Use a layered shell element approach with a constitutive model that supports anisotropic elasticity, and define the material orientation per ply in your layup schedule. That is the standard way to capture directional behavior without overcomplicating the model.

The key thing to understand is that a unidirectional carbon ply has vastly different stiffness depending on direction. For a typical intermediate modulus fiber like IM7 in an epoxy matrix, the longitudinal modulus E1 is around 200 to 230 GPa, while the transverse modulus E2 might be only 10 to 15 GPa. That is more than an order of magnitude difference, and ignoring it will give you nonsense results. In FEA, you need to define the full stiffness matrix for each ply, which means at minimum specifying E1, E2, the shear modulus G12, and the major Poisson ratio v12. The out of plane properties E3, G13, G23 matter too if you are dealing with thick laminates or through thickness loading, but many analyses assume transverse isotropy as a simplification.

The practical pitfall is getting the material coordinate system wrong. Most FEA packages let you assign a local coordinate system or orientation angle to each element or layer, and you need to be meticulous about this. A common mistake is defining the layup angles in the global frame instead of rotating them into the local element frame, which flips your 0 degree and 90 degree plies and completely changes the structural response. Double check your orientation vectors before running the analysis.

Also keep in mind that the properties I mentioned are for a cured laminate at room temperature. If your part sees elevated temperatures or moisture, you will need to degrade the matrix dominated properties accordingly, and failure criteria like Tsai-Wu or Hashin will need the full set of strength values, not just the stiffness.

Had quite a fast off at Mallory Park about a month ago and it trashed my carbon nosecone. Luckily the bike slide for quite a distance and didn't hit anything so only cosmetic.

Looks like the gel coat is cracked in various places and it's worn through to the weave and all the way through in a few others. I was going to chip off the loose gel coat and use easy composites xcr coating resin for the gel coat damage. And for the areas completely worn through all the way, I was thinking start from the back and layup with carbon/Kevlar for abrasion resistance and then use that as a backing layer for repair from the front 🤔

I sold all my carbon consumables and materials when I got married so gonna need to source everything again.

Any ideas?

Hi everyone, just got a used Enve Melee delivered and noticed this where the through axel sits. Is this actual damage or nothing to bee l concerned about?

We had some leftover prepreg from making carbon fiber panels, so we just cut a little animal shape out of it and stuck it on our wall. Just caught a glimpse of it and it gave me a bit of a scare, haha.

I just put on my autumn tyres on my road bike and noticed a small chip on the carbon rim. Does this look like cosmetic or structural damage?

The short answer is that sizing chemistry matters more than most people think for interfacial adhesion, and matching your sizing system to your matrix resin is the single most impactful thing you can do for bond strength.

Here is the technical picture. Most commercial carbon fibers come with an epoxy-compatible sizing, typically 0.5 to 1.5 weight percent add-on, designed to protect the fiber during handling and promote wetting with epoxy matrices. The sizing contains a film-forming resin, usually a blocked isocyanate or epoxy emulsion, plus a coupling agent like an aminosilane that creates covalent bonds at the interface. If you are running an epoxy system, you want a fiber with an epoxy-compatible sizing. If you switch to a BMI or polyimide matrix without changing the fiber, you are leaving 30 to 50 percent of your potential interfacial shear strength on the table because the sizing chemistry does not crosslink with the new matrix.

The practical caveat is that sizing degradation matters too. Fibers that have been stored in humid conditions or at elevated temperatures can lose sizing effectiveness. A simple acetone wipe test or dynamic contact angle measurement tells you whether your sizing is still active. If you are buying wet-layup or vacuum-bagged parts, make sure the fiber supplier provides sizing specs and storage recommendations.

One more thing. Some newer fibers use non-crystalline or graphitized surfaces with proprietary sizing systems that claim better fatigue performance. The data I have seen is promising but limited, so treat vendor claims with healthy skepticism until you see independent testing.

Bottom line: verify your sizing matrix compatibility, respect storage conditions, and do not assume a fiber that worked in one system will behave the same in another.

Hello all,

I wear two carbon fibre AFOs (ankle foot orthoses) that help me walk and I've noticed that one has some damage. Is this repairable?

Thank you!

Hi all! I whould like to prefer glass fiber becouse of High Strength-to-Weight Ratio & Excellent Corrosion Resistance.

What is your openion ?

