Unfilled PEEK tests at roughly 100 MPa tensile strength with a 3.6 GPa elastic modulus, while 30% carbon fiber filled PEEK (CF30) reaches 131-138 MPa tensile strength and 7.58 GPa tensile modulus per ASTM D638 [S4][S5]. The carbon fiber load case is the one designers should default to when stiffness-limited deflection, not just peak stress, drives the section.
The fiber-matrix rule of mixtures is what produces that jump. The reinforcing fiber itself carries 3,500-6,000 MPa tensile strength and 230-600 GPa modulus, dwarfing the 100 MPa / 3.6 GPa PEEK matrix, so even a 30% volumetric loading transfers enough load through the interface to lift the composite well above the neat resin [S4]. The same mass fraction of glass fiber (GF30) closes part of the gap but lands noticeably below CF30 on both axes.
Property Comparison: Unfilled PEEK vs PEEK-CF30 vs PEEK-GF30
On a direct tensile basis, PEEK-CF30 lands at 131 MPa (ASTM D638) for one extruded grade and 138 MPa for a comparable short-carbon compound, with a tensile modulus of 7.58 GPa and a flexural yield of 177.5 MPa per ASTM D790 [S5]. PEEK-CF30 also runs about 7% lighter than the glass-filled equivalent and roughly 3.5x higher in thermal conductivity than unfilled PEEK, which helps bearing surfaces shed frictional heat [S5][S8]. PEEK-GF30 trades that for higher tensile failure strain under impact, so toughness-driven picks still go glass.
For decision-making on a single part, the rule of thumb is simple: specify CF30 when you need the highest specific stiffness and tensile strength in a melt-processable thermoplastic, specify GF30 when you need impact toughness and a lower price, and skip both when the service environment exceeds ~250 °C long-term air exposure where the matrix, not the fiber, is the limit [S1][S5].
Tensile Strength Ranges by Fiber Form and Loading
Short carbon fiber PEEK compounds (P-SCF/PEEK) are bounded by the rule of mixtures. A 40 wt% short carbon fiber / PEEK composite has been measured at 363.96 MPa tensile strength, a 271.50% increase over the 98 MPa pure-PEEK baseline reported in the same study, with the gain driven by both fiber load and interface quality [S3]. That is a research ceiling, not a catalog value; production grades sit well below it because of fiber length distribution and molding-induced orientation.
Continuous unidirectional carbon fiber / PEEK is a different animal. AS4/PEEK at roughly 60% fiber volume reaches about 2,100 MPa tensile strength and 145 GPa modulus along the fiber axis, an order of magnitude above any short-fiber compound but only in the 0° direction, with transverse properties falling back toward matrix-dominated values [S6]. The same part made from neat PEEK would not survive the same load case, and the difference is fiber architecture, not resin chemistry.
Carbon Fiber PEEK: Datasheet Selection Criteria
Three numbers govern almost every CF-PEEK spec: tensile strength, tensile modulus, and elongation at break. PEEK-CF30 typically comes in at 131 MPa tensile strength, 7.58 GPa modulus, and 5.0% elongation at break, which makes it strong and stiff but markedly more brittle than unfilled PEEK [S5]. The other anchor value is Limiting Pressure-Velocity (LPV) at 0.876 MPa·m/sec with a 4:1 safety factor, the bearing-life figure engineers cross-check when the part is a bushing or thrust face [S5].
Beyond mechanics, the auxiliary properties often decide the call. CF30 PEEK cuts the coefficient of thermal expansion to 18.0 µm/m·°C (-40 to 149 °C, ASTM E831) versus ~47 µm/m·°C for neat PEEK, which matters for tight-tolerance parts mating to metals. The same loading also drops volume resistivity below 100,000 ohm/square, enough to qualify as static-dissipative per EOS/ESD S11.11, so CF30 is a common pick for ESD-sensitive semiconductor fixtures [S5].
Who Should and Should Not Specify CF-Reinforced PEEK
CF-reinforced PEEK is the right call for aerospace brackets, semiconductor handling fixtures, pump and valve seats, medical instrument handles that need X-ray transparency with stiffness, and any structural injection-molded or machined part where the section is stiffness-limited. It also dominates in sliding-wear applications at temperatures up to 250 °C long-term air exposure because the carbon fiber both stiffens the part and pulls heat away from the contact zone [S1][S5][S9].
It is the wrong call when impact is the controlling load (use glass-reinforced or unfilled PEEK instead), when the part sees strong mineral acids at high concentration (PEEK itself is only "limited" resistant below pH 3 even with the carbon filler), when the design needs isotropic properties from a short-fiber compound, or when budget drives the spec and ~30% more tensile strength is not worth the 2-3x material cost premium over neat PEEK [S5].
Design Limits and Failure Modes to Watch
The 5.0% elongation at break of CF30 PEEK is the single number that kills the most designs. Unfilled PEEK runs roughly 30-50% elongation, so converting an existing neat-PEEK part to CF30 without re-checking strain can take a ductile, forgiving part and turn it into a brittle one that cracks at the first stress concentration [S4][S5]. The standard mitigation is generous fillet radii, no sharp internal corners, and a strain-based (not stress-based) FOS check on the FEA.
Anisotropy is the second trap. Short carbon fibers in injection-molded parts orient with the melt flow, so a tensile coupon cut transverse to flow can show 30-50% lower strength than the longitudinal value, and that gap is even wider in glass-filled grades because of the larger fiber aspect-ratio distribution. If your section is loaded in multiple directions, the conservative play is to pull coupons from a prototype in the actual flow direction and test to ASTM D638, not to trust the longitudinal datasheet value [S3][S5].
Standards, Spec Format, and What to Put on the Drawing
The baseline spec for a CF30 PEEK shape should name the resin family (PEEK, polyetheretherketone), the filler (30% by weight carbon fiber, often designated "CA30"), the test methods you will accept (ASTM D638 for tensile, ASTM D790 for flexural, ASTM D792 for specific gravity, ASTM E831 for CTE), and the lot-level traceability requirement [S5]. For medical or food contact, add USP Class VI, FDA, or 3-A Dairy compliance lines, none of which the standard CF30 grade carries; you have to specify a compliant compound on the BOM [S5].
For continuous-fiber laminates, the spec is longer: name the fiber (AS4, T700, or equivalent), the fiber areal weight, the layup sequence, the fiber volume fraction target (typically 60% for structural parts), and the test methods for both 0° tension (ASTM D3039) and short-beam shear (ASTM D2344). Unfilled PEEK in this context appears only as the matrix reference, and the 2,100 MPa / 145 GPa AS4/PEEK value should be on the drawing as the design allowables, not the 100 MPa neat-resin number [S6].
Trackable signals to watch: (1) higher-fiber-volume (>50 wt%) short-carbon PEEK compounds with published datasheets in the 350+ MPa tensile range, moving the catalog ceiling up from the 138 MPa CF30 norm [S3]; (2) clearer OEM guidance on transverse-to-flow derating factors for injection-molded CF30, since most published values are longitudinal only. For related reading on fiber architecture trade-offs in composite parts, see the CFRP fatigue vs metal crack propagation selection guide.