Influence of strain induced crystallisation on the dynamic crack propagation resistivity of NR vulcanisates

Ongoing demands on the improvement of service life performance of highly stressed elastomer parts require transferable and efficient laboratory evaluation procedures to estimate the suitability of the materials for safety-relevant usage. For analysis of the mechanical fracture behaviour often dynamic fatigue crack growth (FCG) tests on notched specimens are performed. Recently, company COESFELD MATERIAL TEST and the DIK developed an enhanced evaluation method which combines the automatic Tear- Fatigue-Analysis with Digital Image Correlation (DIC) and Post-Processing to get spatially resolved strain and stress information in the vicinity of the crack tip. While the strain information obtained by DIC are highly accurate, the stress information depends on the assumed material model, which is applied within a mapping procedure to locally transfer local strain to local stress. Since elastomers generally display a complex material behaviour involving e.g., hyper elasticity, relaxation, energy dissipation and in some cases, strain induced crystallisation (SIC), it is necessary to develop constitutive models in respect for these phenomena. Accordingly, an energetical model considering crystallisation is presented which can be applied for the interpretation of FCG experiment results, especially with respect to the significance in the prediction of local strain fields in the crack tip vicinity.