Here are some quick ideas about composites (including Carbon Fiber and glass fiber) for people that may be new to working with them. They may kick-ass in certain in high performance applications but can easily be used incorrectly.
Some Counter-Intuitive lessons about Composites:
Look carefully at this Fig. 9. This shows the Strength-Weight ratio of common materials:
The first thing to note is Carbon Composite’s high Strength-weight ratio. WOW, it’s basically 10x that of steel.
The next thing to remark is that Steels and Aluminum Alloys have VERY similar strength-to-weight. That can be reasoned from the idea that “Aluminum is 1/3 the density of steel and 1/3 the Tensile strength”
Finally, the very impressive thing to note is that WOOD has similar strength-to-weight ratio as steel and aluminum!
(Some of these great insights I learned from this awesome book. It’s enjoyable to read and reinforces tons of fundamentals with tangible examples from the Formula 1 performance realm: Engineer to Win )
Carbon Fiber is NOT ALWAYS YOUR BEST CHOICE:
CF is very stiff (notice the steep stress-strain) which is good if you want very rigid like airplane fuselage or surfboard, however if you need something tougher or more resilient with greater energy absorption, you may prefer something else like Glass Fiber. For Example something where the entire body is intended to flex: Skis, snowboard, longboard, that absorbs bumps for smoother ride would prefer E-glass (glass fiber) over Carbon Fiber. (In other words, Glass fiber can bend further without breaking.)
SpringFree Trampolines utilize FIBERGLASS rods to provide high-cycle energy return
General Design Practices: [Here’s a great overview with much more detail]
Flexibility: Carbon Fiber has 10x the specific Tensile strength of steel… Although Fiberglass (E-glass) has lower Tensile strength than Carbon Fiber, it has much greater elongation meaning that it absorbs much more energy before fracture. Thus contrary to popular assumption, Carbon fiber may not be the ideal material if the structure is intended to undergo large strain.
Shear Buckling is how it will fail: Composites are generally sheets laminated over a light body like foam or honeycomb. When the sheets are in tension, they’ll take load happily, but Compression is often the source of failure because these thin sheets will ultimately have an urge to buckle. They could buckle inward and crush the core material or buckle outward and tear or delaminate from the core material. This is the case even in basic cantilever bending where one side is in tension and the opposite side: compression.
Weave orientation: 45-45, 0-90, 30-60-90 are all typical weave patterns. Align fibers with dominant axis of bending however it is important to supplement with fibers in the 45 or 30-60 to resist out-of-plane buckling and torsion. Furthermore, additional angles avoid warpage of the part during curing.
Taper the strength: When reinforcing a region (such as the center portion of a beam suffering peak bending stress), it is important to taper/feather reinforcement layers so as not to create a hard transition and stress concentration.
Kevlar Fiber can add toughness
TPU layers can add some toughness and improved bonding with core materials
Farbrication, Diagnosis and Repair Methods
Vacuum bagging, autoclave prepreg
Casting molds, applying pressure
Peel-ply stretched plastic to provide smooth surface finish video here
Identifying Fractures:
Thermal dissipation: carbon is a good thermal conductor. Thus applying heating and cooling to a fractured region can highlight hidden fractures when viewed with thermal camera.
Microscopy: Composite fractures will generally splinter and create jagged line. Additionally broken fibers splay and raise up along a fracture. Microscopic imaging while stress loading a region is another effective method for identifying potential fractures
There’re a lot more great tidbits and nuances I’d love to hear so please share comments