Fibre Angle, Stiffness and Strength

Conventional filament winding cannot place fibre at true 0°. The wind angle is a process constraint, not a design choice, and it leaves structural components leaning on resin-dominated properties in exactly the direction the load runs.

CompoTech’s Advanced Winding Technology removes that constraint. Automated Filament Laying (AFL) places continuous fibre along the load axis at a genuine zero degrees, consolidated as it is laid.

Strength is affected roughly twice as much as stiffness by the same off-axis angle. It is consistent with our own verified production figures, where the bending-strength gain (up to 50%) is substantially larger than the axial stiffness gain (10–15%) against conventionally wound composites of equivalent size.

The practical value of recovering that performance is the material it lets you remove. At equal stiffness, a 10–15% higher axial modulus supports a 9–13% thinner wall — and therefore 9–13% less fibre in the part, at the same fibre grade. The gain is best spent on a lighter, cheaper section rather than a stiffer one.

Traditional filament winding builds structural tubes and beams from helical and hoop fibre layers wound at an angle to the part axis. That angle is inherent to the process. Tow is laid onto a rotating mandrel and held there by the tension of the wrap, which only works if the fibre path has a helix component. A conventional winder therefore has no route to a true 0° layer. The nearest available approximation is a low-angle helical wind — and every degree of that angle is paid for in axial performance.

This is efficient for pressure vessels and simple torsional loads. But for components that see primary bending or axial loads — booms, masts, structural beams, robotic arms — angled fibre alone under-delivers. Some of the applied load is carried by the resin matrix rather than the fibre, and resin is orders of magnitude weaker and more compliant than carbon fibre.

Engineers have historically compensated with thicker walls, bonded-in unidirectional tape or pultruded inserts, or oversized sections. All of them add weight, cost, and additional failure modes at the bond lines.

The fundamental issue is a fibre placement problem, not a materials problem.

AFL is the core capability of CompoTech’s Advanced Winding Technology. Rather than approximating an axial layer with a very low helix angle, AFL places continuous fibre tows at a genuine 0° to the part axis — so the finished part has fibre running precisely where the primary load runs.

What makes this possible is that the tow does not rely on wrap tension to stay in position. AFL compresses and consolidates the axial fibre as it is laid, and the cross-wound layers placed over it in the same cycle lock it down. Removing the need for a helix angle to hold the fibre is what allows the angle to go to zero.

Why the angle matters

• Carbon fibre is stiffest and strongest along its length, and a few degrees of wind angle measurably reduces the effective modulus in the load direction.
• True 0° placement means load transfers directly into fibre, not through resin shear between angled plies.
• Cross-wound layers can still be combined with the 0° axial layer, so torsional and hoop performance is retained alongside the axial gain.

The chart below shows the theoretical single-ply relationship, normalised so a true 0° ply reads 100%. At a ±10° equivalent wind angle — a realistic “near-axial” helical layer — the single-ply axial modulus has already fallen to 57% of its true 0° value. A 43% reduction.

The AFL process accepts dry or wet-impregnated fibres and towpreg — carbon, glass, aramid, or ultra-high modulus pitch fibre grades — allowing the same process to be tuned for stiffness-critical, cost-critical, or high-temperature applications. Fibre placement, consolidation and tension are controlled within a single automated cycle, which is what keeps void content and tow straightness tighter than a comparable wet-winding process. Both matter for the mechanisms in the next section.

Wind angle is the largest single factor in the stiffness gap between traditional filament winding and Advanced Winding Technology. It is not the only one. Three further mechanisms, all documented in the composites literature, compound the effect in a real wound component.

Tow crossover and undulation. Helical filament winding is a repeating pattern: fibre tows cross over one another at regular intervals to build up wall thickness. At each crossover a tow rides over or under the previous layer, producing a local geometric undulation. This is a distinct effect from the wind angle itself, and it has been directly modelled and measured in filament-wound cylinders, where fibre undulation and crossover geometry are shown to reduce stiffness and increase the risk of local cracking at the undulation site.

Void content. Wet winding entrains some air during resin impregnation and consolidation. The effect is well quantified: published data on carbon/epoxy laminates puts a 2 percentage-point increase in void content at around a 10% drop in flexural modulus, alongside a roughly 20% drop in interlaminar shear and flexural strength.

Local fibre misalignment. Even nominally straight tow is not perfectly straight — tension variation during winding introduces small-scale waviness. This is a well-established mechanism in the compression-properties literature, where even a few degrees of local misalignment measurably reduces both modulus and strength.

AFL’s straight, in-process consolidated tow placement at true 0° removes the wind-angle penalty and the crossover/undulation penalty entirely, and the consolidated single-pass process is set up to keep void content and fibre tension tighter than a comparable wet-winding process. Voids and minor misalignment remain manufacturing realities that affect any composite process to some degree — Advanced Winding Technology included.