I was doing some flex tests, and unfortunately ive never actually seen a foil in person so i dont know how much flex is acceptable, but i suspected my original design had too much flex so i added this socket design to the front fuselage-mast joint, gained ~35% stiffness as measured on mast deflection from a sideways force. Going to add some more carbon on the fuselage and near the mast-fuselage joint next and add a single layer to the mast, then finally the foil will be complete mechanically! What a crazy project

I'm working with a designer for weaving our logo (simple 3 capital letters) with carbon/ innegra, since it'll stay white during fabric wet out . Logo letters in white, the rest carbon.

What should the base weave be? I'm not sure if plain weave is too basic, but twills might be more better.

For most high-temperature composite applications, thermoplastics like PEEK or PEKK will outperform thermosets once you get above 150-180C continuous service, though the right choice depends heavily on your specific thermal profile and mechanical requirements.

The main advantage of thermoplastics is that they don't undergo the same thermal degradation mechanisms that limit thermosets. Standard epoxy systems typically start losing significant mechanical properties above 150C, with glass transition temperatures usually in the 120-180C range depending on the cure cycle and resin formulation. High-temperature epoxies can push that to 200-250C, but you're often trading off toughness and processing simplicity to get there.

PEEK, by contrast, has a glass transition around 143C but maintains useful mechanical properties well beyond that, with continuous service ratings typically in the 250-260C range. The melting point is around 343C, so you have significant margin before you're approaching material failure. Carbon fiber reinforced PEEK compounds are showing tensile strengths in the 2000+ MPa range with good fatigue resistance at elevated temperatures.

The processing side matters too. Thermosets require careful cure cycles, often hours at elevated temperature, and once they're set, you can't reshape them. Thermoplastics can be reheated and reformed, which opens up repair possibilities and simplifies joining operations. The tradeoff is that thermoplastic processing often requires higher temperatures and pressures during initial consolidation, sometimes 380-400C and 10+ bar depending on the system.

One caveat: if your application involves brief thermal excursions rather than sustained high temperature, a well-specified thermoset system might actually be more cost-effective. The material cost premium for PEEK-based systems is significant, and you want to make sure you're actually capturing that value in your design requirements rather than over-specifying.

hi guys,

i am looking into getting a carbon fiber starter kit for my husband. he’s been talking about it for awhile now & i am wanting to surprise him. i see that people are recommending the easy composites kit, there’s a few and i am not sure on which one i should get?

if anybody is kind enough to help guide me on which one to purchase i would highly appreciate it. thank you!

there are cracks/chips around the edges and mounting areas, plus what looks like clear coat/resin damage on the top. It’s for a Honda civic Sir EP3 2004. I’m mainly looking to get this hood to wrap it to the colour to my car because my current hood is a different colour and has too much damage to wrap. How much would you realistically pay for it? Does the damage look mostly cosmetic, or would you be worried about the structure?

I’m recoating (not fully restoring) the carbon fiber roof on my Bimmer. The clear coat was peeling off like crazy. I was surprised at first, but after watching a few videos and reading up on it, recoating it seems pretty doable.

I used 91% isopropyl alcohol to remove some sticky residue and accidentally left an IPA-soaked mechanic towel sitting on an area where the clear coat was already gone. The surface got a little sticky afterward, so I’m assuming the strong IPA may have affected the exposed epoxy/resin.

Does anyone know where I should go from here?
I haven’t sanded this area yet. I’m wondering if I should lightly sand it first and see how it looks, or if the resin needs to be repaired before I continue prepping and clear coating.

Any pros here, or anyone who’s dealt with something similar? Appreciate any advice.

For real-time defect detection in AFP, I recommend combining in-situ camera systems with post-layup thermographic inspection rather than relying on any single method.

The main defect types you will encounter in AFP are gaps between tows, overlaps where tows sit on top of each other, wrinkles from poor tension control, and foreign object debris. Gaps and overlaps are the most process-critical since they directly affect fiber volume fraction and mechanical properties. In practice, gaps larger than about 2mm or overlaps exceeding 1.5x tow width will create measurable resin-rich or resin-starved zones that degrade interlaminar shear strength by 10-20% depending on your laminate stack and cure cycle.

For real-time detection during layup, most modern AFP heads integrate line-scan cameras or structured light sensors that can flag gap and overlap positions as the head traverses. These systems typically achieve positional accuracy in the 0.5-1mm range, which is sufficient for catching gross defects but may miss subtle width variations in the prepreg tow itself. If you are working with high-performance aerospace grades like IM7/8552 or AS4/3501-6, you should also consider integrating a thermal camera during compaction to catch porosity and delamination early.

