Computational Modeling of Failure at the Fabric Weave Level in Reentry Parachute Energy Modulators
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Energy modulators (EM) are textile mechanical devices designed to dissipate the pull-off loads that occur when parachutes are deployed. Although essential for mitigating shock loads, recent flight tests have shown increasing variability in EM behavior, raising concerns about the predictability of their performance and potential failure under dynamic loading conditions. In response, a new approach was implemented to create a computer model of an EM at the fabric weave level using LS-DYNA simulation software. This work was organized around two primary goals: (1) development of a stitch-per-unit model capturing the geometry and material behavior of the EM sewing pattern, and (2) implementation of a Python script to duplicate the unit model across the entire length of an EM ear, thereby simplifying the process of generating complex patterned geometries in LS-DYNA.
EMs typically consist of a long strip of structural Kevlar webbing that is folded and sewn together with a nylon zigzag stitching pattern to form an EM “ear”. When an EM is pulled above a load threshold during deployment, the nylon seams tear, unfolding the EM and dissipating shock forces. This process is illustrated in Figure 1, illustrating the stages of EM extension during running. In nominal cases, the EM tears cleanly with little damage to the Kevlar webbing. However, abnormal cases have been observed in which the nylon stitches along the ear are skipped during loading, that is, when a row of stitches does not tear in sequence. This results in failure of the surrounding Kevlar webbing, known as EM shredding. The inherent unpredictability of fabric behavior and the high variability of in-flight loading conditions make it difficult to identify a root cause through mechanical testing.
In this study, the development of a computational model of an EM in LS-DYNA was used to better understand the cause of EM shredding. While similar studies of fabric webbing have modeled fabrics on a global level, this approach represents each thread of the Kevlar weave and nylon stitching as individually modeled 3D solid elements. Modeling each thread individually in the weave is essential not only to analyze the failure mechanisms of nylon seams as they tear, but also to understand the failure of the Kevlar weave during EM shredding events.
The first phase of this work focused on modeling individual Kevlar and nylon threads within a representative stitch geometry. A 3D model of the Kevlar weave has been generated for the first time using TexGen, an open source software developed at the University of Nottingham. Using computer-aided design (CAD) software, nylon stitching running through two layers of the Kevlar fabric weave was added. The nylon sewing pattern consisted of a bobbin thread and a needle thread that were looped through the top and bottom layers of the Kevlar weave pattern, respectively, and twisted together at the end of each stitch between the two layers. The unit model was meshed in Hypermesh with 3D tetrahedral solid elements.
In LS-DYNA, material properties, contact, failure conditions, and boundary conditions were defined to evaluate the dynamic response of a point during tensile loading. Material behavior for both fabric types was defined using *MAT_ELASTIC (*MAT_001), and bidirectional surface-to-surface contact with erosion was implemented to capture the progressive failure of the Kevlar weave and nylon yarns. Boundary conditions were applied to reproduce in-flight tensile load scenarios. Additionally, several case studies were carried out to reduce the calculation time, including manual mass scaling, characteristic length analysis, and mesh quality optimization.
Preliminary results from the unit EM model validated the use of solid elements to capture EM behavior, particularly the interaction between Kevlar and nylon yarns. To streamline the construction of full EM models, the second phase of this work focused on developing a Python script to reproduce the LS-DYNA model per unit across the length of an EM ear. This eliminated the need for large CAD assemblies by generating the complete model directly from duplicating the unit model. This model is applicable to 2D and 3D solid and shell elements. Overall, these results will not only help identify the root cause of EM shredding, but also support the evaluation of new EM design variants. This modeling approach has broader implications for other work involving fabrics, enabling more accurate simulations and efficient design flows in aerospace textile applications.
For more information, contact Annika M. Vaidyanathan, Alexander Chin, John Bell and Rumaasha Maasha.
