Creep rupture and time to failure in para-aramid yarns
Time to failure explained
Key findings
- Para-aramid yarns under sustained load accumulate internal damage over time, leading to eventual failure through creep rupture. Load level determines lifetime, and the relationship is non-linear.
- Our time to failure dataset spans over 30 years of continuous measurements across all Teijin Aramid aramid fibers, enabling reliable lifetime predictions across loads and temperatures. These data allow customers to optimize their designs for target load and service life.
- Primary properties such as tensile strength and modulus do not predict long-term creep rupture behavior. Yarns that match on a datasheet can differ by orders of magnitude in time to failure.
- At room temperature, Twaron® yarn is expected to survive a fixed load of 50 % of the breaking tenacity (safety factor of 2) for 30 years.
- High modulus Twaron® 2200 is expected to survive 30 years at 100 °C using a safety factor of only 3 (this safety factor is calculated relative to the breaking tenacity at room temperature).
- For fiber reinforced applications with long service life, the selection of a p-aramid yarn type with appropriate creep rupture properties is important and becomes even more critical in applications at elevated temperature.
- Twaron® 2200 outlasts the majority of competing high-modulus para-aramids by 10 to 1,000 times at fixed dead load.
- Twaron® 3200 shows an even higher performance regarding time to failure than Twaron® 2200
Predicted lifetime of Twaron® 2200 under high sustained load
Temperature at which Twaron® 2200 still lasts 30 years
Technical Paper
What time to failure means for long-life applications
When an aramid yarn carries a sustained load below its breaking force, it does not last indefinitely. Internal damage accumulates over time, visible as creep: irreversible elongation. Once the yarn reaches a critical creep level, it breaks. This failure mechanism is called creep rupture.
Creep rupture can take hours or centuries, depending on the load. The table below shows approximate failure times for Twaron® 2200 yarn at three load levels, indicative for para-aramid yarns in general.
The relationship between load and lifetime is non-linear. A small increase in load reduces time to failure by orders of magnitude. For applications requiring an industrial lifetime of 30 years or more, including a safety factor, yarns are typically designed to operate well below 50% of their breaking tenacity, as lifetime drops sharply above this range.
Specifying aramid for long-life applications therefore requires time to failure data at the actual load and temperature conditions of the design. The breaking strength data alone is not sufficient.
Where creep rupture is a design factor
Twaron® serves many applications where static load acts on the fiber over extended periods. These include all-dielectric self-supporting (ADSS) cables, crane tendons, reinforced thermoplastic pipes (RTP), pressure vessels, optical fiber cables, and mooring lines for floating wind platforms.
Static loading also occurs where it is less immediately obvious. During subsea power cable deployment, the cable hangs over the ocean floor for extended periods. The seabed is uneven, and cables crossing submarine canyons carry constant tension on the aramid armoring long after installation.
In each of these cases, designing for the full economic lifetime of an application requires reliable data on time to failure at operating load and temperature. Without it, aramid content cannot be specified with confidence.
Our testing protocol
Time to failure is measured using tensile testers and levers. Levers are a cost-effective alternative to tensile testers, especially for long running experiments (see photos below). Fibers, which are difficult to see in the photo, are clamped vertically between two clamps and close to the lever’s iron frame (vertical iron bars). Each lever is equipped with a horizontal arm. To keep the lever arm stable, the load on the fiber must neutralize the weight at the tip of the arms. The arm length ratio ensures that the load on the yarn specimens is 5 times the weight fixed at the arm tip.
We use many levers in parallel to boost the number of measurements per unit of time. The clamps mounted on the levers have a similar design to the clamps of the tensile testers. We also use levers operating at elevated temperature, where part of the free length of the clamped yarn specimens is heated.
Photograph 1: detail view of bollard-type clamps with Twaron® yarn
Photograph 2: set of 12 levers for time to failure measurements;
Creep rupture experiments range from a few hours to over a year and everything in between. Typically, dozens of experiments are performed in order to classify the creep rupture performance of a Twaron® yarn type. Due to variability in creep rupture, large datasets are required to produce reliable lifetime predictions.
This depth of data means lifetime curves can be constructed across a range of loads and temperatures, rather than relying on short-term extrapolation. It is the basis on which application-specific predictions are made.
Creep rupture of Twaron® 2200 at various temperatures
Creep rupture experiments are visualized by plotting dead load against survival time (time to failure), where the time scale is compressed such that short-term and long-term performance can be shown on the same graph. Figure 1 shows assembled measurements for Twaron® 2200 in a range of 65 %BT to 95 %BT (BT = Breaking Tenacity) corresponding to failure times from half a minute up to a year.
