Airbag inflators are pyrotechnic devices that have an important role in the passive safety system of cars. In service, they are subjected to very high transient loadings. The design of the inflators must exclude any risk of failure for a wide range of temperatures. It requires the use of mechanical behaviour and advanced fracture models adapted to these loading conditions.

This work deals with the set-up of an experimental characterization of the thermo-visco-plastic behaviour as well as the study of the effect of the stress triaxiality on the ductile fracture mechanisms of the tubular material, made of low-carbon high-strength steel. For that, different tests were performed on specimen machined from tubes with an external diameter of 30 mm and a thickness of 2 mm:

- Uniaxial tensile tests on axisymmetric tubular specimen under quasi-static loading at different temperatures (from ambient to -120°C ) with a strain rate range from 10-3s-1 to 10-1s-1

- Uniaxial split Hopkinson tensile bar (SHTB) tests on axisymmetric tubular specimen at ambient temperature with a strain rate ranging from 100 s-1 to 700 s-1

- Tensile tests on double edge notched specimen with two symmetric circular cut-outs and different notch radii in order to obtain different values of stress triaxiality range from 0.33 up to 0.58

According to the experimental results, a micromechanical ductile fracture model was developed. This model is based on the Gurson-Tvergaard-Needleman approach to describe the nucleation and the cavities growth on the one hand, and on the Thomason model to describe the coalescence mechanisms, on the other. The model takes also into account both temperature and strain rate effects on the material response.

The thermo-viscoplastic model parameters were identified from uniaxial tensile tests and the parameters of the ductile fracture model were identified from tensile tests on double notched specimen and using finite elements computations and inverse analysis.

The modelling results showed good agreement compared to the experimental results obtained with a Charpy impact tests on ring-shaped specimen with two symmetric V-notches, machined from the tubes.