| dc.description.abstract | Semicrystalline thermoplastics (SCTPs) are technologically important for their easy formability, high weight specific strength, and recyclability. However, certain environmental conditions, such as ultraviolet (UV) radiation, high temperatures, or moisture, initialize chemical reactions that degrade the material in a process referred to as chemical aging. Eventually, chemical aging leads to a catastrophic loss of ductility, i.e. embrittlement, often characterized a reduction of the fracture strain in uniaxial tension. Some lifetime criteria propose that embrittlement occurs at a critical molecular state achieved purely through chemical reactions. However, aging creates micromechanical damage (voids), so predictions considering only the molecular state ignore this mesoscale effect. Here, the importance of aging-induced damage to the embrittlement of SCTPs is investigated. Aging-induced damage is investigated using Polyamide-6 (PA-6) exposed to UV radiation. The effects of aging on the mechanical behavior are investigated using mechanical tests and video-based extensometry on both bulk and film specimens. Test videos also enable macroscopic observations of damage. The effects of aging on the development of micromechanical damage is evaluated using both ex situ and in situ synchrotron tomography. A viscoplastic constitutive model for a semicrystalline polymer is formulated, implemented, and calibrated to study the mechanical behavior of the bulk PA-6 material used here. Fracture by the growth and coalescence of micromechanical damage in aged and unaged PA-6 is modeled using a micromechanics-based continuum damage model. Fracture in an aged SCTP is determined to be a growth-controlled phenomenon. During loading, cracks appear on the surface of the aged material before the peak load. The density of the surface cracks increases with both strain and aging time. Additionally, in situ synchrotron tomography reveal that the aging-induced ductile-brittle transition coincides with a damage mechanism transition. Finally, it was found that a micromechanics-based damage model accurately predicts fracture strains of both aged and unaged material. | en |
