Why the true tensile strength of a UD tape is so difficult to measure
Thermoplastic unidirectional tapes (UD tapes) made from carbon fiber reinforced PEEK, PPS, or PA are considered key materials for structural lightweight applications in aerospace, automotive, and industrial engineering. Their axial tensile strength is the critical metric – it determines how much load a structure can bear, how thin the wall thickness can be designed, and whether a component truly realizes its weight-saving potential.
However, this metric is more difficult to determine than it seems at first glance. Current research findings show: Testing artifacts, manufacturing defects, and fiber geometry can reduce the measured tensile strength compared to the theoretical potential of the material by 20 to 50% – without this being visible in the test protocol. Anyone procuring, processing, or evaluating UD tapes should understand why.
Three artifacts dominate the tensile test
Thermoplastic UD tapes combine highly stiff, brittle fibers with a ductile matrix system. This combination makes them mechanically efficient – and testing technically challenging. In tensile testing, three sources of error regularly occur:
1. Clamping effects: Conventional wedge grip clamps create multiaxial stress states at the sample ends. The result: failure does not occur in the measurement length, but directly in the clamping area – and the measured strength is systematically below the actual value. Studies show that this effect is particularly pronounced in unidirectional CFRP samples, as the anisotropic stiffness converts any transverse load into a critical shear component.
2. Fiber misalignment and waviness: Even slight deviations in fiber orientation from the load axis lead to significant strength losses. A misalignment of just 5° reduces the tensile strength to about 75% of the ideal reference value – a loss of a quarter of the load-bearing capacity due to a barely visible geometric deviation. Micro-CT analyses show that local fiber angle deviations of up to 8.9° can occur at tape joints.
3. Microvoids and incomplete fiber wetting: Voids in the laminate interrupt load paths and act as notch stress sources. Even a porosity content of 2.5 vol% reduces the transverse tensile strength by more than a third. Therefore, a limit of less than 1 vol% applies in aerospace – a target that can only be achieved in practice through consistent process control.
Clamping technique: The underestimated influencing factor
The choice of clamping method is not a trivial matter – it determines whether a tensile test reflects the intrinsic material strength or provides a systematically low value.
Wedge grip clamps with ribbed jaws are widely used, but they generate high transverse and shear stresses at the sample ends. Sandpaper inserts reduce the local impression pressure and improve repeatability. However, the most reliable approach is offered by bonded, conically tapering end tabs made of fiberglass fabric: The gentle cross-section transition distributes the introduction forces evenly and shifts the failure into the actual measurement length.
The concept developed at IVW Kaiserslautern goes even further with the continuous carbon fiber/PA6 tabs: The tab extends over the entire sample length, has a tuned stiffness, and completely eliminates the geometric discontinuity at the transition. Compared to conventional tabs, this concept reduces the stress peak at the tab edge by about 10% – and provides measurements that come closest to the theoretical material value.
The core message for practice: Any clamping geometry that avoids abrupt stiffness jumps at the transition to the measurement length delivers more reliable and higher strength values. Those comparing materials based on tensile tests should always check which clamping method was used.
From fiber potential to component strength: A significant gap
Dry carbon fibers achieve intrinsic tensile strengths of 3 to 6 GPa in single fiber tests. In thermoplastic UD tape – embedded in a polymer matrix – practical values remain at 1.5 to 2.5 GPa. This corresponds to only 30 to 50% of the fiber potential. This gap arises from several mechanisms:
Volume dilution: 40 to 50% of the cross-section is accounted for by the matrix system, which contributes little to the strength axially.
Interface efficiency: The bond strength between fiber and matrix affects the strength only with a cube root dependency – high interfacial shear strength therefore brings quickly diminishing additional gains.
Manufacturing defects: Pores and fiber misalignments together cost an additional 20 to 40% of the theoretically achievable strength.
This gap is not a law of nature – it is a measure of optimization potential.
Which manufacturing parameters most strongly influence strength
Research results from recent years allow for a clear ranking of the relevant influencing factors:
Porosity is the most critical individual factor. Even 2.5 vol% void content reduces the transverse strength by 34%. For aerospace applications, a limit of less than 1 vol% is non-negotiable.
Cooling rate influences crystallinity and residual stresses. Rapid quenching – as can occur during in-situ consolidation without subsequent autoclaving – can reduce the transverse strength by up to 44% compared to the autoclaved reference laminate. Process-controlled cooling, tension from winding, and tempering steps after manufacturing can compensate for this loss.
Fiber volume content increases axial strength up to an optimum of 55 to 60 vol% – provided that fiber wetting remains complete. Beyond this range, the negative effects of incomplete impregnation prevail.
Interfacial adhesion – measurable through interlaminar shear strength (ILSS) – is crucial for transverse and shear strength. Plasma treatment of the tape surface before further processing has led in studies to a doubling of the ILSS, from 33 MPa to 75 MPa in LMPAEK/CF systems.
What a reliable tensile test of UD tapes requires
Direct answer for professionals: The tensile strength of thermoplastic UD tapes is reliably measured only when bonded, conically tapering end tabs or continuous tab concepts are used, the fiber angle in the measurement length is below 3°, and the porosity content is kept below 1 vol %. Only then does the measurement value reflect the intrinsic material strength – not an artifact from clamping or manufacturing defects.
Conclusion: Strength is not a material certificate – it is a process result
The tensile strength of a thermoplastic UD tape is not solely a function of the fiber material or the data sheet value. It is the result of a complex interplay of clamping methodology, fiber geometry, porosity content, interface quality, and cooling conditions.
For engineers and buyers, this means: Material comparisons based on tensile strength values are only meaningful if the testing standard, clamping method, and manufacturing history are known. And for manufacturers of thermoplastic composite structures, the quality of the input material – the UD tape – is the first and decisive lever for the strength of the finished component.
Those who consistently use this lever truly harness the potential of thermoplastic high-performance composite materials.
Are you developing a lightweight structure based on thermoplastic UD tapes and want to know which material quality and process management is sensible for your application? Talk to Alformet.
📚 SOURCES USED:
CompositesWorld: Gripping composite test specimens: Options and guidance
ScienceDirect: Specimen designs for accurate tensile testing of unidirectional composite laminates (2023)
SAGE Journals: A novel tab for tensile testing of unidirectional thermoplastic composites – Mohd Tahir et al. (2019)
ScienceDirect: Micro-CT measurement of fibre misalignment
MDPI Polymers: Effects of Void Characteristics on the Mechanical Properties of CF/PEEK Composites (2025)
ScienceDirect: Effect of plasma treatment on LMPAEK/CF tape (2024)
ScienceDirect: Moderation of thermoplastic composite crystallinity through tempering (2021)
PMC: Manufacturing-Induced Defect Taxonomy and Visual Detection in UD Tapes (2025)
IntechOpen: Unidirectional Carbon Fiber Reinforced Thermoplastic Tape in Automated Tape Placement Process