flags something.

This page walks the pipeline in order. The surface step is the same on every inspection. The subsurface step is method-selection by geometry, and the methods are complements not substitutes.

The surface pipeline, in order#

Five steps. Each one calibrates the next, and the order matters.

Step one: clean-frame wipe. The frame is wiped clean to remove dirt, film, and surface contamination that can mask fine surface disruptions. The wipe is the calibration step that makes the next four reliable; under raking light, surface dirt reads as ambiguous textures and slows the technician's pattern-matching against known damage signatures.

Step two: raking light at shallow angle. The frame is swept under a high-intensity, shallow-angle light source (Carbon Bike Doctor). The shallow angle is what makes the method work: light skimming the surface at a low angle casts micro-shadows from surface relief that flat overhead lighting flattens out entirely. Paint bubbling, clearcoat ripples, and localized depressions become visible where they would otherwise blend into the surface.

Step three: cotton-glove pass. Cotton-gloved hands slide across the tubing because the glove's delicate fibers snag on microscopic carbon shards or hairline cracks the eye misses. This is a documented method, not improvisation. The fiber snag is sensitive to surface roughness changes that visual examination alone misses, and it works particularly well at the kind of edge a hairline crack presents at the surface.

Step four: digital microscope and optical loupe at high-stress junctions. The four critical zones (bottom bracket cluster, head tube transitions, seatpost insertion area, dropouts) get separate scrutiny under magnification to separate benign clearcoat scratches from active laminate fractures. A clearcoat scratch looks like a discrete cut through a thin uniform layer with the underlying carbon weave intact; a laminate fracture looks like a broken weave with disturbed fiber ends and altered light scatter.

Step five: fiberscope (borescope) through open ports. A flexible imaging probe is inserted through accessible openings such as the bottom bracket shell or seat tube to inspect internal bladder compaction, wrinkling, voids, and bond integrity at internal junctions. The internal view shows what the external surface cannot.

What surface triage flags decides whether subsurface NDT runs at all, and which subsurface method is the right next step. A clean surface pass with no findings can exit the inspection as Safe or Serviceable. A surface pass with a flagged area enters the subsurface step.

Ultrasonic testing#

Ultrasound is the industry workhorse for carbon bike NDT. A portable transducer coupled to the frame sends high-frequency sound waves into the laminate; the wave reflects off the back wall and returns, and the system measures transit time to calculate wall thickness, resolving down to roughly one one-thousandth of an inch (Evident / Ruckus Composites).

When the wave hits a void, resin pocket, or delaminated layer it reflects early, producing an abnormal echo on the technician's A-scan display. The method is quantitative and entirely safe, but it requires comparative reference standards to interpret signatures correctly across varying tube shapes. The lab's calibration library is what makes the method's quantitative output mean something; a well-calibrated lab can distinguish a normal echo from a manufacturing-artifact echo from a delamination echo on the same tube section.

Where ultrasound excels: conventional tube geometry (down tubes, top tubes, seat tubes, chainstays, seatstays, fork blades, fork steerers). The method maps cleanly onto a curved surface with consistent wall thickness.

Where ultrasound struggles: complex joints with extreme geometry, embedded metallic inserts, or thick build-ups. The wave behavior at such junctions is less interpretable, and the transducer coupling becomes geometry-dependent.

Computed radiography (X-ray)#

Computed radiography is the gold standard for the geometries where ultrasound struggles. Spyder Composites is the US reference for this method (Spyder).

The method captures a digital radiographic image of the frame using X-ray attenuation; the resulting image shows density variation through the laminate thickness. Computed radiography is highly effective at finding trans-laminar cracks, impact stress fractures, fiber misorientation, wrinkles, and resin-to-fiber ratio inconsistencies.

Where computed radiography excels: complex joints with extreme geometry, embedded metallic inserts (the BB junction on some constructions, integrated chainstay-to-BB transitions, internally routed cable ports with metallic guides), and thick build-ups where ultrasonic wave attenuation makes interpretation unreliable.

The trade-off is portability. The equipment is far less portable than an ultrasonic flaw detector, which is why fewer labs offer it. A lab offering computed radiography is typically a fixed-facility operation rather than a shop-floor inspection rig.

Active thermography#

Active thermography applies a brief thermal pulse to the surface and tracks cooling with an infrared camera. Because air pockets, voids, and delaminated plies act as thermal barriers, heat dissipates differently over compromised zones and shows up as distinct thermal patterns (Certify Cycle).

The method scans a wider surface area per pass than point-contact ultrasound and is well-suited to large flat sections like down tubes and top tubes. Where ultrasound moves the transducer across the surface one contact point at a time, thermography reads a large area in one IR-camera frame.

Presidio Composites operates pulsed thermography as its subsurface method, complementary to ultrasound rather than competitive. The two methods read different physics; thermography reads the thermal response of subsurface defects, while ultrasound reads the elastic-wave response.

Fluorescent dye penetrant testing#

Dye penetrant is the method for surface-breaking micro-cracks where direct ultrasonic contact is impractical (Target Composites). A low-viscosity dye is applied to the suspected area; the dye seeps into any surface-breaking crack. The surface is cleaned to remove dye from unaffected paint. Under UV light, the dye trapped in the crack fluoresces and maps the fracture's exact size and direction.

The method has a narrow application (surface-breaking cracks only; subsurface delamination is invisible to it) but a high resolution within that application. It is the right method for a hairline crack at the seat cluster, a surface-breaking crack at a complex junction, or any case where the transducer cannot couple cleanly to the suspected damage zone.

Acoustic-emission (instrumented tap) testing#

The hobbyist "coin tap" survives in professional, objective form. An electronic digital tap hammer with a design originating at Boeing Aerospace (Carbon Bike Doctor) captures contact duration and stiffness response, removing human subjectivity from the hobbyist version of the method.

The instrumented form measures the impulse response of the local laminate. A sound section has a characteristic stiffness signature; a delaminated section has a different one because the disbond changes the effective local stiffness. The contact duration and the rebound profile are quantitative.

Instrumented tap is faster than ultrasound for area coverage and lower-resolution per point. It is best used as a screening method before targeted ultrasonic follow-up at any flagged areas.

Why method selection is geometry-driven#

The methods are not interchangeable. Each one reads different physics, and each one has geometries where it excels and geometries where it struggles.

• Ultrasound: quantitative at a point, slow over area, best on conventional tubes.
• Computed radiography: comprehensive, immobile, best on complex joints with metallic inserts.
• Thermography: fast over area, calibration-dependent, best on large flat sections.
• Dye penetrant: surface-breaking only, high-resolution where it works, best on hairline cracks at inaccessible geometry.
• Instrumented tap: fast and sensitive to stiffness change, lower-resolution per point, best as a screening method.

Many high-end labs combine two or more methods. The combination usually pairs a quantitative point method (ultrasound) with a fast area method (thermography or instrumented tap) so the area method directs the point method to the flagged zones.

The lab decision for an owner is whether the lab runs the method the frame's geometry calls for. A complex BB junction with metallic inserts wants computed radiography. A long flat down tube section wants thermography. A surface-breaking hairline crack at the seat cluster wants dye penetrant. A conventional chainstay tube section wants ultrasound.

What this means for the owner#

The carbon bike frame inspection pipeline is documented and method selection within the pipeline is geometry-driven, not marketing-driven. A lab that runs one method is fine if that method matches the bike's geometry; if it does not, the right answer is a different lab.

The shortest version of the rule: surface triage first, in order; then NDT proper, by geometry. The lab's job is to run the method the frame and the damage pattern call for. The owner's job is to ask which method the lab runs and to confirm it matches the inspection question being asked.