There is considerable variability in the data, yet the strip containing (almost) all the individual measurements (black dots) shows a clear linear relationship between the logarithm of time and load. The ‘prediction strip’ is bordered by prediction boundaries (solid lines), which can straightforwardly be obtained from regression theory. The lower prediction boundary is most important: for a given dead load, 97.5% of the failures are expected to have a survival time exceeding this boundary (95% of the observations will be inside the strip).
Twaron® 2200 yarns are predicted to survive 30 years at a sustained stress of 1.81 GPa (56 %BT) with high probability. Here 1.81 GPa is coined the long-term breaking load, i.e. the maximum load to be applied for which we still expect survival after a given target service life, in this case 30 years. Figure 1 (below) shows the ‘prediction strips’ for Twaron® 2200 at various temperatures. This yarn type is likely to survive 30 years of service life at 100 °C if the dead load does not exceed 1.10 GPa (34 %BT). Here, 34 %BT refers to the breaking tenacity at room temperature. Hence with a working load of 1/3 of the strength (safety factor = 3) combined with very high temperature, Twaron® 2200 yarn retains strong creep rupture performance and is predicted to survive its full 30-year design life.
Figure 1: Schematic representation of time to failure of Twaron® 2200 at various temperatures
Twaron® 2200 compared to alternative para-aramid yarns
The creep rupture behavior is not the same among the members of the Twaron® family. For example, Twaron® 1000 kept at 100 °C will fail within 30 years for any non-zero dead load. So, for long-life applications at elevated temperature, Twaron® 2200 is anyway to be preferred over Twaron® 1000. This is also true if we limit ourselves to high modulus (HM) para-aramid yarns of different brands. In figure 3 2 we plotted a variety of time to failure versus load measurements for high modulus para-aramids and compared these with Twaron® 2200. We see wildly varying creep rupture behavior of alternative para-aramid brands. There is considerable variability in the data points. Most of the points are to the left of Twaron® 2200 and when these measurements are extrapolated are likely to cross the horizontal service lives at lower loads. To get a good overview of fiber performance, it is essential to test samples from multiple batches and production dates. This is done for Twaron® 2200 and the results are confined inside the two dashed lines.
Figure 2 shows that Twaron® 2200 matches or outperforms the best HM para-aramids in the market. For the majority of competitors, the performance gap is substantial. Twaron® 3200 even outperforms our standard Twaron® 2200. Continuous changes in product recipe and spinning settings do gradually change creep rupture behavior (while primary mechanical properties may be retained). It is therefore recommended to periodically check on time to failure performance. For our Twaron® fibers, such quality control is in place.
Figure 2: Time to failure measurements: Twaron® 2200,Twaron® 3200, and five measured competing HM para-aramids.
To appreciate the performance distance between Twaron® 2200 and most of the competitors, simply focus on a specific dead load and analyze the vertical distance between the load curves. Many competitors are 1 to 3 vertical steps below the lower prediction boundary, corresponding to a 10 to 1,000 times shorter time to failure than the worst performing Twaron® 2200 specimens.
Beyond primary properties on datasheets
While primary properties such as tensile strength, modulus, and elongation-at-break appear on datasheets and are sometimes comparable across producers, they do not predict creep rupture behavior. Differences in polymer processing and fiber structure lead to different resistance to long-term damage under load. Yarns that look equivalent on a datasheet can differ by orders of magnitude in time to failure under sustained load. Data on creep rupture is not widely published, particularly at elevated temperatures. If a supplier cannot provide time to failure data for the load and temperature conditions of your application, the design is proceeding without a complete picture of material behavior.
Consequences for application design: opportunities and constraints
Creep rupture is a critical fiber property when it comes to aramid fiber selection since it affects the economic lifetime of end products directly. Yet creep rupture cannot be inferred from datasheets, and it rarely gets disclosed by other manufacturers.
Switching aramid suppliers without verifying time to failure data introduces risk of premature failure in service, particularly in applications where replacement or inspection is difficult or costly.
Our dataset spans over 30 years of continuous measurement across all aramid fiber types in our portfolio. This allows us to construct lifetime curves across a range of loads and temperatures, rather than relying on short-term extrapolation. The same research program that produced this dataset has also been used to optimize Twaron® fiber processing for improved long-term performance under load.
Application-specific data and technical consultation
Lifetime predictions for specific load and temperature conditions are available on request. Contact Teijin Aramid with the yarn type, target load, and operating temperature. Our technical team can provide lifetime data for your specific design conditions.
Head Office Tivolilaan 50, 6824 BW Arnhem P.O. Box 5153, 6802 ED Arnhem The Netherlands Tel: +31 88 268 88 88