Every strength figure for a fibre-reinforced plastic is half a figure. The CFRP entry in this library lists 2,860 MPa along the fibres and 49 MPa across them: one coupon, two numbers, a factor of 58.4 between them.
Steel has no grain, so one tensile number describes it completely. A laminated composite is fibres in a matrix. The fibres carry load only in the direction they run, and turn the load through 90 degrees and what is left is resin. That is not a defect to be apologised for — it is the design variable. The same plate can be stiff one way and soft the other, deliberately. The cost is that no single number means anything on its own.
Two ratios, and they do not agree
| Material | Tensile along / across (MPa) | Strength ratio | Modulus along / across (GPa) | Stiffness ratio | Comp. / tens. |
|---|---|---|---|---|---|
| Glass Fibre Composite (GFRP) | 1,080 / 39 | 27.7 | 39.0 / 8.6 | 4.5 | 0.57 |
| Carbon Fibre Composite (CFRP) | 2,860 / 49 | 58.4 | 177.0 / 10.8 | 16.4 | 0.66 |
| Aramid Fibre Composite (AFRP) | 1,280 / 30 | 42.7 | 87.0 / 5.5 | 15.8 | 0.26 |
Read the strength ratio and the stiffness ratio against each other. For GFRP the strength falls by 27.7 across the fibres while the stiffness falls by only 4.5. For CFRP the same two figures are 58.4 and 16.4. If anisotropy were one property of a material, those ratios would have to be equal. They are not, and the gap is large enough that "how anisotropic is it" is not a question with an answer until you say which measurement you mean.
The reason is that the two are carried by different things. Stiffness across the fibres is shared between the resin and the fibre-matrix interface, both of which still shear elastically under load. Strength across the fibres is not shared: the first crack in the matrix is a crack across the whole load path. So a part loaded across the fibres can be comfortably inside its deflection limit and past its strength limit at the same time, and nothing about the stiffness figure warned you.
The rod test
Ratios hide the trade-off. The useful question is the one a part actually has to answer: same job, how much material? Take a rod 1 m long carrying 100 kN in tension. The section it needs is the load over the strength; the mass is that section times the density.
| Material | Load direction | Tensile (MPa) | Section (mm²) | Mass (kg) |
|---|---|---|---|---|
| Glass Fibre Composite (GFRP) | Along the fibres | 1,080 | 92.6 | 0.194 |
| Glass Fibre Composite (GFRP) | Across the fibres | 39 | 2,564.1 | 5.385 |
| Carbon Fibre Composite (CFRP) | Along the fibres | 2,860 | 35.0 | 0.056 |
| Carbon Fibre Composite (CFRP) | Across the fibres | 49 | 2,040.8 | 3.265 |
| Aramid Fibre Composite (AFRP) | Along the fibres | 1,280 | 78.1 | 0.108 |
| Aramid Fibre Composite (AFRP) | Across the fibres | 30 | 3,333.3 | 4.600 |
| 8.8 steel | Any | 800 | 125.0 | 0.981 |
None of this is a design calculation. There is no safety factor, no buckling, no fatigue, no joint, and no load here but pure tension. It is strength and density divided into each other, and the reason to do it at all is that it puts both directions on one axis.
The across-fibre CFRP row is the whole argument: 3.265 kg, against 0.981 kg for a rod of 8.8 bolt steel. Rotated 90 degrees, the carbon composite is 3.3 times heavier than steel. Along the fibres the same material is 17.5 times lighter. Nothing changed between those two rows except which way the load points.
The comparison is also weighted in the composite's favour, which is why it is worth making. 2,860 MPa is a typical value measured on a unidirectional coupon. 800 MPa is the minimum a bolt of that class must reach, not an average of what arrives in the box. Give the composite the better footing and the across-fibre version is still the heavy one.
Aramid loses in compression
AFRP keeps 26 percent of its tensile strength in compression: 335 MPa against 1,280 MPa along the fibres. GFRP keeps 57 percent and CFRP 66.
That one number explains a rule usually repeated without it: aramid is a fibre for ropes, for ballistics and for tension members, and not for anything that has to push. It is not that aramid is weak — along the fibres it is stronger than glass — it is that a quarter of its strength is unavailable the moment the load reverses.
Note what those figures cover. The records here give compression along the fibres only. There is no transverse compressive strength in this library, so this page does not print one. Each material has a compression figure for one direction and a tension figure for two, and the missing corner is the one a bolted joint would load.
Wood has the same problem, measured another way
Direction dependence is not what makes these materials modern. Wood has it, and had it first. What differs is the measurement.
18 of the 24 species here carry both a radial and a tangential shrinkage figure; the rest are imported species whose row in the source table is a dash. In all 18 the tangential figure is the larger — from 1.3 times on Birch (Yellow Birch) (9.5 percent against 7.3) to 2.2 times on Beech (American) (11.9 against 5.5).
That 2.2 is not a smaller version of the 58.4 above, and this is as close as this article will come to putting them side by side. One is a ratio of shrinkage on drying, the other a ratio of tensile strength, and a number that says "twice" only means something once you know what is being doubled. What the two do share is narrower and more useful: in both materials the direction of measurement is part of the property. A figure quoted without it is not a conservative figure. It is an unknown one.
At the joint
All three composite pages here carry a joining note, and the three say nearly the same thing three ways.
- GFRP — the laminate carries the bolt load poorly through a hole, so washers and edge distance are not detailing preferences.
- AFRP — the fibres buckle in compression around the hole, and the material creeps under a sustained bearing load, so a preloaded joint relaxes.
- CFRP — carbon is cathodic to steel and aluminium, so in a wet joint the metal fastener is the part that corrodes.
None of the three notes carries a number, and neither does this library: there is no bearing strength, no clamp-load limit and no galvanic series in here. What the direction data adds is where to look. A bolt loads the laminate in bearing, bearing runs through the thickness, and through the thickness is the one direction with neither fibres nor figures.
Reading a composite datasheet
- Find the layup. All 3 records here are unidirectional laminates; a woven or quasi-isotropic plate of the same fibre is a different material with different numbers.
- Find the fibre volume fraction. It runs from 55 to 65 percent here, and every figure moves with it.
- Find the direction. "Tensile strength 2,860 MPa" without "along the fibres" is a fibre property, not a plate.
- Find tension or compression. For aramid the two differ by a factor of 3.8.
- If only one direction is quoted, you have been told the best case and not the material.
The composites index lists all 3, and each page prints the pair of numbers rather than one. For the other kind of anisotropic material, the wood stiffness guide reads grain direction out of modulus of rupture and deflection, and the carbon steel entry is there for the comparison above.