Rate-effect on fracture behavior of core-shell-rubber (CSR)-modified epoxies

A bisphenol A diglycidylether (DGEBA) epoxy was modified with up to 20 wt% of a core-shell rubber (CSR), and mechanical properties were investigated a t different crosshead rates (from 0.5 mm/min to 500 mm/min). The yield stre ngth and Young's modulus decreased almost linearly with increasing rubber c ontent or decreasing crosshead rate. Fracture toughness, K-IC, was signific antly improved by the addition of CSR to the pure epoxy. The optimum rubber content was between 15% and 20% at the lowest crosshead rate (i.e. 0.5 mm/ min) and shifted to higher rubber content at higher crosshead rates. K-IC d ecreased slightly as the crosshead rate was increased from 0.5 mm/min and d ropped significantly when the crosshead rate approached 500 mm/min. Rubber cavitation-induced local plastic deformation was identified as the principa l energy absorption mechanism, and it was independent of rubber content and crosshead rate. The variation of K-IC with rubber content and crosshead ra te correlated well with the length of the stress-whitened zone.