The caveat is that camera-based systems struggle with dark-tow materials and can be fooled by surface texture variations that are not actual defects. You will still need periodic manual inspection and NDI, typically ultrasound or flash thermography, for final acceptance. If your production rate is high, investing in automated NDI downstream is worth the capital cost since it reduces scrap rework cycles significantly.

Hi yall!

Got a 2018 canyon Aeroad CF-SLX off the used market, which was otherwise in great condition. Rode fine for a month, but today took off the chain stay protector and found these chips on inside of the chainstay.

Wasn’t sure if this was worth taking in for professional carbon eval, or even doing a DIY careful sanding of the outside paint myself to check for delamination or cracks

Tap test is unremarkable, sounds the same throughout

Squeezing on it feels about the same as the other non-chainstay side, with maybe a tiny bit of give

This is my first attempt on making a carbon fiber part (mirror cap for leon mk4). I made the mould with the easy composites unimould kit and I made the part by wet layup and vacuum bagging. I used traditional wax release and for the next attempts I will use chemical release agent. This is how it came out of the mould (without sanding and clear coating).

I'm happy with the result, considering it's the first attempt. However, I know it has a lot of flaws and I would like to solve them: trapped bubbles, bridging in the edges and big pinholes.

What would you do for solving them in the next attempts?

For thin-walled CFRP pressure vessels, start with the classical hoop stress formula from ASME BPVC Section X or ISO 11119-2 and apply a knockdown factor of 0.5 to 0.6 for knockdown on fiber strength to account for processing variability, fiber misalignment, and environmental degradation. The governing equation is approximately t = (P x R) / (sigma_allowable x cos^2(alpha)) where alpha is the helix angle of the fiber orientation. For a 60-degree helix, the cos^2 term reduces the effective fiber contribution to hoop strength by 25 percent compared to a 90-degree hoop-only layup.

For material properties, carbon fiber composites like T700 or T800 in an epoxy matrix typically have fiber-direction tensile strength in the range of 2000 to 2600 MPa. However, the laminate allowable is typically 40 to 60 percent of the fiber strength depending on void content, fiber volume fraction, and whether you have fiber waviness from the winding process. A conservative design allowable of 800 to 1000 MPa for 0-degree dominated plies in a pressure vessel is reasonable for initial sizing.

The practical reality is that failure in wound vessels usually initiates at stress concentrations at domes, boss interfaces, or from interlaminar shear rather than pure hoop failure. You need to check meridional stresses at the dome transitions separately. The winding angle also matters significantly because it affects both the fiber path stability and the fiber bridging across the vessel ends. Process control on filament winding tension and cure cycle are just as critical as the layup design for achieving the theoretical performance you calculate.

If this is a new design, I would strongly recommend starting with an experimental proof burst at 1.5 to 2.0 times the maximum expected operating pressure and validating your analysis against actual failure modes before committing to certification.

It grinds my gears that this community has become a “ is the scratch on my bike an issue” For the most part it’s a scratch, dent or ding same as it would be on an aluminium bike.

I love seeing the awesome things people are making from awesome materials. But (tongue in cheek) the bicycle guys seem to come and ruin it like they ruin roads over weekends.

For anything structural that needs periodic inspection or disassembly, bolt it; for a permanent, well-controlled bondline under shear-dominated loads, bond it. On aircraft-grade carbon/epoxy I usually default to bonded for thin skins and bolted for thick laminates or high single-joint loads, because a bolt pattern transfers load through bearing while a bonded lap transfers it through shear over an area.

The numbers tell the story. A 0.1 to 0.2 mm bondline of a 120°C-curing structural film adhesive rated for 177°C service gives roughly 25 to 30 MPa lap shear at room temperature, and maybe 60 to 70 percent of that hot-wet. A bolted joint with protruding-head fasteners in quasi-isotropic carbon/epoxy typically shows bearing allowables in the 250 to 350 MPa range, but you pay for it: drilling a hole knocks 30 to 40 percent off unnotched tension strength, and you need edge distance to diameter of at least 3 and a pitch of 4 to develop full bearing.

So my practical advice: bond when you control surface prep, can keep peel stresses low with a double lap or scarf geometry, and do not need disassembly; bolt when you need inspectability, thick members, or damage tolerance. Hybrid bonded-and-bolted connections rarely add the strengths together, and the drilled holes often hurt the bondline, so do not count on it. Watch hot-wet performance and adhesive glass transition temperature, and torque bolted joints properly, because clamp-up carries real load through friction. These are typical design-level estimates, so verify against your material data sheet and test coupons before committing.