A Fabric Material Model with Stress Map Functionality in LS-DYNA

Material 34 (MAT_FABRIC) in LS-DYNA is the material model of choice for fabrics when simulating airbag deployment in the automotive industry. Over the years, the model has been subjected to continuous robustness improvements and incorporation of important application features and today it comes in several forms and with an extensive set of parameters. The state-of-the-art modelling approach is to use the FORM=14 option which allows for specifying uniaxial stress-strain curves in the fabric’s warp and weft directions. The curves are carefully estimated from a combination of uniaxial and biaxial test data to appropriately reflect the inhomogeneous membrane strain state of inflated airbags, which has proved successful for replicating the overall global behavior. Recently an interest for predicting seam failure was raised that requires an accurate description of the local stress-strain relation. The stress response for the FORM=14 option is uncoupled in the warp and weft directions and hence does not account for the observed biaxial stiffening effect arising from the interaction of closely packed orthogonal fibers constituting the weave. To remedy this shortcoming FORM=-14 was developed where independent biaxial stress-strain curves can be added to the input in order to distinguish the biaxial from the uniaxial response. This can be seen as a step on the way towards the recently developed stress map material where an arbitrary stress-strain relation is specified through two tables. The tables should provide warp and weft engineering stress, respectively, for any combination of warp and weft engineering strains. The intention of the model is to take advantage of a wide variety of test data without compromise, for the best possible constitutive representation of the real-life fabric. The present paper introduces the new stress map material (MAT_FABRIC_MAP) including associated features and its potential advantages compared to the standard fabric model. The characteristics of the new model make it prone to dynamic instabilities that are successfully suppressed by introducing rate-independent dissipation through hysteresis modelling. This is not only critical for practical purposes but is also an interesting theoretical observation and will therefore be discussed. Finally we present numerical simulation results from various head form impact tests as a validation of the new stress-map model in combination with coating for bending resistance.