How hydrogen bonds influence the slow crack growth resistance of polyamide 12

Slow Crack Growth (SCG) is considered to be the most critical failure mode for a variety of long-term applications. A key element within this research was to examine the SCG behaviour of polyamide 12 (PA12). Because hydrogen (H) bonds are well-known to affect the mechanical properties of plastics, such as PA12, special focus was put on their influences during quasi-brittle fracture. Therefore, the total fracture energy Gf of PA12 was divided into a pure chain disentanglement fracture energy, driven by creep processes during SCG (Gdis,f), and the additional energy needed to dissociate effective H-bonds that are actively resisting SCG (GH,f) within PA12. In that context, Gf was calculated from the experimentally measured activation energy for SCG via Cracked Round Bar (CRB) tests at different temperatures and the subsequent use of a time-temperature superposition. Subsequently, GH,f, was estimated with the aid of a modified Sequential Debonding Fracture (SDF) model. Subtracting GH,f from Gf, the remaining energy could be classified as Gdis,f and was calculated for different amounts of effective H-bonds. It was demonstrated for the selected material, that GH,f would become the dominating source of energy which has to be overcome, if at least 45% of all H-bonds crossing the crack plane engage in the fracture process and follow a sequential debonding mechanism.