The chart below extends the single comparisons above into a full map — stiffness penalty as a function of both off-axis angle and the proportion of the laminate carrying axial load. It confirms the same conclusion from a different direction: the off-axis angle is the dominant variable, and the laminate’s axial ply fraction only modestly changes the size of the penalty.

Every mechanism discussed so far — off-axis wind angle, tow crossover and undulation, void content, local fibre misalignment — affects strength as well as stiffness. In each case the effect on strength is larger, often considerably larger, than the equivalent effect on stiffness.

The reason is structural rather than incidental.

Stiffness is an averaged property. Load redistributes across the full cross-section, so a laminate’s modulus reflects something close to a rule-of-mixtures average of everything in it.

Strength is set by the weakest local point — a resin-rich pocket, a crossover undulation, a kinked fibre — because that is where failure initiates.

Mechanisms that barely dent an averaged property can be decisive for a localised one.

Void content. Voids act as crack initiation sites rather than simply diluting the load-bearing cross-section, so their effect on strength outpaces their effect on stiffness. Published data on carbon/epoxy laminates shows a 2 percentage-point increase in void content corresponding to roughly a 10% drop in flexural modulus, against a roughly 20% drop in interlaminar shear and flexural strength — about double the impact.

Tow crossover and undulation. The same undulation regions that reduce stiffness in a helically wound part are also documented crack initiation sites. The crossover geometry inherent to traditional filament winding therefore carries a strength and fatigue-life cost beyond its stiffness cost.

Local fibre misalignment. This is the most severe case of all, particularly under compressive loading. A peer-reviewed study of unidirectional composites found that a minor fibre misalignment of around 5° produces a 30–50% drop in compressive strength, worsening to roughly 70% at 10–15° off-axis. This is driven by shear instability in the matrix triggering fibre microbuckling and kink-band formation at stresses well below the fibre’s own compressive capability. The equivalent stiffness knock-down at these angles is a fraction of that size.

The single-ply figures above isolate the fibre-level effect. The reference laminate — 70% axial / 25% at ±45° / 5% hoop, a hoop-to-off-axis ratio of 1:5 — tells the same story at component level, with the magnitudes damped by the presence of the off-axis and hoop groups.

Applying classical lamination theory for stiffness and the Tsai–Hill criterion against fibre-dominated failure of the axial group for strength, true 0° versus a ±10° equivalent:

These figures are verified in production and used in CompoTech’s published technical literature. They compare an Advanced Winding Technology component against a conventionally wound composite of equivalent size.

• 10–15% higher axial stiffness, achieved by orienting high-modulus carbon fibre directly along the beam axis rather than approximating it with a low helix angle.
• Up to 50% greater bending strength, driven by the same true 0° placement plus the elimination of stress concentrations at bonded or bolted fittings.

In practice this allows engineers to hold the same structural performance in a lighter, thinner-walled section — or to hold the same section and carry meaningfully more load — without moving to a different fibre grade or adding mass in bonded reinforcement.

For engineers evaluating whether AFL is the right fit for a component, two questions tend to decide it.

Is the primary load axial or bending — not just torsional or pressure? Pressure vessels and parts dominated by hoop or torsional loading are well served by conventional helical winding, where the fibre angle is already close to optimal. AFL earns its keep on booms, masts, beams, robotic arms, and any structure where load runs along the part length. The greater the axial or bending component of the load case, the greater the benefit of true 0° fibre.

What does the weight or section budget allow? Because AFL recovers stiffness and bending strength without adding wall thickness, it typically allows either a lighter section at the same performance, or a longer, higher-load part at the same section. Either is useful wherever mass is constrained — aerospace, robotics, portable and telescopic structures — or wherever a longer single-piece part avoids a joint altogether, as in long booms and multi-stage telescopic masts where the part length itself is the design driver.

Our engineering team works from load case, envelope and duty cycle rather than a fixed catalogue section. Tube and beam geometry, fibre grade and process are selected against the application, then costed and quoted against our standard sections catalogue or as a bespoke tooling run.

AFL is a fibre placement solution to a structural engineering problem.

Conventional filament winding leaves axial performance on the table, and the loss is not a small one: a few degrees of wind angle costs 43% of single-ply axial modulus and considerably more of its axial strength — before tow crossover, void content and local misalignment are counted.

At laminate level those penalties are damped, but they do not disappear, and they remain asymmetric. For the 70/25/5 reference layup, a ±10° equivalent costs 7.3% in stiffness against 13.8% in strength. That asymmetry is the theoretical basis for CompoTech’s verified production figures, where the bending-strength gain is several times the stiffness gain.

True 0° axial fibre placement recovers that performance directly, at scales from small brackets to 12-metre structural members. The return on it is realised in the section design rather than on the datasheet: taken as permission to thin the wall, a 10–15% stiffness recovery removes 9–13% of the fibre from the part — a lighter structure and a lower material cost, from the same fibre grade and the same process.

That is the argument for treating AFL as an input to structural optimisation rather than a like-for-like substitution for helical winding.

Send us the load case, envelope and duty cycle, and we will tell you whether true 0° fibre placement changes the section — and by how much.